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| is31lt3360 integrated silicon solution, inc. ? www.issi.com 1 rev. c, 12/22/2013 40v/1.2a led driver wi th internal switch january 2014 general description the is31lt3360 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 1.2a. the is31lt3360 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. output current can be adjusted linearly by applying an external control signal to the adj pin. the adj pin will accept either a dc voltage or a pwm waveform. this will provide either a continuous or a gated 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 sot89-5 package. it operates from 6v to 40v over two temperature ranges of -40c to +105c and -40c to +125c. features ? up to 1.2a output current ? high efficiency (up to 98%) ? wide input voltage range: 6v ~ 40v ? internal 40v power switch ? simple low parts count ? typical 3% output current accuracy (-40c to +105c, IS31LT3360-SDLS3-TR) ? typical 5% output current accuracy (-40c to +125c, is31lt3360-sdls4-tr) ? 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 ratio applications ? automotive and avionic lighting ? 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 placed as close as possible to the vin and gnd pins, otherwise the operation might be abnormal.
is31lt3360 integrated silicon solution, inc. ? www.issi.com 2 rev. c, 12/22/2013 pin configuration package pin configuration sot89-5 1 2 34 5 adj isense vin gnd lx thermal pad 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 is31lt3360 integrated silicon solution, inc. ? www.issi.com 4 rev. c, 12/22/2013 absolute maximum ratings (note 1) input voltage, v in -0.3v ~ +50v 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 ~ +50v adjust pin input voltage, v adj -0.3v ~ +6.0v switch output current, i lx 1.5a power dissipation, p d ( max ) (note 2) 0.94w operating temperature, t a = t j -40c ~ +105c, IS31LT3360-SDLS3-TR -40c ~ +125c, is31lt3360-sdls 4 -tr storage temperature, t st -55c ~ +150c junction temperature, t jmax 150c junction to ambient, ja 132.6c/w esd (hbm) esd (cdm) 4kv 750v 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 valid are at v in =12v, typical value at 25c, unless otherwise noted. parameter range based on t a = -40c ~ +125c (note 3) the symbol in the table means these parameters are only available in the above temperature range. symbol parameter conditions temp. min. typ. max. unit v in input voltage 6 40 v i inq_off quiescent supply current with output off adj pin grounded 80 120 160 a 60 120 200 i inq_on quiescent supply current with output switching adj pin floating 450 600 a 450 680 v sense mean current sense threshold voltage 97 100 103 mv 95 100 105 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 0.11 0.2 0.29 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 0.16 0.25 0.34 is31lt3360 integrated silicon solution, inc. ? www.issi.com 5 rev. c, 12/22/2013 electrical characteristics (continued) valid are at v in =12v, typical value at 25c, unless otherwise noted. parameter range based on t a = -40c ~ +125c (note 3) the symbol in the table means these parameters are only available in the above temperature range. symbol parameter conditions temp. min. typ. max. unit r adj resistance between adj pin and v ref 500 k ? i lx_leak lx switch leakage current 1 a 2.5 i lx_mean continuous lx switch current (note 4) 1.2 a r lx lx switch ?on? resistance 0.27 0.4 ? 0.27 0.7 t on_min minimum switch ?on? time lx switch ?on? 200 ns t off_min minimum switch ?off? time lx switch ?off? 200 ns t pd internal comparator propagation delay (note 4) 50 ns t sd thermal shutdown temperature (note 4) 150 c t sd_hys thermal shutdown hysteresis (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. is31lt3360 integrated silicon solution, inc. ? www.issi.com 6 rev. c, 12/22/2013 typical performance characteristics power supply(v) error(%) -3 -2 -1 0 1 2 3 5 10152025303540 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led l = 47 h r s = 0.083 ? figure 2 output current error vs. power supply power supply(v) efficiency (%) 70 75 80 85 90 95 100 5 10152025303540 l = 47 h r s = 0.083 ? 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led figure 4 efficiency vs. power supply power supply(v) supply current (a) 610152025303540 0 100 200 300 400 500 600 operating mode figure 6 supply current vs. power supply (operating mode) power supply(v) error (%) -4 -3 -2 -1 0 1 2 3 4 5 10152025303540 1led 2led 3led 7led 8led 9led 4led 5led 6led 10led l = 47 h r s = 0.17 ? figure 3 output current error vs. power supply power supply(v) efficiency (%) 70 75 80 85 90 95 100 5 10152025303540 l = 47 h r s = 0.17 ? 1led 2led 3led 4led 5led 7led 8led 9led 10led 6led figure 5 efficiency vs. power supply power supply(v) supply current (a) 6 10 15 20 25 30 35 40 0 100 200 300 400 shutdown mode figure 7 supply current vs. power supply (shutdown mode) is31lt3360 integrated silicon solution, inc. ? www.issi.com 7 rev. c, 12/22/2013 power supply(v) v ref vo l t a g e ( m v ) 678 910 1135 1137 1139 1141 1143 1145 low supply voltage figure 8 v ref vs. power supply (low supply voltage) temperature ( c ) v sense v oltage (mv) -40 -25 -10 5 20 35 50 65 80 95 110 125 95 96 97 98 99 100 101 102 103 104 105 v in = 12v figure 10 v sense vs. temperature temperature ( c ) r ds_on (m ? ) 0 50 100 150 200 250 300 350 400 -40 -25 -10 5 20 35 50 65 80 95 110 125 v in = 12v figure 12 r ds_on vs. temperature power supply(v) v ref voltage (mv) 610152025303540 1130 1134 1138 1142 1146 1150 normal supply voltage figure 9 v ref vs. power supply (normal supply voltage) temperature ( c ) v adj (mv) -40 -25 -10 5 20 35 50 65 80 95 110 125 1150 1160 1170 1180 1190 1200 v in = 12v figure 11 v adj vs. temperature time (4s/div) v adj 1.0v/div i l 100ma/div v in = 12v l = 47h r s = 0.2 ? figure 13 adj pin voltage vs. i l is31lt3360 integrated silicon solution, inc. ? www.issi.com 8 rev. c, 12/22/2013 time (1s/div) v lx 5v/div i l 100ma/div v in = 12v l = 47h r s = 0.2 ? led open figure 14 led open-circuit protection time (400ms/div) v led 2v/div i l 100ma/div v in = 12v l = 47h r s = 0.2 ? led short figure 15 led short-circuit protection is31lt3360 integrated silicon solution, inc. ? www.issi.com 9 rev. c, 12/22/2013 functional block diagram is31lt3360 integrated silicon solution, inc. ? www.issi.com 10 rev. c, 12/22/2013 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 =0.083 ? 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) 0.083 1200 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 is31lt3360 integrated silicon solution, inc. ? www.issi.com 11 rev. c, 12/22/2013 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. 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. inductor selection recommended inductor values for the is31lt3360 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 1.2a for 700ma 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 =0.27 ? : s t on ? 564 . 0 ) 27 . 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 570khz 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 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. is31lt3360 integrated silicon solution, inc. ? www.issi.com 12 rev. c, 12/22/2013 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 junction temperature, t a is the ambient temperature, and ja is the junction to ambient thermal resistance. the recommended maximum operating 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 94 . 0 / 6 . 132 25 150 ) ( ? ? ? ? ? ? to ensure the performance, the die temperature (t j ) of is31lt3360 should not exceed 125c. the graph below gives details for power derating. temperature ( c ) power dissipation (w) 0 0.2 0.4 0.6 0.8 1 -40 -25 -10 5 20 35 50 65 80 95 110 125 sot89-5 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. is31lt3360 integrated silicon solution, inc. ? www.issi.com 13 rev. c, 12/22/2013 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 is31lt3360 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. is31lt3360 integrated silicon solution, inc. ? www.issi.com 14 rev. c, 12/22/2013 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 is31lt3360 integrated silicon solution, inc. ? www.issi.com 15 rev. c, 12/22/2013 package information sot89-5 note: all dimensions in millim eters unless otherwise stated. |
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