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lt3695 series 1 3695fa typical application description 1a fault tolerant micropower step-down regulator the lt ? 3695 series are adjustable frequency (250khz to 2.2mhz) monolithic buck switching regulators that accept input voltages up to 36v and can safely sustain transient voltages up to 60v. the devices include a high ef? ciency switch, a boost diode, and the necessary oscillator, control and logic circuitry. current mode topology is used for fast transient response and good loop stability. a sync pin allows the user to synchronize the part to an external clock, and to choose between low ripple burst mode operation and standard pwm operation. the lt3695 regulators tolerate adjacent pin shorts or an open pin without raising the output voltage above its programmed value. low ripple burst mode operation maintains high ef? - ciency at low output currents while keeping output ripple below 15mv in a typical application. 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). protection circuitry senses the current in the power switch and external schottky catch diode to protect the lt3695 regulators against short-circuit conditions. frequency foldback and thermal shutdown provide additional protection. the lt3695 series is available in a thermally enhanced 16-pin msop package. 5v step-down converter features applications n wide input range: operation from 3.6v to 36v overvoltage lockout protects circuits through 60v transients n fmea fault tolerant: output stays at or below regulation voltage during adjacent pin short or when a pin is left floating n 1a output current n low ripple (< 15mv p-p ) burst mode ? operation i q = 75a for 12v in to 3.3v out with no load n adjustable switching frequency: 250khz to 2.2mhz n short-circuit protected n synchronizable between 300khz and 2.2mhz n output voltage: 0.8v to 20v n power good flag n fixed output voltage versions for 3.3v and 5v available n small, thermally enhanced 16-pin msop package n automotive battery regulation n automotive entertainment systems n distributed supply regulation n industrial supplies ef? ciency v in bd lt3695 run/ssv c 0.22f v out 5v0.9a, v in > 6.9v 1a, v in > 12v 102k f = 800khz 536k 10h 10f 3695 ta01a 470pf 2.2f v in 6.9v to 36v transient to 60v rtpg 40.2k 16.2k sync boost on off sw da fb gnd pgnd load current (a) 0 50 efficiency (%) 60 70 80 90 100 0.2 0.4 0.6 0.8 1 v out = 5v v out = 3.3v v in = 12v l = 10hf = 800khz 3695 ta01b l , lt, ltc, ltm, burst mode, linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. downloaded from: http:///
lt3695 series 2 3695fa v in , run/ss voltage (note 3) ...................................60v boost pin voltage ...................................................50v boost pin above sw pin .........................................30v bd voltage (lt3695) .................................................30v rt , v c voltage ............................................................5v rt pin current .........................................................1ma sync voltage ............................................................20v fb voltage (lt3695) ....................................................5v order information lead free finish tape and reel part marking* package description temperature range lt3695emse#pbf lt3695emse#trpbf 3695 16-lead plastic msop C40c to 125c lt3695imse#pbf lt3695imse#trpbf 3695 16-lead plastic msop C40c to 125c lt3695hmse#pbf lt3695hmse#trpbf 3695 16-lead plastic msop C40c to 150c lt3695emse-3.3#pbf lt3695emse-3.3#trpbf 369533 16-lead plastic msop C40c to 125c lt3695imse-3.3#pbf lt3695imse-3.3#trpbf 369533 16-lead plastic msop C40c to 125c lt3695hmse-3.3#pbf lt3695hmse-3.3#trpbf 369533 16-lead plastic msop C40c to 150c lt3695emse-5#pbf lt3695emse-5#trpbf 36955 16-lead plastic msop C40c to 125c lt3695imse-5#pbf lt3695imse-5#trpbf 36955 16-lead plastic msop C40c to 125c lt3695hmse-5#pbf lt3695hmse-5#trpbf 36955 16-lead plastic msop C40c to 150c 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/ absolute maximum ratings (notes 1, 2) lt3695 lt3695-3.3, lt3695-5 12 3 4 5 6 7 8 pgnd danc sw run/ss rt sync v in 1615 14 13 12 11 10 9 boostbd gnd pg nc fb nc v c top view 17 pgnd mse package 16-lead plastic msop ja = 40c/w with exposed pad soldered ja = 110c/w without exposed pad soldered 12 3 4 5 6 7 8 pgnd danc sw run/ss rt sync v in 1615 14 13 12 11 10 9 boostnc out1 out2 gnd pg nc v c top view mse package 16-lead plastic msop 17 pgnd ja = 40c/w with exposed pad soldered ja = 110c/w without exposed pad soldered pin configuration out1, out2 voltage (lt3695-3.3, lt3695-5) ...........16v pg voltage ................................................................30v operating junction temperature range (notes 4, 5) lt3695e ............................................. C40c to 125c lt3695i .............................................. C40c to 125c lt3695h ............................................ C40c to 150c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c downloaded from: http:///
lt3695 series 3 3695fa electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, v run/ss = 10v, unless otherwise noted. (note 4) parameter conditions min typ max units minimum operating voltage (note 6) lt3695 v bd = 3.3v v bd < 3v ll 3.43.4 3.64.3 vv lt3695-3.3 v out1,2 = 3.3v l 3.4 3.6 v lt3695-5 v out1,2 = 5v l 3.4 3.6 v v in overvoltage lockout l 36 38 39.9 v quiescent current from v in lt3695 v run/ss = 0.2v v bd = 3.3v v run/ss = 10v, v bd = 3.3v, not switching v run/ss = 10v, v bd = 0v, not switching l 0.01 3590 0.5 60 160 aa a lt3695-3.3 v run/ss = 10v, v out1,2 = 3.3v, not switching l 35 60 a lt3695-5 v run/ss = 10v, v out1,2 = 5v, not switching l 35 60 a quiescent current from bd pin lt3695 v run/ss = 0.2v, v bd = 3.3v v run/ss = 10v, v bd = 3.3v, not switching v run/ss = 10v, v bd = 0v, not switching l 35 0.01 55 0 0.5 100 C5 aa a quiescent current from out1,2 pins lt3695-3.3 v run/ss = 0.2v v run/ss = 10v, v out1,2 = 3.3v, not switching l 5 43 1065 15 112 aa lt3695-5 v run/ss = 0.2v v run/ss = 10v, v out1,2 = 5v, not switching l 5 43 1065 15 112 aa minimum bd pin voltage: lt3695 2.8 3 v feedback voltage: lt3695 l 792785 800800 808815 mvmv fb pin bias current: lt3695 fb pin voltage = 800mv l C5 C40 na reference voltage line regulation 3.6v < v in < 36v 0.001 0.005 %/v output voltage lt3695-3.3 l 3.273.25 3.33.3 3.333.35 vv lt3695-5 l 4.95 4.925 55 5.05 5.075 vv error amp g m i vc = 1.5a 430 s error amp voltage gain 1300 v/v v c source current 50 a v c sink current 50 a v c pin to switch current gain 1.25 a/v v c switching threshold 0.4 0.6 0.8 v v c clamp voltage 2v switching frequency r rt = 8.06k r rt = 29.4k r rt = 158k 1.98 0.9 225 2.21.0 250 2.42 1.1 275 mhzmhz khz minimum switch off-time e- and i-grades h-grade ll 130130 210250 nsns switch current limit (note 7) sync = 0v sync = 3.3v or clocked 1.451.18 1.71.4 2 1.66 aa downloaded from: http:///
lt3695 series 4 3695fa electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, v run/ss = 10v, unless otherwise noted. (note 4) parameter conditions min typ max units switch v cesat i sw = 1a 350 mv da pin current to stop osc 1.25 1.6 1.95 a switch leakage current v sw = 0v, v in = 36v 0.01 1 a boost schottky diode voltage drop i bsd = 50ma 720 900 mv boost schottky diode reverse leakage v sw = 10v, v bd = 0v 0.1 1 a minimum boost voltage (note 8) l 1.7 2.3 v boost pin current i sw = 0.5a 10.5 17.5 ma run/ss pin current v run/ss = 2.5v v run/ss = 10v l 4.5 12 7.5 20 aa run/ss input voltage high 2.5 v run/ss input voltage low 0.2 v pg leakage current v pg = 5v 0.1 1 a pg sink current v pg = 0.4v l 100 1000 a pg threshold as % of v fb (lt3695) or v out (lt3695-3.3, lt3695-5) measured at fb (lt3695) or out1,2 (lt3695-3.3, lt3695-5) pins (pin voltage rising) 88 90 92 % pg threshold hysteresis lt3695 measured at fb pin 12 mv lt3695-3.3 measured at out1,2, pins 50 mv lt3695-5 measured at out1,2, pins 75 mv sync threshold voltage 300 550 800 mv sync input frequency 0.3 2.2 mhz 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 the device reliability and lifetime. note 2: positive currents ? ow into pins, negative currents ? ow out of pins. minimum and maximum values refer to absolute values.note 3: absolute maximum voltage at v in and run/ss pins is 60v for nonrepetitive 1 second transients, and 36v for continuous operation.note 4: the lt3695e regulators are 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, characterization and correlation with statistical process controls. the lt3695i regulators are guaranteed over the full C40c to 125c operating junction temperature range. the lt3695h regulators are guaranteed over the full C40c to 150c operating junction temperature range. note 5: these ics include overtemperature protection that is intended to protect the devices during momentary overload conditions. junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. continuous operation above the speci? ed maximum operating junction temperature may impair device reliability. note 6: this is the voltage necessary to keep the internal bias circuitry in regulation.note 7: current limit guaranteed by design and/or correlation to static test. slope compensation reduces current limit at higher duty cycles.note 8: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. downloaded from: http:///
lt3695 series 5 3695fa typical performance characteristics no-load supply current maximum load current maximum load current maximum load current maximum load current ef? ciency (v out = 5v, sync = 0v) t a = 25c, unless otherwise noted. ef? ciency (v out = 3.3v, sync = 0v) ef? ciency (v out = 3.3v, sync = 0v) load current (ma) 0.1 efficiency (%) 60 7040 50 80 1000 3695 g01 3010 20 1 10 100 90 v in = 12v v in = 34v l = 10hf = 800khz v in = 24v load current (ma) efficiency (%) 3695 g02 0.1 60 7040 50 80 1000 3010 20 1 10 100 90 v in = 12v v in = 34v l = 10hf = 800khz v in = 24v load current (ma) 0.1 efficiency (%) power loss(w) 7060 80 90 1000 3695 g03 20 30 40 5010 1 10 100 100 0.1 0.010.001 1 v in = 12v v out = 3.3v l = 10hf = 800khz no-load supply current input voltage (v) 05 load current (a) 1.251.00 0.75 1.50 40 3695 g06 0.500.25 10 15 20 30 35 25 1.75 typical minimum v out = 3.3v l = 10hf = 800khz sync = 0vsync = 3.3v input voltage (v) 5 load current (a) 1.000.75 1.25 40 3682 g07 0.500.25 10 15 20 30 35 25 1.50 typical minimum v out = 5v l = 10hf = 800khz sync = 0vsync = 5v input voltage (v) 8 load current (a) 1.00 1.25 20 3695 g08 0.50 0.750.25 10 12 14 16 18 1.50 typical minimum v out = 5v l = 4.7hf = 2mhz sync = 0vsync = 5v input voltage (v) 0 load current (a) 1.501.25 1.75 40 3695 g09 1.000.75 0.50 0.25 5 10 15 20 30 35 25 typical minimum v out = 1.8v l = 10hf = 500khz sync = 0vsync = 3.3v temperature (c) C50 supply current (a) 13001200 500 600 700 800 900 1000 1100 400200 300100 0 05 0 C25 25 100 3695 g05 150 75 125 catch diode: diodes, inc. b140 v in = 12v v out = 3.3v increased supply current due to catch diode leakage at high temperature input voltage (v) 05 supply current (a) 140120 60 80 100 4020 0 10 20 15 30 3695 g04 40 25 35 v out = 3.3v downloaded from: http:///
lt3695 series 6 3695fa boost pin current feedback voltage switching frequency frequency foldback: lt3695 switch current limit switch current limit (sync pin grounded) switch voltage drop typical performance characteristics t a = 25c, unless otherwise noted. duty cycle (%) 0 switch current limit (a) 1.31.1 1.5 1.7 100 3695 g10 0.7 0.90.5 20 40 60 80 1.9 sync < 0.3v sync > 0.8v or clocked switch current (a) 0 voltage drop (mv) 300 1.25 3695 g12 100 200 0 0.25 0.50 0.75 1.00 400 switch current (a) 0 boost pin current (ma) 20 2515 10 30 1.25 3695 g13 50 0.25 0.50 0.75 1.00 35 fb pin voltage (mv) 0 frequency (khz) 1000 900 3695 g17 800600 400 200 0 100 200 300 400 600 700 800 500 1200 r rt = 29.4k temperature (c) C50 feedback voltage (mv) 810790 800780 770 05 0 C25 25 100 3695 g14 150 75 125 temperature (c) C50 frequency (mhz) 1.201.15 1.10 1.05 1.00 0.80 0.85 0.90 0.95 05 0 C25 25 100 3695 g16 150 75 125 r t = 29.4k temperature (c) C50 switch current limit (a) 1.91.7 1.5 1.3 1.1 0.5 0.7 0.9 05 0 C25 25 100 3695 g11 150 75 125 dc = 10% dc = 90% output voltage: lt3695-3.3, lt3695-5 frequency foldback: lt3695-3.3 temperature (c) C50 output volt age (v) output volt age (v) 3.353.25 3.15 3.303.20 3.10 5.155.05 4.95 5.105.00 4.90 05 0 C25 25 100 3695 g15 150 75 125 lt3695-3.3 lt3695-5 frequency (khz) 0 0.5 1.5 1 2 2.5 3695 g18 3 3.5 0 200 400 600 800 1000 1200 output voltage (v) r rt = 29.4k downloaded from: http:///
lt3695 series 7 3695fa minimum switch on-time soft-start typical performance characteristics run/ss pin current t a = 25c, unless otherwise noted. boost diode forward voltage error ampli? er output current: lt3695 run/ss pin voltage (v) 0 run/ss pin current (a) 10 8 35 40 3695 g22 42 60 5 10 15 20 30 25 12 fb pin error voltage (mv) C200 v c pin current (a) 0 10 20 30 C10 40 50 200 3659 g24 C40 C30C50 C20C60 C100 0 100 60 load current (ma) 1 input voltage (v) 4.03.5 4.5 1000 3695 g27 2.5 3.02.0 10 100 5.0 v out = 3.3v l = 10hf = 800khz run/ss pin voltage (v) 0 switch current limit (a) 1.40.8 1.0 1.2 1.6 1.8 3.5 3695 g21 0.40.2 0.6 0 0.5 1.0 1.5 2.0 3.0 2.5 2.0 sync < 0.3v boost diode current (a) 0 boost diode v f (v) 1.00.8 1.2 1 3695 g23 0.40.2 0.6 0 0.25 0.5 0.75 1.4 minimum input voltage temperature (c) C50 minimum switch on time (ns) 120100 8060 40 0 20 05 0 C25 25 100 3695 g20 150 75 125 i out = 1a error ampli? er output current: lt3695-3.3 frequency foldback: lt3695-5 frequency (khz) 0 1 3 24 3695 g19 5 0 200 400 600 800 1000 1200 output voltage (v) r rt = 29.4k 0 10 20 30 C10 40 50 C40 C30C50 C20C60 60 v c pin current (a) C750 C500 C250 0 250 3695 g25 500 750 output error voltage (mv) 0 10 20 30 C10 40 50 C40 C30C50 C20C60 60 v c pin current (a) C900 C600 C300 0 300 3695 g26 600 900 output error voltage (mv) error ampli? er output current: lt3695-5 downloaded from: http:///
lt3695 series 8 3695fa maximum v in for full frequency v c voltages switching waveforms, 60v input voltage transient switching waveforms, burst mode operation switching waveforms, transition from burst mode operation to full frequency typical performance characteristics t a = 25c, unless otherwise noted. load current(a) v in (v) 3695 g31 0 30 35 1 20 2515 10 5 0.1 0.2 0.3 0.4 0.5 0.7 0.9 0.8 0.6 40 t a = 85?c t a = 25?c v out = 5v l = 4.7hf = 2mhz sync = 5v switching waveforms, full frequency continuous operation temperature (c) C50 v c voltage (v) 2.51.5 0.5 2.01.0 0 05 0 C25 25 100 3695 g32 150 75 125 current limit clamp switching threshold 5ms/div v sw 10v/div v out 5v/div v in 20v/div v in = 12v, front page application i load = 500ma 3695 g33 5s/div v sw 5v/div v out 20mv/div i l 0.2a/div v in = 12v, front page application i load = 5ma 3695 g34 1s/div v sw 5v/div v out 20mv/div i l 0.2a/div 3695 g35 v in = 12v, front page application i load = 55ma 1s/div v sw 5v/div v out 20mv/div i l 0.5a/div 3695 g36 v in = 12v, front page application i load = 500ma maximum v in for full frequency maximum v in for full frequency load current(a) 0 v in (v) 30 35 1 3695 g29 20 2515 10 5 0.1 0.2 0.3 0.4 0.5 0.7 0.9 0.8 0.6 40 t a = 85?c v out = 3.3v l = 10hf = 800khz sync = 3.3v t a = 25?c load current(a) v in (v) 3695 g30 0 30 35 1 20 2515 10 5 0.1 0.2 0.3 0.4 0.5 0.7 0.9 0.8 0.6 40 t a = 85?c v out = 5v l = 10hf = 800khz sync = 5v t a = 25?c load current (ma) 1 input voltage (v) 5.55.0 6.0 1000 3695 g28 4.5 4 10 100 6.5 v out = 5v l = 10hf = 800khz minimum input voltage downloaded from: http:///
lt3695 series 9 3695fa pin functions pgnd (pin 1, exposed pad pin 17/pin 1, exposed pad pin 17): this is the power ground used by the catch diode (d1 in the block diagram) when its anode is connected to the da pin. the exposed pad may be soldered to the pcb in order to lower the thermal resistance. da (pin 2/pin 2): connect the anode of the catch diode (d1) to this pin. internal circuitry senses the current through the catch diode providing frequency foldback in extreme situations. nc (pins 3, 10, 12/pins 3, 10, 15): no connects. these pins are not connected to internal circuitry and must be left ? oating to ensure fault tolerance. sw (pin 4/pin 4): the sw pin is the output of the internal power switch. connect this pin to the inductor, catch diode and boost capacitor. run/ss (pin 5/pin 5): the run/ss pin is used to put the lt3695 regulators in shutdown mode. tie to ground to shut down the lt3695 regulators. tie to 2.5v or more for normal operation. run/ss also provides a soft-start function; see the applications information section for more information. rt (pin 6/pin 6): oscillator resistor input. connect a resistor from this pin to ground to set the switching frequency. sync (pin 7/pin 7): this is the external clock synchroni- zation input. ground this pin with a 100k resistor for low ripple burst mode operation at low output loads. tie to 0.8v or more for pulse-skipping mode operation. tie to a clock source for synchronization. clock edges should have rise and fall times faster than 1s. note that the maximum load current depends on which mode is chosen. see the applications information section for more information. v in (pin 8/pin 8): the v in pin supplies current to the internal regulator and to the internal power switch. this pin must be locally bypassed. v c (pin 9/pin 9): 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. fb (pin 11) lt3695: the lt3695 regulates the fb pin to 0.8v. connect the feedback resistor divider tap to this pin.pg (pin 13/pin 11): the pg pin is the open-collector output of an internal comparator. pg remains low until the fb pin (lt3695) or the out1,2 pins (lt3695-3.3, lt3695-5) are within 10% of the ? nal regulation voltage. pg output is valid when v in is above the minimum input voltage and run/ss is high.gnd (pin 14/pin 12): the gnd pin is the ground of all the internal circuitry. tie directly to the local gnd plane. out1, out2, (pins 14, 13) lt3695-3.3, lt3695-5: these pins connect to the anode of the boost schottky diode and also supply current to the internal regulator. they also connect to the internal feedback resistors and must be connected to the output. bd (pin 15) lt3695: this pin connects to the anode of the boost schottky diode and also supplies current to the lt3695s internal regulator. boost (pin 16/pin 16): this pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. connect a capacitor (typically 0.22f) between boost and sw. (lt3695/lt3695-3.3, lt3695-5) downloaded from: http:///
lt3695 series 10 3695fa block diagram v in v in v out c1 r t pg run/ss sync rt +C internal 0.8v ref 11 +C soft-start 0.720v error amp oscillator 250khz to 2.2mhz disable sync out outb +C +C burst mode detect thermal shutdown v c clamp 2 4 15 16 9 5 7 13 8 6 r1 r2 fb gnd ovlo slope comp r s q da pgnd sw boost bd v c c c c3 l1 d1 c2 c f r c 3695 bda 1 pgnd 17 14 v in v in v out c1 r t pg run/ss sync rt +C internal 0.8v ref +C soft-start 0.720v error amp oscillator 250khz to 2.2mhz disable sync out outb +C +C burst mode detect thermal shutdown v c clamp 2 4 14 16 9 5 7 11 8 6 r1 r2 gnd ovlo slope comp r s q da pgnd sw boost out1 13 out2 v c c c c3 l1 d1 c2 c f r c 3695 bd 1 pgnd 17 12 lt3695-3.3/lt3695-5 lt3695 downloaded from: http:///
lt3695 series 11 3695fa operation the lt3695 series are constant-frequency, current mode step-down regulators. an oscillator, with frequency set by r t , enables 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 (lt3695) or through an internal resistor divider con- nected to the output voltage (lt3695-3.3, lt3695-5), and servos the v c pin. 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 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 circuitry. the bias regulator normally draws power from the v in pin, but if the bd pin is connected to an external voltage higher than 3v (lt3695) or if the output voltage connected to the out 1 and out2 pins exceeds 3v (lt3695-3.3, lt3695-5), bias power will be drawn from the external source. this improves ef? ciency. the run/ss pin is used to place the lt3695 regulators 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 and the internal boost diode are 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 lt3695 regulators au- tomatically switch 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 75a in a typical application. the oscillator reduces the lt3695 regulators operating frequency when the voltage at the fb pin (lt3695) or the out1,2 pins (lt3695-3.3, lt3695-5) is low. this frequency foldback helps to control the output current during start-up and overload conditions. internal circuitry monitors the current ? owing through the catch diode via the da pin and delays the generation of new switch pulses if this current is too high (above 1.6a nominal). this mechanism also protects the part during short-circuit and overload conditions by keeping the cur- rent through the inductor under control. the lt3695 regulators contain a power good comparator which trips when the fb pin (lt3695) or the out1,2 pins (lt3695-3.3, lt3695-5) are at 90% of their 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 lt3695 regulators are enabled and v in is above the minimum input voltage. the lt3695 regulators have an overvoltage protection fea- ture which disables switching action when v in goes above 38v (typical) during transients. the lt3695 regulators can then safely sustain transient input voltages up to 60v. downloaded from: http:///
lt3695 series 12 3695fa applications information fb resistor network (lt3695) the output voltage of the lt3695 is programmed with a resistor divider between the output and the fb pin. choose the resistor values according to: rr v v out 12 08 1 = ? ?? ? ?? ? . reference designators refer to the block diagram of the lt3695. 1% resistors are recommended to maintain output voltage accuracy. setting the switching frequency the lt3695 regulators use a constant-frequency pwm architecture that can be programmed to switch from 250khz to 2.2mhz 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) 0.25 158 0.3 127 0.4 90.9 0.5 71.5 0.6 57.6 0.7 47.5 0.8 40.2 0.9 34 1.0 29.4 1.2 22.6 1.4 18.2 1.6 14.7 1.8 12.1 2.0 9.76 2.2 8.06 operating frequency trade-offs selection of the operating frequency is a trade-off 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 tv vv 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 lower 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. input voltage range depends on the switching frequency because the lt3695 regulators switch has ? nite minimum on and off times. an internal timer forces the switch to be off for at least t off(min) per cycle; this timer has a maximum value of 210ns (250ns for t j > 125c). on the other hand, delays associated with turning off the power switch dictate the minimum on-time, t on(min) , before the switch can be turned off; t on(min) has a maximum value of 150ns over temperature. the minimum and maximum duty cycles that can be achieved taking minimum on and off times into account are: dc min = f sw t on(min) dc max = 1 C f sw t off(min) where f sw is the switching frequency, t on(min) is the minimum switch on time (150ns), and t off(min) is the minimum switch off time (210ns, 250ns for t j > 125c). these equations show that the duty cycle range increases when the switching frequency is decreased. downloaded from: http:///
lt3695 series 13 3695fa applications information a good choice of switching frequency should allow an adequate 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 lt3695 regulators minimum operating voltage of ~3.6v (v bd > 3v) or by their maximum duty cycle (see equation in operating frequency trade-offs section). the minimum input voltage due to duty cycle is: v vv ft vv in min out d sw off min ds w () () = + ? ? + 1 where v in(min) is the minimum input voltage, and t off(min) is the minimum switch off time. note that a 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 lt3695 regulator applica- tions depends on switching frequency, the absolute maxi- mum ratings of the v in and boost pins and the operating mode. the lt3695 regulators can operate from continuous input voltages up to 36v. input voltage transients of up to 60v are also safely withstood. however, note that while v in > v ovlo (overvoltage lockout, 38v typical), the lt3695 regulators will stop switching, allowing the output to fall out of regulation. for a given application where the switching frequency and the output voltage are already ? xed, the maximum input voltage that guarantees optimum output voltage ripple for that application can be found by applying the following expression: v vv ft vv in max out d sw on min ds w () () = + ? + where v in(max) is the maximum operating 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 on(min) is the minimum switch on time (~150ns). note that a higher switching frequency will reduce the maximum operating input voltage. conversely, a lower switching frequency will be necessary to achieve optimum operation at high input voltages. special attention must be paid when the output is in start-up, short-circuit or other overload conditions. dur- ing these events, the inductor peak current might easily reach and even exceed the maximum current limit of the lt3695 regulators, especially in those cases where the switch already operates at minimum on-time. the circuitry monitoring the current through the catch diode via the da pin prevents the switch from turning on again if the inductor valley current is above 1.6a nominal. in these cases, the inductor peak current is therefore the maximum current limit of the lt3695 regulators plus the additional current overshoot during the turn off delay due to minimum on time: ia vv l t lpeak in max out ol on min () () () () ? =+ ? 2 where i l(peak) is the peak inductor current, v in(max) is the maximum expected input voltage, l is the inductor value, t on(min) is the minimum on time and v out(ol) is the output voltage under the overload condition. the parts are robust enough to survive prolonged operation under these conditions as long as the peak inductor current does not exceed 3.5a. inductor current saturation and excessive junction temperature may further limit performance. input voltage transients of up to v ovlo are acceptable regardless of the switching frequency. in this case, the lt3695 regulators may enter pulse-skipping operation 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. input voltage transients above v ovlo and up to 60v can be tolerated. however, since the parts will stop switching during these transients, the output will fall out of regulation and the output capacitor may eventually be completely discharged. this case must be treated then as a start-up condition as soon as v in returns to values below v ovlo and the part starts switching again. downloaded from: http:///
lt3695 series 14 3695fa inductor selection and maximum output current a good ? rst choice for the inductor value is: lv v f out d sw =+ () ? . 18 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. 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. for robust operation in fault conditions (start-up or short- circuit) and high input voltage (>30v), the saturation current should be chosen high enough to ensure that the inductor peak current does not exceed 3.5a. for example, an application running from an input voltage of 36v using a 10h inductor with a saturation current of 2.5a will tolerate the mentioned fault conditions. 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, then you can relax the value of the inductor and operate with higher ripple cur- rent. 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 input voltage. in addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. for details of maximum output current and discontinuous mode 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: lvv f min out d sw =+ () ? . 12 the current in the inductor is a triangle wave with an av-erage value equal to the load current. the peak inductor and switch current is: iii i sw peak l peak out max 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 lt3695 regulators limit their switch current in order to protect themselves and the system from overload faults. therefore, the maximum output current that the lt3695 regulators will deliver depends on the switch current limit, the inductor value and the input and output voltages. 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 lt3695 regulators, dc is the duty cycle and l is the value of the inductor. to maintain output regulation, the inductor peak current must be less than the lt3695 regulators switch current limit, i lim . if the sync pin is grounded, i lim is at least 1.45a at low duty cycles and decreases to 1.1a at dc = 90%. if the sync pin is tied to 0.8v or more or if it is tied to a clock source for synchronization, i lim is at least 1.18a at low duty cycles and decreases to 0.85a at dc = 90%. the maximum output current is also a function of the chosen inductor value and can be approximated by the following expressions depending on the sync pin con? guration: for the sync pin grounded: ii i ad c i out max lim ll () .? (.?) = ? = ?? ? 2 145 1 024 2 for the sync pin tied to 0.8v or more, or tied to a clock source for synchronization: ii i ad c i out max lim ll () .? (.?) = ? = ?? ? 2 118 1 029 2 applications information downloaded from: http:///
lt3695 series 15 3695fa choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. table 2. inductor vendors vendor url part series type murata www.murata.com lqh55d open tdk www.componenttdk.com slf7045 slf10145 shieldedshielded toko www.toko.com d62cb d63cb d73c d75f shieldedshielded shielded open coilcraft www.coilcraft.com mss7341 mss1038 shieldedshielded sumida www.sumida.com cr54 cdrh74 cdrh6d38 cr75 open shieldedshielded open one approach to choosing the inductor is to start with the simple rule given above, look at the available inductors, and choose one to meet cost or space goals. then use these equations to check that the lt3695 regulators 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 lt3695 regulators 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 2.2f to 10f ceramic capacitor is adequate to bypass the lt3695 regulators 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 lower 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 lt3695 regulators and to force this very high fre- quency switching current into a tight local loop, minimizing emi. a 2.2f capacitor is capable of this task, but only if it is placed close to the lt3695 regulators (see the pcb layout section for more information). a second precau- tion regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt3695 regulators. a ceramic input capacitor combined with trace or cable inductance forms a high-q (underdamped) tank circuit. if the lt3695 regulators circuit is plugged into a live sup- ply, the input voltage can ring to twice its nominal value, possibly exceeding the lt3695 regulators voltage rating. for details see application note 88. output capacitor and output ripple the output capacitor has two essential functions. along with the inductor, it ? lters the square wave generated by the lt3695 regulators to produce the dc output. in this role it determines the output ripple, and low impedance at the switching frequency is important. the second func- tion is to store energy in order to satisfy transient loads and stabilize the lt3695 regulators control loop. ceramic capacitors have very low equivalent series resistance (esr) and provide the best ripple performance. a good starting value is: c v f out out sw = 50 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 frequency compensation section to choose an appropriate compen- sation network. 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. applications information downloaded from: http:///
lt3695 series 16 3695fa 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 3 lists several capacitor vendors. 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: i d(avg) = i out ? (1 C dc) where dc is the duty cycle. the only reason to consider a diode with larger current rating than necessary for nominal operation is for the case of shorted or overloaded output conditions. for the worst case of shorted output the diode average current will then increase to a value that depends on the following internal parameters: switch current limit, catch diode (da pin) current threshold and minimum on-time. the worst case (taking maximum values for the above mentioned parameters) is given by the following expression: ia v l ns d avg max in () ?? =+ 2 1 2 150 peak reverse voltage is equal to the regulator input voltage if it is below the overvoltage protection threshold. this feature keeps the switch off for v in > v ovlo (39.9v maxi- mum). for inputs up to the maximum operating voltage of 36v, use a diode with a reverse voltage rating greater applications information table 3. capacitor vendors vendor phone url part series commands 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 4. schottky diodes part number v r (v) i ave (a) v f at 1a (mv) v f at 2a (mv) on-semiconducor mbr0520l 20 0.5 mbr0540 40 0.5 620 mbrm120e 20 1 530 595 mbrm140 40 1 550 diodes inc. b0530w 30 0.5 b0540w 40 0.5 620 b120 20 1 500 b130 30 1 500 b140 40 1 500 b220 20 2 500 b230 30 2 500 b140hb 40 1 530 dfls240l 40 2 500 dfls140 40 1.1 510 b240 40 2 500 central semiconductor cmsh1-40m 40 1 500 cmsh1-40ml 40 1 400 cmsh2-40m 40 2 550 cmsh2-40l 40 2 400 cmsh2-40 40 2 500 than the input voltage. if transients at the input of up to 60v are expected, use a diode with a reverse voltage rat- ing of 40v. table 4 lists several schottky diodes and their manufacturers. if operating at high ambient temperatures, consider using a schottky with low reverse leakage. downloaded from: http:///
lt3695 series 17 3695fa audible noise ceramic capacitors are small, robust and have very low esr. however, ceramic capacitors can sometimes cause problems when used with the lt3695 regulators due to their piezoelectric nature. when in burst mode operation, the lt3695 regulators switching frequency depends on the load current, and at very light loads the lt3695 regulators can excite the ceramic capacitor at audio frequencies, gen- erating audible noise. since the lt3695 regulators operate at a lower current limit during burst mode operation, the noise is typically very quiet. if this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. frequency compensation the lt3695 regulators use current mode control to regulate the output. this simpli? es loop compensation. in particular, the lt3695 regulators do 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 1. generally a capacitor (c c ) and a resistor (r c ) in series to ground are used. in addition, there may be a lower value capacitor in parallel. this capacitor (c f ) is used to ? lter noise at the switching frequency, and is required only if a phase-lead capacitor (c pl , lt3695 only) is used or if the output capacitor has high esr. applications information loop compensation determines the stability and transient performance. optimizing the design of the compensation network depends on the application and type of output capacitor. a practical approach is to start with one of the circuits in this data sheet that is similar to your applica- tion 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 1 shows an equivalent circuit for the lt3695 regulators 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 current 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 , lt3695 only) across the feedback divider may improve the transient response. figure 2 shows the transient response when the load current is stepped from 300ma to 650ma and back to 300ma. 0.8v lt3695 c f r c v c 3m 3695 f01 current mode power stage g m = 1.25s c c c pl c1 r2 r1 sw fb gnd output ceramic polymer or tantalum or electrolitic esr + c1 C + g m = 430s figure 1. model for loop response. note that r1 and r2 are integrated in the lt3695-3.3 and lt3695-5 figure 2. transient load response of the lt3695 regulators. a 3.3v out typical application with v in = 12v as the load current is stepped from 300ma to 650ma v out 100mv/div i load 0.5a/div 20s/div 3695 f02 downloaded from: http:///
lt3695 series 18 3695fa low ripple burst mode operation the lt3695 regulators are capable of operating in either low ripple burst mode operation or pulse-skipping mode which are selected using the sync pin. see the synchro- nization section for more information. to enhance ef? ciency at light loads, the lt3695 regulators can be operated in low ripple burst mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. during burst mode operation, the lt3695 regulators deliver 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. because the lt3695 regula- tors deliver power to the output with single, low current pulses, the output ripple is kept below 15mv for a typical application. in addition, v in and bd (lt3695), and out1,2 (lt3695-3.3, lt3695-5) quiescent currents are reduced to typically 35a, 55a and 65a, respectively, during the sleep time. as the load current decreases towards a no-load condition, the percentage of time that the lt3695 regulators operate in sleep mode increases and the average input current is greatly reduced resulting in high ef? ciency even at very low loads (see figure 3). at higher output loads (above about 70ma for the front page application) the lt3695 regulators 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 is seamless, and will not disturb the output voltage. if low quiescent current is not required, tie sync high to select pulse-skipping mode. the bene? t of this mode is that the lt3695 regulators will enter full frequency standard pwm operation at a lower output load current than when in burst mode operation. with the sync pin tied low, the front page application circuit will switch at full frequency at output loads higher than about 100ma. with the sync pin tied high, the front page application circuit will switch at full frequency at output loads higher than about 30ma. the maximum load current that the lt3695 regulators can supply is reduced when sync is high. boost pin considerations capacitor c3 and the internal boost schottky diode (see the block diagram) are used to generate a boost voltage that is higher than the input voltage. in most cases a 0.22f capacitor will work well. figure 4 shows three ways to arrange the boost circuit for the lt3695 regulators. the boost pin must be more than 2.3v above the sw pin for best ef? ciency. for outputs of between 3v and 8v, the standard circuit (figure 4a) 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 4b). for lower output voltages the boost diode can be tied to the input (figure 4c), or to another supply greater than 2.8v. keep in mind that a minimum input voltage of 4.3v is required if the voltage at the bd pin is smaller than 3v. tying bd to v in reduces the maximum input voltage to 25v. the circuit in figure 4a is more ef? cient because the boost pin current and bd pin quiescent current come from a lower voltage source. you must also be sure that the maximum voltage ratings of the boost and bd pins are not exceeded. as mentioned, a minimum of 2.5v across the boost capacitor is required for proper operation of the internal boost circuitry to provide the base current for the power npn switch. for bd pin voltages higher than 3v, the excess voltage across the boost capacitor does not bring an increase in performance but dissipates additional power in the internal boost circuitry instead. the boost circuitry tolerates reasonable amounts of power, however excessive power dissipation on this circuitry may impair reliability. for reliable operation, use no more than 8v on the bd pin for applications information figure 3. switching waveforms, burst mode operation 5s/div v sw 5v/div v out 20mv/div i l 0.2a/div v in = 12v, front page application i load = 5ma 3695 f03 downloaded from: http:///
lt3695 series 19 3695fa 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. figure 5 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. 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 discontinuous and the effective duty cycle can be very high. applications information bd lt3695 v in v in c3d1 v out 3695 f04a boost sw da gnd pgnd (4a) for v out > 2.8v, v in(min) = 4.3v if v out < 3v bd lt3695 v in v in c3d1 d2 v out 3695 f04b boost sw da gnd pgnd (4b) for 2.5v < v out < 2.8v, v in(min) = 4.3v bd lt3695 v in v in c3d1 v out 3695 f04c boost sw da gnd pgnd (4c) for v out < 2.5v, v in(max) = 25v figure 4. three circuits for generating the boost voltage for the lt3695 the circuit in figure 4a. for higher output voltages, make sure that there is no more than 8v at the bd pin either by connecting it to another available supply higher than 3v or by using a zener diode between v out and bd to maintain the bd pin voltage between 3v and 8v. the minimum operating voltage of the lt3695 regulators application is limited by the minimum input voltage and by the maximum duty cycle as outlined previously. for proper start-up, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, or the lt3695 regulators are turned on with their run/ss pin when load current (ma) input voltage (v) 1 4.5 5.0 5.5 1000 3.5 4.03.0 2.5 2.0 10 100 10 100 6.0 load current(ma) input voltage (v) 3695 f05 1 6.0 6.5 7.0 7.5 1000 4.54.0 5.55.0 3.5 2.5 3.02.0 8.0 to run to start (worst case) to start (worst case) v out = 3.3v t a = 25?c l = 10hf = 800khz to run v out = 5v t a = 25?c l = 10hf = 800khz figure 5. the minimum input voltage depends on output voltage, load current and boost circuit downloaded from: http:///
lt3695 series 20 3695fa 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 lt3695 regulators, requiring a higher input voltage to maintain regulation. soft-start the run/ss pin can be used to soft-start the lt3695 regulators, reducing the maximum input current during start-up. the run/ss pin is driven through an external rc network to create a voltage ramp at this pin. figure 6 shows the start-up and shutdown 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 7.5a when the run/ss pin reaches 2.5v. for fault tolerant ap- plications, see the discussion of the run/ss resistor in the fault tolerance section. synchronization to select low ripple burst mode operation, tie the sync pin below 0.3v (this can be ground or a logic output). synchronizing the oscillator of the lt3695 regulators to an external frequency 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 0.3v and peaks that are above 0.8v (up to 6v). the lt3695 regulators will not enter burst mode operation at low output loads while synchronized to an external clock, but instead will skip pulses to maintain regulation. the maximum load current that the part can supply is reduced when a clock signal is applied to sync. the lt3695 regulators may be synchronized over a 300khz to 2.2mhz range. the r t resistor should be chosen to set the lt3695 regulators switching frequency 20% below the lowest synchronization input. for example, if the synchro- nization signal is 360khz, the r t should be chosen for 300khz. to assure reliable and safe operation the lt3695 regulators 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 for more information. it is also important to note that slope compensation is set by the r t value; to avoid subharmonic oscillations, calculate the minimum inductor value using the frequency determined by r t . shorted and reversed input protectionif the inductor is chosen so that it will not saturate exces- sively, the lt3695 regulators will tolerate a shorted output. when operating in short-circuit condition, the lt3695 regulators will reduce their frequency until the valley cur- rent is at a typical value of 1.6a (see figure 7). there is another situation to consider in systems where the output will be held high when the input to the lt3695 regulators is absent. this may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ored with the lt3695 regulators 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 ), applications information 5ms/div v run/ss 5v/div v out 5v/div i l 1a/div v run 5v/div 3695 f05 run 15k 0.22f run/ss gnd figure 6. to soft-start the lt3695 regulators, add a resistor and capacitor to the run/ss pin 3695 f07 i l 500ma/div v sw 20v/div 0v 0a 2s/div figure 7. the lt3695 regulators reduce their frequency to protect against shorted output with 36v input downloaded from: http:///
lt3695 series 21 3695fa then the lt3695 regulators 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 essen- tially zero. however, if the v in pin is grounded while the output is held high, then parasitic diodes inside the lt3695 regulators 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. applications information 3695 f09 lt3695 v in v c backup d4 mbrs140 v out v in bd gnd sw da fb run/ss boost pgnd 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 regulator runs only when the input is present v out gnd c2 d1 r t r c c1 c3 l v in gnd 3695 f09 r1 r2 c c figure 9. a good pcb layout ensures proper, low emi operation (lt3695) pcb layoutfor proper operation and minimum emi, care must be taken during printed circuit board layout. figures 9 and 10 show the recommended component placement with trace, ground plane and via locations. note that large, switched currents ? ow in the lt3695 regulators v in , sw and pgnd pins, the catch diode and the input capacitor (c in ). the loop formed by these components should be as small as possible. these components, along with the inductor and output capacitor (c out ), should be placed on the same side of the circuit board, and their connections should be made on that layer. all connections to gnd should be made at a common star ground point or directly to a local, unbroken ground plane below these components. the sw and boost nodes should be laid out carefully to avoid interference. finally, keep the fb, r t and v c nodes small so that the ground traces will shield them from the sw and boost nodes. to keep thermal resistance low, extend the ground plane as much as possible and add thermal vias under and near the lt3695 regulators to any 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 below the speci? ed absolute maximum temperature. figure 10. a good pcb layout ensures proper, low emi operation (lt3695-3.3, lt3695-5) v out gnd c2 d1 r t r c c1 c3 l v in gnd 3695 f10 c c downloaded from: http:///
lt3695 series 22 3695fa applications information high temperature considerations the pcb must provide heat sinking to keep the lt3695 regulators cool. the exposed pad on the bottom of the package may be soldered to a copper area which should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the lt3695 regu- lators. 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 = 40c/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 resistance. because of the large output current capability of the lt3695 regulators, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum. when operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches these maximums. if the junction temperature reaches the ther- mal shutdown threshold, the parts will stop switching to prevent internal damage due to overheating. power dissipation within the lt3695 regulators can be esti- mated by calculating the total power loss from an ef? ciency measurement. the die temperature rise is calculated by multiplying the power dissipation of the lt3695 regula- tors by the thermal resistance from junction to ambient. die temperature rise was measured on a 2-layer, 10cm 10cm circuit board in still air at a load current of 1a (f sw = 800khz). for a 12v input to 5v output the die temperature elevation above ambient was 22c with the exposed pad soldered and 44c without the exposed pad soldered. fault tolerance the lt3695 regulators are designed to tolerate single fault conditions. shorting two adjacent pins together or leaving one single pin ? oating does not raise v out or cause damage to the lt3695 regulators. however, the application circuit must meet the requirements discussed in this section in order to achieve this tolerance level. tables 5 and 6 show the effects that result from shorting adjacent pins or from a ? oating pin, respectively. for the best fault tolerance to inadvertent adjacent pin shorts, the run/ss pin must not be directly connected to either ground or v in . if there was a short between run/ss and sw then connecting run/ss to v in would tie sw to v in and would thus raise v out . likewise, grounding run/ss would tie sw to ground and would damage the power switch if this is done when the power switch is on. a short between rt and a run/ss pin that is connected to v in would violate the absolute maximum ratings of the rt pin. therefore, the current supplying the run/ss pin must be limited, for example, with resistor r3 in figures 11 and 12. in case of a short between run/ss and sw this resistor charges c2 through the inductor l1 if the current it supplies from v in is not completely drawn by r load , r1 bd v in lt3695 run/ss r3 rt v in c3 l1 d1 d2 r2 r1 c2 v out c ss 220nf 3695 f11 boost sw da fb r ss 47 r t r load figure 11. lt3695: the dashed lines show where a connection would occur if there were an inadvertent short from run/ss to an adjacent pin or from boost to bd. in these cases, r3 protects circuitry tied to the rt or sw pins, and d2 shields boost from v out . if c ss is used for soft start, r ss isolates it from sw figure 12. lt3695-3.3, lt3695-5: the dashed lines show where a connection would occur if there were an inadvertent short from run/ss to an adjacent pin. in these cases, r3 protects circuitry tied to the rt or sw pins. r4 provides an additional load and may be necessary in certain situations (see text). if c ss is used for soft start, r ss isolates it from sw boost v in lt3695-3.3 lt3695-5 run/ss r3 rt v in c3 l1 d1 r4 c2 v out c ss 220nf 3695 f12 sw da out1out2 r ss 47 r t r load downloaded from: http:///
lt3695 series 23 3695fa applications information table 5: effects of pin shorts pins effect pgnd-da no effect if v in < v in(max) . see input voltage range section for description of v in(max) . sw-run/ss the result of this short depends on the load resistance and on r3 (figure 10). see the following discussion. run/ss-rt no effect or v out will fall below regulation voltage if i r3 (figure 10) < 1ma. rt -sync no effect or v out will fall below regulation voltage if the current into the rt pin is less than 1ma. sync-v in no effect if v in does not exceed the absolute maximum voltage of sync (20v). pg-gnd no effect. gnd-bd (lt3695) v out may fall below regulation voltage, power dissipation of the power switch will be increased. note that this short also grounds the voltage source supplying bd. make sure it is safe to short the supply for bd to ground. for this reason bd should not be connected to v in , but it is safe to connect it to v out . bd-boost (lt3695) if diode d2 (see figure 10) is used, no effect or v out may fall below regulation voltage. otherwise the device may be damaged. gnd-out2 (lt3695-3.3, lt3695-5) v out will fall below regulation voltage, because this shorts the output to ground. as a result, the power dissipation of the part may increase. table 6: effects of floating pins pin effect pgnd no effect if the exposed pad is soldered. otherwise: v out may fall below regulation voltage. make sure that v in < v in(max) (see input voltage range section for details) and provide a bypass resistor at the da pin. see the following discussion. da v out may fall below regulation voltage. make sure that v in < v in(max) (see input voltage range section for details) and provide a bypass resistor. see the following discussion. sw v out will fall below regulation voltage. run/ss v out will fall below regulation voltage. rt v out will fall below regulation voltage. sync v out may fall below regulation voltage. a ? oating sync pin con? gures the lt3695 for pulse-skipping mode. however, a ? oating sync pin is sensitive to noise which can degrade device performance. v in v out will fall below regulation voltage. v c v out may fall below regulation voltage. disconnecting the v c pin alters the loop compensation and potentially degrades device performance. the output voltage ripple will increase if the part becomes unstable. fb (lt3695) v out will fall below regulation voltage. pg no effect. gnd output maintains regulation, but potential degradation of device performance. bd (lt3695) v out may fall below regulation voltage. if bd is not connected, the boost capacitor cannot be charged and thus the power switch cannot saturate properly, which increases its power dissipation. out1, out2 (lt3695-3.3, lt3695-5) no effect. boost v out may fall below regulation voltage. if boost is not connected, the boost capacitor cannot be charged and thus the power switch cannot saturate properly, which increases its power dissipation. downloaded from: http:///
lt3695 series 24 3695fa + r2, and the bd pin (if connected to v out ) in the case of the lt3695, or by r load , r4 and the out1,2 pins in the case of the lt3695-3.3 and lt3695-5. since this causes v out to rise, the lt3695 regulators stop switching. the resistive divider formed by r3, r load and r1 + r2 and r4, respectively, must be adjusted for v out not to exceed its nominal value at the required maximum input voltage v in(max) . r3 must supply suf? cient current into run/ss at the required minimum input voltage v in(min) for normal non-fault situations. based on the maximum run/ss cur- rent of 7.5a at v run/ss = 2.5v this gives r vv a in min 3 25 75 () ?. . the current through r3 is maximal at v in(max) with run/ss shorted to sw: i vv r r in max out 3 3 = () ? for the lt3695, this current must be drawn by r load , r1 + r2, and the bd pin, if connected to v out : i v rr r i r out load bd 3 12 + () + || without load (r load = ) and assuming the minimum current of 35a into the bd pin, this leads to rr v vv r a out in max out 12 3 35 + () ? ? as upper limit for the feedback resistors. for v out < 2.5v assume no current drawn by the bd pin, which gives rr vr vv out in max out 12 3 + ? ? () for the lt3695-3.3 and lt3695-5, the current through r3 must be drawn by r load , r4 and the out1,2 pins: i v rr i r out load out 31 2 4 + || , without load (r load = ) and assuming the minimum current of 43a into the out1,2 pins, this leads to: r v vv r a out in max out 4 3 43 () ? ? as upper limit for r4. depending on the required input voltage range, r4 may be omitted. tables 7 and 8 show example values for common appli- cations. r ss must be included as the switch node would otherwise have to charge c ss if the sw pin and the run/ss pin are shorted, which may damage the power switch. if run/ss is controlled by an external circuitry, the current this circuitry can supply must be limited. this can be done as discussed above. in addition, it may be necessary to protect this external circuitry from the voltage at sw, for example by using a diode. table 7. lt3695: example values for r1, r2 and r3 for common combinations of v in and v out . i r1+r2 is the current drawn by r1 + r2 in normal operation v in(max) (v) v in(min) (v) v out (v) r3 (k) r1 (k) r2 (k) i r1+r2 (a) 16 3.8 1.8 169 11.5 9.09 87 36 3.8 1.8 169 4.75 3.74 212 16 4.5 2.5 261 93.1 43.2 18 36 4.5 2.5 261 16.9 7.87 101 16 5.3 3.3 365 432 137 6 36 5.3 3.3 365 43.2 13.7 58 16 7 5 274 536 102 8 36 7 5 590 221 42.2 19 16 10 8 200 562 61.9 13 36 10 8 475 280 30.9 26 27 14 12 301 511 36.5 22 36 14 12 442 511 36.5 22 applications information downloaded from: http:///
lt3695 series 25 3695fa applications information table 8. lt3695-3.3, lt3695-5: example values for r3 and r4 for common combinations of v in and v out . i r4 is the current drawn by r4 in normal operation v in(max) (v) v in(min) (v) v out (v) r3 (k) r4 (k) i r4 (a) 16 5.3 3.3 309 none 24 5.3 3.3 365 215 15 36 5.3 3.3 365 66.5 50 16 7 5 267 none 24 7 5 464 none 36 7 5 590 442 11 the boost pin must not be shorted to a low impedance node like v out that clamps its voltage. for best fault toler- ance of the lt3695, supply current into the bd pin through the schottky diode d2 as shown in figure 10. note that this diode must be able to handle the maximum output current in case there is a short between the bd pin and the gnd pin. a short between run/ss and sw may also increase the output ripple. to suppress this, connect the soft-start network consisting of r ss and c ss to run/ss as shown in figure 10. c ss should not be smaller than 0.22f. the sync pin must not be directly connected to either ground or v in . a short between rt and a sync pin that is connected to v in could violate the absolute maximum ratings of the rt pin. a short between the sync pin and the v in pin could damage an external driver circuit which may be connected to sync or would short v in to ground if sync is grounded. v in lt3695 lt3695-3.3 lt3695-5 c s 100pf r s 100k sync sync rt v in 3695 f13 r t the recommended connection for sync is shown in figure 13. if sync is to be driven by an external circuitry, r s may be used to isolate this circuitry from v in . c s must be used in this case to provide a low impedance path for the synchronization signal. if sync is pulled low, r s prevents v in from being shorted to ground in case of an inadvertent short between sync and v in . if sync is pulled high to v in , then r s protects the rt pin during an inadvertent short between sync and rt. if the da pin or the pgnd pin are inadvertently left ? oat- ing, the current path of the catch diode is interrupted unless a bypass resistor is connected from da to ground. use a 360m (5% tolerance) resistor rated for a power dissipation of: p = i 2 load(max) ? 0.36 ? (1 C dc min ) where i load(max) is the maximum load current and dc min is the minimum duty cycle. for example, this would require a power rating of at least 219mw for an output current of 800ma and a minimum duty cycle of 5%. make sure not to exceed v in(max) (see input voltage range section for details) during start-up or overload conditions. 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. figure 13. the dashed lines show where a connection would occur if there were an inadvertent short from sync to an adjacent pin. in this case, r s protects circuitry connecting to sync downloaded from: http:///
lt3695 series 26 3695fa fully tolerant 3.3v step-down converter with soft-start v in bd lt3695 run/ssv c 0.22f d1b140 d2 b140 v out 3.3v0.9a, v in > 5v 1a, v in > 6.5v f = 800khz 100k 0.36 17.8k 56.2k l 10h 10f 3695 ta02 470pf 2.2f v in 5v to 28.5v transient to 36v rtpg 40.2k 14k 0.22f 47 324k sync boost sw da fb gnd pgnd 1.8v step-down converter v in bd lt3695 run/ssv c 0.22fd1 b140 v out 1.8v1a 102k f = 500khz 127k l1 6.8h 22f 3695 ta03 330pf 4.7f v in 3.6v to 25v rtpg 71.5k 17.4k sync boost on off sw da fb gnd pgnd typical applications downloaded from: http:///
lt3695 series 27 3695fa typical applications fully tolerant 5v step-down converter with soft-start v in lt3695-5 run/ssv c 0.22f d1b140 v out 5v0.9a f = 2mhz 100k 0.36 56.2k l 4.7h 10f 3695 ta04 680pf 2.2f v in 10v to 16.5v transient to 36v rtpg 9.76k 13.3k 0.22f 47 365k sync boost sw da out1 0ut2 gnd pgnd downloaded from: http:///
lt3695 series 28 3695fa package description mse package 16-lead plastic msop, exposed die pad (reference ltc dwg # 05-08-1667 rev a) msop (mse16) 0608 rev a 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 16 16151413121110 12345678 9 9 1 8 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 0.50 (.0197) bsc bottom view of exposed pad option 2.845 p 0.102 (.112 p .004) 2.845 p 0.102 (.112 p .004) 4.039 p 0.102 (.159 p .004) (note 3) 1.651 p 0.102 (.065 p .004) 1.651 p 0.102 (.065 p .004) 0.1016 p 0.0508 (.004 p .002) 3.00 p 0.102 (.118 p .004) (note 4) 0.280 p 0.076 (.011 p .003) ref 4.90 p 0.152 (.193 p .006) detail b detail b corner tail is part of the leadframe feature. for reference only no measurement purpose 0.12 ref 0.35ref downloaded from: http:///
lt3695 series 29 3695fa 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. revision history rev date description page number a 11/09 all sections revised to include lt3695-3.3 and lt3695-5 1-30 downloaded from: http:///
lt3695 series 30 3695fa 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 rev a printed in usa related parts typical application 5v step-down converter v in bd lt3695 run/ssv c 0.22fd1 b140 v out 5v0.9a, v in > 6.9v 1a, v in > 12v 102k f = 800khz 536k 10h 10f 3695 ta05 470pf 2.2f v in 6.9v to 36v transient to 60v rtpg 40.2k 16.2k sync boost on off sw da fb gnd pgnd part number description comments lt3970 40v, 350ma, 2mhz high ef? ciency micropower step-down dc/dc converter v in : 4v to 40v transient to 60v, v out(max) = 1.21v, i q = 2a, i sd < 1a, 3mm 2mm dfn-10, msop-10 packages 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 : 3.6v to 36v transient to 60v, v out(max) = 0.8v, i q = 75a, i sd < 1a, 3mm 3mm qfn-16 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(max) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn-10, msop-10e packages 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(max) = 1.26v, i q = 850a, i sd < 1a, 3mm 3mm dfn-10, msop-10e packages lt3682 36v, 60v max , 1a, 2.2mhz high ef? ciency micropower step-down dc/dc converter v in : 3.6v to 36v, v out(max) = 0.8v, i q = 75a, i sd < 1a, 3mm 3mm dfn-12 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 36v, v out(max) = 0.8v, i q = 4.6ma, i sd = 1a, 4mm 4mm qfn-24, tssop-16e packages 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 : 4v to 36v, v out(max) = 0.8v, i q = 7ma, i sd = 1a, 5mm 7mm qfn-38 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(max) = 0.78v, i q = 2ma, i sd = 2a, 3mm 3mm dfn-8, msop-8e packages lt3500 36v, 40v max , 2a, 2.5mhz high ef? ciency step-down dc/dc converter and ldo controller v in : 3.6v to 36v, v out(max) = 0.8v, i q = 2.5ma, i sd < 10a, 3mm 3mm dfn-10 package lt3493 36v, 1.4a (i out ), 750khz high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(max) = 0.8v, i q = 1.9ma, i sd < 1a, 2mm 3mm dfn-6 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(max) = 1.26v, i q = 50a, i sd < 1a, 3mm 3mm dfn-10, msop-10e packages 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(max) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn-10, msop-10e packages lt3437 60v, 400ma (i out ), micropower step-down dc/dc converter with burst mode operation v in : 3.3v to 60v, v out(max) = 1.25v, i q = 100a, i sd < 1a, 3mm 3mm dfn-10, tssop-16e package lt3434/lt3435 60v, 2.4a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converter with burst mode operation v in : 3.3v to 60v, v out(max) = 1.2v, i q = 100a, i sd < 1a, 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 : 3.3v to 60v, v out(max) = 1.2v, i q = 100a, i sd < 1a, tssop-16e package lt1936 36v, 1.4a (i out ), 500khz high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(max) = 1.2v, i q = 1.9ma, i sd < 1a, ms8e package lt1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(max) = 1.2v, i q = 2.5ma, i sd = 25a, tssop-16/e package downloaded from: http:///

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