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| ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 1 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated 60v 550ma led driver with aec-q100 description the ZXLD1356 is a continuous mode inductive step-down converter, designed for driving single or multiple series connected leds efficiently from a voltage source higher than the led voltage. the device o perates from an input supply between 6v and 60v and provides an externally adjustable output current of up to 55 0ma. depending upon supply voltage and external components, this can provide up to 30 watts of output power. the ZXLD1356 has been qualified to aecq100 grade 1 enabling operation in ambient temperatures from -40 to 125c output current can be adjusted above, or below the set value, by applying an external control signal to the 'adj' pin. enhanced output current dimming can be achieved by applying a pwm signal to the ?adj? pin. pin assignments features ? typically better than 0.8% output current accuracy ? simple and with low part count ? single pin on/off and brightness control using dc voltage or pwm ? pwm resolution up to 1000:1 ? high efficiency (up to 97%) ? wide input voltage range: 6v to 60v ? inherent open-circuit led protection ? available in thermally enhanced packages o dfn3030-6 ja 44 c/w o tsot23-5 ja 82 c/w ? available in green molding (no br, sb) with lead free finish/rohs compliant ? qualified to aec-q100 grade 1 o tsot23-5 ZXLD1356et5ta applications ? low voltage halogen replacement leds ? automotive lighting ? low voltage industrial lighting ? led back-up lighting ? illuminated signs ? emergency lighting ? selv lighting ? refrigeration lights typical application circuit i sense v in adj gnd lx tsot23-5 top view 1 2 3 4 5 6 dfn3030-6 (top view) lx gnd adj v in i sense gnd ? v in i sense lx gnd ZXLD1356 ad j v in (24 v) rs 0.36v 4.7f c1 gnd 100nf d1 l1
ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 2 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated block diagram figure 1. block diagram ? pin connections shown for tsot23-5 package name tsot23-5 dfn3030-6 description lx 1 1 drain of ndmos switch gnd 2 2, 5 ground (0v) adj 3 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 = 0.2v/r s ) ? drive to voltage below 0.2v to turn off output current ? drive with dc voltage (0.3v ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 4 of 29 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 4.85 5.2 v v sd internal regulator shutdown threshold 4.4 4.75 v i inqoff quiescent supply current with output off adj pin grounded 65 108 a i inqon quiescent supply current with output switching (note 3) adj pin floating, l=68mh, 3 leds f=360khz 1.6 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; v in =18v 195 200 205 mv v sensehys sense threshold hysteresis 15 % i sense i sense pin input current v sense =v in -0.2 4 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 (note 4) 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 0.27 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 30 10.4 50 14.2 65 18 k? i lxmean continuous lx switch current 0.55 a r lx lx switch ?on? resistance @ i lx =0.55a 0.5 0.75 ? 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 <300hz pwm amplitude =v ref measured on adj pin 0.001 1 brightness control range 1000:1 dc adj (*) dc brightness control range note 5 5:1 t ss start up time (see graphs for more details) time taken for output current to reach 90% of final value after voltage on adj pin has risen above 0.3v. requires external capacitor 22nf. 2 ms f lx operating frequency (see graphs for more details) adj pin floating l=68mh (0.36v) i out =0.55a @ v led =3.6v driving 3 leds 360 khz f lxmax recommended maximum operating frequency 500 khz notes: 3. static current of device is approximately 700 a, see graph, page 17. 4. 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. 5. ratio of maximum brightness to minimum brightness before shutdown v ref =1.25/0.25. v ref externally driven to 2.5v, ratio 10.1. ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 5 of 29 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 sense resistor (r s ), forms a self-oscillating continuous-mode buck converter. device operation (refer to figure 1 - block diagram and figure 2 operating waveforms) figure 2. theoretical operating waveforms operation can be best understood by assumi ng that the adj pin of the device is unconnected and the voltage on this pin (v adj ) 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 output is hi gh. 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 cu rrent in internal resistors r2 and r3. this produces a ground referred rising voltage at the (-) input of the com parator. when this reaches the threshold voltage (v adj ), the comparator output switches low and mn turns off. the comparator output also drives another nmos switch, which bypasses internal resistor r3 to provide a controlled amount of hysteresis. the hyst eresis 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 voltages to produce a falling vo ltage at the input of the compar ator. when this voltage returns to v adj , the comparator output switches high again. this cycle of events repeats, with the comparator input ramping between limits of v adj 15%. 0v v in 200mv 230mv 0v sense voltage v sense+ v sense- to f f to n 170mv 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 ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 6 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated device description (continued) switching thresholds with v adj = v ref , the ratios of r1, r2 and r3 define an average v sense switching threshold of 200mv (measured on the i sense pin with respect to v in ). the average output current i outnom is then defined by this voltage and r s according to: i outnom = 200mv/r s nominal ripple current is 30mv/r s adjusting output current the device contains a low pass filter between the adj pin and th e threshold comparator and an in ternal current limiting resisto r (50k ? nom) between adj and the internal reference voltage. this al lows the adj pin to be overdriven with either dc or pulse signals to change the v sense switching threshold and adjust the output current. 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 the shutdown circuit. w hen the input voltage to this circuit falls below the threshold (0.2v nom.), the internal regul ator and the output switch are turned off. the voltage reference remains powered during shutdown to provide the bias current for t he shutdown circuit. quiescent supply current during shutdown is nominally 60a and switch leakage is below 5a. ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 7 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated actual operating waveforms [v in =15v, r s =0.36v, l=68h] normal operation. output current (ch3) and lx voltage (ch2) actual operating waveforms [v in =30v, r s =0.36v, l=68h] normal operation. output current (ch3) and lx voltage (ch2) actual operating waveforms [v in =60v, r s =0.36v, l=68h] normal operation. output current (ch3) and lx voltage (ch2) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 8 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions ? 0.500 output current (a) 0.520 0.540 0.560 0.580 0.600 0.620 0.640 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) ZXLD1356 output current l = 68h zxld 1356 output current deviation (normalized) l = 68h 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 10% 8% -10% -8% -6% 6% -4% -2% 0% 2% 4% output current deviation (%) 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) zxld13 56 effic ienc y l = 68h 100% 95% 50% 55% 60% 90% 65% 70% 75% 80% 85% efficiency (%) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 9 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) ZXLD1356 switching frequenc y l = 68h 15 leds 13 leds 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) 700 600 500 400 300 100 200 0 switching frequency (khz) 15 leds 13 leds 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) ZXLD1356 duty cycle l = 68h 0 100 10 90 20 80 30 70 40 60 50 duty cycle (% ) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 10 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) ? ZXLD1356 output current l = 100h 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) 0.500 output current (a) 0.520 0.540 0.560 0.580 0.600 0.620 0.640 ZXLD1356 output current deviation (normalized) l = 100h 10% 8% -10% -8% -6% 6% -4% -2% 0% 2% 4% output current deviation (%) 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 15 leds 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) ZXLD1356 efficienc y l = 100h 100% 95% 50% 55% 60% 90% 65% 70% 75% 80% 85% efficiency (%) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 11 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) ZXLD1356 switching frequenc y l = 100h 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 70 0 600 500 400 300 100 200 0 switching freq uency (khz) 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) ZXLD1356 duty cycle l = 100h 0 100 10 90 20 80 30 70 40 60 50 duty cycle (%) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 12 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) ? ZXLD1356 output current l = 150h 15 led s 13 leds 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) ? 0.500 output current (a) 0.520 0.540 0.560 0.580 0.600 0.620 0.640 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 10% 8% -10% -8% -6% 6% -4% -2% 0% 2% 4% output current deviation (%) zxld 1356 output current deviation (n ormalized) l = 150h 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) ZXLD1356 efficienc y l = 150h 100% 95% 50% 55% 60% 90% 65% 70% 75% 80% 85% efficiency (%) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 13 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) ? 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) ZXLD1356 switching frequenc y l = 150h 700 600 500 400 300 100 200 0 switching frequency (khz) 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 0 100 10 90 20 80 30 70 40 60 50 duty cycle (%) ZXLD1356 duty cycle l = 150h ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 14 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) ZXLD1356 output current l = 220h 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 040 50 60 10 20 30 supply voltage (v) ? 0.500 output current (a) 0.520 0.540 0.560 0.580 0.600 0.620 0.640 ZXLD1356 output current deviation (n ormalized) l = 220h 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 10% 8% -10% -8% -6% 6% -4% -2% 0% 2% 4% output current deviation (%) ZXLD1356 efficienc y l = 220h 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 100% 95% 50% 55% 60% 90% 65% 70% 75% 80% 85% efficiency (%) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 15 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) 15 led s 13 led s 11 le d s 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) ZXLD1356 switching frequency l = 220h ? 700 600 500 400 300 100 200 0 switching frequency (khz) ZXLD1356 duty cycle l = 220h 15 led s 13 led s 11 le ds 9 leds 7 leds 5 leds 3 leds 1 led 04 0 50 60 10 20 30 supply voltage (v) 0 100 10 90 20 80 30 70 40 60 50 duty cycle (%) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 16 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) led current vs v ad j 0 100 200 300 400 500 600 0123 adj pin voltage (v) led current (ma) r=0.36 ? r=0.56 ? r=1.33 ? vref 1.238 1.2385 1.239 1.2395 1.24 1.2405 1.241 1.2415 1.242 1.2425 1.243 0 1020 3040 5060 70 adj pin voltage (v) supply current 0 100 200 300 400 500 600 700 800 0 1 02 03 04 05 06 07 0 supply voltage (v) supply voltage (v) supply voltage (v) supply current ( m a) shutdown current 0 10 20 30 40 50 60 70 80 90 0 1 02 03 0 4 05 06 07 0 shutdown current ( m a) output transistor fully enhanced output transistor not fully enhanced ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 17 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated typical operating conditions (cont.) lx on-resistance vs die temperature 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 -50 0 50 100 150 200 die temperature (c) on-resistance (oh m s) 7v 9v 12v 20v 30v vadj vs temperature 1.244 1.246 1.248 1.25 1.252 1.254 1.256 1.258 1.26 1.262 -50 0 50 100 150 200 temperature (c) vadj (v) 7v 9v 12v 20v 30v 0 0.5 1 1.5 2 2.5 0 1 02 03 04 05 06 07 0 supply voltage (v) on-resistance (ohms) -40 o c 25 o c 125 o c lx on-resistance vs supply voltage ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 18 of 29 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 determine d by the value of the external current sense resistor (r s ) connected between v in and i sense and is given by: i outnom = 0.2/r s [for r s > 0.36 ? ] 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) 0.36 555 0.56 357 1.33 150 the above values assume that the adj pin is floating and at a nominal voltage of v ref (=1.25v). note that r s = 0.36v 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 cu rrent 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 (0.2/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 14.2k ? 25% for voltages above v ref +100mv. gnd ZXLD1356 adj gnd + dc ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 19 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) output current adjustment by pwm control 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 the 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 controllin g the amplitude of the pwm waveform is to use a small npn switching transistor as shown below: this scheme uses the 50k resistor between the adj pin and the internal voltage reference as a pull-up resistor for the external transistor eg mmbt3904. driving the adj input from a microcontroller another possibility is to drive the device from the open drain output of a microcontroller. the diagram below shows one method of doing this: if the nmos transistor within the microcontr oller has high gate / drain capacitance, this arrangement can inject a negative spi ke into adj input of the ZXLD1356 and cause erratic operation but the addition of a schottky clamp diode (eg diodes inc. sd103cws) to ground and inclusion of a series resistor (3.3 k) 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 ZXLD1356 adj pwm gnd ZXLD1356 adj gnd gnd ZXLD1356 adj mcu 3.3k ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 20 of 29 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 approxim ately 100s will turn off the output and supply current to a l ow standby level of 65a 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 page 17 graphs for details). soft-start an external capacitor from the adj pin to ground will provide a soft-start delay, by increasing the time taken for the voltage on this pin to rise to the turn-on threshold and by slowing down t he rate of rise of the control vo ltage at the input of the compa rator. adding capacitance increases this delay by approximately 0.2ms/nf. the graph below show s the variation of soft-start time for different values of capacitor. actual operating waveforms [v in =60v, r s =0.36v, l=68h, 22nf on adj] soft-start operation. lx voltage (ch2) and output current (ch3) using a 22nf external capacitor on the adj pin. soft start time vs capacitance from adj pin to ground -2 0 2 4 6 8 10 12 14 16 0 2 04 06 08 01 0 01 2 0 capacitance (nf) soft start time (ms) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 21 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) v in capacitor selection a low esr capacitor should be used for input decoupling, as the esr of this capacitor appears in se ries with the supply source impedance and lowers overall efficiency. this capacitor has to supply the relatively high peak current to the coil and smooth t he current ripple on the input supply. to avoid transients into the ic, the size of the input capacitor will depend on the v in voltage: v in = 6 to 40v c in = 2.2f v in = 40 to 50v c in = 4.7f v in = 50 to 60v c in = 10f when the input voltage is close to the out put voltage the input current increases wh ich puts more demand on the input capacitor . the minimum value of 2.2f may need to be increased to 4.7f; hi gher values will improve performance at lower input voltages, especially when the source impedance is high. the input c apacitor should be placed as close as possible to the ic. for maximum stability over temperature and voltage, capacitors with x7r, x5r, or be tter dielectric is recommended. capacitors with y5v dielectric are not suitable for decoupling in this application and should not be used. when higher voltages are with c in = 10f, an electrolytic capacitor can be used provided that a suitable 1mf ceramic capacitor is also used and positioned as close to the v in pin as possible. a suitable capacitor would be nacew100m1006.3x8tr13f (nic components). the following web sites are useful when finding alternatives: www.murata.com www.niccomp.com www.kemet.com ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 22 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) inductor selection recommended inductor values for the ZXLD1356 are in the range 68 h to 220 h. 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 va lues of inductance also result in a smaller change in output cu rrent over the supply voltage range. (see graphs pages 10-16). the inductor should be mounted as close to the device as possible with low resistance connections to the lx and v in 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 ZXLD1356 may be selected from the mss range manufactured by co ilcraft, or the npis range manufactured by nic components. the following websit es may be useful in finding suitable components www.coilcraft.com www.niccomp.com www.wuerth-elektronik.de the inductor value should be chosen to maintain operating duty cy cle and switch 'on'/'off' times within the specified limits ov er the supply voltage and load current range. figures 3 and 4 (following) can be used to select a recommended inductor based on maintaining the ZXLD1356 case temperature below 60c for the different package types. for detailed performance characteristics for the inductor values 68, 10 0, 150 and 220h see graphs on pages 10-16. figure 3. ZXLD1356 minimum recommended inductor minimum recommended inductor 2% accuracy, <60c case temperature 0 1 02 03 04 05 06 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 number of leds supply voltage (v) legend 68uh 100uh 150uh 220uh ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 23 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated application information (continued) 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 tim e. it is important to select parts with a peak current rating ab ove the peak coil current and a c ontinuous current rating higher t han the maximum output load current. it is very important to consider the reverse leakage of the di ode when operating above 85c. excess leakage will increase the power dissipation in the device a nd if close to the load may create a thermal runaway conditio n. the higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the l x output. if a silicon diode is used, care should be taken to ensure that the total voltage appearing on the lx pin including sup ply ripple, does not exceed the specified maximum value. *a suitable schottky diode would be b1100b (diodes inc). 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 show n below: a value of 1f will reduce the supply ripple current by a factor three (approx.). proportionally lower ripple can be achieved w ith higher capacitor values. note that the capacitor will not affe ct operating frequency or efficien cy, but it will increase start- up delay, by reducing the rate of rise of led voltage. by adding this capacitor the current wavefo rm 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 below the under-voltage lockout threshold (v sd ) the drive to the output transistor is tu rned off to prevent device operation with excessive on-resistance of the output trans istor. the output transistor is not full enhanced until the supply voltage exceeds approximately 17v. at supply voltages between v sd and 17v care must be taken to avoid excessive power dissipation due to the on-resistance. 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 ZXLD1356 rs l1 cled led d1 ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 24 of 29 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 ambient temperatures, or w hen driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. the graph below give s details for power derating. th is 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 temperature co mpensated current in order to maintain stable and reliable operation at all drive levels. the leds ar e usually mounted remotely from the devic e so, for this reason, the temperature coefficients of the internal circuits for the ZXLD1356 have bee n optimized to minimize the change in output current when no compensation is employed. if output current compensation is r equired, it is possible to use an external temperature sensing network - normally using negative temperature 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. tsot23-5 dfn3030-6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 -40 -25 -10 5 20 35 50 65 80 95 110 125 ambient temperature (c) 140 maximum power dissipation power dissipation (w) ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 25 of 29 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 switch ing 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 dec oupling capacitor as close to the device pins as possible to mi nimize parasitic resistance and inductance, which wi ll degrade efficiency. it is also important to minimize any track resistance in se ries with current sense resistor r s . its best to connect v in directly to one end of r s and isense directly to the opposite end of r s with no other currents flowing in these tracks. it is important that the cathode current of the schottky diode does not flow in a tr ack between r s and v in as this may give an apparent higher measure 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 float ing, pcb tracks to this pin should be as shor t as possible to reduce noise pickup. a 100nf capacitor from the adj pin to ground will reduce frequency modulation of the output under these conditions. an additional series 3.3k ? resistor can also be used when driving the adj pin from an external circuit (see below). this resistor will provide filtering for low freque ncy noise and provide protection against high voltage transient s. high voltage tracks avoid running any high volt age tracks close to the adj pin, to reduce the risk of leakage curre nts due to board contamination. the adj pin is soft-clamped for voltages above 1.35v to desensit ize it to leakage that might ra ise the adj pin voltage and caus e excessive output current. however, a ground ring placed around the adj pin is recommended to minimize changes in output current under these conditions. evaluation boards ZXLD1356 evaluation boards are available on request, which have connection terminals that allow customers to connect their own led products to the board. gnd ZXLD1356 adj 3.3k 100nf gnd ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 26 of 29 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 vadj, causing the input to the shutdown circuit to fall be low its turn-off threshold (200mv nom) when the adj pin is low. this will cause the output current to be switched on and off at the pwm frequency, resulting in an average output current ioutavg proportional to the pwm duty cycle. (see figure 4 - low frequency pwm operating waveforms). low frequency pwm operating waveforms the average value of output current in this mode is given by: s pwm outavg r d2.0 i = for dpwm > 0.001 this mode is preferable if optimum led 'w hiteness' is required. it will also provide the widest possible dimming range (approx. 1000:1) and higher efficiency at the expense of greater output ripple. ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 27 of 29 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 (inches) reel width (mm) quantity per reel part number suffix aec-q100 ZXLD1356dactc 1356 dac dfn3030-6 13 12 3000 tc ZXLD1356et5ta 1356 et5 tsot23-5 7 8 3000 ta grade 1 package outline dimensions tsot23-5 ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 28 of 29 www.diodes.com december 2010 ? diodes incorporated a product line o f diodes incorporated package outline dimensions (continued) dfn3030-6 top view bottom view pin 1 dot by marking side view chamfer 0.300x45 pin #1 identification d a a3 a1 b d2 e e e2 e1 l dim millimeters inches dim millimeters inches min. max. min. max. min. max. min. max. a 0.700 0.800 0.0275 0.0315 d2 1.950 2.050 0.0768 0.0807 a1 0.000 0.050 0.000 0.00197 e 0.950 bsc 0.0374 bsc a3 0.203 ref 0.008 e 2.950 3.050 0.116 0.120 b 0.300 0.400 0.0118 0.0157 e2 1.150 1.250 0.0452 0.0492 d 2.950 3.050 0.116 0.120 e1 1.900ref 0.0748 l 0.450 0.550 0.0177 0.0216 ZXLD1356 ZXLD1356 document number: ds33470 rev. 3 - 2 29 of 29 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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