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lt3760 1 3760fc typical a pplica t ion descrip t ion 8-channel 100ma led driver the lt ? 3760 is an 8- channel led driver with a step-up dc/dc controller capable of driving up to 45 v of leds. each channel contains an accurate current sink with 2% current matching. channels follow a master programmable current to allow between 20 ma to 100 ma of led current per string. channels can be paralleled for higher led current. output voltage adapts to variations in led v f for optimum efficiency and open led faults do not affect the operation of connected led strings. the lt3760 allows a pwm dimming range up to 3000:1 and an analog dimming range up to 25:1. operating frequency can be programmed from 100 khz up to 1mhz using a single resistor or synchronized to an external clock . additional features include: programmable maximum v out for open led protection, a fault flag for open led, programmable led current derating vs temperature, mi- cropower shutdown and internal soft-start. the lt3760 is available in a thermally enhanced 28- pin tssop package. l, lt , lt c and lt m , linear technology and the linear logo are registered trademarks of linear technology corporation. true color pwm is a trademark of linear technology corporation. all other trademarks are the property of their respective owners. protected by u.s. patents , including 7199560, 7321203. fea t ures a pplica t ions n automotive, notebook and tv monitor backlighting n up to 45 v of leds 100 ma , 8-channel led driver n wide input range : 6v to 40v (4.5v to 13v, v in connected to intv cc ) n 2% led current matching at 40ma ( typ 0.7%) n up to 3000:1 true color pwm ? dimming range n single resistor sets led current (20ma to 100ma) n led current regulated even for pv in > v out n output adapts to led v f for optimum efficiency n fault flag + protection for open led strings n protection for led pin to v out short n parallel channels for higher led current n programmable led current derating vs t emperature n accurate undervoltage lockout threshold with programmable hysteresis n programmable frequency (100khz to 1mhz) n synchronizable to an external clock worst-case channels led current matching (normalized to 8-channel average) 92% efficient, 36w backlight led driver lt3760 gate 5 2.2f 10h 0.015 v out sense led1 led2 ctrl pwm ref t set ovp set rt gnd i set v c sync v in v in 8v to 14v pv in 20v to 36v 4.7f 4.7f intv cc 40.2k 499k pgnd ? ? ? ? ? ? ? ? ? ? 11k 30.9k 20k 39.2k 1mhz 5.76k 10k 100k 8 channels 100ma 2.2nf 3760 ta01 shdn/uvlo fault led7 led8 ? ? ? ? ? ? ? ? v in 4.7f ? ? ? ? up to 45v of leds per string 20k junction temerature (c) ?50 ?25 led current matching (%) 0.8 0.4 0.0 ?0.4 ?0.8 100 0 25 3760 ta01 125 50 75 r iset = 14.7k (i(led) = 40ma)
lt3760 2 3760fc p in c on f igura t ion a bsolu t e maxi m u m r a t ings 1 2 3 4 5 6 7 8 9 10 11 12 13 14 top view fe package 28-lead plastic tssop 28 27 26 25 24 23 22 21 20 19 18 17 16 15 ctrl t set v ref i set nc led1 led2 led3 led4 pgnd sense gate intv cc v in ovp set pwm v c rt gnd led8 led7 led6 led5 pgnd v out sync fault shdn/uvlo 29 pgnd t jmax = 125c, ja = 28c/w, jc = 10c/w exposed pad (pin 29) is pgnd, must be soldered to pcb o r d er i n f or m a t ion lead free finish tape and reel part marking* package description temperature range lt3760efe#pbf lt3760efe#trpbf lt3760fe 28-lead plastic tssop C40c to 125c lt3760ife#pbf lt3760ife#trpbf lt3760fe 28-lead plastic tssop C40c to 125c consult lt c marketing for parts specified with wider operating temperature ranges. *the temperature grade is identified by a label on the shipping container. consult lt c 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/ v out , led 1-8 ............................................................ 60 v v in , shdn / uvlo , fa u lt ........................................... 40 v in tv cc ...................................................................... 13 v int v cc above v in .................................................. + 0.3 v p wm , ctrl , sync ..................................................... 6 v v c ............................................................................... 3v v ref , rt, i set , t set , ovp set ....................................... 2v sense ...................................................................... 0. 4 v operating junction temperature range ( notes 2,3) ............................................. - 4 0 c to 125 c storage temperature range .................. - 6 5 c to 150 c lead temperature ( soldering , 10 sec ) ................... 30 0 c (note 1)
lt3760 3 3760fc e lec t rical c harac t eris t ics parameter conditions min typ max units input bias, reference minimum operational v in (to allow gate switching) v c = 1.5v v in = intv cc (shorted) v in intv cc l l 4.2 5.5 4.5 6.0 v v operational v in v in = intv cc (shorted) v in intv cc 4.5 6 13 40 v v v in quiescent current ctrl = 0.1v, pwm = 0v ctrl = 0.1v, pwm = 1.5v, (not switching) led 1C8 = 1.2v 4.2 9.5 5.7 12 ma ma v in shutdown current (v in intv cc ) (not shorted) shdn /uvlo = 0v, v in =6v shdn /uvlo = 0v, v in = 40v 0.1 2 10 a a v in shutdown current (v in = intv cc (shorted)) shdn /uvlo = 0v, v in = intv cc = 4.5v shdn /uvlo = 0v, v in = intv cc = 13v 10 20 20 40 a a shdn/uvlo threshold (micropower) (falling) (v sd ) i vin < 20a l 0.3 0.7 v shdn/uvlo threshold (uvlo) (falling) (stop switching) (v uv ) l 1.414 1.476 1.538 v shdn/uvlo pin current shdn/uvlo = v uv - 50mv shdn/uvlo = v uv + 50mv l 1.6 2.4 0 3.2 a a v ref voltage i vref = 0a l 1.450 1.485 1.524 v v ref line regulation i vref = 0a, 6v < v in < 40v 0.01 0.05 %/v v ref load regulation 0 < i vref < 150a (max) 2 mv oscillator frequency: f osc (100khz) rt = 523k l 92 101 112 khz frequency: f osc (1mhz) rt = 39.2k l 0.90 1 1.10 mhz f osc (1mhz) line regulation rt = 39.2k, 6v < v in < 40v 0.1 0.2 %/v rt pin voltage rt = 39.2k 1.6 v minimum off-time minimum on-time (note 5) (note 5) 170 190 250 250 ns ns sync input high threshold 2.2 v sync input low threshold 0.6 v sync input current sync = 0v sync = 5v 0 25 a a sync frequency range rt = 523k rt = 39.2k 0.12 1.2 1.5 1.5 mhz mhz linear regulator (intv cc ) intv cc regulation voltage v in = 12v 6.65 7 7.35 v dropout (v in - intv cc ) i intvcc = 10ma 250 mv intv cc uvlo (+) (start switching) 3.8 v intv cc uvlo (-) (stop switching) 3.4 v intv cc current limit l 40 57 ma ovp/led error amplifiers transconductance (ovp) ?i vc = 2.5a 4 mhos voltage gain (ovp) 5 v/v transconductance (led) ?i vc = 2.5a 33 mhos the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. v in = v out = 6v, r iset = 14.7k unless otherwise noted.
lt3760 4 3760fc e lec t rical c harac t eris t ics parameter conditions min typ max units voltage gain (led) 45 v/v v c source current (out of pin) v c = 1.5v, v ledx = 0.8v, ovp set = 1.5v 10 a v c sink current (ovp) v c = 1.5v, v ledx = 0.8v, ovp set = 0v 15 a v c sink current (led) v c = 1.5v, v ledx = 1.2v, ovp set = 1.5v 9 a v c output high (clamp) (v coh ) 2.3 v v c output low (clamp) (v col ) 0.8 v v c switching threshold (v csw ) 1.1 v sense amp sense input current (out of pin) 65 a sense current limit threshold l 44 52 60 mv current mode gain ?v(v c )/?v(sense) 6 v/v sense over current limit threshold l 90 100 110 mv led current / control i set pin voltage ctrl = 1.5v 1.00 v ledx current (40ma) (r iset = 14.7k) v ledx = 1v, ctrl = 1.5v 38.3 40.1 41.9 ma ledx current matching (40ma) (r iset = 14.7k) v ledx = 1v, ctrl = 1.5v l 0.7 2 % ledx current (100ma) (r iset = 5.76k) v ledx = 1v, ctrl = 1.5v 95.5 100.7 105.9 ma led pin regulation voltage 1.1 v t set threshold 630 mv analog dimming ctrl input current (out of pin) ctrl = 1v ctrl = 0.04v 40 50 200 200 na na ledx current (dimming 25:1) v ledx = 1v, ctrl = 0.04v 1.6 ma pwm dimming pwm input low threshold 0.7 1 v pwm input high threshold 1.1 1.4 v pwm input current pwm = 1.5v pwm = 6v 6 24 a a v out pin current in pwm mode v(v out ) = 60v pwm = 1.5v, v ledx = 1v pwm = 0v, v ledx = 1v 370 20 a a ledx leakage current ( pwm = 0v) v ledx = 1v, v out = 12v v ledx = 50v, v out = 60v 0.1 0.1 1 2 a a fault diagnostics fault output sink current led1 = open, v fault = 0.3v 0.3 0.6 ma led x short threshold (v sh ) (v out C v ledx ) v out = 12v v out = 60v 6 6 v led open detection threshold v out = 12v 0.5 v the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. v in = v out = 6v, r iset = 14.7k unless otherwise noted.
lt3760 5 3760fc 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 lt3760e is guaranteed to meet performance specifications from 0c to 125c junction temperature. specifications over the -40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the e lec t rical c harac t eris t ics parameter conditions min typ max units gate driver gate driver output rise time v in = 12v, c l = 3300pf (note 4) 30 ns gate driver output fall time v in = 12v, c l = 3300pf (note 4) 30 ns gate output low i gate = 0a 0.1 v gate output high intv cc = v in = 7v i gate = 0a 6.95 v output voltage v out over voltage protection (ovp) regulation voltage ovp set = 0.22v ovp set = 1v 12.5 57 v v ovp set input current (out of pin) ovp set = 0.22v, v out =12v 40 200 na the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. v in = v out = 6v, r iset =14.7k unless otherwise noted. typical p er f or m ance c harac t eris t ics led current vs junction temperature led current vs ctrl pin voltage lt3760i is guaranteed to meet performance specifications from - 40c to 125c junction temperature. note 3: for maximum operating ambient temperature, see thermal calculations in the applications information section. note 4: gate rise and fall times are measured between 10% and 90% of intv cc voltage. note 5: see duty cycle considerations in the applications information. t a = 25c, unless otherwise noted. junction temerature (c) ?50 ?25 led current matching (%) 0.8 0.4 0.0 ?0.4 ?0.8 100 0 25 3760 g01 125 50 75 r iset = 14.7k (i(led) = 40ma) worst-case channels led current matching ( normalized to 8-channel average ) junction temerature (c) ?50 led current (ma) 42 40 41 39 38 75 25 3760 g02 125 50 0 100 ?25 r iset = 14.7k ctrl (v) 0.00 led current (ma) 110 20 10 100 80 60 40 90 70 50 30 0 0.75 1.251.00 3760 g03 1.50 0.500.25 r iset = 5.76k 7.32k 9.76k 14.7k 29.4k
lt3760 6 3760fc typical p er f or m ance c harac t eris t ics shdn/uvlo pin (hysteresis) current vs junction temperature v in shutdown current vs junction temperature v in quiescent current vs v in v in quiescent current vs junction temperature switching frequency vs junction temperature v c high clamp, active and low clamp levels vs junction temperature led current waveforms 3000:1 pwm dimming (100hz) v ref vs junction temperature shdn/uvlo threshold vs junction temperature t a = 25c, unless otherwise noted. 5s/div i(ledx) 40ma/div pwm 10v/div i(l1) 1a/div 3760 g04 (front cover application) junction temperature (c) ?50 v ref voltage (v) 1.525 1.485 1.465 1.505 1.445 25 10075 0 50 3760 g05 125 ?25 junction temerature (c) ?50 shdn /uvlo pin voltage (v) 1.525 1.485 1.465 1.505 1.445 25 10075 0 50 3760 g06 125 ?25 junction temperature (c) ?50 shdn/uvlo pin current (a) 2.80 2.60 2.50 2.40 2.30 2.70 2.20 25 10075 0 50 3760 g07 125 ?25 junction temperature (c) ?50 v in current (a) 5 3 2 1 4 0 25 10075 0 50 3760 g08 125 ?25 v in = 6v, shdn/uvlo = 0v v in (v) 0 v in current (ma) 12 6 4 2 8 10 0 15 30 35 25 10 20 3760 g09 40 5 pwm = 1.5v, no switching, v(led 1-8 ) = 1.2v, ctrl = 0.1v pwm = 0v, ctrl = 0.1v r iset = 14.7k junction temperature (c) ?50 v in current (ma) 15 5 10 0 25 10075 0 50 3760 g10 125 ?25 v in = 6v, r iset = 14.7k, ctrl = 0.1v pwm = 1.5v, no switching, v(led 1-8 ) = 1.2v, ctrl = 0.1v pwm = 0v, ctrl = 0.1v junction temperature (c) ?50 switching frequency (khz) 1100 950 1050 1000 900 25 10075 0 50 3760 g11 125 ?25 r t = 39.2k junction temperature (c) ?50 v c pin voltage (v) 2.5 2.0 1.5 1.0 0.5 0.0 25 10075 0 50 3760 g12 125 ?25 v c high clamp v c low clamp v c active (switching)
lt3760 7 3760fc intv cc current limit vs junction temperature sense threshold vs junction temperature overvoltage protection (ovp) level vs junction temperature v out - v (ledx) short threshold vs junction temperature minimum on and off times vs junction temperature gate rise/fall times vs gate capacitance intv cc vs current, junction temperature intv cc vs current, junction temperature intv cc , uvlo(+), uvlo(C) vs junction temperature typical p er f or m ance c harac t eris t ics t a = 25c, unless otherwise noted. junction temperature (c) ?50 intv cc (v) 7.0 6.9 6.8 6.7 6.6 25 10075 0 50 3760 g13 125 ?25 i load = 10ma, 20ma, 30ma i load = 40ma v in = 8v, pwm = 0v junction temperature (c) ?50 intv cc (v) 6.0 5.00 5.5 4.5 25 10075 0 50 3760 g14 125 ?25 v in = 6v, pwm = 0v i load = 10ma i load = 20ma i load = 30ma i load = 40ma junction temperature (c) ?50 intv cc (v) 8 4 5 6 7 2 3 25 10075 0 50 3760 g15 125 ?25 intv cc uvlo(?) intv cc uvlo(+) v in = 12v intv cc (regulated) junction temperature (c) ?50 intv cc current (ma) 60 45 50 55 40 25 10075 0 50 3760 g16 125 ?25 v in = 6v, intv cc = 0v junction temperature (c) ?50 sense pin voltage (mv) 60.0 57.5 55.0 52.5 50.0 47.5 45.0 42.5 40.0 25 10075 0 50 3760 g17 125 ?25 inductor peak current threshold (cycle-by-cycle) junction temperature (c) ?50 ovp (v) 70 60 50 40 30 20 10 0 25 10075 0 50 3760 g18 125 ?25 ovp set = 1.0v ovp set = 0.22v junction temperature (c) ?50 short threshold (v) 7.00 6.75 6.50 6.25 6.00 5.75 5.50 5.25 5.00 25 10075 0 50 3760 g18 125 ?25 v out = 60v v out = 12v junction temperature (c) ?50 time (ns) 250 225 200 175 150 125 100 25 10075 0 50 3760 g20 125 ?25 minimum on-time minimum off-time c gate (nf) 0 time (ns) 120 100 80 60 40 20 0 15 5 10 3760 g21 20 fall time rise time v in = 8v intv cc = 7v
lt3760 8 3760fc p in func t ions ctrl ( pin 1): ctrl pin voltage below 1 v controls led current. ctrl voltage can be set by a resistor divider from v in , v ref or an external voltage source . led current derating versus temperature is achievable if the voltage programmed at the ctrl pin has a negative temperature coefficient using an external resistor divider from v ref pin to gnd with temperature dependent resistance. t set (pin 2): programs lt3760 junction temperature breakpoint past which led current will begin to derate. program using a resistor divider from v ref to gnd. v ref ( pin 3): 1.485 v reference output pin. this pin can supply up to 150 a. can be used to program ctrl, t set and ovp set pin voltages using resistor dividers to gnd. i set (pin 4): resistor to gnd programs led pin current. see table 6 in the applications information section. nc (pin 5): no connect. okay to leave open or to connect to gnd. led x ( pins 6 to 9, 20 to 23): 8 led driver outputs. each output contains an open collector constant current sink. led currents are programmable from 20ma to 100ma using a single resistor at the i set pin. connect the cathode of each led string to an led pin. connect the anode of each led string to v out . channels can be paralleled for greater led current or individually disabled ( connect led to v out ). pgnd (pins 10, 19, exposed pad pin 29): power grounds for the ic and the converter. the package has an exposed pad (pin 29) underneath the ic which is the best path for heat out of the package. pin 29 should be soldered to a continuous copper ground plane under the device to reduce die temperature and increase the power capability of the lt3760. sense (pin 11): the current sense input for the control loop. connect this pin to the sense resistor in the source of the external power mosfet. gate (pin 12): drives the gate of an n-channel mosfet from 0v to intv cc . intv cc (pin 13): a 7 v ldo supply generated from v in and used to power the gate driver and some control circuitry. must be bypassed with a 4.7f capacitor to pgnd. v in ( pin 14): input supply pin. must be locally bypassed with a 1f capacitor to pgnd. shdn/ uvlo ( pin 15): the shdn/ uvlo pin has an accurate 1.476 v threshold and can be used to program an under volt- age lockout ( uvlo) threshold for system input supply using a resistor divider from supply to gnd. a 2.4 a pin cur- rent hysteresis allows programming of uvlo hysteresis. shdn /uvlo above 1.476 v turns the part on and removes a 2.4 a sink current from the pin. shdn/uvlo < 0.7v reduces v in current < 20 a. if the shutdown function is not required, it should be forced above 1.476 v or con- nected directly to v in . fault (pin 16): active low if any or all led strings have an open fault. if fault(s) removed, fault flag returns high. fault status is only updated during pwm high state and latched during pwm low. sync (pin 17): allows synchronization of boost converter switching frequency to an external clock. rt resistor should be programmed for f osc 20% below sync frequency. if unused, connect to gnd. v out (pin 18): boosted output voltage of the converter. connect a capacitor from this pin to pgnd. connect the anode of each led (string) to v out . gnd (pin 24): signal ground. rt (pin 25): a resistor to gnd programs switching fre- quency f osc between 0.1mhz and 1mhz. v c (pin 26): output of both transconductance error amplifiers for the converter regulation loop. the most commonly used gm error amplifier ( led) regulates v out to ensure no led pin falls below 1v. the other gm error amplifier ( ovp) is activated if all leds fail open and a regulated maximum v out is required. connect a resistor and capacitor in series from the v c pin to gnd. pwm ( pin 27): input pin for pwm dimming control. above 1.4v allows converter switching and below 0.7 v disables switching. the pwm signal can be driven from 0 v to 6 v. if unused, connect to v ref. ovp set (pin 28): programs maximum allowed v out regu- lation level if all leds are open circuit. program using a resistor divider from v ref to gnd.
lt3760 9 3760fc b lock diagra m o pera t ion the operation of the lt3760 is best understood by referring to the typical application circuit on the front page and the block diagram in figure 1. the lt3760 drives 8 strings of leds by using a constant switching frequency, current mode boost controller to generate a single output voltage v out for the top ( anode) of all led strings. led string current is generated and controlled by connection of the bottom led in each string ( cathode) to a current source contained in each corresponding led pin. each led pin contains an accurate current sink to ground, program- mable between 20 ma to 100 ma using a single resistor at the i set pin. led channels can be paralleled to achieve higher led currents. for applications requiring less than 8 strings of leds, channels can be paralleled or disabled (connect led pin to v out before startup). for optimum efficiency, v out regulates to the lowest possible voltage allowable to maintain regulated current in each led string. any open led fault is indicated by the fault pin driven low without effecting the operation of the connected led strings. the block diagram in figure 1 illustrates the key functions of the lt3760. it can be seen that two external supplies, v ref and intv cc , are generated by the lt3760. the v ref pin provides a precision 1.485 v output for use with external resistors to program the ctrl, ovp set and t set input pins. the intv cc pin provides a regulated 7 v output to supply the gate driver for the boost controller gate pin. an accurate 1.476 v threshold on the shdn /uvlo pin combined with a shdn/ uvlo pin current hysteresis allows a programmable resistor divider from v in to shdn/uvlo figure 1. lt3760 block diagram 3760 bd + + + ? channel x overvoltage amp led amp 6v 1.1v + ? + ? + ? + ? led logic r s q + ? intv cc_uv v in_uv v in shdn_uv shdn/uvlo pwm v ref ctrl 1v ss r 56r 600k 15 14 13 1.476v 1.485v slope 100mv over current 52mv peak current hiccup__mode v c en pwm en en 7v(regulated) uvlo(+) = 3.8v, uvlo(?) = 3.4v 4.2v(+) 3.7v(?) + ? ref 1.485v 27 1 2 4 26 24 v ptat + ? en gate sync rt sense v out ledx 6 to 9, 20 to 23 fault soft start + ? 3 16 18 11 osc 12 25 17 28 led current control intv cc ovp set v c i set t set gnd pgnd (10, 19, exposed pad (29)) + ? + ?
lt3760 10 3760fc o pera t ion to define the turn on/off voltages for v in . shdn /uvlo pin current switches from 2.4 a to 0 a when shdn/uvlo pin voltage exceeds 1.476v. the lt3760 constant switching frequency is programmable from 100 khz up to 1 mhz using a single resistor at the rt pin to ground. a sync pin is also provided to allow an external clock to define the converter switching frequency. the gate output provides a 0.8a peak gate drive for an external n-channel power mosfet to generate a boosted output voltage v out using a single inductor, schottky diode and output capacitor. with led strings connected from v out to every led pin, the lowest voltage on each led pin is monitored and compared to an internal 1v reference. v out is regulated to ensure the lowest led pin voltage of any connected led string is maintained at 1v. if any of the led strings are open, the lt3760 will ignore the open led pin. if all of the led strings are open v out charges up until a user programmable ovp ( overvoltage protection) level is reached. this programmable ovp level allows the user to protect against led damage when the led strings are opened and then reconnected. since the lt3760 boost controller uses a current mode topology, the v c pin voltage determines the peak current in the inductor of the converter and hence the duty cycle of the gate switching waveform. the basic loop uses a pulse from an internal oscillator to set an rs flip-flop and turn on the external power mosfet. current increases in the mosfet and inductor until the v c commanded peak switch current is exceeded and the mosfet is then turned off. inductor current is sensed during the gate on period by a sense resistor rs in the source of the external n-channel power mosfet. as with all current mode converters, slope compensation is added to the control path to ensure stability for duty cycles above 50%. any over current fault condition in the mosfet turns off the mosfet and triggers soft start internally. in this fault mode the lt3760 only allows mosfet turn-on approximately every 2 ms. this hiccup mode significantly reduces the power rating required for the mosfet. led current programming and dimming can be achieved using the i set , ctrl and pwm pins. a single resistor at the i set pin programs led current. analog dimming of led brightness is achieved using the ctrl pin below 1v. pwm dimming of led brightness is achieved by control- ling the duty cycle of the pwm pin. for robust operation the lt3760 monitors system conditions and performs soft start for startup after any of the following faults: v in , shdn or intv cc voltages too low or mosfet current too high. the lt3760, when entering these faults, discharges an internal soft start node and prevents switching at the gate pin. when exiting these faults the lt3760 ramps up an internal soft start node to control v c pin voltage rise and hence control mosfet peak switch current rise. in addition the soft start period gradually ramps up switching frequency from approximately 33% to 100% of full scale. the lt3760 monitors each led pin voltage. if the led string has an open fault (v(led x )<0.5v) the fault flag is pulled low. for led protection, the lt3760 ctrl pin allows an led current derating curve to be programmed versus the ambient temperature of the led strings. an ntc resistor placed close to the leds decreases ctrl pin voltage and hence decreases led current as led ambient temperature increases. the lt3760 also allows its own junction temperature to be monitored and regulated by derating led currents when a junction temperature programmed by the t set pin is exceeded.
lt3760 11 3760fc intv cc regulator bypassing and operation the intv cc pin is the output of an internal linear regula- tor driven from v in and is the supply for the lt3760 gate driver. the intv cc pin should be bypassed with a 10v rated 4.7 f low esr, x7r or x5r ceramic capacitor to ensure stability and to provide enough charge for the gate driver. for high enough v in levels the intv cc pin provides a regulated 7 v supply. make sure intv cc voltage does not exceed the v gs rating of the external mosfet driven by the gate pin. for low v in levels the intv cc level will depend on v in and the voltage drop of the regulator. the intv cc regulator has an undervoltage lockout which prevents gate driver switching until intv cc reaches 3.8v and maintains switching until intv cc falls below 3.4 v. this feature prevents excessive power dissipation in the external mosfet by ensuring a minimum gate drive level to keep r ds(on) low. the intv cc regulator has a current limit of 40 ma to limit power dissipation inside the i.c. this current limit should be considered when choosing the n - channel power mosfet and the switching frequency. the average current load on the intv cc pin due to the lt3760 gate driver can be calculated as: i intvcc = q g ? f osc where q g is the gate charge ( at v gs = intv cc ) specified for the mosfet and fosc is the switching frequency of the lt3760 boost converter. it is possible to drive the intv cc pin from a variety of external sources in order to remove power dissipation from the lt3760 and/or to remove the intv cc current limitation of 40 ma. an external supply for intv cc should never exceed the v in pin voltage or the maximum intv cc pin rating of 13 v. if intv cc is shorted to the v in pin, v in operational range is 4.5v to 13v. a pplica t ions i n f or m a t ion inductor a list of inductor manufacturers is given in table 1. how- ever, there are many other manufacturers and inductors that can be used. consult each manufacturer for more detailed information and their entire range of parts. ferrite cores should be used to obtain the best efficiency. choose an inductor that can handle the necessary peak current without saturating. also ensure that the inductor has a low dcr ( copper-wire resistance) to minimize i 2 r power losses. v alues between 2.2 h and 33 h will suffice for most applications. the typical inductor value required for a given application (assuming 50% inductor ripple current for example) can be calculated as: l = 1 - 1 v out v in ? 1 f osc ? v in 0.5 ? v out v in ? i ledx ? 8 where: v out = (n ? v f ) + 1v ( n = number of leds per string), v f = led forward voltage drop, i ledx = led current per string example : for a 12 w led driver application requiring 8 strings of 10 leds each driven with 40 ma , and choos- ing v in = 12 v, v out = (3.75 v ? 10) + 1v = 38.5v, i ledx = 40 ma and f osc = 1 mhz the value for l is calculated as l = (1 - 1 3.2 ) ? 1 10 6 ? 12v 0.5 ? 3.2 ? 40ma ? 8 = 16.5h
lt3760 12 3760fc table 1. inductor manufacturers m anufacturer phone number web sumida 408-321-9660 www.sumida.com wrth elektronik 605-886-4385 www.we-online.com vishay 402-563-6866 www.vishay.com coilcraft 847-639-6400 www.coilcraft.com coiltronics 561-998-4100 www.cooperet.com input capacitor the input capacitor of the lt3760 boost converter will sup- ply the transient input current of the power inductor. values between 2.2 f and 10 f will work well for the lt3760. use only x5r or x7r ceramic capacitors to minimize variation over voltage and temperature. if inductor input voltage is required to operate near the minimum allowed operational v in for the i.c., a larger capacitor value may be required. this is to prevent excessive input voltage ripple causing dips below the minimum operating input voltage. output capacitor low esr ceramic capacitors should be used at the lt3760 converter output to minimize output ripple voltage. use only x5r or x7r dielectrics as these materials retain their capacitance over wider voltage and temperature ranges than other dielectrics. the output capacitance requirements for several led driver application circuits are shown in the applications information section for various i led , v in , v out , l and f osc values. some suggested capacitor manufacturers are listed in table 2. table 2. ceramic capacitor manufacturers m anufacturer phone number web tdk 516-535-2600 www.tdk.com kemet 408-986-0424 www.kemet.com murata 814-237-1431 www.murata.com taiyo yuden 408-573-4150 t-yuden.com avx 843-448-9411 www.avxcorp.com a pplica t ions i n f or m a t ion schottky rectifier the external diode for the lt3760 boost converter must be a schottky diode, with low forward voltage drop and fast switching speed. table 3 lists several schottky manufacturers. the diodes average current rating must exceed the application s average output current. the diode s maximum reverse voltage must exceed the maximum output voltage of the application. for pwm dimming applications be aware of the reverse leakage of the schottky diode. lower leakage current will drain the output capacitor less during pwm low periods, allowing for higher pwm dimming ratios. the companies below offer schottky diodes with high voltage and current ratings. table 3. schottky rectifier manufacturers m anufacturer phone number web diodes, inc. 805-446-4800 www.microsemi.com on semiconductor 888-743-7826 www.onsemi.com zetex 631-360-2222 www.zetex.com vishay siliconix 402-563-6866 www.vishay.com power mosfet selection several mosfet vendors are listed in table 4. consult the factory applications department for other recommended mosfets. the power mosfet selected should have a v ds rating which exceeds the maximum overvoltage protection ( ovp) level programmed for the application. ( see programming ovp level in the applications information section). the mosfet should also have a low enough total gate charge q g (at 7 v v gs ) and a low enough switching frequency (f osc ) to not exceed the intv cc regulator current limit, where loading on intv cc pin due to gate switching should obey, i gate = q g ? f osc 40ma
lt3760 13 3760fc in addition, the current drive required for gate switching should also be kept low in the case of high v in voltages ( see thermal considerations in the applications informa- tion section). the r ds(on) of the mosfet will determine d.c. power losses but will usually be less significant compared to switching losses. be aware of the power dissipation within the mosfet by calculating d.c. and switching losses and deciding if the thermal resistance of the mosfet package causes the junction temperature to exceed maximum ratings. table 4. mosfet manufacturers m anufacturer phone number web vishay siliconix 402-563-6866 www.vishay.com international rectifier 310-252-7105 www.irf.com fairchild 972-910-8000 www.fairchildsemi.com power mosfet: current sense resistor the lt3760 current mode boost converter controls peak current in the inductor by controlling peak mosfet current in each switching cycle. the lt3760 monitors current in the external n-channel power mosfet by sensing the voltage across a sense resistor ( rs) connected between the source of the fet and the power ground in the application. the length of these tracks should be minimized and a kelvin sense should be taken from the top of rs to the sense pin. a 52 mv sense pin threshold combined with the value of rs sets the maximum cycle-by-cycle peak mosfet current. the low 52 mv threshold improves efficiency and determines the value for rs given by: rs 52mv ? 0.7 i l(peak) a pplica t ions i n f or m a t ion where i l(peak) = 1 1 ? d ? 8 ? i ledx ? 1 + 0.5 2 d = mosfet duty cycle = 1 ? v in(min) v out(max) , v out(max) = n ? v f(max) ( ) + 1v n = number of leds in each string, v f(max) = maximum led forward voltage drop, v in(min) = minimum input voltage to the inductor, i led = current in each led pin, and the 0.5 term represents an inductor peak- to- peak ripple current of 50% of average inductor current. the scale factor of ? 0.7 ensures the boost converter can meet the peak inductor requirements of the loop by accounting for the combined errors of the 52 mv sense threshold, i ledx , rs and circuit efficiency. example: for a 12 w led driver application requiring 8 strings of 10 leds each driven with 40 ma , and choosing v in(min) = 8v, v out(max) = (4v ? 10)+1v = 41 v and i ledx = 40ma, the value for rs is chosen as: rs 52mv ? 0.7 i l(peak) 52mv ? 0.7 41 8 ? 8 ? 0.04 ? ? ? ? ? ? ? 1+ 0.25 ( ) 52mv ? 0.7 2.05 17.7 m ?
lt3760 14 3760fc the power rating of rs should be selected to exceed the i 2 r losses in the resistor. the peak inductor current should be recalculated for the chosen rs value to ensure the chosen inductor will not saturate. power mosfet: overcurrent and hiccup mode for severe external faults which may cause the external mosfet to reach currents greater than the peak current defined by rs and the 52mv sense pin threshold described above, the lt3760 has an overcurrent comparator which triggers soft start and turns off the mosfet driver for currents exceeding, i (overcurrent ) = 100mv rs in this fault mode the lt3760 only allows mosfet turn on for approximately 100 ns every 2 ms. this hiccup mode significantly reduces the power rating required for the mosfet. a pplica t ions i n f or m a t ion soft start to limit inductor inrush current and output voltage during startup or recovery from a fault condition, the lt3760 pro- vides a soft start function. the lt3760 when entering these faults will discharge an internal soft start node and prevent switching at the gate pin for any of the following faults: v in , shdn/ uvlo or intv cc voltages too low or mosfet current too high ( see the timing diagram in figure 2). when exiting these faults the lt3760 ramps up an internal soft start node at approximately 0.5 v/ms to control v c pin voltage rise and hence control mosfet switch current rise. in ad- dition the soft start period gradually ramps up switching frequency from approximately 33% to 100% of full scale. the conditions required to exit all faults and allow a soft start ramp of the v c pin are listed in figure 2. an added feature of the lt3760 is that it waits for the first pwm pin active high (minimum 200 ns pulse width) before it allows figure 2. lt3760 fault detection and soft start timing for v c pin and internal ss node gate 3760 f02 v c 0.4v + v be (v c switching threshold) 0.1v + v be 0.5v/ms 0.5v/ms ss (internal) 0.4v 0.1v any of the following faults triggers soft start latch with gate turned off immediately: v in < 3.7v, shdn < 1.476v, intvcc < 3.4v i dss (external mosfet) > 100mv/rs soft-start latch reset requires all conditions satisfied: ss (internal) < 0.2v, v in 4.2v, shdn > 1.476v, intv cc > 3.8v, i dss (external mosfet) < 100mv/rs, pwm > 1.4v (for at least 200ns) soft-start latch set: v c min clamp
lt3760 15 3760fc a pplica t ions i n f or m a t ion shdn/uvlo 1.476v 600k v supply r1 15 r2 3760 f03 onoff + ? figure 3. programming undervoltage lockout (uvlo) with hysteresis figure 4. switching frequency vs rt uvlo pin. after part turn on , 0 a flows from the shdn/ uvlo pin. calculation of the turn on/off thresholds for a system input supply using the lt3760 shdn /uvlo pin can be made as follows : v supply off = 1.476 1 + r1 r2 ? ? ? ? ? ? v supply on = v supply off + 2.4a ? r1 ( ) an open drain transistor can be added to the resistor divider network at the shdn /uvlo pin to independently control the turn off of the lt3760. programming switching frequency the switching frequency of the lt3760 boost converter can be programmed between 100 khz and 1 mhz using a single resistor (r t ) connected from the rt pin to ground (figure 4). connect the r t resistor as close as possible to the rt pin to minimize noise pick up and stray capacitance (see circuit layout considerations in the applications information section). table 5 shows the typical r t values required for a range of frequencies. the soft start of v c pin to begin. this feature ensures that during startup of the lt3760 the soft start ramp has not timed out before pwm is asserted high. without this wait for pwm high feature, systems which apply pwm after v in and shdn /uvlo are valid, can potentially turn on without soft start and experience high inductor currents during wake up of the converters output voltage. it is important to note that when pwm subsequently goes low, the soft start ramp is not held at its present voltage but continues to ramp upwards. if the soft start ramp voltage was held every time pwm goes low, this would cause very slow startup of led displays for applications using very high pwm dimming ratios. shutdown and programming undervoltage lockout the lt3760 has an accurate 1.476 v shutdown threshold at the shdn /uvlo pin. this threshold can be used in conjunction with a resistor divider from the system input supply to define an accurate undervoltage lockout (uvlo) threshold for the system (figure 3). an internal hysteresis current at the shdn /uvlo pin allows programming of hysteresis voltage for this uvlo threshold. just before part turn on, an internal 2.4 a flows from the shdn/ rt (k) 0 switching frequency (khz) 1000 600 700 800 900 500 400 300 200 100 300 500 200 400 3760 f04 600 100
lt3760 16 3760fc a pplica t ions i n f or m a t ion selecting the optimum frequency depends on several factors. higher frequency allows reduction of inductor size but efficiency drops due to higher switching losses. lower frequency allows higher operational duty cycles to drive a larger number of leds per string from a low input supply but require larger magnetics. in each application the switching frequency can be tailored to provide the optimum solution. table 5. switching frequency vs. rt (1% resistors) switching frequency (khz) rt (k?) 100 523 200 249 300 158 400 115 500 90.9 600 73.2 700 60.4 800 51.1 900 44.2 1000 39.2 duty cycle considerations when designing the lt3760 led driver for a given application, the duty cycle requirements should be considered and compared to the minimum/maximum achievable duty cycles for the lt3760 gate pin. if required, the lt3760 switching frequency can be programmed to a lower value to meet the duty cycle requirements. in general, the minimum/maximum gate duty cycles required for a particular application are given by: min duty cycle = gate minimum on-time ? switching frequency f osc max duty cycle = 1 C (gate minimum off-time ? switching frequency f osc ) the typical values for lt3760 gate pin minimum on and off times versus temperature are shown in the typi- cal performance characteristics. the range of gate pin minimum on time and off times are given in the electrical specifications. table 6. led current vs. r iset (1% resistors) led current per channel (ma) r iset (k?) 20 29.4 40 14.7 60 9.76 80 7.32 100 5.76 an extra 50 ns should be added to these tested timings to account for errors in the rise/fall times of the gate and drain of the external mosfet and the d.c. resistance of the external mosfet and inductor. synchronizing to an external clock the sync pin allows the lt3760 oscillator to be synchro- nized to an external clock. the sync pin can be driven from a logic level output, requiring less than 0.6 v for a logic level low and greater than 2.2 v for a logic level high. sync pin high or low periods should exists for at least 100ns. if unused, the sync pin should be tied to ground. to avoid loss of slope compensation during synchroniza- tion, the free running oscillator frequency (f osc ) of the lt3760 should be programmed to 80% of the external clock frequency. programming led current the current source to ground at each led pin is programmed using a single resistor r iset connected from the i set pin to ground according to the following equation: i led x ( ) 590 r iset a ( ) ctrl > 1.1v ( ) see table 6 for resistor values and corresponding pro- grammed led.
lt3760 17 3760fc a pplica t ions i n f or m a t ion turn led currents on/off as quickly as possible. for pwm low, the lt3760 turns off the boost converter, turns off all led channel currents and disconnects the v c pin and internal v out resistor divider connected to the ovp error amplifier. this allows the part to quickly return to the last state of operation when the pwm pin is returned high. some general guidelines for led current dimming using the pwm pin (see figure 5): (1) pwm dimming ratio ( pdr ) = 1/( pwm duty cycle ) = 1/t on( pwm ) ? f pwm (2) lower pwm frequency (f pwm ) allows higher pwm dimming ratios ( typically choose 100 hz to maximize pdr and to avoid visible flicker which can occur for display systems with refresh rates at frequencies below 80hz) (3) higher f osc value improves pdr ( allows lower t on( pwm ) ) but will reduce efficiency and increase internal heating. in general, minimum operational t on( pwm ) = 3 ? (1/f osc ) (4) lower inductor value improves pdr (5) higher output capacitor value improves pdr (6) choose the schottky diode for the lt3760 boost con- verter for minimum reverse leakage current. programming led current derating (breakpoint and slope) versus led ambient temperature (ctrl pin) led data sheets provide curves of maximum allowed led current versus ambient temperature to warn against damaging of the led ( figure 6). the lt3760 led driver improves the utilization and reliability of the led(s) by al- pwm inductor current 3760 f05 led current max i led t pwm t on(pwm) (= 1/f pwm ) figure 5. pwm dimming waveforms analog dimming the lt3760 allows for led dimming ( brightness reduction) by analog dimming or by pwm dimming. analog dimming uses the ctrl pin voltage below 1 v to reduce led brightness by reducing led current. for ctrl pin voltage below 1v, the current in each led pin is given by: i led x ( ) ctrl ? 590 r iset 0.04 < ctrl < 1v ( ) for ctrl pin voltages below 40mv ( greater than 25:1 dimming) the led current will approach zero current. the ctrl pin voltage can be derived from a resistor divider from v ref pin to ground or generated from an external source. if analog dimming is not required, the pin can be directly connected to the v ref pin. the only drawback of analog dimming is that reducing led current to reduce the brightness of the led also changes the perceived color of the led. pwm dimming many applications require an accurate control of the bright- ness of the led(s). in addition, being able to maintain a constant color over the entire dimming range can be just as critical. for constant color led dimming the lt3760 provides a pwm pin and special internal circuitry to achieve up to a 3000:1 wide pwm dimming range. this is achieved by operating the led at its programmed current and then controlling the on time of that led current. the duty cycle of the pwm pin controls the on time of each led pin current source (figure 5). for maximum pwm dimming ratios (low pwm duty cycles) it is important to be able to figure 6. led current derating vs led ambient temperature t a -temperature (c) 0 i f - forward current (ma) 90 80 70 60 50 40 30 20 10 0 40 7060 30 50 3760 f06 80 2010 resistor option a lt3760 programmed led current derating curve
lt3760 18 3760fc lowing the programming of an led current derating curve versus the ambient temperature of the led(s). without the ability to back off led currents as temperature increases, many led drivers are limited to driving the led(s) at 50% or less of their maximum rated currents . this limitation requires more leds to obtain the intended brightness for the application. the lt3760 allows the led(s) to be programmed for maximum allowable current while still protecting the led (s ) from excessive currents at high tem- perature. the temperature breakpoint and the slope of led current versus ambient temperature can be programmed using a simple resistor network shown in figure 7. this is achieved by programming a voltage at the ctrl pin with a negative temperature coefficient using a resistor divider with temperature dependent resistance (figures 7 and 8). a variety of resistor networks and ntc resistors with different temperature coefficients can be used to achieve the desired ctrl pin voltage behavior versus temperature. the current derating curve in figure 6 uses the resistor network shown in option a of figure 7. table 7 shows a list of ntc resistor manufacturers/ distribu - tors. there are several other manufacturers available and the chosen supplier should be contacted for more detailed in formation. to use an ntc resistor to monitor the ambient temperature of the led(s) it should be placed as close as a pplica t ions i n f or m a t ion possible to the led(s). since the temperature dependency of an ntc resistor can be non-linear over a wide range of temperatures it is important to obtain a resistors exact values over temperature from the manufacturer. hand calculations of ctrl voltage can then be performed at each given temperature and the resulting ctrl voltage plotted versus temperature. table 7. ntc resistor manufacturers m anufacturer web murata electronics north america www.murata.com tdk corporation www.tdk.com digi-key www.digikey.com if calculation of ctrl voltage at various temperatures gives a downward slope that is too strong, alternative resistor networks can be chosen ( b,c,d in figure 7) which use temperature independent resistance to reduce the effects of the ntc resistor over temperature. murata electronics provides a selection of ntc resistors with complete data over a wide range of temperatures. in addition, a software tool is available which allows the user to select from dif- ferent resistor networks and ntc resistor values and then simulate the exact output voltage curve ( ctrl pin behav- ior) over temperature. referred to on the website as the murata chip ntc thermistor output voltage simulator , lt3760 v ref ctrl 3 1 r1 3760 f07 r2 r y r y r x r x r ntc r ntc r ntc r ntc d c b a option a to d figure 7. programming led current derating curve vs ambient temperature (r ntc located on led pcb) figure 8. programmed ctrl voltage vs temperature t a - ambient temperature (c) 0 ctrl voltage (v) 1.50 1.25 1.00 0.75 0.50 0.25 30 40 7060 20 50 3760 f08 80 10 resistor option a
lt3760 19 3760fc a pplica t ions i n f or m a t ion users can log onto www.murata.com/designlib and down- load the software followed by instructions for creating an output voltage v out ( lt3760 ctrl pin voltage) from a specified v cc supply ( lt3760 v ref pin voltage). at any time during selection of circuit parameters the user can access data on the chosen ntc resistor by clicking on the link to the murata catalog. for a detailed example of hand calculations using an ntc type resistor divider to program ctrl pin voltage, read the lt3478 led driver data sheet section programming led current derating vs temperature under applications information. using the t set pin for thermal protection the lt3760 contains a special programmable thermal regulation loop that limits the internal junction temperature of the part. since the lt3760 topology consists of a single boost controller with eight linear current sources, any led string voltage mismatch will cause additional power to be dissipated in the package. this topology provides excellent current matching between led strings and allows a single power stage to drive a large number of leds, but at the price of additional power dissipation inside the part (which means a higher junction temperature). being able to limit the maximum junction temperature allows the benefits of this topology to be fully realized. this thermal regulation feature provides important protection at high ambient tem- peratures, and allows a given application to be optimized for typical, not worst-case, ambient temperatures with the assurance that the lt3760 will automatically protect itself and the led strings under worst-case conditions. the operation of the thermal loop is simple. as the ambi- ent temperature increases, so does the internal junction temperature of the part. once the programmed maximum junction temperature is reached, the lt3760 begins to linearly reduce the led current, as needed, to try and maintain this temperature. this can only be achieved when the ambient temperature stays below the desired maximum junction temperature. if the ambient tempera- ture continues to rise past the programmed maximum junction temperature, the leds current will be reduced to approximately 5% of the full led current. while this feature is intended to directly protect the lt3760, it can also be used to derate the led current at high tem- peratures. since there is a direct relationship between the led temperature and lt3760 junction temperature, the tset function also provides some led current derating at high temperatures. tw o external resistors program the maximum ic junction temperature using a resistor divider from the v ref pin, as shown in figure 9. choose the ratio of r1 and r2 for the desired junction temperature. figure 10 shows the relationship of t set voltage to junction temperature, and table 8 shows commonly used values for r1 and r2. figure 9. programming the t set pin 3760 f09 lt3760 v ref t set r2 r1 3 2 figure 10. programing the t set pin threshold junction temperature (c) 0 500 v tset threshold (mv) 600 700 800 50 100 125 25 75 150 950 550 650 750 850 900 3760 f10 v ptat table 8. resistor values to program maximum ic junction temperature (v ref (typical) = 1.485v) t j (c) r1 (k) r2 (k) t set (v) 100 24.9 20 0.824 115 28.0 20 0.866 130 30.9 20 0.902
lt3760 20 3760fc a pplica t ions i n f or m a t ion programming overvoltage protection (ovp) level the lt3760 led driver provides optimum protection to the leds and the external mosfet by providing a pro- grammable maximum regulated output voltage limit using the ovp set pin. the overvoltage protection ( ovp) level is programmed as: ovp( maximum regula ted v out ) = 57 ? ovp set if every led string fails open or the multiple string led display becomes disconnected the lt3760 led driver loop regulates to the programmed ovp level. the ovp level should be programmed to a level high enough to regulate the led strings but low enough to prevent damage to the power switch and to minimize the voltage across the led pins upon reconnection of the led strings. recommended ovp level is given by: ovp(recommended ) = 1.2 ? ((n ? v f ) + 1v) where: n = number of leds in each string, v f = maximum led forward voltage drop and the scaling factor of 1.2 accounts for variation in the generation of ovp from ovp set pin voltage and startup logic requirements. example: for a converter operating with 10 leds per string at a maximum forward voltage of 4 v per led, the ovp level should be programmed to: ovp(recommended) = 1.2 ? (10 ? 4) + 1v ( ) = 49.2v for ovp = 49.2v, ovp set = 49.2 57 = 0.863v the ovp set pin voltage can be generated using a resistor divider from the ref pin. led open circuit and pwm dimming ratios the lt3760 monitors each led pin voltage to determine if the led string has an open fault ( led pin voltage < 0.5v). if an open led fault occurs, the fault flag is pulled low. to avoid false detection of faults during the initial converter startup when v out is low, the lt3760 ignores low led pin voltages until v out reaches 90% of its maximum al- lowed ovp level. once this condition is met, the lt3760 monitors all led pins for open led faults. to avoid false detection of faults during pwm dimming edges ( where led pins can possibly ring and trip fault detection levels) the lt3760 only monitors/updates fault conditions during pwm high ( and only after a blank duration of 2 s following each pwm rising edge). led short circuit a short circuit fault between the positive terminal of an led string (v out ) and the negative terminal of the led string ( ledx pin) causes the channel to be disabled in order to protect the internal current source. a resistive short is allowed as long as (v out -v ledx ) < 6v. loop compensation be sure to check the stability of the loop with the leds connected ( led regulation loop) and disconnected (overvoltage protection ( ovp) regulation loop). various application circuits are shown in the data sheet which cover a range of v in , v out , f osc , output power and inductor current ripple values. for application requirements which deviate from the circuits shown in the data sheet be sure to check the stability of the final application over the full v in range, led current range ( if analog dimming) and temperature range. be aware that if the v c pin components represent a dominant pole for the converter loop and they have been adjusted to achieve stability, the v c pin might move more slowly during load transient conditions such as an all-leds-open fault. a slower moving v c pin will add to v out overshoot during an all-leds-open fault. an alternative compensation approach is to place the dominant pole of the converter loop at the output. this requires an increased output capacitor value but will allow a much reduced vc capacitor. the combination will allow v c to move more quickly and v out to move more slowly resulting in less overshoot during an all-leds-open fault.
lt3760 21 3760fc a pplica t ions i n f or m a t ion thermal considerations the internal power dissipation of the lt3760 comes from 3 main sources: v in quiescent current (i q total), v in current for gate switching (i gate ) and the lt3760 led current sources. since the maximum operational v in voltage is 40 v, care should be taken when selecting the switching frequency and type of external power mosfet since the current required from v in for gate switching is given by, i gate = f osc ? qg where q g is the gate charge ( at v gs = intv cc ) specified for the mosfet and f osc is the programmed switching frequency for the lt3760. a low q g mosfet should al- ways be used when operating the lt3760 from high v in voltages. the internal junction temperature of the lt3760 can be estimated as: t j = t a + [v in ? ( i qtotal + (f osc ? q g )) + (8 ? i( led x ) ? 1.1 v )] ? ja where, t a is the ambient temperature for the lt3760 i qtotal represents the v in quiescent current for the lt3760 (not switching, pwm = 1.5 v and ctrl = 0.1v) - illustrated in the typical characteristics graphs C plus the base cur- rents of active channels (typically 8 ? i(led)/75). ja is the thermal resistance of the package (28 c/w for the 28-pin tssop package). example : for a 12 w led driver application requiring 8 strings of 10 leds each driven with 40 ma, v in = 24 v, f osc = 1 mhz, q g (at 7 v v gs ) = 15 nc, i(led x ) = 40 ma, and an 85c ambient temperature for the lt3760 ic, the lt3760 junction temperature can be approximated as: t j = 85c + [24 ? (9.5ma + (8 ? 40ma/75) + (1 mhz ? 15nc )) + (8 ? 40ma ? 1.1v)] ? 34 = 85c + [(24 ? 28.8ma) + (320ma ? 1.1v)] ? 34 = 85c + (0.691w + 0.35w ) ? 34 = 85c + 35c t j = 120c the exposed pad on the bottom of the package must be soldered to the ground plane. the ground plane should be connected to an internal copper ground plane with vias placed directly under the package to spread out the heat generated by the lt3760. circuit layout considerations as with all switching regulators, careful attention must be given to pcb layout and component placement to achieve optimal thermal, electrical and noise performance . the exposed pad of the lt3760 should be soldered to a continuous copper ground plane underneath the device to reduce die temperature and maximize the power capa- bility of the ic. the signal ground ( gnd, pin 24) is down bonded to the exposed pad near the rt and v c pins. i set , r t and v c components should be connected to an area of ground copper connected to pin 24. the ovp set track should be kept away from fast moving signals and not loaded with an external capacitor. gate pin turn off currents escape through a downbond to the exposed pad and exit the pgnd, pin 10. this area of copper and pin 10 should be the power ground ( pgnd) connection for the inductor input capacitor, intv cc capacitor and output capacitor. a separate bypass capacitor for the v in pin of the ic may be required close the v in pin and connected to the copper area associated with signal ground, pin 24. to minimize mosfet peak current sensing errors the sense resistor ( rs) should have kelvin connections to the sense pin and the power ground copper area near the pin. the mosfet drain rise and fall times are designed to be as short as possible for maximum efficiency. to reduce the effects of both radiated and conducted noise, the area of the copper trace for the mosfet drain should be kept as small as possible. use a ground plane under the switching regulator to minimize interplane coupling. the schottky diode and output capacitor should be placed as close as possible to the drain node to minimize this high switching frequency path.
lt3760 22 3760fc lt3760 gate 2.2f 100v 5 l1 10h d1 m1 0.015 v out sense led1 led2 led3 led4 led5 led6 led7 led8 ctrl sync gnd pwm v ref ovp set t set rt i set v c v in v in 8v to 14v p vin 20v to 36v 4.7f 10v 4.7f 25v 100k intv cc 499k 40.2k 4.7f 50v pgnd ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 11k 30.9k 20k 20k 39.2k 5.76k 10k 2.2nf 3760 ta03 fault shdn/uvlo l1: sumida cdrh8d38 m1: vishay siliconix si7850dp d1: diodes, inc. pds360 v in pwm dimming analog dimming up to 45v of leds per string typical a pplica t ions 5s/div i(ledx) 40ma/div pwm 10v/div i(l1) 1a/div 3760 g04 (front cover application) led current waveforms 3000:1 pwm dimming (100hz) 92% efficient, 36w led driver, 1mhz boost, 8 strings, 100ma per string
lt3760 23 3760fc typical a pplica t ions lt3760 gate 2.2f 100v 7 l1 10h d1 m1 0.0125 v out sense led1 led2 led3 led4 led5 led6 led7 led8 sync gnd ctrl pwm v ref t set ovp set rt i set v c v in v in 11v to 18v 4.7f 10v 4.7f 25v 100k intv cc 1m pgnd ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 10k 30.9k 16.9k 56.2k 7.32k 10k 4.7nf 3760 ta04 fault shdn/uvlo shdn l1: sumida cdrh8d38 m1: vishay siliconix si7308dn d1: diodes, inc. dfls160 up to 44v of leds per string 20k pwm dimming analog dimming 28w led driver, 750khz boost, 8 strings, 80ma per string
lt3760 24 3760fc typical a pplica t ions lt3760 gate 2.2f 50v 5 l1 7.3h d1 m1 0.015 v out sense led1 led2 led3 led4 led5 led6 led7 led8 sync gnd ctrl pwm v ref t set ovp set rt i set v c v in v in 8v to 21v 4.7f 10v 4.7f 50v 100k intv cc 1m ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 10k 30.9k 10k 56.2k 10.7k 5.1k 4.7nf 3760 ta05 fault shdn/uvlo pwm dimming* analog dimming shdn l1: sumida cdrh8d28 m1: vishay siliconix si7308dn d1: diodes, inc. dfls160 *maximum pwm dimming ratio: (a) f pwm = 20khz = 20:1 (v in > 10v) = 5:1 (v in = 8v) (b) f pwm = 100hz = 3000:1 (v in > 10v) = 750:1 (v in = 8v) up to 34v of leds per string 20k pgnd 15w led driver, 750khz boost, 8 strings, 55ma per string
lt3760 25 3760fc typical a pplica t ions 2.2f 100v 10 l1 10h d1 m1 0.007 v in 8v to 36v 4.7f 10v 4.7f 50v 1m 232k ? ? ? 15k 23.2k 30.9k 20k 115k 6.65k 5.1k 4.7nf 3760 ta06 l1: cooper bussmann hc9-100-r m1: vishay siliconix si7850dp d1: diodes, inc. pds560 ? ? ? lt3760 gate v out sense led1 led2 led3 led4 led5 led6 led7 led8 sync gnd pwm ctrl pwm dimming analog dimming v ref t set ovp set rt i set v c v in intv cc fault shdn/uvlo up to 42v of leds per string 100k pgnd 29w led driver, 400khz boost, 2 strings, 350ma per string
lt3760 26 3760fc typical a pplica t ions l1 10h d1 m1 0.007 v in 8v to 36v 4.7f 10v 4.7f 50v 1m 232k ? ? ? 15k 23.2k 30.9k 20k 115k 5.76k 5.1k 4.7nf 3760 ta07 l1: cooper bussmann hc9-100-r m1: vishay siliconix si7850dp d1: diodes, inc. pds560 ? ? ? ? ? ? lt3754 gate v out sense led8 led7 led1 led2 led3 led4 led5 led6 sync gnd pwm ctrl pwm dimming analog dimming v ref t set ovp set rt i set v c v in intv cc fault shdn/uvlo up to 42v of leds per string 100k 2.2f 100v 10 pgnd 25w led driver, 400khz boost, 3 strings, 200ma per string
lt3760 27 3760fc typical a pplica t ions lt3760 gate 2.2f 100v 5 l1 15h d1 m1 0.02 v out sense led1 led2 led7 led8 led5 led6 led3 led4 ctrl sync gnd pwm v ref ovp set t set rt i set v c v in v in 10v to 18v shdn 4.7f 10v 4.7f 25v 100k intv cc 1m ? ? ? ? ? ? ? ? ? ? ? ? 11k 30.9k 20k 20k 60.4k 7.32k 7.5k 4.7nf 3760 ta08 fault shdn/uvlo l1: sumida cdrh8d38 m1: vishay siliconix si7308dn d1: diodes, inc. dfls160 v in pwm dimming analog dimming up to 45v of leds per string pgnd 29w led driver, 700khz boost, 4 strings, 160ma per string
lt3760 28 3760fc typical a pplica t ions lt3760 gate l1 15h d1 m1 0.02 v out sense led1 led2 led7 led8 led5 led6 led3 led4 ctrl sync gnd pwm v ref ovp set t set rt i set v c v in v in 10v to 28v 4.7f 10v 4.7f 50v 100k intv cc 1m ? ? ? ? ? ? ? ? ? ? ? ? 20.5k 30.9k 20k 20k 60.4k 7.32k 10k 820pf 200pf 3760 ta09 fault shdn/uvlo l1: sumida cdrh8d38 m1: vishay siliconix si7308dn m2: vishay siliconix si2309ds m3: vishay siliconix si2312ds q1: mmbta42 d1: diodes, inc. dfls160 v in pwm dimming analog dimming up to 45v of leds per string 1.5k m2 q1 5v/0v on/off 1k pgnd m3 2.2f 100v 5 shdn 10k 140k 14w led driver, 700khz boost, 4 strings, 80ma per string (for machine vision systems with very long off-times)
lt3760 29 3760fc typical a pplica t ions 13w led driver, 1mhz sepic, 8 strings, 100ma per string (survives v out short to pgnd) lt3760 gate 4.7f 25v 4 l1a 15h 2.2f 50v 2 l1b 15h d1 m1 0.015 v out sense led1 led2 led3 led4 led5 led6 led7 led8 ctrl sync gnd pwm v ref ovp set t set rt i set v c v in v in 8v to 14v v out p vin 10v to 32v 4.7f 10v 4.7f 25v 100k intv cc 499k 100k 4.7f 50v pgnd 20k 30.9k 20k 6.34k 39.2k 5.76k 7.5k 4.7nf 3760 ta10 fault shdn/uvlo l1a, l1b: 15h coupled inductor drq125 m1: vishay siliconix si7850dp d1: diodes, inc. pds560 v in pwm dimming analog dimming up to 16v of leds per string ctrl 1m 110k ? ?
lt3760 30 3760fc p ackage descrip t ion fe28 (eb) tssop 0204 0.09 ? 0.20 (.0035 ? .0079) 0 ? 8 0.25 ref 0.50 ? 0.75 (.020 ? .030) 4.30 ? 4.50* (.169 ? .177) 13 4 5 6 7 89 10 11 12 13 14 192022 21 151618 17 9.60 ? 9.80* (.378 ? .386) 4.75 (.187) 2.74 (.108) 28 2726 25 24 23 1.20 (.047) max 0.05 ? 0.15 (.002 ? .006) 0.65 (.0256) bsc 0.195 ? 0.30 (.0077 ? .0118) typ 2 recommended solder pad layout exposed pad heat sink on bottom of package 0.45 0.05 0.65 bsc 4.50 0.10 6.60 0.10 1.05 0.10 4.75 (.187) 2.74 (.108) millimeters (inches) *dimensions do not include mold flash. mold flash shall not exceed 0.150mm (.006") per side note: 1. controlling dimension: millimeters 2. dimensions are in 3. drawing not to scale see note 4 4. recommended minimum pcb metal size for exposed pad attachment 6.40 (.252) bsc fe package 28-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663) exposed pad variation eb please refer to http://www .linear.com/designtools/packaging/ for the most recent package drawings.
lt3760 31 3760fc 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. r evision h is t ory rev date description page number a 1/11 revised fault pin description. 8 b 10/11 updated features section. 1 updated equation in power mosfet: current sense resistor section. 13 updated programming led current derating (breakpoint and slope) versus led ambient temperature (ctrl pin) section. 18 c 3/12 corrected the inductor value formula. 11
lt3760 32 3760fc 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 0312 rev c ? printed in usa r ela t e d p ar t s part number description comments lt3755/lt3755- 1/ lt3755-2 high side 40v, 1mhz led controller with true color 3,000:1 pwm dimming v in = 4.5v to 40v, v out(max) = 75v, 3,000:1 true color pwm dimming, i sd = <1a, 3mm w 3mm qfn-16 msop-16e lt3756/lt3756- 1/ lt3756-2 high side 100v, 1mhz led controller with true color 3,000:1 pwm dimming v in = 6v to 100v, v out(max) = 100v, 3,000:1 true color pwm dimming, i sd = <1a, 3mm w 3mm qfn-16 msop-16e lt3598 44v, 1.5a, 2.5mhz boost 6-channel 20ma led driver v in = 3v to 30v (40v max ), v out(max) = 44v, 1,000:1 true color pwm dimming, i sd = <1a, 4mm w 4mm qfn-24 lt3599 44v, 2a, 2.5mhz boost 4-channel 100ma led driver v in = 3v to 30v (40v max ), v out(max) = 44v, 1,000:1 true color pwm dimming, i sd = <1a, 4mm w 4mm qfn-24 lt3595 45v, 2.5mhz 16-channel full featured led driver v in = 4.5v to 45v, v out(max) = 45v, 5,000:1 true color pwm dimming, i sd = <1a, 5mm w 9mm qfn-56 ltc3783 high side 36v, 1mhz led controller with true color 3,000:1 pwm dimming v in = 3v to 36v, v out(max) = 40v, 3,000:1 true color pwm dimming, i sd = <20a, 4mm w 5mm dfn-16 tssop-16e lt3517 1.5a, 2.5mhz high current led driver with 3,000:1 dimming v in = 3v to 30v, v out(max) = 45v, 3,000:1 true color pwm dimming, i sd = <1a, 4mm w 4mm qfn-16 lt3518 2.3a, 2.5mhz high current led driver with 3,000:1 dimming v in = 3v to 30v, v out(max) = 45v, 3,000:1 true color pwm dimming, i sd = <1a, 4mm w 4mm qfn-16 lt3519/lt3519- 1/ lt3519-2 750ma, 2.2mhz high current led driver v in = 3v to 30v, v out(max) = 45v, 3,000:1 true color pwm dimming, i sd = <1a, msop-16e lt3486 dual 1.3a, 2mhz high current led driver v in = 3v to 40v, v out(max) = 36v, 1,000:1 true color pwm dimming, i sd = <1a, 5mm w 3mm dfn, tssop-16e lt3478/lt3478-1 4.5a, 2mhz high current led driver with 3,000:1 dimming v in = 2.8v to 36v, v out(max) = 60v, 3,000:1 true color pwm dimming, i sd = <10a, 5mm w 7mm qfn-10 lt3496 triple output 750ma, 2.1 mhz high current led driver with 3,000:1 dimming v in = 3v to 30v, v out(max) = 40v, 3,000:1 true color pwm dimming, i sd = <1a, 4mm w 5mm qfn-28 lt3474/lt3474-1 36v, 1a (i led ), 2mhz, step-down led driver v in = 4v to 36v, v out(max) = 13.5v, 400:1 true color pwm dimming, i sd = <1a, tssop-16e lt3475/lt3475-1 dual 1.5a(i led ), 36v, 2mhz, step-down led driver v in = 4v to 36v, v out(max) = 13.5v, 3,000:1 true color pwm dimming, i sd = <1a, tssop-20e lt3476 quad output 1.5a, 2mhz high current led driver with 1,000:1 dimming v in = 2.8v to 16v, v out(max) = 36v, 1,000:1 true color pwm dimming, i sd = <10a, 5mm w 7mm qfn-10 lt3754 16-channel w 50ma led driver v in = 6v to 40v, v out(max) = 60v, 3,000:1 true color pwm dimming, i sd = <2a, 5mm w 5mm qfn-32

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