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_______________general description the max742 dc-dc converter is a controller for dual-out- put power supplies in the 3w to 60w range. relying on simple two-terminal inductors rather than transformers, the max742 regulates both outputs independently to within ?% over all conditions of line voltage, temperature, and load current. the max742 has high efficiency (up to 92%) over a wide range of output loading. two independent pwm current- mode feedback loops provide tight regulation and opera- tion free from subharmonic noise. the max742 can operate at 100khz or 200khz, so it can be used with small and lightweight external components. also ripple and noise are easy to filter. the max742 provides a regulated output for inputs ranging from 4.2v to 10v (and higher with additional components). external power mosfets driven directly from the max742 are protected by cycle-by-cycle overcurrent sensing. the max742 also features undervoltage lockout, thermal shut- down, and programmable soft-start. if 3w of load power or less is needed, refer to the max743 data sheet for a device with internal power mosfets. ________________________applications dc-dc converter module replacement distributed power systems computer peripherals ____________________________features ? specs guaranteed for in-circuit performance ? load currents to ?a ? 4.2v to 10v input-voltage range ? switches from ?5v to ?2v under logic control ? ?% output tolerance max over temp, line, and load ? 90% typ efficiency ? low-noise, current-mode feedback ? cycle-by-cycle current limiting ? undervoltage lockout and soft-start ? 100khz or 200khz operation max742 switch-mode regulator with +5v to ?2v or ?5v dual output ________________________________________________________________ maxim integrated products 1 max742 p r +5v input s -sense -drive pwm n r s +sense +drive -vo +vo pwm osc +2.0v vref cc+ cc- __________simplified block diagram 20 19 18 17 16 15 14 13 1 2 3 4 5 6 7 8 csh+ csl+ gnd ext+ av agnd cc+ fb+ top view pump pdrv ext- v+ ss vref 12/15 100/200 12 11 9 10 csh- csl- fb- cc- dip/so max742 __________________pin configuration 19-3105; rev 2; 8/96 part max742 cpp max742cwp max742c/d 0? to +70? 0? to +70? 0? to +70? temp. range pin-package 20 plastic dip 20 wide so dice* evaluation kit manual follows data sheet max742ewp max742mjp -55? to +125? -40? to +85? 20 wide so 20 cerdip max742epp -40? to +85? 20 plastic dip ______________ordering information * contact factory for dice specifications for free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
max742 switch-mode regulator with +5v to ?2v or ?5v dual output 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (circuit of figure 2, +4.5v < v+ < +5.5v.) 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. v+, av+ to agnd, gnd.........................................-0.3v to +12v pdrv to v+.............................................................+0.3v to -14v fb+, fb- to gnd..................................................................?5v input voltage to gnd (cc+, cc-, csh+, csl+, csh-, csl-, ss, 100/200 , 12/15 ) ..................................-0.3v to (v+ + 0.3v) output voltage to gnd (ext+, pump) ..........................................-0.3v to (v+ + 0.3v) ext- to pdrv................................................-0.3v to (v+ + 0.3v) continuous power dissipation (any package) up to +70? .....................................................................500mw derate above +70? by ..........................................100mw/? operating temperature ranges max742c_ _ .......................................................0? to +70? max742e_ _ ....................................................-40? to +85? max742mjp ..................................................-55? to +125? storage temperature range .............................-65? to +150? lead temperature (soldering, 10sec) .............................+300? 0ma < i l < 100ma, 12/15 = 0v conditions 14.40 15.60 v 14.55 15.45 output voltage, ?5v mode (notes 1, 2) units min typ max symbol parameter t a = +25? t a = t min to t max t a = +25? t a = t min to t max 0ma < i l < 125ma, 12/15 = v+ 11.52 12.48 v 11.64 12.36 output voltage, ?2v mode (notes 1, 2) no ext- or pump load, fb+ = fb- = open circuit i load = 0ma to 100ma v+ = 4.5v to 5.5v, pdrv from pump csl+ = 0v, fb+ = open circuit ext+ or ext- 100/200 = v+ 100/200 = 0v csh- = v+, fb- = open circuit conditions mv 150 225 300 negative current-limit threshold (csh- to csl-) ma 3 no-load supply current mv 30 100 %/% 0.01 0.05 line regulation load regulation (note 2) mv 150 225 300 positive current-limit threshold (csh+ to csl+) % 85 90 duty-cycle limit (note 3) khz f osc /2 pump frequency khz 75 100 125 v 3.8 4.2 uvlo undervoltage lockout v 0.2 undervoltage lockout hysteresis v 2.0 reference output voltage 170 200 230 f osc oscillator frequency units min typ max symbol parameter v+ = 5v electrical characteristics (circuit of figure 2, v+ = 5v, 100/200 = 12/15 = 0v; t a = t min to t max , unless otherwise noted.) 10 v+ = 10v
max742 switch-mode regulator with +5v to ?2v or ?5v dual output _______________________________________________________________________________________ 3 note 1: devices are 100% tested to these limits under 0ma to 100ma and to 125ma conditions using automatic test equipment. the ability to drive loads up to 1a is guaranteed by the current-limit threshold, output swing, and the output current source/sink tests. see figures 2 and 3. note 2: actual load capability of the circuit of figure 2 is ?00ma in ?5v mode and ?50ma in ?2v mode. load regulation is tested at lower limits due to test equipment limitations. note 3: guaranteed by design. note 4: measured at point a, circuit of figure 2, with pdrv disconnected. electrical characteristics (continued) (circuit of figure 2, v+ = 5v, 100/200 = 12/15 = 0v; t a = t min to t max , unless otherwise noted.) ext+, ext-, i l = -1ma, v+ = 4.5v, pdrv= -3v ext+, ext-, i l = 1ma, v+ = 4.5v, pdrv= -3v v+ = 3.8v, ss = 2v cc+, cc- v+ = 4.5v, pdrv = -3v, t a = +25? v+ = 4.5v, i l = -5ma, t a = +25? ss = 0v conditions ma -2 -0.5 soft-start sink current ? 37 soft-start source current v -2.8 v ol v 4.3 v oh output voltage high output voltage low ? 190 thermal-shutdown threshold k 10 compensation pin impedance ma 100 200 output sink current 200 350 v -3 pump output voltage (note 4) units min typ max symbol parameter ext+ = 4.5v ext- = 4.5v ext+ = 0v v+ = 4.5v, pdrv = -3v, t a = +25? ext- = -3v ma -200 -100 output source current -350 -200 ext+, c load = 2nf ns 70 output rise/fall time 100 ext-, c load = 4nf, pdrv = -3v
max742 switch-mode regulator with +5v to ?2v or ?5v dual output 4 _______________________________________________________________________________________ __________________________________________typical operating characteristics (circuit of figure 2, v+ = 5v, t a = +25?, unless otherwise noted.) 25 06 undervoltage lockout hysteresis 5 10 20 max742 -1 supply voltage (v) quiescent supply current (ma) 15 4 23 5 1 ?5v mode, 200khz mode lockout enabled -4.5 0 charge-pump load regulation -2.5 -5.0 -3.0 -4.0 max742 -2 charge-pump load current (ma) charge-pump output voltage (v) -3.5 4 23 7 5 6 10 89 1 measured at point a v+ = 5v v+ = 4.5v 5 0 pdrv current vs. c ext- 1 6 2 4 max742 -3 capacitance at ext- (nf) pdrv current (ma) 3 4 23 1 pdrv forced to -4v pump disconnected 200khz 100khz 50 0 efficiency vs. load current, 22w circuit, ?5v mode 60 70 90 max742 -4 load current (ma) efficiency (%) 80 ?00 ?00 ?00 ?000 ?00 100khz 200khz circuit of figure 3, inductors = gowanda 121-at2502 (mpp core), q2 = two irf9z30 in parallel ?5v mode 0 peak inductor current vs. load current 100 200 400 max742 -7 load current (ma) peak inductor current (ma) 300 500 700 600 800 1000 900 1100 1200 200 100 150 50 100khz 200khz measured at lx-, ?5v mode 50 0 efficiency vs. load current, 6w circuit, ?5v mode 60 70 90 max742 -5 load current (ma) efficiency (%) 80 ?00 ?00 ?50 ?50 ?0 100khz 200khz inductors = gowanda 050-at1003 (mpp core) 50 0 efficiency vs. load current, 6w circuit, ?2v mode 60 70 90 max742 -6 load current (ma) efficiency (%) 80 ?00 ?50 ?25 ?5 100khz 200khz inductors = gowanda 050-at1003 (mpp core) 0 current-limit threshold vs. soft-start voltage 50 100 200 max742 -8 soft-start voltage (v) current-limit threshold (mv) 150 3 12
max742 switch-mode regulator with +5v to ?2v or ?5v dual output _______________________________________________________________________________________ 5 a = gate drive, 5v/div b = switch voltage, 10v/div c = switch current, 0.2a/div switching waveforms, inverting section a b c 2 m s/div _____________________________typical operating characteristics (continued) (circuit of figure 2, i load = 100ma, unless otherwise noted.) a = gate drive, 5v/div b = switch voltage, 10v/div c = switch current, 0.2a/div switching waveforms, step-up section a b c 2 m s/div a = noise with i filter, 1mv/div b = noise without filter, 20mv/div measured at -v out v+ = 5v bw = 5mhz output-voltage noise, filtered and unfiltered a b 2 m s/div a = +vo, 20mv/div b = -vo, 50mv/div load-transient response a b 200 m s/div
max742 switch-mode regulator with +5v to ?2v or ?5v dual output 6 _______________________________________________________________________________________ ______________________________________________________________pin description inverting compensation capacitor cc- 9 inverting section feedback input fb- 10 current-sense low (inverting section) csl- 11 current-sense high (inverting section) csh- 12 supply voltage input (+5v) v+ 13 selects oscillator frequency. ground for 200khz, or tie to v+ for 100khz. 100/200 5 selects v out . ground for ?5v, or tie to v+ for ?2v. 12/15 6 reference voltage output (+2.00v). force to gnd or v+ to disable chip. vref 7 soft-start timing capacitor (sources 5?) ss 8 analog supply voltage input (+5v) av+ 4 analog ground agnd 3 pin step-up compensation capacitor cc+ 2 step-up feedback input fb+ 1 function name charge-pump driverclock output at 1/2 oscillator frequency. pump 16 push-pull outputdrives external logic-level n-channel mosfet. ext+ 17 high-current ground gnd 18 current-sense low (step-up section) csl+ 19 current-sense high (step-up section) csh+ 20 push-pull outputdrives external p-channel mosfet. ext- 14 voltage inputnegative supply for p-channel mosfet driver. pdrv 15 ________________operating principle each current-mode controller consists of a summing amplifier that adds three signals: the current waveform from the power switch fet, an output-voltage error sig- nal, and a ramp signal for ac compensation generated by the oscillator. the output of the summing amplifier resets a flip-flop, which in turn activates the power fet driver stage (figure 1). both external transistor switches are synchronized to the oscillator and turn on simultaneously when the flip- flop is set. the switches turn off individually when their source currents reach a trip threshold determined by the output-voltage error signal. this creates a duty- cycle modulated pulse train at the oscillator frequency, where the on time is proportional to both the output- voltage error signal and the peak inductor current. low peak currents or high output-voltage error signals result in a high duty cycle (up to 90% maximum). ac stability is enhanced by the internal ramp signal applied to the error amplifier. this scheme eliminates regenerative ?taircasing?of the inductor current, which is otherwise a problem when in continuous current mode with greater than 50% duty cycle.
_______________detailed description 100khz/200khz oscillator the max742 oscillator frequency is generated without external components and can be set at 100khz or 200khz by pin strapping. operating the device at 100khz results in lower supply current and improved efficiency, particularly with light loads. however, com- ponent stresses increase and noise becomes more dif- ficult to filter. for a given inductor value, the lower operating frequency results in slightly higher peak cur- rents in the inductor and switch transistor (see typical operating characteristics , peak inductor current vs. load current graph). when the lower frequency is used in conjunction with an lc-type output filter (optional components in figure 2), larger component values are required for equivalent filtering. charge-pump voltage inverter the charge-pump (pump) output is a rail-to-rail square wave at half the oscillator frequency. the square wave drives an external diode-capacitor circuit to generate a negative dc voltage (point a in figure 2), which in turn biases the inverting-output drive stage via pdrv. the charge pump thus increases the gate-source voltage applied to the external p-channel fet. the low on- resistance resulting from increased gate drive ensures high efficiency and guarantees start-up under heavy loads. if a -5v to -8v supply is already available, it can be tied directly to pdrv and all of the charge-pump components removed. for input voltages greater than 8v, ground pdrv to prevent overvoltage. observe pdrv absolute maximum ratings. max742 switch-mode regulator with +5v to ?2v or ?5v dual output _______________________________________________________________________________________ 7 max742 pulse square ramp osc 12/15 select vref soft-start and thermal shutdown s rq s to v+ rq ? ? agnd vref 12/15 cc- fb- cc+ fb+ csh+ csl+ v+ ext+ gnd ss pump ext- pdrv csh- csl- av+ 100/200 figure1. max742 detailed block diagram
max742 switch-mode regulator with +5v to ?2v or ?5v dual output 8 _______________________________________________________________________________________ max742 q2 q1 csh+ vin 4.5v to 6v* csl+ 1 j1 c2 3.3nf +vo optional d1 l3 25 m h l1 100 m h c8 150 m f c6 d4 d3 c9 150 m f c14 2.2 m f gnd ext+ pump pdrv ext- v+ csh- csl- c7 1 m f c13 0.1 m f disc ceramic c1 0.1 m f notes: q1 = motorola mtp15n05l q2 = motorola mtp12p05 l1, l2 = maxl001 c8?12 = maxc001 d1, d2 = 1n5817 d3, d4 = fuji era82-004 or 1n5817 r2, r3 = rcd rsf 1a metal film ?% l3, l4 = wilco mfb 250 r3 0.1 w c10 150 m f c3 10 m f r1 100 w c4 c5 3.3nf l2 100 m h -vo point a optional d2 l4 25 m h c11 150 m f c12 150 m f c15 2.2 m f 1 m f fb+ cc+ agnd av+ 100/200 12/15 vref * for higher input voltage, see supply-voltage range section. ss cc- fb- r2 0.16 w figure 2. standard 6w application circuit
max742 switch-mode regulator with +5v to ?2v or ?5v dual output _______________________________________________________________________________________ 9 max742 q2 q1 csh+ vin 4.5v to 6v* csl+ 1 j1 c2 6.8nf +vo d1 1n5820 l1 25 m h r2 0.02 w c8 1000 m f c6 d4 1n914 d3 1n914 c9 1000 m f gnd ext+ pump pdrv ext- v+ csh- csl- c7 1 m f c14 0.1 m f disc ceramic c1 0.1 m f notes: q1 = motorola mtp25n06l q2 = international rectifier irf9z30 l1, l2 = gowanda 121at2502vc r2, r3 = krl lb4-1 ?% c8?13 = nichicon pl series (25v or 35v) r3 0.02 w c10 1000 m f 10v c3 10 m f r1 100 w c4 2.2 m f c5 6.8nf l2 25 m h -vo d2 1n5820 c11 1000 m f c12 1000 m f 1 m f fb+ cc+ agnd av+ 100/200 12/15 vref * for higher input voltage, see supply-voltage range section. ss cc- fb- c13 330 m f figure 3. high-power 22w application circuit
max742 switch-mode regulator with +5v to ?2v or ?5v dual output 10 ______________________________________________________________________________________ supply-voltage range although designed for operation from a +5v logic supply, the max742 works well from 4.2v (the upper limit of the undervoltage lockout threshold) to +10v (absolute maximum rating plus a safety margin). the upper limit can be further increased by limiting the voltage at v+ with a zener shunt or series regulator. to ensure ac stability, the inductor value should be scaled linearly with the nominal input voltage. for example, if figure 3? application circuit is powered from a nominal 9v source, the inductor value should be increased to 40? or 50?. at high input voltages (>8v), the charge pump can cause overvoltage at pdrv. if the input can exceed 8v, ground pdrv and remove the capacitors and diodes associated with the charge pump. in-circuit testing for guaranteed performance figure 2? circuit has been tested at all extremes of line, load, and temperature. refer to the electrical characteristics table for guaranteed in-circuit specifica- tions. successful use of this circuit requires no compo- nent calculations. soft-start a capacitor connected between soft-start (ss) and ground limits surge currents at power-up. as shown in the typical operating characteristics , the peak switch current limit is a function of the voltage at ss. ss is internally connected to a 5? current source and is diode-clamped to 2.6v (figure 8). soft-start timing is therefore set by the ss capacitor value. as the ss volt- age ramps up, peak inductor currents rise until they reach normal operating levels. typical values for the ss capacitor, when it is required at all, are in the range of 1? to 10?. fault conditions enabling ss reset in addition to power-up, the soft-start function is enabled by a variety of fault conditions. any of the following con- ditions will cause an internal pull-down transistor to dis- charge the ss capacitor, triggering a soft-start cycle: undervoltage lockout thermal shutdown vref shorted to ground or supply vref losing regulation __________________design procedure inductor value an exact inductor value isn? critical. the inductor value can be varied in order to make tradeoffs between noise, efficiency, and component sizes. higher inductor values result in continuous-conduction operation, which maximizes efficiency and minimizes noise. physically smallest inductors (where e = 1/2 li 2 is minimum) are realized when operating at the crossover point between continuous and discontinuous modes. lowering the inductor value further still results in discontinuous cur- rent even at full load, which minimizes the output capacitor size required for ac stability by eliminating the right-half-plane zero found in boost and inverting topologies. ideal current-mode slope compensation where m = 2 x v/l is achieved if l (henries) = r sense ( ) x 0.001, but again the exact value isn? critical and the inductor value can be adjusted freely to improve ac performance. the following equations are given for continuous-conduction operation since the max742 is mainly intended for low-noise analog power supplies. see appendix a in maxim? battery management and dc-dc converter circuit collection for crossover point and discontinuous-mode equations. boost (positive) output: (v in - v sw ) 2 (v out + v d - v in ) l = (v out + v d ) 2 (i load )(f)(lir) inverting (negative) output: (v in - v sw ) 2 l = ? (v out + v d )(i load )(f)(lir) max742 n 8 external ss capacitor 5 m a +5v to current? limit comparator fault ss +2v reference figure 4. soft-start equivalent circuit
max742 switch-mode regulator with +5v to ?2v or ?5v dual output ______________________________________________________________________________________ 11 where: v sw is the voltage drop across the the switch transistor and current-sense resistor in the on state (0.3v typ). v d is the rectifier forward voltage drop (0.4v typ). lir is the ratio of peak-to-peak ripple current to dc offset current in the inductor (0.5 typ). current-sense resistor value the current-sense resistor values are calculated accord- ing to the worst-case-low current-limit threshold voltage from the electrical characteristics table and the peak inductor current. the peak inductor current calculations that follow are also useful for sizing the switches and specifying the inductor current saturation ratings. 150mv r sense = i peak i load (v out + v d ) +i peak (boost) = ? + v in - v sw (v in - v sw ) (v out + v d - v in ) (2)(f)(l)(v out + v d ) i load (v out + v d + v in ) +i peak (inverting) = + v in - v sw (v in - v sw ) (v out + v d + v in ) (2)(f)(l) (v out + v d ) filter capacitor value the output filter capacitor values are generally deter- mined by the effective series resistance (esr) and volt- age rating requirements rather than actual capacitance requirements for loop stability. in other words, the capacitor that meets the esr requirement for noise pur- poses nearly always has much more output capaci- tance than is required for ac stability. output voltage noise is dominated by esr and can be roughly calcu- lated by an ohm? law equation: v noise (peak-to-peak) = i peak x r esr where v noise is typically 0.15v. ensure the output capacitors selected meet the follow- ing minimum capacitance requirements: minimum cf = 60? per output or the following, whichev- er is greater: cf = 0.015/r load (in farads, ?5v mode) cf = 0.01/r load (in farads, ?2v mode) compensation capacitor (cc) value the compensation capacitors (cc+ and cc-) cancel the zero introduced by the output filter capacitors?esr, improving phase margin, and ac stability. the com- pensation poles set by cc+ and cc- should be set to match the esr zero frequencies of the output filter capacitors according to the following: r esr x cf cc (in farads) = (use 1000pf minimum) 10k standard 6w application the 6w supply (figure 2) generates ?00ma at ?5v, or ?50ma at ?2v. output capability is increased to 10w or more by heatsinking the power fets, using cores with higher current capability (such as gowanda #050at1003), and using higher filter capacitance. ferrite and mpp inductor cores optimize efficiency and size. iron-power toroids designed for high frequencies are economical, but larger. ripple is directly proportional to filter capacitor equiva- lent series resistance (esr). in addition, about 250mv transient noise occurs at the lx switch transitions. a very short scope probe ground lead or a shielded enclosure is need for making accurate measurements of transient noise. extra filtering, as shown in figure 2, reduces both noise components. high-power 22w application the 22w application circuit (figure 3) generates ?5v at ?50ma or ?2v at ?50ma. noninductive wire- wound resistors with kelvin current-sensing connec- tions replace the metal-film resistors of the previous (6w) circuit. gate drive for the p-channel fet is boot- strapped from the negative supply via diode d6. the 2.7v zener (d5) is required in 15v mode to prevent overvoltage. the charge pump (d3, d4, and c6) may not be necessary if the circuit is lightly loaded (<100ma) on start-up. aie part #415-0963 is a ferrite pot-core inductor that can be used in place of a small- er, more expensive moly-permalloy toroid inductor (l1, l2). higher efficiencies can be achieved by adding extra mosfets in parallel. load levels above 10w make it necessary to add heatsinks, especially to the p- channel fet.
max742 switch-mode regulator with +5v to ?2v or ?5v dual output 12 ______________________________________________________________________________________ table 1. trouble-shooting chart ___________________chip topography gnd ext+ v+ ext- av+ pump pdrv 12/15 100/200 vref agnd cc+ fb+ csh+ csl+ csh- csl- cc- ss fb- 0.135" (3.45mm) 0.080" (2.03mm) transistor count: 375 substrate connected to v+ symptom correction output is unloaded. apply ?0ma or greater load to observe waveform. no switching. ?o are correct, but no waveform is seen at lx+ or lx-. a. check connections. vref should be +2v. b. when input voltage is less than +4.2v, undervoltage lockout is enabled. no output. +vo = 5v or less. -vo = 0v. a. inductor saturation: peak currents exceed coil ratings. b. mosfet on-resistance too high. c. switching losses: diode is slow or has high forward voltage. inductor has high dc resis- tance. excess capacitance at lx nodes. d. inductor core losses: hysteresis losses cause self-heating in some core materials. e. loop instability: see unstable output above. poor efficiency. supply current is high. output will not drive heavy loads. a. input overvoltage: never apply more than +12v. b. fb+ or fb- disconnected or shorted. this causes runaway and output overvoltage. c. cc+ or cc- shorted. d. output filter capacitor disconnected. self-destruction. transistors or ic die on power-up. a. ground noise: probe ground is picking up switching emi. reduce probe ground lead length (use probe tip shield) or put circuit in shielded enclosure. b. poor hf response: add ceramic or tantalum capacitors in parallel with output filter capacitors. noisy output. switching is steady, but large inductive spikes are seen at the outputs. loop stability problem. a. cc+ or cc- disconnected. b. emi: move inductor away from ic or use shielded inductors. keep noise sources away from cc- and cc+. c. grounding: tie agnd directly to the filter capacitor ground lead. ensure that cur- rent spikes from gnd do not cause noise at agnd or compensation capacitor or reference bypass ground leads. use wide pc traces or a ground plane. d. bypass: tie 10? or larger between agnd and vref. use 150? to bypass the input right at av+. if there is high source resis- tance, 1000? or more may be required. e. current limiting: reduce load currents. ensure that inductors are not saturating. f. slope compensation: inductor value not matched to sense resistor. unstable output. noise or jitter on output ripple waveform. scope may not trigger correctly.

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