MAXIM MAX756/MAX757 3.3V/5V/Adjustable-Output Step-Up DC-DC Converters Manual
Contents
1. 1000 10 8 4 2 0 50 100 150 200 25 0 LOAD CURRENT uA LSLXVW 9SLXVIN MAX756 MAX757 3 3V 5V Adjustable Output Step Up DC DC Converters LOAD TRANSIENT RESPONSE OUTPUT VOLTAGE 50mV div OUTPUT CURRENT OmA to 200mA Typical Operating Characteristics continued Circuit of Figure 1 TA 25 C unless otherwise noted START UP DELAY VSHDN 2V div Vout 2V div Vin 2 5V Vin 2 5V HORIZONTAL 50ps div HORIZONTAL 5ms div 5V Mode 5V Mode Pin Description PIN NAME FUNCTION MAX756 MAX757 1 1 SHDN Shutdown Input disables SMPS when low but the voltage reference and low battery com parator remain active 2 3 5 Selects the main output voltage setting 5V when low 3 3V when high B 2 FB Feedback Input for adjustable output operation Connect to an external voltage divider between OUT and GND 3 3 REF 1 25V Reference Voltage Output Bypass with 0 22uF to GND 0 1pF if there is no external reference load Maximum load capability is 250uA source 20pA sink 4 4 LBO Low Battery Output An open drain N channel MOSFET sinks current when the voltage at LBI drops below 1 25V LBI Low Battery Input When the voltage on LBI drops below 1 25V LBO sinks current 5 5 Connect to if not used 6 6 OUT Connect OUT to the regulator output It provides bootstrapped power to both devices and also senses the output voltage for the MAX756 7 7 GND Power Ground2. Must be low impedance solder directly to ground plane 8 8 LX 1A 0 50 N Channel Power MOSFET Drain MAXIM 3 3V 5V Adjustable Output Step Up DC DC Converters Detailed Description Operating Principle The MAX756 MAX757 combine a switch mode regulator with an N channel MOSFET precision voltage reference and power fail detector in a single monolithic device The MOSFET is a sense FET type for best efficiency and has a very low gate threshold voltage to ensure start up under low battery voltage conditions 1 1V typ Pulse Frequency Modulation Control Scheme A unique minimum off time current limited pulse frequen cy modulation PFM control scheme is a key feature of the MAX756 MAX757 This PFM scheme combines the advantages of pulse width modulation PWM high output power and efficiency with those of a traditional PFM pulse skipper ultra low quiescent currents There is no oscillator at heavy loads switching is accomplished through a constant peak current limit in the switch which allows the inductor current to self oscillate between this peak limit and some lesser value At light loads switching frequency is governed by a pair of one shots which set a minimum off time 1us and a maximum on time 4ps The switching frequency depends on the load and the input voltage and can range as high as 500kHz The peak switch current of the internal MOSFET power switch is fixed at 1A 0 2A The switch s on
3. 3 5 3V Vout 3 47V Note 1 FB 1 3V MAX757 only 20 40 HA Reference Voltage No REF load 0 1uF 1 23 1 25 1 27 V Reference Voltage Regulation 3 5 20UA lt REF load lt 250pA Cngre 0 22uF 0 8 2 0 96 LBI Input Threshold With falling edge 1 22 1 25 1 28 V LBI Input Hysteresis 25 mV LBO Output Voltage Low Isink 2mA 0 4 V LBO Output Leakage Current LBO 5V 1 SHDN 3 5 Input Voltage Low 0 4 V SHDN 3 5 Input Voltage High 1 6 V SHDN 3 5 FB LBI Input Current 3 d Lev ONS 100 nA FB Voltage MAX757 1 22 1 25 1 28 V Output Voltage Range MAX757 lLoap OMA Note 2 27 5 5 V Note 1 Supply current from the 3 3V output is measured with an ammeter between the 3 3V output and OUT pin This current correlates directly with actual battery supply current but is reduced in value according to the step up ratio and efficiency Note 2 Minimum value is production tested Maximum value is guaranteed by design and is not production tested 2 MAAN 3 3V 5V Adjustable Output Step Up DC DC Converters Typical Operating Characteristic s Circuit of Figure 1 TA 25 C unless otherwise noted EFFICIENCY vs LOAD CURRENT 3 3V OUTPUT MODE 90 80 magset Vin 2 0V Vin 1 2V EFFIGIENCY 0 1 1 0 00 000 LOAD CURRE
4. performance a switching Schottky diode such as the 1N5817 is recommended 1N5817 equiv alent diodes are also available in surface mount pack ages from Collmer Semiconductor in Dallas TX phone 214 233 1589 The part numbers are SE014 or SE024 For low output power applications a pn junc tion switching diode such as the 1N4148 will also work well although efficiency will suffer due to the greater forward voltage drop of the pn junction diode ONESHOT MAXIM MAX756 Figure 2 MAX756 Block Diagram MAAN LSLXVW 9SLXVIN MAX756 MAX757 3 3V 5V Adjustable Output Step Up DC DC Converters PC Layout and Grounding The MAX756 MAX757 high peak currents and high fre quency operation make PC layout important for mini mizing ground bounce and noise The distance between the MAX756 MAX757 s GND pin and the ground leads of C1 and C2 in Figure 1 must be kept to less than 0 2 bmm All connections to the FB and LX pins should also be kept as short as possible To obtain maximum output power and efficiency and mini mum output ripple voltage use a ground plane and solder the MAX756 MAX757 GND pin 7 directly to the ground plane 0 004in 0 127mm L 3 5 MAX756 FB MAX757 Chi
5. resistance is typically 0 5Q resulting in a switch voltage drop Vsw of about 500mV under high output loads The value of Vsw decreases with light current loads Conventional PWM converters generate constant fre quency switching noise whereas this architecture pro duces variable frequency switching noise However the noise does not exceed the switch current limit times the filter capacitor equivalent series resistance ESR unlike conventional pulse skippers Voltage Reference The precision voltage reference is suitable for driving external loads such as an analog to digital converter It has guaranteed 250 source current and 20 sink current capability The reference is kept alive even in shutdown mode If the reference drives an external load bypass it with 0 22uF to GND If the ref erence is unloaded bypass it with at least 0 1pF Control Logic Inputs The control inputs 3 5 SHDN are high impedance MOS gates protected against ESD damage by normally reverse biased clamp diodes If these inputs are dri ven from signal sources that exceed the main supply MAAN voltage the diode current should be limited by a series resistor 1MQ suggested The logic input threshold level is the same approximately 1V in both 3 3V and 5V modes Do not leave the control inputs floating Design Procedure Output Voltage Selection The MAX756 output voltage can be selected to 3 3V or 5V under logic control or it can be left
6. 19 0113 Rev 2 1 95 General Description The MAX756 MAX757 are CMOS step up DC DC switch ing regulators for small low input voltage or battery pow ered systems The MAX756 accepts a positive input voltage down to 0 7V and converts it to a higher pin selectable output voltage of 3 3V or 5V The MAX757 is an adjustable version that accepts an input voltage down to 0 7V and generates a higher adjustable output voltage in the range from 2 7V to 5 5V Typical full load efficiencies for the MAX756 MAX757 are greater than 87 The MAX756 MAX757 provide three improvements over previous devices Physical size is reduced the high Switching frequencies up to 0 5MHz made possible by MOSFET power transistors allow for tiny lt 5mm diameter surface mount magnetics Efficiency is improved to 8796 10 better than with low voltage regulators fabricated in bipolar technology Supply current is reduced to 60UA by CMOS construction and a unique constant off time pulse frequency modulation control scheme Applications 3 3V to 5V Step Up Conversion Palmtop Computers Portable Data Collection Equipment Personal Data Communicators Computers Medical Instrumentation 2 Cell amp 3 Cell Battery Operated Equipment Glucose Meters Typical Operating Circuit OUTPUT 22H 5V at 200mA or 1N5817 3 3V at 300mA MAXIM To 100uF MAX756 m 3 5 LOW BATTERY DETECTOR OUTPUT MAXIM MA MAILM 3 3V 5V Adjustable Output Step Up D
7. C DC Converters Features Operates Down to 0 7V Input Supply Voltage 87 Efficiency at 200mA 60pA Quiescent Current 20pA Shutdown Mode with Active Reference and LBI Detector 500kHz Maximum Switching Frequency 1 5 Reference Tolerance Over Temperature Low Battery Detector LBI LBO 8 Pin DIP and SO Packages Ordering Information PART TEMP RANGE PIN PACKAGE MAX756CPA 0 C to 70 C 8 Plastic DIP MAX756CSA 0 C to 70 C 8 SO MAX756C D 0 C to 70 C Dice MAX756EPA 40 C to 85 C 8 Plastic DIP MAX756ESA 40 C to 85 C 8 SO MAX757CPA 0 C to 70 C 8 Plastic DIP MAX757CSA 0 C to 70 C 8 SO MAX757C D 0 C to 70 C Dice MAX757EPA 40 C to 85 C 8 Plastic DIP MAX757ESA 40 C to 85 C 8 SO Dice are tested at TA 25 C only Pin Configurations TOP VIEW e MAXIM MAX756 DIP SO Maxim Integrated Products 1 Call toll free 1 800 998 8800 for free samples or literature LSLXVW 9SLXVIN MAX756 MAX757 3 3V 5V Adjustable Output Step Up DC DC Converters ABSOLUTE MAXIMUM RATINGS Supply Voltage OUT to GND 0 3V 7V Operating Temperature Ranges Switch Voltage LX to GND 75 C 0 C to 70 C Auxiliary Pin Voltages SHDN LBI LBO REF MAX75 E 40 C to 85 C 3 5 FB to GND 0 3V Vout 0 3V Junction Temperature Reference Current IREF eee 2 5mA Storage
8. NT mA SWITCHING FREQUENCY vs LOAD CURRENT MASSA 5V MODE VN 25V m 00m 1 LOAD CURRENT A START UP INPUT VOLTAGE V MAAN 90 80 70 60 EFFICIENCY 50 40 QUIESCENT CUFFENT EFFICIENCY vs LOAD CURRENT 0 1 5V OUTPUT MODE ViN 3 3V i Vin 2 5V Vin 1 25V 1 0 100 1000 LOAD CURRENT mA QUIESCENT CURRENT vs INPUT VOLTAGE CURRENT MEASURED AT INPUT VOLTAGE V MINIMUM START UP INPUT VOLTAGE vs LOAD CURRENT MAKES MAXIMUM OUTPUT CURRENT mA Ss 5 40 30 20 SHUTDOWN QUIESCENT CUFFENT REFERENCE VOLTAGE LOAD REGULATION MAXIMUM OUTPUT CURRENT vs INPUT VOLTAGE 3 3V MODE ey MODE 1 2 3 4 5 INPUT VOLTAGE V SHUTDOWN QUIESCENT CURRENT vs INPUT VOLTAGE URRENT MEASURED AT Vi 2 3 4 5 INPUT VOLTAGE V VFEF LOAD FEGLLATICN mV o 1 10 LOAD CURREN MODE
9. Temperature Range 65 to 160 C Continuous Power Dissipation TA 70 C Lead Temperature soldering 10sec 300 C Plastic DIP derate 9 09mW C above 70 C 727mW SO derate 5 88mW C above 70 C Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device These are stress ratings only and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied Exposure to absolute maximum rating conditions for extended periods may affect device reliability ELECTRICAL CHARACTERISTICS Circuits of Figure 1 and Typical Operating Circuit Vin 2 5V OMA Ta Tmn to Tmax unless otherwise noted PARAMETER CONDITIONS MIN TYP MAX UNITS MAX756 3 5 OMA lt lt 200mA 4 8 5 0 5 2 Output Voltage 2V lt lt 3V MAX756 3 5 3V OMA lt lt 300mA 3 17 3 30 3 43 V MAX757 Vout 5V OMA lt lt 200mA 4 8 5 0 5 2 Minimum Start Up Supply Voltage 10mA 1 1 1 8 V Minimum Operating Supply l 20mA Voltage once started LOAD o Quiescent Supply Current in lLoap OMA 3 5 3V LBI 1 25V Vout 3 47V 60 A 3 3V Mode Note 1 FB 1 3V MAX757 only H Battery Quiescent Current uit 4 Output set for 3 3V 60 UA Measured at Vy in Figure 1 Shutdown Quiescent Current SHDN OV LBI 1 25V
10. chip circuitry for low battery detection If the voltage at LBI falls below the reg ulator s internal reference voltage 1 25V LBO an open drain output sinks current to GND The low battery mon itors threshold is set by two resistors R3 and R4 Figure 1 which forms a voltage divider between the input volt age and the LBI pin The threshold voltage is set by R3 and R4 using the following equation R3 Vin Veer 1 R4 LSLXVW 9SLXVIN MAX756 MAX757 3 3V 5V Adjustable Output Step Up DC DC Converters The inductor s DC resistance significantly affects effi ciency For highest efficiency limit L1 s DC resistance to 0 03Q or less See Table 1 for a list of suggested inductor suppliers Table 1 Component Suppliers LBI LX MAXIM MAX757 Figure 1 Standard Application Circuit where is the desired threshold of the low battery detector R3 and R4 are the input divider resistors at LBI and Vper is the internal 1 25V reference Since the LBI current is less than 100 large resistor values typically 10 to 200kO can be used for and R4 to minimize loading of the input supply When the voltage at LBI is below the internal threshold LBO sinks current to GND A pull up resistor of 10kQ or more connected from LBO to Vour can be used when driving CMOS circuits Any pull up resistor con nected to LBO should not be returned to a voltage source greater than When LBI
11. in one mode or the other by tying 3 5 to GND or OUT Efficiency varies depending upon the battery and the load and is typi cally better than 80 over a 2mA to 200mA load range The device is internally bootstrapped with power derived from the output voltage via OUT When the output is set at 5V instead of 3 3V the higher internal supply voltage results in lower switch transistor on resistance and slightly greater output power Bootstrapping allows the battery voltage to sag to less than 1V once the system is started Therefore the bat tery voltage range is from Vp to less than 1V where Vp is the forward drop of the Schottky rectifier If the battery voltage exceeds the programmed output voltage the output will follow the battery voltage In many systems this is acceptable however the output voltage must not be forced above 7V The output voltage of the MAX757 is set by two resis tors R1 and F2 Figure 1 which form a voltage divider between the output and the FB pin The output voltage is set by the equation Vout Vner R2 R1 R2 where Vger 1 25V To simplify resistor selection R1 R2 Vour Vee 1 Since the input bias current at FB has a maximum value of 100nA large values 10 to 200kQ can be used for R1 and R2 with no significant loss of accuracy For 1 error the current through R1 should be at least 100 times FB s bias current Low Battery Detection The MAX756 MAX757 contain on
12. is above the threshold the LBO output is off The low battery com parator and reference voltage remain active when the MAX756 MAX757 is in shutdown mode If the low battery comparator is not used connect LBI to and leave LBO open Inductor Selection The inductors should have a saturation incremental current rating equal to or greater than the peak switch current limit which is 1 2A worst case However it s generally acceptable to bias the inductor into satura tion by 2096 although this will reduce the efficiency The 22uH inductor shown in the typical applications cir cuit is sufficient for most MAX756 MAX757 application circuits Higher input voltages increase the energy transferred with each cycle due to the reduced input output differential Minimize excess ripple due to increased energy transfer by reducing the inductor value 10H suggested 6 PRODUCTION INDUCTORS CAPACITORS METHOD Surface Mount Sumida AVX CD54 220 22H TPS series CoilCraft DT3316 223 Sprague Coiltronics 595D series CTX20 1 Miniature Sumida Sanyo OS CON Through Hole RCH654 220 OS CON series low ESR organic semiconductor Low Cost CoilCraft Nichicon Through Hole PCH 27 223 PL series low ESR electrolyic United Chemi Con LXF series AVX USA 207 282 5111 FAX 207 283 1941 800 282 9975 CoilCraft USA 708 639 6400 FAX 708 639 1969 Coiltronics USA 407 241 7876 FAX 407 241 9339 Collmer Semiconduct
13. or USA 214 233 1589 Motorola USA 602 244 3576 FAX 602 244 4015 Nichicon USA 708 843 7500 FAX 708 843 2798 Japan 81 7 5231 8461 FAX 81 7 5256 4158 Nihon USA 805 867 2555 FAX 805 867 2556 Japan 81 3 3494 7411 FAX 81 3 3494 7414 Sanyo OS CON USA 619 661 6835 Japan 81 720 70 1005 FAX 81 720 70 1174 Sprague USA 603 224 1961 FAX 603 224 1430 Sumida USA 708 956 0666 Japan 81 3 3607 5111 FAX 81 3 3607 5428 United Chemi Con USA 708 696 2000 FAX 708 640 6311 Capacitor Selection 100uF 10V surface mount SMT tantalum capacitor typically provides 50mV output ripple when stepping up from 2V to 5V at 200mA Smaller capacitors down to 10uF are acceptable for light loads or in applica tions that can tolerate higher output ripple MAXIM 3 3V 5V Adjustable Output Step Up DC DC Converters The ESR of both bypass and filter capacitors affects efficiency Best performance is obtained by using spe cialized low ESR capacitors or connecting two or more filler capacitors in parallel The smallest low ESR SMT tantalum capacitors currently available are Sprague 595D series which are about half the size of competing products Sanyo OS CON organic semiconductor through hole capacitors also exhibit very low ESR and are especially useful for operation at cold tempera tures Table 1 lists suggested capacitor suppliers MINIMUM Rectifier Diode For optimum
14. p Topography 0 080 2 03mm TRANSISTOR COUNT 758 SUBSTRATE CONNECTED TO OUT ey BL GND 0 122 3 10mm GND OUT LBI Package Information MILLIMETERS INCHES MIN MAX MIN MAX gt gt gt 8 PLASTIC SMALL OUTLINE PACKAGE
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MAXIM MAX756/MAX757 3.3V/5V/Adjustable Output Step Up DC DC Converters Manual