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electrical specifications subject to change lt8611 1 8611p typical application features description 42v, 2.5a synchronous step-down regulator with current sense and 2.5a quiescent current the lt ? 8611 is a compact, high efficiency, high speed synchronous monolithic step-down switching regulator that consumes only 2.5a of quiescent current. top and bottom power switches are included with all necessary circuitry to minimize the need for external components. the built-in current sense amplifier with monitor and control pins allows accurate input or output current regulation and limiting. low ripple burst mode operation enables high efficiency down to very low output currents while keeping the output ripple below 10mv p-p . a sync pin allows synchronization to an external clock. internal compensation with peak current mode topology allows the use of small inductors and results in fast transient response and good loop stability. the en/uv pin has an accurate 1v threshold and can be used to program v in undervoltage lockout or to shut down the lt8611 reduc- ing the input supply current to 1a. a capacitor on the tr/ss pin programs the output voltage ramp rate during start-up. the pg flag signals when v out is within 9% of the programmed output voltage as well as fault conditions. the lt8611 is available in a small 24-lead 3mm 5mm qfn package with exposed pad for low thermal resistance. 5v step-down converter with 2.5a output current limit 12v in to 5v out efficiency applications n rail-to-rail current sense amplifier with monitor n wide input voltage range: 3.4v to 42v n ultralow quiescent current burst mode ? operation: 2.5a i q regulating 12v in to 3.3v out output ripple < 10mv p-p n high efficiency synchronous operation: 96% efficiency at 1a, 5v out from 12v in 94% efficiency at 1a, 3.3v out from 12v in n fast minimum switch-on time: 50ns n low dropout under all conditions: 200mv at 1a n allows use of small inductors n low emi n adjustable and synchronizable: 200khz to 2.2mhz n current mode operation n accurate 1v enable pin threshold n internal compensation n output soft-start and tracking n small thermally enhanced 3mm 5mm 24-lead qfn package n automotive and industrial supplies n general purpose step-down n cccv power supplies 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. load current (a) 0 efficiency (%) 80 90 100 2 8611 ta01b 70 60 75 85 95 65 55 50 0.5 1 1.5 2.5 v in = 12v v in = 24v f sw = 700khz bst v in en/uv on off sync imon ictrl intv cc tr/ss rt sw lt8611 gnd pgnd isp bias isn pg 8611 ta01a fb 0.1f v out 5v 2.5a 1f 47f 4.7f v in 5.5v to 42v 0.1f 1f 10pf 4.7h 0.02 1m 243k 52.3k f sw = 800khz
lt8611 2 8611p pin configuration absolute maximum ratings v in , en/uv, pg, isp, isn ...........................................42v bias ..........................................................................30v bst pin above sw pin................................................4v fb, tr/ss, rt, intv cc , imon, ictrl. ........................4v sync voltage . ............................................................6v operating junction temperature range (note 2) lt8611e ................................................. C40 to 125c lt8611i .................................................. C40 to 125c storage temperature range ......................C65 to 150c (note 1) 24 23 22 21 9 10 top view 25 gnd udd package 24-lead (3mm w 5mm) plastic qfn 11 12 6 5 4 3 2 1 15 16 17 18 19 20 sync tr/ss rt en/uv v in v in pgnd pgnd fb pg bias intv cc bst sw sw sw ictrl imon isn isp nc nc nc nc 14 7 13 8 ja = 38c/w, jc(pad) = 10c/w exposed pad (pin 25) is gnd, must be soldered to pcb electrical characteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. order information lead free finish tape and reel part marking* package description temperature range lt8611eudd#pbf lt8611eudd#trpbf 8611 24-lead (3mm 5mm) plastic qfn C40c to 125c lt8611iudd#pbf lt8611iudd#trpbf 8611 24-lead (3mm 5mm) plastic qfn C40c to 125c consult ltc marketing for parts specified with wider operating temperature ranges. *the temperature grade is identified by a la bel on the shipping container. consult ltc marketing for information on non-standard lead based finish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ parameter conditions min typ max units minimum input voltage l 2.9 3.4 v v in quiescent current v en/uv = 0v, v sync = 0v l 1.0 1.0 3 8 a a v en/uv = 2v, not switching, v sync = 0v l 1.7 1.7 4 10 a a v en/uv = 2v, not switching, v sync = 2v 0.46 1 ma v in current in regulation v out = 0.97v, v in = 6v, output load = 100a v out = 0.97v, v in = 6v, output load = 1ma l l 24 210 50 350 a a feedback reference voltage v in = 6v, i load = 0.5a v in = 6v, i load = 0.5a l 0.967 0.956 0.970 0.970 0.973 0.984 v v feedback voltage line regulation v in = 4.0v to 42v, i load = 0.5a l 0.004 0.02 %/v feedback pin input current v fb = 1v C20 20 na intv cc voltage i load = 0ma, v bias = 0v i load = 0ma, v bias = 3.3v 3.23 3.25 3.4 3.29 3.57 3.35 v v
lt8611 3 8611p electrical characteristics note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the lt8611e is guaranteed to meet performance specifications from 0c to 125c junction temperature. specifications over the C40c to 125c operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. the lt8611i is guaranteed over the full C40c to 125c operating junction the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. parameter conditions min typ max units intv cc undervoltage lockout 2.5 2.6 2.7 v bias pin current consumption v bias = 3.3v, i load = 1a, 2mhz 8.5 ma minimum on-time i load = 1a, sync = 0v i load = 1a, sync = 3.3v l l 30 30 50 45 70 65 ns ns minimum off-time 50 80 110 ns oscillator frequency r t = 221k, i load = 1a r t = 60.4k, i load = 1a r t = 18.2k, i load = 1a l l l 180 665 1.85 210 700 2.00 240 735 2.15 khz khz mhz top power nmos on-resistance v intvcc = 3.4v, i sw = 1a 120 m top power nmos current limit v intvcc = 3.4v l 3.5 4.8 5.8 a bottom power nmos on-resistance v intvcc = 3.4v, i sw = 1a 65 m bottom power nmos current limit v intvcc = 3.4v 2.5 3.3 4.8 a sw leakage current v in = 42v, v sw = 0v, 42v C1.5 1.5 a en/uv pin threshold en/uv rising l 0.94 1.0 1.06 v en/uv pin hysteresis 40 mv en/uv pin current v en/uv = 2v C20 20 na pg upper threshold offset from v fb v fb falling l 6 9.0 12 % pg lower threshold offset from v fb v fb rising l C6 C9.0 C12 % pg hysteresis 1.3 % pg leakage v pg = 3.3v C40 40 na pg pull-down resistance v pg = 0.1v l 680 2000 sync threshold sync falling sync rising 0.8 1.6 1.1 2.0 1.4 2.4 v v sync pin current v sync = 2v C40 40 na tr/ss source current l 1.2 2.2 3.2 a tr/ss pull-down resistance fault condition, tr/ss = 0.1v 230 current sense voltage (v isp-isn )v ictrl = 1.5v, v isn = 3.3v v ictrl = 1.5v, v isn = 0v v ictrl = 800mv, v isn = 3.3v v ictrl = 800mv, v isn = 0v v ictrl = 200mv, v isn = 3.3v v ictrl = 200mv, v isn = 0v l l l l l l 48 46.5 39 38 6 5 50 50.5 41 42 10 10.5 52 55.5 15 46 14 16 mv mv mv mv mv mv imon monitor pin voltage v isp-isn = 50mv, v isn = 3.3v v isp-isn = 50mv, v isn = 0v v isp-isn = 10mv, v isn = 3.3v v isp-isn = 10mv, v isn = 0v l l l l 0.965 0.900 150 130 1.00 0.99 220 205 1.035 1.080 290 280 v v mv mv isp, isn pin bias current l C20 20 a temperature range. high junction temperatures degrade operating lifetimes. operating lifetime is derated at junction temperatures greater than 125c. note 3: this ic includes overtemperature protection that is intended to protect the device during overload conditions. junction temperature will exceed 150c when overtemperature protection is active. continuous operation above the specified maximum operating junction temperature will reduce lifetime.
lt8611 4 8611p typical performance characteristics efficiency at 3.3v out efficiency vs frequency reference voltage en pin thresholds load regulation line regulation efficiency at 5v out efficiency at 3.3v out efficiency at 5v out load current (a) 0 efficiency (%) 80 90 100 2 8611 g01 70 60 75 85 95 65 55 50 0.5 1 1.5 2.5 v in = 12v v in = 24v f sw = 700khz load current (a) 0 efficiency (%) 80 90 100 2 8611 g02 70 60 75 85 95 65 55 50 0.5 1 1.5 2.5 v in = 12v v in = 24v f sw = 700khz load current (ma) 30 efficiency (%) 90 100 20 10 80 50 70 60 40 0.001 10 1000 8611 g03 0 0.1 v in = 12v v in = 24v f sw = 700khz load current (ma) 30 efficiency (%) 90 100 20 10 80 50 70 60 40 0.001 10 1000 8611 g04 0 0.1 v in = 12v v in = 24v f sw = 700khz switching frequency (mhz) 0.25 90 92 96 1.75 8611 g05 88 86 0.75 1.25 2.25 84 82 94 efficiency (%) v in = 12v v in = 24v v out = 3.3v temperature (c) C55 0.955 reference voltage (v) 0.958 0.964 0.967 0.970 0.985 0.976 5 65 95 125 8611 g06 0.961 0.979 0.982 0.973 C25 35 155 temperature (c) C55 0.95 en threshold (v) 0.96 0.98 0.99 1.00 65 1.04 8611 g07 0.97 5 C25 95 125 35 155 1.01 1.02 1.03 en rising en falling load current (a) 0 C0.25 change in v out (%) C0.15 C0.05 0.05 0.5 1 1.5 2 8611 g08 2.5 0.15 0.25 C0.20 C0.10 0 0.10 0.20 3 v out = 3.3v v in = 12v input voltage (v) 0 change in v out (%) 0.02 0.06 0.10 40 8611 g09 C0.02 C0.06 0 0.04 0.08 C0.04 C0.08 C0.10 10 5 20 15 30 35 45 25 v out = 3.3v i load = 0.5a
lt8611 5 8611p typical performance characteristics top fet current limit vs duty cycle top fet current limit bottom fet current limit switch drop minimum on-time switch drop no load supply current no load supply current minimum off-time input voltage (v) 0 0 input current (a) 0.5 1.5 2.0 2.5 5.0 3.5 10 20 25 45 8611 g10 1.0 4.0 4.5 3.0 515 30 35 40 v out = 3.3v in regulation temperature (c) C55 C25 0 input current (a) 10 25 5 65 95 8611 g11 5 20 15 35 125 155 v out = 3.3v v in = 12v in regulation input voltage (v) 5 output current (a) 3.8 4.0 4.2 35 8611 g12 3.6 3.4 15 25 10 40 20 30 45 3.2 3.0 4.4 v out = 3.3v temperature (c) C55 2.5 current limit (a) 3.0 3.5 4.0 4.5 5.0 C25 53565 8611 g14 95 125 30% dc 70% dc temperature (c) C55 2.4 current limit (a) 2.6 2.8 3.0 3.2 3.6 C25 53565 8611 g15 95 125 3.4 temperature (c) C55 C25 0 switch drop (mv) 100 250 5 65 95 8611 g40 50 200 150 35 125 155 top sw bot sw switch current = 1a switch current (a) 0 0 switch drop (mv) 50 150 200 250 2 450 8611 g41 100 1 0.5 2.5 1.5 3 300 350 400 top sw bot sw temperature (c) C55 30 minimum on-time (ns) 35 45 50 55 80 65 5 65 95 125 8611 g17 40 70 75 60 C25 35 155 i load = 1a, v sync = 0v i load = 1a, v sync = 3v i load = 2.5a, v sync = 0v i load = 2.5a, v sync = 3v temperature (c) C50 minimum off-time (ns) 95 35 8611 g18 80 70 C25 5 65 65 60 100 90 85 75 95 125 155 v in = 3.3v i load = 0.5a
lt8611 6 8611p typical performance characteristics dropout voltage switching frequency burst frequency frequency foldback minimum load to full frequency (sync dc high) soft-start tracking soft-start current pg high thresholds pg low thresholds load current (a) 0 dropout voltage (mv) 400 8611 g19 200 0 12 0.5 1.5 2.5 600 800 300 100 500 700 3 temperature (c) C55 switching frequency (khz) 730 35 8611 g20 700 680 C25 5 65 670 660 740 r t = 60.4k 720 710 690 95 125 155 load current (ma) 0 switching frequency (khz) 400 500 600 200 8611 g21 300 200 0 50 100 150 100 800 v in = 12v v out = 3.3v 700 input voltage (v) load current (ma) 60 80 100 15 25 40 45 8611 g39 40 20 0 510 20 30 35 v out = 5v f sw = 700khz fb voltage (v) 0 switching frequency (khz) 300 400 500 0.6 1 8611 g22 200 100 0 0.2 0.4 0.8 600 700 800 v out = 3.3v v in = 12v v sync = 0v r t = 60.4k tr/ss voltage (v) 0 fb voltage (v) 0.8 1.0 1.2 0.6 1.0 8611 g23 0.6 0.4 0.2 0.4 0.8 1.2 1.4 0.2 0 temperature (c) C50 ss pin current (a) 2.3 35 8611 g24 2.0 1.8 C25 5 65 1.7 1.6 2.4 2.2 2.1 1.9 95 125 155 v ss = 0.5v temperature (c) C55 7.0 pg threshold offset from v ref (%) 7.5 8.5 9.0 9.5 12.0 10.5 5 65 95 125 8611 g25 8.0 11.0 11.5 10.0 C25 35 155 fb rising fb falling temperature (c) C55 C12.0 pg threshold offset from v ref (%) C11.5 C10.5 C10.0 C9.5 C7.0 C8.5 5 65 95 125 8611 g26 C11.0 C8.0 C7.5 C9.0 C25 35 155 fb rising fb falling
lt8611 7 8611p typical performance characteristics rt programmed switching frequency v in uvlo bias pin current bias pin current switching waveforms switching waveforms switching waveforms transient response transient response switching frequency (khz) 0.2 rt pin resistor (k) 150 200 250 1.8 8611 g27 100 50 125 175 225 75 25 0 0.6 1 1.4 2.2 temperature (c) C55 input voltage (v) 3.4 35 8611 g28 2.8 2.4 C25 5 65 2.2 2.0 3.6 3.2 3.0 2.6 95 125 155 input voltage (v) 5 bias pin current (ma) 4.00 4.50 45 8611 g29 3.50 3.00 15 25 35 10 20 30 40 5.00 3.75 4.25 3.25 4.75 v bias = 5v v out = 5v i load = 1a f sw = 700khz switching frequency (mhz) 0 0 bias pin current (ma) 2 4 6 8 10 12 0.5 1 1.5 2 8611 g30 2.5 v bias = 5v v out = 5v v in = 12v i load = 1a i l 1a/div v sw 5v/div 500ns/div 12v in to 5v out at 1a 8611 g31 i l 200ma/div v sw 5v/div 500s/div 12v in to 5v out at 10ma v sync = 0v 8611 g32 i l 1a/div v sw 10v/div 500ns/div 36v in to 5v out at 1a 8611 g33 i load 1a/div v out 100mv/div 50s/div 0.5a to 1.5a transient 12v in , 5v out c out = 47f 8611 g34 i load 1a/div v out 200mv/div 50s/div 0.5a to 2.5a transient 12v in , 5v out c out = 47f 8611 g35
lt8611 8 8611p typical performance characteristics start-up dropout performance isp-isn limit (v out = 3.3v + 1v) imon vs v isp (isp C isn = 20mv) start-up dropout performance isp-isn limit vs ictrl transient response isp-isn limit (v out = 3.3v + 1v) imon vs isp-isn i l 1a/div v out 200mv/div 50s/div 50ma to 1a transient 12v in , 5v out c out = 47f 8611 g36 v in 2v/div v out 2v/div 100ms/div 2.5 load (2a in regulation) 8611 g37 v in v out v in 2v/div v out 2v/div 100ms/div 20 load (250ma in regulation) 8611 g38 v in v out
lt8611 9 8611p pin functions sync (pin 1): external clock synchronization input. ground this pin for low ripple burst mode operation at low output loads. tie to a clock source for synchronization to an external frequency. apply a dc voltage of 3v or higher or tie to intv cc for pulse-skipping mode. when in pulse- skipping mode, the i q will increase to several hundred a. do not float this pin. tr/ss (pin 2): output tracking and soft-start pin. this pin allows user control of output voltage ramp rate during start-up. a tr/ss voltage below 0.97v forces the lt8611 to regulate the fb pin to equal the tr/ss pin voltage. when tr/ss is above 0.97v, the tracking function is disabled and the internal reference resumes control of the error amplifier. an internal 2.2a pull-up current from intv cc on this pin allows a capacitor to program output voltage slew rate. this pin is pulled to ground with an internal 230 mosfet during shutdown and fault conditions; use a series resistor if driving from a low impedance output. this pin may be left floating if the tracking function is not needed. rt (pin 3): a resistor is tied between rt and ground to set the switching frequency. en/uv (pin 4): the lt8611 is shut down when this pin is low and active when this pin is high. the hysteretic threshold voltage is 1.00v going up and 0.96v going down. tie to v in if the shutdown feature is not used. an external resistor divider from v in can be used to program a v in threshold below which the lt8611 will shut down. v in (pins 5, 6): the v in pins supply current to the lt8611 internal circuitry and to the internal topside power switch. these pins must be tied together and be locally bypassed. be sure to place the positive terminal of the input capaci- tor as close as possible to the v in pins, and the negative capacitor terminal as close as possible to the pgnd pins. pgnd (pins 7, 8): power switch ground. these pins are the return path of the internal bottom-side power switch and must be tied together. place the negative terminal of the input capacitor as close to the pgnd pins as possible. nc (pins 9, 10, 11, 12): no connect. these pins are not connected to internal circuitry. it is recommended that these be connected to gnd so that the exposed pad gnd can be run to the top level gnd copper to enhance thermal performance. sw (pins 13, 14, 15): the sw pins are the outputs of the internal power switches. tie these pins together and con- nect them to the inductor and boost capacitor. this node should be kept small on the pcb for good performance. bst (pin 16): this pin is used to provide a drive voltage, higher than the input voltage, to the topside power switch. place a 0.1f boost capacitor as close as possible to the ic. intv cc (pin 17): internal 3.4v regulator bypass pin. the internal power drivers and control circuits are pow- ered from this voltage. intv cc maximum output cur- rent is 20ma. do not load the intv cc pin with external circuitry. intv cc current will be supplied from bias if v bias > 3.1v, otherwise current will be drawn from v in . voltage on intv cc will vary between 2.8v and 3.4v when v bias is between 3.0v and 3.6v. decouple this pin to power ground with at least a 1f low esr ceramic capacitor placed close to the ic. bias (pin 18): the internal regulator will draw current from bias instead of v in when bias is tied to a voltage higher than 3.1v. for output voltages of 3.3v and above this pin should be tied to v out . if this pin is tied to a supply other than v out use a 1f local bypass capacitor on this pin. pg (pin 19): the pg pin is the open-drain output of an internal comparator. pg remains low until the fb pin is within 9% of the final regulation voltage, and there are no fault conditions. pg is valid when v in is above 3.4v, regardless of en/uv pin state. fb (pin 20): the lt8611 regulates the fb pin to 0.970v. connect the feedback resistor divider tap to this pin. also, connect a phase lead capacitor between fb and v out . typically, this capacitor is 4.7pf to 10pf. isp (pin 21): current sense (+) pin. this is the noninvert- ing input to the current sense amplifier. isn (pin 22): current sense (C) pin. this is the inverting input to the current sense amplifier.
lt8611 10 8611p block diagram imon (pin 23): proportional-to-current monitor output. this pin sources a voltage 20 times the voltage between the isp and isn pins such that: v imon = 20 ? (v isp -v isn ). imon can source 200a and sink 10a. float imon if unused. ictrl (pin 24): current adjustment pin. ictrl adjusts the maximum isp-isn drop before the lt8611 reduces output current. connect directly to intv cc or float for full-scale isp-isn threshold of 50mv or apply values between gnd and 1v to modulate current limit. there is an internal 1.4a pull-up current on this pin. float or tie to intv cc when unused. gnd (exposed pad pin 25): ground. the exposed pad must be connected to the negative terminal of the input capacitor and soldered to the pcb in order to lower the thermal resistance. pin functions + + C + C slope comp internal 0.97v ref oscillator 200khz to 2.2mhz burst detect 3.4v reg m1 m2 c bst c f 8611 bd sw l bst 13-15 switch logic and anti- shoot through error amp shdn 9% v c shdn tsd intv cc uvlo v in uvlo shdn tsd v in uvlo en/uv 1v + + C + C + C 4 6 isp r 20r v out r sen ictrl 1 w 21 isn 1.0v r 22 24 25 imon gnd 23 intv cc 17 bias 18 pgnd 7, 8 pg 19 fb r1 c1 r3 opt r4 opt r2 r t c ss (opt) v out 20 tr/ss 2.2a 2 rt 3 sync 1 v in v in c in c vcc 5, 6 1.4a c out
lt8611 11 8611p operation the lt8611 is a monolithic, constant frequency, current mode step-down dc/dc converter. an oscillator, with frequency set using a resistor on the rt pin, turns on the internal top power switch at the beginning of each clock cycle. current in the inductor then increases until the top switch current comparator trips and turns off the top power switch. the peak inductor current at which the top switch turns off is controlled by the voltage on the internal vc node. the error amplifier servos the vc node by comparing the voltage on the v fb pin with an internal 0.97v reference. when the load current increases it causes a reduction in the feedback voltage relative to the reference leading the error amplifier to raise the vc voltage until the average inductor current matches the new load current. when the top power switch turns off, the synchronous power switch turns on until the next clock cycle begins or inductor current falls to zero. if overload conditions result in more than 3.3a flowing through the bottom switch, the next clock cycle will be delayed until switch current returns to a safe level. the lt8611 includes a current control and monitoring loop using the isn, isp, imon and ictrl pins. the isp/ isn pins monitor the voltage across an external sense resistor such that the v isp -v isn does not exceed 50mv by limiting the peak inductor current controlled by the vc node. the current sense amplifier inputs (isp/isn) are rail- to-rail such that input, output, or other system currents may be monitored and regulated. the imon pin outputs a ground-referenced voltage equal to 20 times the voltage between the isp-isn pins for monitoring system currents. the ictrl pin can be used to override the internal 50mv limit between the isp, isn pin to a lower set point for the current control loop. if the en/uv pin is low, the lt8611 is shut down and draws 1a from the input. when the en/uv pin is above 1v, the switching regulator will become active. to optimize efficiency at light loads, the lt8611 operates in burst mode operation in light load situations. between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 1.7a. in a typical application, 2.5a will be consumed from the input supply when regulating with no load. the sync pin is tied low to use burst mode operation and can be tied to a logic high to use pulse-skipping mode. if a clock is applied to the sync pin the part will synchronize to an external clock frequency and operate in pulse-skipping mode. while in pulse-skipping mode the oscillator operates continuously and positive sw transitions are aligned to the clock. during light loads, switch pulses are skipped to regulate the output and the quiescent current will be several hundred a. to improve efficiency across all loads, supply current to internal circuitry can be sourced from the bias pin when biased at 3.3v or above. else, the internal circuitry will draw current from v in . the bias pin should be connected to v out if the lt8611 output is programmed at 3.3v or above. comparators monitoring the fb pin voltage will pull the pg pin low if the output voltage varies more than 9% (typical) from the set point, or if a fault condition is present. the oscillator reduces the lt8611s operating frequency when the voltage at the fb pin is low. this frequency foldback helps to control the inductor current when the output voltage is lower than the programmed value which occurs during start-up or overcurrent conditions. when a clock is applied to the sync pin or the sync pin is held dc high, the frequency foldback is disabled and the switching frequency will slow down only during overcur- rent conditions.
lt8611 12 8611p applications information achieving ultralow quiescent current to enhance efficiency at light loads, the lt8611 operates in low ripple burst mode operation, which keeps the out- put capacitor charged to the desired output voltage while minimizing the input quiescent current and minimizing output voltage ripple. in burst mode operation the lt8611 delivers single small pulses of current to the output capaci- tor followed by sleep periods where the output power is supplied by the output capacitor. while in sleep mode the lt8611 consumes 1.7a. as the output load decreases, the frequency of single cur- rent pulses decreases (see figure 1a) and the percentage of time the lt8611 is in sleep mode increases, resulting in much higher light load efficiency than for typical convert- ers. by maximizing the time between pulses, the converter quiescent current approaches 2.5a for a typical application when there is no output load. therefore, to optimize the quiescent current performance at light loads, the current in the feedback resistor divider must be minimized as it appears to the output as load current. while in burst mode operation the current limit of the top switch is approximately 400ma resulting in output voltage ripple shown in figure 2. increasing the output capacitance will decrease the output ripple proportionally. as load ramps upward from zero the switching frequency will increase but only up to the switching frequency programmed by the resistor at the rt pin as shown in figure 1a. the out- put load at which the lt8611 reaches the programmed frequency varies based on input voltage, output voltage, and inductor choice. for some applications it is desirable for the lt8611 to operate in pulse-skipping mode, offering two major differ- ences from burst mode operation. first is the clock stays awake at all times and all switching cycles are aligned to the clock. in this mode much of the internal circuitry is awake at all times, increasing quiescent current to several hundred a. second is that full switching frequency is reached at lower output load than in burst mode operation (see figure 1b). to enable pulse-skipping mode, the sync pin is tied high either to a logic output or to the intv cc pin. when a clock is applied to the sync pin the lt8611 will also operate in pulse-skipping mode. figure 1. sw frequency vs load information in burst mode operation (1a) and pulse-skipping mode (1b) figure 2. burst mode operation minimum load to full frequency (sync dc high) burst frequency (1a) (1b) load current (ma) 0 switching frequency (khz) 400 500 600 200 8611 f01a 300 200 0 50 100 150 100 800 v in = 12v v out = 3.3v 700 input voltage (v) load current (ma) 60 80 100 15 25 40 45 8611 f01b 40 20 0 510 20 30 35 5v out 700khz i l 200ma/div v out 10mv/div 5s/div v sync = 0v 8611 f02
lt8611 13 8611p applications information fb resistor network the output voltage is programmed with a resistor divider between the output and the fb pin. choose the resistor values according to: r1 = r2 v out 0.970v ?1 ? ? ? ? ? ? (1) reference designators refer to the block diagram. 1% resistors are recommended to maintain output voltage accuracy. if low input quiescent current and good light-load efficiency are desired, use large resistor values for the fb resistor divider. the current flowing in the divider acts as a load current, and will increase the no-load input current to the converter, which is approximately: i q = 1.7a + v out r1 + r2 ? ? ? ? ? ? v out v in ? ? ? ? ? ? 1 n ? ? ? ? ? ? (2) where 1.7a is the quiescent current of the lt8611 and the second term is the current in the feedback divider reflected to the input of the buck operating at its light load efficiency n. for a 3.3v application with r1 = 1m and r2 = 412k, the feedback divider draws 2.3a. with v in = 12v and n = 80%, this adds 0.8a to the 1.7a quiescent current resulting in 2.5a no-load current from the 12v supply. note that this equation implies that the no-load current is a function of v in ; this is plotted in the typical performance characteristics section. when using large fb resistors, a 4.7pf to 10pf phase-lead capacitor should be connected from v out to fb. setting the switching frequency the lt8611 uses a constant frequency pwm architecture that can be programmed to switch from 200khz 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. the r t resistor required for a desired switching frequency can be calculated using: r t = 46.5 f sw ? 5.2 (3) where r t is in k and f sw is the desired switching fre- quency in mhz. table 1. sw frequency vs r t value f sw (mhz) r t (k) 0.2 232 0.3 150 0.4 110 0.5 88.7 0.6 71.5 0.7 60.4 0.8 52.3 1.0 41.2 1.2 33.2 14 28.0 1.6 23.7 1.8 20.5 2.0 18.2 2.2 15.8 operating frequency selection and trade-offs selection of the operating frequency is a trade-off between efficiency, component size, and input voltage range. the advantage of high frequency operation is that smaller induc- tor and capacitor values may be used. the disadvantages are lower efficiency and a smaller input voltage range. the highest switching frequency (f sw(max) ) for a given application can be calculated as follows: f sw(max) = v out + v sw(bot) t on(min) v in ?v sw(top) + v sw(bot) () (4) where v in is the typical input voltage, v out is the output voltage, v sw(top) and v sw(bot) are the internal switch drops (~0.3v, ~0.15v, respectively at maximum load) and t on(min) is the minimum top switch on-time (see the electrical characteristics). this equation shows that a slower switching frequency is necessary to accommodate a high v in /v out ratio. for transient operation, v in may go as high as the abso- lute maximum rating of 42v regardless of the r t value, however the lt8611 will reduce switching frequency as necessary to maintain control of inductor current to as- sure safe operation.
lt8611 14 8611p applications information the lt8611 is capable of a maximum duty cycle of greater than 99%, and the v in -to-v out dropout is limited by the r ds(on) of the top switch. in this mode the lt8611 skips switch cycles, resulting in a lower switching frequency than programmed by rt. for applications that cannot allow deviation from the pro- grammed switching frequency at low v in /v out ratios use the following formula to set switching frequency: v in(min) = v out + v sw(bot) 1? f sw ?t off(min) ?v sw(bot) + v sw(top) (5) where v in(min) is the minimum input voltage without skipped cycles, v out is the output voltage, v sw(top) and v sw(bot) are the internal switch drops (~0.3v, ~0.15v, respectively at maximum load), f sw is the switching fre- quency (set by rt), and t off(min) is the minimum switch off-time. note that higher switching frequency will increase the minimum input voltage below which cycles will be dropped to achieve higher duty cycle. inductor selection and maximum output current the lt8611 is designed to minimize solution size by allowing the inductor to be chosen based on the output load requirements of the application. during overload or short-circuit conditions the lt8611 safely tolerates opera- tion with a saturated inductor through the use of a high speed peak-current mode architecture. a good first choice for the inductor value is: l = v out + v sw(bot) f sw (6) where f sw is the switching frequency in mhz, v out is the output voltage, v sw(bot) is the bottom switch drop (~0.15v) and l is the inductor value in h. to avoid overheating and poor efficiency, an inductor must be chosen with an rms current rating that is greater than the maximum expected output load of the application. in addition, the saturation current (typically labeled i sat ) rating of the inductor must be higher than the load current plus 1/2 of in inductor ripple current: i l(peak) = i load(max) + 1 2 ? i l (7) where ? i l is the inductor ripple current as calculated in equation 9 and i load(max) is the maximum output load for a given application. as a quick example, an application requiring 1a output should use an inductor with an rms rating of greater than 1a and an i sat of greater than 1.3a. during long duration overload or short-circuit conditons, the inductor rms routing requirement is greater to avoid overheating of the inductor. to keep the efficiency high, the series resistance (dcr) should be less than 0.04, and the core material should be intended for high frequency applications. the lt8611 limits the peak switch current in order to protect the switches and the system from overload faults. the top switch current limit (i lim ) is at least 3.5a at low duty cycles and decreases linearly to 2.8a at dc = 0.8. the inductor value must then be sufficient to supply the desired maximum output current (i out(max) ), which is a function of the switch current limit (i lim ) and the ripple current. i out(max) = i lim ? ? i l 2 (8) the peak-to-peak ripple current in the inductor can be calculated as follows: ? i l = v out l?f sw ?1? v out v in(max) ? ? ? ? ? ? (9) where f sw is the switching frequency of the lt8611, and l is the value of the inductor. therefore, the maximum output current that the lt8611 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. the inductor value may have to be increased if the inductor ripple current does not allow sufficient maximum output current (i out(max) ) given the switching frequency, and maximum input voltage used in the desired application. the optimum inductor for a given application may differ from the one indicated by this design guide. a larger value inductor provides a higher maximum load current and reduces the output voltage ripple. for applications requir- ing smaller load currents, the value of the inductor may be lower and the lt8611 may operate with higher ripple
lt8611 15 8611p applications information current. this allows use of a physically smaller inductor, or one with a lower dcr resulting in higher efficiency. be aware that low inductance may result in discontinuous mode operation, which further reduces maximum load current. for more information about maximum output current and discontinuous operation, see linear technologys application note 44. finally, for duty cycles greater than 50% (v out /v in > 0.5), a minimum inductance is required to avoid sub-harmonic oscillation. see application note 19. input capacitor bypass the input of the lt8611 circuit with a ceramic ca- pacitor of x7r or x5r type placed as close as possible to the v in and pgnd pins. 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 lt8611 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 significant 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 lt8611 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 lt8611 (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt8611. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank cir- cuit. if the lt8611 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt8611s voltage rating. this situation is easily avoided (see linear technology application note 88). output capacitor and output ripple the output capacitor has two essential functions. along with the inductor, it filters the square wave generated by the lt8611 to produce the dc output. in this role it determines the output ripple, thus low impedance at the switching frequency is important. the second function is to store energy in order to satisfy transient loads and stabilize the lt8611s control loop. ceramic capacitors have very low equivalent series resistance (esr) and provide the best ripple performance. for good starting values, see the typical applications section. 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 output capacitor and the addition of a feedforward capacitor placed between v out and fb. increasing the output capacitance will also decrease the output voltage ripple. a lower value of output capacitor can be used to save space and cost but transient performance will suffer and may cause loop instability. see the typical applications in this data sheet for suggested capacitor values. when choosing a capacitor, special attention should be given to the data sheet to calculate the effective capacitance under the relevant operating conditions of voltage bias and temperature. a physically larger capacitor or one with a higher voltage rating may be required. ceramic capacitors ceramic capacitors are small, robust and have very low esr. however, ceramic capacitors can cause problems when used with the lt8611 due to their piezoelectric nature. when in burst mode operation, the lt8611s switching frequency depends on the load current, and at very light loads the lt8611 can excite the ceramic capacitor at audio frequencies, generating audible noise. since the lt8611 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. low noise ceramic capacitors are also available.
lt8611 16 8611p applications information a final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the lt8611. as previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (un- derdamped) tank circuit. if the lt8611 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt8611s rating. this situation is easily avoided (see linear technology application note 88). enable pin the lt8611 is in shutdown when the en pin is low and active when the pin is high. the rising threshold of the en comparator is 1.0v, with 40mv of hysteresis. the en pin can be tied to v in if the shutdown feature is not used, or tied to a logic level if shutdown control is required. adding a resistor divider from v in to en programs the lt8611 to regulate the output only when v in is above a desired voltage (see the block diagram). typically, this threshold, v in(en) , is used in situations where the input supply is current limited, or has a relatively high source resistance. a switching regulator draws constant power from the source, so source current increases as source voltage drops. this looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. the v in(en) threshold prevents the regulator from operating at source voltages where the problems might occur. this threshold can be adjusted by setting the values r3 and r4 such that they satisfy the following equation: v in(en) = r3 r4 + 1 ? ? ? ? ? ? ?1.0v (10) where the lt8611 will remain off until v in is above v in(en) . due to the comparators hysteresis, switching will not stop until the input falls slightly below v in(en) . when operating in burst mode operation for light load currents, the current through the v in(en) resistor network can easily be greater than the supply current consumed by the lt8611. therefore, the v in(en) resistors should be large to minimize their effect on efficiency at low loads. current control loop in addition to regulating the output voltage the lt8611 includes a current regulation loop for setting the aver-age input or output current limit as shown in the typical ap- plications section. the lt8611 measures voltage drop across an external current sense resistor using the isp and isn pins. this resistor may be connected between the inductor and the output capacitor to sense the output current or may be placed between the v in bypass capacitor and the input power source to sense input current. the current loop modulates the internal cycle-by-cycle switch current limit such that the average voltage across isp-isn pins does not exceed 50mv. care must be taken and filters should be used to assure the signal applied to the isn and isp pins has a peak-to- peak ripple of less than 30mv for accurate operation. in addition to high crest factor current waveforms such as the input current of dc/dc regulators, another cause of high ripple voltage across the sense resistor is excessive resistor esl. typically the problem is solved by using a small ceramic capacitor across the sense resistor or using a filter network between the isp and isn pins. the ictrl pin allows the isp-isn set point to be linearly controlled from 50mv to 0mv as the ictrl pin is ramped from 1v down to 0v, respectively and as shown in figure?3. when this functionality is unused the ictrl pin may be tied to intv cc or floated. in addition the ictrl pin includes xxx 0 xxx 2 3 40 ltxxxx ? tpcxx 1 0 10 20 place holder 30 4 figure 3. lt8611 sense voltage vs ictrl voltage
lt8611 17 8611p applications information a 2a pull-up source such that a capacitor may be added for soft-start functionality. the imon pin is a voltage output proportional to the voltage across the current sense resistor such that v imon = 20 ? (isp-isn) as shown in figure 4. this output can be used to monitor the input or output current of the lt8611 or may be an input to an adc for further processing. output voltage tracking and soft-start t he lt8611 allows the user to program its output voltage ramp rate by means of the tr/ss pin. an internal 2.2a pulls up the tr/ss pin to intv cc . putting an external capacitor on tr/ss enables soft starting the output to pre- vent current surge on the input supply. during the soft-start ramp the output voltage will proportionally track the tr/ss pin voltage. for output tracking applications, tr/ss can be externally driven by another voltage source. from 0v to 0.97v, the tr/ss voltage will override the internal 0.97v reference input to the error amplifier, thus regulating the fb pin voltage to that of tr/ss pin. when tr/ss is above 0.97v, tracking is disabled and the feedback voltage will regulate to the internal reference voltage. the tr/ss pin may be left floating if the function is not needed. an active pull-down circuit is connected to the tr/ss pin which will discharge the external soft-start capacitor in the case of fault conditions and restart the ramp when the faults are cleared. fault conditions that clear the soft-start capacitor are the en/uv pin transitioning low, v in voltage falling too low, or thermal shutdown. output power good when the lt8611s output voltage is within the 9% window of the regulation point, which is a v fb voltage in the range of 0.883v to 1.057v (typical), the output voltage is considered good and the open-drain pg pin goes high impedance and is typically pulled high with an external resistor. otherwise, the internal pull-down device will pull the pg pin low. to prevent glitching both the upper and lower thresholds include 1.3% of hysteresis. the pg pin is also actively pulled low during several fault conditions: en/uv pin is below 1v, intv cc has fallen too low, v in is too low, or thermal shutdown. synchronization to select low ripple burst mode operation, tie the sync pin below 0.4v (this can be ground or a logic low output). to synchronize the lt8611 oscillator to an external frequency connect a square wave (with 20% to 80% duty cycle) to the sync pin. the square wave amplitude should have val- leys that are below 0.4v and peaks above 2.4v (up to 6v). xxx 0 xxx 2 3 40 ltxxxx ? tpcxx 1 0 10 20 place holder 30 4 figure 4. lt8611 sense voltage vs imon voltage intv cc regulator an internal low dropout (ldo) regulator produces the 3.4v supply from v in that powers the drivers and the internal bias circuitry. the intv cc can supply enough current for the lt8611s circuitry and must be bypassed to ground with a minimum of 1f ceramic capacitor. good bypassing is necessary to supply the high transient currents required by the power mosfet gate drivers. to improve efficiency the internal ldo can also draw current from the bias pin when the bias pin is at 3.1v or higher. typically the bias pin can be tied to the output of the lt8611, or can be tied to an external supply of 3.3v or above. if bias is connected to a supply other than v out , be sure to bypass with a local ceramic capacitor. if the bias pin is below 3.0v, the internal ldo will consume current from v in . applications with high input voltage and high switching frequency where the internal ldo pulls current from v in will increase die temperature because of the higher power dissipation across the ldo. do not connect an external load to the intv cc pin.
lt8611 18 8611p applications information figure 5. reverse v in protection the lt8611 will not enter burst mode operation at low output loads while synchronized to an external clock, but instead will pulse skip to maintain regulation. the lt8611 may be synchronized over a 200khz to 2.2mhz range. the r t resistor should be chosen to set the lt8611 switching frequency equal to or below the lowest synchronization input. for example, if the synchronization signal will be 500khz and higher, the r t should be selected for 500khz. the slope compensation is set by the r t value, while the minimum slope compensation required to avoid subhar- monic oscillations is established by the inductor size, input voltage, and output voltage. since the synchroniza- tion frequency will not change the slopes of the inductor current waveform, if the inductor is large enough to avoid subharmonic oscillations at the frequency set by r t , then the slope compensation will be sufficient for all synchro- nization frequencies. for some applications it is desirable for the lt8611 to operate in pulse-skipping mode, offering two major differ- ences from burst mode operation. first is the clock stays awake at all times and all switching cycles are aligned to the clock. second is that full switching frequency is reached at lower output load than in burst mode operation. these two differences come at the expense of increased quiescent current. to enable pulse-skipping mode, the sync pin is tied high either to a logic output or to the intvcc pin. the lt8611 does not operate in forced continuous mode regardless of sync signal. never leave the sync pin floating. shorted and reversed input protection the lt8611 will tolerate a shorted output. several features are used for protection during output short-circuit and brownout conditions. the first is the switching frequency will be folded back while the output is lower than the set point to maintain inductor current control. second, the bottom switch current is monitored such that if inductor current is beyond safe levels switching of the top switch will be delayed until such time as the inductor current falls to safe levels. frequency foldback behavior depends on the state of the sync pin: if the sync pin is low the switching frequency will slow while the output voltage is lower than the pro- grammed level. if the sync pin is connected to a clock source or tied high, the lt8611 will stay at the programmed frequency without foldback and only slow switching if the inductor current exceeds safe levels. there is another situation to consider in systems where the output will be held high when the input to the lt8611 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 lt8611s output. if the v in pin is allowed to float and the en pin is held high (either by a logic signal or because it is tied to v in ), then the lt8611s internal circuitry will pull its quiescent current through its sw pin. this is acceptable if the system can tolerate several a in this state. if the en pin is grounded the sw pin current will drop to near 1a. however, if the v in pin is grounded while the output is held high, regard- less of en, parasitic body diodes inside the lt8611 can pull current from the output through the sw pin and the v in pin. figure 5 shows a connection of the v in and en/uv pins that will allow the lt8611 to run only when the input voltage is present and that protects against a shorted or reversed input. v in v in d1 lt8611 en/uv 8611 f05 gnd
lt8611 19 8611p applications information figure 6. recommended pcb layout for the lt8611 pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 6 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents flow in the lt8611s v in pins, pgnd pins, and the input ca- pacitor (c1). the loop formed by the input capacitor should be as small as possible by placing the capacitor adjacent to the v in and pgnd pins. when using a physically large input capacitor the resulting loop may become too large in which case using a small case/value capacitor placed close to the v in and pgnd pins plus a larger capacitor further away is preferred. these components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. place a local, unbroken ground plane under the application circuit on the layer closest to the surface layer. the sw and boost nodes should be as small as possible. finally, keep the fb and rt nodes small so that the ground traces will shield them from the sw and boost nodes. the exposed pad on the bottom of the package must be soldered to ground so that the pad is connected to ground electrically and also acts as a heat sink thermally. to keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the lt8611 to additional ground planes within the circuit board and on the bottom side. high temperature considerations for higher ambient temperatures, care should be taken in the layout of the pcb to ensure good heat sinking of the lt8611. the exposed pad on the bottom of the package must be soldered to a ground plane. this ground should be tied to large copper layers below with thermal vias; these layers will spread heat dissipated by the lt8611. placing additional vias can reduce thermal resistance further. the maximum load current should be derated as the ambient temperature approaches the maximum junction rating. power dissipation within the lt8611 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the inductor loss. the die temperature is calculated by multiplying the lt8611 power dissipation by the thermal resistance from junction to ambient. the lt8611 will stop switching and indicate a fault condition if safe junction temperature is exceeded. 9101112 v out 8611 f06 outline of local ground plane sw gnd 13 14 15 16 17 18 19 pg 20 21 22 23 imon ictrl sync en/uv v in 24 1 2 3 4 5 6 7 8 v out line to bias v out line to isn line to isp vias to ground plane
lt8611 20 8611p typical applications 5v step-down with 1a output current limit 3.3v step-down with 1a input current limit and 7v v in undervoltage lockout 3.3v step-down with 1a input current limit bst v in en/uv on off sync imon ictrl intv cc tr/ss fset sw lt8611 gnd pgnd isp isn bias pg 8611 ta02 fb 0.1f v out 5v 1a 1f 47f 4.7f v in 5.5v to 42v 0.1f 1f 10pf 4.7h 0.050 1m 243k 52.3k f sw = 800khz bst v in en/uv on off sync imon ictrl intv cc tr/ss rt sw lt8611 isn isp pgnd gnd pg bias 8611 ta03 fb 0.10f v out 3.3v 47f 4.7f 1f v in 3.8v to 42v 0.1f 1f 4.7h 1m 0.050 412k 41.2k f sw = 1mhz 10pf bst v in en/uv sync imon ictrl intv cc tr/ss rt sw lt8611 isn isp pgnd gnd pg bias 8611 ta04 fb 0.1f v out 3.3v 47f 4.7f 604k 100k 1f v in 3.8v to 42v 0.1f 1f 4.7h 1m 0.050 412k 60.4k f sw = 700khz 10pf
lt8611 21 8611p typical applications digitally controlled current/voltage source cccv battery charger C3.3v negative converter with 1a output current limit bst v in en/uv on off sync imon ictrl adc c dac intv cc tr/ss fset sw lt8611 gnd isp bias isn pg 8611 ta05 fb 0.1f v out 3.3v 2a 1f 10pf 47f 4.7f v in 3.8v to 42v 1f 4.7h 0.025 1m 412k 60.4k f sw = 700khz pgnd bst v in en/uv on off sync imon ictrl intv cc tr/ss fset sw lt8611 gnd isp isn bias pg 8611 ta06 fb 0.1f v out 4.1v 1a 1f 10pf li-ion battery 47f 4.7f d1 v in 3.8v to 42v 1f 0.1f 4.7h 0.050 324k 100k 60.4k f sw = 700khz pgnd + bst v in en/uv sync imon 60.4k 0.1f ictrl intv cc tr/ss rt sw lt8611 gnd isp isn bias pg 8611 ta07 fb 0.1f 1f v out C3.3v 1a 10pf 47f 4.7f v in 3.8v to 42v 1f 0.1f 4.7h 1m 0.05 412k 60.4k f = 700khz pgnd 4.7f
lt8611 22 8611p 2mhz, 3.3v step-down with power good without current sense 1v step-down with 2a output current limit 12v step-down with 1a output current limit typical applications bst v in en/uv sync imon ictrl intv cc tr/ss rt sw lt8611 gnd isp isn bias pg 8611 ta08 fb 0.1f v out 3.3v 2.5a pgood 10pf 47f v in 3.8v to 42v 1f 0.1f 2.2h 1m 412k 150k 18.2k f = 2mhz pgnd 4.7f on off bst v in en/uv sync imon ictrl intv cc tr/ss rt sw lt8611 gnd isp isn bias pg 8611 ta09 fb 0.1f v out 0.97v 2a 100f v in 3.8v to 42v 1f 0.1f 10h 0.025 150k f = 300khz pgnd 10f on off 1f bst v in en/uv sync imon ictrl intv cc tr/ss rt sw lt8611 gnd isp isn bias pg 8611 ta10 fb 0.1f v out 12v 1a 22f v in 12.5v to 42v 1f 0.1f 10h 0.05 60.4k f = 700khz pgnd 10f on off 1f 1m 10pf 88.7k
lt8611 23 8611p information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. typical applications 3.00 0.10 1.50 ref 5.00 0.10 note: 1. drawing 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 pin 1 top mark (note 6) 0.40 0.10 23 24 1 2 bottom viewexposed pad 3.50 ref 0.75 0.05 r = 0.115 typ pin 1 notch r = 0.20 or 0.25 w 45 chamfer 0.25 0.05 0.50 bsc 0.200 ref 0.00 C 0.05 (udd24) qfn 0808 rev ? r = 0.05 typ 1.65 0.10 3.65 0.10 udd package 24-lead plastic qfn (3mm w 5mm) (reference ltc dwg # 05-08-1833 rev ?) recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 0.70 0.05 0.25 0.05 3.50 ref 4.10 0.05 5.50 0.05 1.50 ref 2.10 0.05 3.50 0.05 package outline 1.65 0.05 3.65 0.05 0.50 bsc 2a led driver bst v in en/uv sync imon ictrl intv cc tr/ss rt sw lt8611 gnd isp isn bias pg 8611 ta11 fb 0.1f 2a 4.7f v in 3.8v to 42v 1f 0.1f 4.7h 0.025 60.4k f = 700khz d1: luminus cbt-40 pgnd 4.7f on off 1f d1 420k 10pf 100k
lt8611 24 8611p linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2012 lt 0512 ? printed in usa related parts typical application coincident tracking step-downs each with 2a output current limit part number description comments lt8610 42v, 2.5a, 96% efficiency, 2.2mhz synchronous micropower step-down dc/dc converter with i q = 2.5a v in : 3.4v to 42v, v out(min) = 0.97v, i q = 2.5a, i sd < 1a, msop-16e package lt3690 36v with 60v transient protection, 4a, 92% efficiency, 1.5mhz synchronous micropower step-down dc/dc converter with i q = 70a v in : 3.9v to 36v, v out(min) = 0.985v, i q = 70a, i sd < 1a, 4mm 6mm qfn-26 package lt3971 38v, 1.2a, 2.2mhz high efficiency micropower step-down dc/dc converter with i q = 2.8a v in : 4.2v to 38v, v out(min) = 1.21v, i q = 2.8a, i sd < 1a, 3mm 3mm dfn-10 and msop-10e packages lt3991 55v, 1.2a, 2.2mhz high efficiency micropower step-down dc/dc converter with i q = 2.8a v in : 4.2v to 55v, v out(min) = 1.21v, i q = 2.8a, i sd < 1a, 3mm 3mm dfn-10 and msop-10e packages lt3970 40v, 350ma, 2.2mhz high efficiency micropower step-down dc/dc converter with i q = 2.5a v in : 4.2v to 40v, v out(min) = 1.21v, i q = 2.5a, i sd < 1a, 3mm 2mm dfn-10 and msop-10 packages lt3990 62v, 350ma, 2.2mhz high efficiency micropower step-down dc/dc converter with i q = 2.5a v in : 4.2v to 62v, v out(min) = 1.21v, i q = 2.5a, i sd < 1a, 3mm 3mm dfn-10 and msop-6e packages LT3480 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high efficiency step-down dc/dc converter with burst mode operation v in : 3.6v to 36v, transient to 60v, v out(min) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn-10 and msop-10e packages lt3980 58v with transient protection to 80v, 2a (i out ), 2.4mhz, high efficiency step-down dc/dc converter with burst mode operation v in : 3.6v to 58v, transient to 80v, v out(min) = 0.78v, i q = 85a, i sd < 1a, 3mm 4mm dfn-16 and msop-16e packages bst v in en/uv sync imon ictrl intv cc tr/ss rt sw lt8611 gnd isp isn bias pg fb 0.1f 47f v in 3.8v to 42v 1f 0.1f 5.6h 0.025 86.6k f = 500khz pgnd 10f on off 1f 232k 10pf 97.6k 16.5k 20k bst v in en/uv sync imon ictrl intv cc tr/ss rt sw lt8611 gnd isp isn bias pg fb 8611 ta12 0.1f v out 1.8v 2a v out 3.3v 2a 68f 1f 5.6h 0.025 88.7k f = 500khz pgnd 10f on off 1f 80.6k 4.7pf 93.1k

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