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lt3597 1 3597fa typical application description 60v triple step-down led driver the lt ? 3597 is a 60v triple step-down led driver capable of achieving 10,000:1 digital pwm dimming at 100hz with fast npn current sources driving up to 10 leds in each channel. led dimming can also be achieved via analog control of the ctrl1-3 pin. the step-down switching frequency is programmed be - tween 200khz and 1mhz. the frequency is also synchr oniz - able to an external clock. the lt3597 provides maximum led brightness while adhering to manufacturers? speci - ?cations for thermal derating. the derate temperature is programmed by placing a negative temperature coef?cient (ntc) resistor on the master control pin. the lt3597 adaptively controls v out in order to achieve optimal ef?ciency. other features include: 2% led cur - rent matching between channels, open led reporting, shorted led protection, programmable led current, and programmable temperature protection. l , lt, ltc and ltm 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. triple step-down rgb single pixel led driver, 100ma current features applications n three 100ma buck regulators, each drives up to 10 leds with fast npn current sources n fast current sources for <1s pulse widths (10,000:1 true color pwm? dimming at 100hz) n leds disconnected in shutdown n adaptive v out for increased ef?ciency n 6v to 60v input voltage range n 2% led current matching n external resistor sets led current for each channel n internal compensation and soft-start n programmable switching frequency (200khz to 1mhz) n synchronizable to external clock n open led detection and reporting n shorted led pin protection and reporting n programmable led thermal derating n programmable temperature protection n 5mm 8mm thermally enhanced qfn package with a 0.6mm high voltage pin spacing n led billboards and signboards n mono, multi, full-color led displays n large screen display led backlighting n automotive, industrial, and medical displays 10,000:1 pwm dimming at 100hz 3597 ta01a lt3597 v in en/uvloboost1 sw1 da1 fb1 v out1 led1bias fault pwm1-3 ctrl1-3 sync rt boost2 sw2 da2 fb2 v out2 led2 boost3 sw3 da3 fb3 v out3 led3 v ref t set ctrlm lt3597 9.1k r 100k 97k 91k 270k 3.3f 0.1f 10f 100h 0.1f 3.3f 100h 9.1k 97k 0.1f 3.3f 100h 9.1k 97k 100k 10k 82.5k v ref 49.9k 10f v cc 5v v out1 v in 48v 33.2k (1mhz) i set1 i set2 i set3 gnd 20k 20k 20k v out2 v out3 3 3 gb i led 50ma/div pwm 2v/div 3597 ta01b 200ns/div downloaded from: http:///
lt3597 2 3597fa pin configuration absolute maximum ratings input voltage (v in ), en/uvlo ................................... 60v boost1-3 ................................................................. 85v boost pin above sw pin ........................................ 25v led1-3, v out1-3 ........................................................ 42v bias, fault .............................................................. 25v v ref , rt, i set1-3 , t set , ctrlm ................................... 3v fb1-3, ctrl1-3, pwm1-3, sync ............................... 6v operating junction temperature range (notes 2, 3) ........................................ ?40c to 125c maximum junction temperature........................... 125c storage temperature range ................... ?65c to 150c (note 1) 18 19 20 21 top view 53 gnd uhg package variation: uhg52(39) 52-lead (5mm 8mm) plastic qfn 22 23 24 25 26 51 50 48 46 44 37 39 41 43 7 6 4 2 bst1 sw1 da1 fb1 en/uvlo t set v ref ctrlm i set1 i set2 i set3 rt da3sw3 bst3 bias fb3 fb2 ctrl1 ctrl2 ctrl3 pwm1 pwm2 pwm3 sync v in v in bst2sw2 da2 gnd v out1 led1led2 v out2 v out3 led3 fault nc 3534 33 32 31 30 29 28 27 9 11 12 13 14 15 16 17 t jmax = 125c, ja = 32c/w exposed pad (pin 53) is gnd, must be soldered to pcb order information lead free finish tape and reel part marking* package description temperature range lt3597euhg#pbf lt3597euhg#trpbf 3597 52-lead (5mm 8mm) plastic qfn ?40c to 125c lt3597iuhg#pbf lt3597iuhg#trpbf 3597 52-lead (5mm 8mm) plastic qfn ?40c to 125c consult ltc marketing for parts speci?ed with wider operating temperature ranges. *the temperature grade is identi?ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ?nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci?cations, go to: http://www.linear.com/tapeandreel/ downloaded from: http:///
lt3597 3 3597fa electrical characteristics parameter conditions min typ max units v in operating voltage l 6 55 v quiescent current from v in en/uvlo = 0.4v bias = 5v, not switching bias = 0v, not switching 2.1 4 2 4 6 a ma ma minimum bias voltage 3 3.1 v quiescent current from bias en/uvlo = 0.4v bias = 5v, not switching bias = 0v, not switching, current out of pin 2 60 2 3 150 a ma a en threshold (falling) 0.4 0.7 v uvlo threshold (falling) 1.47 1.51 1.53 v en/uvlo pin current (hysteresis) en/uvlo = 1.6v en/uvlo = 1.4v 4 10 5.1 6 na a fb1-3 regulation voltage 1.15 1.22 1.25 v fb1-3 pin bias current fb = 6v 200 na maximum duty cycle r t = 220k (200khz) r t = 33.2k (1mhz) 90 78 95 85 % % switch saturation voltage i sw1-3 = 100ma 265 mv switch current limit 400 510 700 ma da pin current to stop osc 225 280 350 ma switch leakage v sw1-3 = 0v 700 na bst1-3 pin current i sw1-3 = 100ma 2 ma switching frequency r t = 220k r t = 33.2k 170 900 200 1000 230 1100 khz khz sync input low 0.4 v sync input high 1.6 v sync frequency range r t = 220k r t = 47k 240 1000 khz khz sync pin bias current v sync = 3.3v 200 na soft-start time (note 4) 2.5 ms v ref voltage i vref = 0a l 1.96 2.0 2.04 v maximum v ref current 200 a i set1-3 pin voltage r iset1-3 = 20k, v ctrl = v ctrlm = t set = 1.5v 1.0 v t set voltage for led current derating 540 mv t set pin leakage current v tset = 1.0v 200 na i led1-3 led current r iset1-3 = 20k l 98 97 100 100 102 103 ma ma the l denotes the speci?cations which apply over the full operating temperature range, otherwise speci?cations are at t a = 25c. v in = 24v, boost = 29v, bias = 5v, en/uvlo = 5v, pwm1-3 = 3.3v, ctrl1-3 = ctrlm = t set = 2.0v, v out1-3 = 24v, sync = 0v, unless otherwise speci?ed. (note 2) downloaded from: http:///
lt3597 4 3597fa electrical characteristics the l denotes the speci?cations which apply over the full operating temperature range, otherwise speci?cations are at t a = 25c. v in = 24v, boost = 29v, bias = 5v, en/uvlo = 5v, pwm1-3 = 3.3v, ctrl1-3 = ctrlm = t set = 2.0v, v out1-3 = 24v, sync = 0v, unless otherwise speci?ed. (note 2) parameter conditions min typ max units led string current matching r iset1-3 = 20k l 0.35 0.35 1.5 2 % % led pin voltage adaptive v out loop enabled 1.1 v led1-3 open detection threshold 0.28 v led1-3 short protection threshold (from gnd) pwm1-3 = 3.3v 10 15 v led1-3 short protection threshold (from v out1-3 ) v out1-3 = 6v 1 1.25 2 v led1-3 pin leakage current v led1-3 = 42v 200 na pwm1-3 input low voltage 0.4 v pwm1-3 input high voltage 1.6 v pwm1-3 pin bias current 200 na ctrl1-3 voltage for full led current 1.2 v ctrl1-3 pin bias current ctrl1-3 = 6v 200 na ctrlm voltage for full led current 1.2 v ctrlm pin bias current ctrlm = 3v 200 na fault output voltage low i fault = 200a 0.11 v fault pin input leakage current fault = 25v 200 na 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 lt3597e is guaranteed to meet performance speci?cations from 0c to 125c junction temperature. speci?cations over the ?40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the lt3597i speci?cations are guaranteed over the full ?40c to 125c operating junction temperature range. note 3: for maximum operating ambient temperature, see thermal considerations in the applications information section.note 4: guaranteed by design. downloaded from: http:///
lt3597 5 3597fa typical performance characteristics uvlo threshold voltage (falling) en/uvlo pin bias current v ref voltage switching frequency current limit switch voltage drop v in quiescent current v bias quiescent current shutdown current t a = 25c, unless otherwise noted. 0 v in current (ma) 1.0 1.50.5 40 60 3597 g01 0 2.0 3.02.5 30 10 20 50 v in voltage (v) ?40c 25c v bias = 5v 125c 0 v bias current (ma) 1.0 1.50.5 0 15 25 3597 g02 ?0.5 2.0 3.02.5 10 5 20 v bias voltage (v) 125c v in = 24v ?40c 25c ?50 current (a) 1.00 1.250.50 0.750.25 75 125 3597 g03 0 1.50 2.001.75 50 25 0 ?25 100 temperature (c) i bias i vin v in = 55v v bias = 25v ?50 uvlo voltage (v) 1.4 1.51.2 1.31.1 1.0 75 125 3597 g04 1.6 1.81.7 50 25 0 ?25 100 temperature (c) ?50 current (a) 4 52 31 75 125 3597 g05 0 6 50 25 0 ?25 100 temperature (c) en/uvlo = 1.4v en/uvlo = 1.6v ?50 v ref voltage (v) 2.02 2.032.00 2.011.99 1.98 1.97 75 125 3597 g06 1.96 2.04 50 25 0 ?25 100 temperature (c) v in = 6v v in = 55v v in = 24v v ref load = 0a ?50 frequency (mhz) 0.8 1.00.6 0.4 0.2 75 125 3597 g07 0 1.2 50 25 0 ?25 100 temperature (c) r t = 33.2k r t = 220k ?50 current (ma) 400 500300 200 100 75 125 3597 g08 0 600 50 25 0 ?25 100 temperature (c) da switch 0 switch voltage drop (mv) 400 500300 200 100 250 3597 g09 0 600 200 150 100 50 switch current (ma) ?40c 25c v bias = 5v 125c downloaded from: http:///
lt3597 6 3597fa typical performance characteristics loop regulation voltage led current led current matching led current vs pwm duty cycle led current vs ctrl adaptive loop operation boost diode v f soft-start 60v switching waveforms t a = 25c, unless otherwise noted. 0 diode v f (mv) 400 500 600 700300 200 100 10 3597 g10 0 800 8 6 4 2 diode current (ma) ?40c 25c 125c v out 20v/div i l 100ma/div v sw 20v/div v en/uvlo 5v/div 3597 g11 400s/div v out 20v/div i l 100ma/div v sw 20v/div 3597 g12 400ns/div ?50 voltage (v) 1.1 1.21.0 75 125 3597 g13 0.9 1.41.3 50 25 0 ?25 100 v fb temperature (c) v led (adaptive loop) ?50 led current (ma) 99 100 98 75 125 3597 g14 97 103101 102 50 25 0 ?25 100 ch2 ch3 temperature (c) ch1 ?50 led current matching (%) 0 0.25 ?0.25 75 125 3597 g15 ?0.50 0.50 50 25 0 ?25 100 ch2 ch3 temperature (c) ch1 0.001 led current (ma) 0.1 1 0.01 0.001 100 3597 g16 0.0001 100 10 10 1 0.1 0.01 duty cycle (%) 0 led current (ma) 60 8040 20 1.5 3597 g17 0 120100 1.25 1 0.75 0.5 0.25 ctrl voltage (v) v out 10v/div v led 10v/div i led 100ma/div v sw 20v/div 3597 g18 400 s/div downloaded from: http:///
lt3597 7 3597fa typical performance characteristics shorted leds fault fault pin voltage low t set voltage for temperature derating t set led current derating 20,000:1 pwm dimming (100hz) 1,000:1 pwm dimming (100hz) open led fault t a = 25c, unless otherwise noted. i led 50ma/div pwm 2v/div 3597 g19 100ns/div i led 50ma/div pwm 2v/div 3597 g20 2s/div v fault 5v/div i led 100ma/div v led 2v/div v out 5v/div 3597 g21 20ms/div v fault 5v/div i led 100ma/div v led 5v/div v out 5v/div 3597 g22 10ms/div 0 fault pin voltage (mv) 100 150 50 0.6 1 3597 g23 0 400200 250 300 350 0.4 0.2 0.8 fault pin current (ma) 125c 25c 40c ?50 t set voltage (v) 0.4 0.5 75 125 3597 g24 0.3 0.80.6 0.7 50 25 0 ?25 100 temperature (c) 25 led current (ma) 4020 60 65 85 125 3597 g25 0 120 80 100 45 105 temperature (c) v tset = 0.675v downloaded from: http:///
lt3597 8 3597fa pin functions boost1, boost2, boost3 (pins 2, 48, 39): boost capacitor pin. this pin is used to provide a voltage that is higher than the input voltage when the switch is on to supply current to the switch driver. sw1, sw2, sw3 (pins 4, 46, 41): switch pin. connect the inductor, catch diode and boost capacitor to this pin. da1, da2, da3 (pins 6, 44, 43): catch diode anode. this pin is used to provide frequency foldback in extreme situations. fb1, fb2, fb3 (pins 7, 34, 35): feedback pin. this pin is regulated to the internal bandgap voltage. the maximum buck output voltage can be set by connecting this pin to a resistor divider from vout1-3. en/uvlo (pin 9): enable and undervoltage lockout pin. accurate 1.51v threshold. uvlo threshold can be pro - grammed by using a resistor divider from v in . if function is not required, pin can be tied to the vin pin.t set (pin 11): thermal regulation pin. programs the lt3597 junction temperature at which led current begins to derate. v ref (pin 12): 2.0v reference output pin. this pin can sources up to 200a and can be used to program tset and ctrlm. ctrlm (pin 13): master control pin. led current derating vs temperature is achievable for all channels if the voltage on ctrlm has a negative temperature coef?cient using an external ntc resistor in a voltage divider from vref. i set1 , i set2 , i set3 (pins 14, 15, 16): led current pro - gramming pin. a resistor to ground programs full-scale led current. rt (pin 17): switching frequency programming pin. a resistor to ground programs the switching frequency between 200khz and 1mhz. gnd (pin 18, exposed pad pin 53): ground pin. this is the ground for both the ic and the switching converters. exposed pad must be soldered to pcb ground. v out1 , v out2 , v out3 (pins 19, 22, 23): buck output. this is the buck regulator output voltage sense into the ic.led1, led2, led3 (pins 20, 21, 24): constant current sink pin. these are 3 led driver outputs, each containing an open collector, constant current sink. all outputs are matched within 2% and are individually programmed up to 100ma using an external resistor at the iset1-3 pin. outputs are rated to allow a maximum vout1-3 of 42v. connect the cathode of the led string to led1-3. connect the anode of the led string to vout1-3. fault (pin 25): fault detection pin. open collector pin used to report open led faults. fault must be externally pulled to a positive supply through a resistor. nc (pin 26): no connection pin. tie to ground. sync (pin 27): external clock synchronization pin. when an external clock drives this pin, the buck regulators are synchronized to that frequency. frequency programmed by the rt pin resistor must be at least 20% less than the sync pin clock frequency. pwm1, pwm2, pwm3 (pins 30, 29, 28): pwm dimming control pin. when driven to a logic high, the led1-3 current sink is enabled. if pwm dimming is not desired, connect the pin to vref. channels can be individually disabled by tying pwm1-3 to ground. ctrl1, ctrl2, ctrl3 (pins 33, 32, 31): analog dimming control pin. this pin is used to dim the led current in an analog fashion. if the pin is tied to a voltage lower than 1.0v, it will linearly reduce the led current. if feature is unused, connect the pin to vref. bias (pin 37): supply pin. this pin is the supply for an internal voltage regulator to internal analog and digital circuitry. bias must be locally bypassed with a 2.2f capacitor. v in (pins 50, 51): input supply pin. v in must be locally bypassed with a 10f capacitor to ground. pins 50 and 51 are internally fused. downloaded from: http:///
lt3597 9 3597fa block diagram figure 1. block diagram 3597 bd bias start-up reference osc slope comp +? s r q + ? ? + 1.22v vc +? 1.1v 540mv ptat gnd, exposed pad v in en/uvlosync rt t set ctrlmctrl n pwm n i set n v ref bias sw n da n gnd fb n led n fault v out n bst n channel n n = 1-3 lt3597 +? soft-start and clamp logic led fault detection conversion and control pwm dimming logic led drive circuitry downloaded from: http:///
lt3597 10 3597fa operation the lt3597 uses a constant frequency, internally com - pensated peak current mode control scheme. operation is best understood by referring to the block diagram in figure 1. enable and undervoltage lockout (uvlo) are both con - trolled by a single pin. if the pin falls below 1.51v, an accurate comparator turns off the led drivers and buck regulators. if the pin continues to fall to less than 0.4v, the part enters shutdown and consumes less than 2a. the lt3597 contains three constant current sink led drivers. each of the three led strings is powered from a dedicated adaptive buck converter in order to achieve maximum ef?ciency. the frequency of the buck regulators is programmed from 200khz to 1mhz using an external resistor. the frequency can also be synchronized to an external clock using the sync pin. internal buck compensation and soft-start requires few external components and permits simple board layout. a high-side switch current limit protects the internal switch during its on time, while a low side current limit prevents the switch from turning on in the case of excessive off phase current. step-down adaptive control adaptive control of the output voltages achieves superior system ef?ciency. when a given channel?s pwm pin is low, the respective buck regulator output will go to a programmable high output voltage. in this case the buck will enter into a pulse-skipping mode since there is no load connected. this guarantees that the buck output voltage is high enough to immediately supply the led current once the string is reactivated. as soon as pwm goes high, the output voltage of the buck will drop until there is 1v across the led current sink. this scheme ensures the best ef?ciency for each led channel. since each led string is independently driven from a separate buck chan - nel, ef?ciency is optimized for all three strings even if the number of leds is mismatched between the channels. another bene?t of this regulation method is that the lt3597 starts up with 10,000:1 dimming even if the pwm1-3 pulse width is 1s. since v out starts up even if pwm1-3 is low, the part achieves high dimming ratios with narrow pulse widths within a couple of pwm1-3 clock cycles. led current each led string current can be individually programmed up to a maximum of 100ma with a 2% matching accuracy between the strings. an external resistor on the i set1-3 pin programs the max current for that string. the ctrl1-3 pin can be used for analog dimming. digital pwm can be programmed using the pwm1-3 pin. a dimming ratio of 10,000:1 can be achieved at a frequency of 100hz. fault protection and reporting the lt3597 protects against both open led and shorted led conditions. if the led1-3 pin voltage exceeds 12v while the led string is sinking current, or if the led1-3 pin voltage is within 1.25v of v out1-3 pin voltage, the channel is disabled until the fault is removed. if the led string opens, the lt3597 will limit the output to the voltage set by the fb resistor divider. the lt3597 reports a fault on the fault pin if any of the led strings is open or shorted. led faults are only reported if the respective string pwm signal is high. a fault is also reported if the internal die temperature reaches the t set programmed derating limit. downloaded from: http:///
lt3597 11 3597fa inductor selection inductor values between 100h and 470h are recom - mended for most applications. it is important to choose an inductor that can handle the peak current without saturating. the inductor dcr (copper wire resistance) must also be low in order to minimize i 2 r power losses. table 1 lists several recommended inductors. table 1. recommended inductors part l (h) dcr () current rating (a) vendor lps6225 mss1038 mss1038 mss1038 100 100 220 470 0.61 0.3 0.76 1.24 0.52 1.46 0.99 0.70 coilcraft www.coilcraft.com cdrh105r cdrw105r cdrh105r cdr6d28mn 100 220 470 100 0.253 0.50 1.29 0.9 1.35 0.94 0.60 0.75 sumida www.sumida.com ds1262c2 ds1262c2 ds1262c2 100 220 470 0.17 0.35 1.243 1.5 1.0 0.7 toko www.toko.com slf10145t slf10145t 100 220 0.26 0.47 1.0 0.7 tdk www.tdk.com dr73 dr73 100 220 0.527 1.05 0.79 0.53 coiltronics www.cooperet.com capacitor selectionlow esr (equivalent series resistance) capacitors should be used at the outputs 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. table 2 lists some suggested manufacturers. consult the manufacturers for detailed information on their entire selection of ceramic surface mount parts. table 2. recommended ceramic capacitor manufacturers taiyo yuden www.t-yuden.com avx www.avxcorp.com murata www.murata.com kemet www.kemet.com tdk www.tdk.com applications information typically 10f capacitors are suf?cient for the v in and bias pins. the output capacitor for the buck regulators depends on the number of leds and switching frequency. refer to table 3 for the proper output capacitor selection. table 3. recommended output capacitor values (v led = 3.5v) switching frequency (khz) # leds c out (f) 1000 1-3 3.3 >3 2.2 500 1-3 4.7 >3 3.3 200 1-3 15 >3 6.8 diode selection schottky diodes, with their low forward voltage drop and fast switching speed, must be used for all lt3597 ap - plications. do not use p-n junction diodes. the diode?s average current rating must exceed the application?s aver - age current. the diode?s maximum reverse voltage must exceed the application?s input voltage. table 4 lists some recommended schottky diodes. table 4. recommended diodes part max current (a) max reverse voltage (v) manufacturer dfls160 b160 1 1 60 60 diodes, inc. www.diodes.com cmmsh1-60 1 60 central www.centralsemi.com esipb 1 100 vishay www.vishay.com downloaded from: http:///
lt3597 12 3597fa applications information undervoltage lockout (uvlo) the en/uvlo can be used to program the input uvlo threshold by connecting it to a resistor divider from the v in pin as shown in figure 2. programming maximum led currentmaximum led current can be programmed by placing a resistor (r iset1-3 ) between the iset1-3 pin and ground. r iset1-3 values between 20k and 100k can be chosen to set the maximum led current between 100ma and 20ma respectively. the led current is programmed according to the follow- ing equation: i led1-3 = 2 ? 1v r iset1-3 (ma) see table 5 and figure 3 for resistor values and corre- sponding programmed led current. table 5. led current programming r iset1-3 value (k) led current (ma) 20 100 25 80 33.3 60 50 40 100 20 figure 3. r iset1-3 value for led current r set1-3 (k) 0 led current (ma) 100 8040 6020 0 25 75 3597 f03 100 50 figure 2. en/uvlo control lt3597 3597 f02 +? 1.51v en/uvlo r2 r1 v in select r1 and r2 according to the following equation: v in(uvlo) = 1.51v ? 1 + r2 r1 ?? ? ?? ? in uvlo an internal 5.1a pull-down current source is con - nected to the pin for programmable uvlo hysteresis. the hysteresis can be set according to the following equation: v uvlo(hyst) = 5.1a ? r2 care must be taken if too much hysteresis is programmed, the pin voltage might drop too far and cause the current source to saturate. once the en/uvlo pin falls below 0.4v, the part enters into shutdown. downloaded from: http:///
lt3597 13 3597fa applications information figure 4. led current vs ctrl1-3 voltage figure 5. led current using pwm dimming ctrl1-3 (v) 0 led current (ma) 120100 8040 6020 0 0.25 1.25 1 0.75 3597 f04 1.5 0.5 led current dimming two different types of dimming control are available with the lt3597. the led current can be dimmed using the ctrl1-3 pin or the pwm1-3 pin. for some applications, a variable dc voltage that adjusts the led current is the preferred method for brightness control. in that case, the ctrl1-3 pin can be modulated to set the led dimming (see figure 4). as the ctrl1-3 pin voltage rises from 0v to 1.0v, the led current increases from 0ma to the maximum programmed led current in a linear fashion. as the ctrl1-3 pin continues to increase past 1.0v, the maximum programmed led current is maintained. if this type of dimming control is not desired, the ctrl1-3 pin can be tied to v ref . for true color pwm dimming, the lt3597 provides up to 10,000:1 pwm dimming range at 100hz. this is achieved by allowing the duty cycle of the pwm1-3 pin to be reduced from 100% to 0.01% for a pwm frequency of 100hz (see figure 5), hence a minimum on-time of 1s and a maxi - mum period of 100ms. pwm duty cycle dimming allows for constant led color to be maintained over the entire dimming range. using the t set pin for thermal protection the lt3597 contains a special programmable thermal regulation loop that limits the internal junction tempera- ture. this thermal regulation feature provides important protection at high ambient temperatures, and allows a given application to be optimized for typical, not worst- case, ambient temperatures with the assurance that the lt3597 will automatically protect itself and the led strings under worst-case conditions. as the ambient temperature increases, so does the internal junction temperature of the part. once the programmed maximum junction temperature is reached, the lt3597 linearly reduces the led current, as needed, to maintain this junction temperature. this can only be achieved when the ambient temperature stays below the maximum pro - grammed junction temperature. if the ambient temperature continues to rise above the programmed maximum junc - tion temperature, the led current will reduce to less than 20% of the full current. a resistor divider from the v ref pin programs the maximum part junction temperature as shown in figure 6. led1-3 current pwm1-3 3597 f06 t pwm t on(pwm) max i led figure 6. programming the t set pin lt3597 3597 f07 t set r2 r1 v ref downloaded from: http:///
lt3597 14 3597fa applications information table 6 shows commonly used values for r1 and r2. choose the ratio of r1 and r2 for the desired junction temperature limit as described in figure 7. table 6. t set programmed junction temperature t j (c) r1 (k) r2 (k) 85 49.9 97.6 100 49.9 90.9 115 49.9 84.5 the t set pin must be tied to v ref if the temperature pro - tection feature is not desired. figure 8. programming the ctrlm pin led current derating using the ctrlm pin another feature of the lt3597 is its ability to program a derating curve for maximum led current versus tem - perature. led data sheets provide curves of maximum allowable led current versus temperature to warn against exceeding this current limit and damaging the led. the lt3597 allows the output leds to be programmed for maximum allowable current while still protecting the leds from excessive currents at high temperature. this is achieved by programming a voltage at the ctrlm pin with a negative temperature coef?cient using a resistor divider with temperature dependent resistance (figure 8). as ambient temperature increases, the ctrlm voltage will fall below the internal 1v voltage reference, causing led currents to be controlled by the ctrlm pin voltage. the led current curve breakpoint and slope versus tem - perature are de?ned by the choice of resistor ratios and use of temperature-dependent resistance in the divider for the ctrlm pin. lt3597 3597 f08 ctrlm r2 r1 (option a to d) v ref a r ntc c r ntc b r ntc r x d r ntc r x r y r y ?50 t set voltage (v) 0.4 0.5 75 125 0.3 0.80.6 0.7 50 25 0 ?25 100 temperature (c) figure 7. t set voltage for temperature derating downloaded from: http:///
lt3597 15 3597fa applications information ctrlm (v) 0 led current (ma) 120100 8040 6020 0 0.25 1.25 1 0.75 3597 f09 1.5 0.5 table 7 shows a list of manufacturers/distributors of ntc resistors. there are several other manufacturers avail - able and the chosen supplier should be contacted for more detailed information. if an ntc resistor is used to indicate led temperature, it is effective only if the resis - tor is placed as closely as possible to the led strings. led derating curves shown by manufacturers are listed for ambient temperature. the ntc resistor should have the same ambient temperature as the leds. since the temperature dependency of an ntc resistor can be non - linear over a wide range of temperatures, it is important to obtain a resistor?s exact value over temperature from the manufacturer. hand calculations of the ctrlm voltage can then be performed at each given temperature, resulting in the ctrlm versus temperature plotted curve. iterations of resistor value calculations may be necessary to achieve the desired break point and slope of the led current derating curve. from the ctrlm voltage, the led current can be found using the curve shown in figure 9. table 7. ntc resistor manufacturers/distributors murata www.murata.com tdk corporation www.tdk.com digi-key www.digikey.com if calculating the ctrlm voltage at various temperatures gives a downward slope that is too strong, use alternative resistor networks (b, c, d in figure 8). they use tempera - ture independent resistance to reduce the effects of the ntc resistor over temperature. figure 10. programming switching frequency 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 volt - age curve (ctrlm behavior) over temperature. referred to as the ?murata chip ntc thermistor output voltage simulator,? users can log onto www.murata.com and download the software followed by instructions for creat - ing an output voltage v out (ctrlm) from a speci?ed v cc supply (v ref ). the ctrlm pin must be tied to v ref if the temperature derating function is not desired.programming switching frequency the switching frequency of the lt3597 can be programmed between 200khz and 1mhz by an external resistor con - nected between the rt pin and ground. do not leave this pin open. see table 8 and figure 10 for resistor values and corresponding frequencies. table 8. r t resistor selection switching frequency (mhz) r t value (k) 1.0 33.2 0.5 80 0.2 220 r t (k) 25 50 switching frequency (mhz) 1.21.0 0.8 0.4 0.60.2 0 175 150 125 100 75 3597 f10 225 200 figure 9. led current vs ctrlm voltage downloaded from: http:///
lt3597 16 3597fa applications information selecting the optimum switching frequency depends on several factors. inductor size is reduced with higher frequency, but ef?ciency drops slightly due to higher switching losses. some applications require very low duty cycles to drive a small number of leds from a high supply. low switching frequency allows a greater range of operational duty cycle and hence a lower number of leds can 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. switching frequency synchronization the nominal operating frequency of the lt3597 is pro - grammed using a resistor from the rt pin to ground over a 200khz to 1mhz range. in addition, the internal oscillator can be synchronized to an external clock applied to the sync pin. the synchronizing clock signal input to the lt3597 must have a frequency between 240khz and 1mhz, a duty cycle between 20% and 80%, a low state below 0.4v and a high state above 1.6v. synchronization signals outside of these parameters will cause erratic switching behavior. for proper operation, an r t resistor is chosen to program a switching frequency 20% slower than the sync pulse frequency. synchronization occurs at a ?xed delay after the rising edge of sync. the sync pin must be grounded if the clock synchroniza - tion feature is not used. when the sync pin is grounded, the internal oscillator controls the switching frequency of the converter. operating frequency trade-offs selection of the operating frequency is a trade-off between ef?ciency, component size, output voltage and maximum input voltage. the advantage of high frequency operation is smaller component sizes and values. the disadvantages are lower ef?ciency and lower input voltage range for a desired output voltage. the highest acceptable switch - ing frequency (f sw(max) ) for a given application can be calculated as follows: f sw(max) = v d + v out t on(min) v d + v in ? v sw ( ) figure 11. programming maximum v out1-3 where v in is the typical input voltage, v out is the output voltage, v d is the catch diode drop (0.5v) and v sw is the internal switch drop (0.5v at max load). this equation shows that slower switching is necessary to accommodate high v in /v out ratios. the reason the input voltage range depends on the switching frequency is due to the ?nite minimum switch on and off times. the switch minimum on and off times are 200ns. adaptive loop control the lt3597 uses an adaptive control mechanism to set the buck output voltage. this control scheme ensures maximum ef?ciency while not compromising minimum pwm pulse widths. when pwm1-3 is low, the output of the respective buck rises to a maximum value set by an external resistor divider to the respective fb pin. once pwm1-3 goes high, the output voltage is adaptively re- duced until the voltage across the led current sink is 1v. figure 11 shows how the maximum output voltage can be set by an external resistor divider. lt3597 3597 f11 fb1-3 r2 r1 v out1-3 v out1-3 the maximum output voltage must be set to exceed the maximum led drop plus 1v by a margin greater than 10%. however, this margin must not exceed a voltage of 10v. this ensures proper adaptive loop control. the equations below are used to estimate the resistor divider ratio. the sum of the resistors should be less than 100k to avoid noise coupling to the fb pin. v out(max) = 1.1 v led(max) + 1.1v ( ) = 1.2v ? 1 + r2 r1 ?? ? ?? ? v out(max) = v led(max) + 1.1v + v margin v margin 10v downloaded from: http:///
lt3597 17 3597fa applications information minimum input voltage the minimum input voltage required to generate an output voltage is limited by the maximum duty cycle and the output voltage (v out ) set by the fb resistor divider. the duty cycle is: dc = v d + v out v in ? v cesat + v d where v d is the schottky forward drop and v cesat is the saturation voltage of the internal switch. the minimum input voltage is: v in(min) = v d + v out(max) dc max ?? ? ?? ? + v cesat ? v d where v out(max) is calculated from the equation in the adaptive loop control section, and dc max is the minimum rating of the maximum duty cycle.fault flag the fault pin is an open-collector output and needs an external resistor tied to a supply. if the led1-3 pin volt - age exceeds 12v or if the led1-3 pin voltage is within 1.25v of v out1-3 pins while pwm1-3 is high, the fault pin will be pulled low. the fault pin will also be pulled low if the internal junction temperature exceeds the t set programmed temperature limit.there is an approximate 3s delay for fault ?ag generation when the pwm1-3 signal is enabled to avoid generating a spurious ?ag signal. the maximum current the fault can sink is typically 200a.thermal considerations the lt3597 provides three channels for led strings with internal npn devices serving as constant current sources. when led strings are regulated, the lowest led pin volt - age is typically 1v. more power dissipation occurs in the lt3597 at higher programmed led currents. for 100ma of led current with a 100% pwm dimming ratio, at least 300mw is dissipated within the ic due to current sources. thermal calculations must include the power dissipation in the current sources in addition to conventional switch dc loss, switch transient loss and input quiescent loss. in addition, the die temperature of the lt3597 must be lower than the maximum rating of 125c. this is generally not a concern unless the ambient temperature is above 100c. care should be taken in the board layout to ensure good heat sinking of the lt3597. the maximum load current should be derated as the ambient temperature ap - proaches 125c. the die temperature rise above ambient is calculated by multiplying the lt3597 power dissipation by the thermal resistance from junction to ambient. power dissipation within the lt3597 is estimated by calculating the total power loss from an ef?ciency measurement and subtracting the losses of the catch diode and the inductor. thermal resistance depends on the layout of the circuit board, but 32c/w is typical for the 5mm 8mm qfn package.board layout as with all switching regulators, careful attention must be paid to the pcb board layout and component placement. to prevent electromagnetic interference (emi) problems, proper layout of high frequency switching paths is essen- tial. minimize the length and area of all traces connected to the switching node pin (sw). always use a ground plane under the switching regulator to minimize inter- plane coupling. good grounding is essential in led fault detection. proper grounding is also essential for the external resistors and resistor dividers that set critical operation parameters. both the lt3597 exposed pad and pin 18 are ground. resistors connected between ground and the ctrl1-3, ctrlm, fb1-3, t set , i set1-3 , rt and en/uvlo pins are best tied to pin 18 and not the ground plane. downloaded from: http:///
lt3597 18 3597fa typical applications 48v 1mhz triple step-down 100ma rgb led driver ef?ciency 3597 ta02 lt3597 v in en/uvloboost1 sw1 da1 fb1 v out1 led1bias fault pwm1-3 ctrl1-3 sync rt boost2 sw2 da2 fb2 v out2 led2 boost3 sw3 da3 fb3 v out3 led3 v ref t set ctrlm lt3597 9.1k 100k 97k 91k 270k 3.3f 0.1f 10f 100h 0.1f 2.2f 100h 4.7k 97k 0.1f 2.2f 100h 3.83k 97k 100k 10k 82.5k v ref 49.9k 10f v cc 5v v in 48v v out1 33.2k i set1 i set2 i set3 gnd 20k 20k 20k v out2 v out3 (1mhz) 3 3 r b g led current per channel (ma) 0 10 efficiency (%) 9080 70 50 6040 20 3010 0 60 50 80 70 40 30 20 3597 ta02b 100 90 downloaded from: http:///
lt3597 19 3597fa typical applications 48v 1mhz triple step-down 10w 100ma white led driver (3.6v leds) ef?ciency 3597 ta03 lt3597 v in en/uvloboost1 sw1 da1 fb1 v out1 led1bias fault pwm1-3 ctrl1-3 sync rt boost2 sw2 da2 fb2 v out2 led2 boost3 sw3 da3 fb3 v out3 led3 v ref t set ctrlm lt3597 3.24k 100k 97k 91k 270k 2.2f 0.1f 10f 100h 0.1f 2.2f 100h 3.24k 97k 0.1f 2.2f 100h 3.24k 97k 100k 10k 82.5k v ref 49.9k 10f v cc 5v v in 48v 33.2k i set1 i set2 i set3 gnd 20k 20k 20k v out1 v out2 v out3 3 3 led current per channel (ma) 0 10 efficiency (%) 9080 70 50 6040 20 3010 0 60 50 80 70 40 30 20 3597 ta03b 100 90 downloaded from: http:///
lt3597 20 3597fa typical applications 24v 200khz triple step-down 100ma rgb led driver ef?ciency 3597 ta04 lt3597 v in en/uvloboost1 sw1 da1 fb1 v out1 led1bias fault pwm1-3 ctrl1-3 sync rt boost2 sw2 da2 fb2 v out2 led2 boost3 sw3 da3 fb3 v out3 led3 v ref t set ctrlm lt3597 9.1k 100k 97k 15f 0.22f 10f 470h 0.22f 15f 470h 9.1k 97k 0.22f 15f 470h 9.1k 97k v ref 10f 220k i set1 i set2 i set3 gnd 20k 20k 20k v in 24v v out1 v out2 v out3 3 3 gb r led current per channel (ma) 0 10 efficiency (%) 9080 70 50 6040 20 3010 0 60 50 80 70 40 30 20 3597 ta04b 100 90 downloaded from: http:///
lt3597 21 3597fa typical applications 48v 1mhz triple step-down 20ma rgb led driver 3597 ta05 lt3597 v in en/uvloboost1 sw1 da1 fb1 v out1 led1bias fault pwm1-3 ctrl1-3 sync rt boost2 sw2 da2 fb2 v out2 led2 boost3 sw3 da3 fb3 v out3 led3 v ref t set ctrlm lt3597 9.1k 100k 97k 91k 270k 3.3f 0.1f 10f 100h 0.1f 2.2f 100h 4.7k 97k 0.1f 2.2f 100h 3.83k 97k 100k 10k 82.5k v ref 49.9k 10f v cc 5v v in 48v 33.2k i set1 i set2 i set3 gnd 100k 100k 100k v out1 v out2 v out3 g b r 3 3 downloaded from: http:///
lt3597 22 3597fa package description uhg package variation: uhg52 (39) 52-lead plastic qfn (5mm 8mm) (reference ltc dwg # 05-08-1846 rev b) 5.00 0.10 note: 1. drawing is not a jedec package outline 2. drawing not to scale 3. all dimensions are in millimeters pin 1top mark (see note 6) 5150 48 46 44 4341 39 37 35 34 33 32 31 30 29 28 27 43 48 46 44 51 50 4139 37 35 34 33 32 31 30 29 28 27 24 6 7 9 11 12 13 14 15 16 17 24 6 7 9 1112 13 14 15 16 17 18 19202122232425 26 bottom view?exposed pad 5.90 0.10 6.40 ref 8.00 0.10 0.75 0.05 0.75 0.05 r = 0.10 typ r = 0.10 typ 0.20 0.05 (uhg39) qfn 0410 rev b 0.40 bsc 0.80 bsc 0.70 typ 0.60 typ 0.200 ref 0.00 ? 0.05 recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 3.20 ref 0.40 0.10 0.00 ? 0.05 18 19 20 21 22 23 24 25 26 0.70 0.05 41 39 37 35 34 33 32 31 30 29 28 27 2625 24 23 22 2120 19 18 17 161514 13 1211 9 7 6 4 2 43 4446 48 50 51 0.40 bsc 0.80 bsc 6.40 ref 3.20 ref 4.10 0.05 5.50 0.05 0.20 0.05 5.90 0.05 2.90 0.05 7.10 0.05 8.50 0.05 packageoutline 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.20mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package pin 1 notch r = 0.30 typ or 0.35 45 chamfer 2.90 0.10 downloaded from: http:///
lt3597 23 3597fa information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa - tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. revision history rev date description page number a 7/11 revised maximum limit for led1-3 pin leakage current in the electrical characteristics section. made text edits in ?led current derating using the ctrlm pin? and ?fault flag? sections in the applications information section. 4 15, 17 downloaded from: http:///
lt3597 24 3597fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 2011 lt 0711 rev a ? printed in usa related parts typical application part number description comments lt3476 quad output 1.5a, 2mhz high current led driver with 1000:1 dimming v in : 2.8v to 16v, v out(max) = 36v, true color pwm dimming = 1000:1, i sd < 10a, 5mm 7mm qfn-10 package lt3492 60v, 2.1mhz 3-channel (i led = 1a) full featured led driver v in : 3v to 30v (40v max ), v out(max) = 60v, true color pwm dimming = 3000:1, i sd < 1a, 4mm 5mm qfn-28 package lt3496 45v, 2.1mhz 3-channel (i led = 1a) full featured led driver v in : 3v to 30v (40v max ), v out(max) = 45v, true color pwm dimming = 3000:1, i sd < 1a, 4mm 3mm qfn-28 package lt3590 48v, 850khz 50ma buck mode led driver v in : 4.5v to 55v, true color pwm dimming = 200:1, i sd < 15a, 2mm 2mm dfn-6 and sc70 packages lt3595 45v, 2.5mhz 16-channel full featured led driver v in : 4.5v to 55v, v out(max) = 45v true color pwm dimming = 5000:1, i sd < 1a, 5mm 9mm qfn-56 package lt3596 60v, 1mhz 3-channel full featured led driver v in : 6v to 60v, v out(max) = 40v, true color pwm dimming = 10,000:1, i sd 2a, 5mm 8mm qfn-52 package lt3598 44v, 1.5a, 2.5mhz boost 6-channel led driver v in : 3v to 30v (40v max ), v out(max) = 44v, true color pwm dimming = 1000:1, i sd < 1a, 4mm 4mm qfn-24 package lt3599 2a boost converter with internal 4-string 150ma led ballaster v in : 3v to 30v, v out(max) = 44v, true color pwm dimming = 1000:1, i sd < 1a, 5mm 5mm qfn-32 and tssop-28 packages lt3754 16-channel x 50ma led driver with 60v boost controller and pwm dimming v in : 6v to 40v, v out(max) = 60v, true color pwm dimming = 3000:1, i sd < 2a, 5mm 5mm qfn-52 package lt3760 8-channel x 100ma led driver with 60v boost controller and pwm dimming v in : 6v to 40v, v out(max) = 60v, true color pwm dimming = 3000:1, i sd < 2a, tssop-28 package triple step-down rgb single pixel led driver, 100ma current 10,000:1 dimming at 100hz 3597 ta06 lt3597 v in en/uvloboost1 sw1 da1 fb1 v out1 led1bias fault pwm1:3 ctrl1:3 sync rt boost2 sw2 da2 fb2 v out2 led2 boost3 sw3 da3 fb3 v out3 led3 v ref t set ctrlm lt3597 9.1k 100k 97k 91k 270k 3.3f 0.1f 10f 100h 0.1f 3.3f 100h 9.1k 97k 0.1f 3.3f 100h 9.1k 97k 100k 10k 82.5k v ref 49.9k 10f v cc 5v 10f 33.2k i set1 i set2 i set3 gnd 20k 20k 20k v out1 v in 48v v out2 v out3 r gb i led 50ma/div pwm 2v/div 3597 ta06b 200ns/div downloaded from: http:///

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