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_______________ge ne ra l de sc ript ion the max761/max762 step-up switching regulators provide high efficiency over a wide range of load c urrents, delivering up to 150ma. a unique, current-limited pulse-frequency-modulated (pfm) control scheme give s the devices the benefits of pulse-width-modulated ( pwm) converters (high efficiency with heavy loads), whil e using less than 110a of supply current (vs. 2ma to 10ma for pwm converters). the result is high efficiency over a wide range of loads. the max761/max762 input voltage range is 2v to 16.5 v. output voltages are preset to 12v (max761) and 15v (max762), or they can be set with two external resi stors. with a 5v input, the max761 guarantees a 12v, 150ma output. its high efficiency, low supply current, fa st start-up time, shdn controlling capability, and small size m ake the max761 ideal for powering flash memory. the max761/max762 have an internal 1a power mos- fet, making them ideal for minimum-component, low- and medium-power applications. these devices use tiny e xter- nal components, and their high switching frequencie s (up to 300khz) allow for small surface-mount magnetics. for increased output drive capability or higher out put volt- ages, use the max770Cmax773, which are similar in design to the max761/max762, but drive external pow er mosfets. for stepping up to 5v, see the max756/ max757 and max856-max859 data sheets. _________________________applic a t ions flash memory programming pcmcia cards battery-powered applications high-efficiency dc-dc converters ____________________________fe a t ure s ? high efficiency for a wide range of load currents ? 12v/150ma flash memory programming supply ? 110a max supply current ? 5a max shutdown supply current ? 2v to 16.5v input voltage range ? 12v (max761), 15v (max762) or adjustable output ? current-limited pfm control scheme ? 300khz switching frequency ? internal, 1a, n-channel power fet ? lbi/lbo low-battery comparator ______________orde ring i nform a t ion m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs ________________________________________________________________ maxim integrated products 1 1 2 3 4 8 7 6 5 v+ lx gnd ref shdn fb lbi lbo max761 max762 dip/so top view __________________pin configura t ion max761 lbi shdn v+ fb ref lx output 12v 150ma low-battery detector output low-battery detector input input 4. 75v to 12v on/off gnd lbo 33f 33f 18h __________typic a l ope ra t ing circ uit ca ll t oll fre e 1 -8 0 0 -9 9 8 -8 8 0 0 for fre e sa m ple s or lit e ra t ure . 19-0201; rev 0; 11/93 part temp. range pin-package max761 cpa 0c to +70c 8 plastic dip max761csa 0c to +70c 8 so max761c/d 0c to +70c dice* max761esa -40c to +85c 8 so max761epa -40c to +85c 8 plastic dip max761mja -55c to +125c 8 cerdip** max762 cpa 0c to +70c 8 plastic dip max762csa 0c to +70c 8 so max762c/d 0c to +70c dice* max762epa -40c to +85c 8 plastic dip max762esa -40c to +85c 8 so max762mja -55c to +125c 8 cerdip** * contact factory for dice specifications. ** contact factory for availability and processing to mil-std-883. evaluation kit available downloaded from: http:///
m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs 2 _______________________________________________________________________________________ absolute maximum ratings supply voltage v+ to gnd ........................... ............-0.3v to 17v ref, lbo, lbi, shdn, fb ............................ -0.3v to (v+ + 0.3v) lx................................................. .............................-0.3v to 17v lx peak current .................................... ................................1.5a lbo current ........................................ ..................................5ma continuous power dissipation (t a = +70c) plastic dip (derate 9.09mw/c above +70c) ......... ...727mw so (derate 5.88mw/c above +70c) .................. .......471mw cerdip (derate 8.00mw/c above +70c) .............. ...640mw operating temperature ranges: max76_c_a .......................................... ..............0c to +70c max76_e_a .......................................... ...........-40c to +85c max76_mja .......................................... ........-55c to +125c junction temperatures: max76_c_a/e_a...................................... ....................+150c max76_mja.......................................... .......................+175c storage temperature range .......................... ...-65c to +160c lead temperature (soldering, 10sec) ................ .............+300c electrical characteristics (v+ = 5v, i load = 0ma, c ref = 0.1f, t a = t min to t max, typical values are at t a = +25c, unless otherwise noted.) stresses beyond those listed under absolute maximu m ratings may cause permanent damage to the device . these are stress ratings only, and functional operation of the device at these or any other condi tions beyond those indicated in the operational sec tions of the specifications is not implied. exposur e to absolute maximum rating conditions for extended per iods may affect device reliability. parameter symbol min typ max 88 110 minimum start-up voltage 1.7 2.0 v minimum operating voltage 1.7 v supply current 300 a shutdown current 15 a 11.52 12.0 12.48 2 16.5 3 16.5 supply voltage v+ 3.1 16.5 v 11.52 12.0 12.48 14.4 15.0 15.6 output voltage (note 1) v out 14.4 15.0 15.6 v peak current at lx i peak 0.75 1.0 1.25 a maximum switch-on time t on 681 0 s minimum switch-off time t off 1.0 1.3 1.6 s load regulation 0.0042 %/ma line regulation 0.08 %/v efficiency 86 % 1.4700 1.50 1.5300 1.4625 1.50 1.5375 reference voltage v ref 1.4550 1.50 1.5450 v conditions figure 2, max761, bootstrapped v+ = 16.5v, normal operation, shdn = 0v, non-bootstrapped figure 2, bootstrapped figure 2, bootstrapped figure 2, max761, v in = 5v, shdn = 0v, normal operation figure 2, max762, bootstrapped v+ = 10.0v, shutdown mode, shdn = v+ see figure 4b figure 2, 0ma i load 200ma, bootstrapped figure 2, bootstrapped figure 2, 4v v in 6v, bootstrapped figure 2, bootstrapped, v out = 12v, 60ma i load 120ma figure 3 or 5 with external resistors. max76_c max76_e max76_m max76_c/e max76_m units 0ma i load 75ma, 3v v+ 12v 0ma i load 150ma, 4.75v v+ 12v 0ma i load 50ma, 3v v+ 15v 0ma i load 100ma, 4.75v v+ 15v downloaded from: http:///
m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs _______________________________________________________________________________________ 3 electrical characteristics (continued) (v+ = 5v, i load = 0ma, c ref = 0.1f, t a = t min to t max , typical values are at t a = +25c, unless otherwise noted.) note 1: see typical operating characteristics for output current capability versus input voltage. guarantees based on correlation to switching on and off times, on-resistance, and p eak-current ratings. parameter voltage trip point symbol min typ max v fb 1.4550 1.50 1.5450 units v -20 20 lx on resistance lx leakage current 1.0 2.2 -30 30 a -10 10 -5 5 shdn input high voltage -40 40 fb leakage current i fb -60 60 na v ih 1.6 v shdn input low voltage v il 1.4700 1.50 1.5300 0.4 v 1.4625 1.50 1.5375 shdn leakage current -1 1 a reference load regulation 1.4700 1.50 1.5300 1.4625 1.50 1.5375 10 lbi threshold voltage 1.4550 1.50 1.5450 v lbi hysteresis 20 mv lbi leakage current reference line regulation -20 20 na 15 mv lbo leakage current -1 1 a lbo voltage v ol 0.4 v lbi to lbo delay 2.5 s conditions max76_m max76_c v+ > 5.0v v+ = 16.5v, lx = 17v max76_e max76_m 2.0v v+ 16.5v 2.0v v+ 16.5v max76_c max76_e v+ = 16.5v, shdn = 0v or v+ lbi falling 0a i load 100a v+ = 16.5v, v lbi = 1.5v 3.0v v+ 16.5v v+ = 16.5v, v lbo = 16.5v v+ = 5.0v, i sink = 1ma overdrive = 5mv max76_c max76_m max76_e 30 100 v/v max76_c/e max76_m max76_c max76_m max76_e downloaded from: http:///
m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs 4 _______________________________________________________________________________________ __________________________________________typic a l o pe ra t ing cha ra c t e rist ic s (circuit of figure 2, t a = +25c, unless otherwise noted.) 100 0 0.1 10 1000 efficiency vs. output current bootstrapped 20 max761-01 output current (ma) efficiency (% ) 40 60 80 1 100 v in = 10v 10 30 50 70 90 v in = 5v v in = 2v v out = 12v 100 0 0.1 10 1000 efficiency vs. output current non-bootstrapped 20 max761-02 output current (ma) efficiency (% ) 40 60 80 1 100 v in = 10v 10 30 50 70 90 v in = 5v v out = 12v 2.00 0 01 3 6 quiescent current vs. input voltage 0.50 1.50 max761-03 input voltage (v) quiescent current (ma) 24 1.00 5 1.75 0.25 1.25 0.75 0.5 1.5 3.5 2.5 4.5 5.5 v out = 12v bootstrapped (internal resistors) bootstrapped (external resistors) non-bootstrapped 400 0 3.0 6.0 m axim um output current vs. input voltage 100 300 max761-04 supply voltage (v) maximum output current (ma) 4.0 200 5.0 350 50 250 150 3.5 4.5 5.5 non-bootstrapped bootstrapped v out = 12v 3.5 0.5 -60 -20 60 140 no-load start-up voltage 1.0 3.0 max761-07 temperature ( c) no-load start-up voltage (v) 20 100 2.0 -40 0 80 40 120 1.5 2.5 v out = 12v bootstrapped (internal resistors) bootstrapped (external resistors) non-bootstrapped (external resistors) 250 0 -60 -20 60 140 reference output resistance vs. tem perature 50 max761-05 temperature ( c) reference output resistance ( w ) 20 100 150 -40 0 80 40 120 100 200 100a 50a 10a 1.502 -60 -20 60 140 reference vs. tem perature coefficient max761-06 temperature ( c) reference output (v) 20 100 -40 0 80 40 120 1.500 1.498 1.496 1.494 1.492 1.504 1.506 2.2 1.3 0.1 10 1000 m ax761 start-up voltage vs. r load max761-08 r load (k w ) start-up voltage (v) 1.4 1.5 1.6 1.7 2.1 2.0 1.8 1.9 1 100 v out = 12v bootstrapped internal resistors 1.6 0.4 -60 -20 60 140 lx on-resistance vs. tem perature 0.6 1.4 max761-09 temperature ( c) lx on-resistance ( w ) 20 100 1.0 -40 0 80 40 120 0.8 1.2 v+ = 12v v+ = 5v downloaded from: http:///
m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs _______________________________________________________________________________________ 5 1000 0.01 20 120 lx leakage vs. tem perature 10 100 max761-10 lx leakage (na) 1 0.1 140 100 80 40 60 temperature ( c) v+ = 15v v lx = 16.5v 1.5 -60 -20 60 140 peak current at lx vs. tem perature max761-11 temperature ( c) i peak (a) 20 100 -40 0 80 40 120 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 v+ = 12v v+ = 5v 4.0 -60 -20 60 140 shutdown current vs. tem perature max761-12 temperature ( c) i cc (a) 20 100 -40 0 80 40 120 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 v+ = 15v v+ = 4v v+ = 8v 8.5 7.5 -60 60 switch-on tim e vs. tem perature 8.0 max761-13 temperature ( c) t on (s) 0 120 v+ = 5v 2.0 1.0 -60 60 switch-off tim e vs. tem perature 1.5 max761-14 temperature ( c) t off (s) 0 120 v+ = 5v 100 80 -60 60 power-supply current vs. tem perature 90 max761-15 temperature ( c) i cc (a) 0 120 v+ = 3v v+ = 16.5v 7 5 -60 60 switch-on/switch-off tim e ratio vs. tem perature 6 max761-16 temperature ( c) t on /t off ratio (s/s) 0 120 v+ = 5v shdn response tim e i load = 100ma, v in = 5v a: v out , 2v/div b: shdn (0v to 4v) 2ms/div 4v 0v 12v 5v ____________________________typic a l ope ra t ing cha ra c t e rist ic s (c ont inue d) (circuit of figure 2, t a = +25c, unless otherwise noted.) downloaded from: http:///
m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs 6 _______________________________________________________________________________________ loadtransient response a: i load , (0ma to 200ma) b: v out , ac coupled, 100mv/div v in = 5v, v out = 12v 5s/div 200ma 0ma a b _____________________________typic a l ope ra t ing cha r a c t e rist ic s (c ont inue d) (circuit of figure 2, t a = +25c, unless otherwise noted.) linetransient response a: v in (4v to 6v) b: v out , ac coupled, 20mv/div i out = 50ma, v out = 12v 5ms/div 6v 4v a b name function lbo low-battery output is an open-drain output that goe s low when lbi is less than 1.5v. connect to v+ through a pull-up resistor. leave lbo floating if not used. lbi input to the internal low-battery comparator. tie t o gnd or v+ if not used. pin 1 2 fb feedback input. for fixed-output bootstrapped opera tion, connect fb to gnd. for adjustable-output bootstrapped operation, connect a resistor divider between v+, fb and gnd. for non-bootstrapped operation, there is no fi xed-output option. connect a resistor divider network between v out , fb and gnd. see bootstrapped/non-bootstrapped modes section. shdn active-high ttl/cmos logic-level input. in shutdown mode (shdn = v+), the internal switch is turned off and the output voltage equals v+ minus a diode drop (due to the dc path from the input to the output). tie to gnd for normal operation. ref 1.5v reference output that can source 100a for ext ernal loads. bypass with 0.1f or larger capacitor. gnd ground 3 4 lx drain of the internal n-channel fet. lx has an outp ut resistance of 1 and a peak current limit of 1a. v+ power-supply input. in bootstrapped mode, v+ is als o the output voltage sense input. 5 6 7 8 ___________________________________________________ ___________pin de sc ript ion downloaded from: http:///
________________de t a ile d de sc ript ion ope ra t ing princ iple the max761/max762 bicmos step-up switch-mode power supplies provide fixed outputs of 12v and 15v , respectively. they have a unique control scheme tha t combines the advantages of pulse-frequency modulati on (low supply current) and pulse-width modulation (hi gh efficiency at high loads). the internal n-channel p ower mosfet allows 1a peak currents, increasing the outp ut current capability over previous pulse-frequency-mo du- lation (pfm) devices. figure 1 shows the max761/ max762 block diagram. the max761/max762 offer three main improvements over prior solutions: (1) the converters operate wi th tiny surface-mount inductors (less than 5mm diameter) because of their 300khz switching frequency, (2) th e current-limited pfm control scheme allows 86% effic ien- cies over a wide range of load currents, and (3) th e max- imum supply current is only 110a. boot st ra ppe d/n on-boot st ra ppe d m ode s figures 2 and 3 show the standard application circu its for bootstrapped and non-bootstrapped modes. in boo t- strapped mode, the ic is powered from the output (v out ). in other words, the current needed to power the bootstrapped circuit is different from the v+ curre nt the chip consumes. the voltage applied to the gate of t he internal n-channel fet is switched from v out to ground, providing more switch-gate drive and increasing the effi- ciency of the dc-dc converter compared with non-boo t- strapped operation. m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs _______________________________________________________________________________________ 7 n n n lbi lbi lbo v+ fb dual-mode ? comparator ref 1.5v reference under voltage comparator low input voltage oscillator current control circuitry current comparator q s r q trig one-shot q trig one-shot 0.1v 0.2v 2.5v 100mv v+ lx gnd error comparator max761 max762 figure 1. simple block diagram downloaded from: http:///
m ax 7 6 1 /m ax 7 6 2 in non-bootstrapped mode, the ic is powered from th e supply voltage, v in , and operates with minimum supply current. since the voltage applied to the gate of t he inter- nal fet is reduced, efficiency declines with low in put voltages. note: in non-bootstrapped mode, there is no fixed-output operation; external resistors must beused to set the output voltage. use 1% external feed- back resistors when operating in non-bootstrapped mode (figure 3). use bootstrapped mode when v in is below approxi- mately 4v. for v in between 4v and 6v, the trade-off is lower supply current in non-bootstrapped mode versu s higher output current in bootstrapped mode (see typical operating characteristics ). pulse -fre que nc y m odula t ion (pfm ) cont rol sc he m e the max761/max762 use a proprietary current-limited pfm control scheme. this control scheme combines the ultra-low supply current of pulse-skipping pfm con- verters with the high full-load efficiency characte ristic of current-mode pulse-width-modulation (pwm) convert- ers. it allows the devices to achieve high efficien cy over a wide range of loads, while the current-sense func tion and high operating frequency allow the use of tiny external components. as with traditional pfm converters, the internal po wer mosfet is turned on when the voltage comparator senses the output is out of regulation (figure 1). however, unlike traditional pfm converters, switchi ng is accomplished through the combination of a peak cur- rent limit and a pair of one-shots that set the max imum on-time (8s) and minimum off-time (1.3s) for the switch. once off, the minimum off-time one-shot hol ds the switch off for 1.3s. after this minimum time, the switch either (1) stays off if the output is in reg ulation, or (2) turns on again if the output is out of regulati on. the max761/max762 also limit the peak inductor cur- rent, allowing the devices to run in continuous-con duc- tion mode (ccm) and maintain high efficiency with heavy loads (figure 4a). this current-limiting feat ure is a key component of the control circuitry. once turn ed on, the switch stays on until either (1) the maximu m on- time one-shot turns it off (8s later), or (2) the current limit is reached. to increase light-load efficiency, the current limi t for the first two pulses is set to half the peak current li mit. if those pulses bring the output voltage into regulati on, the voltage comparator holds the mosfet off, and th e current limit remains at half the peak current limi t. if the output voltage is still out of regulation after two pulses, the current limit for the next pulse is raised to t he full current limit of 1a (figure 4b). i nt e rna l vs. ex t e rna l re sist ors when external feedback resistors are used, an inter nal undervoltage lockout system prevents start-up until v+ rises to about 2.7v. when external feedback resisto rs are 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs 8 _______________________________________________________________________________________ figure 2. bootstrapped operating circuit figure 3. non-bootstrapped operating circuit v in = +5v lx lbo gnd max761 shdn v+ ref l1 18h lbi fb d1 1n5817 c1 33f c3 0.1f c4 33f c2 0.1f r4 r3 100k +12v at 150ma low-battery output 1 8 7 5 4 2 3 6 v in lx lbo gnd max761 max762 shdn v+ ref l1 18h lbi fb d1 1n5817 c1 c2 c4 r4 r3 100k adjustable output (v out ) low-battery detect output c3 r2 8 2 5 4 7 6 1 3 low-battery detect r4 = r3 ( ) v trip - v ref v ref r2 = r1 ( -1 ) v out v ref v ref = 1.5v nominal c1 = 33f c2 = 0.1f c3 = 0.1f c4 = 33f r1 7 downloaded from: http:///
used in a bootstrapped circuit (figure 5), undervol tage lockout prevents start-up at low input voltages; bu t once started, operation can continue down to a lowe r voltage that depends on the load. there is no undervoltage lockout when the internal feed- back resistors are used (figure 2), and special cir cuitry guarantees start-up at 2.0v. the start-up circuitry fixes the duty cycle at 50% until v+ is driven to 2.5v, a bove which the normal control system takes over. shut dow n m ode the max761/max762 enter shutdown mode when shdn is high. in this mode, the internal biasing ci rcuitry is turned off (including the reference) and v out equals v+ minus a diode drop (due to the dc path from the input to the output). in shutdown mode, the supply cur- rent drops to less than 5a. shdn is a ttl/cmos log ic level input. connect shdn to gnd for normal operati on. lbo is high impedance during shutdown. m ode s of ope ra t ion when delivering high output currents, the max761/ max762 operate in ccm. in this mode, current always flows in the inductor, and the control circuit adju sts the switchs duty cycle on a cycle-by-cycle basis to ma intain regulation without exceeding the switch-current cap abili- ty. this provides excellent load-transient response and high efficiency. in discontinuous-conduction mode (dcm), current through the inductor starts at zero, rises to a pea k value, then ramps down to zero on each cycle. although eff i- ciency is still excellent, the switch waveforms con tain ringing (the inductor's self-resonant frequency). t his ringing is normal and poses no operational problems . low -ba t t e ry de t e c t or the max761/max762 provide a low-battery comparator that compares the voltage on lbi to the 1.5v refere nce voltage. when the lbi voltage is below v ref , lbo (an open-drain output) goes low. the low-battery compar a- tors 20mv of hysteresis adds noise immunity, preve nt- ing repeated triggering of lbo. use a resistor-divi der network between v+, lbi, and gnd to set the desired trip voltage v trip (figure 3). when shdn is high, lbi is ignored and lbo is high impedance. the value of resistor r3 should be no larger than 500k to ensure the lbi leakage current does not cause inaccuracies in v trip . __________________de sign proc e dure se t t ing t he out put v olt a ge the max761/max762s output voltage can be adjusted from 5v to 16.5v using external resistors r1 and r2 configured as shown in figures 3 and 5. for adjusta ble- output operation, select feedback resistor r1 in th e 10k to 250k range. higher r1 values within this range give lowest supply current and best light-loa d efficiency. r2 is given by: r2 = (r1)( v out - 1) v ref where v ref = 1.5v. note: tie fb to gnd for fixed-output operation(bootstrapped mode only). m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs _______________________________________________________________________________________ 9 figure 4a. ccm, heavy load current waveform (500ma /div) figure 4b. light/medium load current waveform (500ma/div) 1a 500ma 1a 500ma 0a downloaded from: http:///
m ax 7 6 1 /m ax 7 6 2 se le c t ing t he i nduc t or (l) in both ccm and dcm, practical inductor values rang e from 10h to 50h. if the inductor value is too low , the current in the coil will ramp up to a high level be fore the current-limit comparator can turn off the switch. t he mini- mum on-time for the switch (t on(min) ) is approximately 2.5s, so select an inductance that allows the curr ent to ramp up to i lim / 2 in no less than 2.5s. choosing a value of i lim / 2 allows the half-size pulses to occur, giving high- er light-load efficiency and minimizing ripple. hen ce, cal- culate the minimum inductance value as: l 3 (v in(max) )(t on(min) ) i lim /2 or l 3 (v in(max) )(5) where v in(max) is in volts and l is in microhenries. the coils inductance need not satisfy this criteri on exactly, as the circuit can tolerate a wide range o f val- ues. larger inductance values tend to produce physi cal- ly larger coils and increase the start-up time, but are oth- erwise acceptable. smaller inductance values allow the coil current to ramp up to higher levels before the switch can turn off, producing higher ripple at light load s. in general, an 18h inductor is sufficient for most ap plica- tions (v in 5v). an 18h inductor is appropriate for input voltages up to 3.6v, as calculated above. how ever, the same 18h coil can be used with input voltages up to 5v with only small increases in peak current, as shown in figures 4a and 4b. inductors with a ferrite core or equivalent are rec om- mended. the inductors incremental saturation-curre nt rating should be greater than the 1a peak current l imit. it is generally acceptable to bias the inductor into s atura- tion by approximately 20% (the point where the indu c- tance is 20% below the nominal value). for highest effi- ciency, use a coil with low dc resistance, preferab ly under 100m . to minimize radiated noise, use a toroid, a pot core, or a shielded coil. table 1 lists inductor types and suppliers for vari ous applications. the listed surface-mount inductors e fficien- cies are nearly equivalent to those of the larger t hrough- hole inductors. diode se le c t ion the max761/max762s high switching frequency demands a high-speed rectifier. use a schottky diod e with a 1a average current rating, such as a 1n5817. for high-temperature applications, use a high-speed sil icon diode, such as the mur105 or the ec11fs1. these diodes have lower high-temperature leakage than schottky diodes (table 1). ca pa c it or se le c t ion output filter capacitor the primary criterion for selecting the output filt er capac- itor (c4) is low effective series resistance (esr). the product of the inductor current variation and the o utput filter capacitors esr determines the amplitude of the high-frequency ripple seen on the output voltage. a 33f, 16v sanyo os-con capacitor with 100m esr typically provides 100mv ripple when stepping up fr om 5v to 12v at 150ma. because the output filter capacitors esr affects e fficien- cy, use low-esr capacitors for best performance. th e smallest low-esr smt tantalum capacitors currently available are the sprague 595d series. sanyo os-con organic semiconductor through-hole capacitors and nichicon pl series also exhibit very low esr. table 1 lists some suppliers of low-esr capacitors. input bypass capacitors the input bypass capacitor, c1, reduces peak curren ts drawn from the voltage source, and also reduces noi se at the voltage source caused by the max761/max762s switching action. the input voltage source impedanc e determines the size of the capacitor required at th e v+ input. as with the output filter capacitor, a low-e sr capacitor is recommended. for output currents up to 250ma, 33f (c1) is adequate, although smaller bypa ss capacitors may also be acceptable. bypass the ic se pa- rately with a 0.1f ceramic capacitor, c2, placed c lose to the v+ and gnd pins. 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs 10 ______________________________________________________________________________________ v in lx gnd max761 max762 shdn v+ ref l1 18h lbi fb d1 1n5817 c2 c4 v out c3 r2 8 2 5 4 7 6 3 r2 = r1 ( -1 ) v out v ref c1 = 33f c2 = 0.1f c3 = 0.1f c4 = 33f c1 r1 v ref = 1.5v nominal figure 5. bootstrapped operation with adjustable o utput downloaded from: http:///
reference capacitor bypass ref with a 0.1f capacitor. ref can source u p to 100a. se t t ing t he low -ba t t e ry de t e c t or v olt a ge to set the low-battery detectors falling trip volt age (v trip ), select r3 between 10k and 500k (figures 2 and 3), and calculate r4 as follows: r4 = r3 [ (v trip - v ref ) ] v ref where v ref = 1.5v. the rising trip voltage is higher because of the co mpara- tors hysteresis of approximately 20mv, and can be cal- culated by: v trip (rising) = (v ref + 20mv)(1 + r4/r3). connect a high-value resistor (larger than r3 + r4) between lbi and lbo if additional hysteresis is req uired. connect a pull-up resistor (e.g., 100k ) between lbo and v out . tie lbi to gnd or v+ and leave lbo floating if the low-battery detector is not used. ___________applic a t ions i nform a t ion la yout conside ra t ions proper pc board layout is essential because of high cur- rent levels and fast switching waveforms that radia te noise. minimize ground noise by connecting gnd, the input bypass-capacitor ground lead, and the output filter- capacitor ground lead to a single point (star groun d con- figuration). also minimize lead lengths to reduce s tray capacitance, trace resistance, and radiated noise. the traces connected to fb and lx, in particular, must be short. place bypass capacitor c2 as close as possib le to v+ and gnd. m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs ______________________________________________________________________________________ 11 inductors diodes sumida cd54-180 (22h) coiltronics ctx 100-series matsuo 267 series surface mount sanyo os-con series low-esr organic semiconductor sumida rch855-180m miniature through-hole production method nichicon pl series low-esr electrolytics united chemi-con lxf series renco rl 1284-18 low-cost through-hole capacitors nihon ec10 series motorola 1n5817, mur105 table 1. component suppliers coiltronics (usa) (407) 241-7876 fax (407) 241-9339 matsuo (usa) (714) 969-2491 fax (714) 960-6492 matsuo (japan) 81-6-337-6450 fax 81-6-337-6456 nichicon (usa) (708) 843-7500 fax (708) 843-2798 nihon (usa) (805) 867-2555 fax (805) 867-2556 renco (usa) (516) 586-5566 fax (516) 586-5562 sanyo (usa) (619) 661-6835 fax (619) 661-1055 sanyo (japan) (0720) 70-1005 fax (0720) 70-1174 sumida (usa) (708) 956-0666 sumida (japan) 81-3-607-5111 fax 81-3-607-5144 united chem-con (usa) (714) 255-9500 fax (714) 255-9400 downloaded from: http:///
maxim cannot assume responsibility for use of any c ircuitry other than circuitry entirely embodied in a maxim product. no circuit patent licenses are implied. maxim reserves the right to change the cir cuitry and specifications without notice at any tim e. 12 __________________m a x im i nt e gra t e d produc t s, 1 2 0 sa n ga brie l drive , sunnyva le , ca 9 4 0 8 6 (4 0 8 ) 7 3 7 -7 6 0 0 ? 1993 maxim integrated products printed usa is a reg istered trademark of maxim integrated products. m ax 7 6 1 /m ax 7 6 2 1 2 v /1 5 v or adjust a ble , h igh-effic ie nc y, low i q , st e p-u p dc-dc conve rt e rs transistor count: 492; substrate connected to v+. ___________________chip topogra phy ref lbo 0. 142" (3. 607mm) 0. 080" (2. 030mm) fb shdn gnd lx v+ lbi downloaded from: http:///

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