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lt3681 1 3681f 36v, 2a, 2.8mhz step-down switching regulator with integrated power schottky diode the lt ? 3681 is an adjustable frequency (300khz to 2.8mhz) monolithic buck switching regulator that accepts input voltages up to 34v (36v maximum). a high ef? ciency 0.18 switch is included on the die along with a boost schottky diode and the necessary oscillator, control, and logic circuitry. an undedicated power schottky diode is integrated into the lt3681 to minimize the solution size. current mode topology is used for fast transient response and good loop stability. low ripple burst mode operation maintains high ef? ciency at low output currents while keeping output ripple below 15mv in a typical application. in addition, the lt3681 can further enhance low output current ef? ciency by drawing bias current from the output when v out is above 3v. shutdown reduces input supply current to less than 1a while a resistor and capacitor on the run/ss pin provide a controlled output voltage ramp (soft-start). a power good ? ag signals when v out reaches 90% of the programmed output voltage. the lt3681 is available in 14-pin 4mm x 3mm dfn package with exposed pads for low thermal resistance. automotive battery regulation power for portable products distributed supply regulation industrial supplies wall transformer regulation wide input voltage range: 3.6v to 34v operating, 36v maximum 2a maximum output current low ripple burst mode ? operation 50a i q at 12v in to 3.3v out output ripple < 15mv p-p adjustable switching frequency: 300khz to 2.8mhz low shutdown current: i q < 1a integrated boost diode integrated power schottky diode power good flag saturating switch design: 0.18 on-resistance 1.265v feedback reference voltage output voltage: 1.265v to 20v soft-start capability small 14-pin thermally enhanced (4mm x 3mm) dfn package 5v step-down converter applicatio s u features descriptio u typical applicatio u sw dc da bias fb v c pg rt v in bd v in 6.3v to 34v v out 5v 2a 4.7 f 0.47 f 330pf 22 f 200k 20k 60.4k l 6.8 h 590k gnd off on lt3681 3681 ta01 run/ss boost l: nec plc-0745-6r8 ef? ciency i load (a) 0.0001 efficiency (%) power loss (w) 10 20 0 0.001 0.01 0.1 1 10 3681 ta01b 40 30 100 90 80 60 70 50 0.0001 0.0100 0.0010 10.0000 1.0000 0.1000 v in = 12v v out = 3.3v l = 4.7 h f = 800 khz , lt, ltc and ltm are registered trademarks of linear technology corporation. burst mode is a registered trademark of linear technology corporation. all other trademarks are the property of their respective owners.
lt3681 2 3681f electrical characteristics v in , run/ss voltage .................................................36v boost pin voltage ...................................................56v boost pin above sw pin .........................................30v fb, rt, v c voltage .......................................................5v bias, pg, bd voltage ................................................30v maximum junction temperature .......................... 125c dc above da .............................................................40v operating temperature range (note 2) lt3681e ............................................... C40c to 85c storage temperature range ................... C65c to 150c (note 1) parameter conditions min typ max units minimum input voltage 3 3.6 v quiescent current from v in v run/ss = 0.2v 0.01 0.5 a v bias = 3v, not switching 22 60 a v bias = 0, not switching 75 120 a quiescent current from bias v run/ss = 0.2v 0.01 0.5 a v bias = 3v, not switching 50 120 a v bias = 0, not switching 05 a minimum bias voltage 2.7 3 v feedback voltage 1.25 1.24 1.265 1.265 1.29 1.3 v v fb pin bias current (note 3) v fb = 1.25v, v c = 0.4v 30 100 na fb voltage line regulation 4v < v in < 34v 0.002 0.02 %/v error amp gm 330 mho error amp gain 800 v c source current 65 a v c sink current 85 a v c pin to switch current gain 3.5 a/v v c clamp voltage 2v the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, v runs/ss = 10v, v boost = 15v, v bias = 3.3v unless otherwise noted. (note 2) absolute axi u rati gs w ww u package/order i for atio uu w 1 2 3 4 5 6 7 14 13 12 11 10 9 8 run/ss v in sw boost bd dc da pg bias fb gnd v c r t gnd top view de14ma package 14-lead (4mm 3mm) plastic dfn 15 16 t jmax = 125c, ja = 43c/w exposed pad (pin 15) is gnd, must be soldered to pcb exposed pad pin 16 is dc order part number de part marking lt3681ede 3681 order options tape and reel: add #tr lead free: add #pbf lead free tape and reel: add #trpbf lead free part marking: http://www.linear.com/leadfree/ consult ltc marketing for parts speci? ed with wider operating temperature ranges.
lt3681 3 3681f parameter conditions min typ max units power schottky diode forward voltage i da = 1a i da = 2a 0.50 0.56 v v power schottky diode leakage current v dc-da = 40v 100 a switching frequency rt = 8.66k rt = 29.4k rt = 187k 2.5 1.25 250 2.8 1.4 300 3.1 1.55 350 mhz mhz khz minimum switch off-time 130 200 ns switch current limit duty cycle = 5% 3.2 3.8 4.4 a switch v cesat i sw = 2a 360 mv boost schottky reverse leakage v sw = 10v, v bias = 0v 0.02 2 a minimum boost voltage (note 4) 1.5 2.1 v boost pin current i sw = 1a 18 35 ma run/ss pin current v run/ss = 2.5v 5 10 a run/ss input voltage high 2.5 v run/ss input voltage low 0.2 v pg threshold offset from feedback voltage v fb rising 122 mv pg hysteresis 5 mv pg leakage v pg = 5v 0.1 1 a pg sink current v pg = 3v 100 600 a 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 lt3681e is guaranteed to meet performance speci? cations from 0c to 85c. speci? cations over the C40c to 85c operating temperature range are assured by design, characterization and correlation with statistical process controls. note 3: bias current ? ows into the fb pin. note 4: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, v runs/ss = 10v v boost = 15v, v bias = 3.3v unless otherwise noted. (note 2) electrical characteristics
lt3681 4 3681f duty cycle (%) 0 switch current limit(a) 40 3681 g08 2.5 20 60 1.5 1.0 4.0 3.5 3.0 2.0 80 100 temperature ( c) ?40 supply current ( a) 20 3681 g05 80 60 ?20 0 40 50 40 100 90 70 60 80 v in = 12v v out = 3.3v increased supply current due to catch diode leakage at high temperature ef? ciency (v out = 5.0v) ef? ciency (v out = 3.3v) no load supply current no load supply current vs temperature maximum load current typical perfor a ce characteristics uw load current (a) 0.0001 0 efficiency (%) 40 30 100 0.001 0.01 0.1 1 10 3681 g01 20 10 60 50 90 80 70 v in = 24v v in = 12v l: nec plc-0745-4r7 f: 800khz load current (a) 0.0001 0 efficiency (%) 40 30 100 0.001 0.01 0.1 1 10 3681 g02 20 10 60 50 90 80 70 v in = 12v v in = 24v v in = 7v l: nec plc-0745-4r7 f: 800khz input voltage (v) 0 supply current ( a) 70 80 15 3681 g04 40 20 510 20 10 0 90 60 50 30 25 30 35 front page application input voltage (v) 5 load current (a) 15 3681 g07 2.5 10 20 1.5 1.0 4.0 3.5 3.0 2.0 25 30 v out = 5.0v l = 4.7 h f = 800 khz switch current limit input voltage (v) 5 load current (a) 15 3681 g06 2.5 10 20 1.5 1.0 4.0 3.5 3.0 2.0 25 30 v out = 3.3v l = 4.7 h f = 800 khz temperature ( c) ?50 switch current limit (a) 2.0 2.5 3.0 125 3681 g09 1.5 1.0 0 0 ?25 50 25 100 75 0.5 4.5 4.0 duty cycle = 10 % duty cycle = 90 % maximum load current switch current limit switching frequency (mhz) 0 efficiency (%) 85 1.5 3681 g03 70 60 0.5 1 2 55 50 90 80 75 65 2.5 3 v in = 12v v in = 24v v out = 3.3v l = 10 h load = 1a ef? ciency vs switching frequency t a = 25c unless otherwise noted.
lt3681 5 3681f boost diode current (a) 0 boost diode v f (v) 0.8 1.0 1.2 2.0 3681 g18 0.6 0.4 0 0.5 1.0 1.5 0.2 1.6 1.4 run/ss pin voltage (v) 0 run/ss pin current ( a) 8 10 12 15 25 3681 g17 6 4 510 20 30 35 2 0 switch voltage drop boost pin current feedback voltage switching frequency frequency foldback minimum switch on-time soft start run/ss pin current boost diode switch current (ma) 0 400 500 700 1500 2500 3681 g10 300 200 500 1000 2000 3000 3500 100 0 600 voltage drop (mv) switch current (ma) 0 0 boost pin current (ma) 10 30 40 50 2000 90 3681 g11 20 1000 500 2500 3000 1500 3500 60 70 80 temperature ( c) ?50 feedback voltage (v) 1.270 1.280 125 4681 g12 1.260 1.250 0 50 100 ?25 25 75 1.290 1.265 1.275 1.255 1.285 temperature ( c) ?50 frequency (mhz) 1.00 1.10 125 4681 g13 0.90 0.80 0 50 100 ?25 25 75 1.20 r t = 45.3k ? 0.95 1.05 0.85 1.15 fb pin voltage (mv) 0 switching frequency (khz) 800 1000 1200 600 1000 3681 g14 600 400 200 400 800 1200 1400 200 0 r t = 45.3k ? temperature (?c) ?50 minimum switch on time (ns) 80 100 120 25 3681 g15 60 40 ?25 0 50 75 100 125 20 0 140 run/ss pin voltage (v) 0 switch current limit (a) 3.5 1.5 3681 g16 2.0 1.0 0.5 1 2 0.5 0 4.0 3.0 2.5 1.5 2.5 3 3.5 typical perfor a ce characteristics uw t a = 25c unless otherwise noted.
lt3681 6 3681f temperature ( c) ?50 threshold voltage (v) 1.50 2.00 2.50 25 75 125 3681 g22 1.00 0.50 0 ?25 0 50 100 current limit clamp switching threshold v c voltages 3681 g24 i l 0.5a/div v sw 5v/div v out 10mv/div 2 s/div v in = 12v; front page application i load = 10ma fb pin voltage (v) 1.065 ?80 v c pin current ( a) ?60 ?20 0 20 1 .265 1.465 100 3681 g19 ?40 1.165 1.365 40 60 80 load current (a) 0.001 input voltage (v) 3.0 3.5 10 3681 g20 2.5 2.0 0.01 0.1 1 4.5 4.0 v out = 3.3v l = 4.7 f = 800khz load current (a) 0.001 input voltage (v) 5.0 5.5 10 3681 g20 4.5 4.0 0.01 0.1 1 6.5 6.0 v out = 5.0v l = 4.7 f = 800khz temperature ( c) ?50 threshold voltage (v) 1.160 1.180 1.200 25 75 125 3681 g23 1.140 1.120 1.100 ?25 0 50 100 pg rising 3681 g25 i l 0.5a/div v run/ss 5v/div v out 10mv/div v in = 12v; front page application i load = 140ma 1 s/div 3681 g26 i l 0.5a/div v run/ss 5v/div v out 10mv/div v in = 12v; front page application i load = 1a 1 s/div error amp output current minimum input voltage minimum input voltage power good threshold switching waveforms; transition from burst mode to full frequency switching waveforms; full frequency continuous operation switching waveforms; burst mode typical perfor a ce characteristics uw power schottky diode forward voltage vs current forward voltage (v) 0 current (ma) 0.2 3681 g29 3000 0.1 0.3 1000 0 6000 5000 4000 2000 0.4 0.5 0.6 0.7 t a = 25c unless otherwise noted.
lt3681 7 3681f pi fu ctio s uuu pg (pin 1): the pg pin is the open collector output of an internal comparator. pg remains low until the fb pin is within 10% of the ? nal regulation voltage. pg output is valid when v in is above 3.5v and run/ss is high. bias (pin 2): the bias pin supplies the current to the lt3681s internal regulator. tie this pin to the lowest available voltage source above 3v (typically v out ). this architecture increases ef? ciency especially when the input voltage is much higher than the output. fb (pin 3): the lt3681 regulates the fb pin to 1.265v. connect the feedback resistor divider tap to this pin. v c (pin 5): the v c pin is the output of the internal error ampli? er. the voltage on this pin controls the peak switch current. tie an rc network from this pin to ground to compensate the control loop. rt (pin 6): oscillator resistor input. connecting a resistor to ground from this pin sets the switching frequency. da (pin 8): this is the anode of the integrated power schottky diode. high frequency, large amplitude currents ? ow through this pin, so tie it to ground through a low impedance connection. dc (pin 9, exposed pad 16): these pins connect to the cathode of the integrated power schottky diode. high fre- quency, large amplitude currents ? ow through these pins, so tie them to sw through a low impedance connection. bd (pin 10): this pin connects to the anode of the internal boost schottky diode. boost (pin 11): this pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. connect a capacitor between this pin and sw. sw (pin 12): the sw pin is the output of the internal power switch. connect this pin to the inductor, dc and boost capacitor. v in (pin 13): the v in pin supplies current to the lt3681s internal regulator and to the internal power switch. this pin must be locally bypassed. run/ss (pin 14): the run/ss pin is used to put the lt3681 in shutdown mode. tie to ground to shut down the lt3681. tie to 2.3v or more for normal operation. if the shutdown feature is not used, tie this pin to the v in pin. run/ss also provides a soft-start function; see the applications information section. gnd (pins 4, 7, exposed pad 15): all three of these terminals internally connect to the lt3681 control ics signal return, while exposed pad 15 performs the added function of providing a low thermal resistance heat ? ow path between the ic and the system heatsink. tie all of these terminals to a copper pour on the top layer of the printed circuit board. please refer to the applications information section for more details.
lt3681 8 3681f the lt3681 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 . an error ampli? er measures the output voltage through an external resistor divider tied to the fb pin and servos the v c pin. if the error ampli? ers output increases, more current is delivered to the output; if it decreases, less cur- rent is delivered. an active clamp on the v c pin provides current limit. the v c pin is also clamped to the voltage on the run/ss pin; soft-start is implemented by generating a voltage ramp at the run/ss pin using an external resistor and capacitor. an internal regulator provides power to the control cir- cuitry. the bias regulator normally draws power from the v in pin, but if the bias pin is connected to an external voltage higher than 3v bias power will be drawn from the external source (typically the regulated output voltage). this improves ef? ciency. the run/ss pin is used to place the lt3681 in shutdown, disconnecting the output and reducing the input current to less than 1a. the switch driver operates from either the input 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 lt3681 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 55a in a typical application. the oscillator reduces the lt3681s operating frequency when the voltage at the fb pin is low. this frequency foldback helps to control the output current during startup and overload. the lt3681 contains a power good comparator which trips when the fb pin is at 91% of its regulated value. the pg output is an open-collector 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 lt3681 is enabled and v in is above 3.6v. the lt3681 integrates a high quality, 36v, 2a power schottky diode to reduce the overall solution size. operation block diagra w + ? + ? + ? oscillator 300khz?2.8mhz burstmode detect v c clamp soft-start slope comp internal 1.265v ref r v in v in bias run/ss boost sw switch latch v c v out c2 c3 c f l1 dc dc da disable c c r c bd rt r2 error amp r1 fb r t c1 pg 1.12v s q 3681 bd 13 2 14 6 1 10 11 12 9 16 8 5 gnd 15 gnd 7 gnd 43
lt3681 9 3681f fb resistor network the output voltage is programmed with a resistor divider between the output and the fb pin. choose the 1% resis- tors according to: rr v out 12 1 265 1 = ? ? ? ? ? ? . ? reference designators refer to the block diagram. setting the switching frequency the lt3681 uses a constant frequency pwm architecture that can be programmed to switch from 300khz to 2.8mhz by using a resistor tied from the rt pin to ground. a table showing the necessary rt value for a desired switching frequency is in figure 1. switching frequency (mhz) r t value (k ) 0.3 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 187 133 84.5 60.4 45.3 36.5 29.4 23.7 20.5 16.9 14.3 12.1 10.2 8.66 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 d out on min dinsw () () = + + () ? where v in is the typical input voltage, v out is the out- put voltage, v d is the power schottky catch diode drop (~0.55v), 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 next section, lower frequency al- lows a lower dropout voltage. the reason input voltage range depends on the switching frequency is because the lt3681 switch has ? nite minimum on and off times. the switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. this means that the minimum and maximum duty cycles 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 (~150ns), and the t off(min) is the minimum switch off time (~150ns). 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 next section) and keep the inductor and capacitor values small. input voltage range the maximum input voltage for lt3681 applications de- pends on switching frequency, the absolute maximum rat- ings on v in and boost pins, and on operating mode. if the output is in start-up or short-circuit operating modes, then v in must be below 34v and below the result of the following equation: v vv ft vv in max out d sw on min dsw () () = + + ? where v in(max) is the maximum operating input voltage, v out is the output voltage, v d is the catch diode drop (~0.55v), v sw is the internal switch drop (~0.5v at max load), f sw is the switching frequency (set by r t ), and t on(min) is the minimum switch on time (~150ns). note that a higher switching frequency will depress the maximum operating input voltage. conversely, a lower switching figure 1. switching frequency vs. rt value applications information
lt3681 10 3681f frequency will be necessary to achieve safe operation at high input voltages. if the output is in regulation and no short-circuit or start-up events are expected, then input voltage transients of up to 36v are acceptable regardless of the switching frequency. in this mode, the lt3681 may enter pulse skipping opera- tion where some switching pulses are skipped to maintain output regulation. in this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. the minimum input voltage is determined by either the lt3681s minimum operating voltage of ~3.6v or by its maximum duty cycle (see equation in previous 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, and t off(min) is the minimum switch off time (150ns). note that higher switching frequency will increase the minimum input voltage. if a lower dropout voltage is desired, a lower switching frequency should be used. inductor selection for a given input and output voltage, the inductor value and switching frequency will determine the ripple current. the ripple current i l increases with higher v in or v out and decreases with higher inductance and faster switch- ing frequency. a reasonable starting point for selecting the ripple current is: i l = 0.4(i out(max) ) where i out(max) is the maximum output load current. to guarantee suf? cient output current, peak inductor current must be lower than the lt3681s switch current limit (i lim ). the peak inductor current is: i l(peak) = i out(max) + i l /2 where i l(peak) is the peak inductor current, i out(max) is the maximum output load current, and i l is the inductor ripple current. the lt3681s switch current limit (i lim ) is at least 3.5a at low duty cycles and decreases linearly to 2.5a at dc = 0.8. the maximum output current is a func- tion of the inductor ripple current: i out(max) = i lim C i l /2 be sure to pick an inductor ripple current that provides suf? cient maximum output current (i out(max) ). the largest inductor ripple current occurs at the highest v in . to guarantee that the ripple current stays below the speci? ed maximum, the inductor value should be chosen according to the following equation: l vv fi vv v out d l out d in max = + ? ? ? ? ? ? + ? ? ? ? ? ? ? () ? 1? ?? where v d is the voltage drop of the integrated schottky diode (~0.55v), v in(max) is the maximum input voltage, v out is the output voltage, f sw is the switching frequency (set by rt), and l is in the inductor value. 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 3.5a. 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 1 lists several vendors and suitable types. table 1. 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 d75c d75f fdv0620 shielded open shielded sumida www.sumida.com cdrh74 cdrh6d38 cr75 cdrh8d43 shielded shielded open shielded nec www.nec.tokin.com plc-0745 shielded applications information
lt3681 11 3681f of course, such a simple design guide will not always re- sult in the optimum inductor for your application. a larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. if your load is lower than 2a, then you can decrease 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. there are several graphs in the typical performance characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. for details of maximum output current and discontinuous mode opera- tion, see linear technology application note 44. finally, for duty cycles greater than 50% (v out /v in > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. see application note 19. input capacitor bypass the input of the lt3681 circuit with a ceramic capaci- tor 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 lt3681 and will easily handle the ripple current. note that larger input capacitance is required when a lower switching frequency is used. 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 lt3681 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 lt3681 and the catch diode (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt3681. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt3681 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt3681s voltage rating. this situation is easily avoided (see the hot plugging safely section). for space sensitive applications, a 2.2f ceramic capaci- tor can be used for local bypassing of the lt3681 input. however, the lower input capacitance will result in in- creased input current ripple and input voltage ripple, and may couple noise into other circuitry. also, the increased voltage ripple will raise the minimum operating voltage of the lt3681 to ~3.7v. output capacitor and output ripple the output capacitor has two essential functions. along with the inductor, it ? lters the square wave generated by the lt3681 to produce the dc output. in this role it determines the output ripple, and low impedance at the switching frequency is important. the second function is to store energy in order to satisfy transient loads and stabilize the lt3681s 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 if the compensation network is also adjusted to maintain the loop bandwidth. a lower value of output capacitor can be used to save space and cost but transient performance will suffer. see the fre- quency compensation section to choose an appropriate compensation network. applications information
lt3681 12 3681f 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. high performance tantalum or electrolytic capacitors can be used for the output capacitor. low esr is important, so choose one that is intended for use in switching regulators. the esr should be speci? ed by the supplier, and should be 0.05 or less. such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low esr. table 2 lists several capacitor vendors. catch diode the integral power schottky catch diode conducts current only during switch off time. average forward current in normal operation can be calculated from: i d(avg) = i out (v in C v out )/v in where i out is the output load current. ceramic capacitors ceramic capacitors are small, robust and have very low esr. however, ceramic capacitors can cause problems when used with the lt3681 due to their piezoelectric nature. when in burst mode operation, the lt3681s switching frequency depends on the load current, and at very light loads the lt3681 can excite the ceramic capacitor at audio frequencies, generating audible noise. since the lt3681 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 lt3681. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt3681 circuit is plugged into a live supply, the input volt- age can ring to twice its nominal value, possibly exceeding the lt3681s rating. this situation is easily avoided (see the hot plugging safely section). frequency compensation the lt3681 uses current mode control to regulate the output. this simpli? es loop compensation. in particular, the lt3681 does not require the esr of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. frequency compensation is provided by the components tied to the v c pin, as shown in figure 2. generally a capacitor (c c ) and a resistor (r c ) in series to ground are used. in ad- dition, there may be a lower value capacitor in parallel. this capacitor (c f ) is not part of the loop compensation but is used to ? lter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high esr. vendor phone url part series comments panasonic (714) 373-7366 www.panasonic.com ceramic, polymer, tantalum eef series kemet (864) 963-6300 www.kemet.com ceramic, tantalum t494, t495 sanyo (408) 749-9714 www.sanyovideo.com ceramic, polymer, tantalum poscap murata (408) 436-1300 www.murata.com ceramic avx www.avxcorp.com ceramic, tantalum tps series taiyo yuden (864) 963-6300 www.taiyo-yuden.com ceramic table 2. capacitor vendors applications information
lt3681 13 3681f loop compensation determines the stability and transient performance. designing the compensation network is a bit complicated and the best values depend on the ap- plication and in particular the type of output capacitor. a practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the compensation network to optimize the performance. stability should then be checked across all operating conditions, including load current, input voltage and temperature. the lt1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. figure 2 shows an equivalent circuit for the lt3681 control loop. the error ampli? er is a transconductance ampli? er with ? nite output impedance. the power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance ampli? er generating an output cur- rent proportional to the voltage at the v c pin. note that the output capacitor integrates this current, and that the capacitor on the v c pin (c c ) integrates the error ampli- ? er output current, resulting in two poles in the loop. in most cases a zero is required and comes from either the output capacitor esr or from a resistor r c in series with c c . this simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. a phase lead capacitor (c pl ) across the feedback divider may improve the transient response. figure 3 shows the transient response when the load current is stepped from 500ma to 1500ma and back to 500ma. burst mode operation to enhance ef? ciency at light loads, the lt3681 auto- matically switches to burst mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. during burst mode operation, the lt3681 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. in addition, v in and bias quiescent currents are reduced to typically 20a and 50a respectively during the sleep time. as the load current decreases towards a no load condition, the percentage of time that the lt3681 operates in sleep mode increases and the average input current is greatly reduced resulting in higher ef? ciency. see figure 4. ? + 1.265v sw v c gnd 3meg lt3681 3681 f02 r1 output esr c f c c r c error amplifier fb r2 c1 c1 current mode power stage g m = 3.5mho g m = 330 mho + polymer or tantalum ceramic c pl figure 2. model for loop response figure 3. transient load response of the lt3681 3.3v application as the load current is stepped from 500ma to 1500ma. 3681 f03 i l 1a/div v out 100mv/div 10 s/div v in = 12v applications information figure 4. burst mode operation 3681 f04 i l 0.5a/div v sw 5v/div v out 10mv/div 5 s/div v in = 12v; v out = 3.3v i load = 10ma
lt3681 14 3681f 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 discontinuous and the effective duty cycle at the boost pin (not the sw pin) can be very high. this reduces the minimum input voltage to approximately 300mv 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 lt3681, requiring a higher input voltage to maintain regulation. boost and bias 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 0.22f capacitor will work well. figure 5 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. for outputs of 2.8v and above, the standard circuit (figure 5a) 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 (see figure 5b). for lower output voltages the boost diode can be tied to the input (figure 5c), or to another supply greater than 2.8v. the circuit in figure 5a is more ef? cient because the boost pin current and bias pin quiescent current comes from a lower voltage source. you must also be sure that the maximum voltage ratings of the boost and bias pins are not exceeded. the minimum operating voltage of an lt3681 application is limited by the minimum input voltage (3.6v) and by the maximum duty cycle as outlined in a previous section. for proper startup, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, or the lt3681 is turned on with its run/ss pin when the output is already in regulation, then 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. if, however, the lt3681 is started by the run/ss pin and the output is discharged, the discharged output capacitance will often present enough of a load to allow the circuit to start. figure 6 gives plots of the input voltage required for three different situations: the worst case situation where run/ss is tied to v in and v in is ramped up very slowly, the minimum input voltage at which the circuit will regulate when start-up is controlled by run/ss, and the minimum input voltage required to maintain output regulation. for lower start-up voltage, the v in boost sw bd v in v out 4.7 f c3 gnd dc lt3681 v in boost sw bd v in v out 4.7 f c3 d2 gnd lt3681 v in boost sw bd v in v out 4.7 f c3 gnd da da da lt3681 3681 fo5 c out c out c out (5a) for v out > 2.8v (5b) for 2.5v < v out < 2.8v (5c) for v out < 2.5v dc dc figure 5. three circuits for generating the boost voltage applications information
lt3681 15 3681f soft-start the run/ss pin can be used to soft-start the lt3681, reducing the maximum input current during start-up. the run/ss pin is driven through an external rc ? lter to create a voltage ramp at this pin. figure 7 shows the start- up and shut-down waveforms with the soft-start circuit. by choosing a large rc time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. choose the value of the resistor so that it can supply 20a when the run/ss pin reaches 2.3v. synchronization the internal oscillator of the lt3681 can be synchronized to an external 275khz to 475khz clock by using a 5pf to 20pf capacitor to connect the clock signal to the rt pin. the resistor tying the rt pin to ground should be chosen such that the lt3681 oscillates 20% lower than the intended synchronization frequency (see setting the switching frequency section). the lt3681 should not be synchronized until its output is near regulation as indicated by the pg ? ag. this can be done with the system microcontroller/microprocessor or with a discrete circuit by using the pg output. if a sync signal is applied while the pg is low, the lt3681 may exhibit erratic operation. when applying a sync signal, positive clock transitions reset lt3681s internal clock and negative transitions initiate a switch cycle. the amplitude of the sync signal must be at least 2v. the sync signal duty cycle can range from 5% up to a maximum value given by the following equation: dc vv vv v f sync max out d in sw d sw () = + + ? ? ? ? ? ? 16 ? ? ?0 00ns where v out is the programmed output voltage, v d is the diode forward drop, v in is the typical input voltage, v sw is the switch drop, and f sw is the desired switching fre- quency. for example, a 24v input to 5v output at 300khz can be synchronized to a square wave with a maximum duty cycle of 60%. for some applications, such as 12v in to 5v out at 350khz, the maximum allowable sync duty cycle will be less than 50%. if a low duty cycle clock cannot be obtained from the system, then a one-shot should be used between the sync signal and the lt3681. the value of the coupling capacitor which connects the clock signal to the rt pin should be chosen based on the clock signal amplitude. good starting values for 3.3v and 5v clock signals are 10pf and 5pf, respectively. these values should be tested and adjusted for each individual application to assure reliable operation. figure 6. the minimum input voltage depends on output voltage, load current and boost circuit load current (a) 0.001 input voltage (v) 4.0 4.5 5.0 10 3.5 3.0 2.0 0.01 0.1 1 2.5 6.0 5.5 3681 f06 load current (a) 0.001 input voltage (v) 5.0 4.5 6.0 5.5 7.0 6.5 10 4.0 3.5 2.0 0.01 0.1 1 3.0 2.5 8.0 7.5 v out = 3.3v t a = 25 c l = 4.7 f = 800 khz to start (run/ss = v in ) to start (run/ss control) to run to start (run/ss = v in ) to start (run/ss control) to run v out = 5.0v t a = 25 c l = 4.7 f = 800 khz figure 7. to soft-start the lt3681, add a resistor and capacitor to the run/ss pin 3681 f07 i l 1a/div v run/ss 2v/div v out 2v/div run/ss gnd 0.22 f run 15k 2ms/div applications information
lt3681 16 3681f caution should be used when synchronizing more than 50% above the initial switching frequency (as set by the rt resistor) because at higher clock frequencies the amplitude of the internal slope compensation used to prevent subharmonic switching is reduced. this type of subharmonic switching only occurs at input voltages less than twice output voltage. higher inductor values will tend to reduce this problem. reversed input protection in some systems, the output may be held high when the input to the lt3681 is absent. this may occur in battery charging applications or in battery backup systems where 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 lt3681 runs only when the input is present v in boost da bd fb run/ss v c sw dc d4 mbrs140 v in lt3681 3681 f08 v out backup gnd applications information a battery or some other supply is diode ored with the lt3681s output. if the v in pin is allowed to ? oat and the run/ss pin is held high (either by a logic signal or because it is tied to v in ), then the lt3681s internal circuitry will pull its quiescent current through its sw pin. this is ? ne if your system can tolerate a few ma in this state. if you ground the run/ss pin, the sw pin current will drop to essentially zero. however, if the v in pin is grounded while the output is held high, then parasitic diodes inside the lt3681 can pull large currents 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.
lt3681 17 3681f 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 lt3681s v in and sw pins, the integrated schottky diode the input capacitor (c in ) and the output capacitor (c out ). 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, figure 9. a good pcb layout ensures proper, low emi operation 1 2 14 13 3 4 12 11 10 9 8 vias to gnd vias to v out vias to v in vias to dc heatsink 5 6 7 v in c in c out 3681 f11 unbroken ground plane below these components. the sw and boost nodes should be as small as possible. finally, keep the fb and v c nodes small so that the ground traces will shield them from the sw and boost nodes. each of the exposed pads on the bottom of the package must be soldered to copper pours 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 lt3681 to additional ground planes within the circuit board and on the bottom side. keep in mind that the thermal design must keep the junctions of the ic and power diode below the speci? ed absolute maximum temperature of 125c. applications information
lt3681 18 3681f hot plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt3681 circuits. however, these capacitors can cause problems if the lt3681 is plugged into a live supply (see linear technology application note 88 for a complete discussion). 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 lt3681 can ring to twice the nominal input voltage, possibly exceeding the lt3681s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt3681 into an energized supply, the input network should be designed to prevent this overshoot. figure 10 shows the waveforms that result when an lt3681 circuit is connected to a 24v supply through six feet of 24-gauge twisted pair. the ? rst plot (10a) is the response with a 4.7f ceramic capacitor at the input. the input voltage rings as high as 50v and the input current peaks at 26a. a good solution is shown in figure 10b. a 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak inrush current). a 0.1f capacitor improves high frequency ? ltering. for high input voltages its impact on ef? ciency is minor, reducing ef? ciency by 1.5 percent for a 5v output at full load operating from 24v. another effective method of reducing the overshoot is to add a 22f aluminum electrolytic capacitor, as shown in figure 10c. high temperature considerations the pcb must provide heat sinking to keep the lt3681 cool. the exposed pads on the bottom of the package must be soldered to copper pours, which in turn should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the lt3681. place additional vias to reduce thermal resistance further. with these steps, the thermal resistance from die (or junction) to ambient can be reduced to ja = 35c/w or less. with 100 lfpm air? ow, this resistance can fall by another 25%. further increases in air? ow will lead to lower thermal resis- tance. because of the large output current capability of the lt3681, it is possible to dissipate enough power to raise the junction temperature beyond the absolute maximum of applications information 125c. when operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125c. power dissipation within the lt3681 can be estimated by calculating the total power loss from an ef? ciency measurement. the die temperature is calculated by multiplying the lt3681 power dissipation by the thermal resistance from junction to ambient. also keep in mind that the leakage current of the integrated power schottky diode, like all schottky diodes, goes up with junction temperature. the curves in figure 11 show how the leakage current in the power schottky diode varies with temperature and reverse voltage. 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. figure 11. like all schottky diodes, the lt3681 integrated power diode leakage current varies with temperature and applied reverse voltage v r . temperature ( c) ?50 1 leakage current ( a) 100 10000 0 50 100 150 3681 f12 10 1000 v r = 10v v r = 25v v r = 40v other linear technology publications application notes 19, 35, 44 and 76 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 100 shows how to generate a bipolar output supply using a buck regulator.
lt3681 19 3681f applications information figure 10. a well chosen input network prevents input voltage overshoot and ensures reliable operation when the lt3681 is connected to a live supply + lt3681 4.7 f v in 20v/div i in 10a/div 20 s/div v in closing switch simulates hot plug i in (10a) (10b) low impedance energized 24v supply stray inductance due to 6 feet (2 meters) of twisted pair + lt3681 4.7 f 0.1 f 0.7 v in 20v/div i in 10a/div 20 s/div danger ringing v in may exceed absolute maximum rating (10c) + lt3681 4.7 f 22 f 35v ai.ei. 3681 f10 v in 20v/div i in 10a/div 20 s/div +
lt3681 20 3681f typical applications 5v step-down converter sw dc bias fb v c pg rt v in bd v in 6.3v to 34v v out 5v 2a 4.7 f 0.47 f 22 f 200k f = 800khz l: nec plc-0745-6r8 20k 60.4k l 6.8 h 590k gnd da 330pf on off lt3681 3681 ta02 run/ss boost 3.3v step-down converter sw bias fb v c pg rt v in bd v in 4.4v to 34v v out 3.3v 2a 4.7 f 0.47 f 22 f 200k f = 800khz l: nec plc-0745-4r7 20k 60.4k l 4.7 h 324k gnd 330pf on off lt3681 3681 ta03 run/ss boost dc da
lt3681 21 3681f typical applications 5v, 2mhz step-down converter sw bias fb v c pg rt v in bd v in 8.6v to 22v transient to 36v v out 5v 2a 2.2 f 0.47 f 10 f 200k f = 2mhz l: toko fdv0620-2r2 6.8k 16.9k l 2.2 h 590k gnd 470pf on off lt3681 3681 ta05 run/ss boost dc da sw bias fb v c pg rt v in bd v in 4v to 34v v out 2.5v 2a 4.7 f 1 f 47 f 200k f = 600khz d2: mbr0540 l: sumida cdrh8d43-4r7 22k 84.5k l 4.7 h 196k gnd 680pf on off lt3681 d2 3681 ta04 run/ss boost dc da 2.5v step-down converter
lt3681 22 3681f 12v step-down converter sw bias fb v c pg rt v in bd v in 15v to 34v v out 12v 2a 10 f 0.47 f 22 f 100k f = 800khz l: sumida cdrh8d43-100 24k 60.4k l 10 h 845k gnd 470pf on off lt3681 3681 ta06 run/ss boost dc da typical applications 1.8v step-down converter sw dc da bias fb v c pg rt v in bd v in 3.6v to 27v v out 1.8v 2a 10 f 0.47 f 47 f 200k f = 500khz l: sumida cdrh8d28-3r3 15k 105k l 3.3 h 84.5k gnd 330pf on off lt3681 3681 ta09 run/ss boost
lt3681 23 3681f package description de14ma package 14-lead plastic dfn, multichip (4mm 3mm) (reference ltc dwg # 05-08-1731 rev 0) 3.00 0.10 (2 sides) 4.00 0.10 (2 sides) note: 1. drawing proposed is not a jedec package outline 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 0.40 0.10 1.65 0.10 1.65 0.10 bottom view?exposed pad 0.75 0.05 r = 0.115 typ r = 0.05 typ 3.00 ref 1 7 14 8 pin 1 top mark (see note 6) 0.200 ref 0.00 ? 0.05 (de14ma) dfn 1106 rev ? pin 1 notch r = 0.20 or 0.25 45 chamfer 3.00 ref recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 2.10 0.05 0.70 0.05 3.50 0.05 package outline 0.25 0.05 0.25 0.05 0.50 bsc 1.78 0.10 0.10 typ 0.10 typ 1.07 0.10 1.07 0.05 0.51 typ 0.50 bsc 1.78 0.05 1.65 0.05 1.65 0.05 0.51 typ 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.
lt3681 24 3681f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2006 lt 0407 ? printed in usa typical applicatio u related parts sw bias fb v c pg rt v in bd v in 3.6v to 27v v out 1.265v 2a 4.7 f 0.47 f 47 f f = 500khz l: nec plc-0745-3r3 13k 105k l 3.3 h gnd 330pf on off lt3681 3681 ta10 run/ss boost dc da 1.265v step-down converter part number description comments lt1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in = 5.5v to 60v, v out = 1.20v, i q = 2.5ma, i sd 25 a tssop16e package lt1767 25v, 1.2a (i out ), 1.2mhz, high ef? ciency step-down dc/dc converter v in = 3.0v to 25v, v out = 1.20v, i q = 1ma, i sd < 6 a ms8e package lt1933 500ma (i out ), 500khz step-down switching regulator in sot-23 v in = 3.6v to 36v, v out = 1.2v, i q = 1.6ma, i sd < 1 a thinsot package lt1936 36v, 1.4a (i out ), 500khz high ef? ciency step-down dc/dc converter v in = 3.6v to 36v, v out = 1.2v, i q = 1.9ma, i sd < 1 a ms8e package lt1940 dual 25v, 1.4a (i out ), 1.1mhz, high ef? ciency step-down dc/dc converter v in = 3.6v to 25v, v out = 1.20v, i q = 3.8ma, i sd < 30 a tssop16e package lt1976/lt1977 60v, 1.2a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converter with burst mode v in = 3.3v to 60v, v out = 1.20v, i q = 100 a, i sd < 1 a tssop16e package lt3434/lt3435 60v, 2.4a (i out ), 200/500khz, high ef? ciency step-down dc/dc converter with burst mode v in = 3.3v to 60v, v out = 1.20v, i q = 100 a, i sd < 1 a tssop16e package lt3437 60v, 400ma (i out ), micropower step-down dc/dc converter with burst mode v in = 3.3v to 60v, v out = 1.25v, i q = 100 a, i sd < 1 a (3mm 3mm) dfn-10 tssop16e 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 = 3.6v to 38v, v out = 0.78v, i q = 70 a, i sd < 1 a (3mm 3mm) dfn-10 msop10e 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 = 3.6v to 34v, v out = 1.265v, i q = 50 a, i sd < 1 a (3mm 3mm) dfn-10 msop10e package lt3493 36v, 1.4a (i out ), 750khz high ef? ciency step-down dc/dc converter v in = 3.6v to 36v, v out = 0.8v, i q = 1.9ma, i sd < 1 a (2mm 3mm)dfn-6 package lt3505 36v with transient protection to 40v, 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in = 3.6v to 34v, v out = 0.78v, i q = 2ma, i sd < 2 a (3mm 3mm) dfn-8 msop8e package lt3508 36v with transient protection to 40v, dual 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in = 3.7v to 37v, v out = 0.8v, i q = 4.6ma, i sd < 1 a (4mm 4mm) qfn-24 tssop16e package lt3684 34v with transient protection to 36v, 2a (i out ), 2.8mhz, high ef? ciency step-down dc/dc converter v in = 3.6v to 34v, v out = 01.26v, i q = 850ma, i sd < 1 a (3mm 3mm) dfn-10 msop10e package lt3685 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high ef? ciency step-down dc/dc converter v in = 3.6v to 38v, v out = 0.78v, i q = 70ma, i sd < 1 a (3mm 3mm) dfn-10 msop10e package

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