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IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 1 40v/1.2a led driver with switch dimming general description the IS31LT3380 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 voltage between 8.5v and 40v and provides an output current up to 1.2a. the IS31LT3380 includes a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. the IS31LT3380 includes an integrated output switch which has a very low conducting impedance to ensure high system efficiency. the IS31LT3380 has switch dimming function. the chip detects external switch action to adjust output current, allowing for dimming functionality to be achieved without changing the original lighting system circuitry. the switch dimming is implemented in either two-level mode or three-level mode. the output current of every level and the total number of levels are customer selected by setting the corresponding input conditions of dim1 and dim2 pin. the output current is set at the initial value the first time that power is supplied to the chip. after the initial power up sequence, the chip adjusts the output current according to the external switch action. after the lowest current level, the current cycles back to the initial value if more switch action is detected. if the power is switched off for longer than 2 seconds, the device will return to it?s initial state, and the output current will be set to the initial value at the next time that power is applied. features ? multi-modes switch dimming ? up to 1.2a output current ? high efficiency (up to 98%) ? wide input voltage range: 8.5v to 40v ? internal 40v power switch ? simple low parts count ? typical 5% output current accuracy ? up to 1mhz switching frequency ? inherent led open-circuit/sho rt-circuit protection ? thermal shutdown protection circuitry ? sop8 package applications ? mr16, mr11 led spotlight ? led street lighting ? par led bulb ? other led lighting october 2011 copyright ? ? ? 2011 ? integrated ? silicon ? solution, ? inc. ? all ? rights ? reserved. ? issi ? reserves ? the ? right ? to ? make ? changes ? to ? this ? specification ? and ? its ? products ? at ? any ? time ? without ? notice. ? issi ? assumes ? no ? liability ? arising ? out ? of ? the ? application ? or ? use ? of ? any ? information, ? products ? or ? services ? described ? herein. ? customers ? are ? advised ? to ? obtain ? the ? latest ? version ? of ? this ? device ? specification ? before ? relying ? on ? any ? published ? information ? and ? before ? placing ? orders ? for ? products. ? integrated ? silicon ? solution, ? inc. ? does ? not ? recommend ? the ? use ? of ? any ? of ? its ? products ? in ? life ? support ? applications ? where ? the ? failure ? or ? malfunction ? of ? the ? product ? can ? reasonably ? be ? expected ? to ? cause ? failure ? of ? the ? life ? support ? system ? or ? to ? significantly ? affect ? its ? safety ? or ? effectiveness. ? products ? are ? not ? authorized ? for ? use ? in ? such ? applications ? unless ? integrated ? silicon ? solution, ? inc. ? receives ? written ? assurance ? to ? its ? satisfaction, ? that: ? a.) ? the ? risk ? of ? injury ? or ? damage ? has ? been ? minimized; ? b.) ? the ? user ? assume ? all ? such ? risks; ? and ? c.) ? potential ? liability ? of ? integrated ? silicon ? solution, ? inc ? is ? adequately ? protected ? under ? the ? circumstances
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 2 application circuit figure 1-a figure 1-b
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 3 pin configurations package pin configurations sop-8 pin description pin name no. description v in 1 input voltage (8.5v to 40v). decouple to ground with 0.1 f x7r ceramic capacitor close to device. while using the switch dimming function, connect a 220 uf or higher capacitor close the device. i sense 2 connect resistor r s from this pin to v in to define nominal average output current i outnom =0.1/r s dim1 6 set the current-levels number of and current ratio of each level of switch dimming function as below: dim1=?floating? dim2=?floating? not dimming dim1=?floating? dim2=?gnd? three-levels 100%---50%---20% dim1=?gnd? dim2=?floating? three-levels 100%---60%---30% dim1=?gnd? dim2=?gnd? two-levels 100%---30% dim2 5 gnd 7 ground (0v) lx 8 drain of power switch nc 3,4 not connect. float required ordering information industrial range: -40 c to 105 c order number package qty/reel IS31LT3380-grls3-tr sop-8, lead-free 2500
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 4 absolute maximum ratings (note1) symbol parameter rating v in input voltage -0.3v to +50v v isense i sense voltage vin+0.3v to vin-5v ,vin>5v vin+0.3v to -0.3v,vin<5v v l x lx output voltage -0.3v to +50v v dim dim1 ,dim2 pin input voltage -0.3v to +6v i l x switch output current 1.5a p to t power dissipation 0.5w t op operating temperature -40 c to 105c t st storage temperature -55 to 150c t j ma x junction temperature 150c r ja junction to ambient 100c/w esd rating (hbm) 3kv electrical characteristics (v in =12v, t a =25c unless otherwise stated) (note 2) symbol parameter conditions min. typ. max. unit v in input voltage 8.5 40 v i inqoff quiescent supply current with output off 70 120 140 a i inqon quiescent supply current with output switching 450 600 a v isense mean current sense threshold voltage 95 100 105 mv uvlo undervoltage lockout threshold v in falling 5.05 5.85 6.65 v uvlo undervoltage lockout hysteresis 1.65 v v sensehys sense threshold hysteresis 15 % i sense i sense pin input current v sense =v in -0.1 8 a ts the interval time between the external switch action 2 s kmode1 in dimming mode 1,the ratio of different output current level to the initial current (dim1=?floating? dim2=?gnd?) level 1 100% level 2 50% level 3 17% 20% 23%
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 5 electrical characteristics (v in =12v, ta=25c unless otherwise stated) (note 2) (continued) symbol parameter conditions min. typ. max. unit kmode2 in dimming mode 2,the ratio of different output current level to the initial current (dim1=?gnd? dim2=?floating?) level 1 100% level 2 60% level 3 28% 30% 32% kmode3 in dimming mode 3,the ratio of different output current level to the initial current (dim1=?gnd? dim2=?gnd?) level 1 100% level 2 28% 30% 32% i lxmean continuous lx switch current 1.2 a i lx ( leak ) lx switch leakage current 1 a r lx lx switch ?on? resistance 0.3 0.4 ? t onmin minimum switch ?on? time lx switch ?on? 200 ns t offmin minimum switch ?off? time lx switch ?off? 200 ns f lxmax recommended maximum operating frequency 1 mhz t pd internal comparator propagation delay 25 ns t sd thermal shutdown temperature 150 c t sd-hys thermal shutdown hysteresis 20 c notes: 1. work beyond maximum absolute voltage and current will cause permanent damage to the device. long time working under absolut e conditions will lower the reliability of the dice. 2. production testing of the chip is performed at 25 c. functional operation of the chip and par ameters specified are guaranteed by design, characterization and process control in other temperature.
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 6 typical operating conditions for typical application circuit and t a =25c unless otherwise stated. supply current vs. vin (operating) efficiency vs. no. of leds l=47uh, rs=0.10ohm efficiency vs. no. of leds l=47uh, rs=0.30ohm efficiency vs. no. of leds l=47uh, rs=0.15ohm output current variation with output voltage l=47uh, rs=0.15ohm
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 7 typical operating conditions temperature vs vsense voltage l=47uh, rs=0.10ohm
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 8 switch dimming waveform led open waveform vin=24vdc l=47uh, rs=0.30ohm led short waveform vin=24vdc l=47uh, rs=0.30ohm
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 9 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 out nom = 0.1/r s the table below gives values of nominal average output current for several preferred values of current setting resistor (r s ) in the typical application circuit shown on page 2: r s ( ? ) nominal average output current (ma) 0.082 1200 0.15 667 0.3 333 the resistance tolerance of rs should be 1%, and the power rating must be high enough to continuously handle the average output curre nt. good over-temperature performance of the resistor is also recommended. multi-modes switch dimming the IS31LT3380 detects external switch action to adjust output current. there are two types of switch action: a normal switch, which means the off-time between each subsequent switching is longer than 2s; and a dimming-switching, which means the off-time between each subsequent switch actions is less than 2s. the nominal average output current is given by: i out = k*0.1/r s [k is the current ratio to initial current] the input condition of the dim1 and dim2 pins set the number of current-levels and cu rrent ratio for the switch dimming function as shown below: dim1 dim2 dim-ratio floating floating not dimming floating gnd 100%---50%---20%---100% gnd floating 100%---60%---30%---100% gnd gnd 100%---30%---100% for instance, suppose the dim1 pin is floating and the dim2 pin is connected to gnd. the first time the switch is turned on, the initial current is i outnom. after the 1 st dimming-switch, i out(1) = 50% * i outnom after the 2 nd dimming-switch, i out(2) = 20% * i outnom after the 3 rd dimming-switch, i out(3) = 100%* i outnom this process recycles. if a normal-switch is detected or dim1 and dim2 are both floating, the output current goe s back to the initial state of 100%. since the chip needs to count the time for more than 2 seconds after the switch is off, during the off time one capacitor bigger than 220uf is required to keep the chip work in low quiescent current mode. inherent open-circuit led protection if the connection to the led( s) is open-circuited, the circuit is open, 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 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 a dc supply, the capacitor is decided by ripple of the source, the value is given by: max on f u t i c ? ? * min i f is the value of output current max u ? is the ripple of power supply. ton is the ?on? time of the mosfet, normally set at about twice the ?off? time. if the source is an ac supply, the output voltage ripple from a nominal 12v ac transformer can be 10%. if the input capacitor value is lower than 200 f, the ac input waveform will be distorted, and sometimes the lowest input voltage can be lower t han the forward voltage of the led strings. this lowers the average current of the
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 10 leds. therefore, it is recommended to select the value of the capacitor bigger than 200uf. for maximum stability over temperature and 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. inductor selection recommended inductor values for the IS31LT3380 are in the range of 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 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. 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,etc. 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 s avg led in on r rl r i v v i l t ? ? ? ? ? ? note: t onmin >200ns lx switch 'off' time ) ( s avg d led off r rl i v v i l t ? ? ? ? ? note: t offmin >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 and v d =0.36v r lx =0.27 ? t on = (47e-6 0.105)/(12 - 3.4 - 0.274) = 0.59 s t off = (47e-6 0.105)/(3.4 + 0.36 + 0.188)= 1.25 s this gives an operating frequency of 543khz and a duty cycle of 0.32 optimum performance will be achieved by setting the duty cycle close to 0.5 at t he 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 (d1) 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 above 85c. excess leakage will increase the power dissipation in the device. the higher forward voltage and overshoot due to reverse recovery time in silicon di odes 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 peak to peak ripple current in the led can be reduced, if required, by shunting a capacitor c3 across the led(s) as shown below: a c3 value of 1 f will reduce nominal ripple current by a factor of approximately three. 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 start-up delay, by reducing the rate of rise of led voltage.
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 11 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. note that the ch ip power dissipation will be a maximum at the 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 on the switch output. layout considerations vin/gnd pin the gnd of the power supply usually has some distance between it and the chip gnd pin, causing parasitic resistance and inductance, resulting in ground voltage bounce when the mosfet switches. to minimize ground bounce, the ground pin of the chip should be soldered directly to the ground plane. connecting a 0.1uf capacitor between the vin and gnd pins as close to the chip as possible minimizes the effects of 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. coil and decoupling capacitors it is particularly important to mount the coil and the input decoupling capacitor close to the chip to minimize parasitic resistance and indu ctance, which will degrade efficiency. it is also important to take account of any trace resistance in series with current sense resistor rs. dim pin the dim pin is a high impedance input, when it is left floating; this pin is pull up to 3.3v by internal circuit. avoid running any high voltage traces close to the dim pins.
IS31LT3380 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 12 package information sop-8 3 d f n d j h 2 x w o l q h ' u d z l q j ' % 6 &

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