View LT3478EFE-1PBF_8576485.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

lt3478/lt3478-1 1 34781f v in (v) 8 efficiency (%) 100 9590 85 80 12 10 3478 ta01b 14 16 i led = 700ma f osc = 500khz pwm duty cycle = 100% 6 leds luxeon iii (white) 4.5a monolithic led drivers with true color pwm dimming high power led driver automotive lighting true color pwm? dimming delivers constant led color with up to 3000:1 range wide input voltage range: 2.8v to 36v 4.5a, 60m , 42v internal switch drives leds in boost, buck-boost or buck modes integrated resistors for inductor and led current sensing program led current: 100ma to 1050ma (lt3478-1) (10mv to 105mv)/r sense (lt3478) program led current de-rating vs temperature separate inductor supply input inrush current protection programmable soft-start fixed frequency operation from 200khz to 2.25mhz open led protection (programmable ovp) accurate shutdown/uvlo threshold with programmable hysteresis 16-pin thermally enhanced tssop package automotive tft lcd backlight applicatio s u features descriptio u typical applicatio u ef? ciency vs v in lt3478-1 shdnv ref out led ctrl2 ovpsetctrl1 pwm ss v c r t v in v s ls w 10 f 4.7 f 45.3k 54.9k 69.8k 130k 0.1 ? r sense (lt3478) pwm dimmingcontrol 700ma15w 6 leds (white) 3478 ta01 10 h v in 8v to 16v 0.1 f 1 f , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. patents pending. the lt ? 3478/lt3478-1 are 4.5a step-up dc/dc convert- ers designed to drive leds with a constant current over a wide programmable range. series connection of the leds provides identical led currents for uniform bright- ness without the need for ballast resistors and expensive factory calibration. the lt3478-1 reduces external component count and cost by integrating the led current sense resistor. the lt3478 uses an external sense resistor to extend the maximum programmable led current beyond 1a and also to achieve greater accuracy when programming low led currents. operating frequency can be set with an external resistor from 200khz up to 2.25mhz. unique circuitry allows a pwm dimming range up to 3000:1 while maintaining constant led color. the lt3478/lt3478-1 are ideal for high power led driver applications such as automotive tft lcd backlights, courtesy lighting and heads-up displays. one of two ctrl pins can be used to program maximum led current. the other ctrl pin can be used to program a reduction in maximum led current vs temperature to maximize led usage and improve reliability. additional features include inrush current protection, programmable open led protection and programmable soft-start. each part is available in a 16-pin thermally enhanced tssop package. downloaded from: http:///
lt3478/lt3478-1 2 34781f sw ............................................................................42v v out , led ..................................................................42v v in , v s , v l , ? s ? h ? d ? n (note 5) .......................................36v pwm .........................................................................15v ctrl1, 2 .....................................................................6v ss, r t , v c , v ref , ovpset ............................................2v operating junction temperature range (notes 2, 3, 4) .................................... C40c to 125c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c (note 1) the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. sw = open, v in = v s = l = v out = ? s ? h ? d ? n = 2.7v, led = open, ss = open, pwm = ctrl1, ctrl2 = 1.25v, v ref = open, v c = open, r t = 31.6k. electrical characteristics absolute axi u rati gs w ww u package/order i for atio uu w fe package 16-lead plastic tssop 12 3 4 5 6 7 8 top view 1615 14 13 12 11 10 9 17 swsw v in v s l v out led ovpset ssr t pwmctrl2 ctrl1 shdn v ref v c t jmax = 125c, ja = 35c/w exposed pad (pin 17) is pgnd, must be soldered to pcb. order part number fe part marking lt3478efe lt3478efe-1 lt3478ife lt3478ife-1 3478fe3478fe-1 3478fe 3478fe-1 order options tape and reel: add #tr lead free: add #pbf lead free tape and reel: add #trpbf lead free part marking: http://www.linear.com/leadfree/ consult ltc marketing for parts speci? ed with wider operating temperature ranges. parameter conditions min typ max units minimum operating voltage (rising) 2.4 2.8 v operational input voltage v s v in (note 5) 2.82.8 3636 vv v in quiescent current v c = 0v (no switching) 6.1 ma v in shutdown current ? s ? h ? d ? n = 0v 3 6 a ? s ? h ? d ? n pin threshold (v sd_p ) (micropower) 0.1 0.4 0.7 v ? s ? h ? d ? n pin threshold (v sd_uvlo ) (switching) 1.3 1.4 1.5 v ? s ? h ? d ? n pin current ? s ? h ? d ? n = v sd_uvlo C 50mv ? s ? h ? d ? n = v sd_uvlo + 50mv 81 0 0 12 a a v ref voltage i(v ref ) = 0a, v c = 0v 1.213 1.240 1.263 v v ref line regulation i(v ref ) = 0a, 2.7v < v in < 36v 0.005 0.015 %/v v ref load regulation 0 < i(v ref ) < 100a (max) 8 12 mv frequency: f osc 200khz r t = 200k 0.18 0.2 0.22 mhz frequency: f osc 1mhz r t = 31.6k 0.88 1.12 mhz downloaded from: http:///
lt3478/lt3478-1 3 34781f the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. sw = open, v in = v s = l = v out = ? s ? h ? d ? n = 2.7v, led = open, ss = open, pwm = ctrl1, ctrl2 = 1.25v, v ref = open, v c = open, r t = 31.6k. electrical characteristics parameter conditions min typ max units frequency: f osc 2.25mhz r t = 9.09k 2 2.25 2.6 mhz line regulation f osc r t = 31.6k, 2.7v < v in < 36v 0.05 0.2 %/v nominal r t pin voltage 0.64 v maximum duty cycle r t = 31.6k r t = 200k r t = 9.09k 80 88 9773 %% % led current to v c current gain (note 6) 770 a/a led current to v c voltage gain (note 6) 400 v/a v c to switch current gain 13 a/v v c source current (out of pin) ctrl1 = 0.4v, v c = 1v 40 a v c sink current ctrl1 = 0v, v c = 1v 40 a v c switching threshold 0.65 v v c high level (v oh ) ctrl1 = 0.4v 1.5 v v c low level (v ol ) ctrl1 = 0v 0.2 v inductor current limit 2.7v < v s < 36v 4.5 6 6.8 a switch current limit 4.5 6.3 7.5 a switch v ce sat i sw = 4.5a 270 mv switch leakage current sw = 42v, v c = 0v 1 a v out overvoltage protection (ovp) (rising) ovpset = 1vovpset = 0.3v 41 12.3 vv full scale led current (lt3478-1) ctrl1 = v ref , current out of led pin 1010 1050 1090 ma 700ma led current (lt3478-1) ctrl1 = 700mv, current out of led pin 655 700 730 ma 350ma led current (lt3478-1) ctrl1 = 350mv, current out of led pin 325 350 375 ma 100ma led current (lt3478-1) ctrl1 = 100mv, current out of led pin 70 100 130 ma full scale led current v sense (lt3478) ctrl1 = v ref , v sense = v vout C v led 101 105 109 mv ctrl1 = 700mv, v sense (lt3478) ctrl1 = 700mv, v sense = v vout C v led 67 70.5 74 mv ctrl1 = 350mv, v sense (lt3478) ctrl1 = 350mv, v sense = v vout C v led 33 35.5 38 mv ctrl1 = 100mv, v sense (lt3478) ctrl1 = 100mv, v sense = v vout C v led 71 01 3 m v ctrl1, 2 input currents ctrl1 = 100mv, ctrl2 = 1.25v or ctrl2 = 100mv, ctrl1 = 1.25v (current out of pin) 40 na ovpset input current ovpset = 1v, v out = 41v (current out of pin) 200 na pwm switching threshold 0.8 1 1.2 v v c pin current in pwm mode v c = 1v, pwm = 0 1 50 na out pin current in pwm mode pwm = 0 1 100 na ss low level (v ol )i (ss) = 20a 0.15 v ss reset threshold v c = 0v 0.25 v ss high level (v oh )v c = 0v 1.5 v soft-start (ss) pin charge current ss = 1v, current out of pin, v c = 0v 12 a soft-start (ss) pin discharge current ss = 0.5v, v c = 0v 350 a downloaded from: http:///
lt3478/lt3478-1 4 34781f junction temperature ( c) ?0 current limit (a) 7.06.5 6.0 5.5 5.0 4.5 ?5 0 25 3478 g06 50 75 100 125 inductor switch switch current (a) 0.0 switch v ce (sat) (mv) 240210 180 120 60 0 1.0 2.0 2.5 4.5 3478 g05 0.5 1.5 3.0 3.5 4.0 t a = 25 c junction temperature ( c) ?0 ctrl1 pin current x (?) (na) 5040 30 20 10 0 ?5 0 25 3478 g04 50 75 100 125 ctrl1 = 0.9v ctrl1 = 0.7v ctrl1 = 0.35v ctrl1 = 0.1v ctrl2 = v ref ctrl1 and ctrl2 pinsinterchangeable ctrl1 (v) 0 led current (ma) 1.40 3478 g01 0.35 0.70 1.05 14001050 700350 0 t a = 25 c ctrl2 = v ref (for lt3478 scale by 0.1 ? /r sense ) lt3478-1 v ref junction temperature ( c) ?0 led current (ma) 14001050 700350 0 ?5 0 25 3478 g02 50 75 100 125 i led = 1050ma, ctrl1 = ctrl2 = v ref i led = 100ma, ctrl1 = 100mv, ctrl2 = v ref (for lt3478 scale by 0.1 ? /r sense ) lt3478-1 led current vs ctrl1 led current vs temperature led current vs pwm duty cycle wide pwm dimming range (3000:1) ctrl1 pin current vs temperature switch v ce (sat) vs switch current switch and inductor peak current limits vs temperature typical perfor a ce characteristics uw pwm duty cycle (%) 1000 100 10 10 0.01 1 10 100 3478 g03 0.1 led current (ma) t a = 25 c v in = v s = 12v 6 leds at 500mapwm freq = 100hz ctrl1 = 0.5v ctrl2 = v ref f osc = 1.6mhz l = 2.2 h 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 lt3478efe/lt3478efe-1 are guaranteed to meet performance speci? cations from 0c to 125c junction temperature. speci? cations over the C40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the lt3478ife/lt3478ife-1 are guaranteed over the full C40c to 125c operating junction temperature range. note 3: this ic includes over-temperature protection that is intended to protect the device during momentary overload conditions. junction temperature will exceed 125c when over-temperature protection is active. continuous operation above the speci? ed maximum operating junction temperature may impair device reliability. note 4: for maximum operating ambient temperature, see the thermal calculations section in the applications information section.note 5: the maximum operational voltage for v in is limited by thermal and ef? ciency considerations. power switch base current is delivered from v in and should therefore be driven from the lowest available power supply in the system. see thermal calculations in the applications information section. note 6: for lt3478, parameter scales ? (r sense /0.1 ). electrical characteristics downloaded from: http:///
lt3478/lt3478-1 5 34781f junction temperature ( c) ?0 v in current (ma) ?5 0 25 3478 g12 50 75 100 125 v in = 2.8v v c = 0v 1412 10 86 4 2 0 v in (v) 0 v in current (ma) 1412 10 86 4 2 0 36 12 3478 g11 18 27 24 91 52 1 3 03 33 6 t a = 25 c v c = 0v junction temperature ( c) ?0 v in current ( a) 5040 30 20 10 0 ?5 0 25 3478 g10 50 75 100 125 v in = 36v v in = 20v v in = 2.8v shdn = 0v junction temperature ( c) ?0 shdn pin current ( a) 1510 50 ?5 0 25 3478 g09 50 75 100 125 just before part turns on after part turns on junction temperature ( c) ?0 shdn (v) 1.601.50 1.40 1.30 1.20 ?5 0 25 3478 g08 50 75 100 125 junction temperature ( c) ?0 v ref (v) 1.281.26 1.24 1.22 1.20 1.18 ?5 0 25 3478 g07 50 75 100 125 v ref vs temperature ? s ? h ? d ? n threshold vs temperature ? s ? h ? d ? n pin (hysteresis) current vs temperature v in shutdown current vs temperature v in quiescent current vs v in v in quiescent current vs temperature typical perfor a ce characteristics uw duty cycle (%) 0 switch peak current limit (a) 76 5 4 3 2 1 0 20 3478 g19 40 60 80 100 t a = 25 c junction temperature ( c) ?0 pin current ( a) 42 0 ?5 0 25 3478 g18 50 75 100 125 i(sw pin) i(v s pin) = i(l pin) shdn = 0vv s = l = sw = 36v v s , l, sw shutdown currents vs temperature switch peak current limit vs duty cycle downloaded from: http:///
lt3478/lt3478-1 6 34781f junction temperature ( c) ?0 v c (v) 1.81.5 1.2 0.6 0.9 0 0.3 ?5 0 25 3478 g17 50 75 100 125 v c clamp v c active threshold junction temperature ( c) ?0 v out clamp (v) 43.042.5 42.0 41.5 41.0 40.5 39.0 39.5 40.0 ?5 0 25 3478 g15 50 75 100 125 ovpset = 1v junction temperature ( c) ?0 ss pin current ( a) (out of pin) 1413 12 11 10 ?5 0 25 3478 g16 50 75 100 125 junction temperature ( c) ?0 switching frequency (mhz) 1.201.15 1.10 1.05 1.00 0.95 0.90 0.80 0.85 ?5 0 25 3478 g14 50 75 100 125 r t = 31.6k r t (k ? ) 1 100 switching frequency (khz) 1000 10000 10 100 1000 3478 g13 t a = 25 c switching frequency vs r t switching frequency vs temperature open-circuit output clamp voltage vs temperature ss pin charge current vs temperature v c pin active and clamp voltages vs temperature typical perfor a ce characteristics uw downloaded from: http:///
lt3478/lt3478-1 7 34781f pi fu ctio s uuu sw (pins 1, 2): switch pin. collector of the internal npn power switch. both pins are fused together inside the ic. connect the inductor and diode here and minimize the metal trace area connected to this pin to minimize emi. v in (pin 3): input supply. must be locally bypassed with a capacitor to ground.v s (pin 4): inductor supply. must be locally bypassed with a capacitor to ground. can be shorted to v in if only one supply is available (see l (pin 5) function).l (pin 5): inductor pin. an internal resistor between v s and l pins monitors inductor current to protect against inrush current. exceeding 6a immediately turns off the internal npn power switch and discharges the soft-start pin. input current monitoring can be disabled by connect- ing the inductor power supply directly to the l pin and leaving the v s pin open (requires local bypass capacitor to gnd on l pin; not v s pin). v out (pin 6): output voltage of the converter. connect a capacitor from this pin to ground. internal circuitry moni- tors v out for protection against open led faults. led (pin 7): connect the led string from this pin to ground. an internal (lt3478-1)/external (lt3478) resistor between the v out and led pins senses led current for accurate control.ovpset (pin 8): programs v out overvoltage protection level (ovp) to protect against open led faults. ovp = (ovpset ? 41)v. ovpset range is 0.3v to 1v for an ovp range of typically 12.3v to 41v. v c (pin 9): output of the transconductance error ampli? er and compensation pin for the converter regulation loop.v ref (pin 10): bandgap voltage reference. this pin can supply up to 100a. can be used to program ctrl1, ctrl2, ovpset pin voltages using resistor dividers to ground. ? s ? h ? d ? n (pin 11): the ? s ? h ? d ? n pin has an accurate 1.4v threshold and can be used to program an undervoltage lockout (uvlo) threshold for system input supply using a resistor divider from supply to ground. a 10a pin current hysteresis allows programming of undervoltage lockout (uvlo) hysteresis. ? s ? h ? d ? n above 1.4v turns the part on and removes a 10a sink current from the pin. ? s ? h ? d ? n = 0v reduces v in current < 3a. ? s ? h ? d ? n can be directly connected to v in . if left open circuit the part will be turned off. ctrl1 (pin 12): ctrl1 pin voltage is used to program maximum led current (ctrl2 = v ref ). ctrl1 voltage can be set by a resistor divider from v ref or an external voltage source. maximum led current is given by: (lt3478-1) max led current = min(ctrl1, 1.05) amps () (, .) . lt max led current min ctrl r se 3478 105 01 = n nse amps (linear for 0.1v < ctrl1< 0.95v ; ctrl2 = v ref ) for maxi- mum led current, short ctrl1 and ctrl2 pins to v ref . ctrl2 (pin 13): the ctrl2 pin is available for program- ming a decrease in led current versus temperature (setting temperature breakpoint and slope). this feature allows the output led(s) to be programmed for maximum allowable current without damage at higher temperatures. this maximizes led usage and increases reliability. a ctrl2 voltage with negative temperature coef? cient is created using an external resistor divider from v ref with temperature dependant resistance. if not used, ctrl2 should be tied to v ref . pwm (pin 14): input pin for pwm dimming control. above 1v allows converter switching and below 1v disables switching with v c pin level maintained. with an external mosfet placed in series with the ground side of the led string, a pwm signal driving the pwm pin and mosfet gate provides accurate dimming control. the pwm signal can be driven from 0v to 15v. if unused, the pin should be connected to v ref . r t (pin 15): a resistor to ground programs switching frequency between 200khz and 2.25mhz.ss (pin 16): soft-start pin. placing a capacitor here pro- grams soft-start timing to limit inductor inrush current during start-up due to the converter. when inductor current downloaded from: http:///
lt3478/lt3478-1 8 34781f v in v s v c ledled led led 3478 f01 3 ctrl2 ctrl1 uvlo inrushcurrent protection v ref 10 4 l 5 ss 16 15 r t exposed pad (gnd) 17 ovpset to overvoltagedetect circuit 8 v c 9 sw 1, 2 pwm 14 led 7 v out 6 shdn 11 + + + + ++ + 1.4v + 1.05v 1000 ? r s 100 ? r sense 0.1 ? (internal for lt3478-1) r sense (external for lt3478) 10 a 9.5m ? 57mv ref 1.24v soft-start overvoltage detect ovpset 12 13 + pwm detect osc sq q1 gm r slope comp pwm + 1v q2 block diagra w figure 1 pi fu ctio s uuu exceeds 6a or v out exceeds ovp, an internal soft-start latch is set, the power npn is immediately turned off and the ss pin is discharged. the soft-start latch is also set if v in and/or ? s ? h ? d ? n do not meet their turn on thresholds. the ss pin only recharges when all faults are removed and the pin has been discharged below 0.25v. exposed pad (pin 17): the ground for the ic and the con- verter. the fe package has an exposed pad underneath the ic which is the best path for heat out of the package. pin 17 should be soldered to a continuous copper ground plane under the device to reduce die temperature and increase the power capability of the lt3478/lt3478-1. downloaded from: http:///
lt3478/lt3478-1 9 34781f operatio u the lt3478/lt3478-1 are high powered led drivers with a 42v, 4.5a internal switch and the ability to drive leds with up to 1050ma for lt3478-1 and up to 105mv/r sense for lt3478. the lt3478/lt3478-1 work similarly to a conventional current mode boost converter but use led current (instead of output voltage) as feedback for the control loop. the block diagram in figure 1 shows the major functions of the lt3478/lt3478-1. for the part to turn on, the v in pin must exceed 2.8v and the ? s ? h ? d ? n pin must exceed 1.4v. the ? s ? h ? d ? n pin threshold allows programming of an undervoltage lockout (uvlo) threshold for the system input supply using a simple resistor divider. a 10a current ? ows into the ? s ? h ? d ? n pin before part turn on and is removed after part turn on. this current hysteresis allows programming of hysteresis for the uvlo threshold. see shutdown pin and programming undervoltage lockout in the applications information section. for micropower shutdown the ? s ? h ? d ? n pin at 0v reduces v in supply current to approximately 3a. each led driver is a current mode step-up switch- ing regulator. a regulation point is achieved when the boosted output voltage v out across the output led(s) is high enough to create current in the led(s) equal to the programmed led current. a sense resistor connected in series with the led(s) provides feedback of led current to the converter loop. the basic loop uses a pulse from an internal oscillator to set the rs ? ip-? op and turn on the internal power npn switch q1 connected between the switch pin, sw, and ground. current increases in the external inductor until switch current limit is exceeded or until the oscillator reaches its maximum duty cycle. the switch is then turned off, causing inductor current to lift the sw pin and turn on an external schottky diode connected to the output. inductor current ? ows via the schottky diode charging the output capacitor. the switch is turned back on at the next reset cycle of the internal oscillator. during normal operation the v c voltage controls the peak switch current limit and hence the inductor current available to the output led(s). as with all current mode converters, slope compensation is added to the control path to ensure stability. the ctrl1 pin is used to program maximum led current via q2. the ctrl2 pin can be used to program a decrease in led current versus temperature for maximum reliability and utilization of the led(s). a ctrl2 voltage with negative temperature coef? cient can be created using an external resistor divider from v ref with temperature dependant resistance. unused ctrl2 is tied to v ref . for true color pwm dimming, the lt3478/lt3478-1 provide up to a 3000:1 wide pwm dimming range by al- lowing the duty cycle of the pwm pin (connected to the ic and an external n-channel mosfet in series with the led(s)) to be reduced from 100% to as low as 0.033% for a pwm frequency of 100hz. dimming by pwm duty cycle, allows for constant led color to be maintained over the entire dimming range. for robust operation, the lt3478/lt3478-1 monitor system performance for any of the following faults : v in or ? s ? h ? d ? n pin voltages too low and/or inductor current too high and/or boosted output voltage too high. on detection of any of these faults, the lt3478/lt3478-1 stop switching immediately and a soft-start latch is set discharging the ss pin (see timing diagram for ss pin in figure 11). all faults are detected internally and do not require external components. when all faults no longer exist, an internal 12a supply charges the ss pin with a timing programmed using a single external capacitor. a gradual ramp up of ss pin voltage limits switch current during startup. for optimum component sizing, duty cycle range and ef- ? ciency the lt3478/lt3478-1 allow for a separate inductor supply v s and for switching frequency to be programmed from 200khz up to 2.25mhz using a resistor from the r t pin to ground. the advantages of these options are covered in the applications informations section. downloaded from: http:///
lt3478/lt3478-1 10 34781f applicatio s i for atio wu u u inductor selection several inductors that work well with the lt3478/lt3478-1 are listed in table 1. however, there are many other manu- facturers 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 ef? ciency. 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. values between 4.7h and 22h will suf? ce for most applications. inductor manufacturers specify the maximum current rating as the current where inductance falls by a given percentage of its nominal value. an inductor can pass a current greater than its rated value without damaging it. aggressive designs where board space is precious will exceed the maximum current rating of the inductor to save space. consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct. capacitor selectionlow esr (equivalent series resistance) ceramic capaci- tors should be used at the output to minimize the output ripple voltage. use only x5r or x7r dielectrics, as these materials retain their capacitance over wider voltage and temperature ranges than other dielectrics. a 4.7f to 10f output capacitor is suf? cient for most high output current designs. some suggested manufacturers are listed in table 2. diode selection schottky diodes, with their low forward voltage drop and fast switching speed, are ideal for lt3478/lt3478-1 ap- plications. table 3 lists several schottky diodes that work well. the diodes average current rating must exceed the applications average output current. the diodes maximum reverse voltage must exceed the applications output volt- age. a 4.5a diode is suf? cient for most designs. for pwm dimming applications, be aware of the reverse leakage current of the diode. lower leakage current will drain the output capacitor less, allowing for higher dimming range. the companies below offer schottky diodes with high voltage and current ratings. table 1. suggested inductors manufacturer part number idc (a) inductance (h) max dcr (m )l w h (mm) manufacturer cdrh104r-100nc cdrh103rnp-4r7nc-b cdrh124r-100mc cdrh104r-5r2nc 3.8 4 4.55.5 10 4.7 10 5.2 3530 28 22 10.5 10.3 4.0 10.5 10.3 3.1 12.3 12.3 4.5 10.5 10.3 4.0 sumida www.sumida.com fdv0630-4r7m 4.2 4.7 49 7.0 7.7 3.0 toko www.toko.com up4b-220 7.6 22 34 22 15 7.9 cooper www.cooperet.com table 2. ceramic capacitor manufacturers manufacturer phone number web taiyo yuden (408) 573-4150 www.t-yuden.com avx (803) 448-9411 www.avxcorp.com murata (714) 852-2001 www.murata.com table 3. suggested diodes manufacturer part number max current (a) max reverse voltage web ups340 3 40 microsemi www.microsemi.com b520cb530c b340a b540c pds560 55 3 5 5 3030 40 40 60 diodes, inc. www.diodes.com downloaded from: http:///
lt3478/lt3478-1 11 34781f shutdown and programming undervoltage lockout the lt3478/lt3478-1 have an accurate 1.4v shutdown threshold at the ? s ? h ? d ? n pin. this threshold can be used in conjunction with a resistor divider from the system input supply to de? ne an accurate undervoltage lockout (uvlo) threshold for the system (figure 2). ? s ? h ? d ? n pin current hysteresis allows programming of hysteresis voltage for this uvlo threshold. just before part turn on, 10a ? ows into the ? s ? h ? d ? n pin. after part turn on, 0a ? ows from the ? s ? h ? d ? n pin. calculation of the on/off thresholds for a system input supply using the lt3478/lt3478-1 ? s ? h ? d ? n pin can be made as follows: v supply off = 1.4 [1 + r1/r2)] v supply on = v supply off + (10a ? r1) an open drain transistor can be added to the resistor divider network at the ? s ? h ? d ? n pin to independently control the turn off of the lt3478/lt3478-1. programming switching frequencythe switching frequency is programmed using an external resistor (r t ) connected between the r t pin and ground. the internal free-running oscillator is programmable between 200khz and 2.25mhz. table 4 shows the typical r t values required for a range of switching frequencies. selecting the optimum switching frequency depends on several factors. inductor size is reduced with higher frequency but ef? ciency drops due to higher switching losses. in addition, some applications require very high duty cycles to drive a large number of leds from a low supply. low switching frequency allows a greater operational duty cycle and hence a greater number of leds to be driven. in each case the switching frequency can be tailored to provide the optimum solution. when programming the switching frequency the total power losses within the ic should be considered. see thermal calculations in the applications information section. applicatio s i for atio wu u u with the ? s ? h ? d ? n pin connected directly to the v in pin, an internal undervoltage lockout threshold exists for the v in pin (2.8v max). this prevents the converter from operat- ing in an erratic mode when supply voltage is too low. the lt3478/lt3478-1 provide a soft-start function when recovering from such faults as ? s ? h ? d ? n <1.4v and/or v in <2.8v. see details in the applications information section soft-start. figure 2. programming undervoltage lockout (uvlo) with hysteresis table 4. switching frequencies vs r t values switching frequency (mhz) r t (k ) 2.25 9.09 1 31.6 0.2 200 figure 3. switching frequency vs r t resistor value r t (k ? ) 1 100 switching frequency (khz) 1000 10000 10 100 1000 3478 f03 t a = 25 c shdn 1.4v 10 a v supply r1 11 r2 3478 f02 on off + downloaded from: http:///
lt3478/lt3478-1 12 34781f ctrl1 (v) 0 led current (ma) 1.40 3478 f04 0.35 0.70 1.05 14001050 700350 0 t a = 25 c ctrl2 = v ref (for lt3478 scaleby 0.1 ? /r sense ) lt3478-1 v ref programming maximum led current maximum led current can be programmed using the ctrl1 pin with ctrl2 tied to the v ref pin (see figures 4 and 5). the maximum allowed led current is de? ned as: (lt3478-1) max led current = min(ctrl1, 1.05) amps () (, . ) . lt max led current min ctrl r s 3478 1105 01 = e ense amps led current vs ctrl1 is linear for approximately 0.1v < ctrl1 < 0.95v for maximum possible led current, connect ctrl1 and ctrl2 to the v ref pin. without the ability to back off led current as temperature increases, many led drivers are limited to driving the led(s) at only 50% or less of their maximum rated currents. this limitation requires more leds to obtain the intended brightness for the application. the lt3478/lt3478-1 al- low the output led(s) to be programmed for maximum allowable current while still protecting the led(s) from excessive currents at high temperature. this is achieved by programming a voltage at the ctrl2 pin with a nega- tive temperature coef? cient using a resistor divider with temperature dependent resistance (figures 7 and 8). ctrl2 voltage is programmed higher than ctrl1 voltage. this allows initial led current to be de? ned by ctrl1. as temperature increases, ctrl2 voltage will fall below ctrl1 voltage causing led currents to be controlled by ctrl2 pin voltage. the choice of resistor ratios and use of temperature dependent resistance in the divider for the ctrl2 pin will de? ne the led current curve breakpoint and slope versus temperature (figure 8). a variety of resistor networks and ntc resistors with differ- ent temperature coef? cients can be used for programming applicatio s i for atio wu u u figure 6. led current derating curve vs ambient temperature maximum allowed led current versus temperature to warn against exceeding this current limit and damaging the led (figure 6). figure 5. programming led current figure 4. led current vs ctrl1 voltage lt3478/lt3478-1 (lt3478) v ref ctrl2ctrl1 v out led 1013 12 r2 r sense r1 3478 f05 t a ambient temperature ( c) 0 i f forward current (ma) 900 luxeon v emitter(green, cyan, blue, royal blue) ja = 20 c/w 700 800 0 500400 300 200 100 600 25 3478 f06 50 75 100 examplelt3478-1 programmed led current derating curve luxeon v emittercurrent derating curve programming led current derating vs temperature a useful feature of the lt3478/lt3478-1 is the ability to program a derating curve for maximum led current versus temperature. led data sheets provide curves of luxeon v (maximum) and lt3478-1 (programmed) current derating curves vs temperature downloaded from: http:///
lt3478/lt3478-1 13 34781f lt3478/lt3478-1 v ref ctrl2ctrl1 1013 12 r4 3478 f07 r2 r1 r3 r y r y r x r x r ntc r ntc r ntc r ntc d c b a option a to d applicatio s i for atio wu u u ctrl2 to achieve the desired ctrl2 curve vs temperature. the current derating curve shown in figure 6 uses the resistor network shown in option c of figure 7. to obtain a resistors exact values over temperature from the manufacturer. hand calculations of ctrl2 voltage can then be performed at each given temperature and the resulting ctrl2 curve plotted versus temperature. several iterations of resistor value calculations may be required to achieve the desired breakpoint and slope of the led current derating curve. table 5. ntc resistor manufacturers/distributors manufacturer murata electronics north america www.murata.com tdk corporation www.tdk.com digi-key www.digikey.com if calculation of ctrl2 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 different resistor networks and ntc resistor values and then simulate the exact output voltage curve (ctrl2 behavior) over temperature. referred to as the murata chip ntc thermistor output voltage simulator, users can log onto www.murata.com/designlib and down- load the software followed by instructions for creating an output voltage v out (ctrl2) from a speci? ed v cc supply (v ref ). at any time during selection of circuit parameters the user can access data on the chosen ntc resistor by clicking on a link to the murata catalog. the following example uses hand calculations to derive the resistor values required for ctrl1 and ctrl2 pin voltages to achieve a given led current derating curve. the resistor values obtained using the murata simulation tool are also provided and were used to create the derating curve shown in figure 6. the simulation tool illustrates the non-linear nature of the ntc resistor temperature coef? cient at temperatures exceeding 50c ambient. in addition, the resistor divider technique using an ntc resistor to derive ctrl2 voltage inherently has a ? atten- ing characteristic (reduced downward slope) at higher temperatures. to avoid led current exceeding a maximum table 5 shows a list of manufacturers/distributors of ntc resistors. there are several other manufacturers available and the chosen supplier should be contacted for more detailed information. to use an ntc resistor to indicate led temperature it is only effective if the resistor is con- nected as close as possible to the led(s). led derating curves shown by manufacturers are listed for ambient temperature. the ntc resistor should be submitted to the same ambient temperature as the led(s). since the temperature dependency of an ntc resistor can be non- linear over a wide range of temperatures it is important figure 7. programming led current derating curve vs temperature (r ntc located on leds pcb) figure 8. ctrl1, 2 programmed voltages vs temperature t a ambient temperature ( c) 0 ctrl1, ctrl2 pin voltages (mv) 11001000 900700 800 0 500400 300 200 100 600 25 3478 f08 50 75 100 led current = minimumof ctrl1, ctrl2 r3 = option c ctrl1 ctrl2 downloaded from: http:///
lt3478/lt3478-1 14 34781f applicatio s i for atio wu u u allowed level at higher temperatures, the ctrl2 voltage curve may require a greater downward slope between 25c and 50c to compensate for that loss of slope at higher temperatures. example: calculate the resistor values required for generat- ing ctrl1 and ctrl2 from v ref based on the following requirements:(a) i led = 700ma at 25c (b) i led derating curve breakpoint occurs at 25c (c) i led derating curve has a slope of C200ma/25c be- tween 25c and 50c ambient temperaturestep1: choose ctrl1 = 700mv for i led = 700ma ctrl1 = v ref /(1 + r2/r1) r2 = r1 ? [(v ref /ctrl1) C 1] for v ref = 1.24v and choosing r1 = 22.1k, r2 = 22.1k [(1.24/0.7) C 1] r2 = 17k (choose 16.9k) ctrl1 = 1.24/(1 + (16.9/22.1)) ctrl1 = 703mv (i led = 703ma) step 2: choose resistor network option a (figure 7) and ctrl2 = ctrl1 for 25c breakpoint start with r4 = r2 = 16.9k, r ntc = 22k (closest value available) ctrl2 = 701mv (i led = min(ctrl1, ctrl2) ? 1a = 701ma) step 3: calculate ctrl2 slope between 25c and 50c ctrl2 (t) = 1.24/(1 + r4/r ntc (t)) at t = t o = 25c, ctrl2 = 701mv at t = 50c, r ntc (t) = r ntc (t o ).e x , x = b [(1/(t + 273) C 1/298)] (b = b-constant; linear over the 25c to 50c temperature range) for r ntc b-constant = 3950 and t = 50c x = 3950 [(1/323) C 1/298] = C1.026 r ntc (50c) = r ntc (25c).e C1.026 r ntc (50c) = 22k ? 0.358 r ntc (50c) = 7.9k ctrl2(50c) = 1.24/(1 + 16.9/7.9) = 395mv ctrl2 slope (25c to 50c) = [ctrl2(50c) C ctrl2(25c)]/25c = (395 C 701)/25 = C306mv/25c i led slope = C306ma/25c the required i led slope is C200ma/25c. to reduce the slope of ctrl2 versus temperature it is easier to keep the exact same ntc resistor value and b-constant (there are limited choices) and simply adjust r4 and the type of resistor network used for the ctrl2 pin. by changing the resistor network to option c it is possible to place a temperature independent resistor in series with r ntc to reduce the effects of r ntc on the ctrl2 pin voltage over temperature.step 4: calculate the resistor value required for r y in resistor network option (c) (figure 7) to provide an i led slope of C200ma/25c between 25c and 50c ambient temperature. ctrl2 (25c) = 0.7v = 1.24/(1 + (r4/(r ntc (25c)+ r y )) r4 = 0.77 (r ntc (25c) + r y ) (a) for C200ma/25c slope ctrl2(50c) = 0.7 C 0.2 = 0.5 ctrl2(50c) = 0.5v = 1.24/(1 + (r4/(r ntc + r y )) r4 = 1.48 (r ntc (50c) + r y ) (b) equating (a) = (b) and knowing r ntc (25c) = 22k and r ntc (50c) = 7.9k gives, 0.77 (22k + r y ) = 1.48 (7.9k + r y ) 17k + 0.77 r y = 11.7 k + 1.48 r y r y = (17k C 11.7k)/(1.48 C 0.77) r y = 7.5k downloaded from: http:///
lt3478/lt3478-1 15 34781f the value for r4 can now be solved using equation (a) where, r4 = 0.77 (r ntc (25c) + r y ) = 0.77 (22k + 7.5k) r4 = 22.7k (choose 22.6k) i led slope can now be calculated from, i led slope = [ctrl2(50c) C ctrl2(25c)]/25c where ctrl2 (50c) = 1.24/(1 + 22.6/(7.9 + 7.5)) = 503mv and ctrl2 (25c) = 1.24/(1 + 39.2/(22 + 28.7)) = 699mv giving i led slope (from 25c to 50c) = 503mv C 699mv/25c = C196mv/25c => i led slope = C196ma/25c using the murata simulation tool for the resistor network and values in the above example shows a ctrl2 volt- age curve that ? attens out as temperatures approach 100c ambient. the ? nal resistor network chosen for the derating curve in figure 6 used option c network with r4 = 19.3k, r ntc = 22k (ncp15xw223j0src) and r y = 3.01k. although the ctrl2 downward slope is greater than C200ma/25c initially, the slope is required to avoid exceeding maximum allowed led currents at high ambient temperatures (see figure 6). pwm dimming many led applications require an accurate control of the brightness of the led(s). in addition, being able to main- tain a constant color over the entire dimming range can be just as critical. for constant color led dimming, the lt3478/lt3478-1 provide a pwm pin and special internal circuitry to allow up to a 3000:1 wide pwm dimming range. with an n-channel mosfet connected between the led(s) and ground and a pwm signal connected to the gate of the mosfet and the pwm pin (figure 9), it is possible to control the brightness of the led(s) based on pwm signal duty cycle only. this form of dimming is superior to dimming control using an analog input voltage (reducing ctrl1 voltage) because it allows constant color to be maintained during dimming. the maximum current for the output led(s) is programmed for a given bright-ness/color and chopped over a pwm duty cycle range (figure 10) from 100% to as low as 0.033%. applicatio s i for atio wu u u figure 9. pwm dimming control using the lt3478/lt3478-1 figure 10. pwm dimming waveforms using the lt3478/lt3478-1 lt3478/ lt3478-1 shdn v in v ref v out led ctrl2ovpset pwm v c v s ls w pwm dimmingcontrol 3478 f09 ctrl1 r t r sense c out d1 d2 (lt3478) pwm inductor current 3478 f10 led current max i led t pwm ton pwm (= 1/f pwm ) some general guidelines for led current dimming using the pwm pin (see figure 10): (1) pwm dimming ratio (pdr) = 1/(pwm duty cycle) = 1/(ton pwm ? f pwm ) (2) lower f pwm allows higher pwm dimming ratios (use minimum f pwm = 100hz to avoid visible ? icker and to maximize pdr)(3) higher f osc value improves pdr (allows lower ton pwm ) but will reduce ef? ciency and increase internal heating. in general, minimum operational ton pwm = 3 ? (1/f osc ). (4) lower inductor value improves pdr downloaded from: http:///
lt3478/lt3478-1 16 34781f applicatio s i for atio wu u u (5) higher output capacitor value improves pdr(6) choose the schottky diode (d2, figure 9) for minimum reverse leakage see typical performance characteristics graph led cur- rent vs pwm duty cycle. soft-start to limit inrush current and output voltage overshoot dur- ing startup/recovery from a fault condition, the lt3478/ lt3478-1 provide a soft-start pin ss. the ss pin is used to program switch current ramp up timing using a ca- pacitor to ground. the lt3478/lt3478-1 monitor system parameters for the following faults: v in <2.8v, ? s ? h ? d ? n <1.4, inductor current >6a and boosted output voltage >ovp. on detection of any of these faults, the lt3478/lt3478-1 stop switching immediately and a soft-start latch is set causing the ss pin to be discharged (see timing diagram for the ss pin in figure 11). when all faults no longer ex- ist and the ss pin has been discharged to at least 0.25v, the soft-start latch is reset and an internal 12a supply charges the ss pin. a gradual ramp up of ss pin voltage is equivalent to a ramp up of switch current limit until ss exceeds v c . the ramp rate of the ss pin is given by: v ss / t = 12a/c ss to limit inductor current overshoot to <0.5a when ss charges past the v c level required for loop control, the c ss capacitor should be chosen using the following formula: c ss(min) = c c (7.35 C 0.6(i led ? v out /v s )) example: v s = 8v, v out = 16v, i led = 1.05a, c c = 0.1f, c ss(min) = 0.1f (7.35 C 0.6(1.05 ? 16/8)) = 0.612f (choose 0.68f). high inductor current inrush protection the lt3478/lt3478-1 provide an integrated resistor between the v s and l pins to monitor inductor current (figure 1). during startup or hotplugging of the induc- tor supply, it is possible for inductor currents to exceed the maximum switch current limit. when inductor current exceeds 6a, the lt3478/lt3478-1 protect the internal power switch by turning it off and triggering a soft-start latch. this protection prevents the switch from repetitively turning on during excessive inductor currents by delay- ing switching until the fault has been removed. to defeat inductor current sensing the inductor supply should be connected to the l pin and the v s pin left open. see details in the applications information section soft-start.led open circuit protection and maximum pwm dimming ratios the lt3478/lt3478-1 led drivers provide optimum pro- tection from open led faults by clamping the converter output to a programmable overvoltage protection level (ovp). in addition, the programmable ovp feature draws zero current from the output during pwm = 0 to allow higher pwm dimming ratios. this provides an advantage over other led driver applications which connect a resistor divider directly from v out . an open led fault occurs when the connection to the led(s) becomes broken or the led(s) fails open. for an led driver using a step-up switching regulator, an open circuit led fault can cause the converter output to exceed the voltage capabilities of the regulators power switch, causing permanent damage. when v out exceeds ovp, the figure 11. lt3478 fault detection and ss pin timing diagram sw 3478 f11 ss 0.65v (active threshold)0.25v (reset threshold) 0.15v soft-start latch reset: faults triggeringsoft-start latch with sw turned off immediately: v in < 2.8v or shdn < 1.4v orv out > ovp or i (inductor) > 6a ss < 0.25v andv in > 2.8v and shdn > 1.4v andv out < ovp and i (inductor) < 6a soft-start latch set: downloaded from: http:///
lt3478/lt3478-1 17 34781f applicatio s i for atio wu u u lt3478/lt3478-1 immediately stop switching, a soft-start latch is set and the ss pin is discharged. the ss latch can only be reset when v out falls below ovp and the ss pin has been discharged below 0.25v (figure 11). if the led(s) simply go open circuit and are reconnected, however, the ovp used to protect the switch might be too high for the reconnected led(s). the lt3478/lt3478-1 therefore allow ovp to be programmable to protect both the led driver switch and the led(s). (the minimum allowable ovp for normal operation for a given led string depends on the number of leds and their maximum forward voltage rat- ings.) ovp is programmed using the ovpset pin (front page), given by, ovp = (ovpset ? 41)v where the programmable range for the ovpset pin is 0.3v to 1v resulting in an ovp range of 12.3v to 41v. the ovpset pin can be programmed with a single resistor by tapping off of the resistor divider from v ref used to program ctrl1. if both ctrl1 and ctrl2 are connected directly to v ref (maximum led current setting) then ovp- set requires a simple 2 resistor divider from v ref . thermal calculations to maximize output power capability in an application without exceeding the lt3478/lt3478-1 125c maximum operational junction temperature, it is useful to be able to calculate power dissipation within the ic. the power dissipation within the ic comes from four main sources: switch dc loss, switch ac loss, inductor and led cur- rent sensing and input quiescent current. these formulas assume a boost converter architecture, continuous mode operation and no pwm dimming. (1) switch dc loss = p sw(dc) = (r sw ? i l(ave) 2 ? d) r sw = switch resistance = 0.07 (at t j = 125c) i l(ave) = p out /( ? v s ) p out = v out ? i led = converter ef? ciency = p out /(p out + p loss ) v s = inductor supply input d = switch duty cycle = (v out + v f C v s )/(v out + v f C v sat ) v f = forward voltage drop of external schottky diode v sat = i l(ave) ? r sw (2) switch ac loss = p sw(ac) = t eff (1/2)i l(ave) (v out + v f )(f osc ) t eff = effective switch current and switch v ce voltage overlap time during turn on and turn off = 2 ? (t isw + t vsw ) t isw = i switch rise/fall time = i l(ave) ? 2ns t vsw = sw fall/rise time = (v out + v f ) ? 0.7ns f osc = switching frequency (3) current sensing loss = p sense = p sense(il) + p sense(iled) p sense(il) = i l(ave) 2 ? 9.5m p sense(iled) = i led 2 ? 0.1 (4) input quiescent loss = p q = v in ? i q where i q = (6.2ma + (100ma ? d)) example (using lt3478-1): for v in = v s = 8v, i led = 700ma, v out = 24.5v (7 leds), v f = 0.5v and f osc = 0.2mhz, = 0.89 (initial assumption) i l(ave) = (24.5 ? 0.7)/(0.89 ? 8) = 2.41a d = (24.5 + 0.5 C 8)/(24.5 + 0.5 C 0.17) = 0.684 t eff = 2 ? ((2.41 ? 2)ns + (24.5 + 0.5) ? 0.7)ns = 45ns total power dissipation: p ic = p sw(dc) + p sw(ac) + p sense + p q p sw(dc) = 0.07 ? (2.41) 2 ? 0.684 = 0.278w p sw(ac) = 45ns ? 0.5 ? 2.41 ? 25 ? 0.2mhz = 0.271w p sense = ((2.41) 2 ? 0.0095) + ((0.7) 2 ? 0.1) = 0.104w p q = 8 ? (6.2ma + (100ma ? 0.684)) = 0.597w p ic = 0.278 + 0.271 + 0.104 + 0.597 = 1.25w downloaded from: http:///
lt3478/lt3478-1 18 34781f local heating from the nearby inductor and schottky diode will also add to the ? nal junction temperature of the ic. based on empirical measurements, the effect of diode and inductor heating on the lt3478-1 junction temperature can be approximated as: t j (lt3478-1) = 5c/w ? (p diode + p inductor ) p diode = (1 C d) ? v f ? i l(ave) 1 C d = 0.316 v f = 0.5v i l(ave) = 2.41 p diode = 0.316 ? 0.5 ? 2.41 = 0.381w p inductor = i l(ave) 2 ? dcr dcr = inductor dc resistance (assume 0.05 ) p inductor = (2.41) 2 ? 0.05 = 0.29w the lt3478/lt3478-1 use a thermally enhanced fe pack- age. with proper soldering to the exposed pad on the underside of the package combined with a full copper plane underneath the device, thermal resistance ( ja ) will be about 35c/w. for an ambient temperature of t a = 70c, the junction temperature of the lt3478-1 for the example application described above, can be calculated as: t j (lt3478-1) = t a + ja (p tot ) + 5(p diode + p inductor ) = 70 + 35(1.25) + 5(0.671) = 70 + 44 + 4 = 118c in the above example, ef? ciency was initially assumed to be = 0.89. a lower ef? ciency ( ) for the converter will increase i l(ave) and hence increase the calculated value for t j . can be calculated as: = p out /(p out + p loss ) p out = v out ? i led = 17.15w p loss (estimated) = p ic + p diode + p inductor = 1.92w = 17.15/(17.15 + 1.92) = 0.9 if an application is built, the inductor current can be mea-sured and a new value for junction temperature estimated. ideally a thermal measurement should be made to achieve the greatest accuracy for t j . note: the junction temperature of the ic can be reduced if a lower v in supply is available C separate from the inductor supply v s . in the above example, driving v in from an available 3v source (instead of v s = 8v) reduces input quiescent losses in item(4) from 0.597w to 0.224w, resulting in a reduction of t j from 118c to 105c. layout considerationsas with all switching regulators, careful attention must be given to pcb layout and component placement to achieve optimal thermal,electrical and noise performance (figure 12). the exposed pad of the lt3478/lt3478-1 (pin 17) is the only gnd connection for the ic. the exposed pad should be soldered to a continuous copper ground plane underneath the device to reduce die temperature and maximize the power capability of the ic. the ground path for the r t resistor and v c capacitor should be taken from nearby the analog ground connection to the exposed pad (near pin 9) separate from the power ground connection to the exposed pad (near pin 16). the bypass capacitor for v in should be placed as close as possible to the v in pin and the analog ground connection. sw pin voltage rise and fall times are designed to be as short as possible for maximum ef? ciency. to reduce the effects of both radiated and conducted noise, the area of the sw trace 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 sw node to minimize this high frequency switching path. to minimize led current sensing errors for the lt3478, the terminals of the external sense resistor r sense should be tracked to the v out and led pins separate from any high current paths. applicatio s i for atio wu u u downloaded from: http:///
lt3478/lt3478-1 19 34781f typical applicatio s u 15w, 6 leds at 700ma, boost led driver lt3478-1 shdnv ref out led ctrl2 ovpsetctrl1 pwm ss v c r t v in v s ls w c210 f 25v r1 45.3k r4 54.9k r t 69.8k f osc = 500khz 100hz r2 130k pwmdimming ratio = 1000:1 l1: cdrh104r-100ncd1: pds560 q1: si2318ds leds: luxeon iii (white) 700ma d1 3478 ta02a l1 10 h 3.3v 0v v in 8v to 16v c1 4.7 f 25v c c 0.1 f c ss 1 f r3 10k q1 pwm dimming ratio = 1000:1(see efficiency on page 1) pwm 5v/div inductor current 1a/div i led 0.5a/div 3478 ta02b 2 s/div f pwm = 100hz lt3478-1 pwm dimming waveforms applicatio s i for atio wu u u 3252 f08 output capacitor c vs v s c vin v in inductor 12 3 4 5 6 7 8 lt3478/lt3478-1 1615 14 13 12 11 10 9 exposed pad pin 17 power gnd analog gnd v in bypass cap v out gnd (connect multiple ground planesthrough vias underneath the ic) solder the exposed pad (pin 17) to the entire copper ground plane underneath the device swsw v in v s l v out led ovpset l sw ssr t pwmctrl2 ctrl1 shdn v ref v c c ss c c r t r r c c f c r sense (lt3478 only) r schottky diode r rr r figure 12. recommended layout for lt3478/lt3478-1 (boost con? guration) downloaded from: http:///
lt3478/lt3478-1 20 34781f typical applicatio s u 16w, 12 leds at 350ma, buck-boost mode led driver plus lt3003 lt3478-1 shdnv ref out led ctrl2 ctrl1ovpset pwm ss v c r t v in v s ls w c210 f 50v c3 3.3 f 10v r1 24k r t 69.8k f osc = 500khz 100hz r2 100k pwmdimming ratio = 200:1 l1: cdrh105r-8r2d1: pds560 d2: 7.5v zener leds: luxeon i (white) 1.05a d1 3478 ta04a l1 8.2 h v in 5v 3.3v 0v v s 12v to 16v c1 4.7 f 25v v c v c v out v out c c 0.1 f c ss 1 f c4 1 f lt3003 v max ot1ot2 v in pwm gnd v ee led1 led2 led3 d2 v s (v) efficiency (%) 9085 80 75 70 50 55 60 65 3478 ta04b 12 leds(4 series x 3 channels) luxeon i (white) 12 14 16 15 13 v in = 5v i led = 350ma f osc = 500khz pwm duty cycle = 100% ef? ciency vs input v s lt3478-1 shdnv ref out led ctrl2 ctrl1ovpset pwm ss v c r t v in v s ls w c23.3 f 25v c3 3.3 f 10v r1 24k r t 31.6k f osc = 1mhz 100hz r2 100k pwmdimming ratio = 3000:1 l1: cdrh104r-5r2d1: pds560 leds: luxeon i (white) 1.05a d1 3478 ta03a l1 5.2 h v in 3.3v 3.3v 0v v s 8v to 14v c1 4.7 f 16v v c v c v in v out v out c c 0.1 f c ss 1 f lt3003 v max ot1ot2 v in pwm gnd v ee led1 led2 led3 v s (v) 8 efficiency (%) 9085 80 75 70 3478 ta03b 10 14 12 v in = 3.3v i led = 350ma f osc = 1mhz pwm duty cycle = 100% 15 leds(5 series x 3 channels) luxeon i (white) 17w, 15 leds at 350ma, boost led driver plus lt3003 ef? ciency vs input v s downloaded from: http:///
lt3478/lt3478-1 21 34781f typical applicatio s u lt3478-1 shdnv ref out led ctrl2 ctrl1ovpset pwm ss v c r t v in v s ls w c24.7 f 16v r1 100k r4 510 ? r5 510 ? r t 69.8k f osc = 500khz 1khz r2 34k pwmdimming ratio = 200:1 l1: cdrh105r-6r8d1: b320 q1: si2302ads q2: si2315bds led: luxeon iii (white) 1a q1 q2 d1 3478 ta06a l1 6.8 h 3.3v 0v v in 3.8v to 6.5v nimh 4 c1 10 f 10v c c 0.1 f c ss 1 f r3 10k off on 357 6 4 v in (v) efficiency (%) 8075 70 50 55 60 65 3478 ta06b single ledluxeon iii (white) i led = 1a f osc = 500khz pwm duty cycle = 100% 4w, 1 led at 1a, buck-boost mode led driver ef? ciency vs v in downloaded from: http:///
lt3478/lt3478-1 22 34781f typical applicatio s u lt3478 shdnv ref pwm ctrl2 ctrl1ovpset ss v c r t v in v s l out led sw r1 24k r sense 0.068 ? 1.5a r4 365 ? r t 69.8k f osc = 500khz 100hz r2 100k pwmdimming ratio = 3000:1 l1: cdrh105r-100d1: pds560 q1: 2n7002 q2: si2319ds leds: lxk2 (white) typical efficiency = 90%for conditions/components shown (pwm duty cycle = 100%, t a =25 c) q2 d1 3478 ta07a l110 h 4 leds 3.3v 0v pv in 32v c1 3.3 f 50v c3 10 f 25v c c 0.1 f c ss 1 f c2 4.7 f 10v v in 3.3v q1 r3 10k r5 510 ? 24w, 4 leds at 1.5a, buck mode led driver downloaded from: http:///
lt3478/lt3478-1 23 34781f 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 descriptio u fe package 16-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663) exposed pad variation bc fe16 (bc) 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) 134 5 6 7 8 10 9 4.90 ?5.10* (.193 ?.201) 16 1514 13 12 11 1.10 (.0433) max 0.05 ?0.15 (.002 ?.006) 0.65 (.0256) bsc 2.94 (.116) 0.195 ?0.30 (.0077 ?.0118) typ 2 recommended solder pad layout 0.45 0.05 0.65 bsc 4.50 0.10 6.60 0.10 1.05 0.10 2.94 (.116) 3.58 (.141) 3.58 (.141) 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 downloaded from: http:///
lt3478/lt3478-1 24 34781f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2007 lt 0107 ? printed in usa typical applicatio u related parts lt3478 shdnv ref out led ctrl2 ctrl1ovpset pwm ss v c r t v in v s ls w c23.3 f 25v c3 3.3 f 10v r1 8.25k r sense 0.42 ? r t 10k f osc = 2mhz 100hz r2 10k pwmdimming ratio = 1000:1 l1: cdrh6d28d1: zlls1000 q1: si2318ds leds: luxeon i (white) 250ma d1 3478 ta05a l1 10 h v in 3.3v 3.3v 0v v s 8v to 16v c1 4.7 f 25v c c 0.1 f c ss 1 f r3 10k q1 v s (v) 8 efficiency (%) 100 9590 85 80 75 70 60 65 3478 ta05b 12 16 14 10 v in = 3.3v i led = 250ma f osc = 2mhz pwm duty cycle = 100% 6 leds = luxeon i (white) 6w, 6 leds at 250ma, boost led driver ef? ciency vs input v s part number description comments lt1618 constant current, 1.4mhz, 1.5a boost converter with analog/pwm dimming v in : 5v to 18v, v out(max) = 36v, i sd <1a, ms10 package lt3003 three channel led ballaster with 3,000:1 true color pwm dimming v in : 3v to 48v, i sd <5a, msop10 package lt3474 36v, 1a (i led ), 2mhz,step-down led driver with 400:1 true color pwm dimming v in : 4v to 36v, v out(max) = 13.5v, i sd <1a, tssop16e package lt3475 dual 1.5a(i led ), 36v, 2mhz,step-down led driver 3,000:1 true color pwm dimming v in : 4v to 36v, v out(max) = 13.5v, i sd <1a, tssop20e package lt3476 quad output 1.5a, 2mhz high current led driver with 1,000:1 true color pwm dimming v in : 2.8v to 16v, v out(max) = 36v, i sd <10a, 5mm 7mm qfn package lt3477 42v, 3a, 3.5mhz boost, buck-boost, buck led driver with analog/ pwm dimming v in : 2.5v to 25v, v out(max) = 40v, i sd <1a, qfn, tssop20e packages lt3479 3a, 3.5mhz full featured dc/dc converter with soft-start and inrush current protection and analog/pwm dimming v in : 2.5v to 24v, v out(max) = 40v, i sd <1a, 4mm 3mm dfn, tssop16e packages lt3486 dual 1.3a , 2mhz high current led driver with 1,000:1 true color pwm dimming v in : 2.5v to 24v, v out(max) = 36v, i sd <1a, 5mm 3mm dfn, tssop16e packages ltc3783 high current led controller with 3,000:1 true color pwm dimming v in : 3v to 36v, v out(max) = ext fet, i sd <20a, 5mm 4mm dfn, tssop16e packages downloaded from: http:///

▲Up To Search▲

Price & Availability of LT3478EFE-1PBF

	To Download LT3478EFE-1PBF Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .