National LM1578A/LM2578A/LM3578A Manual(1)(1)
Contents
1. Switching Regulator General Description The LM1578A is a switching regulator which can easily be set up for such DC to DC voltage conversion circuits as the buck boost and inverting configurations The LM1578A fea tures a unique comparator input stage which not only has separate pins for both the inverting and non inverting inputs but also provides an internal 1 0V reference to each input thereby simplifying circuit design and p c board layout The output can switch up to 750 mA and has output pins for its collector and emitter to promote design flexibility An external current limit terminal may be referenced to either the ground or the Vi terminal depending upon the application In addi tion the LM1578A has an on board oscillator which sets the switching frequency with a single external capacitor from 1 Hz to 100 kHz typical The LM1578A is an improved version of the LM1578 offer ing higher maximum ratings for the total supply voltage and output transistor emitter and collector voltages Functional Diagram LM1578A LM2578A LM3578A Eo REFERENCE INTERNAL SUPPLIES OSCILLATOR TIMING CAPACITOR PIN 3 April 1998 National Semiconductor Features m inverting and non inverting feedback inputs m 1 0V reference at inputs m Operates from supply voltages of 2V to 40V m Output current up to 750 mA saturation less than 0 9V m Cu2. 160 _ 160 COLLECTOR OUTPUT 15 COLLECTOR OUTPUT SWING 15V a SWING 15V MO 15 10 140 tT 25 a gt 1 RES 5 e i 120 OND REFERR We qn GND REFERRED x x 3 ee ee 5 15 Tree a Z 100 2 GND REFERRED 100 amp 10 Viy REFERRED 2 E _ COLLECTOR 2 a EN a 56 8 5 a OUTPUT o 80 0 15V R 1 60 60 50 25 0 25 50 75 100 125 150 0 1 LIS UID L8 0 10 20 30 40 TEMPERATURE 9C TIME SUPPLY VOLTAGE V DS008711 38 DS008711 39 DS008711 40 Supply Current Supply Current Collector Current with 1 4 Emitter Output Below Ground EMITTER GND Vs in 15 2 0 111 1 OUTPUT ON EMITTER GND COLLECTOR 15V z Ll 3 3 OUTPUT ON Jg 5 ko 9 4 a amp g g E 2 amp 2 e Boca o OUTPUT OFF gt 25 OUTPUT OFF E amp amp j 0 8 amp o 5 1 5 1 E gt amp 0 4 0 0 0 10 20 30 40 50 25 0 25 50 75 100 125 150 SUPPLY VOLTAGE V DS008711 41 Test Circuit Parameter tests can be made using the test circuit shown Select the desired Vin collector voltage and duty cycle with adjustable power supplies A digital volt meter with an input resistance greater than 100 MQ should be used to measure the following Input Reference Voltage to Ground S1 in either position Level Shift Accuracy Tp3 V 1V x
3. CAPACITANCE nF 0 1 1 10 100 FREQUENCY kHz DS008711 4 FIGURE 1 Value of Timing Capacitor vs Oscillator Frequency OUTPUT TRANSISTOR The output transistor is capable of delivering up to 750 mA with a saturation voltage of less than 0 9V see Collector Saturation Voltage and Emitter Saturation Voltage curves The emitter must not be pulled more than 1V below ground this limit is 0 6V for T 2 100 C Because of this limit an ex ternal transistor must be used to develop negative output voltages see the Inverting Regulator Typical Application Other configurations may need protection against violation of this limit see the Emitter Output section of the Applica tions Information CURRENT LIMIT The LM1578A s current limit may be referenced to either the ground or the Vi pins and operates a cycle by cycle ba sis The current limit section consists of two comparators one with its non inverting input referenced to a voltage 110 mV below Vin the other with its inverting input referenced 110 mV above ground see FUNCTIONAL DIAGRAM The current limit is activated whenever the current limit terminal is pulled 110 mV away from either Vin or ground Applications Information CURRENT LIMIT As mentioned in the functional description the current limit terminal may be referenced to either the Vin or the ground termin
4. 350 mA Step 2 Calculate the inductor Volts sec product E T aC cording to the equations given from the chart For the Buck E Top Vi Vo Vo Vin 1000 1 15 5 5 15 1000 50 66V us with the oscillator frequency foso expressed in kHz DS008711 6 Vin 15V R3 0 150 Vo 5V C1 1820 pF Viigple 10 mV C2 220 uF lo 350 mA C3 20 pF foso 50 kHz L1 470 uH R1 40 D1 1N5818 R2 10 FIGURE 15 Buck or Step Down Regulator Step 3 Using the graph with axis labeled Discontinuous At lour and limax pc find the point where the desired maximum inductor current pc intercepts the desired discontinuity percentage In this example the point of interest is where the 0 35A line intersects with the 20 line This is nearly the midpoint of the horizontal axis Step 4 This last step is merely the translation of the point found in Step 3 to the graph directly below it This is accom plished by moving straight down the page to the point which intercepts the desired E T For this example E T is 66V us and the desired inductor value is 470 pH Since this example was for 20 discontinuity the bottom chart could have been used directly as noted in step 3 of the chart instructions www national com Typical Applications continued 1 112800SG 8861 4 05 WOLOnaNOOIW3S TWNOILYN vo o voL S Joje no e eouejonpu 9
5. MIN 40 040 NOM 0 018 0 003 0 325 _ 0 015 0 457 0 076 1 016 0 100 0 010 2 540 0 254 ey o Re 143 0 0 060 e 1 524 iE x 1 270 REV Molded Dual In Line Package Order Number LM2578AN or LM3578AN NS Package Number NO8E LIFE SUPPORT POLICY NATIONAL S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DE VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI CONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or sys 2 A critical component in any component of a life support tems which a are intended for surgical implant into device or system whose failure to perform can be rea the body or b support or sustain life and whose fail sonably expected to cause the failure of the life support ure to perform when properly used in accordance device or system or to affect its safety or effectiveness with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user National Semiconductor National Semiconductor National Semiconductor National Semiconductor Corporation Europe Asia Pacific Customer Japan Ltd Americas Fax 449 0 1 80 530 85 86 Response Group Tel 81 3 5620 6175 Tel 1 800 272 9959 Email europe support nsc com Tel 65 2544466 Fax 81 3 5620 6179 Fax 1 800 737 7018 Deutsch Tel 49 0
6. 1 80 530 85 85 Fax 65 2504466 Email support nsc com English Tel 49 0 1 80 532 78 32 Email sea support nsc com Frangais Tel 49 0 1 80 532 93 58 www national com Italiano Tel 49 0 1 80 534 16 80 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
7. Va is the forward voltage drop of the diodes Vsa iS the saturation voltage of the LM1578A output transis tor Vasa is the saturation voltage of transistor Q1 L12 Vin id Vsat ud Vsat ton lp where ta 1 fosc 2Va On Vo Vin 2Vq Vsat im 2lo Vin Vo 29 Vsat Vsatt B Vin Vsat Vsati RS 232 LINE DRIVER POWER SUPPLY The power supply shown in Figure 23 operates from an in put voltage as low as 4 2V 5V nominal and delivers an out put of 12V at 40 mA with better than 70 efficiency The circuit provides a load regulation of 150 mV from 10 to 100 of full load and a line regulation of 10 mV Other no table features include a cycle by cycle current limit and an output voltage ripple of less than 40 mVp p A unique feature of this circuit is its use of feedback from both outputs This dual feedback configuration results in a sharing of the output voltage regulation by each output so that neither side becomes unbalanced as in single feedback Systems In addition since both sides are regulated it is not necessary to use a linear regulator for output regulation The feedback resistors R2 and R3 may be selected as fol lows by assuming a value of 10 kO for R1 R2 V 1V 45 8 pA 240 R3 V 1 54 2 uA 240 Actually the currents used to program the values for the feedback resistors may vary from 40 pA to 60 pA as long as their sum is equal
8. 0 737 1 143 0 028 0 034 0 115 0 145 1156 0148 0 711 0 86 ix 2 821 3 683 DIA EQUALLY SPACED gt Metal Can Package H REV E Order Number LM1578AH 883 or SMD 5962 8958602 NS Package Number H08C 0 22 0 228 0 244 5 791 6 198 0 150 0 157 3 810 3 988 45 8 MAX TYP 0 053 0 069 0 010 0 020 0 053 0 069 1 346 1 753 0 254 0 508 0 189 0 197 4 800 5 004 0 004 0 010 0 102 0 254 WH SEATING A 1 PLANE 10 102 014 208 090 yen rs 0 016 0 050 0 356 0 950 PE E 0 014 0 020 qyp PALL LEAT 0 406 1 270 m ds 356 0 508 ALL LEADS SL Plastic Surface Mount Package M Order Number LM3578AM NS Package Number M08A 0 203 MOBA REV H 17 www national com LM1578A LM2578A LM3578A Switching Regulator Physical Dimensions inches millimeters unless otherwise noted Continued 0 373 0 400 9 474 10 16 0 090 t 0 032 0 005 0 813 0 127 0 250 0 005 RAD 6 35 0 127 PIN NO 1 iE c 0 280 7 112 MIN OPTION 2 0 300 0 320 0 039 0 145 0 200 0 991 TETTE 7 62 8 128 3 683 5 080 0 130 0 005 23010127 wis d 0 125 0 140 A i B Y 3175 3 556 ita 0 009 0 015 mam 155 90 4 10 508 1009 0015 _ 3 175 0 508 0 229 0 381
9. 100 S1 at 1mA Input Current mA 1V V 1 MQ St atl l5 0 mA Oscillator parameters can be measured at using a fre quency counter or an oscilloscope TEMPERATURE 0 50 25 0 25 50 75 100 125 150 DS008711 42 TEMPERATURE DS008711 43 The Current Limit Sense Voltage is measured by connecting an adjustable 0 to 1V floating power supply in series with the current limit terminal and referring it to either the ground or the Vi terminal Set the duty cycle to 90 and monitor test point Tps while adjusting the floating power supply voltage until the LM1578A s duty cycle just reaches 0 This voltage is the Current Limit Sense Voltage The Supply Current should be measured with the duty cycle at 0 and S1 in the I 1 0 mA position LM1578A specifications are measured using automated test equipment This circuit is provided for the customer s convenience when checking parameters Due to possible variations in testing conditions the measured values from these testing procedures may not match those of the factory www national com Test Circuit continued Vin VcoLLECTOR 2 TO 40V 5 TO 40V 15V INPUT REFERENCE R4 VOLTAGES 10k LEVEL SHIFT ACCURACY AND INPUT CURRENT Te4 LI OSCILLATOR FREQUENCY AMPLITUDE AND SCOPE SYNC Op amp supplies are 15V DVM input resistance 2100 MQ LM1578 max duty cycle is 90 Definition of Te
10. Dissipation Note 3 Internally limited Junction Temperature Range Output Current 750 mA LM1578A Storage Temperature 65 C to 150 C LM2578A Lead Temperature LM3578A soldering 10 seconds 260 C Maximum Junction Temperature 150 C LM1578A LM2578A Symbol Parameter Conditions Typical Limit LM3578A Units Note 5 Note 6 Limit Note 11 Note 7 OSCILLATOR fosc Frequency 20 kHz 224 24 kHz max 17 6 16 kHz min Afosc AT Frequency Drift with 0 13 SIC Temperature Amplitude 550 mV REFERENCE COMPARATOR Note 8 Vn Input Reference L l 0 mA and 1 0 V Voltage l 1 1 mA 1 Note 9 1 035 1 050 1 050 1 070 V max 0 965 0 950 0 950 0 930 V min AVnR AViN Input Reference Volt 1 0 mA and 0 003 IN age Line Regulation l lp 1 mA 41 Note 9 0 01 0 02 0 01 0 02 max lnv Inverting Input l l 0 mA duty cycle 25 0 5 uA Current Level Shift Accuracy Level Shift Current 1 mA 1 0 5 8 10 13 AVg At Input Reference 100 ppm 1000h Voltage Long Term Stability OUTPUT Vc sat Collector Saturation lc 750 mA pulsed Emitter 0 7 V Voltage grounded 0 85 1 2 0 90 1 2 V max Ve sat Emitter Saturation lo 80 mA pulsed 1 4 V Voltage Vin 40V 1 6 2 1 1 7 2 0 V max Ices Collector Leakage Vin 40V Emitter 0 1 uA Current grounded Output OFF 50 100 200 250 uA max BVceoisus Collector Emitter lsusr 0 2A pu
11. IVol Viw R5 is defined in the Buck with Boosted Output Current sec tion R6 serves the same purpose as R4 in the Boost Regulator circuit and is typically 220 C1 C3 and C4 are defined in the Boost Regulator section C2 gt lo IVol fose Vol L1 is found as outlined in the section on buck converters us ing the inductance chart of Figure 16 for the invert configura tion and 20 discontinuity www national com Typical Applications continued Vin 5V Vo 15V Viipple 5 mV lo 300 mA Imin 60 mA foso 50 kHz R1 160 kQ R2 10 R3 0 010 DS008711 12 R4 1902 R5 820 R6 220 C1 1820 pF C2 1000 uF C3 20 pF C4 0 0022 uF L1 150 pH D1 1N5818 FIGURE 21 Inverting Regulator BUCK BOOST REGULATOR The Buck Boost Regulator shown in Figure 22 may step a voltage up or down depending upon whether or not the de sired output voltage is greater or less than the input voltage In this case the output voltage is 12V with an input voltage from 9V to 15V The circuit exhibits an efficiency of 75 with a load regulation of 60 mV 10 mA to 100 mA and a line regulation of 52 mV R1 V 1 R2 where R2 10 V 0 75A R4 C1 and C4 are defined in the Boost Regulator sec tion D1 and D2 are Schottky type diodes such as the 1N5818 or 1N5819 lo Vripple Vo 2Va C22 fosc Vin Vo 2Va where
12. a squarewave This squarewave is then con verted back into a DC voltage of lower magnitude by the low pass filter consisting of L1 and C1 The duty cycle D of the squarewave relates the output voltage to the input voltage by the following equation Vout Dx Vin Vin x ton ton lor www national com Typical Applications continued 05008711 5 FIGURE 14 Basic Buck Regulator Figure 15 is a 15V to 5V buck regulator with an output cur rent lo of 350 mA The circuit becomes discontinuous at 20 of lo max has 10 mV of output voltage ripple an effi ciency of 75 a load regulation of 30 mV 70 mA to 350 mA and a line regulation of 10 mV 12 x Vi x 18V Component values are selected as follows R1 V 1 x R2 where R2 10 R3 R3 0 150 where V is the current limit sense voltage 0 11V lsw max IS the maximum allowable current thru the output transistor L1 is the inductor and may be found from the inductance cal culation chart Figure 16 as follows Given Vin 15V Vo 5V 350 mA fosc 50 kHz Discontinuous at 20 of loa Note that since the circuit will become discontinuous at 2096 Of max the load current must not be allowed to fall below 70 mA Step 1 Calculate the maximum DC current through the in ductor max The necessary equations are indicated at the top of the chart and show that 1 for the buck configuration Thus
13. to the 100 necessary to establish the 1V threshold across R1 Ideally these currents should be equal 50 pA each for optimal control However as was done here they may be mismatched in order to use standard resistor values This results in a slight mismatch of regulation between the two outputs The current limit resistor R4 is selected by dividing the cur rent limit threshold voltage by the maximum peak current level in the output switch For our purposes R4 110 mV 750 mA 0 150 A value of 0 1Q was used www national com Typical Applications continued DS008711 13 QV lt Vin lt 15V R5 270 Vo 12V C1 1820 pF lo 100 mA C2 220 uF Viipple 50 mV C3 20 pF foso 50 kHz 0 0022 pF R1 110k L1 220 uH R2 10k D1 D2 1N5819 0 15 Q1 D44 4 220k FIGURE 22 Buck Boost Regulator LM 15784 s 05008711 14 Vin 5V R4 0 152 Vo t12V C1 820 pF lo 40 mA C2 10 pF fose 80 kHz C3 220 uF R1 10 D1 02 D3 135819 R2 240 1 64287 240 FIGURE 23 RS 232 Line Driver Power Supply Capacitor C1 sets the oscillator frequency and is selected from Figure 1 Capacitor C2 serves as a compensation capacitor for syn chronous operation and a value of 10 to 50 pF should be suf ficient for most applications A minimum value for an ideal output capacitor C3 could be calculated as C x where I is the l
14. 0 94 9 AYNSIA sany 90Xvn Ei e ALIINILNOOSIQ 07 304 vro 80 zo vioo a 93 vool 8 z v g 3 vio 80 wo 3 ol 0c og 0 E 05 OL 001 S00 0 zHA g DANA L 0 d0 NI 2913 A NIA 0 a eor A 0001 CA NI NI 1 A_ 0 21 _1 I2 o 1 11 A 183ANI 901 3 S N 1Y SNONNILNOOSIA 90 XYW li uo 301 3 1Y INVA JONVLONGNI avs 8 383H T 2 XYN avis 20 x 303 301 3 1Y INWA JONVLONGNI 9 301 3 OL ATINOLISA 033208d 90 1 woud S CD noui 2 7 OL ATIVINOZINOH 4 0330094 1330044 40 31 20 XIN Tz _ 001 19 ALINNILNOOSIG ana 901 3 ayn Z 2 avino 7 lNVH2 SIHL ISN OL MOH www national com 11 Typical Applications continued where is the peak to peak output voltage ripple C3 is necessary for continuous operation and is generally in For a full line of standard inductor values contact Pulse En the 10 pF to 30 pF range gineering San Diego Calif regarding their PEB26XX se ries or A Magnetics Nashville Tenn D1 should be a
15. 5096 Applying the maximum Current Limit Sense Voltage to pin 7 is certain to reduce the duty cycle below 5096 Increasing this voltage by 15 mV may be required to reduce the duty cycle to 0 when the Collector output swing is 40V or greater see Ground Referred Current Limit Sense Voltage typical curve Note 11 A military RETS specification is available on request At the time of printing the LM1578A RETS spec complied with the boldface limits in this column The LM1578AH may also be procured as a Standard Military Drawing 3 www national com Connection Diagram and Ordering Information Metal Can CURRENT LIMIT DS008711 28 Top View Order Number LM1578AH 883 or SMD 5962 8958602 See NS Package Number HO8C Typical Performance Characteristics Oscillator Frequency Change INPUT Dual In Line Package INPUT osc GND e N gt Vin CURRENT LIMIT COLLECTOR EMITTER an DS008711 29 Order Number LM3578AM LM2578AN or LM3578AN See NS Package Number or NOSE Oscillator Voltage Swing with Temperature 800 18 750 UPPER LIMIT o 12 700 T Sos 2 300 g 5 M 250 g 200 09 LOWER LIMIT S 150 XE 0 8 er 4 nF 100 57 50 25 0 25 50 75 100 125 150 50 25 0 25 50
16. 75 100 125 150 TEMPERATURE C TEMPERATURE 9C Pt DS008711 32 Collector Saturation Voltage Emitter Saturation Voltage Sinking Current Sourcing Current Emitter Grounded Collector at Vin 1 0 1 0 15 0 8 P 0 8 0 6 amp 0 6 3 n o o S 04 8 2 0 4 at I S8 02 Z 0 2 0 0 0 02 04 06 0 8 1 0 0 04 08 12 16 20 COLLECTOR EMITTER VOLTAGE V DS008711 35 EMITTER COLLECTOR VOLTAGE V DS008711 36 Input Reference Voltage Drift with Temperature 1 030 1 020 1 010 1 000 n EE SS 0 980 0 970 REFERENCE VOLTAGE V 0 960 0 950 50 25 0 25 50 75 100 125 150 TEMPERATURE DS008711 34 Ground Referred Current Limit Sense Voltage 100 1 f 20 kHz C 40V SWING 80 E GROUND INV INPUT TO GND 60 4 2 2 5 4 5 20 60 80 100 120 140 160 SENSE VOLTAGE mV DS008711 37 www national com Typical Performance Characteristics Continued Current Limit Sense Voltage Drift with Temperature Current Limit Response Time for Various Over Drives Current Limit Sense Voltage vs Supply Voltage
17. GURE 19 Boost or Step Up Regulator R1 V 1 R2 where R2 10 ko R3 V l max 0 5 Al where Al 2 1 Vo Vin Al is 200 mA in this example R4 C3 and C4 are necessary for continuous operation and are typically 220 20 pF and 0 0022 uF respectively C1 is the timing capacitor found in Figure 1 C22 lo Vo r2 Vin fosc Vo D1 is a Schottky type diode such as a IN5818 or IN5819 L1 is found as described in the buck converter section using the inductance chart for Figure 16 for the boost configuration and 20 discontinuity INVERTING REGULATOR Figure 20 shows the basic configuration for an inverting regulator The input voltage is of a positive polarity but the output is negative The output may be less than equal to or greater in magnitude than the input The relationship be tween the magnitude of the input voltage and the output volt age is Vs Vin X ton tor D1 Vin 05008711 10 FIGURE 20 Basic Inverting Regulator Figure 21 shows an LM1578A configured as a 5V to 15V polarity inverter with an output current of 300 mA a load regulation of 44 mV 60 mA to 300 mA and a line regulation of 50 mV 4 5V lt Vin lt 8 5V R1 1 R2 where R2 10 R3 V li max pc 0 5 Ali R4 10VsgBV ly max 0 5 where V Veer B are defined in the Buck Converter with Boosted Output Current section Al 2 lioAp min Vin
18. Schottky type diode such as the 1N5818 or 1N5819 A more precise inductance value may be calculated for the Buck Boost and Inverting Regulators as follows BUCK WITH BOOSTED OUTPUT CURRENT BUCK For applications requiring a large output current an external L V Vi V5 Al Vin fosc transistor may be used as shown in Figure 17 This circuit BOOST steps a 15V supply down to 5V with 1 5A of output current The output ripple is 50 mV with an efficiency of 80 a load L Vin Vo Vi Al foso Vo regulation of 40 mV 150 mA to 1 5A and a line regulation INVERT of 20 mV 12V lt Vi lt 18V L Vin Vol fosc Component values are selected as outlined for the buck where Al is the current ripple through the inductor Al is regulator with a discontinuity factor of 10 with the addition usually chosen based on the minimum load current expected of R4 and R5 of the circuit For the buck regulator since the inductor cur R4 10Vsg Bil rent I equals the load current lo 5 Vin V Vago Vea Br ias Ina Al 2 login where Al 140 mA for this circuit Al can also be interpreted as V is the Vac of transistor Q1 4 BE1 BE Discontinuity Factor l s Vsa iS the saturation voltage of the LM1578A output transis where the Discontinuity Factor is the ratio of the minimum tor load current to the maximum load current For this example the Discontinuity Factor is 0 2 The remainder of t
19. al Resistor R3 converts the current to be sensed into a voltage for current limit detection Vin Q 05008711 15 FIGURE 2 Current Limit Ground Referred Vin LM1578A le DS008711 16 FIGURE 3 Current Limit Vin Referred CURRENT LIMIT TRANSIENT SUPPRESSION When noise spikes and switching transients interfere with proper current limit operation R1 and C1 act together as a low pass filter to control the current limit circuitry s response time Because the sense current of the current limit terminal varies according to where it is referenced R1 should be less than 2 when referenced to ground and less than 1000 when referenced to Vin Vin QOQ LM1578A le DS008711 17 FIGURE 4 Current Limit Transient Suppressor Ground Referred www national com Applications Information continued Vin LM1578A 05008711 18 FIGURE 5 Current Limit Transient Suppressor Vi Referred C L SENSE VOLTAGE MULTIPLICATION When a larger sense resistor value is desired the voltage di vider network consisting of R1 and R2 may be used This effectively multiplies the sense voltage by 1 R1 R2 Also R1 can be replaced by a diode to increase current limit sense voltage to about 800 mV diode V 110 mV 05008711 19 FIGURE 6 Current Limit Sense Voltage Multiplication Ground Referred Vin LM1578A DS008711 20 FIGURE 7 Current Limit Sense Voltage Mul
20. he components of Figure 15 are chosen V is the current limit sense voltage B is the forced current gain of transistor Q1 B 30 for Fig ure 17 as follows es feed Ina Vgg R4 C1 is the timing capacitor found in Figure 1 2 gt Vo Vin Dx Vo 8fose 2VinViippteL 1 Pag hime D S DS008711 8 Vin 15V R4 2002 Vo 5V R5 3300 Viipgie 50 mV C1 1820 pF lo 1 5A C2 330 uF foso 50 kHz C3 20 pF R1 40 L1 220 pH R2 10 D1 1N5819 R3 0 050 Q1 D45 FIGURE 17 Buck Converter with Boosted Output Current BOOST REGULATOR The boost regulator converts a low input voltage into a higher output voltage The basic configuration is shown in Figure 18 Energy is stored in the inductor while the transis tor is on and then transferred with the input voltage to the output capacitor for filtering when the transistor is off Thus Vo Vin Vin ton toti www national com 12 Typical Applications continued DS008711 9 FIGURE 18 Basic Boost Regulator The circuit of Figure 19 converts a 5V supply into a 15V sup ply with 150 mA of output current a load regulation of 14 mV 30 mA to 140 mA and a line regulation of 35 mV 4 5V lt Vin 8 5V DS008711 11 Vin 5V R4 200 ka Vo 15V C1 1820 pF Viipple 10 mV C2 470 pF l 140 mA C3 20 pF fose 50 kHz 0 0022 pF R1 140 ka L1 330 pH R2 10 D1 1N5818 R3 0 150 FI
21. ing Voltage The collector emitter breakdown voltage of the output transistor measured at a specified current 25 1 35V Rig 0 2V Rig 10k DS008711 3 Current Limit Sense Voltage The voltage at the Current Limit pin referred to either the supply or the ground terminal which via logic circuitry will cause the output transistor to turn OFF and resets cycle by cycle at the oscillator fre quency Current Limit Sense Current The bias current for the Cur rent Limit terminal with the applied voltage equal to the Cur rent Limit Sense Voltage Supply Current The IC power supply current excluding the current drawn through the output transistor with the oscilla tor operating Functional Description The LM1578A is a pulse width modulator designed for use as a switching regulator controller It may also be used in other applications which require controlled pulse width volt age drive A control signal usually representing output voltage fed into the LM1578A s comparator is compared with internally generated reference The resulting error signal and the oscillator s output are fed to a logic network which determines when the output transistor will be turned ON or OFF The following is a brief description of the subsections of the LM1578A COMPARATOR INPUT STAGE The LM1578A s comparator input stage is unique in that both the inverting and non inverting inputs are available to the user a
22. lsed V 0 60 V Sustaining Voltage 50 50 V min www national com Electrical Characteristics Continued These specifications apply for 2V lt Vin lt 40V 2 2V lt Vin lt 40V for T lt 25 C timing capacitor 3900 pF and 25 lt duty cycle 7595 unless otherwise specified Values in standard typeface are for T 25 values in boldface type apply for operation over the specified operating junction temperature range LM1578A LM2578A Symbol Parameter Conditions Typical Limit LM3578A Units Note 5 Note 6 Limit Note 11 Note 7 CURRENT LIMIT VeL Sense Voltage Referred to Vin or Ground 110 mV Shutdown Level Note 10 95 80 mV min 140 160 mV max AVa AT Sense Voltage 0 3 AFC Temperature Drift lc Sense Bias Current Referred to Vin 4 0 pA Referred to ground 0 4 DEVICE POWER CONSUMPTION ls Supply Current Output OFF Ve OV 2 0 mA 3 0 3 3 3 5 4 0 mA max Output ON lc 750 mA pulsed 14 mA Ve 0V Note 1 Absolute Maximum Ratings indicate limits beyond which damage to the device may occur DC and AC electrical specifications do not apply when operating the device beyond its rated operating conditions Note 2 For Ty gt 100 C the Emitter pin voltage should not be driven more than 0 6V below ground see Application Information Note 3 At elevated temperatures devices must be derated based on package thermal resistance The de
23. ltage drop of the catch diode see Figure 2 For better efficiency of a high output current buck regulator an external PNP transistor should be used as shown in Figure 16 DS008711 30 FIGURE 12 D1 Prevents Output Transistor from Improperly Turning ON due to D2 s Forward Voltage SYNCHRONIZING DEVICES When several devices are to be operated at once their oscil lators may be synchronized by the application of an external signal This drive signal should be a pulse waveform with a minimum pulse width of 2 us and an amplitude from 1 5V to 2 0V The signal source must be capable of 1 driving ca pacitive loads and 2 delivering up to 500 pA for each LM1578A Capacitors C1 thru CN are to be selected for a 20 slower frequency than the synchronization frequency LM1578A LM1578A LM1578A 3 3 3 DS008711 25 FIGURE 13 Synchronizing Devices Typical Applications The LM1578A may be operated in either the continuous or the discontinuous conduction mode The following applica tions except for the Buck Boost Regulator are designed for continuous conduction operation That is the inductor cur rent is not allowed to fall to zero This mode of operation has higher efficiency and lower EMI characteristics than the dis continuous mode ALL DIODES ARE 1N914 BUCK REGULATOR The buck configuration is used to step an input voltage down to a lower level Transistor Q1 in Figure 14 chops the input DC voltage into
24. nd both contain a 1 0V reference This is accom plished as follows A 1 0V reference is fed into a modified voltage follower circuit see FUNCTIONAL DIAGRAM When both input pins are open no current flows through R1 www national com Functional Description continued and R2 Thus both inputs to the comparator will have the po tential of the 1 0V reference V4 When one input for ex ample the non inverting input is pulled AV away from V4 a current of AV R1 will flow through R1 This same current flows through R2 and the comparator sees a total voltage of 2AV between its inputs The high gain of the system through feedback will correct for this imbalance and return both in puts to the 1 0V level This unusual comparator input stage increases circuit flex ibility while minimizing the total number of external compo nents required for a voltage regulator system The inverting switching regulator configuration for example can be set up without having to use an external op amp for feedback polar ity reversal see TYPICAL APPLICATIONS OSCILLATOR The LM1578A provides an on board oscillator which can be adjusted up to 100 kHz Its frequency is set by a single exter nal capacitor C4 as shown in Figure 1 and follows the equation fosc 8x1079 C The oscillator provides a blanking pulse to limit maximum duty cycle to 9096 and a reset pulse to the internal circuitry 100
25. oad current t is the transistor on time typically 0 4 f and AV is the peak to peak output voltage ripple A larger output capacitor than this theoretical value should be used since electrolytics have poor high frequency performance Experience has shown that a value from 5 to 10 times the calculated value should be used For good efficiency the diodes must have a low forward volt age drop and be fast switching 1N5819 Schottky diodes work well Transformer selection should be picked for an output transis tor on time of 0 4 f and a primary inductance high enough to prevent the output transistor switch from ramping higher than the transistors rating of 750 mA Pulse Engi neering San Diego Calif and Renco Electronics Inc Deer Park N Y can provide further assistance in selecting the proper transformer for a specific application need The transformer used in Figure 23 was a Pulse Engineering PE 64287 www national com 16 Physical Dimensions inches millimeters unless otherwise noted 0 350 0 370 8 890 9 398 DIA 0 315 0 335 DIA 8 001 8 509 UNCONTROLLED irt 635 LEAD DIA SEATING PLANE A o 015 0 040 0 025 MAX 0 165 0 185 4 191 4 699 Y REFERENCE PLANE 0 035 0 500 70 889 0 889 X ES 1 016 0 016 0 019 DIA TYP 0 406 0 483 0 195 0 205 DIA Vin 0952 5207 2 540 nr 000 0045
26. rms Input Reference Voltage The voltage referred to ground that must be applied to either the inverting or non inverting input to cause the regulator switch to change state ON or OFF Input Reference Current The current that must be drawn from either the inverting or non inverting input to cause the regulator switch to change state ON or OFF Input Level Shift Accuracy This specification determines the output voltage tolerance of a regulator whose output con trol depends on drawing equal currents from the inverting and non inverting inputs see the Inverting Regulator of Fig ure 21 and the RS 232 Line Driver Power Supply of Figure 23 Level Shift Accuracy is tested by using two equal value re sistors to draw current from the inverting and non inverting input terminals then measuring the percentage difference in the voltages across the resistors that produces a controlled duty cycle at the switch output Collector Saturation Voltage With the inverting input ter minal grounded thru a 10 resistor and the output transis tor s emitter connected to ground the Collector Saturation Voltage is the collector to emitter voltage for a given collector current Emitter Saturation Voltage With the inverting input termi nal grounded thru a 10 kQ resistor and the output transistor s collector connected to Vin the Emitter Saturation Voltage is the collector to emitter voltage for a given emitter current Collector Emitter Sustain
27. rrent limit and thermal shut down m Duty cycle up to 90 Applications Switching regulators in buck boost inverting and single ended transformer configurations Motor speed control m Lamp flasher Vin REGULATOR CURRENT LIMIT PIN 7 COLLECTOR GATES AND DRIVER THERMAL LIMIT EMITTER PIN 5 GROUND PIN 4 DS008711 1 1998 National Semiconductor Corporation DS008711 www national com Joyeinbay v87seiW1 v8 SscW1 V8ZS 0 07 Absolute Maximum Ratings Note 1 If Military Aerospace specified devices are required please contact the National Semiconductor Sales Office Distributors for availability and specifications ESD Tolerance Note 4 Operating Ratings Ambient Temperature Range 2kV 55 C lt T4 lt 125 40 C lt TA lt 85 lt TA lt 70 55 C lt lt 150 40 C lt T lt 125 0 lt Tj lt 125 operation over the specified operating Electrical Characteristics These specifications apply for 2V lt Vin lt 40V 2 2V lt Vin lt 40V for T lt 25 C timing capacitor 3900 pF and 25 lt duty cycle 7595 unless otherwise specified Values in standard typeface are for T 25 values in boldface type apply for junction temperature range Total Supply Voltage 50V LM1578A Collector Output to Ground 0 3V to 50V LM2578A Emitter Output to Ground Note 2 1V to 50V LM3578A Power
28. shown R3 can be used to adjust the duty cycle from 0 to 90 When the sum of R2 and R3 is twice the value of R1 the duty cycle will be about 50 C1 may be a large electrolytic capacitor to lower the oscillator frequency below 1 Hz www national com Applications Information continued Vin DS008711 23 FIGURE 10 Duty Cycle Adjustment REMOTE SHUTDOWN The LM1578A may be remotely shutdown by sinking a greater current from the non inverting input than from the in verting input This may be accomplished by selecting resistor R3 to be approximately one half the value of R1 and R2 in parallel DS008711 24 FIGURE 11 Shutdown Occurs when V is High EMITTER OUTPUT When the LM1578A output transistor is in the OFF state if the Emitter output swings below the ground pin voltage the output transistor will turn ON because its base is clamped near ground The Collector Current with Emitter Output Be low Ground curve shows the amount of Collector current drawn in this mode vs temperature and Emitter voltage When the Collector Emitter voltage is high this current will cause high power dissipation in the output transistor and should be avoided This situation can occur in the high current high voltage buck application if the Emitter output is used and the catch diode s forward voltage drop is greater than 0 6V A fast recovery diode can be added in series with the Emitter output to counter the forward vo
29. tiplication Vi Referred UNDER VOLTAGE LOCKOUT Under voltage lockout is accomplished with few external components When Vin becomes lower than the zener breakdown voltage the output transistor is turned off This occurs because diode D1 will then become forward biased allowing resistor R3 to sink a greater current from the non inverting input than is sunk by the parallel combination of R1 and R2 at the inverting terminal R3 should be one fifth of the value of R1 and R2 in parallel Vin Q LM1578A DS008711 22 FIGURE 8 Under Voltage Lockout MAXIMUM DUTY CYCLE LIMITING The maximum duty cycle can be externally limited by adjust ing the charge to discharge ratio of the oscillator capacitor with a single external resistor Typical values are 50 pA for the charge current 450 pA for the discharge current and a voltage swing from 200 mV to 750 mV Therefore R1 is se lected for the desired charging and discharging slopes and C1 is readjusted to set the oscillator frequency Vin DS008711 21 FIGURE 9 Maximum Duty Cycle Limiting DUTY CYCLE ADJUSTMENT When manual or mechanical selection of the output transis tors duty cycle is needed the cirucit shown below may be used The output will turn on with the beginning of each os cillator cycle and turn off when the current sunk by R2 and R3 from the non inverting terminal becomes greater than the current sunk from the inverting terminal With the resistor values as
30. vice in the TO 99 package must be derated at 150 C W junction to ambient or 45 C W junction to case The device in the 8 pin DIP must be derated at 95 C W junction to ambient The device in the surface mount package must be derated at 150 C W junction to ambient Note 4 Human body model 1 5 kO in series with 100 pF Note 5 Typical values are for 25 C and represent the most likely parametric norm Note 6 All limits guaranteed and 100 production tested at room temperature standard type face and at temperature extremes bold type face All limits are used to calculate Average Outgoing Quality Level AOQL Note 7 All limits guaranteed at room temperature standard type face and at temperature extremes bold type face Room temperature limits are 100 production tested Limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control SQC methods All limits are used to calculate AOQL Note 8 Input terminals are protected from accidental shorts to ground but if external voltages higher than the reference voltage are applied excessive current will flow and should be limited to less than 5 mA Note 9 1 and Ip are the external sink currents at the inputs refer to Test Circuit Note 10 Connection of a 10 kQ resistor from pin 1 to pin 4 will drive the duty cycle to its maximum typically 9096 Applying the minimum Current Limit Sense Voltage to pin 7 will not reduce the duty cycle to less than
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National LM1578A/LM2578A/LM3578A Manual(1)(1)