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_______________general description the max1649/max1651 bicmos, step-down, dc-dc switching controllers provide high efficiency over loads ranging from 1ma to more than 2.5a. a unique, current- limited pulse-frequency-modulated (pfm) control scheme gives these devices the benefits of pulse-width-modula- tion (pwm) converters (high efficiency at heavy loads), while using only 100? of supply current (vs. 2ma to 10ma for pwm converters). dropout performance down to 300mv is provided by a high switch duty cycle (96.5%) and a low current-sense threshold (110mv). a high switching frequency (up to 300khz) allows these devices to use miniature external components. the max1649/max1651 have dropout voltages less than 0.3v at 500ma and accept input voltages up to 16v. output voltages are preset at 5v (max1649), or 3.3v (max1651). they can also be adjusted to any voltage from 1.5v to the input voltage by using two resistors. these step-down controllers drive external p-channel mosfets at loads greater than 12.5w. if less power is required, use the max639/max640/max653 step-down converters with on-chip fets, which allow up to a 225ma load current. ________________________applications pdas high-efficiency step-down regulation 5v-to-3.3v green pc applications battery-powered applications ____________________________features ? more than 90% efficiency (10ma to 1.5a loads) ? more than 12.5w output power ? less than 0.3v dropout voltage at 500ma ? 100 a max quiescent supply current ? 5 a max shutdown supply current ? 16v max input voltage ? 5v (max1649), 3.3v (max1651), or adjustable output voltage ? current-limited control scheme ? up to 300khz switching frequency ? up to 96.5% duty cycle ______________ordering information *dice are tested at t a = +25?. max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers ________________________________________________________________ maxim integrated products 1 1 2 3 4 8 7 6 5 gnd ext cs v+ ref shdn fb out dip/so top view max1649 max1651 __________________pin configuration __________typical operating circuit 19-0305; rev 3; 3/09 part temp range pin-package max1649 cpa 0? to +70? 8 plastic dip max1649csa 0? to +70? 8 so max1649c/d 0? to +70? dice* max1649epa -40? to +85? 8 plastic dip max1649esa -40? to +85? 8 so max1651 v+ cs shdn fb gnd on/off p ext ref out output 3.3v input 3.6v to 16v max1651 cpa 0? to +70? 8 plastic dip max1651csa 0? to +70? 8 so max1651c/d 0? to +70? dice* max1651epa -40? to +85? 8 plastic dip max1651esa -40? to +85? 8 so evaluation kit available for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim's website at www.maxim-ic.com.
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v+ = 5v, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. parameter symbol min typ max units 1.4625 1.5 1.5375 fb trip point 1.470 1.5 1.530 v i+ 15 fb input current i fb ?0 na ?0 output voltage v out 4.80 5.0 5.20 v supply current v+ input voltage range v+ 3.0 16 v 78 100 ? 2 3.17 3.3 3.43 reference voltage v ref 1.470 1.5 1.530 v 1.4625 1.5 1.5375 ref load regulation mv 410 conditions max1649e, max1651e max1649, v+ = 5.5v to 16v max1649c, max1651c v+ = 10v, shdn 1.6v (shutdown) max1649c, max1651c max1649c, max1651c, i ref = 0 a max1649e, max1651e max1649e, max1651e, i ref = 0 a v out < v+ v+ = 16v, shdn 0.4v (operating, switch off) v+ = 16v, shdn 1.6v (shutdown) supply voltage, v+ to gnd.......................................-0.3v, +17v ref, shdn, fb, cs, ext, out .......................-0.3v, (v+ + 0.3v) continuous power dissipation (t a = +70?) plastic dip (derate 9.09mw/? above +70?) .............727mw so (derate 5.88mw/? above +70?) ..........................471mw operating temperature ranges max1649c_a, max1651c_a ..............................0? to +70? max1649e_a, max1651e_a ............................-40? to +85? storage temperature range .............................-65? to +160? lead temperature (soldering, 10sec) .............................+300? 0? i ref 100?, sourcing only 3v v+ 16v ref line regulation 40 100 ?/v 2.6 mv/v 1.7 output voltage line regulation circuit of figure 1 max1649, 5.5v v+ 16v, i load = 1a max1651, 3.6v v+ 16v, i load = 1a -47 mv/a max1649, 0a i load 1.5a, v in = 10v max1651, 0a i load 1.5a, v in = 5v -45 output voltage load regulation circuit of figure 1 90 % max1649, v+ = 10v, i load = 1a max1651, v+ = 5v, i load = 1a 90 efficiency circuit of figure 1 shdn input current v+ = 16v, shdn = 0v or v+ 1 ? shdn input voltage high v ih 3v v+ 16v 1.6 v shdn input voltage low v il 3v v+ 16v 0.4 v max1651, v+ = 3.6v to 16v
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers _______________________________________________________________________________________ 3 electrical characteristics (continued) (v+ = 5v, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) parameter symbol min typ max units conditions 3v v+ 16v cs input current ? ? c ext = 0.001?, v+ = 12v ext rise time 25 ns c ext = 0.001?, v+ = 12v ext fall time 25 ns current-limit trip level (v+ to cs) mv v cs 3v v+ 16v 80 110 140 t on t on + t off x 100% maximum duty cycle 95 96.5 % 80 66 supply current vs. temperature 68 78 i+ ( a) 76 74 -60 -20 60 140 max1649-toc06 temperature (c) 20 100 -40 0 80 40 120 72 70 v+ = 10v v+ = 16v v+ = 4v 4.0 0 shutdown current vs. temperature 0.5 3.5 i+ ( a) 3.0 2.5 -60 -20 60 140 max1649-toc05 temperature (?) 20 100 -40 0 80 40 120 2.0 1.5 v+ = 8v v+ = 16v v+ = 4v 1.0 v+ = 12v switch maximum on-time 24 32 40 ? t on (max) v+ = 12v switch minimum off-time 0.8 1.1 1.8 ? t off (min) -60 -20 60 140 ext rise and fall times vs. temperature (1nf) max1649/51-01 temperature (?) t rise & t fall (ns) 20 100 -40 0 80 40 120 60 55 50 45 40 35 30 25 20 15 v+ = 15v, t fall v+ = 15v, t rise v+ = 5v, t fall v+ = 5v, t rise c ext = 1nf 1 10 10k load current (ma) efficiency (%) 100 90 80 70 60 50 40 100 1k 0.1 max1649/51-a1 efficiency vs. load current (v out = 5v) v out = 5v circuit of figure 1 top to bottom: v in = 6v v in = 8v v in = 10v v in = 12v v in = 15v __________________________________________typical operating characteristics (t a = +25?, unless otherwise noted.) 40 -60 -20 60 140 ext rise and fall times vs. temperature (5nf) 80 max1649/51-02 temperature (?) t rise & t fall (ns) 20 100 -40 0 80 40 120 60 100 140 120 160 200 180 240 220 v+ = 15v, t fall v+ = 15v, t rise v+ = 5v, t fall v+ = 5v, t rise c ext = 5nf 1 10 10k load current (ma) efficiency (%) 100 90 80 70 60 50 40 100 1k 0.1 max1649/51-a2 efficiency vs. load current (v out = 3.3v) v out = 3.3v circuit of figure 1 top to bottom: v in = 4.3v v in = 5v v in = 8v v in = 10v v in = 12v v in = 15v
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers 4 _______________________________________________________________________________________ -60 -20 60 140 switch on-time vs. temperature max1649/51-03 temperature (?) t on ( s) 20 100 -40 0 80 40 120 34.0 33.5 33.0 32.5 32.0 31.5 31.0 30.0 30.5 -60 -20 60 140 switch off-time vs. temperature max1649/51-04 temperature (?) t off ( s) 20 100 -40 0 80 40 120 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 -60 -20 60 140 maximum duty cycle vs. temperature max1649/51-05 temperature (?) duty cycle (%) 20 100 -40 0 80 40 120 100 99 98 97 96 95 94 93 -60 -20 60 140 cs trip level vs. temperature max1649/51-06 temperature (?) cs trip level (mv) 20 100 -40 0 80 40 120 120 115 110 100 105 95 0 0.5 1.0 1.5 2.0 dropout voltage vs. load current max1649/51-a3 load current (a) dropout voltage (mv) 600 500 400 300 200 100 0 v out = 4.80v v out = 3.17v circuit of figure 1 250 0 reference output resistance vs. temperature 50 200 reference output resistance ( ) 150 100 -60 -20 60 140 max1649-toc07 temperature (?) 20 100 -40 0 80 40 120 i ref = 10 a i ref = 50 a i ref = 100 a 1.506 1.492 reference output voltage vs. temperature 1.494 1.504 reference output voltage (v) 1.502 1.500 -60 -20 60 140 max1649-toc01 temperature (?) 20 100 -40 0 80 40 120 1.498 1.496 i ref = 10 a ____________________________typical operating characteristics (continued) (t a = +25?, unless otherwise noted.)
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers _______________________________________________________________________________________ 5 ____________________________typical operating characteristics (continued) (t a = +25?, unless otherwise noted.) circuit of figure 1, i load = 1a a: v out = 5v, 100mv/div, ac-coupled b: v+ = 6v to 16v, 5v/div max1649 line-transient response a b 5ms/div 16v 6v circuit of figure 1, v+ = 10v a: v out = 5v, 100mv/div, ac-coupled b: i load = 30ma to 1.6a, 1a/div max1649 load-transient response a 1.6a 0a b 200 s/div circuit of figure 1, v+ = 10v, i load = 1a max1649 shdn response time 5v 4v output 0v 0v shdn input 1ms/div
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers 6 _______________________________________________________________________________________ _______________detailed description the max1649/max1651 are bicmos, step-down, switch-mode power-supply controllers that provide adjustable and fixed outputs of 5v and 3.3v, respec- tively. their unique control scheme combines the advantages of pulse-frequency-modulation (low supply current) and pulse-width-modulation (high efficiency at high loads). an external p-channel power mosfet allows peak currents in excess of 3a, increasing the output current capability over previous pfm devices. figure 2 is the block diagram. the max1649/max1651 offer four main improvements over prior solutions: 1) the converters operate with miniature surface-mount inductors, due to their 300khz switching frequency. 2) the current-limited pfm control scheme allows greater than 90% efficiencies over a wide range of load currents (10ma to 1.5a). 3) dropout voltage has been reduced to less than 300mv for many applications. 4) the quiescent supply current is only 100?. pfm control scheme the max1649/max1651 use a proprietary, current-limit- ed pfm control scheme. as with traditional pfm con- verters, the external power mosfet is turned on when the voltage comparator senses that the output is out of regulation. however, unlike traditional pfm converters, switching is accomplished through the combination of a peak current limit and a pair of one-shots that set the maximum switch on-time (32s) and minimum switch off-time (1.1?). once off, the off-time one-shot holds the switch off for 1.1?. after this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. the max1649/max1651 also limit the peak inductor cur- rent, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads (figure 3). this current-limiting feature is a key compo- nent of the control circuitry. once turned on, the switch stays on until either 1) the maximum on-time one-shot turns it off (32? later), or 2) the current limit is reached. ext swings from v+ to gnd and provides the drive out- put for an external p-channel power mosfet. max1649 max1651 v+ cs fb gnd 5 6 28 3 v in c2 330 f 7 1 ext out shdn 4 c3 0.1 f c4 0.1 f c1 100 f r1 0.05 d1 nsq03a02l l1 47 h ** p1 si9430 * output @ 1.5a * siliconix surface-mount mosfet ** sumida cdrh125-470 ref figure 1. typical application circuit ______________________________________________________________pin description name function 1 out sense input for fixed 5v or 3.3v output operation. out is internally connected to the on-chip voltage divider. although it is connected to the output of the circuit, the out pin does not supply current. leave out unconnected for adjustable-output operation. 2 fb feedback input. connect to gnd for fixed-output operation. connect a resistor divider between out, fb, and gnd for adjustable-output operation. see setting the output voltage section. pin 3 shdn active-high shutdown input. part is placed in shutdown when shdn is driven high. in shutdown mode, the refer- ence, output, and external mosfet are turned off. connect to gnd for normal operation. 4 ref 1.5v reference output that can source 100?. bypass with 0.1?. 8 gnd ground 7 ext gate drive for external p-channel mosfet. ext swings between v+ and gnd. 6 cs current-sense input. connect current-sense resistor between v+ and cs. when the voltage across the resistor equals the current-limit trip level, the external mosfet is turned off. 5 v+ positive power-supply input
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers _______________________________________________________________________________________ 7 shutdown mode when shdn is high, the max1649/max1651 enter shut- down mode. in this mode, the internal biasing circuitry is turned off (including the reference) and the supply cur- rent drops to less than 5a. ext goes high, turning off the external mosfet. shdn is a logic-level input. connect shdn to gnd for normal operation. quiescent current in normal operation, the device's typical quiescent cur- rent is 78?. in an actual application, even with no load, additional current is drawn to supply external feedback resistors (if used) and the diode and capacitor leakage currents. in the circuit of figure 1, with v+ at 5v and v out at 3.3v, typical no-load supply current for the entire circuit is 90?. max1649 max1651 q minimum off-time one-shot trig q maximum on-time one-shot trig dual-mode comparator error comparator current comparator 110mv q s r from v+ from v+ cs ext out gnd ref shdn fb v+ 1.5v reference f/f n dual-mode is a trademark of maxim integrated products 50mv figure 2. block diagram
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers 8 _______________________________________________________________________________________ modes of operation when delivering high output currents, the max1649/ max1651 operate in continuous-conduction mode. in this mode, current always flows in the inductor, and the control circuit adjusts the switch duty cycle to main- tain regulation without exceeding the switch current capability (figure 3). this provides excellent load-tran- sient response and high efficiency. in discontinuous-conduction mode, current through the inductor starts at zero, rises to a peak value, then ramps down to zero. although efficiency is still excel- lent, the output ripple increases slightly, and the switch waveform exhibits ringing (at the inductor's self-reso- nant frequency). this ringing is to be expected and poses no operational problems. dropout the max1649/max1651 are in dropout when the input voltage (v+) is low enough that the output drops below the minimum output voltage specification (see electrical characteristics ). the dropout voltage is the difference between the input and output voltage when dropout occurs. see the typical operating characteristics for the dropout voltage vs. load current and dropout voltage vs. temperature graphs. __________________design procedure setting the output voltage the max1649/max1651 are preset for 5v and 3.3v out- put voltages, respectively; tie fb to gnd for fixed-output operation. they may also be adjusted from 1.5v (the reference voltage) to the input voltage, using external resistors r2 and r3 configured as shown in figure 4. for adjustable-output operation, 150k is recommended for resistor r3?igh enough to avoid wasting energy, yet low enough to avoid rc delays caused by parasitic capacitance at fb. r2 is given by: v out r2 = r3 x ( -1 ) v ref where v ref = 1.5v. when using external resistors, it does no harm to con- nect out and the output together, or to leave out unconnected. current-sense resistor selection the current-sense resistor limits the peak switch cur- rent to 110mv/r sense , where r sense is the value of the current-sense resistor, and 110mv is the current- limit trip level (see electrical characteristics ). v+ = 10v, i load = 1.3a circuit of figure 1, r1 = 75m 1.5a 0a 1a 2 s/div figure 3. max1649 continuous-conduction mode, heavy load-current waveform (500ma/div) ( ) max1649 max1651 v+ cs gnd 5 6 2 8 3 v in c2 330 f 7 1 ext out shdn 4 c3 0.1 f c4 0.1 f c1 100 f r1 0.05 d1 1n5820 l1 47 h p1 si9430 output @ 1.5a ref fb r2 r3 150k r2 = r3 v out v ref ?1 v ref = 1.5v figure 4. adjustable-output operation
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers _______________________________________________________________________________________ 9 to maximize efficiency and reduce the size and cost of external components, minimize the peak current. however, since the available output current is a func- tion of the peak current, the peak current must not be too low. to choose the proper current-sense resistor for a par- ticular output voltage, determine the minimum input voltage and the maximum load current. next, refer- ring to figures 5a or 5b, using the minimum input volt- age, find the curve with the largest sense resistor that provides sufficient output current. it is not necessary to perform worst-case calculations. these curves take into account the sense-resistor (5%) and inductor (47? ?0%) values, the diode drop (0.4), and the ic? current-sense trip level (85mv); an external mos- fet on-resistance of 0.07 is assumed for v gs = -5v. standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. surface-mount (chip) resistors have very little inductance and are well suited for use as current-sense resistors. a u-shaped wire resistor made by irc works well in through-hole applications. because this resistor is a band of metal shaped as a ?? its inductance is less than 10nh (an order of magnitude less than metal film resistors). resistance values between 5m and 0.1 are available (see table 1). inductor selection the max1649/max1651 operate with a wide range of inductor values, although for most applications coils between 10? and 68? take best advantage of the con- trollers?high switching frequency. with a high inductor value, the max1649/max1651 will begin continuous-cur- rent operation (see detailed description ) at a lower frac- tion of full-load current. in general, smaller values pro- duce higher ripple (see below) while larger values require larger size for a given current rating. in both the continuous and discontinuous modes, the lower limit of the inductor is important. with a too-small inductor value, the current rises faster and overshoots the desired peak current limit because the current-limit com- parator has a finite response time (300ns). this reduces efficiency and, more importantly, could cause the current rating of the external components to be exceeded. calculate the minimum inductor value as follows: (v+(max) - v out ) x 0.3? l(min) = i x i lim where i is the inductor-current overshoot factor, i lim = v cs /r sense , and 0.3? is the time it takes the com- parator to switch. set i = 0.1 for an overshoot of 10%. for highest efficiency, use a coil with low dc resis- tance; a value smaller than 0.1v/i lim works best. to minimize radiated noise, use a toroid, pot core, or shielded-bobbin inductor. inductors with a ferrite core or equivalent are recommended. make sure the induc- tor? saturation-current rating is greater than i lim (max). however, it is generally acceptable to bias the inductor into saturation by about 20% (the point where the inductance is 20% below its nominal value). 5.0 5.4 5.8 6.2 6.6 16.0 1649 fig05a input voltage (v) maximum output current (a) 3.0 2.5 2.0 1.5 1.0 0 0.5 r s = 0.030 v out = 5v r s = 0.040 r s = 0.050 r s = 0.060 r s = 0.080 r s = 0.100 figure 5a. max1649 current-sense resistor graph 3.0 3.4 3.8 4.2 4.6 16.0 1651 fig05b input voltage (v) maximum output current (a) 3.0 2.5 2.0 1.5 1.0 0 0.5 r s = 0.030 r s = 0.040 r s = 0.050 r s = 0.060 r s = 0.080 r s = 0.100 v out = 3.3v figure 5b. max1651 current-sense resistor graph
max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers 10 ______________________________________________________________________________________ table 1. component selection guide the peak current of figure 1 is 2.35a for a 1.5a output. the inductor used in this circuit is specified to drop by 10% at 2.2a (worst case); a curve provided by the manufacturer shows that the inductance typically drops by 20% at 2.7a. using a slightly underrated inductor can sometimes reduce size and cost, with only a minor impact on efficiency. table 1 lists inductor types and suppliers for various applications. the efficiencies of the listed surface- mount inductors are nearly equivalent to those of the larger size through-hole versions. diode selection the max1649/max1651? high switching frequency demands a high-speed rectifier. schottky diodes, such as the 1n5817 through 1n5823 (and their surface- mount equivalents), are recommended. choose a diode with an average current rating equal to or greater than i lim (max) and a voltage rating higher than v+(max). external switching transistor the max1649/max1651 drive p-channel enhancement- mode mosfet transistors only. the choice of power transistor is primarily dictated by the input voltage and the peak current. the transistor? on-resistance, gate- source threshold, and gate charge must also be appro- priately chosen. the drain-to-source and gate-to- source breakdown voltage ratings must be greater than v+. the total gate-charge specification is normally not critical, but values should be less than 100nc for best efficiency. the mosfet should be capable of handling the peak current and, for maximum efficiency, have a very low on-resistance at that current. also, the on- resistance must be low for the minimum available v gs , which equals v+(min). select a transistor with an on- resistance between 50% and 100% of the current- sense resistor. the si9430 transistor chosen for the typical operating circuit has a drain-to-source rating of -20v and a typical on-resistance of 0.070 at 2a with v gs = -4.5v. tables 1 and 2 list suppliers of switching transistors suitable for use with these devices. capacitor selection output filter capacitor the primary criterion for selecting the output filter capacitor is low equivalent series resistance (esr), rather than high capacitance. an electrolytic capacitor with low enough esr will automatically have high enough capacitance. the product of the inductor-cur- rent variation and the output filter capacitor? esr determines the amplitude of the high-frequency ripple seen on the output voltage. when a 330?, 10v sprague surface-mount capacitor (595d series) with esr = 0.15 is used, 40mv of output ripple is typically observed when stepping down from 10v to 5v at 1a. the output filter capacitor's esr also affects efficiency. again, low-esr capacitors perform best. table 1 lists some suppliers of low-esr capacitors. production method inductors capacitors diodes current-sense resistors mosfets surface mount avx tps series sprague 595d series motorola mbrs340t3 nihon nsq series dale wsl series irc lrc series miniature through-hole sumida rch875-470m (1.3a) sanyo os-con series low-esr organic semiconductor irc oar series motorola low-cost through-hole coilcraft pch-45-473 (3.4a) motorola 1n5817 to 1n5823 motorola tmos power mosfets sumida cdrh125-470 (1.8a) cdrh125-220 (2.2a) coilcraft do3316-473 (1.6a) do3340-473 (3.8a) siliconix little foot series motorola medium-power surface-mount products nichicon pl series low-esr electrolytics united chemi-con lxf series
input bypass capacitor the input bypass capacitor reduces peak currents drawn from the voltage source, and also reduces the amount of noise at the voltage source caused by the switching action of the max1649/max1651. the input voltage source impedance determines the size of the capacitor required at the v+ input. as with the output fil- ter capacitor, a low-esr capacitor is recommended. bypass the ic separately with a 0.1? ceramic capac- itor placed close to the v+ and gnd pins. reference capacitor bypass ref with a 0.1? or larger capacitor. max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers ______________________________________________________________________________________ 11 ___________________chip topography transistor count: 428 substrate connected to v+ 0.106" (2.692mm) 0.081" (2.057mm) out gnd cs ext v+ fb shdn ref table 2. component suppliers company phone fax (207) 282-5111 avx usa or (207) 283-1941 (800) 282-4975 coiltronics usa (516) 241-7876 (516) 241-9339 coilcraft usa (708) 639-6400 (708) 639-1469 dale usa (402) 564-3131 (402) 563-1841 international usa (310) 322-3331 (310) 322-3332 rectifier irc usa (512) 992-7900 (512) 992-3377 (602) 244-3576 motorola usa or (602) 244-4015 (602) 244-5303 nichicon usa (708) 843-7500 (708) 843-2798 japan 81-7-5231-8461 81-7-5256-4158 nihon usa (805) 867-2555 (805) 867-2556 japan 81-3-3494-7411 81-3-3494-7414 sanyo usa (619) 661-6835 (619) 661-1055 japan 81-7-2070-6306 81-7-2070-1174 (408) 988-8000 siliconix usa or (408) 970-3950 (800) 554-5565 sprague usa (603) 224-1961 (603) 224-1430 sumida usa (708) 956-0666 (708) 956-0702 japan 81-3-3607-5111 81-3-3607-5144 united usa (714) 255-9500 (714) 255-9400 chemi-con layout considerations proper pc board layout is essential because of high current levels and fast switching waveforms that radi- ate noise. minimize ground noise by connecting the anode of the rectifier, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (?tar?ground configuration). a ground plane is recommended. also minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. in particular, the traces connected to fb (if an external resistor divider is used) and ext must be short. place the 0.1? ceramic bypass capac- itor as close as possible to the v+ and gnd pins. max1649/max1651 vs. max649/max651 the max1649 and max1651 are pin compatible with the max649 and max651, but have been optimized for improved dropout performance and efficiency?artic- ularly with low input voltages. the max1649/max1651 feature increased maximum switch duty cycle (96.5%) and reduced current-limit sense voltage (110mv). their predecessors, the max649/max651, use a high- er two-step (210mv/110mv) current-limit sense voltage to provide tighter current-sense accuracy and reduced inductor peak current at light loads. package information for the latest package outline information and land patterns, go to www.maxim-ic.com/packages . package type package code document no. 8 pdip p8-2 21-0041 8 so s8-4 21-0043
maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 12 __________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 (408) 737-7600 2009 maxim integrated products maxim is a registered trademark of maxim integrated products, inc. max1649/max1651 5v/3.3v or adjustable, high-efficiency, low-dropout, step-down dc-dc controllers revision history revision number revision date description pages changed 3 3/09 corrected output voltage conditions and figure 1 title 2, 6

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