max756/MAX757 3.3v/5v/adjustable-output, step-up dc-dc converters ________________________________________________________________ maxim integrated products 1 call toll free 1-800-998-8800 for free samples or literature. 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?he high switching frequencies (up to 0.5mhz) made possible by mosfet power transistors allow for tiny (<5mm diameter) surface-mount magnetics. efficiency is improved to 87% (10% better than with low-voltage regulators fabricated in bipolar technology). supply current is reduced to 60? 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 & 3-cell battery-operated equipment glucose meters ____________________________features ? operates down to 0.7v input supply voltage ? 87% efficiency at 200ma ? 60? quiescent current ? 20? shutdown mode with active reference and lbi detector ? 500khz maximum switching frequency ? ?.5% reference tolerance over temperature ? low-battery detector (lbi/lbo) ? 8-pin dip and so packages ______________ordering information * dice are tested at t a = +25? only. 1 2 3 4 8 7 6 5 lx gnd out lbi lbo ref 3/5 shdn max756 dip/so top view 1 2 3 4 8 7 6 5 lx gnd out lbi lbo ref fb shdn MAX757 dip/so _________________pin configurations max756 shdn 1 3/5 2 ref 3 lbi 5 lx 7 150 m f gnd out 6 input
2v to v out 1n5817 output
5v at 200ma
or
3.3v at 300ma 100 m f lbo 4 8 0.1 m f 22 m h low-battery
detector output __________typical operating circuit part temp. range pin-package max756 cpa 0? to +70? 8 plastic dip max756csa 0? to +70? 8 so max756c/d 0? to +70? dice* max756epa -40? to +85? 8 plastic dip max756esa -40? to +85? 8 so MAX757 cpa 0? to +70? 8 plastic dip MAX757csa 0? to +70? 8 so MAX757c/d 0? to +70? dice* MAX757epa -40? to +85? 8 plastic dip MAX757esa -40? to +85? 8 so evaluation kit manual follows data sheet
max756/MAX757 3.3v/5v/adjustable-output, step-up dc-dc converters 2 _______________________________________________________________________________________ supply voltage (out to gnd) ....................................-0.3v, +7v switch voltage (lx to gnd) ........................................-0.3v, +7v auxiliary pin voltages (shdn , lbi, lbo, ref, 3/5 , fb to gnd) ........................................-0.3v, (v out + 0.3v) reference current (i ref ) ....................................................2.5ma continuous power dissipation (t a = +70?) plastic dip (derate 9.09mw/? above +70?) .............727mw so (derate 5.88mw/? above +70?) ..........................471mw operating temperature ranges: max75_c_ _ ........................................................0? to +70? max75_e_ _......................................................-40? to +85? junction temperature ......................................................+150? storage temperature range............................... -65?o +160? lead temperature (soldering, 10sec) ........................... +300? electrical characteristics (circuits of figure 1 and typical operating circuit, v in = 2.5v, i load = 0ma, t a = t min to t max , unless otherwise noted.) stresses beyond those listed under ?bsolute 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. absolute maximum ratings 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. lbo output leakage current shdn , 3/5 , fb, lbi input current v 2.7 5.5 MAX757, i load = 0ma (note 2) output voltage range v 1.22 1.25 1.28 MAX757 fb voltage na ?00 lbi = 1.25v, fb = 1.25v, shdn = 0v or 3v, 3/5 = 0v or 3v v 1.6 shdn , 3/5 input voltage high v 0.4 shdn , 3/5 input voltage low shutdown quiescent current (note 1) battery quiescent current measured at v in in figure 1 v 1.22 1.25 1.28 with falling edge lbi input threshold mv 25 lbi input hysteresis v 0.4 i sink = 2ma lbo output voltage low ? 1 lbo = 5v ? 20 40 shdn = 0v, lbi = 1.25v, 3/5 = 3v, v out = 3.47v, fb = 1.3v (MAX757 only) ? quiescent supply current in 3.3v mode (note 1) MAX757, v out = 5v, 0ma < i load < 200ma max756, 3/5 = 0v, 0ma < i load < 200ma max756, 3/5 = 3v, 0ma < i load < 300ma % 0.8 2.0 3/5 = 3v, -20? < ref load < 250?, c ref = 0.22? reference-voltage regulation v 1.23 1.25 1.27 no ref load, c ref = 0.1? 60 i load = 0ma, 3/5 = 3v, lbi = 1.25v, v out = 3.47v, fb = 1.3v (MAX757 only) ? v 60 output set for 3.3v 1.1 1.8 i load = 10ma minimum start-up supply voltage 4.8 5.0 5.2 2v < v in < 3v reference voltage 3.17 3.30 3.43 v 4.8 5.0 5.2 output voltage units min typ max conditions parameter v 0.7 i load = 20ma minimum operating supply voltage (once started)
max756/MAX757 3.3v/5v/adjustable-output, step-up dc-dc converters _________________________________________________________________________________________________ 3 minimum start-up input voltage
vs. load current max756-7 load current (ma) start-up input voltage (v) 10 100 1000 1 1.8 1.6 1.4 1.2 1.0 0.8 3.3v mode 40 0.1 10 1000 efficiency vs. load current
3.3v output mode max756-1 load current (ma) efficiency (%) 60 80 90 50 70 1 100 v in = 2.0v v in = 1.2v 40 0.1 10 1000 efficiency vs. load current
5v output mode max756-2 load current (ma) efficiency (%) 60 80 90 50 70 1 100 v in = 3.3v v in = 2.5v v in = 1.25v 800 0 02 maximum output current
vs. input voltage 200 600 max756-3 input voltage (v) maximum output current (ma) 4 400 100 300 500 700 13 5 3.3v mode 5v mode 1m 10 10 m 10m 1 switching frequency
vs. load current 100 max756-4 load current (a) switching frequency (hz) 1k 10k 100k 100 m 1m 100m 5v mode 3.3v mode v in = 2.5v 0 2 quiescent current
vs. input voltage max756-5 input voltage (v) quiescent current ( m a) 4 100 200 300 400 500 1 3 5 v out = 3.3v v out = 5v current measured at v in 50 0 12 5 shutdown quiescent current
vs. input voltage 20 max756-6 input voltage (v) shutdown quiescent current ( m a) 4 40 10 30 3 current measured at v in 10 0 0 reference voltage
load regulation 2 8 max756-8 load current ( m a) vref load regulation (mv) 6 4 50 100 150 200 250 v out = 3.3v __________________________________________typical operating characteristics (circuit of figure 1, t a = +25?, unless otherwise noted.)
max756/MAX757 3.3v/5v/adjustable-output, step-up dc-dc converters 4 _______________________________________________________________________________________ ______________________________________________________________pin description name function 1 shdn 2 3/5 selects the main output voltage setting; 5v when low, 3.3v when high. fb 3 ref 4 lbo 5 lbi 6 out 7 gnd power ground. must be low impedance; solder directly to ground plane. 8 lx 1a, 0.5 n-channel power mosfet drain 1 2 3 4 5 6 7 8 pin max756 MAX757 shutdown input disables smps when low, but the voltage reference and low-battery com- parator remain active. feedback input for adjustable output operation. connect to an external voltage divider between out and gnd. 1.25v reference voltage output. bypass with 0.22? to gnd (0.1? if there is no external reference load). maximum load capability is 250? source, 20? sink. low-battery output. an open-drain n-channel mosfet sinks current when the voltage at lbi drops below +1.25v. low-battery input. when the voltage on lbi drops below +1.25v, lbo sinks current. connect to v in if not used. connect out to the regulator output. it provides bootstrapped power to both devices, and also senses the output voltage for the max756. output
voltage
50mv/div v in = 2.5v
horizontal = 50 m s/div
5v mode load-transient response output
current
0ma to 200ma _____________________________typical operating characteristics (continued) (circuit of figure 1, t a = +25?, unless otherwise noted.) v shdn
2v/div v in = 2.5v
horizontal = 5ms/div
5v mode start-up delay v out
2v/div 3v 0v 5v 0v
max756/MAX757 3.3v/5v/adjustable-output, step-up dc-dc converters _______________________________________________________________________________________ 5 _______________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 ?ense-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 (1?) and a maximum on-time (4?). 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 ?.2a. the switch's on resistance is typically 0.5 , resulting in a switch voltage drop (v sw ) of about 500mv under high output loads. the value of v sw 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 0.22? to gnd. if the ref- erence is unloaded, bypass it with at least 0.1?. 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 voltage, the diode current should be limited by a series resistor (1m 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 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 v out + v d to less than 1v (where v d 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 r2 (figure 1), which form a voltage divider between the output and the fb pin. the output voltage is set by the equation: v out = (v ref ) [(r2 + r1) / r2] where v ref = 1.25v. to simplify resistor selection: r1 = (r2) [(v out / v ref ) - 1] since the input bias current at fb has a maximum value of 100na, large values (10k to 200k ) 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? bias current. low-battery detection the max756/MAX757 contain on-chip circuitry for low- battery detection. if the voltage at lbi falls below the reg- ulator? internal reference voltage (1.25v), lbo (an open- drain output) sinks current to gnd. the low-battery mon- itor's 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 = [(v in / v ref ) - 1] (r4)
max756/MAX757 3.3v/5v/adjustable-output, step-up dc-dc converters 6 _______________________________________________________________________________________ where v in is the desired threshold of the low-battery detector, r3 and r4 are the input divider resistors at lbi, and v ref is the internal 1.25v reference. since the lbi current is less than 100na, large resistor values (typically 10k to 200k ) can be used for r3 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 10k or more connected from lbo to v out 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 v out . when lbi 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 v in 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? generally acceptable to bias the inductor into satura- tion by 20%, although this will reduce the efficiency. the 22? 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 (10? suggested). the inductor? dc resistance significantly affects effi- ciency. for highest efficiency, limit l1? dc resistance to 0.03 or less. see table 1 for a list of suggested inductor suppliers. table 1. component suppliers 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 semiconductor 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 a 100?, 10v surface-mount (smt) tantalum capacitor typically provides 50mv output ripple when stepping up from 2v to 5v at 200ma. smaller capacitors, down to 10?, are acceptable for light loads or in applica- tions that can tolerate higher output ripple. MAX757 ref 3 lx 7 c1
150 m f gnd out 6 v in d1
1n5817 v out lbo 4 8 c3
0.1 m f l1
22 m h lbi 5 c2
100 m f shdn 1 fb 2 r1 r2 r3 r4 figure 1. standard application circuit production method inductors capacitors surface-mount avx tps series sprague 595d series miniature through-hole sumida rch654-220 low-cost through-hole sumida cd54-220 (22?) coilcraft dt3316-223 coiltronics ctx20-1 sanyo os-con os-con series low-esr organic semiconductor coilcraft pch-27-223 nichicon pl series low-esr electrolyic united chemi-con lxf series
max756/MAX757 3.3v/5v/adjustable-output, step-up dc-dc converters _______________________________________________________________________________________ 7 n n max756 gnd v out v in trig q one-shot minimum
off-time
one-shot q f/f s r maximum
on-time
one-shot trig q one-shot lx out reference ref lbi lbo 3/5 shdn figure 2. max756 block diagram 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 filter 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. rectifier diode for optimum 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.
max756/MAX757 3.3v/5v/adjustable-output step-up dc-dc converters 3.3v/5v/adjustable-output, step-up dc-dc converters gnd gnd lbi out 3/5 (max756)
fb (MAX757) ref shdn lx lbo 0.122"
(3.10mm) 0.080"
(2.03mm) ___________________chip topography ________________________________________________________package information l dim
a
a1
b
c
d
e
e
h
h
l
a min
0.053
0.004
0.014
0.007
0.189
0.150
0.228
0.010
0.016
0? max
0.069
0.010
0.019
0.010
0.197
0.157
0.244
0.020
0.050
8? min
1.35
0.10
0.35
0.19
4.80
3.80
5.80
0.25
0.40
0? max
1.75
0.25
0.49
0.25
5.00
4.00
6.20
0.50
1.27
8? inches millimeters a 8-pin plastic
small-outline
package h e d e a a1 c h x 45? 0.127mm
0.004in. b 1.27 bsc 0.050 bsc 21-325a 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? gnd pin and the ground leads of c1 and c2 in figure 1 must be kept to less than 0.2" (5mm). 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. transistor count: 758 substrate connected to out 8 _______________________________________________________________________________________
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