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_______________general description the MAX1771 step-up switching controller provides 90% efficiency over a 30ma to 2a load. a unique cur- rent-limited pulse-frequency-modulation (pfm) control scheme gives this device the benefits of pulse-width- modulation (pwm) converters (high efficiency at heavy loads), while using less than 110? of supply current (vs. 2ma to 10ma for pwm converters). this controller uses miniature external components. its high switching frequency (up to 300khz) allows sur- face-mount magnetics of 5mm height and 9mm diame- ter. it accepts input voltages from 2v to 16.5v. the output voltage is preset at 12v, or can be adjusted using two resistors. the MAX1771 optimizes efficiency at low input voltages and reduces noise by using a single 100mv current-limit threshold under all load conditions. a family of similar devices, the max770?ax773, trades some full-load efficiency for greater current-limit accuracy; they provide a 200mv current limit at full load, and switch to 100mv for light loads. the MAX1771 drives an external n-channel mosfet switch, allowing it to power loads up to 24w. if less power is required, use the max756/max757 or max761/max762 step-up switching regulators with on-board mosfets. an evaluation kit is available. order the MAX1771evkit-so. ________________________applications positive lcd-bias generators flash memory programmers high-power rf power-amplifier supply palmtops/hand-held terminals battery-powered applications portable communicators ____________________________features ? 90% efficiency for 30ma to 2a load currents ? up to 24w output power ? 110? max supply current ? 5? max shutdown current ? 2v to 16.5v input range ? preset 12v or adjustable output voltage ? current-limited pfm control scheme ? up to 300khz switching frequency ? evaluation kit available ______________ordering information MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller ________________________________________________________________ maxim integrated products 1 1 2 3 4 8 7 6 5 cs gnd agnd ref shdn fb v+ ext top view MAX1771 dip/so __________________pin configuration fb agnd gnd v+ cs ext n ref shdn on/off output 12v input 2v to v out MAX1771 __________typical operating circuit call toll free 1-800-998-8800 for free samples or literature. part temp. range pin-package MAX1771cpa 0? to +70? 8 plastic dip MAX1771csa 0? to +70? 8 so MAX1771c/d 0? to +70? dice* MAX1771epa -40? to +85? 8 plastic dip MAX1771esa -40? to +85? 8 so MAX1771mja -55? to +125? 8 cerdip** 19-0263; rev 1; 7/95 evaluation kit manual follows data sheet * contact factory for dice specifications. ** contact factory for availability and processing to mil-std-883b. .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 2 _______________________________________________________________________________________ absolute maximum ratings supply voltage v+ to gnd ...............................................................-0.3v, 17v ext, cs, ref, shdn, fb to gnd ...................-0.3v, (v+ + 0.3v) gnd to agnd.............................................................0.1v, -0.1v 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 MAX1771c_ a .....................................................0? to +70? MAX1771e_ a ..................................................-40? to +85? MAX1771mja ................................................-55? to +125? junction temperatures MAX1771c_ a/e_ a.......................................................+150? MAX1771mja ..............................................................+175? storage temperature range .............................-65? to +160? lead temperature (soldering, 10sec) .............................+300? 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. electrical characteristics (v+ = 5v, i load = 0ma, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) parameter symbol conditions min typ max units supply current 85 110 m a standby current 25 m a 4 output voltage (note 1) v v+ = 2.0v to 12.0v, over full load range, circuit of figure 2a 11.52 12.0 12.48 v+ = 5v to 7v, v out = 12v i load = 700ma, circuit of figure 2a 5 mv/v v+ = 6v, v out = 12v, i load = 0ma to 500ma, circuit of figure 2a 20 mv/a maximum switch on-time t on (max) 12 16 20 m s minimum switch off-time t off (min) 1.8 2.3 2.8 m s % reference voltage v ref i ref = 0 m a MAX1771c 1.4700 1.5 1.5300 v MAX1771e 1.4625 1.5 1.5375 MAX1771m 1.4550 1.5 1.5450 output voltage line regulation (note 2) output voltage load regulation (note 2) v+ = 5v, v out = 12v, i load = 500ma, circuit of figure 2a v+ = 10.0v, shdn 3 1.6v (shutdown) v+ = 16.5v, shdn 3 1.6v (shutdown) v+ = 16.5v, shdn = 0v (normal operation) minimum start-up voltage 1.8 2.0 v MAX1771 (internal feedback resistors) 2.0 12.5 MAX1771c/e (external resistors) 3.0 16.5 MAX1771mja (external resistors) 3.1 16.5 input voltage range v efficiency 92 ref load regulation 0? i ref 100? MAX1771c/e 410 mv MAX1771m 415 3v v+ 16.5v 40 100 fb trip point voltage v fb MAX1771c 1.4700 1.5 1.5300 v MAX1771e 1.4625 1.5 1.5375 MAX1771m 1.4550 1.5 1.5450 ?/v ref line regulation .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller _______________________________________________________________________________________ 3 electrical characteristics (continued) (v+ = 5v, i load = 0ma, t a = t min to t max , unless otherwise noted. typical values are at t a = +25?.) parameters symbol conditions min typ max units fb input current i fb MAX1771c ?0 na MAX1771e ?0 MAX1771m ?0 shdn input high voltage v ih v+ = 2.0v to 16.5v 1.6 v shdn input low voltage v il v+ = 2.0v to 16.5v 0.4 v shdn input current ? m a v+ = 16.5v, shdn = 0v or v+ current-limit trip level v cs v+ = 5v to 16v 85 100 115 mv cs input current 0.01 ? m a ext rise time v+ = 5v, 1nf from ext to ground 55 ns ext fall time v+ = 5v, 1nf from ext to ground 55 ns note 1: output voltage guaranteed using preset voltages. see figures 4a?d for output current capability versus input voltage. note 2: output voltage line and load regulation depend on external circuit components. 75 100 125 __________________________________________typical operating characteristics (t a = +25?, unless otherwise noted.) 60 65 75 70 80 85 90 95 100 1 10 100 10,000 1000 efficiency vs. load current (bootstraped mode) load current (ma) efficiency ( % ) v in = 5v v in = 3v v in = 8v v in = 10v v out = 12v circuit of figure 2a MAX1771?1 60 65 75 70 80 85 90 95 100 1 10 100 10,000 1000 efficiency vs. load current (non-bootstraped mode) load current (ma) efficiency ( % ) v in = 5v v in =10v v in = 8v v out = 12v circuit of figure 2b MAX1771?2 0 2.00 load current vs. minimum start-up input voltage MAX1771-toc3 minimum start-up input voltage (v) load current (ma) 100 200 300 400 500 600 700 2.25 2.50 2.75 3.00 3.25 3.50 external fet threshold limits full-load start-up below 3.5v v out = 12v, circuit of figure 2a MAX1771c/e MAX1771m .com .com .com .com 4 .com u datasheet
250 0 -60 -20 60 140 reference output resistance vs. temperature 50 MAX1771-07 temperature ( c) reference output resistance ( w ) 20 100 150 -40 0 80 40 120 100 200 100? 50? 10? 1.502 -60 -20 60 140 reference vs. temperature MAX1771-08 temperature ( c) reference (v) 20 100 -40 0 80 40 120 1.500 1.498 1.496 1.494 1.492 1.504 1.506 15.5 16.0 16.5 -60 -30 0 30 60 90 120 150 maximum switch on-time vs. temperature temperature (?) t on(max) (?) MAX1771-09 MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 4 _______________________________________________________________________________________ 4.0 -60 -20 60 140 shutdown current vs. temperature MAX1771-10 temperature ( c) shutdown current (?) 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 2.20 2.25 2.30 -60 -30 0 30 60 90 120 150 minimum switch off-time vs. temperature temperature (?) t off(min) (?) MAX1771-11 6.0 7.0 8.0 -60 -30 0 30 60 90 120 150 maximum switch on-time/ minimum switch off-time ratio vs. temperature temperature (?) t on(max)/ t off(min) ratio MAX1771-12 7.5 6.5 ____________________________typical operating characteristics (continued) (t a = +25?, unless otherwise noted.) 0 1 2 3 4 -75 -50 -25 0 25 50 75 100 125 supply current vs. temperature temperature (?) supply current (ma) v out = 12v, v in = 5v circuit of figure 2a bootstrapped mode entire circuit schottky diode leakage excluded MAX1771-04 0 0.2 0.4 0.6 0.8 2 4 68 10 12 supply current vs. supply voltage supply voltage (v) supply current (ma) v out = 12v non-bootstrapped circuit of figure 2b bootstrapped circuit of figure 2a MAX1771-05 0 100 150 200 250 50 2 4 68 10 12 ext rise/fall time vs. supply voltage supply voltage (v) ext rise/fall time (ns) c ext = 2200pf c ext = 1000pf c ext = 446pf c ext = 100pf MAX1771-06 .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller _______________________________________________________________________________________ 5 2?/div v in = 5v, i out = 900ma, v out = 12v a: ext voltage, 10v/div b: inductor current, 1a/div c: v out ripple, 50mv/div, ac-coupled a b c v out 0v i lim 0a heavy-load switching waveforms ____________________________typical operating characteristics (continued) (circuit of figure 2a, t a = +25?, unless otherwise noted.) 10?/div v in = 5v, i out = 500ma, v out = 12v a: ext voltage, 10v/div b: inductor current, 1a/div c: v out ripple, 50mv/div, ac-coupled a b c v out 0v i lim 0a medium-load switching waveforms 5ms/div i out = 700ma, v out = 12v a: v in , 5v to 7v, 2v/div b: v out ripple, 100mv/div, ac-coupled a b 5v 7v 0v line-transient response 5ms/div v in = 6v, v out = 12v a: load current, 0ma to 500ma, 500ma/div b: v out ripple, 100mv/div, ac-coupled a b 500ma 0a load-transient response .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 6 _______________________________________________________________________________________ 2ms/div i out = 500ma, v in = 5v a: shdn, 5v/div b: v out , 5v/div a b 0v 0v entering/exiting shutdown 5v ______________________________________________________________pin description ____________________________typical operating characteristics (continued) (circuit of figure 2a, t a = +25?, unless otherwise noted.) pin name function 1 ext gate drive for external n-channel power transistor 2 v+ 3 fb 4 shdn 5 ref 6 agnd analog ground 7 gnd high-current ground return for the output driver 8 cs power-supply input. also acts as a voltage-sense point when in bootstrapped mode. feedback input for adjustable-output operation. connect to ground for fixed-output operation. use a resistor divider network to adjust the output voltage. see setting the output voltage section. active-high ttl/cmos logic-level shutdown input. in shutdown mode, v out is a diode drop below v+ (due to the dc path from v+ to the output) and the supply current drops to 5 m a maximum. connect to ground for normal operation. 1.5v reference output that can source 100 m a for external loads. bypass to gnd with 0.1 m f. the reference is disabled in shutdown. positive input to the current-sense amplifier. connect the current-sense resistor between cs and gnd. .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller _______________________________________________________________________________________ 7 _______________detailed description the MAX1771 is a bicmos, step-up, switch-mode pow- er-supply controller that provides a preset 12v output, in addition to adjustable-output operation. its unique control scheme combines the advantages of pulse-fre- quency modulation (low supply current) and pulse- width modulation (high efficiency with heavy loads), providing high efficiency over a wide output current range, as well as increased output current capability over previous pfm devices. in addition, the external sense resistor and power transistor allow the user to tai- lor the output current capability for each application. figure 1 shows the MAX1771 functional diagram. the MAX1771 offers three main improvements over prior pulse-skipping control solutions: 1) the converter operates with miniature (5mm height and less than 9mm diameter) surface-mount inductors due to its 300khz switching frequency; 2) the current-limited pfm control scheme allows 90% efficiencies over a wide range of load currents; and 3) the maximum supply current is only 110?. the device has a shutdown mode that reduces the supply current to 5? max. bootstrapped/non-bootstrapped modes figure 2 shows the standard application circuits for bootstrapped and non-bootstrapped modes. in boot- strapped mode, the ic is powered from the output (v out , which is connected to v+) and the input voltage range is 2v to v out . the voltage applied to the gate of the external power transistor is switched from v out to ground, providing more switch gate drive and thus reducing the transistor? on-resistance. in non-bootstrapped mode, the ic is powered from the input voltage (v+) and operates with minimum supply current. in this mode, fb is the output voltage sense point. since the voltage swing applied to the gate of the external power transistor is reduced (the gate swings from v+ to ground), the power transistor? on-resistance 1.5v reference q trig q s f/f r q trig low-voltage oscillator 2.5v 0.1v max on-time one-shot min off-time one-shot current-sense amplifier dual-mode comparator fb ref 50mv error comparator shdn v+ ext cs bias circuitry n MAX1771 2.3 m s 16 m s figure 1. functional diagram .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 8 _______________________________________________________________________________________ increases at low input voltages. however, the supply current is also reduced because v+ is at a lower volt- age, and because less energy is consumed while charging and discharging the external mosfet? gate capacitance. the minimum input voltage is 3v when using external feedback resistors. with supply voltages below 5v, bootstrapped mode is recommended. note: when using the MAX1771 in non-boot- strapped mode, there is no preset output operation because v+ is also the output voltage sense point for fixed-output operation. external resistors must be used to set the output voltage. use 1% external feedback resistors when operating in adjustable-output mode (figures 2b, 2c) to achieve an overall output volt- age accuracy of ?%. to achieve highest efficiency, operate in bootstrapped mode whenever possible. external power-transistor control circuitry pfm control scheme the MAX1771 uses a proprietary current-limited pfm control scheme to provide high efficiency over a wide range of load currents. this control scheme combines the ultra-low supply current of pfm converters (or pulse skip- pers) with the high full-load efficiency of pwm converters. unlike traditional pfm converters, the MAX1771 uses a sense resistor to control the peak inductor current. the device also operates with high switching frequencies (up to 300khz), allowing the use of miniature external components. as with traditional pfm converters, the power transistor is not turned on until the voltage comparator senses the output is out of regulation. however, unlike tradition- al pfm converters, the MAX1771 switch uses the com- bination of a peak current limit and a pair of one-shots that set the maximum on-time (16?) and minimum off- time (2.3?); there is no oscillator. once off, the mini- mum 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. figure 2a. 12v preset output, bootstrapped figure 2b. 12v output, non-bootstrapped figure 2c. 9v output, bootstrapped MAX1771 v in = 5v ref shdn agnd gnd n mtd20n03hdl 7 ext cs fb l1 22 m h d1 1n5817-22 r1 18k c4 300 m f c5 100pf c3 0.1 m f 5 4 6 1 8 3 2 v+ c1 68 m f v out = 12v @ 0.5a r2 127k r sense 40m w c2 0.1 m f v out v ref r2 = (r1) ( -1 ) v ref = 1.5v MAX1771 ref shdn agnd gnd n 7 ext cs fb c1 47 m f l1 22 m h d1 1n5817-22 r1 28k c4 200 m f c5 100pf c3 0.1 m f 5 4 6 1 8 3 2 v+ v out = 9v r2 140k r sense 40m w c2 0.1 m f v out v ref r2 = (r1) ( -1 ) v ref = 1.5v si9410dy/ mtd20n03hdl v in = 4v MAX1771 v in = 5v ref shdn fb agnd gnd n 7 ext cs c2 0.1 m f c1 68 m f l1 22 m h d1 1n5817-22 si9410dy/ mtd20n03hdl r sense 40m w c4 300 m f c3 0.1 m f 5 4 3 6 1 8 2 v+ v out = 12v @ 0.5a .com .com .com .com 4 .com u datasheet
the control circuitry allows the ic to operate in continu- ous-conduction mode (ccm) while maintaining high efficiency with heavy loads. when the power switch is turned on, it stays on until either 1) the maximum on- time one-shot turns it off (typically 16? later), or 2) the switch current reaches the peak current limit set by the current-sense resistor. the MAX1771 switching frequency is variable (depend- ing on load current and input voltage), causing variable switching noise. however, the subharmonic noise gen- erated does not exceed the peak current limit times the filter capacitor equivalent series resistance (esr). for example, when generating a 12v output at 500ma from a 5v input, only 100mv of output ripple occurs using the circuit of figure 2a. low-voltage start-up oscillator the MAX1771 features a low input voltage start-up oscil- lator that guarantees start-up with no load down to 2v when operating in bootstrapped mode and using inter- nal feedback resistors. at these low voltages, the supply voltage is not large enough for proper error-comparator operation and internal biasing. the start-up oscillator has a fixed 50% duty cycle and the MAX1771 disre- gards the error-comparator output when the supply volt- age is less than 2.5v. above 2.5v, the error-comparator and normal one-shot timing circuitry are used. the low- voltage start-up circuitry is disabled if non-bootstrapped mode is selected (fb is not tied to ground). shutdown mode when shdn is high, the MAX1771 enters shutdown mode. in this mode, the internal biasing circuitry is turned off (including the reference) and v out falls to a diode drop below v in (due to the dc path from the input to the output). in shutdown mode, the supply current drops to less than 5a. shdn is a ttl/cmos logic-level input. connect shdn to gnd for normal operation. __________________design procedure setting the output voltage to set the output voltage, first determine the mode of operation, either bootstrapped or non-bootstrapped. bootstrapped mode provides more output current capability, while non-bootstrapped mode reduces the supply current (see typical operating characteristics ). if a decaying voltage source (such as a battery) is used, see the additional notes in the low input voltage operation section. the MAX1771? output voltage can be adjusted from very high voltages down to 3v, using external resistors r1 and r2 configured as shown in figure 3. for adjustable-output operation, select feedback resistor r1 in the 10k w to 500k range. r2 is given by: v out r2 = (r1) ( -1 ) v ref where v ref equals 1.5v. for preset-output operation, tie fb to gnd (this forces bootstrapped-mode operation. figure 2 shows various circuit configurations for boot- strapped/non-bootstrapped, preset/adjustable operation. determining r sense use the theoretical output current curves shown in figures 4a?d to select r sense . they were derived using the minimum (worst-case) current-limit compara- tor threshold value over the extended temperature range (-40? to +85?). no tolerance was included for r sense . the voltage drop across the diode was assumed to be 0.5v, and the drop across the power switch r ds(on) and coil resistance was assumed to be 0.3v. determining the inductor (l) practical inductor values range from 10? to 300?. 22? is a good choice for most applications. in appli- cations with large input/output differentials, the ic? output current capability will be much less when the inductance value is too low, because the ic will always operate in discontinuous mode. if the inductor value is too low, the current will ramp up to a high level before the current-limit comparator can turn off the switch. the minimum on-time for the switch (t on (min)) is MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller _______________________________________________________________________________________ 9 MAX1771 r1 r2 c5* gnd fb v out r1 = 10k to 500k * see text for value v out v ref r2 = r1 ( -1 ) v ref = 1.5v figure 3. adjustable output circuit .com .com .com .com 4 .com u datasheet
MAX1771 approximately 2?; select an inductor that allows the cur- rent to ramp up to i lim . the standard operating circuits use a 22? inductor. if a different inductance value is desired, select l such that: v in (max) x 2? l 3 ?---?- i lim larger inductance values tend to increase the start-up time slightly, while smaller inductance values allow the coil current to ramp up to higher levels before the switch turns off, increasing the ripple at light loads. inductors with a ferrite core or equivalent are recom- mended; powder iron cores are not recommended for use with high switching frequencies. make sure the inductor? saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current rating set by r sense . however, it is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). for highest efficiency, use a coil with low dc resistance, preferably under 20m . to minimize radiated noise, use a toroid, a pot core, or a shielded coil. table 1 lists inductor suppliers and specific recom- mended inductors. 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 10 ______________________________________________________________________________________ maximum output current (a) 0 input voltage (v) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2345 r sense = 20m w r sense = 25m w r sense = 35m w r sense = 100m w r sense = 50m w v out = 5v l = 22 m h maximum output current (a) 0 input voltage (v) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2 4 6 8 10 12 r sense = 100m w r sense = 50m w r sense = 20m w r sense = 25m w r sense = 35m w v out = 12v l = 22 m h figure 4a. maximum output current vs. input voltage (v out = 5v) figure 4b. maximum output current vs. input voltage (v out = 12v) figure 4c. maximum output current vs. input voltage (v out = 15v) figure 4d. maximum output current vs. input voltage (v out = 24v) maximum output current (a) 0 input voltage (v) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 2 4 6 8 10 12 14 16 r sense = 100m w r sense = 50m w v out = 15v l = 22 m h r sense = 20m w r sense = 25m w r sense = 35m w maximum output current (a) 0 2 input voltage (v) 0.8 61014 0.2 0.4 0.6 v out = 24v l =150 m h r sense = 50m w r sense = 100m w r sense = 200m w .com .com .com .com 4 .com u datasheet
power transistor selection use an n-channel mosfet power transistor with the MAX1771. to ensure the external n-channel mosfet (n-fet) is turned on hard, use logic-level or low-threshold n-fets when the input drive voltage is less than 8v. this applies even in bootstrapped mode, to ensure start-up. n-fets provide the highest efficiency because they do not draw any dc gate-drive current. when selecting an n-fet, three important parameters are the total gate charge (q g ), on-resistance (r ds(on) ), and reverse transfer capacitance (c rss ). q g takes into account all capacitances associated with charging the gate. use the typical q g value for best results; the maximum value is usually grossly over- specified since it is a guaranteed limit and not the mea- sured value. the typical total gate charge should be 50nc or less. with larger numbers, the ext pins may not be able to adequately drive the gate. the ext rise/fall time varies with different capacitive loads as shown in the typical operating characteristics . the two most significant losses contributing to the n-fet? power dissipation are i 2 r losses and switching losses. select a transistor with low r ds(on) and low c rss to minimize these losses. determine the maximum required gate-drive current from the q g specification in the n-fet data sheet. the MAX1771? maximum allowed switching frequency during normal operation is 300khz; but at start-up, the maximum frequency can be 500khz, so the maximum current required to charge the n-fet? gate is f(max) x q g (typ). use the typical q g number from the transistor data sheet. for example, the si9410dy has a q g (typ) of 17nc (at v gs = 5v), therefore the current required to charge the gate is: i gate (max) = (500khz) (17nc) = 8.5ma. the bypass capacitor on v+ (c2) must instantaneously furnish the gate charge without excessive droop (e.g., less than 200mv): q g d v+ = c2 continuing with the example, d v+ = 17nc/0.1? = 170mv. figure 2a? application circuit uses an 8-pin si9410dy surface-mount n-fet that has 50m on-resistance with 4.5v v gs , and a guaranteed v th of less than 3v. figure 2b? application circuit uses an mtd20n03hdl logic- level n-fet with a guaranteed threshold voltage (v th ) of 2v. diode selection the MAX1771? high switching frequency demands a high-speed rectifier. schottky diodes such as the 1n5817?n5822 are recommended. make sure the schottky diode? average current rating exceeds the peak current limit set by r sense , and that its break- down voltage exceeds v out . for high-temperature applications, schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes such as the mur105 or ec11fs1 can be used instead. at heavy loads and high temperatures, the benefits of a schottky diode? low forward voltage may outweigh the disadvantages of its high leakage current. capacitor selection output filter capacitor the primary criterion for selecting the output filter capac- itor (c4) is low effective series resistance (esr). the product of the peak inductor current and the output filter capacitor? esr determines the amplitude of the ripple seen on the output voltage. two os-con 150?, 16v output filter capacitors in parallel with 35m of esr each typically provide 75mv ripple when stepping up from 5v to 12v at 500ma (figure 2a). smaller-value and/or high- er-esr capacitors are acceptable for light loads or in applications that can tolerate higher output ripple. since the output filter capacitor? esr affects efficien- cy, use low-esr capacitors for best performance. see table 1 for component selection. input bypass capacitors the input bypass capacitor (c1) reduces peak currents drawn from the voltage source and also reduces noise at the voltage source caused by the switching action of the MAX1771. 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. for output currents up to 1a, 68? (c1) is adequate, although smaller bypass capacitors may also be acceptable. bypass the ic with a 0.1? ceramic capacitor (c2) placed as close to the v+ and gnd pins as possible. reference capacitor bypass ref with a 0.1? capacitor (c3). ref can source up to 100? of current for external loads. feed-forward capacitor in adjustable output voltage and non-bootstrapped modes, parallel a 47pf to 220pf capacitor across r2, as shown in figures 2 and 3. choose the lowest capac- itor value that insures stability; high capacitance values may degrade line regulation. MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller ______________________________________________________________________________________ 11 .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 12 ______________________________________________________________________________________ table 1. component suppliers production inductors capacitors transistors surface mount sumida cd54 series cdr125 series coiltronics ctx20 series coilcraft do3316 series do3340 series matsuo 267 series sprague 595d series avx tps series siliconix si9410dy si9420dy (high voltage) motorola mtp3055el mtd20n03hdl mmft3055elt1 mtd6n1o mmbt8099lt1 mmbt8599lt1 through hole sumida rch855 series rch110 series sanyo os-con series nichicon pl series motorola 1n5817?n5822 mur115 (high voltage) mur105 (high-speed silicon) central semiconductor cmpsh-3 cmpz5240 nihon ec11 fs1 series (high- speed silicon) motorola mbrs1100t3 mmbz5240bl diodes avx usa: (803) 448-9411 (803) 448-1943 supplier phone fax coiltronics usa: (407) 241-7876 (407) 241-9339 sumida usa: (708) 956-0666 (708) 956-0702 japan: 81-3-3607-5111 81-3-3607-5144 matsuo usa: (714) 969-2491 (714) 960-6492 japan: 81-6-337-6450 81-6-337-6456 coilcraft usa: (708) 639-6400 (708) 639-1469 motorola usa: (800) 521-6274 (602) 952-4190 central semiconductor usa: (516) 435-1110 (516) 435-1824 nihon usa: (805) 867-2555 (805) 867-2556 sanyo usa: (619) 661-6835 (619) 661-1055 japan: 81-7-2070-1005 81-7-2070-1174 siliconix usa: (800) 554-5565 (408) 970-3950 sprague usa: (603) 224-1961 (603) 224-1430 nichicon usa: (708) 843-7500 (708) 843-2798 .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller ______________________________________________________________________________________ 13 __________applications information low input voltage operation when using a power supply that decays with time (such as a battery), the n-fet transistor will operate in its linear region when the voltage at ext approaches the threshold voltage of the fet, dissipating excessive power. prolonged operation in this mode may damage the fet. this effect is much more significant in non- bootstrapped mode than in bootstrapped mode, since bootstrapped mode typically provides much higher v gs voltages. to avoid this condition, make sure v ext is above the v th of the fet, or use a voltage detector (such as the max8211) to put the ic in shutdown mode once the input supply voltage falls below a predeter- mined minimum value. excessive loads with low input voltages can also cause this condition. starting up under load the typical operating characteristics show the start- up voltage vs. load current graph for bootstrapped- mode operation. this graph depends on the type of power switch used. the MAX1771 is not designed to start up under full load in bootstrapped mode with low input voltages. layout considerations due to high current levels and fast switching wave- forms, which radiate noise, proper pc board layout is essential. protect sensitive analog grounds by using a star ground configuration. minimize ground noise by connecting gnd, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). also, minimize lead lengths to reduce stray capacitance, trace resis- tance, and radiated noise. place input bypass capaci- tor c2 as close as possible to v+ and gnd. excessive noise at the v+ input may falsely trigger the timing circuitry, resulting in short pulses at ext. if this occurs it will have a negligible effect on circuit efficien- cy. if desired, place a 4.7? directly across the v+ and gnd pins (in parallel with the 0.1? c2 bypass capaci- tor) to reduce the noise at v+. other application circuits 4 cells to 5v (or 3 cells to 3.3v), 500ma step-up/down converter the circuit shown in figure 5 generates 5v (or 3.3v) at 500ma with 85% efficiency, from an input voltage that varies above and below the output. the output couples to the switching circuitry via a capacitor. this configu- ration offers two advantages over flyback-transformer and step-up linear-regulator circuits: smooth regulation as the input passes through the output, and no output current in shutdown. this circuit requires two inductors, which can be wound on one core with no regard to coupling since they do not work as a transformer. l1 and l2 can either be wound together (as with the coiltronics ctx20-4) or kept as two separate inductors; both methods provide equal performance. capacitors c2 and c3 should be low-esr types for best efficiency. a 1f ceramic capacitor will work at c2, but with about 3% efficiency loss. c2? voltage rating must be greater than the maxi- mum input voltage. also note that the lx switch must withstand a voltage equal to the sum of the input and output voltage; for example, when converting 11v to 5v, the switch must withstand 16v. lx switch pulses are captured by schottky diode d2 to boost v+ to (v out + v in ). this improves efficiency with a low input voltage, but also limits the maximum input supply to 11v. if the input voltage does not fall below 4v and if a 3v logic threshold fet is used for q1, you may omit d2 and connect v+ directly to the input supply. 12v output buck/boost the circuit in figure 6 generates 12v from a 4.5v to 16v input. higher input voltages are possible if you MAX1771 shdn r1 0.1 w ref agnd r2 ? r3 ? c5 47pf gnd 4 3v = off 5 fb q1** see text for further component info * * v in may be lower than indicated if the supply is not ** required to start under full load **motorola mmft3055elt1 ? for 5v: r2 = 200k w , r3 = 470k w 3.3v: r2 = 100k w , r3 = 20k w 3 6 ext cs 1 l1 20 m h 1 ctx20-4 8 2 d2 1n5817 d1 1n5817 c3 220 m f 10v c1 2.2 m f c2 47 m f 16v c4 0.1 m f v+ v in * 3v to 11v v out 5v 500ma 7 l2 figure 5. step-up/down for a 5v/3.3v output .com .com .com .com 4 .com u datasheet
MAX1771 carefully observe the component voltage ratings, since some components must withstand the sum of the input and output voltage (27v in this case). the circuit oper- ates as an ac-coupled boost converter, and does not change operating modes when crossing from buck to boost. there is no instability around a 12v input. efficiency ranges from 85% at medium loads to about 82% at full load. also, when shutdown is activated (shdn high) the output goes to 0v and sources no cur- rent. a 1? ceramic capacitor is used for c2. a larger capacitor value improves efficiency by about 1% to 3%. d2 ensures start-up for this ac-coupled configuration by overriding the MAX1771? dual-mode feature, which allows the use of preset internal or user-set external feedback. when operating in dual-mode, the ic first tries to use internal feedback and looks to v+ for its feedback signal. however, since v+ may be greater than the internally set feedback (12v for the MAX1771), the ic may think the output is sufficiently high and not start. d2 ensures start-up by pulling fb above ground and forcing the external feedback mode. in a normal (not ac-coupled) boost circuit, d2 isn? needed, since the output and fb rise as soon as input power is applied. transformerless -48v to +5v at 300ma the circuit in figure 7 uses a transformerless design to supply 5v at 300ma from a -30v to -75v input supply. the MAX1771 is biased such that its ground connec- tions are made to the -48v input. the ic? supply volt- age (at v+) is set to about 9.4v (with respect to -48v) by a zener-biased emitter follower (q2). an n-channel fet (q1) is driven in a boost configuration. output reg- ulation is achieved by a transistor (q3), which level shifts a feedback signal from the 5v output to the ic? fb input. conversion efficiency is typically 82%. when selecting components, be sure that d1, q1, q2, q3, and c6 are rated for the full input voltage plus a reasonable safety margin. also, if d1 is substituted, it should be a fast-recovery type with a t rr less than 30ns. r7, r9, c8, and d3 are optional and may be used to soft start the circuit to prevent excessive current surges at power-up. battery-powered lcd bias supply the circuit in figure 8 boosts two cells (2v min) to 24v for lcd bias or other positive output applications. output power is boosted from the battery input, while v+ voltage for the MAX1771 is supplied by a 5v or 3.3v logic supply. 5v, 1a boost converter the circuit in figure 9 boosts a 2.7v to 5.5v input to a regulated 5v, 1a output for logic, rf power, or pcmcia applications. efficiency vs. load current is shown in the adjacent graph. 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 14 ______________________________________________________________________________________ MAX1771 shdn r1 0.1 w ref agnd r2 200k 1% r3 28k 1% gnd 4 on off 5 fb note: high- current gnd q1** *see text for further component information **q1 = motorola mmft3055elt1 ? l1 + l2 = one coiltronics ctx20-4 3 6 ext cs 1 l1 ? 20 m h 8 2 d1 1n5819 d2* 1n4148 c3 100 m f 16v c1 33 m f 16v c2* 1 m f c5 0.1 m f v+ v in 4.5v to 15v v out 12v 250ma l2* 20 m h 7 c4 100 m f 16v note: keep all traces connected to pin 3 as short as possible figure 6. 12v buck/boost from a 4.5v to 15v input .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller ______________________________________________________________________________________ 15 MAX1771 shdn r1 0.15 w r7 200 w ref agnd gnd 4 5 q1 mtd6n10 6 -48v ext cs 1 l1 d03340 220 m h?80 m h 8 3 1 2 d1 mbrs1100t3 d2 cmpz5240/ mmbz5240bl d3 cmpsh-3 c8 1 m f +5v 300ma 7 c5 0.1 m f c6 10 m f 100v fb 3 v+ q3 mmbt8599lt1 2 c7 220pf r2 47k 1% r3 16k 1% c4 2.2 m f 20v r5 1k r6 200k r4 100k c1 220 m f 10v c3 0.33 m f q2 mmbt8099lt1 r9 5.1k c2 220 m f 10v gnd ref MAX1771 v+ extl cs 4 shdn 0.1 m f 3.3v or 5v logic supply battery input 2v to 12v 8 6, 7 5 r sense 0.2 w 1n5817 1 2 l1 22 m h output adj. = 12v to 24v (as shown) n 47 m f 0.1 m f fb 3 r3 10k 10k r2 150k mmft3055elt1 off on figure 7. -48v input to 5v output at 300ma, without a transformer figure 8. 2v input to 24v output lcd bias .com .com .com .com 4 .com u datasheet
MAX1771 12v or adjustable, high-efficiency, low i q , step-up dc-dc controller 16 ______________________________________________________________________________________ ___________________chip topography v+ fb 0.126" (3.200mm) 0.080" (2.032mm) ext cs gnd agnd shdn ref transistor count: 501 substrate connected to v+ 100 50 1m 10m 100m 1 efficiency vs. load current 60 load current (a) efficiency (%) 70 80 90 v in = 3v MAX1771 gnd 76 1n5820 330 m f 0.1 m f 4 5 1 8 2 3 output 5v 1a 0.1 m f 232k 100k 0.04 w 150 m f 22 m h mtd20n03hdl input 2.7v to 5.5v cs ext v+ fb 100pf agnd shdn ref off on v in = 4v figure 9. 5v/1a boost converter .com .com .com 4 .com u datasheet

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