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LT3971 1 3971f typical application description 38v, 1.2a, 2mhz step-down regulator with 2.8a quiescent current the lt ? 3971 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 38v. low quiescent current design consumes only 2.8a of supply current while regulating with no load. low ripple burst mode operation maintains high ef? ciency at low output currents while keeping the output ripple below 15mv in a typical application. an internally compensated current mode topology is used for fast transient response and good loop stability. a high ef? ciency 0.33 switch is included on the die along with a boost schottky diode and the necessary oscillator, control and logic circuitry. an accurate 1v threshold enable pin can be used to shut down the LT3971, reducing the input supply current to 700na. a capacitor on the ss pin provides a controlled inrush current (soft-start). a power good ? ag signals when v out reaches 91% of the programmed output volt- age. the LT3971 is available in small 10-pin msop and 3mm 3mm dfn packages with exposed pads for low thermal resistance. l , lt, ltc, ltm, linear technology, the linear logo and burst mode are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. 3.3v step down converter features applications n ultralow quiescent current: 2.8a i q regulating 12v in to 3.3v out n low ripple burst mode ? operation: output ripple < 15mv p-p n wide input voltage range: 4.3v to 38v n 1.2a maximum output current n adjustable switching frequency: 200khz to 2mhz n synchronizable between 250khz to 2mhz n fast transient response n accurate 1v enable pin threshold n low shutdown current: i q = 700na n power good flag n soft-start capability n internal compensation n saturating switch design: 0.33 on-resistance n output voltage: 1.19v to 30v n small thermally enhanced 10-pin msop package and (3mm 3mm) dfn packages n automotive battery regulation n power for portable products n industrial supplies no load supply current sw fb ss rt v in v in 4.5v to 38v v out 3.3v 1.2a 4.7f 0.47f 22f 1m 49.9k 4.7h 1.78m gnd bd sync off on 10pf LT3971 3480 ta01 en boost pg input voltage (v) 0 input current (a) 1.0 10 20 30 40 3971 ta01b 2.0 1.5 4.0 3.5 3.0 2.5
LT3971 2 3971f absolute maximum ratings v in , en voltage .........................................................38v boost pin voltage ...................................................55v boost pin above sw pin .........................................30v fb, rt, sync, ss voltage ...........................................6v pg, bd voltage .........................................................30v boost diode current....................................................1a (note 1) top view dd package 10-lead (3mm s 3mm) plastic dfn 10 11 gnd 9 6 7 8 4 5 3 2 1 sync pg rt ss fb bd boost sw v in en ja = 45c, jc = 10c/w exposed pad (pin 11) is gnd, must be soldered to pcb 1 2 3 4 5 bd boost sw v in en 10 9 8 7 6 11 gnd sync pg rt ss fb top view mse package 10-lead plastic msop ja = 45c, jc = 10c/w exposed pad (pin 11) is gnd, must be soldered to pcb pin configuration order information lead free finish tape and reel part marking* package description temperature range LT3971edd#pbf LT3971edd#trpbf lfjf 10-lead (3mm 3mm) plastic dfn C40c to 125c LT3971idd#pbf LT3971idd#trpbf lfjf 10-lead (3mm 3mm) plastic dfn C40c to 125c LT3971emse#pbf LT3971emse#trpbf ltfjg 10-lead plastic msop C40c to 125c LT3971imse#pbf LT3971imse#trpbf ltfjg 10-lead plastic msop C40c to 125c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ operating junction temperature range (note 2) LT3971e ............................................. C40c to 125c LT3971i .............................................. C40c to 125c storage temperature range .............. C65c to 150c lead temperature (soldering, 10 sec) (mse only) ....................................................... 300c
LT3971 3 3971f electrical characteristics note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the LT3971e is guaranteed to meet performance speci? cations from 0c to 125c junction temperature. speci? cations over the C40c to 125c operating junction temperature range are assured by design, the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 12v, v en = 12v, v bd = 3.3v unless otherwise noted. (note 2) characterization, and correlation with statistical process controls. the LT3971i is guaranteed over the full C40c to 125c operating junction temperature range. high junction temperatures degrade operating lifetimes. operating lifetime is derated at junction temperatures greater than 125c. note 3: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. parameter conditions min typ max units minimum input voltage l 4 4.3 v quiescent current from v in v en low v en high, v sync low v en high, v sync low l 0.7 1.7 1.2 2.7 4.5 a a a fb pin current v fb = 1.19v l 0.1 12 na feedback voltage l 1.175 1.165 1.19 1.19 1.205 1.215 v v fb voltage line regulation 4.3v < v in < 40v 0.0002 0.01 %/v switching frequency r t = 11k r t = 35.7k r t = 255k 1.6 0.8 160 2 1 200 2.4 1.2 240 mhz mhz khz minimum switch on time 80 ns minimum switch off time 110 150 ns switch current limit 1.8 2.4 3 a switch v cesat i sw = 1a 330 mv switch leakage current 0.02 1 a boost schottky forward voltage i sh = 100ma 770 mv boost schottky reverse leakage v reverse = 12v 0.02 1 a minimum boost voltage (note 3) v in = 5v l 1.4 1.8 v boost pin current i sw = 1a, v boost = 15v 20 28 ma en voltage threshold en rising l 0.95 1.01 1.07 v en voltage hysteresis 30 mv en pin current 0.2 20 na pg threshold offset from v fb v fb rising 60 100 140 mv pg hysteresis 20 mv pg leakage v pg = 3v 0.02 1 a pg sink current v pg = 0.4v l 300 570 a sync threshold 0.6 0.8 1.0 v sync pin current 0.1 na ss source current v ss = 1v 0.6 1 1.6 a
LT3971 4 3971f typical performance characteristics ef? ciency, v out = 3.3v no load supply current no load supply current feedback voltage ef? ciency, v out = 5v ef? ciency, v out = 3.3v ef? ciency, v out = 5v t a = 25c, unless otherwise noted. maximum load current maximum load current load current (a) efficiency (%) 3971 g01 100 30 40 50 60 70 80 90 20 0 1.2 1 0.8 0.6 0.4 0.2 v in = 12v v in = 24v v in = 36v front page application v out = 5v r1 = 1m r2 = 309k load current (a) front page application efficiency (%) 3971 g02 100 30 40 50 60 70 80 90 20 0 1.2 1 0.8 0.6 0.4 0.2 v in = 12v v in = 24v v in = 36v load current (ma) efficiency (%) 3971 g03 100 30 20 10 40 50 60 70 80 90 0 0.01 1000 100 10 1 0.1 v in = 36v front page application v out = 5v r1 = 1m r2 = 309k v in = 24v v in = 12v load current (ma) efficiency (%) 3971 g04 90 30 20 10 40 50 60 70 80 0 0.01 1000 100 10 1 0.1 v in = 12v v in = 24v v in = 36v front page application temperature (c) input current (a) 3971 g05 100 10 1 C55 C25 155 125 95 65 35 5 diodes, inc. dfls2100 input voltage (v) 0 input current (a) 1.0 10 front page application v out = 3.3v 20 30 40 3971 g06 2.0 1.5 4.0 3.5 3.0 2.5 temperature (c) feedback voltage (v) 3971 g07 1.205 1.180 1.185 1.190 1.195 1.200 1.175 C55 155 65 95 125 35 5 C25 input voltage (v) load current (a) 3971 g08 3.0 0.5 1.0 1.5 2.0 2.5 0 540 25 30 35 typical minimum 20 15 10 front page application v out = 3.3v input voltage (v) load current (a) 3971 g09 2.5 0.5 1.0 1.5 2.0 0 540 25 30 35 typical minimum 20 15 10 front page application v out = 5v
LT3971 5 3971f typical performance characteristics switch current limit switch current limit switch v cesat boost pin current frequency foldback minimum switch on-time/ switch off-time load regulation switching frequency t a = 25c, unless otherwise noted. load current (ma) load regulation (%) 3971 g10 0.30 C0.25 C0.20 C0.15 C0.10 C0.05 0.20 0.15 0.10 0.05 0 0.25 C0.30 0 1200 600 800 1000 400 200 front page application referenced from v out at 0.5a load temperature (c) frequency (khz) 3971 g11 1000 650 900 850 800 750 700 950 600 C55 155 35 65 125 95 5 C25 duty cycle (%) switch current limit (a) 3971 g12 3.0 2.0 1.5 1.0 0.5 2.5 0 0 100 60 80 40 20 temperature (c) switch current limit (a) 3971 g13 2.5 2.4 2.3 2.2 2.1 2.0 1.5 1.6 1.7 1.8 1.9 C55 C25 155 125 95 65 35 5 duty cycle = 30% fb pin voltage (v) 0 switching frequency (khz) 900 800 400 200 500 600 700 300 100 0 0.8 0.4 3971 g16 1.2 0.6 0.2 1 switch current (ma) boost pin current (ma) 3971 g15 30 20 15 10 5 25 0 0 1500 500 750 1000 1250 250 switch current (ma) v cesat (mv) 3971 g14 600 400 300 200 100 500 0 0 1500 500 750 1000 1250 250 soft-start ss pin voltage (v) 0 switch current limit (a) 2.5 2.0 0.5 1.0 1.5 0 1.25 0.75 3971 g18 2 1 0.5 0.25 1.75 1.5 temperature (c) C55 switch on/off time (ns) 400 350 150 50 200 250 300 100 0 65 5 3971 g17 155 35 C25 95 125 min t off 1a load min t off 0.5a load min t on
LT3971 6 3971f typical performance characteristics en threshold boost diode forward voltage power good threshold switching waveforms; burst mode operation switching waveforms; full frequency continuous operation minimum input voltage t a = 25c, unless otherwise noted. transient load response, load current stepped from 25ma (burst mode operation) to 525ma transient load response, load current stepped from 0.5a to 1a temperature (c) threshold voltage (v) 3971 g21 1.05 0.97 0.96 1.03 1.02 1.01 1.00 0.99 0.98 1.04 0.95 C55 155 35 65 125 95 5 C25 rising threshold falling threshold boost diode current (ma) boost diode vf (v) 3971 g22 1.6 0.4 0.2 1.2 1.0 0.8 0.6 1.4 0 0 1500 500 750 1250 1000 250 temperature (c) threshold voltage (%) 3971 g23 95 88 87 86 93 92 91 90 89 94 85 C55 155 35 65 125 95 5 C25 10s/div 3971 g24 v out 100mv/div i l 500ma/div front page application v in = 12v, v out = 3.3v c out = 47f 10s/div 3971 g25 v out 100mv/ div i l 500ma/ div front page application v in = 12v, v out = 3.3v c out = 47f 5s/div 3971 g26 v sw 5v/div v out 20mv/div i l 500ma/div front page application v in = 12v, v out = 3.3v i load = 10ma c out = 22f 1s/div 3971 g27 v sw 5v/div v out 20mv/div i l 500ma/div front page application v in = 12v, v out = 3.3v i load = 1a c out = 22f minimum input voltage load current (ma) 0 input voltage (v) 5.0 4.8 4.0 3.4 3.6 4.2 4.4 4.6 3.8 3.2 3.0 800 400 3971 g19 1200 600 200 1000 to start to run front page application v out = 3.3v load current (ma) 0 input voltage (v) 6.4 6.2 5.4 5.6 6.0 5.8 5.2 5.0 800 400 3971 g20 1200 600 200 1000 to start to run front page application v out = 5v
LT3971 7 3971f pin functions bd (pin 1): this pin connects to the anode of the boost diode. the bd pin is normally connected to the output. boost (pin 2): this pin is used to provide a drive volt- age, higher than the input voltage, to the internal bipolar npn power switch. sw (pin 3): the sw pin is the output of an internal power switch. connect this pin to the inductor, catch diode, and boost capacitor. v in (pin 4): the v in pin supplies current to the LT3971s internal circuitry and to the internal power switch. this pin must be locally bypassed. en (pin 5): the part is in shutdown when this pin is low and active when this pin is high. the hysteretic threshold voltage is 1.005v going up and 0.975v going down. the en threshold is only accurate when v in is above 4.3v. if v in is lower than 4.2v, ground en to place the part in shutdown. tie to v in if shutdown feature is not used. fb (pin 6): the LT3971 regulates the fb pin to 1.19v. connect the feedback resistor divider tap to this pin. also, connect a phase lead capacitor between fb and v out . typically this capacitor is 10pf. ss (pin 7): a capacitor is tied between ss and ground to slowly ramp up the peak current limit of the LT3971 on start-up. the soft-start capacitor is only actively discharged when en is low. the ss pin is released when the en pin goes high. float this pin to disable soft-start. for applica- tions with input voltages above 25v, add a 100k resistor in series with the soft-start capacitor. rt (pin 8): a resistor is tied between rt and ground to set the switching frequency. pg (pin 9): the pg pin is the open-drain output of an internal comparator. pgood remains low until the fb pin is within 9% of the ? nal regulation voltage. pgood is valid when the LT3971 is enabled and v in is above 4.3v. sync (pin 10): this is the external clock synchronization input. ground this pin for low ripple burst mode operation at low output loads. tie to a clock source for synchroni- zation, which will include pulse-skipping at low output loads. when in pulse-skipping mode, quiescent current increases to 1.5ma. gnd (exposed pad pin 11): ground. the exposed pad must be soldered to pcb.
LT3971 8 3971f block diagram + C + C + C oscillator 200khz to 2mhz burst mode detect v c clamp v c slope comp r v in v in en boost sw shdn switch latch ss 1a v out c2 c3 c4 r3 l1 d1 bd rt r2 gnd error amp r1 fb r t c1 pg 1.09v 1v s q 3991 bd internal 1.19v ref sync 3 + C shdn c5
LT3971 9 3971f operation the LT3971 is a constant frequency, current mode step- down regulator. an oscillator, with frequency set by rt, sets an rs ? ip-? op, turning on the internal power switch. an ampli? er and comparator monitor the current ? owing between the v in and sw pins, turning the switch off when this current reaches a level determined by the voltage at v c (see block diagram). an error ampli? er measures the output voltage through an external resistor divider tied to the fb pin and servos the v c node. if the error ampli? ers output increases, more current is delivered to the output; if it decreases, less current is delivered. an active clamp on the v c node provides current limit. the v c node is also clamped by the voltage on the ss pin; soft-start is implemented by generating a voltage ramp at the ss pin using an external capacitor. if the en pin is low, the LT3971 is shut down and draws 700na from the input. when the en pin exceeds 1v, the switching regulator will become active. the switch driver operates from either v in or from the boost pin. an external capacitor is used to generate a voltage at the boost pin that is higher than the input supply. this allows the driver to fully saturate the internal bipolar npn power switch for ef? cient operation. to further optimize ef? ciency, the LT3971 automatically switches to burst mode operation in light load situations. between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 1.7a. in a typical application, 2.8a will be con- sumed from the supply when regulating with no load. the oscillator reduces the LT3971s operating frequency when the voltage at the fb pin is low. this frequency foldback helps to control the output current during start- up and overload. the LT3971 contains a power good comparator which trips when the fb pin is at 91% of its regulated value. the pg output is an open-drain transistor that is off when the output is in regulation, allowing an external resistor to pull the pg pin high. power good is valid when the LT3971 is enabled and v in is above 4.2v. applications information achieving ultralow quiescent current to enhance ef? ciency at light loads, the LT3971 operates in low ripple burst mode, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current. in burst mode operation the LT3971 delivers single pulses of current to the output ca- pacitor followed by sleep periods where the output power is supplied by the output capacitor. when in sleep mode the LT3971 consumes 1.7a, but when it turns on all the circuitry to deliver a current pulse, the LT3971 consumes 1.5ma of input current in addition to the switch current. therefore, the total quiescent current will be greater than 1.7a when regulating. as the output load decreases, the frequency of single cur- rent pulses decreases (see figure 1) and the percentage of time the LT3971 is in sleep mode increases, resulting in much higher light load ef? ciency. by maximizing the time between pulses, the converter quiescent current figure 1. switching frequency in burst mode operation gets closer to the 1.7a ideal. therefore, to optimize the quiescent current performance at light loads, the current in the feedback resistor divider and the reverse current in the catch diode must be minimized, as these appear to the output as load currents. use the largest possible load current (ma) switching frequency (khz) 3971 f01 1000 200 400 600 800 0 0 120 100 80 60 40 20 front page application v in = 12v v out = 3.3v
LT3971 10 3971f applications information feedback resistors and a low leakage schottky catch diode in applications utilizing the ultralow quiescent current performance of the LT3971. the feedback resistors should preferably be on the order of m and the schottky catch diode should have less than 1a of typical reverse leak- age at room temperature. these two considerations are reiterated in the fb resistor network and catch diode selection sections. it is important to note that another way to decrease the pulse frequency is to increase the magnitude of each single current pulse. however, this increases the output voltage ripple because each cycle delivers more power to the output capacitor. the magnitude of the current pulses was selected to ensure less than 15mv of output ripple in a typical application. see figure 2. to ensure proper burst mode operation, the sync pin must be grounded. when synchronized with an external clock, the LT3971 will pulse skip at light loads. the qui- escent current will signi? cantly increase to 1.5ma in light load situations when synchronized with an external clock. holding the sync pin high yields no advantages in terms of output ripple or minimum load to full frequency, so is not recommended. fb resistor network the output voltage is programmed with a resistor divider between the output and the fb pin. choose the resistor values according to: rr v v out 12 119 1 =? ? ? ? ? ? ? . reference designators refer to the block diagram. 1% resistors are recommended to maintain output voltage accuracy. the total resistance of the fb resistor divider should be selected to be as large as possible to enhance low current performance. the resistor divider generates a small load on the output, which should be minimized to optimize the low supply current at light loads. when using large fb resistors, a 10pf phase lead capacitor should be connected from v out to fb. setting the switching frequency the LT3971 uses a constant frequency pwm architecture that can be programmed to switch from 200khz to 2mhz by using a resistor tied from the rt pin to ground. a table showing the necessary r t value for a desired switching frequency is in table 1. table 1. switching frequency vs r t value switching frequency (mhz) r t value (k) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 255 118 71.5 49.9 35.7 28.0 22.1 17.4 14.0 11.0 figure 2. burst mode operation 5s/div 3971 f02 v out 20mv/div v sw 5v/div i l 500ma/div front page application v in = 12v v out = 3.3v i load = 10ma while in burst mode operation, the burst frequency and the charge delivered with each pulse will not change with output capacitance. therefore, the output voltage ripple will be inversely proportional to the output capacitance. in a typical application with a 22f output capacitor, the output ripple is about 10mv, and with a 47f output capacitor the output ripple is about 5mv. the output voltage ripple can continue to be decreased by increasing the output capacitance. at higher output loads (above 92ma for the front page application) the LT3971 will be running at the frequency programmed by the r t resistor, and will be operating in standard pwm mode. the transition between pwm and low ripple burst mode operation will exhibit slight frequency jitter, but will not disturb the output voltage.
LT3971 11 3971f applications information operating frequency tradeoffs selection of the operating frequency is a tradeoff between ef? ciency, component size, minimum dropout voltage, and maximum input voltage. the advantage of high frequency operation is that smaller inductor and capacitor values may be used. the disadvantages are lower ef? ciency, lower maximum input voltage, and higher dropout voltage. the highest acceptable switching frequency (f sw(max) ) for a given application can be calculated as follows: f vv tvvv sw max out d on min in sw d () () () = + ?+ where v in is the typical input voltage, v out is the output voltage, v d is the catch diode drop (~0.5v), and v sw is the internal switch drop (~0.5v at max load). this equation shows that slower switching frequency is necessary to safely accommodate high v in /v out ratio. also, as shown in the input voltage range section, lower frequency allows a lower dropout voltage. the input voltage range depends on the switching frequency because the LT3971 switch has ? nite minimum on and off times. the minimum switch on and off times are strong functions of temperature. use the typical minimum on and off curves to design for an applications maximum temperature, while adding about 30% for part-to-part variation. the minimum and maximum duty cycles that can be achieved taking minimum on and off times into account are: dc f t dc f t min sw on min max sw off min = =? () () 1 where f sw is the switching frequency, the t on(min) is the minimum switch on-time, and the t off(min) is the minimum switch off-time. these equations show that duty cycle range increases when switching frequency is decreased. a good choice of switching frequency should allow ad- equate input voltage range (see input voltage range sec- tion) and keep the inductor and capacitor values small. input voltage range the minimum input voltage is determined by either the LT3971s minimum operating voltage of 4.3v or by its maximum duty cycle (see equation in operating frequency tradeoffs section). the minimum input voltage due to duty cycle is: v vv ft vv in min out d sw off min dsw () () = + ? ?+ 1 where v in(min) is the minimum input voltage, v out is the output voltage, v d is the catch diode drop (~0.5v), v sw is the internal switch drop (~0.5v at max load), f sw is the switching frequency (set by r t ), and t off(min) is the minimum switch off-time. note that higher switch- ing frequency will increase the minimum input voltage. if a lower dropout voltage is desired, a lower switching frequency should be used. the maximum input voltage for LT3971 applications depends on switching frequency, the absolute maximum ratings of the v in and boost pins, and the operating mode. for a given application where the switching fre- quency and the output voltage are already selected, the maximum input voltage (v in(op-max) ) that guarantees optimum output voltage ripple for that application can be found by applying the following equation: v vv ft vv in op max out d sw on min dsw () () ? ? - = + + where t on(min) is the minimum switch on-time. note that a higher switching frequency will decrease the maximum operating input voltage. conversely, a lower switching frequency will be necessary to achieve normal operation at higher input voltages. the circuit will tolerate inputs above the maximum op- erating input voltage and up to the absolute maximum ratings of the v in and boost pins, regardless of chosen switching frequency. however, during such transients
LT3971 12 3971f where v in is higher than v in(op-max) , the LT3971 will enter pulse-skipping operation where some switching pulses are skipped to maintain output regulation. the output voltage ripple and inductor current ripple will be higher than in typical operation. do not overload when v in is greater than v in(op-max) . inductor selection and maximum output current a good ? rst choice for the inductor value is: l vv f out d sw = + where f sw is the switching frequency in mhz, v out is the output voltage, v d is the catch diode drop (~0.5v) and l is the inductor value in h. the inductors rms current rating must be greater than the maximum load current and its saturation current should be about 30% higher. for robust operation in fault conditions (start-up or short-circuit) and high input voltage (>30v), the saturation current should be above 2.8a. to keep the ef? ciency high, the series resistance (dcr) should be less than 0.1, and the core material should be intended for high frequency applications. table 2 lists several vendors and suitable types. the inductor value must be suf? cient to supply the desired maximum output current (i out(max) ), which is a function of the switch current limit (i lim ) and the ripple current. ii i out max lim l () ? = 2 the LT3971 limits its peak switch current in order to protect itself and the system from overload faults. the LT3971s switch current limit (i lim ) is at least 2.4a at low duty cycles and decreases linearly to 1.75a at dc = 0.8. table 2. inductor vendors vendor url part series type murata www.murata.com lqh55d open tdk www.componenttdk.com slf7045 slf10145 shielded shielded toko www.toko.com d62cb d63cb d73c d75f shielded shielded shielded open coilcraft www.coilcraft.com mss7341 mss1038 shielded shielded sumida www.sumida.com cr54 cdrh74 cdrh6d38 cr75 open shielded shielded open when the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. this gives the peak-to-peak ripple current in the inductor: i dc v v lf l out d sw = ?+ ()?( ) ? 1 where f sw is the switching frequency of the LT3971, dc is the duty cycle and l is the value of the inductor. therefore, the maximum output current that the LT3971 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. the inductor value may have to be increased if the inductor ripple current does not allow suf? cient maximum output current (i out(max) ) given the switching frequency, and maximum input voltage used in the desired application. the optimum inductor for a given application may differ from the one indicated by this simple design guide. a larger value inductor provides a higher maximum load current and reduces the output voltage ripple. if your load is lower than the maximum load current, than you can relax the value of the inductor and operate with higher ripple current. this allows you to use a physically smaller inductor, or one with a lower dcr resulting in higher ef? ciency. be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on the input voltage. in addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. applications information
LT3971 13 3971f for details of maximum output current and discontinuous operation, see linear technologys application note 44. finally, for duty cycles greater than 50% (v out /v in >0.5), a minimum inductance is required to avoid sub-harmonic oscillations. see application note 19. one approach to choosing the inductor is to start with the simple rule given above, look at the available induc- tors, and choose one to meet cost or space goals. then use the equations above to check that the LT3971 will be able to deliver the required output current. note again that these equations assume that the inductor current is continuous. discontinuous operation occurs when i out is less than i l /2. input capacitor bypass the input of the LT3971 circuit with a ceramic capacitor of x7r or x5r type. y5v types have poor performance over temperature and applied voltage, and should not be used. a 4.7f to 10f ceramic capacitor is adequate to bypass the LT3971 and will easily handle the ripple current. note that larger input capacitance is required when a lower switching frequency is used (due to longer on-times). if the input power source has high impedance, or there is signi? cant inductance due to long wires or cables, additional bulk capacitance may be necessary. this can be provided with a low performance electrolytic capacitor. step-down regulators draw current from the input sup- ply in pulses with very fast rise and fall times. the input capacitor is required to reduce the resulting voltage ripple at the LT3971 and to force this very high frequency switching current into a tight local loop, minimizing emi. a 4.7f capacitor is capable of this task, but only if it is placed close to the LT3971 (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3971. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank cir- cuit. if the LT3971 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3971s voltage rating. this situation is easily avoided (see the hot plugging safely section). output capacitor and output ripple the output capacitor has two essential functions. along with the inductor, it ? lters the square wave generated by the LT3971 to produce the dc output. in this role it determines the output ripple, so low impedance (at the switching frequency) is important. the second function is to store energy in order to satisfy transient loads and stabilize the LT3971s control loop. ceramic capacitors have very low equivalent series resistance (esr) and provide the best ripple performance. a good starting value is: c vf out out sw = 100 where f sw is in mhz, and c out is the recommended output capacitance in f. use x5r or x7r types. this choice will provide low output ripple and good transient response. transient performance can be improved with a higher value capacitor. increasing the output capacitance will also decrease the output voltage ripple. a lower value of output capacitor can be used to save space and cost but transient performance will suffer. when choosing a capacitor, look carefully through the data sheet to ? nd out what the actual capacitance is under operating conditions (applied voltage and temperature). a physically larger capacitor or one with a higher voltage rating may be required. table 3 lists several capacitor vendors. table 3. recommended ceramic capacitor vendors manufacturer website avx www.avxcorp.com murata www.murata.com taiyo yuden www.t-yuden.com vishay siliconix www.vishay.com tdk www.tdk.com catch diode selection the catch diode (d1 from block diagram) conducts cur- rent only during switch off time. average forward current in normal operation can be calculated from: ii vv v d avg out in out in () ? = where i out is the output load current. the only reason to consider a diode with a larger current rating than necessary applications information
LT3971 14 3971f for nominal operation is for the worst-case condition of shorted output. the diode current will then increase to the typical peak switch current. peak reverse voltage is equal to the regulator input voltage. use a diode with a reverse voltage rating greater than the input voltage. table 4. schottky diodes. the reverse current values listed are estimates based off of typical curves for reverse current vs reverse voltage at 25c. part number v r (v) i ave (a) v f at 1a (mv) v f at 2a (mv) i r at v r = 20v 25c (a) on semiconductor mbr0520l 20 0.5 30 mbr0540 40 0.5 620 0.4 mbrm120e 20 1 530 595 0.5 mbrm140 40 1 550 20 diodes inc. b0530w 30 0.5 15 b0540w 40 0.5 620 1 b120 20 1 500 1.1 b130 30 1 500 1.1 b140 40 1 500 1.1 b150 50 1 700 0.4 b220 20 2 500 20 b230 30 2 500 0.6 b140hb 40 1 1 dfls240l 40 2 500 4 dfls140 40 1.1 510 1 dfls160 60 1 500 2.5 dfls2100 100 2 770 860 0.01 b240 40 2 500 0.45 central semiconductor cmsh1 - 40m 40 1 500 cmsh1 - 60m 60 1 700 cmsh1 - 40ml 40 1 400 cmsh2 - 40m 40 2 550 cmsh2 - 60m 60 2 700 cmsh2 - 40l 40 2 400 cmsh2 - 40 40 2 500 cmsh2 - 60m 60 2 700 applications information an additional consideration is reverse leakage current. when the catch diode is reversed biased, any leakage current will appear as load current. when operating under light load conditions, the low supply current consumed by the LT3971 will be optimized by using a catch diode with minimum reverse leakage current. low leakage schottky diodes often have larger forward voltage drops at a given current, so a trade-off can exist between low load and high load ef? ciency. often schottky diodes with larger reverse bias ratings will have less leakage at a given output voltage than a diode with a smaller reverse bias rating. therefore, superior leakage performance can be achieved at the expense of diode size. table 4 lists several schottky diodes and their manufacturers. ceramic capacitors ceramic capacitors are small, robust and have very low esr. however, ceramic capacitors can cause problems when used with the LT3971 due to their piezoelectric nature. when in burst mode operation, the LT3971s switching frequency depends on the load current, and at very light loads the LT3971 can excite the ceramic capacitor at audio frequencies, generating audible noise. since the LT3971 operates at a lower current limit during burst mode op- eration, the noise is typically very quiet to a casual ear. if this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. a ? nal precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT3971. as pre- viously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the LT3971 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3971s rating. this situation is easily avoided (see the hot plugging safely section). boost and bd pin considerations capacitor c3 and the internal boost schottky diode (see the block diagram) are used to generate a boost volt- age that is higher than the input voltage. in most cases a 0.47f capacitor will work well. figure 3 shows three ways to arrange the boost circuit. the boost pin must be more than 2.3v above the sw pin for best ef? ciency.
LT3971 15 3971f applications information for outputs of 3v and above, the standard circuit (figure 3a) is best. for outputs between 2.8v and 3v, use a 1f boost capacitor. a 2.5v output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. for reliable boost pin operation with 2.5v outputs use a good external schottky diode (such as the on semi mbr0540), and a 1f boost capacitor (figure 3b). for output voltages below 2.5v, the boost diode can be tied to the input (figure 3c), or to another external supply greater than 2.8v. however, the circuit in figure 3a is more ef? cient because the boost pin current comes from a lower voltage source. you must also be sure that the maximum voltage ratings of the boost and bd pins are not exceeded. the minimum operating voltage of an LT3971 application is limited by the minimum input voltage (4.3v) and by the maximum duty cycle as outlined in the input voltage range section. for proper start-up, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, the boost capacitor may not be fully charged. because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. this minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. the minimum load generally goes to zero once the circuit has started. figure 4 shows a plot of minimum load to start and to run as a function of input voltage. in many cases the discharged output capacitor will present a load to the switcher, which will allow it to start. the plots show the worst-case situation where v in is ramping very slowly. v in boost sw bd v in v out 4.7f c3 gnd LT3971 v in boost sw bd v in v out 4.7f c3 d2 gnd LT3971 v in boost sw bd v in v out 4.7f c3 gnd LT3971 3971 fo3 (3a) for v out > 2.8v (3b) for 2.5v < v out < 2.8v (3c) for v out < 2.5v; v in(max) = 27v figure 4. the minimum input voltage depends on output voltage, load current and boost circuit 3971 f04 load current (ma) 10 input voltage (v) 4.0 4.4 4.2 4.6 1000 3.6 3.8 3.4 3.0 100 10 1000 100 3.2 5.0 4.8 load current (ma) input voltage (v) 5.8 6.0 6.2 5.6 5.0 5.4 5.2 6.4 to run v out = 3.3v t a = 25c l = 4.7h f = 800khz v out = 5v t a = 25c l = 4.7h f = 800khz to start to run to start figure 3. three circuits for generating the boost voltage
LT3971 16 3971f applications information for lower start-up voltage, the boost diode can be tied to v in ; however, this restricts the input range to one-half of the absolute maximum rating of the boost pin. at light loads, the inductor current becomes discontinu- ous and this reduces the minimum input voltage to ap- proximately 400mv above v out . at higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the LT3971, requiring a higher input voltage to maintain regulation. enable pin the LT3971 is in shutdown when the en pin is low and active when the pin is high. the rising threshold of the en comparator is 1.01v, with 30mv of hysteresis. the en pin can be tied to v in if the shutdown feature is not used. adding a resistor divider from v in to en programs the LT3971 to regulate the output only when v in is above a desired voltage (see figure 5). typically, this threshold, v in(en) , is used in situations where the input supply is cur- rent limited, or has a relatively high source resistance. a switching regulator draws constant power from the source, so source current increases as source voltage drops. this looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. the v in(en) threshold prevents the regulator from operating at source voltages where the problems might occur. this threshold can be adjusted by setting the values r3 and r4 such that they satisfy the following equation: v r r in en () =+ 3 4 1 where output regulation should not start until v in is above v in(en) . due to the comparators hysteresis, switching will not stop until the input falls slightly below v in(en) . be aware that when the input voltage is below 4.3v, the input current may rise to several hundred a. and the part may be able to switch at cold or for v in(en ) thresholds less than 7v. figure 6 shows the magnitude of the increased input current in a typical application with different pro- grammed v in(en) . when operating in burst mode for light load currents, the current through the v in(en) resistor network can easily be greater than the supply current consumed by the LT3971. therefore, the v in(en) resistors should be large to minimize their effect on ef? ciency at low loads. figure 5. programmed enable threshold + C 1v shdn 3971 f05 LT3971 en v in r3 r4 3971 f06 input voltage (v) 12v v in(en) input current 6v v in(en) input current 01234 input current (a) 300 400 12 200 100 0 6 5 7 8 9 10 11 500 300 400 200 100 0 500 input voltage (v) 01234 input current (a) 6 5 v in(en) = 6v r3 = 5m r4 = 1m v in(en) = 12v r3 = 11m r4 = 1m figure 6. input current vs input voltage for a programmed v in(en) of 6v and 12v
LT3971 17 3971f applications information soft-start the ss pin can be used to soft-start the LT3971 by throttling the maximum input current during start-up. an internal 1a current source charges an external capacitor generating a voltage ramp on the ss pin. the ss pin clamps the internal v c node, which slowly ramps up the current limit. maximum current limit is reached when the ss pin is about 1.5v or higher. by selecting a large enough capacitor, the output can reach regulation without overshoot. for applications with input voltages above 25v, a 100k resistor in series with the soft-start capacitor is recommended. figure 7 shows start-up waveforms for a typical application with a 10nf capacitor on ss for a 3.3 load when the en pin is pulsed high for 13ms. the external ss capacitor is only actively discharged when en is low. with en low, the external ss cap is discharged through approximately 150. the en pin needs to be low long enough for the external cap to completely discharge through the 150 pull-down prior to start-up. the LT3971 will not enter burst mode operation at low output loads while synchronized to an external clock, but instead will pulse skip to maintain regulation. the LT3971 may be synchronized over a 250khz to 2mhz range. the r t resistor should be chosen to set the LT3971 switching frequency 20% below the lowest synchronization input. for example, if the synchronization signal will be 250khz and higher, the r t should be selected for 200khz. to assure reliable and safe operation the LT3971 will only synchronize when the output voltage is near regulation as indicated by the pg ? ag. it is therefore necessary to choose a large enough inductor value to supply the required output current at the frequency set by the r t resistor (see the inductor selection section). the slope compensation is set by the r t value, while the minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size, input voltage, and output voltage. since the synchronization frequency will not change the slopes of the inductor current waveform, if the inductor is large enough to avoid subharmonic oscillations at the frequency set by r t , than the slope compensation will be suf? cient for all synchronization frequencies. shorted and reversed input protection if the inductor is chosen so that it wont saturate exces- sively, a LT3971 buck regulator will tolerate a shorted output. there is another situation to consider in systems where the output will be held high when the input to the LT3971 is absent. this may occur in battery charging ap- plications or in battery backup systems where a battery or some other supply is diode ored with the LT3971s output. if the v in pin is allowed to ? oat and the en pin is held high (either by a logic signal or because it is tied to v in ), then the LT3971s internal circuitry will pull its quiescent current through its sw pin. this is ? ne if your system can tolerate a few a in this state. if you ground the en pin, the sw pin current will drop to essentially zero. however, if the v in pin is grounded while the output is held high, regardless of en, parasitic diodes inside the LT3971 can pull current from the output through the sw pin and the v in pin. figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. figure 7. soft-start waveforms for front-page application with 10nf capacitor on ss. en is pulsed high for about 13ms with a 3.3 load resistor 2ms/div 3971 f07 v ss 1v/div v out 2v/div i l 0.5a/div synchronization to select low ripple burst mode operation, tie the sync pin below 0.6v (this can be ground or a logic low output). synchronizing the LT3971 oscillator to an external fre- quency can be done by connecting a square wave (with 20% to 80% duty cycle) to the sync pin. the square wave amplitude should have valleys that are below 0.6v and peaks above 1.0v (up to 6v).
LT3971 18 3971f applications information pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 9 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents ? ow in the LT3971s v in and sw pins, the catch diode (d1), and the input capacitor (c1). the loop formed by these components should be as small as possible. these components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. place a local, unbroken ground plane below these components. the sw and boost nodes should be as small as possible. finally, keep the fb and r t nodes small so that the ground traces will shield them from the sw and boost nodes. the exposed pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. to keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT3971 to additional ground planes within the circuit board and on the bottom side. hot plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT3971 circuits. however, these ca- pacitors can cause problems if the LT3971 is plugged into a live supply. the low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the v in pin of the LT3971 can ring to twice the nominal input voltage, possibly exceeding the LT3971s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the LT3971 into an energized supply, the input network should be designed to prevent this overshoot. see linear technology application note 88 for a complete discussion. high temperature considerations for higher ambient temperatures, care should be taken in the layout of the pcb to ensure good heat sinking of the LT3971. the exposed pad on the bottom of the package must be soldered to a ground plane. this ground should be tied to large copper layers below with thermal vias; these layers will spread heat dissipated by the LT3971. placing additional vias can reduce thermal resistance further. the maximum load current should be derated as the ambient temperature approaches the maximum junction rating. power dissipation within the LT3971 can be estimated by calculating the total power loss from an ef? ciency measure- ment and subtracting the catch diode loss and inductor figure 8. diode d4 prevents a shorted input from discharging a backup battery tied to the output. it also protects the circuit from a reversed input. the LT3971 runs only when the input is present figure 9. a good pcb layout ensures proper, low emi operation LT3971 boost v in en v in v out backup 3971 f07 sw bd d4 mbrs140 fb gnd + vias to local ground plane vias to v out vias to run/ss vias to pg vias to v in outline of local ground plane 3971 f09 l1 c2 v out d1 c1 c3 c5 c4 r1 r2 r t r pg gnd gnd vias to sync
LT3971 19 3971f applications information 5v step-down converter 3.3v step-down converter sw fb ss rt v in v in 7v to 38v v out 5v 1.2a 4.7f 0.47f 22f 10pf 309k 49.9k f = 800khz 4.7h 1m gnd bd sync off on LT3971 3971 ta02 en boost pg 10pf sw fb ss rt v in v in 4.3v to 38v v out 3.3v 1.2a 4.7f 0.47f 22f 562k 71.5k f = 600khz 4.7h 1m gnd bd sync off on LT3971 3971 ta03 en boost pg typical applications loss. the die temperature is calculated by multiplying the LT3971 power dissipation by the thermal resistance from junction to ambient. also keep in mind that the leakage current of the power schottky diode goes up exponentially with junction tem- perature. when the power switch is closed, the power schottky diode is in parallel with the power converters output ? lter stage. as a result, an increase in a diodes leakage current results in an effective increase in the load, and a corresponding increase in input power. therefore, the catch schottky diode must be selected with care to avoid excessive increase in light load supply current at high temperatures. other linear technology publications application notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. the lt1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. design note 318 shows how to generate a bipolar output supply using a buck regulator.
LT3971 20 3971f 2.5v step-down converter 10pf sw fb ss rt v in v in 4.3v to 38v v out 2.5v 1.2a 4.7f 1f 47f 909k 118k f = 400khz 4.7h 1m gnd sync off on LT3971 3971 ta04 en boost pg bd typical applications 1.8v step-down converter 10pf sw fb ss rt v in v in 4.3v to 27v v out 1.8v 1.2a 4.7f 0.47f 100f 1m 118k f = 400khz 4.7h 511k gnd sync off on LT3971 3971 ta05 en boost pg bd 12v step-down converter 3.3v step-down converter with undervoltage lockout, soft-start, and power good 10pf sw fb ss rt v in v in 15v to 38v v out 12v 1.2a 10f 0.47f 10f 110k 49.9k f = 800khz 10h 1m gnd bd sync off on LT3971 3971 ta06 en boost pg 10pf sw fb ss rt v in v in 6v to 38v v out 3.3v 1.2a 4.7f 0.47f 22f 562k 49.9k 4.7h 1m gnd pg pgood sync LT3971 3971 ta07 en boost bd 150k 5m 1m 100k 1nf f = 800khz
LT3971 21 3971f typical applications 5v, 2mhz step-down converter with soft-start 10pf sw fb ss rt v in v in 9v to 25v v out 5v 1.2a 2.2f 0.47f 22f 309k 11k f = 2mhz 2.2h 1m gnd bd sync off on LT3971 3971 ta08 en boost pg 1nf 4v step-down converter with a high impedance input source 10pf sw fb ss rt v in 24v v out 4v 1.2a* 4.7f c bulk 100f 0.47f 100f 412k 49.9k f = 800khz 4.7h 1m gnd bd sync 11m LT3971 3971 ta09a en boost pg 1nf 1m + C + * average output power cannot exceed that which can be provided by high impedance source. namely, where v is voltage of source, r is internal source impedance, and n is LT3971 efficiency. maximum output current of 1.2a can be supplied for a short time based on the energy which can be sourced by the bulk input capacitance. p out(max) = ? h v 2 4r 500s/div 3971 ta09b v in 5v/div v out 200mv/div i l 1a/div sourcing a maximum load pulse start-up from high impedance input source 2ms/div 3971 ta09c v in 1v/div v out 2v/div i l 500ma/div
LT3971 22 3971f dd package 10-lead plastic dfn (3mm 3mm) (reference ltc dwg # 05-08-1699) package description 3.00 0.10 (4 sides) note: 1. drawing to be made a jedec package outline m0-229 variation of (weed-2). check the ltc website data sheet for current status of variation assignment 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.38 0.10 bottom viewexposed pad 1.65 0.10 (2 sides) 0.75 0.05 r = 0.115 typ 2.38 0.10 (2 sides) 1 5 10 6 pin 1 top mark (see note 6) 0.200 ref 0.00 C 0.05 (dd) dfn 1103 0.25 0.05 2.38 0.05 (2 sides) recommended solder pad pitch and dimensions 1.65 0.05 (2 sides) 2.15 0.05 0.50 bsc 0.675 0.05 3.50 0.05 package outline 0.25 0.05 0.50 bsc
LT3971 23 3971f information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description mse package 10-lead plastic msop , exposed die pad (reference ltc dwg # 05-08-1664 rev c) msop (mse) 0908 rev c 0.53 p 0.152 (.021 p .006) seating plane 0.18 (.007) 1.10 (.043) max 0.17 C 0.27 (.007 C .011) typ 0.86 (.034) ref 0.50 (.0197) bsc 12 3 45 4.90 p 0.152 (.193 p .006) 0.497 p 0.076 (.0196 p .003) ref 8 9 10 10 1 7 6 3.00 p 0.102 (.118 p .004) (note 3) 3.00 p 0.102 (.118 p .004) (note 4) note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.254 (.010) 0 o C 6 o typ detail a detail a gauge plane 5.23 (.206) min 3.20 C 3.45 (.126 C .136) 0.889 p 0.127 (.035 p .005) recommended solder pad layout 0.305 p 0.038 (.0120 p .0015) typ 2.083 p 0.102 (.082 p .004) 2.794 p 0.102 (.110 p .004) 0.50 (.0197) bsc bottom view of exposed pad option 1.83 p 0.102 (.072 p .004) 2.06 p 0.102 (.081 p .004) 0.1016 p 0.0508 (.004 p .002) detail b detail b corner tail is part of the leadframe feature. for reference only no measurement purpose 0.05 ref 0.29 ref
LT3971 24 3971f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2009 lt 1109 ? printed in usa related parts part description comments lt3980 58v, 80v transient protection, 2a, 2.4mhz high ef? ciency micropower step-down dc/dc converter v in(min) = 3.6v, v in(max) = 58v, transient to 80v, v out(min) = 0.79v, i q = 75a, i sd <1a, msop-16e 3mm 4mm dfn-16 package lt3970 40v, 350ma high ef? ciency micropower step-down dc/dc converter with i q = 2.5a v in(min) = 4.2v, v in(max) = 40v, v out(min) = 1.21v, i q = 2.2a, i sd <1a, msop-10 3mm 2mm dfn-10 package lt3695 36v, 60v transient protection, 1a, 2.2mhz high ef? ciency micropower step-down dc/dc converter with 1a fault tolerance v in(min) = 3.6v, v in(max) = 36v, transient to 60v, v out(min) = 0.8v, i q = 75a, i sd <1a, msop-16e package lt3689 36v, 60v transient protection, 800ma, 2.2mhz high ef? ciency micropower step-down dc/dc converter with por reset and watchdog timer v in(min) = 3.6v, v in(max) = 36v, transient to 60v, v out(min) = 0.8v, i q = 75a, i sd <1a, 3mm 3mm qfn-16 package lt3682 36v, 60v max , 1a, 2.2mhz high ef? ciency micropower step-down dc/dc converter v in(min) = 3.6v, v in(max) = 36v, v out(min) = 0.8v, i q = 75a, i sd <1a, 3mm 3mm dfn-12 package lt3480 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high ef? ciency step-down dc/dc converter with burst mode operation v in(min) = 3.6v, v in(max) = 38v, v out(min) = 0.78v, i q = 70a, i sd <1a, 3mm 3mm dfn-10, msop-10e package lt3685 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high ef? ciency step-down dc/dc converter v in(min) = 3.6v, v in(max) = 38v, v out(min) = 0.78v, i q = 70a, i sd <1a, 3mm 3mm dfn-10, msop-10e package lt3481 34v with transient protection to 36v, 2a (i out ), 2.8mhz, high ef? ciency step-down dc/dc converter with burst mode operation v in(min) = 3.6v, v in(max) = 34v, v out(min) = 1.26v, i q = 50a, i sd <1a 3mm 3mm dfn-10, msop-10e package lt3684 34v with transient protection to 36v, 2a (i out ), 2.8mhz, high ef? ciency step-down dc/dc converter v in(min) = 3.6v, v in(max) = 34v, v out(min) = 1.26v, i q = 850a, i sd <1a, 3mm 3mm dfn-10, msop-10e package lt3508 36v with transient protection to 40v, dual 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in(min) = 3.7v, v in(max) = 37v, v out(min) = 0.8v, i q = 4.6ma, i sd = 1a, 4mm 4mm qfn-24, tssop-16e package lt3505 36v with transient protection to 40v, 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in(min) = 3.6v, v in(max) = 34v, v out(min) = 0.78v, i q = 2ma, i sd = 2a, 3mm 3mm dfn-8, msop-8e package lt3500 36v, 40v max , 2a, 2.5mhz high ef? ciency step-down dc/dc converter and ldo controller v in(min) = 3.6v, v in(max) = 36v, v out(min) = 0.8v, i q = 2.5ma, i sd <10a, 3mm 3mm dfn-10 package lt3507 36v 2.5mhz, triple (2.4a + 1.5a + 1.5a (i out )) with ldo controller high ef? ciency step-down dc/dc converter v in(min) = 4.0v, v in(max) = 36v, v out(min) = 0.8v, i q = 7ma, i sd = 1a, 5mm 7mm qfn-38 package lt3437 60v, 400ma (i out ), micropower step-down dc/dc converter with burst mode operation v in(min) = 3.3v, v in(max) = 60v, v out(min) = 1.25v, i q = 100a, i sd <1a, 3mm 3mm dfn-10, tssop-16e package lt1976/ lt1977 60v, 1.2a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converter with burst mode operation v in(min) = 3.3v, v in(max) = 60v, v out(min) = 1.20v, i q = 100a, i sd <1a, tssop16e package lt3434/ lt3435 60v, 2.4a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converter with burst mode operation v in(min) = 3.3v, v in(max) = 60v, v out(min) = 1.20v, i q = 100a, i sd <1a, tssop16e package lt1936 36v, 1.4a(i out ) , 500khz high ef? ciency step-down dc/dc converter v in(min) = 3.6v, v in(max) = 36v, v out(min) = 1.2v, i q = 1.9ma, i sd <1a, ms8e package lt3493 36v, 1.4a(i out ), 750khz high ef? ciency step-down dc/dc converter v in(min) = 3.6v, v in(max) = 36v, v out(min) = 0.8v, i q = 1.9ma, i sd <1a, 2mm 3mm dfn-6 package lt1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in(min) = 5.5v, v in(max) = 60v, v out(min) = 1.20v, i q = 2.5ma, i sd = 25a, tssop16e package

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