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| IS31LT3353 integrated silicon solution, inc. ? www.issi.com 1 rev. a, 10/13/2014 40v/1a buck led driver with internal switch october 2014 general description the IS31LT3353 is a continuous mode inductive step-down converter, designed for driving a single led or multiple series connected leds efficiently from a voltage source higher than the led voltage. the chip operates from an input supply between 6v and 40v and provides an externally adjustable output current of up to 1a. the IS31LT3353 includes an integrated output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. the output current can be dynamically adjusted by adding either a digital pwm or analog voltage level signal to the adj pin. a pwm signal will provide a gated output current while a voltage signal will generate a continuously linear output current. applying a voltage less than 0.2v to the adj pin turns the output off and switches the chip into a low current standby state. the chip is assembled in sot23-5 package. it operates from 6v to 40v over the temperature range of -40c to +125c. features ? up to 1a output current ? high efficiency (up to 97% ) ? wide input voltage range: 6v to 40v ? internal 40v power switch ? simple low parts count ? typical 3% output current accuracy ? single pin on/off and brightness control using dc voltage or pwm ? up to 1mhz switching frequency ? inherent led open-circuit/short-circuit protection ? thermal shutdown protection circuitry ? up to 1200: 1 dimming rate applications ? led mr16, mr11 spot light ? led street light ? par light ? industrial lighting ? refrigeration lights ? other led lighting application circuit figure 1 typical application circuit note: the capacitor, c 2 , can?t be removed. and it should be pl aced as close as possible to the vi n and gnd pins, otherwise the operation might be abnormal.
IS31LT3353 integrated silicon solution, inc. ? www.issi.com 2 rev. a, 10/13/2014 pin configuration package pin configuration sot23-5 1 2 34 5 adj isense vin gnd lx pin description no. pin description 1 lx drain of power switch. 2 gnd ground (0v). 3 adj multi-function on/off and brightness control pin: * leave floating for normal operation.(v adj = v ref = 1.2v giving nominal average output current i out_nom =0.1/r s ) * drive to voltage below 0.2v to turn off output current. * drive with dc voltage (0.3v IS31LT3353 integrated silicon solution, inc. ? www.issi.com 4 rev. a, 10/13/2014 absolute maximum ratings (note 1) input voltage, v in -0.3v ~ +43v isense voltage, v sense v in -5v ~ v in +0.3v (v in 5v) -0.3v ~ v in +0.3v (v in <5v) lx output voltage, v lx -0.3v ~ +43v adjust pin input voltage, v adj -0.3v ~ +6v switch output current, i lx 1.2a power dissipation, p d ( max ) (note 2) 0.46w operating temperature, t a -40c ~ +125c storage temperature, t st -55c ~ +150c junction temperature, t jmax 150c junction to ambient, ja 271c/w esd (hbm) at lx pin esd (hbm) at other pins esd (cdm) 1kv 3kv 1kv note 1: stresses beyond those listed under ?absolute maximum ratings? may cause permanent damage to the device. these are stress rating s only and functional operation of the device at these or any other condition beyond those indicat ed in the operational sections of the sp ecifications is not implied. exposure to absolute maximu m rating conditions for extended peri ods may affect device reliability. note 2: detail information please refer to package thermal de-rating curve on page 12. electrical characteristics test conditions: v in = 12v, t a =t j = 25c, unless otherwise stated. (note 3) symbol parameter conditions min. typ. max. unit v in input voltage 6 40 v i inq_off quiescent supply current with output off adj pin grounded 70 120 160 a i inq_on quiescent supply current with output switching adj pin floating 450 600 a v sense mean current sense threshold voltage 97 100 103 mv v sensehys sense threshold hysteresis 15 % i sense i sense pin input current v sense =v in -0.1v 8 a v ref internal reference voltage measured on adj pin with pin floating 1.2 v v adj external control voltage range on adj pin for dc brightness control 0.3 1.2 v v adj_off dc voltage on adj pin to switch chip from active (on) state to quiescent (off) state v adj falling 0.15 0.2 0.25 v v adj_on dc voltage on adj pin to switch chip 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 500 k ? i lx_mean continuous lx switch current (note 4) 1 a IS31LT3353 integrated silicon solution, inc. ? www.issi.com 5 rev. a, 10/13/2014 electrical characteristics (continued) test conditions: v in = 12v, t a =t j = 25c, unless otherwise stated. (note 3) symbol parameter conditions min. typ. max. unit i lx_leak lx switch leakage current 1 a r lx lx switch ?on? resistance 0.5 1 ? t on_min minimum switch ?on? time lx switch ?on? (note 4) 200 ns t off_min minimum switch ?off? time lx switch ?off? (note 4) 200 ns d dim typical contrast ratio f pwm =100hz, v in =15v, 1led, l=27h 1200:1 1 f lx_max recommended maximum operating frequency 1 mhz d lx recommended duty cycle range of output switch at f lx_max 30 70 90 % t pd internal comparator propagation delay (note 4) 50 ns t sd thermal shutdown temperature (note 4) 150 c t sd_hys thermal shutdown hysteres is (note 4) 20 c note 3: production testing of the device is perform ed at 25c. functional operation of the dev ice and parameters specified over -40c t o +125c temperature range, are guaranteed by de sign, characterization and process control. note 4: guaranteed by design. IS31LT3353 integrated silicon solution, inc. ? www.issi.com 6 rev. a, 10/13/2014 typical performance characteristics power supply(v) error(%) -4 -3 -2 -1 0 1 2 3 4 5 101520 25303540 l = 47 h r s = 0.1 ? 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led figure 2 output current error vs. power supply power supply(v) efficiency (%) 6 10 20 30 40 75 80 85 90 95 100 l = 47 h r s = 0.1 ? 1led 2led 4led 6led 9led 10led 15 25 35 3led 5led 8led 7led figure 4 efficiency vs. power supply power supply(v) supply current (a) 610152025303540 0 100 200 300 400 500 600 700 operating mode figure 6 supply current vs. power supply (operating mode) power supply(v) error (%) -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 5 10152025303540 l = 47 h r s = 0.2 ? 1led 2led 3led 7led 8led 9led 4led 5led 6led 10led figure 3 output current error vs. power supply power supply(v) efficiency (%) 75 80 85 90 95 100 6 10 15 20 25 30 35 40 3led 4led 5led 6led 7led 8led 9led 10led 1led 2led l = 47 h r s = 0.2 ? figure 5 efficiency vs. power supply power supply(v) supply current (a) 610152025303540 0 20 40 60 80 100 120 140 shutdown mode figure 7 supply current vs. power supply (shutdown mode) IS31LT3353 integrated silicon solution, inc. ? www.issi.com 7 rev. a, 10/13/2014 power supply(v) v ref voltage (v) 1.15 1.17 1.19 1.21 1.23 1.25 66.577.588.599.510 low supply voltage figure 8 v ref vs. power supply (low supply voltage) temperature ( c ) v adj (mv) -40 -25 -10 5 20 35 50 65 80 95 110 125 1100 1120 1140 1160 1180 1200 v in = 12v figure 10 v adj vs. temperature temperature ( c ) v sense v oltage (mv) -40 -25 -10 5 20 35 50 65 80 95 110 125 90 94 98 102 106 110 v in = 12v figure 12 v sense vs. temperature power supply(v) v ref v oltage (v) 1.15 1.17 1.19 1.21 1.23 1.25 6 101418222630343840 normal supply voltage figure 9 v ref vs. power supply (normal supply voltage) temperature ( c ) r ds_on (m ? ) -40 -25 -10 5 20 35 50 65 80 95 110 125 0 100 200 300 400 500 600 700 v in = 12v figure 11 r ds_on vs. temperature time (10s/div) v adj 2.0v/div i l 200ma/div v in = 12v l = 47h r s = 0.13 ? figure 13 adj pin voltage vs. i l IS31LT3353 integrated silicon solution, inc. ? www.issi.com 8 rev. a, 10/13/2014 time (100ms/div) v lx 10v/div i l 500ma/div v in = 12v l = 47h r s = 0.13 ? led open figure 14 led open-circuit protection time (100ms/div) v led 5v/div i l 500ma/div v in = 12v l = 47h r s = 0.13 ? led short figure 15 led short-circuit protection IS31LT3353 integrated silicon solution, inc. ? www.issi.com 9 rev. a, 10/13/2014 functional block diagram IS31LT3353 integrated silicon solution, inc. ? www.issi.com 10 rev. a, 10/13/2014 application information setting nominal average output current with exter nal resistor r s the nominal average output current in the led(s) is determined by the value of t he external current sense resistor (r s ) connected between vin and isense pins and in is given by equation (1): s nom out r i 1 . 0 _ ? (1) note that r s =0.1 ? 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 external voltage. the table below gives values of nominal average output current for several pr eferred values of current setting resistor (r s ) in the typical application circuit figure 1: r s ( ? ) nominal average output current (ma) 0.1 1000 0.15 667 0.3 333 the above values assume that the adj pin is floating and at a nominal voltage of v ref =1.2v. rs need to be chosen 1% accuracy resistor with enough power tolerance and good temperature characteristic to ensure stable output current. output current adjustment by external dc control voltage the adj pin can be driven by an external dc voltage (v adj ), as shown in figure 16, to adjust the output current to a value above or below the nominal average value defined by r s . figure 16 dimming by external dc voltage the nominal average output current in this case is given by equation (2): s adj dc out r v i ? ? 083 . 0 _ (2) for 0.3v< v adj <1.2v. note that 100% brightness setting corresponds to v adj = v ref . when driving the adj pin above 1.2v, the current will be clamped to 100% brightness automatically. the input impedance of the adj pin is 500k ? (typ.). 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 in figure 17, to adjust the output current to a value below the nominal average value set by resistor r s , the signal range is from 0v~5v.the logic ?high? is higher than 1.2v, the logic ?low? is lower than 0.2v.the pwm signal must have the driving ability to drive internal 500k ? pull-up resistor. figure 17 pwm dimming control via adj pin driving the adj input from a microcontroller another possibility is to drive the chip from the open drain output of a microcontroller. the figure 18 below shows one method of doing this: figure 18 dimming by mcu the diode and resistor suppress possible high amplitude negative spikes on the adj input resulting from the drain-source capacitance of the fet. negative spikes at the input to the chip should be avoided as they may cause errors in output current or erratic device operation. shutdown mode taking the adj pin to a voltage below 0.2v will turn off the output and supply current will fall to a low standby level of 120 a nominal. inherent open-circuit led protection if the connection to the led( s) is open-circuited, the coil is isolated from the lx pin of the chip, so the chip 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 the esr of this capacitor appears in series with the supply source impedance and lowers IS31LT3353 integrated silicon solution, inc. ? www.issi.com 11 rev. a, 10/13/2014 overall efficiency. this capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. if the source is dc supply, the capacitor is decided by ripple of the source, the value is given by equation (3): max on f min u t i c ? ? ? (3) i f is the value of output current, max u ? is the ripple of power supply. t on is the ?on? time of mosfet. the value is higher than the minimum value. a 100f capacitor is recommended. if the source is an ac supply, typical output voltages ripple from a nominal 12v ac transformer can be 10%.if the input capacitor value is lower than 220 f, the ac input waveform is distorted, sometimes the lowest value will be lower than the forward voltage of led strings. this lower t he average current of the leds. so it is recommended to set the value of the capacitor bigger than 220f. to minimize the ground bounce, it must connect a 0.1f capacitor as close to device as possible. this capacitor can?t be removed, otherwise the operation might be abnormal. inductor selection recommended inductor values for the IS31LT3353 are in the range 47 h to 220 h. higher values of inductance are recommended at higher supply voltages and low output current 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. the inductor should be mounted as close to the chip 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. it is recommended to use inductor with saturation current bigger than 1.5a for 1a output current and inductor with saturation current bigger than 500ma for 350ma output current. the inductor value should be chosen to maintain operating duty cycle and switch 'on/off' times within the specified limits over the supply voltage and load current range. the following equations can be used as a guide. lx switch 'on' time: ) ( lx l s avg led in on r r r i v v i l t ? ? ? ? ? ? ? (4) note: t on_min > 200ns. lx switch 'off' time: ) ( s l avg d led off r r i v v i l t ? ? ? ? ? ? (5) note: t off_min > 200ns. where: l is the coil inductance (h) r l is the coil resistance ( ? ) i avg is the required led current (a) ? i is the coil peak-peak ripple current (a) {internally set to 0.3 i avg } v in is the supply voltage (v) v led is the total led forward voltage (v) r lx is the switch resistance ( ? ) v d is the diode forward voltage at the required load current (v) example: for v in =12v, l=47 h, r l =0.26 ? , v led =3.4v, i avg =333ma, v d =0.36v, r s = 0.3 ? , r lx =0.5 ? : s t on ? 569 . 0 ) 5 . 0 26 . 0 3 . 0 ( 333 . 0 4 . 3 12 333 . 0 3 . 0 47 ? ? ? ? ? ? ? ? ? s t off ? 19 . 1 ) 3 . 0 26 . 0 ( 333 . 0 36 . 0 4 . 3 333 . 0 3 . 0 47 ? ? ? ? ? ? ? ? this gives an operating frequency of 569khz and a duty cycle of 32%. optimum performance will be achieved by setting the duty cycle close to 50% at the nominal supply 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 (d 1 ) should be a fast low capacitance schottky diode with low reverse leakage at the maximum operating voltage and temperature. if alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. it is very important to consider the reverse leakage of the diode when operating at high temperature. excess leakage will increase the power dissipation in the device. the higher forward voltage and overshoot due to IS31LT3353 integrated silicon solution, inc. ? www.issi.com 12 rev. a, 10/13/2014 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 ensure that the total voltage appearing on the lx pin including supply ripple, does not exceed the specified maximum value. reducing output ripple a value of 1 f will reduce nominal ripple current by a factor three (approx.). pro portionally lower ripple can be achieved with higher capacitor values. note that the capacitor will not affect operating frequency or efficiency, but it will in crease start-up delay, by reducing the rate of rise of led voltage. operation at low supply voltage the internal regulator disables the drive to the switch until the supply has risen above the startup threshold set internally which makes power mosfet on-resistance small enough. above this threshold, the chip will start to operate. however, with the supply voltage below the specified minimum value, the switch duty cycle will be high and the chip power dissipation will be at a maximum. care should be taken to avoid operating the chip 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). note that when driving loads of two or more leds, the forward drop will normally be sufficient to prevent the chip from switching below approximately 6v. this will minimize the risk of damage to the chip. thermal considerations when operating the chip at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. the maximum power dissipation can be calculated using the following equation (6): ja a max j max d t t p ? ? ? ) ( ) ( (6) where t j(max) is the maximum operating junction temperature, t a is the ambient temperature, and ja is the junction to ambient thermal resistance. the recommended maximum junction temperature, t j(max) , is 150c and so maximum ambient temperature is determined by the junction to ambient thermal resistance, ja . therefore the maximum power dissipation at t a = 25c is: w w c c c p max d 46 . 0 / 271 25 150 ) ( ? ? ? ? ? ? the graph below gives details for power derating. temperature ( c ) power dissipation (w) -40 -25 -10 5 20 35 50 65 80 95 110 125 0 0.1 0.2 0.3 0.4 0.5 sot23-5 figure 19 p d vs. t a 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 on the switch output. layout considerations vin pin the gnd of power supply usually have some distance to the chip gnd pin, which cause parasitic resistance and inductance. it causes ground voltage bounce while the mosfet is switching. connect a 0.1f capacitor c 2 as close to device as possible to minimize the ground bounce. lx pin the lx pin of the chip is a fast switching node, so pcb traces should be kept as short as possible. to minimize ground 'bounce', the ground pin of the chip should be soldered directly to the ground plane. coil and decoupling capacitor c 1 it is particularly important to mount the coil and the input decoupling capacitor close to the chip to minimize parasitic resistance and inductance, which will degrade efficiency. it is also important to take account of any trace resistance in series with current sense resistor r s . adj pin the adj pin is a high impedance input, so when left floating, pcb traces to this pin should be as short as possible to reduce noise pickup. adj pin can also be connected to a voltage between 1.2v~5v. in this case, the internal circuit will clamp the output current at the value which is set by v adj =1.2v. IS31LT3353 integrated silicon solution, inc. ? www.issi.com 13 rev. a, 10/13/2014 high voltage traces avoid running any high voltage traces close to the adj pin, to reduce the risk of leakage due to board contamination. any such leakage may affect the adj pin voltage and cause unexpectable output current. the IS31LT3353 has external protection circuitry to prevent excessive output current if adj voltage rises above 1.2v. a ground ring placed around the adj pin will minimize changes in output current under these conditions. IS31LT3353 integrated silicon solution, inc. ? www.issi.com 14 rev. a, 10/13/2014 classification reflow profiles profile feature pb-free assembly preheat & soak temperature min (tsmin) temperature max (tsmax) time (tsmin to tsmax) (ts) 150c 200c 60-120 seconds average ramp-up rate (tsmax to tp) 3c/second max. liquidous temperature (tl) time at liquidous (tl) 217c 60-150 seconds peak package body temperature (tp)* max 260c time (tp)** within 5c of the specified classification temperature (tc) max 30 seconds average ramp-down rate (tp to tsmax) 6c/second max. time 25c to peak temperature 8 minutes max. figure 20 classification profile IS31LT3353 integrated silicon solution, inc. ? www.issi.com 15 rev. a, 10/13/2014 package information sot23-5 |
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