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National LM2599 Manual

1. DS012582 7 Vout 1 23V lLoap 120 mA 50 25 0 25 75 100 125 JUNCTION TEMPERATURE 9C DS012582 10 www national com 66S2IN1 LM2599 Feedback Pin Bias Current 10 Flag Saturation Voltage a in gn o ADJUSTABLE VERSIO N ONLY FEEDBACK BIAS CURRENT nA m Cn 50 25 0 25 50 75 100 125 JUNCTION TEMPERATURE 9C DS012582 11 0 7 0 6 0 5 Vy gt 1 5V Voeray 0 3V SATURATION VOLTAGE V 0 4 0 3 0 2 0 1 0 1 2 4 5 CURRENT mA DS012582 12 Shutdown Soft start Soft start Response DS012582 14 E Ty 25 C toms 120ms 12s B 100 Csoft start 80 x 0 01 uF e 60 1 0 1 uF 5 40 a 1 0 uF 2 20 0 2 4 4 ms 44 ms 440 ms 2 V z 0 0 1m Im 10m 100m 10 TIME seconds DS012582 18 Soft start Current 100 6 T 125 C e 80 5 E 60 gt a 4 i 25 C E 40 cg amp 40 C o 20 amp o 2 0 1 0 1 2 3 4 5 o VOLTAGE V Typical Performance Characteristics Circuit of Figure 1 Continued Switching Frequency 160 155 150 145 QUENCY kHz 140 FR 135 130 50 25 0 25 50 75 100 125 JUNCTION
2. WARS RAR Rh BURR LM2599 General Description The LM2599 series of regulators are monolithic integrated circuits that provide all the active functions for a step down buck switching regulator capable of driving a 3A load with excellent line and load regulation These devices are avail able in fixed output voltages of 3 3V 5V 12V and an adjust able output version This series of switching regulators is similar to the LM2596 series with additional supervisory and performance features added Requiring a minimum number of external components these regulators are simple to use and include internal frequency compensationt improved line and load specifications fixed frequency oscillator Shutdown Soft start error flag de lay and error flag output The LM2599 series operates at a switching frequency of 150 kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators Available in a standard 7 lead TO 220 package with several different lead bend options and a 7 lead TO 263 Surface mount package A standard series of inductors both through hole and sur face mount types are available from several different manu facturers optimized for use with the LM2599 series This feature greatly simplifies the design of switch mode power supplies Other features include a guaranteed 4 tolerance on out put voltage under all conditions
3. 47 uH L39 Renco Through Hole Ry 1 kQ 1 R Use formula in Design Procedure Crr See Figure 4 Ref See Application Information Section Cpp Section RpuLLup 10k CpELAY 0 1 uF Cspiss 0 1 pF Thermalloy Heat Sink 7020 FIGURE 33 PC Board Layout www national com 30 Physical Dimensions inches millimeters unless otherwise noted 0 100 0 120 2 54 3 05 0 149 0 153 0 025 0 035 3 78 3 89 0 64 0 89 TYP i Wi 0 015 M 0 040 0 060 0 38 0 040 0 10 16 535 T D 02 1 52 TYP E E a 0 240 0 260 b 33010250 PIN 1 ID 6 10 6 60 8 38 8 89 0 840 0 860 mo 0 21 34 21 84 0 015 C35 40 184 TYP R 0 049 TYP 0 38 0 03 0 190 0 210 4 83 5 33 0 175 0 185 4 45 4 70 0 155 0 165 I 3 3 94 4 19 0 255 0 275 0 048 0 052 0 105 sos 6 48 6 98 B 40 25 1 22 1 32 TM 2 67 ys SEATING PLANE 0 880 0 900 22 35 22 86 TA07B REV A 7 Lead TO 220 Bent and Staggered Package Order Number LM2599T 3 3 LM2599T 5 0 LM2599T 12 or LM2599T ADJ NS Package Number TA07B 31 www national com 66ScW1 Features LM2599 SIMPLE SWITCHER Power Converter 150 kHz 3A Step Down Voltage Regulator with Physical Dimensions inches millimeters unless otherwise noted Continued 0 260 0 280 0 040 0 060 6 60 7 11 10 1 30 TYP 1 02 2 52 TP sacot 1 0 575 0 25 10 167513 d 4 wae 8 wp 0 0
4. KEEP FEEDBACK WIRING AWAY FROM INDUCTOR FLUX REGULATED OUTPUT FIGURE 1 Standard Test Circuits and Layout Guides As in any switching regulator layout is very important Rap idly switching currents associated with wiring inductance can generate voltage transients which can cause problems For minimal inductance and ground loops the wires indicated by heavy lines should be wide printed circuit traces and should be kept as short as possible For best results external components should be located as close to the switcher IC as possible using ground plane construction or single point grounding If open core inductors are used special care must be taken as to the location and positioning of this type of induc tor Allowing the inductor flux to intersect sensitive feedback IC groundpath and Cour wiring can cause problems When using the adjustable version special care must be taken as to the location of the feedback resistors and the associated wiring Physically locate both resistors near the IC and route the wiring away from the inductor especially an open core type of inductor See application section for more information DS012582 25 www national com 66S2IN1 LM2599 LM2599 Series Buck Regulator Design Procedure Fixed Output PROCEDURE Fixed Output Voltage Version EXAMPLE Fixed Output Voltage Version Given Vout Regulated Output Voltage 3 3V 5V or 12V Vin max Maximum DC Inpu
5. Note 2 Voltage internally clamped If clamp voltage is exceeded limit current to a maximum of 1 mA Note 3 The human body model is a 100 pF capacitor discharged through a 1 5k resistor into each pin Note 4 Typical numbers are at 25 C and represent the most likely norm Note 5 All limits guaranteed at room temperature standard type face and at temperature extremes bold type face All room temperature limits are 100 production tested All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control SQC methods All limits are used to calculate Average Outgoing Quality Level AOQL Note 6 External components such as the catch diode inductor input and output capacitors can affect switching regulator system performance When the LM2599 is used as shown in the Figure 1 test circuit system performance will be as shown in system parameters section of Electrical Characteristics Note 7 The switching frequency is reduced when the second stage current limit is activated The amount of reduction is determined by the severity of current overload www national com 4 All Output Voltage Versions Electrical Characteristics continued Note 8 No diode inductor or capacitor connected to output pin Note 9 Feedback pin removed from output and connected to OV to force the output transistor switch ON Note 10 Feedback pin removed from output and connected to 12V for the 3 3V 5V and the ADJ
6. Output 1 24 Minimum Load Cour 220 pF 35V Vout Unregulated Negative C1 C2 27 uF 25N Output Voltage D2 D3 MUR110 D2 DS012582 48 FIGURE 32 Charge Pump for Generating a Low Current Negative Output Voltage www national com 28 66ScW1 Application Information continued TYPICAL THROUGH HOLE PC BOARD LAYOUT FIXED OUTPUT 1X SIZE DOUBLE SIDED Bottom Side Top Side Component Side Renco o Ju o REGULATED OUTPUT VOLTAGE 4 Pre GROUND H Lu EE 69 a t221 318 Ze e LL Lu ta 1 E 9 DS012582 51 Cin 470 pF 50V Aluminum Electrolytic Panasonic HFQ Series Cour 330 pF 35V Aluminum Electrolytic Panasonic HFQ Series D1 5A 40V Schottky Rectifier 1N5825 Li 47 uH L39 Renco Through Hole RpuLL uP 10k Cpetay 0 1 pF Csp ss 0 1 pF Thermalloy Heat Sink 7020 29 www national com LM2599 Application Information continued TYPICAL THROUGH HOLE PC BOARD LAYOUT ADJUSTABLE OUTPUT 1X SIZE DOUBLE SIDED Bottom Side Top Side Component Side Renco L o T0900 9 REGULATED OUTPUT VOLTAGE a GROUND e ru eo e EET ETT a g l E gt DS012582 52 Cin 470 pF 50V Aluminum Electrolytic Panasonic HFQ Series Cour 220 uF 35V Aluminum Electrolytic Panasonic HFQ Series D1 5A 40V Schottky Rectifier 1N5825 Li
7. it is essential that a short low inductance scope probe ground connection be used Most scope probe manufacturers provide a special probe terminator which is soldered onto the regulator board preferable at the output capacitor This provides a very short scope ground thus eliminating the problems associated with the 3 inch ground lead normally provided with the probe and provides a much cleaner and more accurate picture of the ripple voltage waveform The voltage spikes are caused by the fast switching action of the output switch the diode and the parasitic inductance of the output filter capacitor and its associated wiring To mini mize these voltage spikes the output capacitor should be designed for switching regulator applications and the lead lengths must be kept very short Wiring inductance stray capacitance as well as the scope probe used to evaluate these transients all contribute to the amplitude of these spikes Inductance Region NT mA p p L L WA S INDUCTOR RIPPLE CURR 0 5 1 0 1 5 2 0 2 5 3 0 DS012582 49 FIGURE 21 Peak to Peak Inductor Ripple Current vs Load Current When a switching regulator is operating in the continuous mode the inductor current waveform ranges from a triangu lar to a sawtooth type of waveform depending on the input voltage For a given input and output voltage the peak to peak amplitude of this inductor current waveform remains con
8. version and 15V for the 12V version to force the output transistor switch OFF Note 11 ViN 40V Note 12 Junction to ambient thermal resistance no external heat sink for the package mounted TO 220 package mounted vertically with the leads soldered to a printed circuit board with 1 oz copper area of approximately 1 in Note 13 Junction to ambient thermal resistance with the TO 263 package tab soldered to a single sided printed circuit board with 0 5 in of 1 oz copper area Note 14 Junction to ambient thermal resistance with the TO 263 package tab soldered to a single sided printed circuit board with 2 5 in of 1 oz copper area Note 15 Junction to ambient thermal resistance with the TO 263 package tab soldered to a double sided printed circuit board with 3 in of 1 oz copper area on the LM25998 side of the board and approximately 16 in of copper on the other side of the p c board See application hints in this data sheet and the thermal model in Switchers Made Simple version 4 2 1 or later software Typical Performance Characteristics circuit of Figure 1 Normalized Output Voltage 1 5 n Vin 20V B 1 0 Loap 3A Fa Normalized at Z 05 Tj 25 C X o o tai gt 05 i 5 a 5 1 0 o 1 5 50 25 50 75 100 125 JUNCT EMPERATURE C DS012582 2 Switch Saturation Voltage 1 4 My 12V E
9. 15 20 25 30 35 40 NPUT VOLTAGE V DS012582 39 JUNCTION TEMPERATURE RISE ABOVE AMBIENT 9C Circuit Data for Temperature Rise Curve TO 263 Package S Surface mount tantalum molded D size Surface mount Pulse engineering 68 pH Surface mount 5A 40V Schottky 9 square inches single sided 2 oz copper 0 0028 Capacitors Inductor Diode PC board FIGURE 23 Junction Temperature Rise TO 263 For the best thermal performance wide copper traces and generous amounts of printed circuit board copper should be used in the board layout One exception to this is the output switch pin which should not have large areas of copper Large areas of copper provide the best transfer of heat lower thermal resistance to the surrounding air and moving air lowers the thermal resistance even further Package thermal resistance and junction temperature rise numbers are all approximate and there are many factors that will affect these numbers Some of these factors include board size shape thickness position location and even board temperature Other factors are trace width total printed circuit copper area copper thickness single or double sided multilayer board and the amount of solder on the board The effectiveness of the pc board to dissipate heat also depends on the size quantity and spacing of other components on the board as well as whether the surround ing air is still or moving Furt
10. C The ability of the capacitor to dissipate this heat to the surround ing air will determine the amount of current the capacitor can safely sustain Capacitors that are physically large and have a large surface area will typically have higher RMS current ratings For a given capacitor value a higher voltage elec trolytic capacitor will be physically larger than a lower voltage capacitor and thus be able to dissipate more heat to the surrounding air and therefore will have a higher RMS cur rent rating www national com 20 Application Information continued E w 2200 E Typical RMS current ratings 2000 of low ESR Electrolytic 25 Capacitors 100 kHz 105 C L 3 1800 10 470 uF CRM 680 pF 330 uF H o 1400 la z 270 uF 1200 amp L 5 1000 180 uF ij 800 E 120 uF 3 600 400 E e 200 E 0 10 20 30 40 50 60 70 o CAPACITOR VOLTAGE RATING V DS012582 33 FIGURE 16 RMS Current Ratings for Low ESR Electrolytic Capacitors Typical 400 TYPICAL ESR 25 C gt 350 FOR LOW ESR SERIES E ELECTROLYTIC CAPACITORS 300 O o 2 250 e e 200 E 82 uF g 100 120 uF a S 50 220 uF H 0 10 20 30 40 50 60 70 CAPACITOR VOLTAGE RATING V DS012582 34 FIGURE 17 Capacitor ESR vs Capacitor Voltage Rating Typical Low ESR Electrolytic Capacitor The consequences
11. If unstable operation is seen and an open core inductor is used it s possible that the location of the inductor with respect to other PC traces may be the problem To deter mine if this is the problem temporarily raise the inductor away from the board by several inches and then check circuit operation If the circuit now operates correctly then the magnetic flux from the open core inductor is causing the problem Substituting a closed core inductor such as a tor roid or E core will correct the problem or re arranging the PC layout may be necessary Magnetic flux cutting the IC device ground trace feedback trace or the positive or nega tive traces of the output capacitor should be minimized Sometimes locating a trace directly beneath a bobbin in ductor will provide good results provided it is exactly in the center of the inductor because the induced voltages cancel themselves out but if it is off center one direction or the other then problems could arise If flux problems are present even the direction of the inductor winding can make a difference in some circuits This discussion on open core inductors is not to frighten the user but to alert the user on what kind of problems to watch out for when using them Open core bobbin or stick induc tors are an inexpensive simple way of making a compact efficient inductor and they are used by the millions in many different applications THERMAL CONSIDERATIONS The LM2599 is a
12. TEMPERATURE 9C DS012582 13 Daisy Pin Current 3 5 1 3V FLAG l 3 0 b RESE 4 peak VALUE 16 pA VT THRESHOLD a 5 1 40 C bel 20 40 C z i 40 C 15 t 5 25 C 4 e I 1 0 125 C 0 5 1 l l 0 0 05 10 15 20 25 0 1 2 3 4 5 6 7 8 VOLTAGE V VOLTAGE V DS012582 16 DS012582 15 Shutdown Soft start Threshold Voltage 2 5 gt 20 m Soft start 2 A 15 o e 8 10 Y a Shutdown 05 0 50 25 0 25 50 75 100 125 JUNCTION TEMPERATURE C DS012582 53 www national com Typical Performance Characteristics circuit ot Figure 1 Continued Continuous Mode Switching Waveforms Vin 20V Vour 5V li oap 2A L 32 uH Cour 220 pF Cour ESR 50 mo 20V 10V A OV 2A B 1A Ue e E am div DS012582 20 A Output Pin Voltage 10V div B Inductor Current 1A div C Output Ripple Voltage 50 mV div Horizontal Time Base 2 us div Load Transient Response for Continuous Mode Vin 20V Vour 5V lioap 500 mA to 2A L 32 uH Cour 220 pF Cour ESR 50 ma 0A DS012582 21 A Output Voltage 100 mV div AC B 500 mA to 2A Load Pulse Horizontal Time Base 50 us div Discontinuous Mode Switching Waveforms Vin 20V Vout 5V li goAp 500 mA L 10 HH Cour 330 pF Cour ESR 45 ma 20V 10V DS012582 19 A Output Pin Voltage 10V div B Inductor Current
13. border to the upper border for a given load current within an inductance region The upper border rep resents a higher input voltage while the lower border repre sents a lower input voltage see Inductor Selection Guides These curves are only correct for continuous mode opera tion and only if the inductor selection guides are used to select the inductor value Consider the following example Vout 5V maximum load current of 2 5A Vin 12V nominal varying between 10V and 16V The selection guide in Figure 5 shows that the vertical line for a 2 5A load current and the horizontal line for the 12V input voltage intersect approximately midway between the upper and lower borders of the 33 uH inductance region A 33 uH inductor will allow a peak to peak inductor current Alino to flow that will be a percentage of the maximum load current Referring to Figure 21 follow the 2 5A line approxi mately midway into the inductance region and read the peak to peak inductor ripple current Alinp on the left hand axis approximately 620 mA p p As the input voltage increases to 16V it approaches the upper border of the inductance region and the inductor ripple current increases Referring to the curve in Figure 21 it can be seen that for a load current of 2 5A the peak to peak inductor ripple current Aliup is 620 mA with 12V in and can range from 740 mA at the upper border 16V in to 500 mA at the lower border 10V in Once the
14. closely resemble a buck configuration thus pro viding good closed loop stability A Schottky diode is recom mended for low input voltages because of its lower voltage drop but for higher input voltages a IN5400 diode could be used Because of differences in the operation of the inverting regulator the standard design procedure is not used to select the inductor value In the majority of designs a 33 uH 3 5A inductor is the best choice Capacitor selection can also be narrowed down to just a few values Using the values shown in Figure 27 will provide good results in the majority of inverting designs This type of inverting regulator can require relatively large amounts of input current when starting up even with light loads Input currents as high as the LM2599 current limit approximately 4 5A are needed for 2 ms or more until the output reaches its nominal output voltage The actual time depends on the output voltage and the size of the output capacitor Input power sources that are current limited or sources that can not deliver these currents without getting loaded down may not work correctly Because of the rela tively high startup currents required by the inverting topology the Soft start feature shown in Figure 27 is recommended Also shown in Figure 27 are several shutdown methods for the inverting configuration With the inverting configuration some level shifting is required because the ground pin of the regulator is n
15. goes 6820 68 25 FIGURE 2 LM2599 Fixed Voltage Quick Design Component Selection Table LM2599 Series Buck Regulator Design Procedure Adjustable Output PROCEDURE Adjustable Output Voltage Version Given Vout Regulated Output Voltage Vin max Maximum Input Voltage li oap max Maximum Load Current F Switching Frequency Fixed at a nominal 150 kHz 1 Programming Output Voltage Selecting R and R3 as shown in Figure f Use the following formula to select the appropriate resistor values R Vout VREF 1 gh Fe where Vngr 1 23V Select a value for R between 2400 and 1 5 kQ The lower resistor values minimize noise pickup in the sensitive feed back pin For the lowest temperature coefficient and the best stability with time use 196 metal film resistors EXAMPLE Adjustable Output Voltage Version Given Vou 20V Vin max 28V l oap max 3A F Switching Frequency Fixed at a nominal 150 kHz 1 Programming Output Voltage Selecting R and Ra as shown in Figure 1 Select R to be 1 kQ 1 Solve for Ro VouT 20V Ra Ry be b 1 23V Ro 1k 16 26 1 15 26k closest 1 value is 15 4 KQ Ro 15 4 kQ www national com LM2599 Series Buck Regulator Design Procedure Adjustable Output Continued PROCEDURE Adjustable Output Voltage Version EXAMPLE Adjustable Output Voltage Version 2 Inductor Selection L1 A Calculate the inductor Volt
16. temperatures below 25 C CATCH DIODE Buck regulators require a diode to provide a return path for the inductor current when the switch turns off This must be a fast diode and must be located close to the LM2599 using short leads and short printed circuit traces Because of their very fast switching speed and low forward voltage drop Schottky diodes provide the best performance especially in low output voltage applications 5V and lower Ultra fast recovery or High Efficiency rectifiers are also a good choice but some types with an abrupt turnoff charac 21 www national com 66ScW1 LM2599 Application Information continued teristic may cause instability or EMI problems Ultra fast recovery diodes typically have reverse recovery times of 50 ns or less Rectifiers such as the IN5400 series are much too slow and should not be used 500 mr 400 TYPICAL 100 kHz ESR 4 300 OF 220 uF 35V LOW ESR ELECTROLYTIC CAPACITOR 200 100 80 60 50 40 30 ESR mQ 20 40 20 0 20 40 60 80 100 TEMPERATURE C DS012582 35 FIGURE 18 Capacitor ESR Change vs Temperature INDUCTOR SELECTION All switching regulators have two basic modes of operation continuous and discontinuous The difference between the two types relates to the inductor current whether it is flowing continuously or if it drops to zero for a period of time in the normal switching cy
17. than the output voltage and often much higher volt age ratings are needed to satisfy the low ESR requirements needed for low output ripple voltage 4 Feedforward Capacitor Cp See Figure 1 For output voltages greater than approximately 10V an ad ditional capacitor is required The compensation capacitor is typically between 100 pF and 33 nF and is wired in parallel with the output voltage setting resistor Ro It provides addi tional stability for high output voltages low input output volt ages and or very low ESR output capacitors such as solid tantalum capacitors 1 Cee l FF 31 X 103 X Ro This capacitor type can be ceramic plastic silver mica etc Because of the unstable characteristics of ceramic capaci tors made with Z5U material they are not recommended 2 Inductor Selection L1 A Calculate the inductor Volt e microsecond constant E e T EwT 28 20 1 16 e L25 ou OUO yo us CH 28 116 05 150 E o T 6 84 e 202 o 6 67 V us 34 2 V us UM a734 B aan B E T 34 2 V us C l oAp max 3A D From the inductor value selection guide shown in Figure 7 the inductance region intersected by the 34 V 7 us horizon tal line and the 3A vertical line is 47 uH and the inductor code is L39 E From the table in Figure 8 locate line L39 and select an inductor part number from the list of manufacturers part numbers 3 Output Capacitor Selection
18. 0 5A div C Output Ripple Voltage 100 mV div Horizontal Time Base 2 us div Load Transient Response for Discontinuous Mode Vin 20V Vour 5V li gAp 500 mA to 2A L 10 pH Cour 330 pF Cour ESR 45 ma 100mV A AC div DS012582 22 A Output Voltage 100 mV div AC B 500 mA to 2A Load Pulse Horizontal Time Base 200 ps div Connection Diagrams and Order Information Bent and Staggered Leads Through Hole Package 7 Lead TO 220 T Pins 1 3 5 and 7 Pins 2 4 and 6 7 SD SS 6 Feed Back 5 Delay 4 Ground 3 Flag 2 Output 1 ViN DS012582 50 Order Number LM2599T 3 3 LM2599T 5 0 LM2599T 12 or LM2599T ADJ See NS Package Number TA07B Surface Mount Package 7 Lead TO 263 S SD SS Feed Back Metal Delay Tab GND FO Q4 RUSO 1 e zh o c a DS012582 23 Order Number LM2599S 3 3 LM2599S 5 0 LM2599S 12 or LM2599S ADJ See NS Package Number TS7B 7 www national com 66ScW1 LM2599 Test Circuit and Layout Guidelines Fixed Output Voltage Versions KEEP FEEDBACK WIRING AWAY FROM INDUCTOR FLUX FEEDBACK r 1 ERROR OUTPUT 3 uH LM2599 OUTPUT REGULATED OUTPUT 1 00 1 FIXED OUTPUT l I L Lfi l ha oft 1 LI UNREGULATED l 180 uF 1 DC INPUT D I 1 I LOW ESR i HEAVY LINES MUST BE KEPT SHORT AND USE l l SHORT LEADS LA GROUND PLANE CONSTRUCTION FOR BEST RESULTS SHUTDOWN IN
19. 22 0 032 0 425 m 0 92 n TYP 10 80 co 0 92 0 56 0 81 0 330 0 350 IS PIN 1 ID 1 27 co 8 38 8 89 0 030 TYP 0 050 yay R p 76 MAX TYP 1 27 ge 0 015 0 030 0 173 0 183 0 89 4 45 4 65 LEAD POSITION OVERLAY SF C 0 004 0 10 TAPERED 0 048 0 052 SIDES M 1 22 1 32 0 250 MIN P STAND orr l 6 35 N 0 490 yay 12 45 0 565 0 200 565 MAX MIN 14 35 5 08 CONTROLLING DIMENSION INCH BACK VIEW TS7B REV B 7 Lead TO 263 Bent and Formed Package Order Number LM2599S 3 3 LM2599S 5 0 LM2599S 12 or LM2599S ADJ NS Package Number TS7B LIFE SUPPORT POLICY NATIONAL S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or 2 A critical component is any component of a life Systems which a are intended for surgical implant support device or system whose failure to perform into the body or b support or sustain life and can be reasonably expected to cause the failure of whose failure to perform when properly used in the life support device or system or to affect its accordance with instructions for use provided in the safety or effectiveness labeling can be reasonably expected to result in a significant injury to the user National Semicon
20. 320 MURS620 MUR620 SK35 30WF10 50WF10 HER601 MBRS360 MBR350 50WQ05 30WQ05 31DQ05 50SQ080 17 www national com 66S2IN1 LM2599 Block Diagram CURRENT SOURCE BIAS 1 235V REFERENCE FEEDBACK LI ACTIVE CAPACITOR 3 3V R2 4 2k SV R2 7 6k 12V R2 21 8k ADJ R2 00 R1 OPEN Veer X 0 95 ERROR FLAG SHUTDOWN SOFT START EMITTER OUTPUT DS012582 30 FIGURE 12 Application Information PIN FUNCTIONS Vin Pin 1 This is the positive input supply for the IC switching regulator A suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the switching currents needed by the regulator Ground Pin 4 Circuit ground Output Pin 2 Internal switch The voltage at this pin switches between approximately 4 Vi Vsar and approxi mately 0 5V with a duty cycle of VourMin To minimize coupling to sensitive circuitry the PC board copper area connected to this pin should be kept to a minimum Feedback Pin 6 Senses the regulated output voltage to complete the feedback loop Shutdown Soft start Pin 7 This dual function pin pro vides the following features a Allows the switching regula tor circuit to be shut down using logic level signals thus dropping the total input supply current to approximately 80 pA b Adding a capacitor to this pin provides a soft start feature which minimizes startup current an
21. 8380 67148490 RL 1283 15 43 PE 53825 PE 53824 S DO3316 153 L26 330 0 80 67144100 67144480 RL 5471 1 PE 53826 PE 53826 S DOS022P 334 L28 150 1 20 67144120 67144500 RL 5471 3 PE 53828 PE 53828 S DOS022P 154 L29 100 1 47 67144130 67144510 RL 5471 4 PE 53829 PE 53829 S DOS022P 104 L30 68 1 78 67144140 67144520 RL 5471 5 PE 53830 PE 53830 S DOS022P 684 L31 2 2 67144150 67144530 RL 5471 6 PE 53831 PE 53831 S DOS022P 473 L32 33 2 5 67144160 67144540 RL 5471 7 PE 53932 PE 53932 S DOS022P 334 L33 22 3 1 67148390 67148500 RL 1283 22 43 PE 53933 PE 53933 S DOSO022P 223 L34 15 34 67148400 67148790 RL 1283 15 43 PE 53934 PE 53934 S DOS022P 153 L35 220 1 70 67144170 RL 5473 1 PE 53935 PE 53935 S L36 2 1 67144180 RL 5473 4 PE 54036 PE 54036 S L37 100 2 5 67144190 RL 5472 1 PE 54037 PE 54037 S L38 68 3 1 67144200 RL 5472 2 PE 54038 PE 54038 S L39 47 3 5 67144210 RL 5472 3 PE 54039 PE 54039 S L40 8 5 67144220 67148290 RL 5472 4 PE 54040 PE 54040 S L41 22 8 5 67144230 67148300 RL 5472 5 PE 54041 PE 54041 S L42 150 2 7 67148410 RL 5473 4 PE 54042 PE 54042 S L43 100 34 67144240 RL 5473 2 PE 54043 L44 68 34 67144250 RL 5473 3 PE 54044 FIGURE 8 Inductor Manufacturers Part Numbers Coilcraft Inc Coilcr
22. Alinp value is known the following formulas can be used to calculate additional information about the switching regulator circuit 1 Peak Inductor or peak switch current Al 0 62 Icono iO 25 2 81A 2 Minimum load current before the circuit becomes dis continuous AliND 2 3 Output Ripple Voltage Alinp X ESR of Cour 0 62Ax0 19 62 mV p p 0 62 0 31A 2 4 ESR of Cout Output Ripple Voltage AVour AliND 0 062V ota 0 62A OPEN CORE INDUCTORS Another possible source of increased output ripple voltage or unstable operation is from an open core inductor Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end These magnetic lines of flux will induce a voltage into any wire or PC board copper trace that comes within the induc tor s magnetic field The strength of the magnetic field the orientation and location of the PC copper trace to the mag netic field and the distance between the copper trace and the inductor determine the amount of voltage generated in the copper trace Another way of looking at this inductive coupling is to consider the PC board copper trace as one turn of a transformer secondary with the inductor winding as the primary Many millivolts can be generated in a copper trace located near an open core inductor which can cause stability problems or high output ripple voltage problems
23. Cour A See section on Cour in Application Information section B From the quick design table shown in Figure 3 locate the output voltage column From that column locate the output voltage closest to the output voltage in your application In this example select the 24V line Under the output capacitor section select a capacitor from the list of through hole elec trolytic or surface mount tantalum types from four different capacitor manufacturers It is recommended that both the manufacturers and the manufacturers series that are listed in the table be used In this example through hole aluminum electrolytic capaci tors from several different manufacturers are available 220 35 Panasonic HFQ Series 150 35 Nichicon PL Series C For a 20V output a capacitor rating of at least 30V or more is needed In this example either a 35V or 50V capaci tor would work A 50V rating was chosen because it has a lower ESR which provides a lower output ripple voltage Other manufacturers or other types of capacitors may also be used provided the capacitor specifications especially the 100 kHz ESR closely match the types listed in the table Refer to the capacitor manufacturers data sheet for this information 4 Feedforward Capacitor Cpp The table shown in Figure 3 contains feed forward capacitor values for various output voltages In this example a 560 pF capacitor is needed 13 www national com 66ScW1 LM2599 LM2599
24. E 5 a gt z o T Z 2 E a 0 2 3 4 SWITCH CURRENT A DS012582 5 Operating Quiescent Current 24 T 4 5V lt Vin lt 40V 20 Iswirek 9 SUPPLY CURRENT mA S 0 50 25 JUNCT 50 75 100 125 ON TEMPERATURE CC DS012582 8 Line Regulation CURRENT A 80 60 40 20 125 C 0 4 Vout 5V amp 03 lgap 100 mA S LOAD w 02 Ty 25 C 8 01 5 E 0 0 1 gt 5 0 2 a 03 04 0 25 30 35 40 LTAGE V DS012582 3 Switch Current Limit 5 5 Vy 12V z Your 5V 5 0 a E ee ee oe 5 B 45 amp 5 3 x 2 40 zm a 35 50 50 75 100 125 ON TEMPERATURE 9C DS012582 6 Shutdown Quiescent Current 120 184 Vsp ss OV 100 30 40 SUPPLY VOLTAGE V DS012582 9 EFFICIENCY Efficiency 95 I 3A Load 20V 90 12V 85 80 5V 75 3 3V 70 65 5 10 15 20 25 30 35 40 NPUT VOLTAGE V SUPPLY VOLTAGE V 5 0 Minimum Operating Supply Voltage DS012582 4 Dropout Voltage 1 6 L 33 uH Ryp 0 040 eit Vout Vreg 750 mV E Z TIT1L LI TJ Fa 1 2 loan 3A a 5 10 z 5 F Loan 14 5 08 a z 0 6 50 25 0 25 75 100 125 JUNCTION TEMPERATURE C
25. Limits Note 4 Note 5 SYSTEM PARAMETERS Note 6 Test Circuit Figure 1 Vour Output Voltage 4 75V Vin 40V 0 2A l oAp 3A 3 3 V 3 168 3 135 V min 3 432 3 465 V max i LM2599 5 0 Electrical Characteristics Specifications with standard type face are for T 25 C and those with boldface type apply over full Operating Tempera ture Range Parameter Conditions Units Limit Limits Note 4 Note 5 SYSTEM PARAMETERS Note 6 Test Circuit Figure 1 Vour Output Voltage 7V Vin 40V 0 2A l oap 3A 5 V 4 800 4 750 V min 5 200 5 250 V max a Efficiency EEATT E BL www national com 2 LM2599 12 Electrical Characteristics Specifications with standard type face are for T 25 C and those with boldface type apply over full Operating Tempera ture Range Symbol Parameter Conditions LM2599 12 Units Typ Limit Limits Note 4 Note 5 SYSTEM PARAMETERS Note 6 Test Circuit Figure 1 Vour Output Voltage 15V lt Vin 40V 0 2A lioap 3A 12 V 11 52 11 40 V min 12 48 12 60 V max n Efficiency Vin 25V l oap 3A 90 LM2599 ADJ Electrical Characteristics Specifications with standard type face are for T 25 C and those with boldface type apply over full Operating Tempera ture Range Symbol Parameter Conditions LM2599 ADJ Units Typ Limit Limits Note 4 Note 5 SYSTEM PARAMETERS Note 6 Test Cir
26. PLE SWITCHER and Switchers Made Simple are registered trademarks of National Semiconductor Corporation 6 5 0V O Regulated Cour Output T 220 uF 3A Load Output 4 Gnd D1 1N5824 DS012582 1 2001 National Semiconductor Corporation DS012582 www national com S91NJE94 YUM JOjejn oq eyo uwoq deis YE ZH OGL 19M9AUOD J9MOd H3HO LIMS AIdINIS 66SZNI LM2599 Absolute Maximum Ratings note 1 ESD Susceptibility If Military Aerospace specified devices are required Human Body Model Note 3 2 kV please contact the National Semiconductor Sales Office Lead Temperature Distributors for availability and specifications S Package Maximum Supply Voltage Vin 45V Vapor Phase 60 sec 215 C SD SS Pin Input Voltage Note 2 6V Infrared 10 sec 245 C Delay Pin Voltage Note 2 1 5V T Package Soldering 10 sec 260 C Flag Pin Voltage 0 3 lt V 45V Maximum Junction Temperature 150 C Feedback Pin Voltage 0 3 lt V lt 25V i Output Voltage to Ground Operating Conditions Steady State 1V Temperature Range 40 C lt T lt 125 C Power Dissipation Internally limited Supply Voltage 4 5V to 40V Storage Temperature Range 65 C to 150 C LM2599 3 3 Electrical Characteristics Specifications with standard type face are for T 25 C and those with boldface type apply over full Operating Tempera ture Range Symbol Parameter Conditions LM2599 3 3 Units Typ Limit
27. PUT OPTIONAL POST RIPPLE FILTER DS012582 24 Component Values shown are for Vi 15V Vout 5V ILoap 3A Cin 470 pF 50V Aluminum Electrolytic Nichicon PL Series Cour 220 uF 25V Aluminum Electrolytic Nichicon PL Series Di 5A 40V Schottky Rectifier 1N5825 Li 68 uH L38 Typical Values Css 0 1 UF Cppav 0 1 pF RPuiup 47k www national com 8 Test Circuit and Layout Guidelines Continued Adjustable Output Voltage Versions CFF TO REGULATED OUTPUT VOLTAGE Roull up ERROR OUTPUT UNREGULATED DC INPUT SHUTDOWN INPUT Vout VREF i zx Ry where Vper 1 23V Vi R2 Ry You VREF Select R4 to be approximately 1 kQ use a 1 resistor for best stability Component Values shown are for Viy 20V Vout 10V ILoap 3A Cmn 470 pF 35V Aluminum Electrolytic Nichicon PL Series Cour 220 pF 35V Aluminum Electrolytic Nichicon PL Series D1 5A 30V Schottky Rectifier 1N5824 L1 68 pH L38 Ry 1 kQ 1 Ro 7 15k 196 Crr 3 3 nF See Application Information Section Rrr 3 kQ See Application Information Section Typical Values Css 0 1 uF CpgLav 0 1 pF RpULL up 4 7k FEEDBACK LM2599 ADJUSTABLE OUTPUT HEAVY LINES MUST BE KEPT SHORT AND USE GROUND PLANE CONSTRUCTION FOR BEST RESULTS LOCATE THE PROGRAMMING RESISTORS NEAR THE FEEDBACK PIN USING SHORT LEADS
28. Series Buck Regulator Design Procedure Adjustable Output Continued PROCEDURE Adjustable Output Voltage Version EXAMPLE Adjustable Output Voltage Version 5 Catch Diode Selection D1 A The catch diode current rating must be at least 1 3 times greater than the maximum load current Also if the power supply design must withstand a continuous output short the diode should have a current rating equal to the maximum current limit of the LM2599 The most stressful condition for this diode is an overload or shorted output condition B The reverse voltage rating of the diode should be at least 1 25 times the maximum input voltage C This diode must be fast short reverse recovery time and must be located close to the LM2599 using short leads and short printed circuit traces Because of their fast switching speed and low forward voltage drop Schottky diodes provide the best performance and efficiency and should be the first choice especialy in low output voltage applications Ultra fast recovery or High Efficiency rectifiers are also a good choice but some types with an abrupt turn off charac teristic may cause instability or EMI problems Ultra fast re covery diodes typically have reverse recovery times of 50 ns or less Rectifiers such as the 1N4001 series are much too slow and should not be used 6 Input Capacitor Cj Alow ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground to
29. aeu m s L33 x X 2 L15 3 L23 D 125 L24 sy L16 14V L34 0 6 0 8 1 0 15 2 253 0 6 0 8 1 0 1 5 2 253 MAXIMUM LOAD CURRENT A MAXIMUM LOAD CURRENT A DS012582 26 DS012582 28 FIGURE 4 LM2599 3 3 FIGURE 6 LM2599 12 40V 70 r L29 L38 60 TA L35 DE L43 20V KS L30 L39 50 M L44 Z aKO y 40 Fah L28 L37 S adi L31 T _ G0 A L29 L38 5 25 0 V L30 39 4 S qv t2 my L32 E 20 68 H AC L31 L40 S 9v uns Z 15 L21 NES L32 141 z L33 E of 2 35 p L33 2 o d fa 21r J 2 L23 dm L34 B L23 1344 z L24 6f L15 L25 L25 5 7V 4 0 6 0 8 1 0 15 2 2 5 3 0 6 0 8 1 0 15 2 253 MAXIMUM LOAD CURRENT A DS012582 27 FIGURE 5 LM2599 5 0 MAXIMUM LOAD CURRENT A DS012582 29 FIGURE 7 LM2599 ADJ 15 www national com LM2599 LM2599 Series Buck Regulator Design Procedure Continued Inductance Current Schott Renco Pulse Engineering Coilcraft pH A Through Surface Through Surface Through Surface Surface Hole Mount Hole Mount Hole Mount Mount L15 22 0 99 67148350 67148460 RL 1284 22 43 RL1500 22 PE 53815 PE 53815 S DO3308 223 L21 68 0 99 67144070 67144450 RL 5471 5 RL1500 68 PE 53821 PE 53821 S DO3316 683 L22 147 67144080 67144460 RL 5471 6 PE 53822 PE 53822 S DO3316 473 L23 33 1 40 67144090 67144470 RL 5471 7 PE 53823 PE 53823 S DO3316 333 L24 22 1 70 67148370 67148480 RL 1283 22 43 PE 53824 PE 53825 S DO3316 223 L25 15 2 1 6714
30. aft Inc Europe Phone 800 322 2645 708 639 1469 11 1236 730 595 444 1236 730 627 Pulse Engineering Inc 619 674 8100 619 674 8262 Pulse Engineering Inc Europe Renco Electronics Inc Schott Corp 353 93 24 107 Phone FAX FAX 353 93 24 459 800 645 5828 516 586 5562 612 475 1786 Phone 612 475 1173 FIGURE 9 Inductor Manufacturers Phone Numbers www national com LM2599 Series Buck Regulator Design Procedure Continued Nichicon Corp 708 843 7500 Sprague Vishay 708 843 2798 Panasonic 714 373 7857 714 373 7102 AVX Corp 803 448 941 1 803 448 1943 207 207 fe SI I Od od eS Se Paus Dice Eia 324 4140 324 7223 FIGURE 10 Capacitor Manufacturers Phone Numbers FIGURE 11 Diode Selection Table 3 Amp Diodes 4 to 6 Amp Diodes Surface Mount Through Hole Surface Mount Through Hole Ultra Fast Schot Ultra Fast Ultra Fast Ultra Fast Schottky tky Recovery Recovery Recovery Recovery All of 1N5820 All of All of All of these these these 1N5823 these diodes MBR320 diodes diodes diodes 30WQ03 are rated 1N5821 are rated are rated are rated to at MBR330 to at 50WQ03 to at to at least 31DQ03 least least 1N5824 least 50V 1N5822 50V 50V 50V 50WQ04 MBRS340 MBR340 1N5825 30WQ04 MURS320 31DQ04 MUR
31. ator ON but the output voltage is zero With the SD SS pin voltage between approximately 1 3V and 1 8V the internal regulator circuitry is operating the quiescent current rises to approximately 5 mA but the output voltage is still zero Also as the 1 3V threshold is exceeded the Soft start capacitor charging current decreases from 5 uA down to approximately 1 6 A This decreases the slope of capacitor voltage ramp 3 Soft start Region When the SD SS pin voltage is be tween 1 8V and 2 8V 25 C the regulator is in a Soft start condition The switch Pin 2 duty cycle initially starts out very low with narrow pulses and gradually get wider as the capacitor SD SS pin ramps up towards 2 8V As the duty cycle increases the output voltage also increases at a con trolled ramp up See the center curve in Figure 13 The input supply current requirement also starts out at a low level for www national com Application Information continued the narrow pulses and ramp up in a controlled manner This 20 F Ta 25 C a is a very useful feature in some switcher topologies that 10 Paga require large startup currents such as the inverting configu e 40 ration which can load down the input power supply war AS Clamps at 1 7V Note The lower curve shown in Figure 13 shows the Soft start region from 1 0 0 to 100 This is not the duty cycle percentage but the output a Delay Pi
32. capacitors should be at least 1 5 times greater than the output voltage and often much higher voltage ratings are needed to satisfy the low ESR requirements for low output ripple voltage D For computer aided design software see Switchers Made Simple version 4 2 1 or later Given Vout 5V Vin max 12V lLoAp max 3A 1 Inductor Selection L1 A Use the inductor selection guide for the 5V version shown in Figure 5 B From the inductor value selection guide shown in Figure 5 the inductance region intersected by the 12V horizontal line and the 3A vertical line is 33 pH and the inductor code is L40 C The inductance value required is 33 WH From the table in Figure 8 go to the L40 line and choose an inductor part number from any of the four manufacturers shown In most instance both through hole and surface mount inductors are available 2 Output Capacitor Selection Cour A See section on output capacitors in application infor mation section B From the quick design component selection table shown in Figure 2 locate the 5V output voltage section In the load current column choose the load current line that is closest to the current needed in your application for this example use the 3A line In the maximum input voltage column select the line that covers the input voltage needed in your application in this example use the 15V line Continuing on this line are recommended inductors and capacitors t
33. cle Each mode has distinctively different operating characteristics which can affect the regulators performance and requirements Most switcher designs will operate in the discontinuous mode when the load current is low The LM2599 or any of the Simple Switcher family can be used for both continuous or discontinuous modes of opera tion In many cases the preferred mode of operation is the con tinuous mode It offers greater output power lower peak switch inductor and diode currents and can have lower output ripple voltage But it does require larger inductor values to keep the inductor current flowing continuously especially at low output load currents and or high input volt ages To simplify the inductor selection process an inductor selec tion guide nomograph was designed see Figure 4 through 7 This guide assumes that the regulator is operating in the continuous mode and selects an inductor that will allow a peak to peak inductor ripple current to be a certain percent age of the maximum design load current This peak to peak inductor ripple current percentage is not fixed but is allowed to change as different design load currents are selected See Figure 19 Alyy OF MAX DESIGN LOAD CURRENT 05 10 15 20 25 3 0 MAXIMUM LOAD CURRENT A DS012582 36 FIGURE 19 Alyyp Peak to Peak Inductor Ripple Current as a Percentage of the Load Current vs Load Current By allowing the percentage o
34. cuit Figure 1 Veg Feedback Voltage 4 5V lt Vn 40V 0 2A lt loan 3A 1 230 V Vour programmed for 3V Circuit of Figure 1 1 193 1 180 V min 1 267 1 280 V max n Vin 12V Vout 3V ligap 3A 73 All Output Voltage Versions Electrical Characteristics Specifications with standard type face are for T 25 C and those with boldface type apply over full Operating Tempera ture Range Unless otherwise specified Vi 12V for the 3 3V 5V and Adjustable version and Vi 24V for the 12V ver sion lL oap 500 mA Symbol Parameter Conditions LM2599 XX Units Typ Limit Limits Note Note 5 4 DEVICE PARAMETERS lb Feedback Bias Current Adjustable Version Only Veg 1 3V 10 nA 50 100 nA max fo Oscillator Frequency Note 7 150 kHz 127 110 kHz min 173473 kHz max VsaT Saturation Voltage lout 3A Note 8 Note 9 1 16 V V max DC Max Duty Cycle ON Note 9 100 Min Duty Cycle OFF Note 10 0 lot Current Limit Peak Current Note 8 Note 9 4 5 A A min A max IL Output Leakage Current Note 8 Note 10 Note 11 Output OV HA max Output 1V 2 mA mA max www national com 66ScIN 1 LM2599 All Output Voltage Versions Electrical Characteristics continued Specifications with standard type face are for T 25 C and those with boldface type apply over full Operating Tempera ture Range Unless otherwise specified Vi 12V for the 3 3V 5V an
35. d Adjustable version and Vi 24V for the 12V ver sion l Loap 500 mA Parameter Conditions LM2599 XX Units Limits DEVICE PARAMETERS mA 10 mA max Standby Quiescent SD SS pin 0V Note 11 yA Current 200 250 HA max Thermal Resistance TO220 or TO263 Package Junction to Case C W TO220 Package Juncton to Ambient Note 12 C W TO263 Package Juncton to Ambient Note 13 C W TO263 Package Juncton to Ambient Note 14 C W TO263 Package Juncton to Ambient Note 15 CAN Shutdown Threshold V Voltage Low Shutdown Mode 0 6 V max High Soft start Mode 2 V min Vou 20 of Nominal Output Voltage V Vout 100 of Nominal Output Voltage VsnurbowN 0 5V HA 10 HA max pA 5 HA max Regulator Dropout Detector Low Flag ON 96 96 Threshold Voltage 92 min 98 max VF gat Flag Output Saturation Isink 9 mA 0 3 V Voltage Vpgiav 0 5V 0 7 1 0 V max IFL pA Delay Pin Threshold l V Voltage Low Flag ON 1 21 V min High Flag OFF and Voy Regulated 1 29 V max Delay Pin Source Current Vpgrav 0 5V HA 6 HA max Delav Pin Saturation Low Flag ON mV 350 400 mV max Note 1 Absolute Maximum Ratings indicate limits beyond which damage to the device may occur Operating Ratings indicate conditions for which the device is intended to be functional but do not guarantee specific performance limits For guaranteed specifications and test conditions see the Electrical Characteristics
36. d provides a con trolled ramp up of the output voltage Error Flag Pin 3 Open collector output that provides a low signal flag transistor ON when the regulated output voltage drops more than 5 from the nominal output volt age On start up Error Flag is low until Vo reaches 95 of the nominal output voltage and a delay time determined by the Delay pin capacitor This signal can be used as a reset to a microprocessor on power up Delay Pin 5 At power up this pin can be used to provide a time delay between the time the regulated output voltage reaches 9596 of the nominal output voltage and the time the error flag output goes high Special Note If any of the above three features Shutdown Soft start Error Flag or Delay are not used the respective pins should be left open EXTERNAL COMPONENTS SOFT START CAPACITOR Css A capacitor on this pin provides the regulator with a Soft start feature slow start up When the DC input voltage is first applied to the regulator or when the Shutdown Soft start pin is allowed to go high a constant current approximately 5 pA begins charging this capacitor As the capacitor voltage rises the regulator goes through four op erating regions See the bottom curve in Figure 13 1 Regulator in Shutdown When the SD SS pin voltage is between OV and 1 3V the regulator is in shutdown the output voltage is zero and the IC quiescent current is ap proximately 85 pA 2 Regul
37. d tantalum capacitors are often used for input bypassing but several precautions must be observed A small percentage of solid tantalum capacitors can short if the inrush current rating is exceeded This can happen at turn on when the input voltage is suddenly applied and of course higher input voltages produce higher inrush currents Sev eral capacitor manufacturers do a 10096 surge current test ing on their products to minimize this potential problem If high turn on currents are expected it may be necessary to limit this current by adding either some resistance or induc tance before the tantalum capacitor or select a higher volt age capacitor As with aluminum electrolytic capacitors the RMS ripple current rating must be sized to the load current OUTPUT CAPACITOR Cour An output capacitor is required to filter the output and provide regulator loop stability Low impedance or low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used When select ing an output capacitor the important capacitor parameters are the 100 kHz Equivalent Series Resistance ESR the RMS ripple current rating voltage rating and capacitance value For the output capacitor the ESR value is the most important parameter The output capacitor requires an ESR value that has an upper and lower limit For low output ripple voltage a low ESR value is needed This value is determined by the maxi mum allo
38. ductor National Semiconductor National Semiconductor National Semiconductor Corporation Europe Asia Pacific Customer Japan Ltd Americas Fax 49 0 180 530 85 86 Response Group Tel 81 3 5639 7560 Tel 1 800 272 9959 Email europe support nsc com Tel 65 2544466 Fax 81 3 5639 7507 Fax 1 800 737 7018 Deutsch Tel 449 0 69 9508 6208 Fax 65 2504466 Email support nsc com English Tel 444 0 870 24 0 2171 Email ap support nsc com www national com Fran ais Tel 433 0 1 41 91 8790 National does not assume any responsibility for use of any circuitry described no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications
39. e IC using short leads and the voltage rating should be approximately 1 5 times the maximum input voltage If solid tantalum input capacitors are used it is recomended that they be surge current tested by the manufacturer Use caution when using ceramic capacitors for input bypass ing because it may cause severe ringing at the Vin pin For additional information see section on input capaci tors in Application Information section 3 Catch Diode Selection D1 A Refer to the table shown in Figure 11 In this example a 5A 20V 1N5823 Schottky diode will provide the best perfor mance and will not be overstressed even for a shorted output 4 Input Capacitor C The important parameters for the Input capacitor are the input voltage rating and the RMS current rating With a nominal input voltage of 12V an aluminum electrolytic ca pacitor with a voltage rating greater than 18V 1 5 x Vin would be needed The next higher capacitor voltage rating is 25V The RMS current rating requirement for the input capacitor in a buck regulator is approximately 1 the DC load current In this example with a 3A load a capacitor with a RMS current rating of at least 1 5A is needed The curves shown in Figure 16 can be used to select an appropriate input capacitor From the curves locate the 35V line and note which capaci tor values have RMS current ratings greater than 1 5A A 680 uF 35V capacitor could be used For a through hole desi
40. e maximum switch current will still be less than the switch current limit Discontinuous operation can have voltage waveforms that are considerable different than a continuous design The output pin switch waveform can have some damped sinu soidal ringing present See Typical Performance Character istics photo titled Discontinuous Mode Switching Wave forms This ringing is normal for discontinuous operation and is not caused by feedback loop instabilities In discon tinuous operation there is a period of time where neither the switch or the diode are conducting and the inductor current has dropped to zero During this time a small amount of energy can circulate between the inductor and the switch diode parasitic capacitance causing this characteristic ring ing Normally this ringing is not a problem unless the ampli tude becomes great enough to exceed the input voltage and even then there is very little energy present to cause dam age Different inductor types and or core materials produce differ ent amounts of this characteristic ringing Ferrite core induc tors have very little core loss and therefore produce the most ringing The higher core loss of powdered iron inductors produce less ringing If desired a series RC could be placed in parallel with the inductor to dampen the ringing The computer aided design software Switchers Made Simple version 4 3 will provide all component values for continu ous and discontinuous m
41. e microsecond constant E T V us from the following formula ii _ Vout Vp 1000 E T Vin Vout Vsa Vn von p Aso KHz a where Vgar internal switch saturation voltage 1 16V and Vp diode forward voltage drop 0 5V B Use the E T value from the previous formula and match it with the E T number on the vertical axis of the Inductor Value Selection Guide shown in Figure 7 C on the horizontal axis select the maximum load current D Identify the inductance region intersected by the E T value and the Maximum Load Current value Each region is identified by an inductance value and an inductor code LXX E Select an appropriate inductor from the four manufactur er s part numbers listed in Figure 8 3 Output Capacitor Selection Cour A In the majority of applications low ESR electrolytic or solid tantalum capacitors between 82 uF and 820 pF provide the best results This capacitor should be located close to the IC using short capacitor leads and short copper traces Do not use capacitors larger than 820 uF For additional informa tion see section on output capacitors in application information section B To simplify the capacitor selection procedure refer to the quick design table shown in Figure 3 This table contains different output voltages and lists various output capacitors that will provide the best design solutions C The capacitor voltage rating should be at least 1 5 times greater
42. ed startup functions If a very slow output voltage ramp is desired the Soft start capacitor can be made much larger Many seconds or even minutes are possible If only the shutdown feature is needed the Soft start capaci tor can be eliminated Error Output LM2599 12 0 47 uH 24V Unregulated 12 0V DC Input O Regulated 680 uF I Cour Output V TTE nd Pre 1 OFF ON 25V SU 1N4148 Css E 0 068 uF omi mm mmus L Shutdown Soft start L B ger Shutdown Soft start kell k i ese N 4 ON OFF D r ee ed Css l n 0 068 uF i ben P d Ssa ip DS012582 42 FIGURE 26 Typical Circuit Using Shutdown Soft start and Error Flag Features www national com 26 Application Information continued 1N5823 INPUT VOLTAGE 4 5V to 20V D3 5V R2 orr on gt I 100k ov Q1 1N4148 2 SHUTDOWN OPTION 1 GROUND REFERENCED ele eee ee oad FEEDBACK LM2599 FLAG 5 0 1N5825 D2 c2 5V OUTPUT ERROR FLAG OUTPUT peewee ee ee ee ee ee ee ee ee ee OR POWER OK DS012582 43 FIGURE 27 Inverting 5V Regulator With Shutdown and Soft start INVERTING REGULATOR The circuit in Figure 27 converts a positive input voltage to a negative output voltage with a common ground The circuit operates by bootstrapping the regulator s ground pin to the negative output voltage th
43. en grounding the feedback pin the regulator senses the inverted output voltage and regu lates it This example uses the LM2599 5 to generate a 5V output but other output voltages are possible by selecting other output voltage versions including the adjustable version Since this regulator topology can produce an output voltage that is either greater than or less than the input voltage the maximum output current greatly depends on both the input and output voltage The curve shown in Figure 28 provides a guide as to the amount of output load current possible for the different input and output voltage conditions The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage and this must be limited to a maximum of 40V In this example when converting 420V to 5V the regulator would see 25V be tween the input pin and ground pin The LM2599 has a maximum input voltage rating of 40V Inverting Regulator L 32 uH 5V Out 12V Out 20V Out MAXIMUM LOAD CURRENT A 0 5 10 15 20 25 INPUT VOLTAGE V DS012582 44 FIGURE 28 Maximum Load Current for Inverting Regulator Circuit An additional diode is required in this regulator configuration Diode D1 is used to isolate input voltage ripple or noise from coupling through the Cj capacitor to the output under light or no load conditions Also this diode isolation changes the topology to
44. f inductor ripple current to increase for low load currents the inductor value and size can be kept relatively low When operating in the continuous mode the inductor current waveform ranges from a triangular to a sawtooth type of waveform depending on the input voltage with the average value of this current waveform equal to the DC output load current Inductors are available in different styles such as pot core toroid E core bobbin core etc as well as different core materials such as ferrites and powdered iron The least expensive the bobbin rod or stick core consists of wire wound on a ferrite bobbin This type of construction makes for an inexpensive inductor but since the magnetic flux is not completely contained within the core it generates more Electro Magnetic Interference EMI This magnetic flux can induce voltages into nearby printed circuit traces thus caus ing problems with both the switching regulator operation and nearby sensitive circuitry and can give incorrect scope read ings because of induced voltages in the scope probe Also see section on Open Core Inductors When multiple switching regulators are located on the same PC board open core magnetics can cause interference between two or more of the regulator circuits especially at high currents A torroid or E core inductor closed magnetic structure should be used in these situations The inductors listed in the selection chart include ferrite E co
45. gn a 680 uF 35V electrolytic capaci tor Panasonic HFQ series or Nichicon PL series or equiva lent would be adequate other types or other manufacturers capacitors can be used provided the RMS ripple current ratings are adequate For surface mount designs solid tantalum capacitors are recommended The TPS series available from AVX and the 593D series from Sprague are both surge current tested www national com 66S2IN1 LM2599 LM2599 Series Buck Regulator Design Procedure Fixed Output Continued Conditions Inductor Output Capacitor Through Hole Electrolytic Surface Mount Tantalum Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague Voltage Current Voltage pH f HFQ Series PL Series Series 595D Series V A V pF V HF V HF V HF V 5 L41 470 25 560 16 330 6 3 390 6 3 56035 38065 3006 3 68085 38063 3006 3 3 3 470 35 330 6 3 390 6 3 ei aroas 33063 39063 2 10 330 35 330 35 330 6 3 390 6 3 40 330 35 270 50 220 10 330 10 8 470 25 560 16 220 10 330 10 10 560 25 560 25 220 10 330 10 i 15 330 35 330 35 220 10 330 10 5 a705 220m0 33070 o e e poma 560 16 22010 3900 2 18085 100m0 270 10 40 180 35 180 35 100 10 270 10 15 470 25 470 25 100 16 180 16 18 330 25 330 25 100 16 180 16 30 180 25 180 25 100 16 120 20 12 40 180 35 180 35 100 16 120 20 33025 10016 180 16 2 18025 10016 12020
46. hat will provide the best overall performance The capacitor list contains both through hole electrolytic and surface mount tantalum capacitors from four different capaci tor manufacturers It is recommended that both the manufac turers and the manufacturer s series that are listed in the table be used In this example aluminum electrolytic capacitors from several different manufacturers are available with the range of ESR numbers needed 330 uF 35V Panasonic HFQ Series 330 uF 35V Nichicon PL Series C For a 5V output a capacitor voltage rating at least 7 5V or more is needed But even a low ESR switching grade 220 uF 10V aluminum electrolytic capacitor would exhibit ap proximately 225 mQ of ESR see the curve in Figure 17 for the ESR vs voltage rating This amount of ESR would result in relatively high output ripple voltage To reduce the ripple to 196 of the output voltage or less a capacitor with a higher value or with a higher voltage rating lower ESR should be selected A 16V or 25V capacitor will reduce the ripple volt age by approximately half www national com LM2599 Series Buck Regulator Design Procedure Fixed Output Continued PROCEDURE Fixed Output Voltage Version EXAMPLE Fixed Output Voltage Version 3 Catch Diode Selection D1 A The catch diode current rating must be at least 1 3 times greater than the maximum load current Also if the power supply design must withstand a continuou
47. hermore some of these com ponents such as the catch diode will add heat to the pc board and the heat can vary as the input voltage changes For the inductor depending on the physical size type of core material and the DC resistance it could either act as a heat sink taking heat away from the board or it could add heat to the board SHUTDOWN SOFT START The circuit shown in Figure 26 is a standard buck regulator with 20V in 12V out 1A load and using a 0 068 uF Soft start capacitor The photo in Figure 24 Figure 25 show the effects of Soft start on the output voltage the input current with and without a Soft start capacitor The reduced input current required at startup is very evident when comparing the two photos The Soft start feature reduces the startup current from 2 6A down to 650 mA and delays and slows down the output voltage rise time 25 www national com 66S2IN1 LM2599 Application Information continued 15V Output 10V Voltage 5V ov 3A Input 2A Current 1A 0A 500 usec div DS012582 41 10 msec div FIGURE 25 Output Voltage Input Current DS012582 40 at Start Up WITHOUT Soft start FIGURE 24 Output Voltage Input Current KADI T at Start Up WITH Soft start This reduction in start up current is useful in situations where the input power source is limited in the amount of current it can deliver In some applications Soft start can be used to replace undervoltage lockout or delay
48. ine ot switch ing regulators Switchers Made Simple version 4 2 1 or later is available on a 312 diskette for IBM compatible computers www national com LM2599 Series Buck Regulator Design Procedure Adjustable Output 66ScW1 Continued Output Through Hole Output Capacitor Surface Mount Output Capacitor Voltage Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward HFQ Series Series Capacitor Series 595D Series Capacitor F V pF V pF V F V 820 35 820 35 33 nF 330 6 3 470 4 33 nF 560 35 470 35 10 nF 330 6 3 390 6 3 10 nF 6 47025 470 25 3 3 nF 220 10 330 10 3 3 nF 9 33025 330 25 1 5 nF 100 16 180 16 1 5 nF 12 330 25 330 25 1 nF 100 16 180 16 1 nF 15 220 35 220 35 680 pF 68 20 120 20 680 pF 24 220 35 150 35 560 pF 33 25 33 25 220 pF 28 100 50 100 50 390 pF 10 35 15 50 220 pF FIGURE 3 Output Capacitor and Feedforward Capacitor Selection Table LM2599 Series Buck Regulator Design Procedure INDUCTOR VALUE SELECTION GUIDES For Continuous Mode Operation 40V 40V 2v SS L31 HA L40 30V L7 L36 Aa S 15V o S asy 6 ys L37 du at A f L41 R5 i 10V k a L32 o 20V i28 As SS L38 4 19V AS L39 g av L22 os L33 S iav 129 7 L30 5 MW 5 WW e P L31 L40 Eo w L23 L34 devhizi mama f L24 KD 2
49. ing greater than 42V 1 5 x Vin would be needed Since the the next higher capacitor voltage rating is 50V a 50V capacitor should be used The capacitor voltage rating of 1 5 x Vn is a conser vative guideline and can be modified somewhat if desired The RMS current rating requirement for the input capacitor of a buck regulator is approximately 12 the DC load current In this example with a 3A load a capacitor with a RMS current rating of at least 1 5A is needed The curves shown in Figure 16 can be used to select an appropriate input capacitor From the curves locate the 50V line and note which capacitor values have RMS current ratings greater than 1 5A Either a 470 uF or 680 uF 50V capacitor could be used For a through hole design a 680 uF 50V electrolytic capaci tor Panasonic HFQ series or Nichicon PL series or equiva lent would be adequate Other types or other manufacturers capacitors can be used provided the RMS ripple current ratings are adequate For surface mount designs solid tantalum capacitors can be used but caution must be exercised with regard to the capacitor sure current rating see Application Information or input capacitors in this data sheet The TPS series available from AVX and the 593D series from Sprague are both surge current tested To further simplify the buck regulator design procedure Na tional Semiconductor is making available computer design software to be used with the Simple Switcher l
50. l tilayer pc board with large copper areas and or airflow are recommended The curves shown in Figure 23 show the LM2599S TO 263 package junction temperature rise above ambient tempera ture with a 2A load for various input and output voltages This data was taken with the circuit operating as a buck switching regulator with all components mounted on a pc board to simulate the junction temperature under actual operating conditions This curve can be used for a quick check for the approximate junction temperature for various conditions but be aware that there are many factors that can affect the junction temperature When load currents higher than 2A are used double sided or multilayer pc boards with large copper areas and or airflow might be needed especially for high ambient temperatures and high output voltages n TO 263 Package V 20V 2A Load OUT 9 in Copper 80 T 25 C still Air 7 7 714 0 5 10 15 20 25 30 35 40 INPUT VOLTAGE V DS012582 38 JUNCTION TEMPERATURE RISE ABOVE AMBIENT 9C Circuit Data for Temperature Rise Curve TO 220 Package T Through hole electrolytic Through hole Renco Through hole 5A 40V Schottky 3 square inches single sided 2 oz copper 0 0028 Capacitors Inductor Diode PC board FIGURE 22 Junction Temperature Rise TO 220 70 D T0 220 Package Thermalloy Heat Sink 7020 0 5 10
51. n 20 Ir ix Regulator ON l But Vour OV gt 10 Reaulator i egulator In 8 0 5 Shutdown 5 lg 80 pA 2 e 0 0 TIME gt TIME DEPENDENT UPON SOFT START CAPACITOR VALUE DS012582 31 FIGURE 13 Soft start Delay Error Output 1 INPUT VOLTAGE cael CN ME MMC Vi Aaa tii 5V OUTPUT l WITH SS CAPACITOR f EM 01 p 1 U 1 ot I l SOFT START 1 pI l SOFT START 1 REGION I p REGION l ERROR FLAG OUTPUT 1 POWER 0K NO DELAY CAPACITOR ERROR FLAG OUTPUT 1 POWER OK WITH DELAY CAPACITOR ov OV REGULATED 4 75V 95 MIT A a NA ov OV OV OV OV DELAY TIME TIME TIME DS012582 32 FIGURE 14 Timing Diagram for 5V Output 19 www national com 66S2IN1 LM2599 Application Information continued LM2599 SD SS DS012582 65 FIGURE 15 External 3 7V Soft Start Clamp DELAY CAPACITOR CbeLay Provides delay for the error flag output See the upper curve in Figure 13 and also refer to timing diagrams in Figure 14 A capacitor on this pin provides a time delay between the time the regulated output voltage when it is increasing in value reaches 95 of the nominal output voltage and the time the error flag output goes high A 3 pA constant current from the delay pin charges the delay ca pacitor resulting in a voltage ramp When this voltage reaches a threshold of approximately 1 3V the open collec tor error flag output or power OK goes high This
52. n voltage percentage Also the Soft start voltage range has a negative ii 0 5 Voltage temperature coefficient associated with it See the Soft start curve in Mi 0 i the electrical characteristics section TIME 4 Normal operation Above 2 8V the circuit operates as a TIME DEPENDENT UPON DELAY CAPACITOR VALUE standard Pulse Width Modulated switching regulator The capacitor will continue to charge up until it reaches the internal clamp voltage of approximately 7V If this pin is driven from a voltage source the current must be limited to about 1 mA If the part is operated with an input voltage at or below the internal soft start clamp voltage of approximately 7V the voltage on the SD SS pin tracks the input voltage and can be disturbed by a step in the voltage To maintain proper func tion under these conditions it is strongly recommended that the SD SS pin be clamped externally between the 3V maxi mum soft start threshold and the 4 5V minimum input volt age Figure 15 is an example of an external 3 7V approx LM2599 ADJ set for 10V or curve OUTPUT VOLTAGE V E 45 clamp that prevents a line step related glitch but does not i Shutdown Clamps at 7V interfere with the soft start behavior of the device a EE Soft start Q 3 5 Pin Voltage Normal Z 30 j Operation C 2gRV 1002z3 amp 2 5 Soft start E Regio
53. nominal output voltage this ripple can be coupled to the feedback pin through the feedforward capacitor and cause the error com parator to trigger the error flag In this situation adding a resistor Ree in series with the feedforward capacitor ap proximately 3 times R1 will attenuate the ripple voltage at the feedback pin INPUT CAPACITOR Cin A low ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground pin It must be located near the regulator using short leads This capacitor prevents large voltage transients from appearing at the in put and provides the instantaneous current needed each time the switch turns on The important parameters for the Input capacitor are the voltage rating and the RMS current rating Because of the relatively high RMS currents flowing in a buck regulator s input capacitor this capacitor should be chosen for its RMS current rating rather than its capacitance or voltage ratings although the capacitance value and voltage rating are di rectly related to the RMS current rating The RMS current rating of a capacitor could be viewed as a capacitor s power rating The RMS current flowing through the capacitors internal ESR produces power which causes the internal temperature of the capacitor to rise The RMS current rating of a capacitor is determined by the amount of current required to raise the internal temperature approxi mately 10 C above an ambient temperature of 105
54. nt than the turn OFF voltage The amount of hysteresis is approximately equal to the value of the output voltage Since the SD SS pin has an internal 7V zener clamp R2 is needed to limit the current into this pin to approximately 1 mA when Q1 is on LM2599 5 0 SD SS 4 GND Q1 2N3906 Vout DS012582 47 FIGURE 30 Undervoltage Lockout Without Hysteresis for an Inverting Regulator LM2599 5 0 SD ss 4 GND Vout V DS012582 46 FIGURE 31 Undervoltage Lockout With Hysteresis for an Inverting Regulator NEGATIVE VOLTAGE CHARGE PUMP Occasionally a low current negative voltage is needed for biasing parts of a circuit A simple method of generating a negative voltage using a charge pump technique and the switching waveform present at the OUT pin is shown in Figure 32 This unregulated negative voltage is approxi mately equal to the positive input voltage minus a few volts and can supply up to a 600 mA of output current There is a requirement however that there be a minimum load of 1 2A on the regulated positive output for the charge pump to work correctly Also resistor R1 is required to limit the charging current of C1 to some value less than the LM2599 current limit typically 4 5A This method of generating a negative output voltage without an additional inductor can be used with other members of the Simple Switcher Family using either the buck or boost topology Feedback 6 Regulated
55. o longer at ground but is now at the negative output voltage The shutdown methods shown accept ground referenced shutdown signals UNDERVOLTAGE LOCKOUT Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage Figure 29 contains a undervoltage lockout circuit for a buck configu ration while Figure 30 and 30 are for the inverting types only the circuitry pertaining to the undervoltage lockout is 27 www national com 66S2IN1 LM2599 Application Information continued shown Figure 29 uses a zener diode to establish the threshold voltage when the switcher begins operating When the input voltage is less than the zener voltage resistors R1 and R2 hold the Shutdown Soft start pin low keeping the regulator in the shutdown mode As the input voltage ex ceeds the zener voltage the zener conducts pulling the Shutdown Soft start pin high allowing the regulator to begin switching The threshold voltage for the undervoltage lockout feature is approximately 1 5V greater than the zener voltage LM2599 5 0 SD SS 4 GND DS012582 45 FIGURE 29 Undervoltage Lockout for a Buck Regulator Figure 30 and 30 apply the same feature to an inverting circuit Figure 30 features a constant threshold voltage for turn on and turn off zener voltage plus approximately one volt If hysteresis is needed the circuit in Figure 31 has a turn ON voltage which is differe
56. odes of operation Before Ripple Filter 5 mV div Alter Ripple Filter 2 usec div DS012582 37 FIGURE 20 Post Ripple Filter Waveform OUTPUT VOLTAGE RIPPLE AND TRANSIENTS The output voltage of a switching power supply operating in the continuous mode will contain a sawtooth ripple voltage at the switcher frequency and may also contain short voltage spikes at the peaks of the sawtooth waveform The output ripple voltage is a function of the inductor saw tooth ripple current and the ESR of the output capacitor A typical output ripple voltage can range from approximately 0 596 to 396 of the output voltage To obtain low ripple voltage the ESR of the output capacitor must be low how ever caution must be exercised when using extremely low ESR capacitors because they can affect the loop stability resulting in oscillation problems If very low output ripple voltage is needed less than 20 mV a post ripple filter is recommended See Figure 1 The inductance required is typically between 1 uH and 5 uH with low DC resistance to maintain good load regulation A low ESR output filter ca pacitor is also required to assure good dynamic load re sponse and ripple reduction The ESR of this capacitor may be as low as desired because it is out of the regulator feedback loop The photo shown in Figure 20 shows a typical output ripple voltage with and without a post ripple filter When observing output ripple with a scope
57. of input voltage and output load conditions and 15 on the oscillator frequency Ex ternal shutdown is included featuring typically 80 HA National Semiconductor SIMPLE SWITCHER Power Converter 150 kHz 3A Step Down Voltage Regulator with Features December 2000 standby current Self protection features include a two stage current limit for the output switch and an over temperature shutdown for complete protection under fault conditions Features m 3 3V 5V 12V and adjustable output versions m Adjustable version output voltage range 1 2V to 37V 4 max over line and load conditions m Guaranteed 3A output current m Available in 7 pin TO 220 and TO 263 surface mount Package Input voltage range up to 40V 150 kHz fixed frequency internal oscillator Shutdown Soft start Out of regulation error flag Error output delay Low power standby mode lo typically 80 pA High Efficiency Uses readily available standard inductors Thermal shutdown and current limit protection Applications m Simple high efficiency step down buck regulator m Efficient pre regulator for linear regulators m On card switching regulators W Positive to Negative converter Note t Patent Number 5 382 918 12V Unregulated DC Input ik LM2599 5 0 Error Flag 1 Flag Shutdown Soft start ri Lea Cin Css 680 uF 0 1 uF SD SS 5 Delay CpeLAY 0 1 uF Typical Application Fixed Output Voltage Versions Vin Feedback SIM
58. of operating an electrolytic capacitor above the RMS current rating is a shortened operating life The higher temperature speeds up the evaporation of the capacitor s electrolyte resulting in eventual failure Selecting an input capacitor requires consulting the manu facturers data sheet for maximum allowable RMS ripple current For a maximum ambient temperature of 40 C a general guideline would be to select a capacitor with a ripple current rating of approximately 50 of the DC load current For ambient temperatures up to 70 C a current rating of 75 of the DC load current would be a good choice for a conservative design The capacitor voltage rating must be at least 1 25 times greater than the maximum input voltage and often a much higher voltage capacitor is needed to satisfy the RMS current requirements A graph shown in Figure 16 shows the relationship between an electrolytic capacitor value its voltage rating and the RMS current it is rated for These curves were obtained from the Nichicon PL series of low ESR high reliability electro lytic capacitors designed for switching regulator applications Other capacitor manufacturers offer similar types of capaci tors but always check the capacitor data sheet Standard electrolytic capacitors typically have much higher ESR numbers lower RMS current ratings and typically have a shorter operating lifetime Because of their small size and excellent performance sur face mount soli
59. prevent large voltage transients from appearing at the input In addition the RMS current rating of the input capacitor should be selected to be at least 12 the DC load current The capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded The curve shown in Figure 16 shows typical RMS current ratings for several different aluminum electro lytic capacitor values This capacitor should be located close to the IC using short leads and the voltage rating should be approximately 1 5 times the maximum input voltage If solid tantalum input capacitors are used it is recomended that they be surge current tested by the manufacturer Use caution when using a high dielectric constant ceramic capacitor for input bypassing because it may cause severe ringing at the Vn pin For additional information see section on input capaci tor in application information section 5 Catch Diode Selection D1 A Refer to the table shown in Figure 11 Schottky diodes provide the best performance and in this example a 3A 40V 1N5825 Schottky diode would be a good choice The 3A diode rating is more than adequate and will not be over stressed even for a shorted output 6 Input Capacitor Cn The important parameters for the Input capacitor are the input voltage rating and the RMS current rating With a nominal input voltage of 28V an aluminum electrolytic alumi num electrolytic capacitor with a voltage rat
60. re construction for Schott ferrite bobbin core for Renco and Coilcraft and powdered iron toroid for Pulse Engineer ing Exceeding an inductor s maximum current rating may cause the inductor to overheat because of the copper wire losses or the core may saturate If the inductor begins to saturate the inductance decreases rapidly and the inductor begins to look mainly resistive the DC resistance of the winding This can cause the switch current to rise very rapidly and force the switch into a cycle by cycle current limit thus reducing the DC output load current This can also result in overheat ing of the inductor and or the LM2599 Different inductor types have different saturation characteristics and this should be kept in mind when selecting an inductor The inductor manufacturer s data sheets include current and energy limits to avoid inductor saturation www national com 22 Application Information continued DISCONTINUOUS MODE OPERATION The selection guide chooses inductor values suitable for continuous mode operation but for low current applications and or high input voltages a discontinuous mode design may be a better choice It would use an inductor that would be physically smaller and would need only one half to one third the inductance value needed for a continuous mode design The peak switch and inductor currents will be higher in a discontinuous design but at these low load currents 1A and below th
61. s output short the diode should have a current rating equal to the maximum current limit of the LM2599 The most stressful condition for this diode is an overload or shorted output condition B The reverse voltage rating of the diode should be at least 1 25 times the maximum input voltage C This diode must be fast short reverse recovery time and must be located close to the LM2599 using short leads and short printed circuit traces Because of their fast switching speed and low forward voltage drop Schottky diodes provide the best performance and efficiency and should be the first choice especially in low output voltage applications Ultra fast recovery or High Efficiency rectifiers also provide good results Ultra fast recovery diodes typically have re verse recovery times of 50 ns or less Rectifiers such as the IN5400 series are much too slow and should not be used 4 Input Capacitor Cj Alow ESR aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large voltage transients from appearing at the input In addition the RMS current rating of the input capacitor should be selected to be at least 12 the DC load current The capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded The curve shown in Figure 16 shows typical RMS current ratings for several different aluminum electro lytic capacitor values This capacitor should be located close to th
62. signal can be used to indicate that the regulated output has reached the correct voltage and has stabilized If for any reason the regulated output voltage drops by 596 or more the error output flag Pin 3 immediately goes low internal transistor turns on The delay capacitor provides very little delay if the regulated output is dropping out of regulation The delay time for an output that is decreasing is approximately a 1000 times less than the delay for the rising output For a 0 1 uF delay capacitor the delay time would be approximately 50 ms when the output is rising and passes through the 95 threshold but the delay for the output dropping would only be approximately 50 us Rpun up The error flag output or power OK is the col lector of a NPN transistor with the emitter internally grounded To use the error flag a pullup resistor to a positive voltage is needed The error flag transistor is rated up to a maximum of 45V and can sink approximately 3 mA If the error flag is not used it can be left open FEEDFORWARD CAPACITOR Adjustable Output Voltage Version Cpr A Feedforward Capacitor C shown across R2 in Figure 1 is used when the output voltage is greater than 10V or when Cour has a very low ESR This capacitor adds lead compensation to the feedback loop and increases the phase margin for better loop stability For Cer selection see the design procedure section If the output ripple is large gt 5 of the
63. stant As the load current increases or de creases the entire sawtooth current waveform also rises and falls The average value or the center of this current waveform is equal to the DC load current If the load current drops to a low enough level the bottom of the sawtooth current waveform will reach zero and the switcher will smoothly change from a continuous to a discon tinuous mode of operation Most switcher designs irregard less how large the inductor value is will be forced to run discontinuous if the output is lightly loaded This is a per fectly acceptable mode of operation In a switching regulator design knowing the value of the peak to peak inductor ripple current Al up can be useful for 23 www national com 66S2IN1 LM2599 Application Information continued determining a number of other circuit parameters Param eters such as peak inductor or peak switch current mini mum load current before the circuit becomes discontinuous output ripple voltage and output capacitor ESR can all be calculated from the peak to peak Aliup When the inductor nomographs shown in Figure 4 through 7 are used to select an inductor value the peak to peak inductor ripple current can immediately be determined The curve shown in Figure 21 shows the range of Alinp that can be expected for different load currents The curve also shows how the peak to peak inductor ripple current Alinp changes as you go from the lower
64. t Voltage lLoap max Maximum Load Current 1 Inductor Selection L1 A Select the correct inductor value selection guide from Figure 4 Figure 5 or 6 Output voltages of 3 3V 5V or 12V respectively For all other voltages see the design proce dure for the adjustable version B From the inductor value selection guide identify the in ductance region intersected by the Maximum Input Voltage line and the Maximum Load Current line Each region is identified by an inductance value and an inductor code LXX C Select an appropriate inductor from the four manufactur er s part numbers listed in Figure 8 2 Output Capacitor Selection Cour A In the majority of applications low ESR Equivalent Series Resistance electrolytic capacitors between 82 uF and 820 uF and low ESR solid tantalum capacitors between 10 uF and 470 pF provide the best results This capacitor should be located close to the IC using short capacitor leads and short copper traces Do not use capacitors larger than 820 pF For additional information see section on output capaci tors in application information section B To simplify the capacitor selection procedure refer to the quick design component selection table shown in Figure 2 This table contains different input voltages output voltages and load currents and lists various inductors and output capacitors that will provide the best design solutions C The capacitor voltage rating for electrolytic
65. vailable in two packages a 7 pin TO 220 T and a 7 pin surface mount TO 263 S The TO 220 package needs a heat sink under most condi tions The size of the heat sink depends on the input voltage the output voltage the load current and the ambient tem perature The curves in Figure 22 show the LM2599T junc tion temperature rises above ambient temperature for a 3A load and different input and output voltages The data for these curves was taken with the LM2599T TO 220 pack age operating as a buck switching regulator in an ambient temperature of 25 C still air These temperature rise num bers are all approximate and there are many factors that can affect these temperatures Higher ambient temperatures re quire more heat sinking The TO 263 surface mount package tab is designed to be soldered to the copper on a printed circuit board The copper and the board are the heat sink for this package and the other heat producing components such as the catch diode and inductor The pc board copper area that the package is soldered to should be at least 0 4 in and ideally should have 2 or more square inches of 2 oz 0 0028 in copper Additional copper area improves the thermal characteristics www national com 24 Application Information continued but with copper areas greater than approximately 6 in only small improvements in heat dissipation are realized If fur ther thermal improvements are needed double sided mu
66. wable output ripple voltage typically 196 to 296 of the output voltage But if the selected capacitors ESR is extremely low there is a possibility of an unstable feedback loop resulting in an oscillation at the output Using the capacitors listed in the tables or similar types will provide design solutions under all conditions If very low output ripple voltage less than 15 mV is re quired refer to the section on Output Voltage Ripple and Transients for a post ripple filter An aluminum electrolytic capacitors ESR value is related to the capacitance value and its voltage rating In most cases higher voltage electrolytic capacitors have lower ESR values see Figure 17 Often capacitors with much higher voltage ratings may be needed to provide the low ESR values re quired for low output ripple voltage The output capacitor for many different switcher designs often can be satisfied with only three or four different capaci tor values and several different voltage ratings See the quick design component selection tables in Figure 2 and 3 for typical capacitor values voltage ratings and manufactur ers capacitor types Electrolytic capacitors are not recommended for tempera tures below 25 C The ESR rises dramatically at cold tem peratures and typically rises 3X 25 C and as much as 10X at 40 C See curve shown in Figure 18 Solid tantalum capacitors have a much better ESR spec for cold temperatures and are recommended for

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National LM2599 Manual

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