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LT3486 1 3486f wide (1000:1) pwm dimming range with no colorshift independent dimming and shutdown control of the led drivers drives up to 16 white leds at 25ma (8 per driver) from a single li-ion cell drives up to 16 white leds at 100ma (8 per driver) from 12v supply 3% led current programming accuracy open led protection: 36v clamp voltage fixed frequency operation: up to 2mhz wide input voltage range: 2.5v to 24v low shutdown current: i cc < 1a overtemperature protection available in (5mm 3mm 0.75mm) 16-pin dfn and 16-pin thermally enhanced tssop packages dual 1.3a white led step-up converters with wide dimming the lt ? 3486 is a dual step-up dc/dc converter speci? cally designed to drive up to 16 white leds (8 in series per converter) at constant current from a single li-ion cell. series connection of the leds provides identi- cal led currents resulting in uniform brightness. the two independent converters are capable of driving asymmetric led strings. the dimming of the two led strings can be controlled independently via the respective ctrl pins. an internal dimming system allows the dimming range to be extended up to 1000:1 by feeding a pwm signal to the respective pwm pins. the LT3486 operating frequency can be set with an external resistor over a 200khz to 2mhz range. a low 200mv feedback voltage (3% accuracy) minimizes power loss in the current setting resistor for better ef? ciency. additional features include output voltage limiting when leds are disconnected and overtemperature protection. the LT3486 is available in a space saving 16-pin dfn (5mm 3mm 0.75mm) and 16-pin thermally enhanced tssop packages. li-ion powered driver for camera flash and lcd backlighting ef? ciency vs v in , ltc and lt are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. patent pending. features descriptio u applicatio s u typical applicatio u notebook pc display led camera light for cell phones car dashboard lighting avionics displays v in (v) 3 65 efficiency (%) 70 75 80 85 90 3.2 3.4 3.6 3.8 3486 ta01b 4 4.2 movie mode i led1 = 175ma flash mode i led1 = 320ma 8 leds/25ma v in 3v to 4.2v 10 f l1 10 h l2 10 h 0.1 f 2.2 f 63.4k 2.8k 4.7nf r fb2 8.06 ? r fb1 0.62 ? 2.2 f 0.1 f 3486 ta01a 100k led1 aot3218 25ma 8 leds off on off on dimming 2 v c1 v c2 r t sw1 sw2 v in LT3486 ovp2 ctrl2 ref pwm2 fb2 ovp1 ctrl1 shdn pwm1 fb1 dimming 1
LT3486 2 3486f 16 15 14 13 12 11 10 9 17 dhc package 16-lead (5mm 3mm) plastic dfn exposed pad (pin 17) is gnd must be soldered to pcb 1 2 3 4 5 6 7 8 sw2 ref ovp2 shdn v c2 fb2 ctrl2 pwm2 sw1 v in ovp1 r t v c1 fb1 ctrl1 pwm1 input voltage (v in ) ...................................................25v ? s ? h ? d ? n voltage ..........................................................25v sw1, sw2 voltages .................................................40v ovp1, ovp2 voltages ...............................................40v ctrl1, ctrl2 voltages ...........................................10v pwm1, pwm2 voltages ...........................................10v fb1, fb2 voltages .....................................................10v order part number dhc part marking consult ltc marketing for parts speci? ed with wider operating temperature ranges. (note 1) the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 3v, v ctrl1 = 3v, v ctrl2 = 3v, v pwm1 = 3v, v pwm2 = 3v, v shdn = 3v, unless otherwise noted. operating temperature range (note 2) ...C 40c to 85c storage temperature range dfn ...................................................C 65c to 125c tssop ............................................... C65c to 150c maximum junction temperature .......................... 125c lead temperature (soldering, 10sec, tssop) ...... 300c order part number fe part marking 3486efe LT3486efe sw2 ref ovp2 shdn v c2 fb2 ctrl2 pwm2 sw1 v in ovp1 r t v c1 fb1 ctrl1 pwm1 fe package 16-lead plastic tssop exposed pad is gnd (pin 17) must be soldered to pcb 1 2 3 4 5 6 7 8 top view 16 15 14 13 12 11 10 9 17 t jmax = 125c, ja = 43c/w, jc = 4c/w t jmax = 125c, ja = 38c/w, jc = 10c/w 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/ LT3486edhc 3486 parameter conditions min typ max units minimum operating voltage 2.5 v maximum operating voltage 24 v feedback voltage (fb1, fb2) 194 200 206 mv offset between fb1 and fb2 v os = |fb1-fb2| 0 3 6 mv feedback pin bias current (fb1, fb2) v fb1 = v fb2 = 0.2v (note 3) 10 45 100 na quiescent current v fb1 = v fb2 = 1v 9 14 ma ? s ? h ? d ? n = 0v, ctrl1 = ctrl2 = 0v 0.1 1 a switching frequency r t = 53.6k 0.75 1 1.25 mhz oscillator frequency range (note 4) 200 2000 khz nominal r t pin voltage r t = 53.6k 0.54 v for atio package/order i uu w electrical characteristics absolute axi u rati gs w ww u
LT3486 3 3486f the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 3v, v ctrl1 = 3v, v ctrl2 = 3v, v pwm1 = 3v, v pwm2 = 3v, v shdn = 3v, unless otherwise noted. note 1: absolute maximum ratings are those beyond which the life of a device may be impaired. note 2: the LT3486e is guaranteed to meet speci? ed performance from 0c to 70c and is designed, characterized and expected to meet these extended temperature limits, but is not tested at C 40c and 85c. parameter conditions min typ max units maximum duty cycle r t = 53.6k 90 96 % r t = 20.5k 90 % r t = 309k 98 % switch current limit (sw1, sw2) 1 1.3 a switch v cesat i sw1 = i sw2 = 0.75a 300 mv switch leakage current v sw1 = v sw2 = 10v 0.1 5 a error ampli? er transconductance ?i = 5a 220 a/v error ampli? er voltage gain 120 v c1 , v c2 switching threshold 0.85 v v c1 , v c2 clamp voltage 1.5 v v c1 , v c2 source current v fb1 = v fb2 = 0v 25 a v c1 , v c2 sink current v fb1 = v fb2 = 1v 25 a v c1 , v c2 pin leakage current v c1 = v c2 = 1v, v pwm1 = v pwm2 = 0v 1 10 na ovp1, ovp2 overvoltage threshold voltage 34 36 38 v ctrl1, ctrl2 voltages to turn off led1, 2 currents 75 mv ctrl1, ctrl2 voltages to turn on led1, 2 currents 150 mv ctrl1, ctrl2 voltages for full led1, 2 currents 1.8 v ctrl1, ctrl2 pin bias current v ctrl1 = v ctrl2 = 3v 20 30 40 a pwm1, pwm2 voltage high 0.9 v pwm1, pwm2 voltage low 0.4 v pwm1, pwm2 pin bias current v pwm1 = v pwm2 = 3v 0.1 1 a shdn voltage high 1.6 v shdn voltage low 0.4 v shdn pin bias current v shdn = 3v 20 a ref voltage i ref = 10a 1.2 1.25 1.3 v ref source current 50 80 a electrical characteristics note 3: current ? ows out of the pin. note 4: guaranteed by design and test correlation, not production tested.
LT3486 4 3486f typical perfor uw ce characteristics a t a = 25c unless otherwise speci? ed. v sw2 50v/div v sw1 10v/div i l2 500ma/div i l1 1a/div 0.5 s/div 3486 g17 v in = 3.6v 8 leds/25ma 2 leds/320ma circuit of front page application pwm duty cycle (%) 0.1 i led (ma) 1 10 100 0.01 1 10 100 3486 g01 0.01 0.1 v in = 12v 8/8 leds pwm freq = 100hz switching waveforms pwm dimming wavforms led current vs pwm duty cycle wide dimming range (1000:1) v fb vs v ctrl v fb vs v ctrl (temperature variation) shdn pin bias current (ctrl1 = ctrl2 = 3v) open-circuit output clamp voltage vs temperature open-circuit output clamp voltage vs v in control voltage (v) 0 feedback voltage (mv) 100 150 2 3486 g03 50 0 0.5 1 1.5 250 200 v in = 3.6v t a = 25 c control voltage (v) 0 feedback voltage (mv) 100 150 2 3486 g04 50 0 0.5 1 1.5 250 200 t a = ?50 c t a = 85 c t a = 25 c 5mv shdn pin voltage (v) 140 120 100 80 60 40 20 0 3486 g05 shdn pin bias current ( a) 01220 48 16 24 t a = 50 c t a = 25 c t a = 100 c v in = 3.6v temperature ( c) ?50 33 output clamp voltage (v) 34 35 36 37 ?25 0 25 50 3486 g06 75 100 125 v out2 v out1 v in = 3.6v r t = 63.4k v in (v) 2 output clamp voltage (v) 35 36 18 3486 g07 34 33 6 10 12 22 24 37 14 4 8 20 16 v out2 v out1 v in = 3.6v r t = 63.4k i l 500ma/div i led 200ma/div pwm 5v/div 0.2ms/div 3486 g18 v in = 12v 8/8 leds pwm freq = 1khz
LT3486 5 3486f t a = 25c unless otherwise speci? ed. input current with output 1 and output 2 open circuit r t vs oscillator frequency oscillator frequency vs v in oscillator frequency vs temperature quiescent current vs v in pwm pin input bias current switch current limit vs duty cycle ref voltage vs temperature typical perfor a ce characteristics uw v in (v) 2 input current (ma) 10 15 18 3486 g08 5 0 6 10 12 22 24 20 14 4 8 20 16 t a = 25 c r t = 63.4k oscillator frequency (khz) r t (k ? ) 1000 3486 g09 10 100 500 2500 2000 1500 1000 0 v in (v) 2 oscillator frequency (khz) 1000 1050 18 3486 g10 950 900 6 10 12 22 24 1100 14 4 8 20 16 r t = 53.6k temperature ( c) 100 10000 1000 3486 g11 oscillator frequency (khz) ?50 125 ?25 0 25 50 75 100 r t = 22.1k r t = 53.6k r t = 309k v in (v) 0 0 quiescent current (ma) 2 4 6 8 12 4 81216 3486 g12 20 2 6 10 14 18 22 24 10 shdn = 3v ctrl1 = ctrl2 = 3v uvlo pwm pin voltage (v) 0 pwm pin current ( a) 0 0.5 8 3486 g13 ? 0.5 ?1.0 2 4 6 1 0 1.0 v in = 3.6v pwm 1 pwm 2 duty cycle (%) 20 1200 current limit (ma) 1300 1400 1500 1600 40 60 80 100 3486 g14 1700 1800 30 50 70 90 v in = 3.6v temperature ( c) ?50 ?25 1.20 ref voltage (v) 1.24 1.30 0 50 75 3486 g15 1.22 1.28 1.26 25 100 125 v in = 3.6v ref voltage load regulation ref load current ( a) 0 ref voltage (v) 1.30 1.25 1.20 1.15 1.10 1.05 1.00 0.95 0.90 160 3468 g16 40 80 120 200 140 20 60 100 180 v in = 3.6v t a = 25 c t a = ?50 c t a = 25 c t a = 85 c
LT3486 6 3486f pi fu ctio s uuu sw1, sw2 (pins 1, 16): the sw pins are the collectors of the internal power transistors. connect the inductors and schottky diodes to these pins. minimize trace area at these pins to minimize emi. v in (pin 2): input supply pin. must be locally bypassed with an x5r or x7r type ceramic capacitor. ovp1, ovp2 (pins 3, 14): output overvoltage protection pins. connect these pins to the output capacitors. the on-chip voltage detectors monitor the voltages at these pins and limit it to 36v (typ) by turning off the respective switcher and pulling its v c pin low. r t (pin 4): timing resistor to program the switching frequency. the switching frequency can be programmed from 200khz to 2mhz. v c1 , v c2 (pins 5, 12): the v c pins are the outputs of the internal error ampli? er. the voltages at these pins control the peak switch currents. connect a resistor and capacitor compensation network from these pin to ground. fb1, fb2 (pins 6, 11): the LT3486 regulates the voltage at each feedback pin to 200mv. connect the cathode of the lowest led in the string and the feedback resistor (r fb ) to the respective feedback pin. the led current in each string can be programmed by: i led ? 200mv/r fb , when v ctrl > 1.8v i led ? v ctrl /(5r fb ), when v ctrl < 1v ctrl1, ctrl2 (pins 7, 10): the ctrl pins are used to provide dimming and shutdown control for the individual switching converters. connecting these to ground shuts down the respective converter. as the voltages on these pins is ramped from 0v to 1.8v, the led current in each converter ramps from 0 to i led = (200mv/r fb ). any volt- age above 1.8v does not affect the led current. pwm1, pwm2 (pins 8, 9): the pwm control pins can be used to extend the dimming range for the individual switching converter. the led current in each string can be controlled down to a levels by feeding a pwm signal to these pins. when the pwm pin voltage is taken below 0.4v, the respective converter is turned off and its v c pin is disconnected from the internal circuitry. taking it higher than 0.9v resumes normal operation. connect these pins to 0.9v supply or higher, if not in use. shdn (pin 13): shutdown pin for the device. connect it to 1.6v or higher to enable device; 0.4v or less to disable device. ref (pin 15): the internal bandgap reference (1.25v) is available at this pin. bypass with a 0.1f x5r or x7r ce- ramic capacitor. draw no more than 50a from this pin. exposed pad (pin 17): the exposed pad of the package provides an electrical contact to ground and good thermal connection to the printed circuit board (pcb). solder the exposed pad to the pcb system ground.
LT3486 7 3486f block diagra w ? + ? + ? + + ea en2 en1 ea pwm comp r sns1 r sns2 osc driver sw2 0.2v 20k 20k v c1 converter 2 control 0.2v 80k 80k shdn shdn ref fb2 15 11 ctrl2 17 10 pwm2 exposed pad 9 ctrl1 pwm1 fb1 13 8 7 6 ? + ? + + + + start-up control ref 1.25v pwm logic 16 v in 2 r t 4 3486 f01 ? + osc en1 sw1 converter1 converter2 3 1 5 ovp1 ovp2 ov2 en2 ov1 ov1 ov2 pwm logic osc ramp gen osc overvolt detection 14 q1 q2 a2 a1 a1 a2 a3 pwm comp a3 converter1 control v c2 12 overvolt detection figure 1. LT3486 block diagram
LT3486 8 3486f operatio u main control loop the LT3486 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. it incorporates two identical, but fully independent pwm converters. operation can be best understood by referring to the block diagram in figure 1. the oscillator, start-up bias and the bandgap reference are shared between the two converters. the control circuitry, power switch, dimming control etc., are all identical for both converters. at power-up, the output capacitors of both converters are charged up to v in (input supply voltage) via their respective inductor and the schottky diode. if the shdn pin is taken above 1.6v, the bandgap reference, start-up bias and the oscillator are turned on. grounding the shdn pin shuts down the part. the ctrl1 and ctrl2 pins perform independent dimming and shutdown control for the two converters. taking the ctrl pins high, enables the respective converters. connecting these pins to ground, shuts down each converter by pulling their respective v c pin low. working of the main control loop can be understood by following the operation of converter 1. at the start of each oscillator cycle, the power switch q1 is turned on. a voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the pwm comparator a2. when this voltage exceeds the level at the negative input of a2, the pwm logic turns off the power switch. the level at the negative input of a2 is set by the error ampli? er a1, and is simply an ampli? ed version of the difference between the feedback voltage and the 200mv reference voltage. in this manner, the error ampli? er a1 regulates the feedback voltage to 200mv reference voltage. the output of the error ampli? er a1 sets the correct peak current level in inductor l1 to keep the output in regulation. the ctrl1 pin voltage is used to adjust the reference voltage. the pwm1, 2 control pins are used to extend the dimming range for the individual converter. the led current in each string can be controlled down to a levels by feeding a pwm signal to these pins. refer to the applications information section for more detail. if only one of the converters is turned on, the other converter will stay off and its output will remain charged up to v in (input supply voltage). minimum output current the LT3486 can drive an 8-led string at 4ma led current without pulse skipping. as current is further reduced, the device may begin skipping pulses. this will result in some low frequency ripple, although the led current remains regulated on an average basis down to zero. the photo in figure 2 shows circuit operation with 8 white leds at 4ma current driven from 3.6v supply. peak inductor current is less than 200ma and the regulator operates in discontinuous mode implying that the inductor current reached zero during the discharge phase. after the inductor current reaches zero, the switch pin exhibits ringing due to the lc tank circuit formed by the inductor in combination with switch and diode capacitance. this ringing is not harmful; far less spectral energy is contained in the ringing than in the switch transitions. the ringing can be damped by application of a 300 resistor across the inductors, although this will degrade ef? ciency. figure 2. switching waveforms v out2 10mv/div v sw2 20v/div i l2 200ma/div 0.5 s/div 3486 f02 v in = 3.6v i led2 = 4ma (8 leds) circuit of front page application open-circuit protection the LT3486 has internal open-circuit protection for both the converters. connect the overvoltage protection pins (ovp1, ovp2) to the output of the respective converter. when the leds are disconnected from the circuit or fail open, the on-chip voltage detectors monitor the voltages at the ovp1 and ovp2 pins and limits these voltages to 36v (typ) by turning off the respective switcher. the converter will then switch at a very low frequency to minimize the input current. output voltage and input current during
LT3486 9 3486f output open circuit are shown in the typical performance characteristics graphs. figure 3a shows the transient response of switcher 1 with the leds disconnected from the output. when the led1 string is disconnected from the output, the voltage at the feedback pin (fb1) drops to 0v. as a result, the error ampli? er charges up the v c node to the clamp voltage level of 1.5v (typ). the converter starts switching at peak current limit and ramps up the output voltage. when the output voltage reaches the ovp clamp voltage level of 36v (typ), the LT3486 shuts off the converter by pulling the v c node to ground. the converter then regulates the output voltage at 36v (typ) by switching at a very low frequency. in the event one of the converters has an output open- circuit, its output voltage will be clamped at 36v (typ). however, the other converter will continue functioning properly. the photo in figure 3b shows circuit operation with converter 1 output open-circuit and converter 2 driving eight leds at 25ma. converter 1 starts switching at a very low frequency, reducing its input current. soft-start the LT3486 has a separate internal soft-start circuitry for each converter. soft-start helps to limit the inrush current during start-up. soft-start is achieved by clamping the output of the error ampli? er during the soft-start period. this limits the peak inductor current and ramps up the output voltage in a controlled manner. the converter enters into soft-start mode whenever the respective ctrl pin is pulled from low to high. figure 4 shows the start-up waveforms with converter 2 driving eight leds at 25ma. the ? ltered input current, as shown in figure 4, is well controlled. the soft-start circuit is more effective when driving a smaller number of leds. undervoltage lockout the LT3486 has an undervoltage lockout circuit which shuts down both the converters when the input voltage drops below 2.1v (typ). this prevents the converter to operate in an erratic mode when powered from low supply voltages. overtemperature protection the maximum allowable junction temperature for LT3486 is 125c. in normal operation, the ics junction temperature should be kept below 125c at an ambient temperature of 85c or less. if the junction temperature exceeds 150c, the internal thermal shutdown circuitry kicks in and turns-off both the converters. the converters will remain off until the die temperature falls below 150c. operatio u v out1 20v/div v c1 2v/div i l1 1a/div 100 s/div 3486 f03a v in = 3.6v circuit of front page application led1 disconnected at this instant v out1 1v/div ac coupled i l1 1a/div i l2 500ma/div 2ms/div 3486 f03b v in = 3.6v circuit of front page application led1 disconnected v out2 10v/div v fb2 200mv/div i in 200ma/div ctrl2 5v/div 0.5ms/div 3486 f04 v in = 3.6v 8 leds, 25ma circuit of front page application figure 3a. transient response of switcher 1 with led1 disconnected from the output figure 3b. switching waveforms with output 1 open circuit figure 4. start-up waveforms
LT3486 10 3486f figure 5. timing resistor (r t ) value operating frequency selection the choice of operating frequency is determined by sev- eral factors. there is a tradeoff between ef? ciency and component size. higher switching frequency allows the use of smaller inductors albeit at the cost of increased switching losses and decreased ef? ciency. another consideration is the maximum duty cycle achievable. in certain applications the converter needs to operate at the maximum duty cycle in order to light up the maximum number of leds. the LT3486 has a ? xed oscillator off-time and a variable on-time. as a result, the maximum duty cycle increases as the switching frequency is decreased. the circuit of figure 6a is operated with different values of timing resistor (r t ). r t is chosen so as to run the converters at 800khz (r t = 63.4k), 1.25mhz (r t = 39.1k) and 2mhz (r t = 21.5k). the ctrl pins are used to provide dimming for the respective led strings. the ef? ciency comparison for different r t values is shown in figure 6b. applicatio s i for atio wu u u figure 6a. 5v to 8/8 white leds duty cycle the duty cycle for a step-up converter is given by: d vvv vvv out d in out d cesat = + + ? ? where: v out = output voltage v d = schottky forward voltage drop v cesat = saturation voltage of the switch v in = input battery voltage the maximum duty cycle achievable for LT3486 is 96% (typ) when running at 1mhz switching frequency. it in- creases to 98% (typ) when run at 200khz and drops to 90% (typ) at 2mhz. always ensure that the converter is not duty-cycle limited when powering the leds at a given switching frequency. setting the switching frequency the LT3486 uses a constant frequency architecture that can be programmed over a 200khz to 2mhz range with a single external timing resistor from the r t pin to ground. the nominal voltage on the r t pin is 0.54v, and the current that ? ows into the timing resistor is used to charge and discharge an internal oscillator capacitor. a graph for selecting the value of r t for a given operating frequency is shown in the figure 5. ref 1.25v 25ma 25ma off on 8.06 8.06 v c1 v c2 r t r t 2.8k 2.8k c in : 10v, x7r c out1 , c out2 : 35v, x5r d1, d2: zetex zhcs400 l1, l2: toko d53lc type a 4.7nf 4.7nf sw1 sw2 v in 5v d1 d2 l1 10 h l2 10 h c in 10 f c ref 0.1 f c out1 2.2 f c out2 2.2 f LT3486 3486 f06a ovp2 ctrl2 ref pwm2 fb2 ovp1 ctrl1 shdn pwm1 fb1 oscillator frequency (khz) r t (k ? ) 1000 3486 g09 10 100 500 2500 2000 1500 1000 0
LT3486 11 3486f figure 6b. ef? ciency comparison for different r t resistors inductor selection the choice of the inductor will depend on the selection of switching frequency of LT3486. the switching frequency can be programmed from 200khz to 2mhz. higher switch- ing frequency allows the use of smaller inductors albeit at the cost of increased switching losses. the inductor current ripple ( i l ), neglecting the drop across the schottky diode and the switch, is given by: ?= () i vv v vfl l in min out max in min out max () ( ) () () ? where: l = inductor f = operating frequency v in(min) = minimum input voltage v out(max) = maximum output voltage the i l is typically set to 20% to 40% of the maximum inductor current. the inductor should have a saturation current rating greater than the peak inductor current required for the application. also, ensure that the inductor has a low dcr (copper wire resistance) to minimize i 2 r power losses. recommended inductor values range from 4.7h to 22h. several inductors that work well with the LT3486 are listed in table 1. consult each manufacturer for more detailed in- formation and for their entire selection of related parts. table 1. recommended inductors max current l dcr rating part (h) ( ) (a) vendor lqh55dn150m 15 0.150 1.40 murata lqh55dn220m 22 0.190 1.20 (814) 237-1431 www.murata.com a915ay-4r7m 4.7 0.045 2.49 toko a915ay-6r8m 6.8 0.068 2.01 (847) 297-0070 a915ay-100m 10 0.090 1.77 www.toko.com a918cy-100m 10 0.098 1.22 a918cy-150m 15 0.149 0.94 cdrh4d28-100 10 0.048 1.30 sumida cdrh5d18-150 15 0.145 0.97 (847) 956-0666 www.sumida.com capacitor selection the small size of ceramic capacitors make them ideal for LT3486 applications. use only x5r and x7r types because they retain their capacitance over wider voltage and temperature ranges than other types such as y5v or z5u. a 4.7f or larger input capacitor is suf? cient for most applications. always use a capacitor with suf? cient voltage rating. table 2 shows a list of several ceramic capacitor manufac- turers. consult the manufacturers for detailed information on their entire selection of ceramic parts. table 2. ceramic capacitor manufacturers taiyo yuden (408) 573-4150 www.t-yuden.com avx (803) 448-9411 www.avxcorp.com murata (714) 852-2001 www.murata.com diode selection schottky diodes with their low forward voltage drop and fast reverse recovery, are the ideal choices for LT3486 applications. the diode conducts current only during the switch off time. the peak reverse voltage that the diode must withstand is equal to the regulator output voltage. applicatio s i for atio wu u u led current (ma) 0 30 efficiency (%) 40 50 60 70 80 90 5101520 3486 f06b 25 v in = 5v 8/8 leds r t = 63.4k r t = 21.5k r t = 39.1k
LT3486 12 3486f the average forward current in normal operation is equal to the output current, and the peak current is equal to the peak inductor current. a schottky diode rated at 1a is suf- ? cient for most LT3486 applications. some recommended schottky diodes are listed in table 3. table 3. recommended schottky diodes part number v r (v) i avg (a) manufacturer mbr0530 30 0.5 on semiconductor mbrm120e 20 1 www.onsemi.com zlls400 40 0.4 zetex zlls1000 40 1 www.zetex.com zhcs400 40 0.4 zhcs1000 40 1 when the LT3486 is set up for pwm dimming operation, choose a schottky diode with low reverse leakage current. during pwm dimming operation, the output capacitor is required to hold up the charge in the pwm off period. a low reverse leakage schottky helps in that mode of op- eration. the zetex zlls400 and zlls1000 are available in a small surface mount package and are a good ? t for this application. mosfet selection the power mosfet used in LT3486 applications with wide dimming range requirements should be chosen based on the maximum drain-source voltage. the maximum drain current i d(max) and gate-to-source voltages should also be considered when choosing the fet. choose a mosfet with maximum v ds (drain source) volt- age greater than the output clamp voltage i.e., 36v (typ). fairchild semiconductors fdn5630 (60v, 1.7a n-channel fet) is a good ? t for most LT3486 applications. for dim- ming low current leds (~25ma), fairchild 2n7002 is a good alternative. programming led current the current in each led string can be set independently by the choice of resistors r fb1 and r fb2 respectively (see front page application). the feedback reference is 200mv. in order to have accurate led current, precision resistors are preferred (1% is recommended). r mv i r mv i fb led fb led 1 1 2 2 200 200 = = table 4. r fb value selection i led (ma) r fb ( ) 5 40.2 10 20.0 15 13.3 20 10.0 25 8.06 most low power white leds are driven at maximum cur- rents of 15ma to 25ma. the LT3486 can be used to power high power leds as well. refer to the typical applications for more detail. dimming control the dimming of the two led strings can be controlled independently by modulating the respective ctrl and pwm pins. there are two ways to control the intensity of the leds. adjusting the led current value controlling the current ? owing through the leds controls the intensity of the leds.this is the easiest way to control the intensity of the leds. the led forward current can be controlled by modulating the dc voltage at the respective crtl pin. the pwm pins are not in use when appying this scheme. they must be connected to a 0.9v supply or higher. the dc voltage at the ctrl pin can be modulated in two ways. (a) using a dc voltage source for some applications, the preferred method of brightness control is a variable dc voltage fed to the ctrl pins. the ctrl1, ctrl2 pin voltage can be modulated to set the dimming of the respective led string. as the voltage on the ctrl1, ctrl2 pin increases from 0v to 1.8v, the led current increases from 0 to i led . as the ctrl1, ctrl2 pin voltage increases beyond 1.8v, it has no effect on the led current. applicatio s i for atio wu u u
LT3486 13 3486f the led current can be set by: i led (200mv/r fb ), when v ctrl > 1.8v i led (v ctrl /5 ? r fb ), when v ctrl < 1v feedback voltage variation versus control voltage is given in the typical performance characteristics graphs. (b) using a filtered pwm signal a variable duty cycle pwm can be used to control the brightness of the led string. the pwm signal is ? ltered (figure 7) by an rc network and fed to the ctrl1, ctrl2 pins. the corner frequency of r1, c1 should be much lower than the frequency of the pwm signal. r1 needs to be much smaller than the internal impedance in the ctrl pins, which is 100k . pulse-width modulation (pwm) adjusting the forward current ? owing in the leds changes the intensity of the leds, as explained in the previous sec- tion. however, a change in forward current also changes the color of the leds. the chromaticity of the leds changes with the change in forward current. many applications can- not tolerate any shift in the color of the leds. controlling the intensity of the leds via applying a pwm signal allows dimming of the leds without changing the color. dimming the leds via a pwm signal essentially involves turning the leds on and off at the pwm frequency. the human eye has a limit of 60 frames per second. by in- creasing the pwm frequency to say, 80hz, the eye can be deceived into believing that the pulsed light source is continously on. additionally by modulating the duty cycle (amount of on-time), the intensity of the leds can be controlled. the color of the leds remains unchanged in this scheme since the led current value is either zero or a constant value. figure 8(a) shows a 12v to 8/8 white led driver. the pwm dimming control method requires an external nmos tied to the cathode of the lowest led in the string, as shown in 3486 f07 c1 1 f pwm 10khz typ r1 10k LT3486 ctrl1,2 figure 7. dimming control using a filtered pwm signal applicatio s i for atio wu u u 3486 ta10a 12v (typ) 9v to 15v c out1 2.2 f c out2 2.2 f 100ma pwm freq 1khz r fb1 2 ? r fb2 2 ? 100ma off on d1 q1 100k 100k 3.65k 3.65k 21.5k q2 5v d2 c in 10 f c1 1 f l1 10 h l2 10 h v in v in dimming input 1 dimming input 2 2.2nf 22pf 2.2nf luxeon leds lxcl-pwf1 luxeon leds lxcl-pwf1 c out1 , c out2 : 35v, x5r or x7r c in : 25v, x5r or x7r c1: 10v, x5r or x7r c ref : 6.3v, x5r or x7r c ref 0.1 f v c1 v c2 r t sw1 sw2 v in LT3486 ovp2 ctrl2 ref pwm2 fb2 ovp1 ctrl1 shdn pwm1 fb1 pwm freq 1khz d1, d2: zetex zlls1000 l1, l2: toko d53lc (type a) q1, q2: fairchild fdn5630 i l 500ma/div i led 200ma/div pwm 5v/div 0.2ms/div 3486 g18 v in = 12v 8/8 leds pwm freq = 1khz figure 8a. 12v to 8/8 white leds figure 8b. pwm dimming waveforms
LT3486 14 3486f s i for atio applicatio wu u u figure 10. recommended layout for LT3486 the ? gure. a pwm logic input is applied to the gate of the nmos and the pwm pin of the LT3486. when the pwm input is taken high, the leds are connected to the r fb resistor and a current i led = 200mv/r fb ? ows through the leds. when the pwm input is taken low, the leds are disconnected and turn off. the low pwm input applied to the LT3486 ensures that the respective converter turns off and its v c pin goes high impedance. this ensures that the capacitor connected to the v c pin retains its voltage which in turn allows the leds to turn on faster, as shown in figure 8(b). the ctrl pin is not used to modulate the led current in the scheme. it can be connected to a supply voltage greater than 1.8v. the dimming control pins (pwm1, pwm2) can be used to extend the dimming range for the individual switching converters. the led current can be controlled down to a levels by feeding a pwm signal with frequencies in the range of 80hz to 50khz. the led current can be controlled by pwm frequencies above 50khz but the controllable current decreases with increasing frequency. pulling the pwm pins below 0.4v disables the respective switcher. taking it higher than 0.9v resumes normal operation. connect these pins to 0.9v or higher if not in use. figure 9 shows the led current variation vs pwm duty cycle. the led current is controlled by applying a pwm of frequency 100hz, 1khz and 25khz to the circuit of figure 8a. as seen in the curves, the led string is able to get a wide (1000:1) dimming range with pwm frequency of 100hz. the dimming range decreases as pwm frequency goes up. board layout consideration 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 pins (sw1 and sw2). keep the feedback pins (fb1 and fb2) away from the switching nodes. the dfn and fe packages both have an exposed paddle that must be connected to the system ground. the ground connection for the feedback resistors should be tied directly to the ground plane and not shared with any other compo- nent, except the r t resistor, ensuring a clean, noise-free connection. recommended component placement for the dfn package is shown in the figure 10. figure 9. led current variation vs pwm duty cycle pwm duty cycle (%) 0.1 led current (ma) 1 10 100 0.01 1 10 100 3486 f09 0.01 0.1 pwm freq = 100hz pwm freq = 1khz pwm freq = 25khz 16 15 14 13 12 11 10 9 17 1 2 3 4 5 6 7 8 3486 f10 ovp1 v out1 v out2 fb1 fb2 led1 led2 ctrl1 ctrl2 v in sw1 sw2 v in v in r t v c1 pwm1 ref ovp2 shdn v c2 pwm2 vias to v in plane vias to ground plane vias to v in plane
LT3486 15 3486f typical applicatio s u li-ion cell powered driver for camera flash and lcd backlighting 3486 ta02a v in 3v to 5v d1 l1 10 h l2 10 h d2 c in 10 f c ref 0.1 f c out2 2.2 f c out1 2.2 f 4.7nf 0.1 f 25ma dimming 2 dimming 1 320ma off on off on r fb2 8.06 r fb1 0.62 63.4k 100k 2.8k q1 led1 aot3218 5v 0v c in : 6.3v, x5r or x7r dielectric c out1 , c out2 : 35v, x5r or x7r d1: zetex zhcs1000 d2: zetex zhcs400 l1, l2: toko d53lc (type a) q1: fairchild fdn5630 v c1 v c2 r t sw1 sw2 v in LT3486 ovp2 ctrl2 ref pwm2 fb2 ovp1 ctrl1 shdn pwm1 fb1 v in (v) 3 65 efficiency (%) 70 75 80 85 90 3.2 3.4 3.6 3.8 3486 ta01b 4 4.2 movie mode i led1 = 175ma flash mode i led1 = 320ma 8 leds/25ma ef? ciency vs v in
LT3486 16 3486f 3486 ta05a 3v to 5v c out1 2.2 f c out2 2.2 f 25ma pwm2 100hz 8.06 ? 8.06 ? 25ma off on d1 q1 5v 100k 100k 2.8k 2.8k 63.4k q2 d2 c in 10 f l1 10 h l2 10 h v in v in 4.7nf 4.7nf 8 leds 8 leds c out1 , c out2 : 35v, x5r or x7r c in : 10v, x5r or x7r c ref 0.1 f v c1 v c2 r t sw1 sw2 v in LT3486 ovp2 ctrl2 ref pwm2 fb2 ovp1 ctrl1 shdn pwm1 fb1 pwm1 100hz d1, d2: zetex zlls400 l1, l2: toko d53lc (type a) q1, q2: fairchild 2n7002 1 li-ion cell to 8/8 white leds pwm duty cycle (%) 0 75 80 85 80 3486 ta05b 70 65 20 40 60 100 60 55 50 25 30 35 20 15 10 5 0 efficiency (%) led current (ma) v in = 3.6v 8/8 leds efficiency led current led c urren t an d ef? c i ency vs pwm d u t y c yc l e wid e (250 : 1) di mm i ng r ange (led current 0.1ma to 25ma) i l 200ma/div led current 20ma/div pwm 5v/div 2ms/div 3486 ta05c v in = 3.6v ctrl1 = 3.6v 8 leds/25ma pwm freq = 100hz duty cycle (%) 0.1 led current (ma) 10 100 1 10 100 3486 ta05d 1 0.10 0.01 v in = 3.6v 8/8 leds pwm freq = 100hz pwm di mm i ng w ave f orms typical applicatio s u
LT3486 17 3486f typical applicatio s u 5v to 16/16 white leds led c urren t an d ef? c i ency vs pwm d u t y c yc l e pwm di mm i ng w ave f orms 3486 ta08a 5v d5 d6 d3 d4 d1 l2 15 h l1 15 h d2 25ma 25ma off on 16 leds 16 leds v in v in q1 8.06 8.06 q2 100k 4.02k 4.02k 4.7nf 4.7nf 63.4k 22pf pwm freq 200hz 100k c out2 2.2 f c out1 2.2 f c2 0.1 f c4 1 f c3 1 f c1 0.1 f c in 1 f c ref 0.1 f c in : 6.3v, x5r or x7r c out1 , c out2 : 35v, x5r or x7r c1-c4: 50v, x5r or x7r c ref : 6.3v, x5r or x7r d1, d2: zetex zlls400 d3-d6: philips bav99w l1, l2: toko d53lc (type a) q1, q2: fairchild 2n7002 v c1 v c2 r t sw1 sw2 v in LT3486 ovp2 ctrl2 ref pwm2 fb2 ovp1 ctrl1 shdn pwm1 fb1 pwm freq 200hz pwm duty cycle (%) 0 75 80 85 80 3486 ta08b 70 65 20 40 60 100 60 55 50 25 30 35 20 15 10 5 0 efficiency (%) led current (ma) v in = 5v 16/16 leds efficiency led current i l 500ma/div i led 50ma/div pwm 5v/div 1ms/div 3486 ta08c l = 15 h pwm freq = 200hz
LT3486 18 3486f 3.00 0.10 (2 sides) 5.00 0.10 (2 sides) note: 1. drawing proposed to be made variation of version (wjed-1) in jedec package outline mo-229 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.40 0.10 bottom view?exposed pad 1.65 0.10 (2 sides) 0.75 0.05 r = 0.115 typ r = 0.20 typ 4.40 0.10 (2 sides) 1 8 16 9 pin 1 top mark (see note 6) 0.200 ref 0.00 ? 0.05 (dhc16) dfn 1103 0.25 0.05 pin 1 notch 0.50 bsc 4.40 0.05 (2 sides) recommended solder pad pitch and dimensions 1.65 0.05 (2 sides) 2.20 0.05 0.50 bsc 0.65 0.05 3.50 0.05 package outline 0.25 0.05 package descriptio u dhc package 16-lead plastic dfn (5mm 3mm) (reference ltc dwg # 05-08-1706)
LT3486 19 3486f package descriptio u fe package 16-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663) exposed pad variation bb fe16 (bb) 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 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.
LT3486 20 3486f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2005 lt/tp 0705 500 ? printed in usa typical applicatio u related parts 12v to 8/8 white leds led current and ef? ciency vs pwm duty cycle 3486 ta10a 12v (typ) 9v to 15v c out1 2.2 f c out2 2.2 f 100ma pwm freq 1khz r fb1 2 ? r fb2 2 ? 100ma off on d1 q1 100k 100k 3.65k 3.65k 21.5k q2 5v d2 c in 10 f c1 1 f l1 10 h l2 10 h v in v in dimming input 1 dimming input 2 2.2nf 22pf 2.2nf luxeon leds lxcl-pwf1 luxeon leds lxcl-pwf1 c out1 , c out2 : 35v, x5r or x7r c in : 25v, x5r or x7r c1: 10v, x5r or x7r c ref : 6.3v, x5r or x7r c ref 0.1 f v c1 v c2 r t sw1 sw2 v in LT3486 ovp2 ctrl2 ref pwm2 fb2 ovp1 ctrl1 shdn pwm1 fb1 pwm freq 1khz d1, d2: zetex zlls1000 l1, l2: toko d53lc (type a) q1, q2: fairchild fdn5630 pwm duty cycle (%) 0 60 efficiency (%) led current (ma) 65 70 75 80 85 90 0 20 40 60 80 100 120 20 40 60 80 3486 ta10b 100 efficiency led current v in = 12v 8/8 leds part number description comments lt1618 constant current, constant voltage 1.24mhz, high ef? ciency up to 16 white leds, v in : 1.6v to 18v, v out(max) = 34v, boost regulator i q = 1.8ma, i sd < 1a, ms package lt1932 constant current, 1.2mhz, high ef? ciency white led boost up to 8 white leds, v in : 1v to 10v, v out(max) = 34v, regulator i q = 1.2ma, i sd < 1a, thinsot tm package lt1937 constant current, 1.2mhz, high ef? ciency white led boost up to 4 white leds, v in : 2.5v to 10v, v out(max) = 34v, regulator i q = 1.9ma, i sd < 1a, thinsot, sc70 packages ltc3200 low noise, 2mhz, regulated charge pump white led driver up to 6 white leds, v in : 2.7v to 4.5v, i q = 8ma, i sd < 1a, ms package ltc3200-5 low noise, 2mhz, regulated charge pump white led driver up to 6 white leds, v in : 2.7v to 4.5v, i q = 8ma, i sd < 1a, thinsot package ltc3201 low noise, 1.7mhz, regulated charge pump white led driver up to 6 white leds, v in : 2.7v to 4.5v, i q = 6.5ma, i sd < 1a, ms package ltc3202 low noise, 1.5mhz, regulated charge pump white led driver up to 8 white leds, v in : 2.7v to 4.5v, i q = 5ma, i sd < 1a, ms package ltc3205 high ef? ciency, multidisplay led controller up to 4 (main), 2 (sub) and rgb, v in : 2.8v to 4.5v, i q = 50a, i sd < 1a, qfn-24 package lt3465/lt3465a constant current, 1.2mhz/2.7mhz, high ef? ciency white led up to six white leds, v in : 2.7v to 16v, v out(max) = 34v, boost regulator with integrated schottky diode i q = 1.9ma, i sd < 1a, thinsot package lt3466 dual full function white led boost regulator with integrated drives up to 20 leds, v in : 2.7v to 24v, v out(max) = 40v, schottky diode i q = 5ma, i sd < 16a, dfn package

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