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ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 1 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated 30v 1a led driver with aec-q100 description the ZXLD1360 is a continuous mode inductive step-down converter with integrated switch and high side current sense. it operates from an input supply from 7v to 30v driving single or multiple series connected leds efficiently externally adjustable output current up to 1ma. the ZXLD1360 has been qualified to aec-q100 grade 1 enabling operation in ambient temperatures from -40c to 125c. the output current can be adju sted by applying a dc voltage or a pwm waveform to the adj pin; 100:1 adjustment of output current is possible using pwm control. applying 0.2v or lower to the adj pin turns the output off and switches the device into a low current standby state. pin assignments features ? simple low parts count ? single pin on/off and brightness control using dc voltage or pwm ? high efficiency (up to 95%) ? wide input voltage range: 7v to 30v ? 40v transient capability ? qualified to aec-q100 grade 1 ? available in thermally enhanced packages o tsot23-5 ja 82 c/w ? available in green molding (no br, sb) with lead free finish/rohs compliant ? up to 1mhz switching frequency ? typical 4% output current accuracy typical application circuit i sense v in adj gnd lx tsot23-5 top view ? v in i sense lx gnd ZXLD1360 adj v in (7 v - 3 0v) rs 0.1v 4.7f c1 gnd n/c d 1 47mhl1
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 2 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated block diagram figure 1. block diagram ? pin connection for tsot23-5 package block diagram name pin no. description lx 1 drain of ndmos switch gnd 2 ground (0v) adj 3 multi-function on/off and br ightness control pin: ? leave floating for normal operation.(v adj = v ref = 1.25v giving nominal average output current o i outnom = 0.1/rs) ? drive to voltage below 0.2v to turn off output current ? drive with dc voltage (0.3v < v adj < 2.5v) to adjust output current from 25% to 200% of i outnom ? drive with pwm signal from open-collector or open-drain transistor, to adjust output current. ? adjustment range 25% to 100% of i outnom for f>10khz and 1% to 100% of i outnom for f < 500hz ? connect a capacitor from this pin to ground to in crease soft-start time. (d efault soft-start time = 0.5ms. additional soft-start time is approx.0.5ms/nf) i sense 4 connect resistor r s from this to v in to define nominal average output current i outnom =0.1/r s (note: r smin =0.1v with adj pin open circuit) v in 5 input voltage (7v to 30v). decouple to ground with 4.7f of higher x7r ceramic capacitor close to device low voltage detector voltage regulator lx mn l1 d1 i sense adj gnd v in v in 200k 20k 1.25v 1.35v 0.2v + - + - + - r4 r5 r2 r3 r1 2 3 54 1 d1 + - r s c1 4.7mf 5v
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 3 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated absolute maximum ratings (voltages to gnd unless otherwise stated) symbol parameter rating unit v in input voltage -0.3 to +30 (40v for 0.5 sec) v v sense i sense voltage +0.3 to -5 (measured with respect to v in ) v v lx lx output voltage -0.3 to +30 (40v for 0.5 sec) v v adj adjust pin input voltage -0.3 to +6 v i lx switch output current 1.25 a p tot power dissipation (refer to package thermal de-rating curve on page 20) 1 w t st storage temperature -55 to 150 c t j max junction temperature 150 c these are stress ratings only. operation above the absolute maxi mum rating may cause device failure. operation at the absolute maximum ratings, for extended periods, may reduce device reliability. esd susceptibility rating unit human body model 500 v machine model <100 v semiconductor devices are esd sensitive and may be damaged by expos ure to esd events. suitable esd precautions should be taken when handling and transporting these devices. the human body model is a 100pf capacitor discharge through a 1.5k ? resistor pin. the machine model is a 200pf capacitor discharged directly into each pin thermal resistance symbol parameter rating unit ja junction to ambient 82 c/w jb junction to board 33 c/w recommended operating conditions symbol parameter min max units v in input voltage range 7 30 v t offmin minimum switch off-time 800 ns t onmin minimum switch on-time 800 ns t onmin _ rec recommended minimum swit ch ?on? time 800 ns f lx max recommended maximum operating frequency 625 ? khz d lx duty cycle range 0.01 0.99 t op operating temperature range -40 125 c ? ZXLD1360 will operate at higher frequencies but due to propagation delays accuracy will be affected.
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 4 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated electrical characteristics (test conditions: v in = 12v, t amb = 25c, unless otherwise specified.) symbol parameter condition min. typ. max. unit v su internal regulator start-up threshold v in rising 5.65 v v sd internal regulator shutdown threshold v in falling 5.55 v i inqoff quiescent supply current with output off adj pin grounded 20 40 a i inqon quiescent supply current with output switching adj pin floating f=250khz 1.8 5.0 ma v sense mean current sense threshold voltage (defines led current setting accuracy) measured on i sense pin with respect to v in v adj =1.25v 95 100 105 mv v sensehys sense threshold hysteresis 15 % i sense i sense pin input current v sense =v in -0.1 1.25 10 a v ref internal refe rence voltage measured on adj pin with pin floating 1.25 v v ref / t temperature coefficient of v ref 50 ppm/c v adj external control voltage range on adj pin for dc brightness control (b) 0.3 2.5 v v adjoff dc voltage on adj pin to switch device from active (on) state to quiescent (off) state v adj falling 0.15 0.2 0.25 v v adjon dc voltage on adj pin to switch device from quiescent (off) state to active (on) state v adj rising 0.2 0.25 0.3 v r adj resistance between adj pin and v ref 0< v adj < v ref v adj >v ref +100mv 135 13.5 250 25 k? i lxmean continuous lx switch current 1 a r lx lx switch ?on? resistance @ i lx =0.55a 0.5 1.0 ? i lx(leak) lx switch leakage current 5 a d pwm(lf) duty cycle range of pwm signal applied to adj pin during low frequency pwm dimming mode pwm frequency <500hz pwm amplitude = v ref measured on adj pin 0.01 1 brightness control range 100:1 d pwm(hf) duty cycle range of pwm signal applied to adj pin during high frequency pwm dimming mode pwm frequency >10khz pwm amplitude = v ref measured on adj pin 0.16 1 brightness control range 5:1 t ss soft start time time taken for output current to reach 90% of final value after voltage on adj pin has risen above 0.3v 500 ms f lx operating frequency (see graphs for more details) adj pin floating l=33mh (0.093v) i out =1a @ v led =3.6v driving 1 led 280 khz t pd internal comparator propagation delay 50 ns notes: (a). production testing of the device is performed at 25 c. functional operation of t he device and parameters specified over a -40c to +125c temperature range, are guaranteed by desig n, characterization and process control. (b). 100% brightness corresponds to v adj = v adj(nom) = v ref . driving the adj pin above v ref will increase the v sense threshold and output current proportionally. .
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 5 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated device description the device, in conjunction with the coil (l1) and current sens e resistor (rs), forms a self-oscillating continuous-mode buck converter. device operation (refer to figure 1 - block diagram and figure 2 operating waveforms) operation can be best understood by assuming that the adj pin of the device is unconnected and the voltage on this pin (vadj) appears directly at the (+) input of the comparator. when input voltage v in is first applied, the initial current in l1 and r s is zero and there is no ou tput from the current sense circuit. under this condition, the (-) input to the comparator is at ground and its out put is high. this turns mn on and switch es the lx pin low, causing current to flow from v in to ground, via r s , l1 and the led(s). the current rises at a rate determined by v in and l1 to produce a voltage ramp (v sense ) across r s . the supply referred voltage v sense is forced across internal resistor r1 by the current sense circuit and produces a proportional current in internal resistors r2 and r3. this produces a ground referred rising voltage at the (-) input of the co mparator. when this reache s the threshold voltage (v adj ), the comparator output swit ches low and mn turns off. the comparator output also drives another nmos switch, which bypasses internal resistor r3 to provide a controlled amount of hyster esis. the hysteresis is set by r3 to be nominally 15% of v adj . when mn is off, the current in l1 continues to flow via d1 and the led(s) back to v in . the current decays at a rate determined by the led(s) and diode forward vo ltages to produce a falling voltage at the input of the co mparator. when this voltage returns to v adj , the comparator output switches high again. this cycle of events re peats, with the comparator input ramping between limits of v adj 15%. switching thresholds with v adj = v ref , the ratios of r1, r2 and r3 define an average v sense switching threshold of 100mv (measured on the i sense pin with respect to v in ). the average output current i outnom is then defined by this voltage and rs according to: i outnom = 100mv/rs nominal ripple current is 15mv/rs adjusting output current the device contains a low pass filter bet ween the adj pin and the threshold comparator and an internal current limiting resistor (200kv nom) between adj and the internal reference voltage. this allows the adj pin to be overdriven with either dc or pulse signals to change the v sense switching threshold and adjust the outp ut current. the filter is third order, comprising three sections, each with a cut-off frequency of nominally 4khz. details of the different modes of adjusting output current are give n in the applications section. output shutdown the output of the low pass filter drives th e shutdown circuit. when the input voltage to this circuit falls below the threshold (0.2v nom.), the internal regulator and the output switch are turned off. the vo ltage reference remains powered during shutdown to provide the bias current for the shutdown circuit. quiescent supply current during shutdown is nominally 20ma and switch leakage is below 5ma.
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 6 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated device description figure 2. operating waveforms 0v v in 100mv 115mv 0v sense voltage v sense+ v sense- to f f to n 85mv 0v 5v v in 0.15v adj 0.15v adj i outnom i outnom +15% i outnom -15% v adj lx voltage coil current comparator input voltage comparator output
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 7 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated device description (continued) actual operating waveforms [v in =15v, r s =0.1v, l=33h] normal operation. output current (ch1) and lx voltage (ch2) actual operating waveforms [v in =30v, r s =0.1v, l=33h] normal operation. output current (ch1) and lx voltage (ch2)
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 8 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating characteristics 0 510 15 20 25 30 35 supply voltage v (v) in ZXLD1360 output current l = 33h 1060 900 1040 1020 1000 980 960 940 920 output current (ma) 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 output current l = 33h 10% -10% 8% 6% 4% 2% 0% -4% -6% output current deviation (%) -8% -2% 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 switching frequenc y l = 33h 600 500 400 300 200 100 0 switching frequency (khz) 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 duty cycle l = 33h 100 0 90 80 70 60 50 30 20 d uty cycle (%) 10 40
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 9 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating characteristics (cont.) 0 510 15 20 25 30 35 supply voltage v (v) in ZXLD1360 output current l = 47h 1060 900 1040 1020 1000 980 960 940 920 output current (ma) 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 output current l = 47h 10% -10% 8% 6% 4% 2% 0% -4% -6% output current deviation (%) -8% -2% 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 switching frequenc y l = 47h 600 500 400 300 200 100 0 switching frequency (khz) 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 duty cycle l = 47h 100 0 90 80 70 60 50 30 20 d uty cycle (%) 10 40
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 10 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating characteristics (cont.) 0 510 15 20 25 30 35 supply voltage v (v) in ZXLD1360 output current l = 100h 1060 900 1040 1020 1000 980 960 940 920 output current (ma) 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 output current l = 100h 10% -10% 8% 6% 4% 2% 0% -4% -6% output current deviation (%) -8% -2% 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 switching frequenc y l = 10 0h 600 500 400 300 200 100 0 switching frequency (khz) 0 510 15 20 25 30 35 supply voltage v (v) in 8 led s 7 led s 6 led s 5 led s 4 led s 3 led s 2 led s 1 led ZXLD1360 duty cycle l = 100h 100 0 90 80 70 60 50 30 20 d uty cycle (%) 10 40
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 11 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating characteristics (cont.) 012 3 4567 supply voltage v (v) in v vs. supply voltage ref 8 1.4 1.2 1 0.8 0.6 0.4 0.2 0 v (v) ref 0510 15 20 25 30 35 supply voltage v (v) in v vs. supply voltage ref v (v) ref 1.2364 v (v) ref 1.237 2 1.2365 1.2371 1.2366 1.2370 1.2367 1.2368 1.2369 0510 15 20 25 30 35 supply voltage v (v) in supply current vs. supply voltage 0 100 600 500 400 300 200 0510 15 20 25 30 35 su pply vo ltag e v (v) in shutdown current vs. supply voltage 0 i (a) in 18 4 16 6 14 8 10 12 2 12 00 12 00 80 0 60 0 40 0 20 0 0 0123 adj pin voltage (v) r = 330m r = 150m r = 100m led curren t (ma)
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 12 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating characteristics (cont.) -55 -35 -15 5 25 45 65 85 105 125 ambient temperature ( c) min lx off min lx on 0 50 100 150 200 250 300 350 response time (ns) ZXLD1360 response time vs. temperature typical minimum lx ?on? and ?off? time lx switch ?on? resistance vs. temperature -50 ambient temperature ( c) 0 50 100 150 200 0.20 0.30 0.40 0.50 0.60 0.70 0.80 v vs. temperature adj l = 470h, r = 0.33 s -55 -35 -15 5 25 45 65 85 10 5 125 ambient temperature ( c) 24v, three led 24v, single led 12v, three led 1 2v, s in g le le d 1.215 1.225 1.235 1.23 1.24 1.22 1.21 v (v) adj -55 -35 -15 5 25 45 65 85 105 125 ambient temperature ( c) 24v, three led 24v, single led 12v, three led 12v, single led 98.6 99.6 99.8 100.4 99.4 98.4 v (v) se ns e 100.2 100 99.2 99 98.8 v oltage across r (0.333 ) vs. temperatur e sense -55 -35 -15 5 25 45 65 85 105 125 ambient temperature ( c) 12v, three led 12v, single led output current change vs. temperatur e v = 12v, l = 470h, r = 0.33 in s 0.5 deviation from nominal set value (%) 0.4 0.3 0.2 0.1 0 -0.2 -0.1 -0.3 -0.4 -0.5 output current change vs. temperatur e v = 24v, l = 470h, r = 0.33 in s -55 -35 -15 5 25 45 65 85 105 125 ambient temperature ( c) 24v, three led 24v, single led deviation from nominal set value (%) 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 13 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information setting nominal average output current with external resistor r s the nominal average output current in the led(s) is determined by the value of the external current sense resistor (r s ) connected between v in and i sense and is given by: i outnom = 0.1/r s [for r s > 0.1 ? ] the table below gives values of nominal average output current for several prefe rred values of current setting resistor (r s ) in the typical application circuit shown on page 1: r s ( ) nominal average output current (ma) 0.1 1000 0.13 760 0.15 667 the above values assume that the adj pin is floating and at a nominal voltage of v ref (=1.25v). note that r s = 0.1v is the minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value. it is possible to use different values of r s if the adj pin is driven from an exte rnal voltage. (see next section). output current adjustment by external dc control voltage the adj pin can be driven by an external dc voltage (v adj ), as shown, to adjust the output current to a value above or below the nominal average value defined by r s . the nominal average output current in this case is given by: i outdc = (v adj /1.25) x (100mv/r s ) [for 0.3< v adj <2.5v] note that 100% brightness setting corresponds to v adj = v ref . when driving the adj pin above 1.25v, r s must be increased in proportion to prevent i outdc exceeding 550ma maximum. the input impedance of the adj pin is 50k ? 25% for voltages below v ref and 20k ? 25% for voltages above v ref +100mv. gnd ZXLD1360 adj gnd + dc
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 14 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) directly driving adj input a pulse width modulated (pwm) signal with duty cycle d pwm can be applied to the adj pin, as shown below, to adjust the output current to a value above or below t he nominal average value set by resistor r s : driving the adj input via open collector transistor the recommended method of driving the adj pin and controlling the amplitude of the pwm waveform is to use a small npn switching transistor as shown below: this scheme uses the 200k resistor between the adj pin and th e internal voltage reference as a pull-up resistor for the external transistor. driving the adj input from a microcontroller another possibility is to drive the device from the open drain output of a micr ocontroller. the diagram below shows one method of doing this: if the nmos transistor within the microcontroller has high drai n / source capacitance, this arrangement can inject a negative spike into adj input of the 1360 and cause erratic operation bu t the addition of a schottky clamp diode (cathode to adj) to ground and inclusion of a series resistor (10k) will prevent this. see the section on pwm dimming for more details of the various modes of control using high frequency and low frequency pwm signals. ? pwm gnd 0v v adj gnd ZXLD1360 adj pwm gnd ZXLD1360 adj gnd gnd ZXLD1360 adj mcu 10k
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 15 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) shutdown mode taking the adj pin to a voltage below 0.2v for more than approximately 100s will turn off the output and supply current to a low standby level of 20a nominal. note that the adj pin is not a logic input. taking the adj pin to a voltage above v ref will increase output current above the 100% nominal average value. (see graphs for details). soft-start the device has inbuilt soft-start action due to the delay through the pwm filter. an external capacitor from the adj pin to ground will provide additional soft-start delay, by increasing the time taken for the voltage on this pin to rise to the turn-o n threshold and by slowing down the rate of rise of the cont rol voltage at the input of the comparator. with no external capacitor, the time taken for the output to reac h 90% of its final value is approximately 500 s. adding capacitance increases this delay by approximately 0.5ms/nf. the graph below shows the va riation of soft-start time for different values of capacitor. actual operating waveforms [v in =15v, r s =0.1v, l=33h, 0nf on adj] soft-start operation. output curr ent (ch2) and lx voltage (ch1) the trace above shows the typical soft startup time (t ss ) of 500msec with no additional capacitance added to the adj pin. dnuorgotnipjdamorfecnaticapacsvemittratstfos 0 2 4 6 8 01 52 02 51 01 5 0 )fn(ecnaticapac soft start time (ms)
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 16 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) this time has been extended on the trace below by adding a 100nf ceramic capacitor which gives a soft start time of 40 milliseconds approximately. actual operating waveforms [v in =15v, r s =0.1 v , l=33 h ,100nf on adj] soft-start operation. output curr ent (ch2) and lx voltage (ch1) inherent open-circuit led protection if the connection to the led(s) is open-circuit ed, the coil is isolated fr om the lx pin of the chip, so the device will not be damaged, unlike in many boost converters, where the back emf may damage the internal switch by forcing the drain above its breakdown voltage. capacitor selection a low esr capacitor should be used for input decoupling, as t he esr of this capacitor appears in series with the supply source impedance and lowers overall efficiency. this capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. a minimum value of 4.7 f is acceptable if the input source is close to the device, but higher values will improve pe rformance at lower input voltages, especia lly when the source impedance is high. the input capacitor should be placed as close as possible to the ic. for maximum stability over temperature a nd voltage, capacitors with x7r, x5r, or better dielectric are recommended. capacitors with y5v dielectric are not suitable for decoupling in this application and should not be used. a suitable murata capacitor would be grm42-2x7r475k-50. the following web sites are useful when finding alternatives: www.murata.com www.t-yuden.com www.kemet.com www.avxcorp.com
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 17 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) inductor selection recommended inductor values for the ZXLD1360 are in the range 33mh to 100mh. higher values of inductance are recommended at higher supply volt ages in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. higher values of inductance also result in a smaller change in output current over the supply voltage range. (s ee graphs). the inductor should be mounted as close to the device as possible with low resistance connections to the lx and vin pins. the chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. suitable coils for use with the ZXLD1360 are listed in the table below: part no. l ( mh) dcr (v) isat (a) manufacturer mss1038-333 33 0.093 2.3 coilcraft www.coilcraft.com mss1038-683 68 0.213 1.5 npis64d330mtrf 33 0.124 1.1 nic www.niccomp.com the inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times within the specified limits ov er the supply voltage and load current range. the following equations can be used as a guide, wi th reference to figure 1 - operating waveforms. lx switch ?on? time t on = t off = note: t offmin > 200ns where: l is the coil inductance (h) rl is the coil resistance (v) r s is the current sense resistance i avg is the required led current (a) di is the coil peak-peak ripple current (a) {internally set to 0.3 x iavg} v in is the supply voltage (v) v led is the total led forward voltage (v) r lx is the switch resistance (v) {=0.5v nominal} v d is the diode forward voltage at the required load current (v) l i v in ? v led ?i avg (r s + rl + r lx ) l i v led + vd + i avg (r s + rl)
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 18 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) example: for v in =12v, l=33mh, rl=0.093, r s =0.1 , r lx =0.15v, v led =3.6v, i avg =1a and v d =0.49v t on = (33e-6 x 0.3)/(12 - 3.6 - 0.693) = 1.28ms t off = (33e-6 x 0.3)/(3.6 + 0.49 + 0.193)= 2.31ms this gives an operating frequency of 280khz and a duty cycle of 0.35. these and other equations are available as a spre adsheet calculator from the diodes website at www.diodes.com note that, in practice, the duty cycle and operating frequency will deviate from the calculated values due to dynamic switching delays, switch rise/fall times and losses in the external components. optimum performance will be achieved by setting the duty cycle clos e to 0.5 at the nominal supp ly voltage. this helps to equalize the undershoot and overshoot and improves temperature stability of the output current. diode selection for maximum efficiency and performance, the rectifier (d1) should be a fast low capacitance schottky diode with low reverse leakage at the maximum operati ng voltage and temperature. they also provide better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time. it is important to select parts with a peak current rating above the peak coil current and a cont inuous current rating higher than the maximum output load current. it is very important to c onsider the reverse leakage of the diode when operating above 85c. excess leakage will increase the power dissipation in the dev ice and if close to the load may create a thermal runaway condition. the higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the lx output. if a silicon diode is used, care should be taken to en sure that the total voltage appearing on the lx pin including supply ripple, does not exceed the specified maximum value.
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 19 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) reducing output ripple peak to peak ripple current in the led(s) can be reduced, if required, by shunting a capacitor cled across the led(s) as shown below: a value of 1mf will reduce the supply ripple current by a factor three (approx.). proportionally lower ripple can be achieved with higher capacitor values. note that the capacitor will not affect operating frequency or efficiency, but it will increase s tart- up delay, by reducing the rate of rise of led voltage. by adding this capacitor the current waveform through the led( s) changes from a triangular ramp to a more sinusoidal version without altering the mean current value. operation at low supply voltage the internal regulator disables the dr ive to the switch until the supply ha s risen above the start-up threshold (v su ). above this threshold, the device will start to operate. however, with t he supply voltage below the specified minimum value, the switch duty cycle will be high and the device power dissipation will be at a maximum. care should be taken to avoid operating the device under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (see next section on thermal considerations). the drive to the switch is turned off when the supply voltage falls below the under-voltage threshold (v sd ). this prevents the switch working with ex cessive 'on' resistance under conditions where the duty cycle is high. note that when driving loads of two or more leds, the forwar d drop will normally be sufficient to prevent the device from switching below approximately 6v. this will minimize the risk of damage to the device. ? v in v in i sense lx ZXLD1360 rs l1 cled led d1
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 20 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) thermal considerations when operating the device at high ambien t temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipati on limits. the graph below gives details for power derating. this assumes the device to be mounted on a 25mm 2 pcb with 1oz copper standing in still air. note that the device power dissipation will most often be a ma ximum at minimum supply voltage. it will also increase if the efficiency of the circuit is low. this may result from the use of unsuitable coils, or excessive parasitic output capacitance o n the switch output. thermal compensation of output current high luminance leds often need to be supplied with a temperat ure compensated current in order to maintain stable and reliable operation at all drive levels. the leds are usually mounted remotely from the device so, for this reason, the temperature coefficients of the internal circuits for t he ZXLD1360 have been optimized to minimize the change in output current when no compensation is employed. if output current com pensation is required, it is possible to use an external temperature sensing network - normally using negative temper ature coefficient (ntc) thermistors and/or diodes, mounted very close to the led(s). the output of the sensing network can be used to drive the adj pin in order to reduce output current with increasing temperature. m axi m u m power dissipation 0 100 200 300 400 500 600 700 800 900 1000 110 0 -50 -30 -10 10 30 50 70 90 110 130 150 ambient temperature (deg c) power ( m w)
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 21 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) layout considerations lx pin the lx pin of the device is a fast swit ching node, so pcb tracks should be kept as short as possible. to minimize ground 'bounce', the ground pin of the device should be soldered directly to the ground plane. coil and decoupling capacitors and current sense resistor it is particularly important to mount the coil and the input decou pling capacitor as close to the device pins as possible to minimize parasitic resistance and inductance, which will degra de efficiency. it is also important to minimize any track resistance in series with current sense resistor rs. its best to connect vin directly to one end of rs and isense directly to t he opposite end of rs with no other currents flowing in these tracks. it is im portant that the cathode current of the schottky diode does not flow in a track between rs and vin as this ma y give an apparent higher measur e of current than is actual because of track resistance. adj pin the adj pin is a high impedance input for voltages up to 1.35v so, when left fl oating, pcb tracks to this pin should be as short as possible to reduce noise pickup. a 100nf capacitor fr om the adj pin to ground will reduce frequency modulation of the output under these conditions. an additional series 10k ? resistor can also be used when driving the adj pin from an external circuit (see below). this resist or will provide filtering for low frequenc y noise and provide protection against high voltage transients. high voltage tracks avoid running any high voltage tracks close to the adj pin, to reduce the risk of leakage current s due to board contamination. the adj pin is soft-clamped for voltages above 1.35v to desensitize it to leakage that might raise the adj pin voltage and cause excessive output current. however, a ground ring placed around the adj pin is recommended to minimize changes in output current under these conditions. evaluation pcb the ZXLD1360ev1, 2 or 3 evaluation boards are available on requ est. these boards contain a lumileds k2 or multiple ostar lew type leds to allow quick testing of the 1360 device. a dditional terminals allow for interfacing to customers own led products. gnd ZXLD1360 adj 10k 100nf gnd
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 22 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) dimming output current using pwm low frequency pwm mode when the adj pin is driven with a low frequency pwm signal (eg 100hz), with a high level voltage vadj and a low level of zero, the output of the internal low pass filter will swing between 0v and v adj , causing the input to the shutdown circuit to fall below its turn-off threshold (200mv nom) when the adj pin is lo w. this will cause the output current to be switched on and off at the pwm frequency, resulting in an average output current i out avg proportional to the pwm duty cycle. (see figure 2 - low frequency pwm operating waveforms). figure 3. low frequency pwm operating waveforms the average value of output current in this mode is given by: i out avg = 0.1d pwm /r s [for d pwm >0.01] this mode is preferable if optimum led 'whiteness' is r equired. it will also provide the widest possible dimming range (approx. 100:1) and higher efficiency at the expense of greater output ripple. note that the low pass filter introduces a small error in the output duty cycl e due to the difference between the start-up and shut-down times. this time difference is a result of the 200mv shutdown threshold an d the rise and fall times at the output of the filter. to minimize this error, the pwm frequency should be as low as possible consistent with avoiding flicker in the led(s). v adj v adj pwm voltage to n ioutavg filter output 0v 0v 0 to ff 0.1/rs ioutnom 200mv 300mv output current
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 23 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) high frequency pwm mode at pwm frequencies above 10khz and for duty cycles above 0.16, the output of the internal low pass filter will contain a dc component that is always above the shutdown threshold. th is will maintain continuous device operation and the nominal average output current will be proportional to the average voltage at t he output of the filter, which is directly proportional to the duty cycle. (see figure 3 ? high frequency pwm operating waveforms). for best results, the pwm frequency should be maintained above the minimum specified value of 10khz, in order to minimize ripple at the output of the filter. the shutdown comparator has approximately 50mv of hyst eresis, to minimize erratic switching du e to this ripple. an upper pwm frequency limit of approximately one tenth of the operating frequency is recommend ed, to avoid excessive output modulation and to avoid injecting excessive noise into the internal reference. figure 4. high frequency pwm operating waveforms the nominal average value of output current in this mode is given by: i out nom ? 0.1d pwm /r s [for d pwm >0.16] this mode will give minimum output ripple and reduced radiated emission, but with a reduced dimming range (approx.5:1). the restricted dimming range is a result of the device being turned off when the dc component on the filter output falls below 200mv. pwm voltage v adj to n 0v v adj to ff 200mv 0v output current 0.1/r s 0 i outnom filter output
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 24 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated ordering information device part mark package code packaging (note 4) reel size (mm) reel width (mm) quantity per reel part number suffix aec-q100 level ZXLD1360et5ta 1360 et5 tsot23-5 180 8 3000 ta grade1 package outline diminsions tsot23-5
ZXLD1360 ZXLD1360 document number: ds33471 rev. 3 - 2 25 of 25 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated important notice diodes incorporated makes no warranty of any kind, express or implied, with regards to this document, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose (and their equivalents under the laws of any jurisdiction). diodes incorporated and its subsidiaries reserve the right to ma ke modifications, enhancements, im provements, corrections or ot her changes without further notice to this document and any product described herein. diodes incorporated does not assume any liability ari sing out of the application or use of this document or any product described herein; neither does diodes incorporated convey any license under its patent or trademark rights, nor the rights of others. any customer or user of this document or products described herein in such applica tions shall assume all risks of such use and will agree to hold diodes incorporat ed and all the companies whose products are represented on diodes incorporated website, harmless against all damages. diodes incorporated does not warrant or accept any liability wh atsoever in respect of any products purchased through unauthoriz ed sales channel. should customers purchase or use diodes inco rporated products for any unintended or unauthorized application, customers shall i ndemnify and hold diodes incorporated and its representat ives harmless against all claims, damages, expenses, and attorney fees arising out of, directly or indirectly, any claim of personal injury or death a ssociated with such unintended or unauthorized application. products described herein may be covered by one or more united states, international or foreign patents pending. product names and markings noted herein may also be covered by one or more united states, international or foreign trademarks. life support diodes incorporated products are specifically not authorized for use as critical comp onents in life support devices or systems without the express written approval of the chief executive offi cer of diodes incorporated. as used herein: a. life support devices or syst ems are devices or systems which: 1. are intended to implant into the body, or 2. support or sustain life and whose failure to perform when proper ly used in accordance with inst ructions for use provided in the labeling can be reasonably expected to re sult in significant injury to the user. b. a critical component is any component in a life support devic e or system whose failure to perform can be reasonably expect ed to cause the failure of the life support device or to affect its safety or effectiveness. customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support dev ices or systems, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning the ir products and any use of diodes incorporated products in such safety-critica l, life support devices or systems, notwithstanding any devices- or systems-related information or support that may be provided by diodes incorporated. further, customers must fully indemnify diodes incorporate d and its representatives against any damages arising out of the use of diodes incorporated products in such safety-critical, life suppor t devices or systems. copyright ? 2010, diodes incorporated www.diodes.com

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