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lt3598 1 3598fb typical application features applications description 6-string 30ma led driver with 1.5% current matching the lt ? 3598 is a ? xed frequency step-up dc/dc converter designed to drive up to six strings of leds at an output voltage up to 44v. led dimming can be achieved with analog dimming on the ctrl pin, and with pulse width modulation dimming on the pwm pin. the lt3598 accurately regulates led current even when the input voltage is higher than the led output voltage. the switching frequency is programmable from 200khz to 2.5mhz through an external resistor. additional features include programmable overvoltage protection, switching frequency synchronization to an external clock, led current derating based on junction temperature and/or led temperature, led string disable control, openled alert pin and output voltage limiting when all led strings are disconnected. the lt3598 is available in a thermally enhanced 24-lead (4mm 4mm) qfn and 24-lead tssop packages. 90% ef? cient led driver for 60 white leds n true color pwm? dimming delivers up to 3000:1 dimming ratio n drives six strings of leds at up to 30ma n 1.5% accurate led current matching n wide input voltage range: 3.2v to 30v n output voltage up to 44v n regulates current even when v in > v out n disconnects leds in shutdown n programmable open led protection (regulated) n openled alert pin n programmable led current derating n adjustable frequency: 200khz to 2.5mhz n synchronizable to an external clock n parallel channels for higher current per led string n thermally enhanced 4mm 4mm qfn and 24-lead tssop packages n notebook computer display n medium size displays n automotive lcd display led current matching 2.61k 2.2f 10h 4.7f 15nf 20ma 3598 ta01a lt3598 v o_sw fb openled pwm shdnrt syncctrl v ref led1led2 led3 led4 led5 led6 v out sw v in v c ss gnd t set i set 1.00m 30.9k pv in 8v to 40v 2.2f v in 5v 14.7k 10nf 47pf 100k 100k 10k 51.1k 100k 60.4k pwm shdn sync temperature (c) C50 matching (%) 0.5 1.0 0 C0.5 05 0 C25 25 75 100 125 C1.0 C1.5 1.5 3598 ta01b all six led strings l , lt, ltc, ltm, linear technology and the linear logo are registered trademarks of linear technology corporation. true color pwm is a trademark of linear technology corporation. all other trademarks are the property of their respective owners. protected by u.s. patents including 7199560, 7321203. downloaded from: http:///
lt3598 2 3598fb pin configuration absolute maximum ratings shdn ................................................................v in + 3v v in , openled ...........................................................30v sw voltage ...............................................................45v v out , v o_sw voltage .................................................44v led1 to led6 ............................................................44v pwm, sync, ctrl, rt, ss, v c ...................................6v (note 1) 12 3 4 5 6 7 8 9 1011 12 top view fe package 24-lead plastic tssop 2423 22 21 20 19 18 17 16 15 14 13 sw v out v o_sw led1led2 led3 led4 led5 led6 openled i set ctrl v in shdngnd v ref ssrt pwm sync nc t set fbv c 25 t jmax = 125c, ja = 38c/w exposed pad (pin 25) is pgnd, must be soldered to pcb 24 23 22 21 20 19 25 7 8 9 top view uf package 24-lead (4mm s 4mm) plastic qfn 10 11 12 6 5 4 3 2 1 13 14 15 16 17 18 led1led2 led3 led4 led5 led6 v ref ssrt pwm sync nc v o_sw v out swv in shdngnd openled i set ctrl v c fb t set t jmax = 125c, ja = 37c/w exposed pad (pin 25) is gnd, must be soldered to pcb order information lead free finish tape and reel part marking* package description temperature range lt3598euf#pbf lt3598euf#trpbf 3598 24-lead (4mm 4mm) plastic qfn C40c to 125c lt3598iuf#pbf lt3598iuf#trpbf 3598 24-lead (4mm 4mm) plastic qfn C40c to 125c lt3598efe#pbf lt3598efe#trpbf lt3598 24-lead plastic tssop C40c to 125c lt3598ife#pbf lt3598ife#trpbf lt3598 24-lead plastic tssop C40c 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/ v ref , fb voltage..........................................................6v i set , t set .....................................................................6v operating junction temperature range (note 2) ..................................................C40c to 125c maximum junction temperature........................... 125c storage temperature range ...................C65c to 150c downloaded from: http:///
lt3598 3 3598fb 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 = 5v, v shdn = v in unless otherwise noted. (note 2) parameter conditions min typ max units minimum operating voltage 3 3.2 v maximum operating voltage 30 v reference voltage l 1.2161.210 1.230 1.260 1.260 vv reference voltage line regulation 3.2v < v in < 30v, v c = 0.3v 0.01 0.03 %/v maximum v ref pin current out of pin 200 a fb pin bias current v fb = 1.230v (note 3) 100 250 na fb error amp transconductance i = 5a 300 mhos fb error amp voltage gain 600 v/v fb pin voltage 1.22 1.24 1.26 v current loop amp transconductance 21 mhos current loop amp voltage gain 80 v/v v c sink current 10 a quiescent current v shdn = 5v, pwm = 0v, not switching 3.5 5 ma quiescent current in shutdown v shdn = 0v 0 1 a i set voltage v ctrl = 1.5v, v tset = 1.5v, r iset = 14.7k 0.985 1.000 1.015 v led current r iset = 14.7k 19.5 20 20.7 ma led string current matching 20ma led current l 0.5 1.5 % led open detection threshold 0.2 0.25 v openled sink current 2m a minimum led regulation voltage 0.8 v led1-6 leakage current v led1-6 = 1v, v out = 5v, pwm = 0v v led1-6 = 42v, v out = 44v, pwm = 0v 0.10.2 12 aa ctrl pin bias current v ctrl = 0.8v (note 4) 50 125 na switching frequency rt = 309k rt = 51.1k rt = 14.7k 171 0.9 2.25 190 1 2.5 209 1.1 2.75 khz mhzmhz t set voltage 602 mv maximum switch duty cycle rt = 309k rt = 51.1k rt = 14.7k l 9087 80 9590 86 %% % switch current limit (note 5) 1.5 2 2.5 a switch v cesat i sw = 0.5a 0.12 v switch leakage current v sw = 40v 0.2 5 a downloaded from: http:///
lt3598 4 3598fb parameter conditions min typ max units shdn pin current v shdn = 0v v shdn = 5v 0.1 30 1 60 aa shdn voltage high 1.6 v shdn voltage low 0.4 v soft-start charging current v ss = 0.1v 5 10 15 a pwm input high voltage 1v pwm input low voltage 0.4 v pwm pin bias current pwm = 3.3v 0.1 1 a sync input high voltage 1.5 v sync input low voltage 0.4 v sync pin bias current sync = 0v sync = 3.3v 25 0.1 50 1 aa v o_sw switch resistance 1000 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 = 5v, v shdn = v in unless otherwise noted. (note 2) 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 lt3598e is 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 lt3598i is guaranteed over the full C40c to 125c operating junction temperature range. note 3: current ? ows out of fb pin. note 4: current ? ows out of ctrl pin. note 5: current limit guaranteed by design and/or correlation to static test. current limit is independent of duty cycle and is guaranteed by design. downloaded from: http:///
lt3598 5 3598fb typical performance characteristics shdn pin turn-on threshold shdn pin current quiescent current reference voltage oscillator frequency switch current limit switch v cesat soft-start pin current feedback pin voltage temperature (c) C50 shdn threshold (v) 1.4 1.3 05 0 C25 25 75 100 125 1.2 1.5 3598 g01 v shdn (v) 0 0 shdn pin current (a) 5 15 20 25 5035 10 20 25 10 40 4530 5 15 30 35 3598 g02 125c C50c 25c temperature (c) C50 quiescent current (ma) 5 4 05 0 C25 25 75 100 125 3 2 6 3598 g03 temperature (c) C50 reference voltage (v) 1.230 1.235 1.225 1.220 05 0 C25 25 75 100 125 1.215 1.240 3598 g04 v in = 40v v in = 30v v in = 5v temperature (c) C50 oscillator frequency (mhz) 2.0 2.5 1.5 1.0 05 0 C25 25 75 100 125 0.5 0 3.0 3598 g05 2.5mhz 1mhz 200khz temperature (c) C50 switch current (a) 2.0 2.4 1.6 1.2 05 0 C25 25 75 100 125 0.8 0.4 2.8 3598 g06 temperature (c) C50 i ss (a) 12.5 10.0 05 0 C25 25 75 100 125 7.5 5.0 15.0 3598 g08 temperature (c) C50 feedback voltage (v) 1.23 1.22 05 0 C25 25 75 100 125 1.21 1.20 1.24 3598 g09 v c = 1.5v v c = 1v switch current (a) 0 v cesat (v) 0.30 0.35 0.40 0.250.20 0.25 0.75 0.50 1.00 1.25 1.5o 0.05 0 0.15 0.10 3598 g07 125 c C50 c 25 c downloaded from: http:///
lt3598 6 3598fb typical performance characteristics led current vs pwm duty cycle led current vs temperature led current vs ctrl voltage led current matching vs temperature openled sink current led current waveforms (0.1% pwm) led current waveforms (90% pwm) led current waveforms (0.1% pwm) led current waveforms (90% pwm) pwm duty cycle (%) 0.01 0.001 led current (ma) 1 100 11 0 0.1 100 3598 g10 0.1 0.01 10 temperature (c) C50 led current (ma) 20.1 20.2 20.0 19.9 05 0 C25 25 75 100 125 19.8 19.7 20.3 3598 g11 ctrl voltage (v) 0 led current (ma) 15 20 25 1.0 10 5 0 0.2 0.4 0.6 0.8 1.2 3598 g12 temperature (c) C50 matching (%) 0.5 1.0 0.0 C0.5 05 0 C25 25 75 100 125 C1.0 C1.5 1.5 3598 g13 temperature (c) C50 openled current (ma) 2.0 2.5 1.5 1.0 05 0 C25 25 75 100 125 0.5 0 3.0 3598 g14 20s/div pwm 5v/div sw 20v/div i led1 50ma/div i l 1a/div 3598 g15 2s/div pwm 5v/div sw 20v/div i led1 50ma/div i l 1a/div 3598 g16 100s/div pwm 5v/div sw 20v/div i led1 50ma/div i l 1a/div 3598 g17 5s/div pwm 5v/div sw 20v/div i led1 50ma/div i l 1a/div 3598 g18 downloaded from: http:///
lt3598 7 3598fb pin functions led1-6 (pins 1, 2, 3, 4, 5, 6/pins 4, 5, 6, 7, 8, 9): led string output. connect the bottom cathode of each led string to these pins. tie pins to v out if the string is not used. openled (pin 7/pin 10): open led flag when any led string opens. the output is open-collector. tie a resistor to other supply for open led ? ag function. i set (pin 8/pin 11): programs led current for each string. connect a 14.7k resistor between i set and gnd to program each led string current to 20ma. a 47pf capacitor on the i set pin reduces current ripple in each led string. ctrl (pin 9/pin 12): led current control. if the ctrl pin is not used, tie this pin to v ref through a 10k to 20k resistor. v c (pin 10/pin 13): error ampli? er output pin. tie the external compensation network to this pin. fb (pin 11/pin 14): feedback pin for overvoltage protection. reference voltage is 1.230v. connect the resistive divider tap here. minimize trace area at fb. set v out according to v out = 1.230(1 + r2/r1) when overvoltage protection occurs (see figure 2).t set (pin 12/pin 15): an external resistor divider from v ref programs a decrease in led current versus internal junction temperature (setting temperature breakpoint and slope). if the t set pin is not used, tie this pin to v ref . nc (pin 13/pin 16): no connection. sync (pin 14/pin 17): frequency synchronization pin. this input allows for synchronizing the operating frequency to an external clock. the r t resistor should be chosen to program a switching frequency 20% slower than sync pulse frequency. this pin should be grounded if this feature is not used. pwm (pin 15/pin 18): input pin for pwm dimming control. above 1v allows converter switching and below 0.4v disables switching with v c pin level maintained. a pwm signal driving the pwm pin provides accurate dimming control. the pwm signal can be driven from 0v to 5v. if unused, the pin should be connected to v ref . rt (pin 16/pin 19): a resistor to ground programs switching frequency between 200khz and 2.5mhz. for sync function, choose the resistor to program a frequency 20% slower than the sync pulse frequency. do not leave this pin open. ss (pin 17/pin 20): soft-start pin. place a soft-start capacitor here. upon start-up, a 10a current charges the capacitor. use a larger capacitor for slower start-up. leave open if not used. v ref (pin 18/pin 21): bandgap voltage reference. internally set to 1.230v. this pin can supply up to 100a. can be used to program the ctrl pin voltage using resistor dividers to ground. gnd (pin 19/pin 22): ground. tie directly to local ground plane.shdn (pin 20/pin 23): shutdown pin. tie to 1.6v or more to enable the device. tie below 0.4v or less to disable device. do not ? oat this pin. v in (pin 21/pin 24): input supply pin. must be locally bypassed with a capacitor to ground. sw (pin 22/pin 1): switch pin. this is the collector of the internal npn power switch. minimize the metal trace area connected to this pin to minimize emi. v out (pin 23/ pin 2): output pin. this pin provides power to all leds. v o_sw (pin 24/ pin 3): drain of an internal pmos. the internal pmos disconnects the feedback resistors from the v out pin during shutdown and the pwm transitioned to low. exposed pad (pin 25/ pin 25): ground. the exposed pad must be soldered to the pcb. (qfn/ tssop) downloaded from: http:///
lt3598 8 3598fb block diagram 3598 f01 t set ctrl v ref pwm q1 C + C + C + openled detection openled led1led2 led3 led4 led5 led6 r v o_sw v out led disable detection fb gnd gnd i set ss sw sync rt v in shdn C + led g m 0.8v ovp g m a3 a1 v c C + v out pwm dimming logic slope oscillator 1.230v ref 3 s q a2 v ptat led drive circuitry figure 1. block diagram downloaded from: http:///
lt3598 9 3598fb operation the lt3598 uses a constant-frequency, peak current mode control scheme to provide excellent line and load regulation. each string can drive up to 30ma with 1.5% matching ac- curacy between strings. operation can be best understood by referring to the block diagram in figure 1. lt3598 has a built-in boost converter which converts the input voltage to a higher output voltage to drive leds. the led strings are connected to current sources where the current level is set with an external resistor on the i set pin. the led1 to led6 voltages are monitored for output voltage regulation. during normal operation, when all leds are used, the lowest led pin voltage (led1 to led6) is used to regulate the output voltage to ensure all led strings have enough voltage to run the programmed current. for any unused led strings, tie their led pins to v out . an unused led string is no longer in the regulation loop, nor does it affect open led detection. never allow unused led strings to be left open. the basic loop uses a pulse from an internal oscillator to set the sr latch and turn on the internal power npn switch q1. the signal at the noninverting input of the pwm comparator (a2 slope) is proportional to the sum of the switch current and oscillator ramp. when slope exceeds v c (the output of the g m ampli? er), the pwm comparator resets the latch. the switch is then turned off, causing the 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 capaci- tor. the switch is turned on again 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 leds. dimming of the leds is accomplished by pulsing the led current using the pwm pin. when the pwm pin is low, switching is disabled and the error ampli? er is turned off so that it does not drive the v c pin. also, all internal loads on the v c pin are disabled so that the state of the v c pin is maintained on the external compensation capacitor. this feature reduces transient recovery time. when the pwm input again transitions high, the peak switch current returns to the correct value. the lt3598 uses the fb pin to provide overvoltage protec- tion when all led strings are open. there is an internal pmos switch between v out and v o_sw that is controlled by the pwm signal. during the pwm off-period, this pmos is turned off, allowing for higher dimming range and lower current during shutdown. a resistor divider is connected between the v o_sw pin and ground, which sets the overvoltage protection voltage. if the led1-6 pin voltage is below 0.2v (for a certain delay after 80% of the programmed output voltage is reached), the string is treated as an open led string. as a result, openled ? ag is set. if a led string is open in the middle of the operation, the regulation will continue. openled detection is disabled during the start-up phase to avoid erratic ? ag generation. an led string that is disabled by connecting its led pin to v out is not an open led condition. during normal operation, if an led string is open and has the lowest led pin voltage, the output voltage will regulate itself to ? nd another led string that has the lowest led pin voltage at about 0.8v. if the open led string has an led voltage above 0.8v, the output voltage will remain the same. when the led string is open, it is no longer in the regulation loop. the openled detection is active only when the pwm signal is enabled. to avoid spurious openled detection and high pwm dimming ratio, more output capacitance is recommended to allow less voltage drop on v out . during start-up, 10a of current charges the external soft-start capacitor. the ss pin directly limits the rate of voltage rise on the v c pin, which in turn, limits the peak switch current. soft-start also enables the switching frequency foldback to provide a clean start-up for the lt3598. current limit protects the power switch and external components. downloaded from: http:///
lt3598 10 3598fb inductor selection table 1 lists several inductors that work well with the lt3598, however, there are many other manufacturers and devices that can be used. consult each manufacturer for detailed information on their entire range of parts. ferrite core inductors 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. consult each manufacturer to determine how the maximum inductor current is measured and how much more current the inductor can reliably conduct. table 1. recommended inductors part l (h) max dcr () current rating (a) vendor b1015as-100m #817fy-4r7m 1123as-4r7m 10 4.74.7 0.070.06 0.12 2.2 2.261.90 toko www.toko.com 7445406874454010 7447745100 6.8 1010 0.0550.065 0.12 2.2 2 1.7 wrth electronics www.we-online.com cdh74np-120lcdh74np-150l cdrh6d38-100 1215 10 0.0650.083 0.038 2.452.10 2.00 sumida www.sumida.com ihlp-2525bd-01 10 0.129 2.5 vishay www.vishay.com sd25-4r7-r 4.7 0.056 1.83 cooper www.cooperet.com lps4018-472ml 4.7 0.200 1.8 coilcraft www.coilcraft.com capacitor selectionlow esr (equivalent series resistance) ceramic capacitors 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 applications information temperature ranges than other dielectrics. a 4.7f to 10f output capacitor is suf? cient for most high output current designs. table 2 lists some suggested manufacturers. consult the manufacturers for detailed information on their entire selection of ceramic parts. table 2. recommended ceramic capacitor manufacturers taiyo yuden 408-573-4150 www.t-yuden.com avx 843-448-9411 www.avxcorp.com murata 770-436-1300 www.murata.com kemet 408-986-0424 www.kemet.com united chemi-con 847-696-2000 www.chemi-con.com diode selection schottky diodes, with their low forward voltage drop and fast switching speed, must be used for all lt3598 applications. do not use p-n diodes. 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 voltage. a 2a 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 3. suggested diodes part max current (a) max reverse voltage (v) manufacturer b250ab240a sbr140s3 sbm340, pds340 22 1 3 5040 40 40 diodes, inc. www.diodes.com hsm150ghsm150j 11 5050 microsemi www.microsemi.com ss3h9 3 90 vishay www.vishay.com downloaded from: http:///
lt3598 11 3598fb overvoltage protection the lt3598 uses the fb pin to provide regulated overvoltage protection when all led strings are open. a resistor divider is connected between the v o_sw pin and ground (figure 2). there is an internal pmos switch between v out and v o_sw , which is controlled by the pwm signal. the pmos switch addition prevents the feedback resistor divider from draining the output capacitor during pwm off-period, allowing for a higher dimming range without falsely tripping the openled ? ag. it also reduces the system current in shutdown. this pmos has about 1k resistance, so select fb resistors taking this resistance into account. to set the maximum output voltage, select the values of r1 and r2 (see figure 2) according to the following equation: vv r r out max () . =+ ?? ? ?? ? 1 230 1 2 1 applications information figure 2. overvoltage protection voltage feedback connections the output voltage should be set 15% higher than the normal led string operating voltage. under normal operation, led1 to led6 pin voltages are monitored and provide feedback information to the converter for output voltage regulation given the programmed led current. the maximum output regulation loop is activated only when all leds are open. programming maximum led current maximum led current is programmed by placing a resistor between the i set pin and ground (r iset ). the i set pin resistor can be selected from 10k to 100k.the led current is programmed according to the following equation: i v r led iset 294 see table 4 and figure 3 for resistor values and corr- esponding programmed led current. led current can also be adjusted by programming the ctrl pin voltage. table 4. r iset value selection for led current led current (ma) resistor on i set pin (k) 3ma 97.6 10ma 29.4 20ma 14.7 30ma 9.76 led current dimming two different types of dimming control can be used with the lt3598. the led current can be set by modulating the ctrl pin or the pwm pin. for some applications, a variable dc voltage that adjusts the led current is the preferred method of brightness control. the ctrl pin voltage can be modulated to set the dimming of the led string (see figures 4 and 5). as the voltage on the ctrl pin increases from 0v to 1.0v, the led current increases from 0 to the programmed led current level. as the ctrl pin voltage increases beyond 1v, it has no effect on the led current . figure 3. r iset value selection for led current 3598 f02 lt3598 v o_sw fb v out r2 r1 r iset (k) 0 led current (ma) 20 25 1510 40 80 20 60 100 5 0 30 3598 f03 downloaded from: http:///
lt3598 12 3598fb applications information for true color pwm tm dimming, the lt3598 provides up to a 3000:1 pwm dimming range. this is achieved by allowing the duty cycle of the pwm pin (connected to the ic and an internal switch in series with the led(s)), to be reduced from 100% to as low as 0.1% for a pwm frequency of 100hz (figure 6). pwm duty cycle dimming allows for constant led color to be maintained over the entire dimming range. for wide pwm dimming range, higher switching freq- uency a nd lower pwm frequency con? guration are needed. special considerations are required for component selection and compensation network. please contact factory for optimized components selection if very high dimming ratio is desired. led current derating using the ctrl pin a useful feature of the lt3598 is its ability to program a derating curve for maximum led current versus temperature. led data sheets provide curves of maximum- allowable led current versus temperature to warn against exceeding this current limit and damaging the led. the lt3598 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 ctrl pin with a negative temperature coef? cient using a resistor divider with temperature dependent resistance (figure 7). as ambient temperature increases, the ctrl voltage will fall below the internal 1v voltage reference, causing led currents to be controlled by the ctrl pin voltage. the led current curve breakpoint and slope versus temperature is de? ned by the choice of resistor ratios and use of temperature-dependent resistance in the divider for the ctrl pin. 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. if an ntc resistor is used to indicate led temperature, it is effective only if the resistor is connected 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 nonlinear over a wide range of temperatures, it is important to obtain a resistors exact value over temperature from the manufacturer. hand calculations of ctrl voltage can then be performed at each given temperature, resulting in the ctrl versus temperature plotted curve. several iterations of resistor value calculations may be required to achieve the desired breakpoint and slope of the led current derating curve. figure 4. led current vs ctrl voltage figure 5. led current vs ctrl figure 6. led current using pwm dimming 3598 f05 lt3598 v ref ctrl r2 r1 pwm inductor current 3598 f06 led current max i led t pwm ton pwm (= 1/f pwm ) ctrl (v) 0 led current (ma) 20 25 30 1.4 1510 0.4 0.8 0.2 0.6 1 1.2 1.6 5 0 35 3598 f04 r iset = 9.76k downloaded from: http:///
lt3598 13 3598fb table 5. ntc resistor manufacturers/distributors murata electronics north america 770-436-1300 www.murata.com tdk corporation 516-535-2600 www.tdk.com digi-key 800-344-4539 www.digikey.com if calculating the ctrl voltage at various temperatures gives a downward slope that is too strong, alternative resistor networks can be chosen (b, c, d in figure 7) which use temperature independent resistance to reduce the effects of the ntc resistor overtemperature. 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 (ctrl behavior) overtemperature. 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 creating an output voltage v out (ctrl) from a speci? ed v cc supply (v ref ). using the t set pin for thermal protection the lt3598 contains a special programmable thermal regulation loop that limits the internal junction temperature of the part. since the lt3598 topology consists of a single boost converter with six linear current sources, any led string voltage mismatch will cause additional power to be dissipated in the package. this topology provides excellent current matching between led strings and allows a single power stage to drive a large number of leds, but at the price of additional power dissipation inside the part applications information (which means a higher junction temperature). being able to limit the maximum junction temperature allows the bene? ts of this topology to be fully realized. 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 lt3598 will automatically protect itself and the led strings under worst-case conditions. the operation of the thermal loop is simple. as the ambient temperature increases, so does the internal junction temperature of the part. an internal voltage is developed thats proportional to the junction temperature (v ptat ). once the programmed maximum junction temperature is reached, the lt3598 begins to linearly reduce the led current, as needed, to try and maintain this temperature. this can only be achieved when the ambient temperature stays below the desired maximum junction temperature. if the ambient temperature continues to rise past the programmed maximum junction temperature, the leds current will be reduced to approximately 5% of the full led current. while this feature is intended to directly protect the lt3598, it can also be used to derate the led current at high temperatures. since there is a direct relationship between the led temperature and lt3598 junction temperature, the tset function also provides some led current derating at high temperatures. two external resistors program the maximum ic junction temperature using a resistor divider from the v ref pin, as shown in figure 8. choose the ratio of r1 and r2 for the desired junction temperature. figure 9 shows the relationship of t set voltage to junction temperature, and table 6 shows commonly used values for r1 and r2. figure 7 . led current derating vs temperature using ntc resistor 3598 f07 r y r y r x r x r ntc r ntc r ntc r ntc d c b a lt3598 v ref ctrl r2 r1(option a to d) downloaded from: http:///
lt3598 14 3598fb applications information table 6. t set junction temperature t j (c) r1 r2 90 100k 68.1k 100 100k 63.4k 110 100k 59k 120 100k 54.9k programming switching frequency the switching frequency of the lt3598 should be programmed between 200khz and 2.5mhz by an external resistor connected between the rt pin and ground. do not leave this pin open. see table 7 and figure 10 for resistor values and corresponding frequencies. 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. 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. table 7. switching frequency switching frequency (mhz) r t (k) 2.5 14.7 2 20.5 1.5 29.4 1 51.1 0.5 105 0.2 301 switching frequency synchronization the nominal operating frequency of the lt3598 is programmed using a resistor from the rt pin to ground and can be controlled over a 200khz to 2.5mhz 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 lt3598 must have a frequency between 250khz and 3mhz, a duty cycle between 20% and 80%, a low state below 0.4v and a high state above 1.5v. synchronization signals outside of these parameters will cause erratic switching behavior. for proper operation, an r t resistor should be 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. figure 8. programming the t set pin figure 9. t set pin threshold 3598 f08 lt3598 v ref t set r2 r1 junction temperature (c) 0 500 v tset threshold (mv) 600 700 800 50 100 125 25 75 150 900 550 650 750 850 3598 f09 figure 10. switching frequency r t (k) switching frequency (mhz) 2.52.0 1.5 0 1.00.5 1000 10 100 3598 f10 downloaded from: http:///
lt3598 15 3598fb the sync pin should be grounded if the clock sync-hronization feature is not used. when the sync pin is grounded, the internal oscillator generates switching frequency to the converter. soft-start and switching frequency foldback for many applications, it is necessary to minimize the inrush current at start-up. the lt3598s soft-start circuit signi? cantly reduces the start-up current spike and output voltage overshoot. before the ss pin voltage reaches 1v, the switching frequency will also fold back proportional to the ss pin voltage. a typical value for the soft-start capacitor is 10nf. openled flag the openled pin is an open-collector output and needs an external resistor tied to a supply (see figure 11). if any led string is open during normal operation, the openled pin will be pulled down. loop compensation the lt3598 has an internal transconductance error ampli? er for led current regulation whose v c output compensates the control loop. during an open led event where all led strings are open, the v c node also compensates the control loop. the external inductor, output capacitor, and the compensation resistor and capacitor determine the loop stability. the inductor and output capacitor are chosen based on performance, size and cost. the compensation resistor and capacitor at v c are selected to optimize control loop stability. for typical led applications, a 15nf compensation capacitor in series with a 3k resistor at v c is adequate. thermal considerations the lt3598 provides six channels for led strings with internal npn devices serving as constant-current sources. when led strings are regulated, the lowest led pin voltage is typically 0.8v. the higher the programmed led current, the more power dissipation in the lt3598. for 30ma led programming current with a 100% pwm dimming ratio, at least 144mw is dissipated within the ic due to current sources. if the forward voltages of the six led strings are very dissimilar, there can be signi? cant power dissipation. thermal calculations shall include the power dissipation on current sources in addition to conventional switch dc loss, switch ac loss and input quiescent loss. for best ef? ciency, it is recommended that all channels have the same number of leds, and each string has a similar voltage drop across the leds. board layout considerations 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 essential. 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 interplane coupling. good grounding is essential in led fault detection. applications information figure 11. openled connection 3598 f11 lt3598 openled r1 the openled ? ag is only activated after the ? rst pwm edge. the open led detection is enabled only when the pwm signal is enabled. there is a delay for openled ? ag generation when the pwm signal is enabled to avoid generating a spurious ? ag signal. the maximum current the openled can sink is typically 2ma. during start-up (see the operation section), the open led detection is disabled. if an led string is not used and tied to v out , the string will not be in any fault detection. downloaded from: http:///
lt3598 16 3598fb typical applications l1 10h d1 c24.7f 3598 ta02a lt3598 v o_sw fb openled pwm shdnsync led1led2 led3 led4 led5 led6 v out sw v in r41.00m r530.9k r314.7k r7100k c1: taiyo yuden gmk325bj225mlc2: murata grm32er71h475k c3: taiyo yuden lmk212bj225mg d1: diodes, inc. b240a l1: w rth elektronik 744777410 r ntc : murata ncp18wf104j03rb r ntc 100k r hot 10k r6100k pwm shdn sync c1 2.2f pv in 6v to 40v c3 2.2f v in 5v r c 2.61k c c 15nf rtctrl v ref v c ss gnd t set i set c40.1f c547pf r151.1k r8 60.4k 20ma led driver for 40 white leds with two channels unused ef? ciency (pwm dimming) total led current (ma) 0 efficiency (%) 80 85 90 70 75 70 20 40 10 90 30 50 80 60 60 50 65 55 95 3598 ta02b pv in = 25v pv in = 12v downloaded from: http:///
lt3598 17 3598fb typical applications led driver for 30 white leds with 60ma each string l1 10h d1 c24.7f 3598 ta03a lt3598 v o_sw fb openled pwm shdnsync led1led2 led3 led4 led5 led6 v out sw v in r41.00m r530.9k pwm shdn sync ctrl ctrl r6100k c1 2.2f pv in 6v to 40v c3 2.2f v in 5v r39.76k r7100k c1: taiyo yuden gmk325bj225mlc2: murata grm32er71h475ka88l c3: taiyo yuden lmk212bj225mg d1: vishay ss3h9 l1: w rth elektronik 744777410 r ntc : murata ncp18wf104j03rb r hot 10k rtctrl v ref v c ss gnd t set i set c40.1f c547pf r151.1k 60ma r ntc 100k r c 2.61k c c 15nf r8 60.4k ef? ciency (pwm dimming) dimming range (1000:1 pwm) at 125c junction temperature, 10ms period total led current (ma) 0 efficiency (%) 80 85 90 140 75 70 40 80 20 180 60 100 160 120 65 60 100 95 3598 ta03b pv in = 25v pv in = 12v 2s/div pwm 5v/div sw 20v/div i led1 100ma/div 359 8 downloaded from: http:///
lt3598 18 3598fb typical applications auto battery powered driver for 20 leds with 90ma each string ef? ciency l1 4.7h c24.7f 3598 ta04a lt3598 v o_sw fb openledrt led1led2 led3 led4 led5 led6 v out sw v in r1020k ctrl v ref pwm shdnsync pwm shdn ctrl ctrl 100k pv in 6v to 40v c3 2.2f v in 5v d1 r41.00m r530.9k c1 2.2f r39.76k c1: nippon chemi-con kts500b225m32notooc2: murata grm32er71h475ka88l c3: taiyo yuden lmk212bj225mg d1: vishay ss3h9 l1: w rth elektronik 7447785004 v c ss gnd t set i set c40.1f c556pf r151.1k 90ma r7100k r c 5.11k c c 6.8nf r860.4k dimming range 1000:1 pwm, 10ms period (125c junction temperature) total led current (ma) 0 efficiency (%) 80 85 90 140 75 70 40 80 20 180 60 100 160 120 65 60 95 3598 ta04b 5s/div pwm 5v/div i led1 100ma/div 3598 ta04c downloaded from: http:///
lt3598 19 3598fb typical applications l1 10h d1 c24.7f 3598 ta02a lt3598 v o_sw fb openled pwm shdnsync led1led2 led3 led4 led5 led6 v out sw v in r42.4m r5140k r314.7k r7178k c1: murata grm21br71e225kc2: murata grm32er71h475k c3: taiyo yuden lmk212bj225mg d1: diodes, inc. sbr140s3 l1: toko 1123as-100m r910k r6100k pwm shdn sync c1 2.2f pv in 11.4v to 12.6v c3 2.2f v in 3.2v to 5.5v r c 2k c c 10nf rtctrl v ref v c ss gnd t set i set c40.1f c547pf r121.5k r8 107k 20ma c6100pf 2 mhz led driver for 20 white leds pwm dimming (3000:1) pwm 5v/div i l 200ma/divi led, total 50ma/div 3598 ta07a pv in = 12v, v in = 3.3v 1s/div downloaded from: http:///
lt3598 20 3598fb pwm 5v/div i l 500ma/divi led, total 50ma/div 3598 ta06 pv in = v in = 5v 5s/div typical applications 1 mhz led driver for 36 white leds pwm dimming (20s pulse width) l1 10h d1 c24.7f s 2 3598 ta07 lt3598 v o_sw fb openled pwm shdnsync led1led2 led3 led4 led5 led6 v out sw v in r42.4m r576.8k r311.5k c1: murata grm21br71a225kc2: murata grm32er71h475k d1: diodes, inc. sbr140s3 l1: vishay ihlp-2525bd-01 r910k r6100k pwm shdn sync c1 2.2f v in 5v c3 1f r c 3k c c 10nf rtctrl v ref v c ss gnd t set i set c40.1f c547pf c6100pf r153.6k r8 107k 25ma downloaded from: http:///
lt3598 21 3598fb package description fe24 (aa) tssop 0208 rev ? 0.09 C 0.20 (.0035 C .0079) 0 o C 8 o 0.25 ref recommended solder pad layout 0.50 C 0.75 (.020 C .030) 4.30 C 4.50* (.169 C .177) 134 5 6 7 8 9 10 11 12 14 13 7.70 C 7.90* (.303 C .311) 3.25 (.128) 2.74 (.108) 20 21 2223 24 19 18 17 16 15 1.20 (.047) max 0.05 C 0.15 (.002 C .006) 0.65 (.0256) bsc 0.195 C 0.30 (.0077 C .0118) typ 2 2.74 (.108) 0.45 p 0.05 0.65 bsc 4.50 p 0.10 6.60 p 0.10 1.05 p 0.10 3.25 (.128) 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 sie for exposed pad attachment 6.40 (.252) bsc fe package 24-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1771 rev ?) exposed pad variation aa downloaded from: http:///
lt3598 22 3598fb package description 4.00 0.10 (4 sides) note:1. drawing proposed to be made a jedec package outline mo-220 variation (wggd-x)to be approved 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side, if present 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 1top mark (note 6) 0.40 0.10 24 23 12 bottom viewexposed pad 2.45 0.10 (4-sides) 0.75 0.05 r = 0.115 typ 0.25 0.05 0.50 bsc 0.200 ref 0.00 C 0.05 (uf24) qfn 0105 recommended solder pad pitch and dimensions 0.70 0.05 0.25 0.05 0.50 bsc 2.45 0.05 (4 sides) 3.10 0.05 4.50 0.05 package outline pin 1 notchr = 0.20 typ or 0.35 45 chamfer uf package 24-lead plastic qfn (4mm 4mm) (reference ltc dwg # 05-08-1697) downloaded from: http:///
lt3598 23 3598fb 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 b 7/10 updated data sheet title and 3rd bullet under features to 1.5% changed v in condition in reference voltage line regulation to 3.2v deleted v in = 5v conditions from shdn pin current revised voltage in pwm description in pin functions fixed minor typo added 15% to ? rst sentence of third paragraph added information to using t set pin for thermal protection section changed to 250khz in switching frequency synchronization section 13 4 7 9 1113 14 (revision history begins at rev b) downloaded from: http:///
lt3598 24 3598fb linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2008 lt 0710 rev b printed in usa l1, 10h d1 c24.7f 3598 ta04a lt3598 v o_sw fb openled pwm shdnsync led1led2 led3 led4 led5 led6 v out sw v in pwm shdn sync 100k pv in 8v to 40v c3 2.2f v in 5v 20ma r41.00m r5 30.9k r314.7k r7100k r hot 10k c1 2.2f rtctrl v ref v c ss gnd t set i set c547pf r151.1k c410nf r ntc 100k r c 2.61k c c 15nf r8 60.4k related parts typical application part number description comments lt3474/ lt3474-1 36v, 1a (i led ), 2mhz, step-down led driver v in : 4v to 36v, v out(max) = 13.5v, true color pwm dimming = 400:1, i sd < 1a, tssop-16e package lt3475/ lt3475-1 dual 1.5a (i led ), 36v, 2mhz, step-down led driver v in : 4v to 36v, v out(max) = 13.5v, true color pwm dimming = 3000:1, i sd < 1a, tssop-20e package 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 lt3477 3a, 42v, 3mhz boost, buck-boost, buck led driver v in : 2.5v to 25v, v out(max) = 40v, dimming = analog/pwm, i sd < 1a, qfn and tssop20e packages lt3478/lt3478-1 4.5a, 42v, 2.5mhz high current led driver with 3000:1 dimming v in : 2.8v to 36v, v out(max) = 42v, true color pwm dimming = 3000:1, i sd < 3a, tssop16e package lt3486 dual 1.3a, 2mhz high current led driver v in : 2.5v to 24v, v out(max) = 36v, true color pwm dimming = 1000:1, i sd < 1a, 5mm 3mm dfn and tssop-16e packages 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 lt3497 dual 2.3mhz, full function led driver with integrated schottkys and 250:1 true color pwm dimming v in : 2.5v to 10v, v out(max) = 32v, i q = 6ma, i sd < 12a, 2mm 3mm dfn-10 package lt3498 2.3mhz, 20ma led driver and oled driver with integrated schottky v in : 2.5v to 12v, v out(max) = 32v, i q = 1.65ma, i sd < 9a, 2mm 3mm dfn-10 package lt3518/lt3517 2.3a/1.3a 45v, 2.5mhz full featured led driver with true color pwm dimming v in : 3v to 30v (40v max ), v out(max) = 42v, true color pwm dimming = 3000:1, i sd < 5a, 4mm 4mm qfn-16 package lt3590 48v, 850khz 50ma buck mode led driver v in : 4.5v to 55v, dimming = 200:1 true color pwm, i sd < 15a, 2mm 2mm dfn-6 and sc70 packages lt3592 36v, 2.2mhz, 500ma buck mode led driver v in : 3.6v to 36v, true color pwm dimming = 10:1, i sd < 1a, 2mm 3mm dfn-10 and msop-10e 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 90% ef? cient led driver for 60 white leds ef? ciency vs total led current total led current (ma) 0 60 efficiency (%) 70 80 90 20 40 80 60 100 100 65 75 85 95 120 3598 ta05b v in = 25v v in = 16v downloaded from: http:///

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