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_______________general description the max649/MAX651/max652 bicmos, step-down dc- dc switching controllers provide high efficiency over three decades of load current. a unique, current-limited pulse-frequency-modulated (pfm) control scheme gives these devices the benefits of pulse-width-modulation (pwm) converters (high efficiency at heavy loads), while using only 100? of supply current (vs. 2ma to 10ma for pwm converters). the result is high efficiency over loads ranging from 10ma to more than 2.5a. these devices use miniature external components. their high switching frequency (up to 300khz) allows for less than 9mm diameter surface-mount inductors. the max649/MAX651/max652 have dropout voltages less than 1v and accept input voltages up to 16.5v. output voltages are preset at 5v (max649), 3.3v (MAX651), and 3v (max652). these controllers 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 10w. 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 5v-to-3.3v green pc applications high-efficiency step-down regulation minimum-component dc-dc converters battery-powered applications ____________________________features ? more than 90% efficiency (10ma to 1.5a loads) ? more than 12.5w output power ? 100? max quiescent supply current ? 5? max shutdown supply current ? less than 1.0v dropout voltage ? 16.5v max input voltage ? 5v (max649), 3.3v (MAX651), 3v (max652), or adjustable output voltage ? current-limited control scheme ? up to 300khz switching frequency ______________ordering information ordering information continued at end of data sheet. * dice are tested at t a = +25?. **contact factory for availability and processing to mil-std-883. max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , 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 max649 MAX651 max652 __________________pin configuration __________typical operating circuit 19-0225; rev 3; 9/97 part temp. range pin-package max649 cpa 0? to +70? 8 plastic dip max649csa 0? to +70? 8 so max649c/d 0? to +70? dice* max649epa -40? to +85? 8 plastic dip max649esa -40? to +85? 8 so max649mja -55? to +125? 8 cerdip** MAX651 v+ cs shdn fb gnd on/off p ext ref out output 3.3v input 4v to 16.5v evaluation kit available for free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. for small orders, phone 1-800-835-8769. http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , 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 1.455 1.5 1.545 1.4625 1.5 1.5375 fb trip point 1.470 1.5 1.530 v i q 25 fb input current i fb ?0 na ?0 ?0 output voltage v out 4.80 5.0 5.20 v supply current v+ input voltage range v+ 4.0 16.5 v 80 100 ? 4 3.17 3.3 3.43 2.88 3.0 3.12 reference voltage v ref 1.470 1.5 1.530 v 1.4625 1.5 1.5375 1.455 1.5 1.545 ref load regulation 410 mv 415 conditions max649m, max65_m circuit of figure 1 max649e, max65_e max649c, max65_c v+ = 10v, shdn 3 1.6v (shutdown) max649c, max65_c max649e, max65_e max649c, max65_c, i ref = 0 max649e, max65_e, i ref = 0 max649m, max65_m max649m, max65_m, i ref = 0 v+ = 16.5v, shdn 0.4v (operating, switch off) v+ = 16.5v, shdn 3 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 cerdip (derate 8.00mw/? above +70?) ..................640mw operating temperature ranges max649c_a, max65_c_a ..................................0? to +70? max649e_a, max65_e_a ................................-40? to +85? max649mja, max65_mja ............................-55? to +125? storage temperature range .............................-65? to +160? lead temperature (soldering, 10sec) .............................+300? max649, v+ = 6v to 16.5v MAX651, v+ = 4v to 16.5v max652, v+ = 4v to 16.5v max649c/e, max65_c/e max649m, max65_m 0 i ref 100?, sourcing only 4v v+ 16.5v ref line regulation 40 100 ?/v 2.6 mv/v 1.7 1.9 output voltage line regulation circuit of figure 1 max649, 6v v+ 16v, i load = 1a MAX651, 4.5v v+ 16v, i load = 1a max652, 4v v+ 16v, i load = 1a http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , 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 -45 units v+ = 16.5v, shdn = 0v or v+ -45 shdn input current 1 ? output voltage load regulation -47 mv/a conditions circuit of figure 1 max649, 0 i load 1.5a, v in = 10v MAX651, 0 i load 1.5a, v in = 5v max652, 0 i load 1.5a, v in = 5v max649, v+ = 10v, i load = 1a MAX651, v+ = 5v, i load = 1a 89 max652, v+ = 5v, i load = 1a 88 efficiency 92 % circuit of figure 1 4v v+ 16.5v shdn input voltage high 1.6 v v ih 4v v+ 16.5v 0.4 v v il shdn input voltage low 4v v+ 16.5v cs input current ? ? v+ = 12v 12 16 20 ? t on (max) switch maximum on-time c ext = 0.001?, v+ = 12v ext rise time 50 ns c ext = 0.001?, v+ = 12v ext fall time 50 ns v+ = 12v switch minimum off-time 1.8 2.3 2.8 ? t off (min) max649c/e, max65_c/e current-limit trip level (v+ to cs) 180 210 240 mv v cs max649m, max65_m 160 210 260 4v v+ 16.5v http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers 4 _______________________________________________________________________________________ __________________________________________typical operating characteristics (t a = +25?, unless otherwise noted.) 80 66 supply current vs. temperature 68 78 i+ (ma) 76 74 -60 -20 60 140 max649-a01 temperature (c) 20 100 -40 0 80 40 120 72 70 v+ = 10v v+ = 16.5v v+ = 4v 4.0 0 shutdown current vs. temperature 0.5 3.5 i+ (ma) 3.0 2.5 -60 -20 60 140 max649-a02 temperature (?) 20 100 -40 0 80 40 120 2.0 1.5 v+ = 8v v+ = 16.5v v+ = 4v 1.0 2500 0 0 1 2 3 4 5 6 7 8 9 10 1112131415 max649 maximum load current vs. supply voltage 500 2000 max649-a03 input voltage (v) maximum load current (ma) 1500 1000 v out = 5v circuit of figure 1 100 90 0 100 m 1m 10m 100m 1 max649 efficiency vs. load current 20 max649-a04 load current (a) efficiency (%) 40 60 80 70 50 30 10 v in = 6v v in = 8v v in = 10v v in = 12v v in = 15v v out = 5v top to bottom: 17 15 switch on-time vs. temperature t on (ms) -60 -40 -20 60 max649-a07 temperature (?) 0 20 40 80 100 120 16 v+ = 5v 100 90 0 100 m 1m 10m 100m 1 MAX651 efficiency vs. load current 20 max649-a05 load current (a ) efficiency (%) 40 60 80 70 50 30 10 v in = 4.3v v in = 5v v in = 8v v in = 10v v in = 12v v in = 15v v out = 3.3v top to bottom: 100 90 0 100 m 1m 10m 100m 1 max652 efficiency vs. load current 20 max649-a06 load current (a) efficiency (%) 40 60 80 70 50 30 10 v in = 4.3v v in = 5v v in = 8v v in = 10v v in = 12v v in = 15v v out = 3v top to bottom: 2.5 1.5 switch off-time vs. temperature t off (ms) max649-a08 temperature (?) 2.0 v+ = 5v -60 -40 -20 60 0 20 40 80 100 120 8.0 6.0 switch on-time/off-time ratio vs. temperature 6.4 7.6 t on /t off ratio 7.2 6.8 6.2 6.6 7.8 7.4 7.0 v+ = 5v -60 -20 60 140 max649-a9 temperature (?) 20 100 -40 0 80 40 120 http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers _______________________________________________________________________________________ 5 130 20 ext rise and fall times vs. temperature (1nf) 120 t rise & t fall (ns) 110 90 -60 -20 60 140 max649-a10 temperature (?) 20 100 -40 0 80 40 120 v+ = 5v, t rise 100 80 70 60 40 50 30 v+ = 5v, t fall v+ = 12v, t rise v+ = 12v, t fall c ext = 1nf 500 50 ext rise and fall times vs. temperature (5nf) 450 t rise & t fall (ns) 400 350 -60 -20 60 140 max649-a11 temperature (?) 20 100 -40 0 80 40 120 v+ = 5v, t rise 300 250 150 200 100 v+ = 5v, t fall v+ = 12v, t rise c ext = 5nf v+ = 12v, t fall 1000 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 dropout voltage vs. load current 200 300 100 800 900 max649-a12 load current (a) dropout voltage (mv) 600 500 700 400 max649, v out = 5v max652, v out = 3v MAX651, v out = 3.3v 1100 600 dropout voltage vs. temperature 700 1000 dropout voltage (mv) 900 800 MAX651 -60 -20 60 140 max649-a13 temperature (?) 20 100 -40 0 80 40 120 i load = 1a circuit of figure 1 max649 max652 250 0 reference output resistance vs. temperature 50 200 refrence output resistance ( w ) 150 100 -60 -20 60 140 max649-a16 temperature (?) 20 100 -40 0 80 40 120 i ref = 10 m a i ref = 50 m a i ref = 100 m a 235 185 cs trip level vs. temperature 195 225 cs trip level (mv) 215 205 -60 -20 60 140 max649-a14 temperature (?) 20 100 -40 0 80 40 120 190 200 230 220 210 1.506 1.492 reference output voltage vs. temperature 1.494 1.504 refrence output (v) 1.502 1.500 -60 -20 60 140 max649-a15 temperature (?) 20 100 -40 0 80 40 120 1.498 1.496 ____________________________typical operating characteristics (continued) (t a = +25?, unless otherwise noted.) http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers 6 _______________________________________________________________________________________ max649 line-transient response 250 m s/div a b i load = 1a a: input voltage (7v & 12v), 5v/div b: 5v out, ac coupled, 100mv/div _____________________________typical operating characteristics (continued) max649 load-transient response 250 m s/div a b a: load current (100ma & 1a), 500ma/div b: 5v output voltage, ac coupled, 50mv/div ______________________________________________________________pin description positive power-supply input v+ 5 cs 6 gate drive for external p-channel mosfet. ext swings between v+ and gnd. ext 7 ground gnd 8 1.5v reference output that can source 100?. bypass with 0.1?. ref 4 shdn 3 pin fb 2 out 1 function name sense input for fixed 5v, 3.3v, or 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. 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. active-high ttl/cmos logic-level input. part is placed in shutdown when shdn is driven high. in shutdown mode, the reference and the external mosfet are turned off, and out = 0v. connect to gnd for normal operation. 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. max649 shutdown response time 1ms/div a b i load = 1a a: shdn input voltage (0v & 5v), 2v/div b: 5v output voltage , 2v/div http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers _______________________________________________________________________________________ 7 _______________detailed description the max649/MAX651/max652 are bicmos, step- down, switch-mode power-supply controllers that pro- vide fixed outputs of 5v, 3.3v, and 3v, respectively. 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 max649/MAX651/max652 offer three main improvements over prior solutions: 1) the converters operate with tiny (less than 9mm diameter) 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 (1.0ma to 1.5a). 3) the maximum supply current is only 100?. pfm control scheme the max649/MAX651/max652 use a proprietary, cur- rent-limited pfm control scheme. as with traditional pfm converters, 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 (16?) and minimum switch off-time (2.3?). once off, the minimum off-time one-shot holds the switch off for 2.3?. 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 max649/MAX651/max652 also limit the peak induc- tor current, which allows them to run in continuous-con- duction mode and maintain high efficiency with heavy loads (figure 3a). 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 (16? later), or 2) the current limit is reached. to increase light-load efficiency, the current limit for the first two pulses is set to half the peak current limit. if those pulses bring the output voltage into regulation, the voltage comparator holds the mosfet off and the current limit remains at half its peak. if the output vol- tage is still out of regulation after two pulses, the current limit for the next pulse is raised to its peak (figure 3b). calculate the peak current limit by dividing the current-limit trip level (see electrical characteristics ) by the value of the current-sense resistor. shutdown mode when shdn is high, the max649/MAX651/max652 enter shutdown mode. in this mode, the internal biasing circuit- ry is turned off (including the reference) and the supply current drops to less than 5?. ext goes high, turning off the external mosfet. shdn is a ttl/cmos logic-level input. connect shdn to gnd for normal operation. quiescent current in normal operation, the quiescent current is less than 100?. however, this current is measured by forcing the external transistor switch off. in an actual applica- tion, 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, the typical quiescent current is 90?. ext drive voltage range ext swings from v+ to gnd and provides the drive out- put for an external p-channel power mosfet. modes of operation when delivering high output currents, the max649/ MAX651/max652 operate in continuous-conduction mode (ccm). in this mode, current always flows in the max649 MAX651 max652 v+ cs fb gnd 5 6 28 3 v in c2 330 m f 7 1 ext out shdn 4 c3 0.1 m f c4 0.1 m f c1 100 m f r1 0.1 w d1 nsq03a02l l1 22 m h ** p1 si9430 * output @ 1.5a * siliconix surface-mount mosfet ** sumida cdr125-220 ref figure 1. test circuit http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers 8 _______________________________________________________________________________________ max649 MAX651 max652 q minimum off-time one-shot trig q maximum on-time one-shot trig current control circuits dual-mode comparator error comparator current comparator 0.2v (full current) 0.1v (half current) q s r from v+ from v+ cs ext out gnd ref shdn fb v+ 1.5v reference n figure 2. block diagram http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers _______________________________________________________________________________________ 9 inductor, and the control circuit adjusts the switch duty cycle to maintain regulation without exceeding the switch current capability (figure 3a). this provides excellent load-transient response and high efficiency. in discontinuous-conduction mode (dcm), current through the inductor starts at zero, rises to a peak value, then ramps down to zero. although efficiency is still excellent, the output ripple increases slightly, and the switch waveforms exhibit ringing (the self-resonant frequency of the inductor). this ringing is to be expect- ed and poses no operational problems. dropout the max649/MAX651/max652 are said to be 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. v+ = 10v, i load = 1.3a circuit of figure 1, r1 = 150m w 1.5a 0a 1a 2 m s/div figure 3a. max649 continuous-conduction mode, heavy load-current waveform (500ma/div) v+ = 10v, i load = 1.4a circuit of figure 1, r1 = 100m w 2.5a 0a 1.5a 0.5a 1.0a 2.0a 5 m s/div figure 3b. max649 light/medium load-current waveform (500ma/div) ( ) max649 MAX651 max652 v+ cs gnd 5 6 2 8 3 v in c2 330 m f 7 1 ext out shdn 4 c3 0.1 m f c4 0.1 m f c1 100 m f r1 0.1 w d1 1n5820 l1 22 m 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 http://
__________________design procedure setting the output voltage the max649/MAX651/max652 are preset for 5v, 3.3v, and 3v output 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. 150k is a good value?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 210mv/r sense , where r sense is the value of the current-sense resistor, and 210mv is the current- limit trip level (see electrical characteristics ). 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, referring to figures 5a, 5b, or 5c, 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 worst-case values for sense resistor (?%), inductor (22? 10%), diode drop (0.6v), and the ic? current-sense trip level; an external mosfet on-resistance of 0.13 is assumed for v gs = -4.5v. max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers 10 ______________________________________________________________________________________ 3.0 0 2.5 maximum output current (a) 2.0 16 max649-a26 input voltage (v) 14 13 15 0.5 1.0 1.5 12 MAX651 v out = 3.3v 10 911 8 6 57 4 3 r s = 0.06 w r s = 0.07 w r s = 0.08 w r s = 0.10 w r s = 0.12 w r s = 0.14 w figure 5b. MAX651 current-sense resistor graph 3.0 0 2.5 maximum output current (a) 2.0 16 max649-a27 input voltage (v) 14 13 15 0.5 1.0 1.5 12 max652 v out = 3.0v 10 911 8 6 57 4 3 r s = 0.06 w r s = 0.07 w r s = 0.08 w r s = 0.10 w r s = 0.12 w r s = 0.14 w figure 5c. max652 current-sense resistor graph 3.0 0 2.0 2.5 maximum output current (a) 916 max649-a25 input voltage (v) 712 10 11 813 0.5 1.0 1.5 6 15 14 45 3 max649 v out = 5v r s = 0.06 w r s = 0.07 w r s = 0.08 w r s = 0.10 w r s = 0.12 w r s = 0.14 w figure 5a. max649 current-sense resistor graph http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers ______________________________________________________________________________________ 11 standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. surface-mount (chip) resistors have very little induc- tance and are well suited for use as current-sense resistors. a 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 practical inductor values range from 10? to 50? or more. the circuit operates in discontinuous-conduction mode if: v out x (r + 1) v d v + + + v sw rr r, the switch on-time/off-time ratio, equals 6.7. v d is the diode? drop, and v sw is the voltage drop across the p-channel fet. to get the full output capability in discontinuous-conduction mode, choose an inductor value no larger than: r sense x 12? x (v+ - v sw - v out ) l(max) = v cs where v cs is the current-sense voltage. in both the continuous and discontinuous modes, the lower limit of the inductor is more important. with a small inductor value, the current rises faster and over- shoots the desired peak current limit because the cur- rent-limit comparator cannot respond fast enough. this reduces efficiency slightly 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 sw - v out ) x 0.3? l(min) = d i x i lim (min) where d i is the percentage of inductor-current over- shoot, where i lim = v cs /r sense and 0.3? is the time it takes the comparator to switch. an overshoot of 10% is usually not a problem. inductance values above the minimum work well if the maximum value defined above is not exceeded. smaller inductance values cause higher output ripple because of overshoot. larger val- ues tend to produce physically larger coils. 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). 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 3.1a. using a slightly underrated inductor can sometimes reduce size and cost, with only a minor impact on efficiency. the max649/MAX651/max652 current limit prevents any damage from an underrated inductor? low inductance at high currents. 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 max649/MAX651/max652? high switching fre- quency demands a high-speed rectifier (commonly called a catch diode when used in switching-regulator circuits). schottky diodes, such as the 1n5817 through 1n5822 families (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). for high-tempera- ture applications, where schottky diodes can be inadequate because of high leakage currents, use high-speed silicon diodes instead. at heavy loads and high temperatures, the disadvantages of a schottky diode? high leakage current may outweigh the benefits of its low forward voltage. table 1 lists diode types and suppliers for various applications. external switching transistor the max649/MAX651/max652 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's on-resistance, gate-source threshold, and gate capacitance must also be appropriately 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 http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers 12 ______________________________________________________________________________________ table 1. component selection guide production method inductors capacitors diodes current-sense resistors mosfets surface mount matsuo 267 series sprague 595d series nihon nsq series irc lrc series miniature through-hole sumida rch855-220m sanyo os-con series low-esr organic semiconductor irc oar series motorola low-cost through-hole renco rl 1284-22 motorola 1n5820, 1n5823 motorola tmos power mosfets sumida cdr125-220 (22?) coiltronics ctx 100 series siliconix little foot series motorola medium-power surface-mount products nichicon pl series low-esr electrolytics united chemi-con lxf series a drain-to-source rating of -20v and a typical on-resis- tance of 0.115 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 esr of the output filter capacitor 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 w 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. use low-esr capacitors for best performance. the smallest low-esr smt tantalum capacitors currently available are from the sprague 595d series. sanyo os- con organic semiconductor through-hole capacitors and the nichicon pl series also exhibit very low esr. table 1 lists some suppliers of low-esr capacitors. 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 max649/MAX651/max652. the input voltage source impedance determines the size of the capacitor required at the v+ input. as with the output filter capacitor, a low-esr capacitor is recommended. bypass the ic separately with a 0.1? ceramic capacitor placed close to the v+ and gnd pins. reference capacitor bypass ref with a 0.1? or larger capacitor. ref can source at least 100?. layout considerations proper pc board layout is essential because of high current levels and fast switching waveforms that radiate noise. minimize ground noise by connecting the anode of the catch diode, 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 radiat- ed 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 capacitor as close as possible to v+ and gnd. http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers ______________________________________________________________________________________ 13 table 2. component suppliers company phone fax coiltronics usa (407) 241-7876 (407) 241-9339 harris usa (800) 442-7747 (407) 724-3937 international rectifier usa (310) 322-3331 (310) 322-3332 irc usa (704) 264-8861 (704) 264-8866 matsuo usa (714) 969-2491 (714) 960-6492 japan 81-6-337-6450 81-6-337-6456 motorola usa (800) 521-6274 (602) 244-4015 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 renco usa (516) 586-5566 (516) 586-5562 sanyo usa (619) 661-6835 (619) 661-1055 japan 81-7-2070-6306 81-7-2070-1174 siliconix usa (408) 988-8000 (408) 970-3950 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 chemi-con usa (714) 255-9500 (714) 255-9400 __ordering information (continued) 8 cerdip** -55? to +125? max652mja 8 so -40? to +85? max652esa 8 plastic dip -40? to +85? max652epa dice* 0? to +70? max652c/d 8 so 0? to +70? max652csa 8 plastic dip 0? to +70? max652 cpa 8 cerdip** -55? to +125? MAX651mja 8 so -40? to +85? MAX651esa 8 plastic dip -40? to +85? MAX651epa dice* 0? to +70? MAX651c/d 8 so 0? to +70? MAX651csa 8 plastic dip 0? to +70? MAX651 cpa pin-package temp. range part ___________________chip topography transistor count: 442; substrate connected to v+. * dice are tested at t a = +25?. **contact factory for availability and processing to mil-std-883. 0.109" (2.769mm) 0.080" (2.032mm) out gnd cs ext v+ fb shdn ref http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers 14 ______________________________________________________________________________________ ________________________________________________________package information pdipn.eps soicn.eps http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers ______________________________________________________________________________________ 15 ___________________________________________package information (continued) cdips.eps http://
max649/MAX651/max652 5v/3.3v/3v or adjustable, high-efficiency, low i q , step-down dc-dc controllers 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. 16 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 1997 maxim integrated products printed usa is a registered trademark of maxim integrated products. notes http://

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