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lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 descri pt i on k ey feat u r es ? white led backlight controller for middle size display panels ? up to 6 led strings with 0.5% precision current matching ? wide input voltage range from 6v to 28v ? temperature compensated led current ? ultrasonic pulse skipping for audible noise reduction ? direct ambient light sensor interface for brightness control ? high efficiency over full dimming range ? low standby current ? on-chip thermal shut-down ? over-voltage protection ? packaged in a thermally efficient 24 p in 4x4mm mlpq packa g e appli cat i on s lx1996 is an integrated white led driver for notebook size displays. it is designed to drive up to six strings of leds. the lx1996 consists of a boost converter and six precision current sources. it also includes a direct analog light sensor interface and can be dimmed by a pwm signal or by application of a dc control voltage. precision current mirror circuitry sets all channels to the same current with a single programming input. the current value can be programmed up to 30ma per channel with a single resistor from the prg pin to ground. channel to channel matching is within 0.5 percent typical at 25c ambient of the channels mean at the rated output current. unique design features include temperature compensated led current, and audible noise reduction using ultrasonic pulse skipping. temperature compensated led current is accomplished by a dedicated input pin (ot) that in conjunction with an external thermistor measures the leds ambient temperature and thereby prevents over temperature stress to the leds by keeping them within their safe operating area. additional features include low sleep mode current (6a typ.) for extending battery time, multi-mode dimming via ambient light sensor interface, analog dimming, direct pwm dimming and combination of analog and direct pwm dimming for up to a 25000:1 dimming range. the lx1996 is designed to work over a wide input voltage range from 6v to 28vdc with adjustable over voltage protection (ovp), and short and open led protection. the lx1996 comes in a compact thermal efficient 4x4mm 24pin mlpq package. ? notebook & monitor displays ? entertainment systems ? automotive li ghting & panels important: for the most current data, consult microsemi s website: http://www.microsemi.com patents pending produ ct h i gh li gh t 6v to 28v large lcd backlight 5v lx1972 light sensor pwm ot ovp gnd di pwm vldo vin drv src n/c ct dsel1 cs1 cs2cs3 cs4 cs5 cs6 prg current programing clf thermistor dsel2 pgnd en rt led panel temperature compensation of led current multi-mode dimming via ambient light sensor and/or direct pwm dimming lx1996 5v pack age order i n fo t h erm al dat a lq plastic 4x4 mm mlpq 24-pin j a = 30 c/w t a ( c) rohs compliant / pb-free thermal resistance - junction to ambient -40 to 85 LX1996ILQ note: available in tape & reel. append the letters tr to the part number. (i.e. LX1996ILQ-tr) junction temperature calculation: t j = t a + (p d x ja ). the ja numbers are guidelines for the thermal performance of the device attached to a pc-board system. all of the above assume no ambient airflow. obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 2 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 absolu t e m ax i m u m rat i n gs pack age pi n ou t supply input voltage (vin), en ........................................................................ -0.3v to 30v cs# pins .............................................................................................................. -0.3v to 28v dsel1, dsel2, di and pwm pi ns .................................................................. -0.3 v to 6.0v ot, drv, src, ovp, prg, clf, ct, rt pins .................................. -0.3v to v ldo + 0.3v cs# curre nt ................................................................................................................... 35ma operating temperatur e range ............................................................................... - 40 to 85 c maximum operating junc tion temper ature ...................................................................150 c storage temperat ure rang e............................................................................. -65 c to 150 c package peak temp. for solder reflow ( 40 seconds maximum exposure) ....... 260c (+0 -5) notes: exceeding these ratings could cause damage to the de vice. all voltages are with respect to ground. currents are positive into, negativ e out of specified terminal. solder reflow to follow: ipc/jede c j-std-020b 7/02 pb-sn small body profile dsel2 dsel1 msc lx1996 xxxx ct pwm clf prg cs1 cs2 cs4 cs5 vin n/c src cs6 1 2 3 4 7 8 9 10 15 16 17 18 21 22 23 24 connect bottom pad to gnd 14 11 20 5 ovp vldo gnd cs3 en 6 12 13 19 pgnd drv di ot rt lq p ackage (top view) rohs / pb-free 100% matte tin pin finish xxxx = date code / lot code identifier fu n ct i on al pi n descri pt i on name pin # description en 1 enable input - logic input. pull low to put the sy stem into sleep mode; if not used must be tied to vin. vldo 2 internal 5v regulator output C power pin. provides a regu lated 5v for internal 5v circuits. it can also power an external light sensor and/or sm buss chip (limited to 1ma). must be decoupled closely to the gnd with a ceramic capacitor typical value of 4.7 f. dsel1 3 dimming mode select input 1. three level input. refe r to table 1 in the applications section for functionality dsel2 4 dimming mode select input 2. C two level logic inpu t. refer to table 1 in the applications section for functionality di 5 dimming input C signal pin. connect a dc dimmi ng voltage or the light sensor output to this pin. ot 6 led ambient over temperature protecti on C signal pin C connect to an ex ternal ntc thermistor to monitor the led light bar temperature. if nt c is not used, then tie this pin to the ics vldo pin. the ntc needs to be 100k ? at 25c and b constant of 4000. please see the applications section for more details regarding the ot option. pwm 7 pulse width modulated dimming signal C signal input. pwm input used for multi-dimming mode. with dsel1 pin tied low, a pwm signal input in the range of 100hz to 25khz provides direct digital dimming for the led outputs. with dsel1 pin open and dsel2 pin ti ed high, this pin is used as a sync input for synchronizing the lx1996s internal pwm dimming clock with an external clock signal of 400ns or greater width. rt 8 current source setting resistor. must be 100k between this pin and ground when analog to digital dimming or ultrasonic pulse skipping modes are used. if neit her mode is used, this pin may be left open or tied to v ldo . ct 9 internal digital dimming frequency setting capacitor. a capacitor between this pin and ground sets the internal digital dimming frequency. the relationship between the c t pin capacitor and internal frequency is; f(hz) = 5e-6/c t (farads) this pin should be left open or grounded when the internal oscillator is not used. clf 10 pll loop filter connection pin. a loop filter capacitor should be conn ected between this pin and ground when the internal dimming is used with synchronization mode engaged. typical capacitor values are; 0.1uf for 200hz; 1nf for 20khz. this pin should be left open or grounded when the synchroniz ation option is not used. prg 11 led current programming. connect a resistor between this pin and ground to set the led current. refer to applications section for functionality gnd 12 signal ground. should be tied at a single common point to power ground (pgnd pin 20). cs1-6 13 to18 current sink pins C controls individual led string current. cs pins can be wired in parallel for higher currents. the lowest cs voltage is k ept at approx. 0.8v at 20ma. obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 3 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 fu n ct i on al pi n descri pt i on name pin # description ovp 19 over voltage protection C signal input. an external vo ltage divider sets the maximum boost voltage limit in case of open led strings. refer to applications section for functionality. pgnd 20 power ground. should be tied at a single common point to signal ground (gnd pin 12). src 21 boost converter current sense pin. this connects to the junction of the exter nal n-fet source and the current sense resistor. it is used to set the peak inductor current. drv 22 external n-fet gate driv e. connect to the gate of the external n-channel mosfet n/c 23 no connection. vin 24 power supply input. must be closely decoupled to pgnd with a 4.7uf ceramic capacitor. obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 4 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 fu n ct i on al block di agram drv gnd cs1 pgnd + - led open detection boost fb digital dimming clock ngate driver + - cs open + - cs over voltage + - cs over voltage limit off timer toff=f(vcc) ultrasonic timer + - src coil current sense vin vldo 5v regulator 4-bit dac boost converter control logic mode control en dsel1 dsel2 led current reference prg triangle wave generator pll rt ct clf analog dimming 0.2v/2v limit analog to digital dimming pwm di cs2 cs3 cs4 cs5 cs6 0.2v 15v 25v fbref led short detection output control logic fb led current output, led open/short detection, boost converter regulation reference fb led current reference mu x led current temprature profiler ot ovp + - 2v fbref gen. fbref current source output #6 current source output #5 current source output #4 current source output #3 current source output #2 current source output #1 output enable obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 5 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 elect ri cal ch a ract eri st i cs unless otherwise specified, the following specificati ons apply over the operating am bient temperature of -40 0 c < temp <85 0 c, and the following test conditions: vin=12v, v ot = 3v, en = vin , r prg = 4.99 k ? , l=6.8 h, r src = 0.075 ? , c output = 4.7uf lx1996 parameter symbol test conditions / comment min typ max units power supply input voltage v in 6 12 28 v quiescent current i in on en 2.4v, output not switching, v cs = 0.8v 2.5 ma sleep current i in sleep en 0.4v 6 15 a logic dsel1 input high voltage v dsel1-h 2.4 v dsel1 input low voltage v dsel1-l 0.8 v dsel1 input current high i dsel1_h dsel1 = 2.4v 4 10 a dsel1 input current low i dsel1_l dsel1 = 0.8v -10 -4 a dsel1 open voltage 1.2 1.6 2.0 v dsel2 input high voltage 2.4 v dsel2 input low voltage 0.8 v dsel2 pull-down resistor 600 k ? en input high voltage 2.4 v en input low voltage 0.8 v en input current i en_h 12v en 2.4v 4.0 10 a en input current i en_l en < 0.4v 0.4 1 a pwm input high voltage v pwm-h 2.4 v pwm input low voltage v pwm-l 0.8 v pwm pull-down resistor 600 k ? dimming di input bias current v di =0.2 to 2.0v, analog dimming 1 a maximum prg voltage v th_di_h v di =2.0v, analog dimming 1.83 2.0 2.17 v minimum prg voltage v th_di_l v di =0.2v, analog dimming 0.2 v direct digital dimming frequency f pwm 100 25000 hz direct digital dimming min. pwm pulse width (note 2) f pwm 4 us rt reference voltage rt = 100k ? 2 v ct = 200pf, rt = 100k ? 25 khz internal pwm frequency f intpwm ct =.047 f, rt = 100k ? 100 hz sync. freq. capture range percent of internal pwm frequency -25 +25 % minimum pwm input pulse width for synchronization 400 ns device protection ovp threshold voltage v th_ovp 1.9 2.0 2.1 v obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 6 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 elect ri cal ch a ract eri st i cs unless otherwise specified, the following specificati ons apply over the operating am bient temperature of -40 0 c < temp <85 0 c, and the following test conditions: vin=12v, v ot = 3v, en = vin , r prg = 4.99 k ? , l=6.8 h, r src = 0.075 ? , c output = 4.7uf led short protection threshold vcs voltage at cs pin that latches off its respective string. 15 v cs pin over-voltage limit v cs voltage at cs pin that clamps boost output 23 25 27 v led current sink outputs v cs = 0.8v; di 2.0v; direct analog dimming mode 19.2 20 20.8 ma led dc current programming accuracy (including string to string matching) i cs r prg = 6.65k (led current set for15ma);, v cs = 0.8v; di 2.0v; direct analog dimming mode 14.4 15 15.6 ma analog dimming accuracy i cs di=0.5v; direct analog dimming mode 5 ma direct digital dimming accuracy i csavg direct digital dimming mode; v cs = 0.8v; pwm input at 25khz , 25% duty cycle; average (dc) led current. 5 ma current matching between strings (note 3). i cs-match vin = 6v to 28v; -40c to 85c; vcs = 0.8v to 5v; di 2.0v; direct analog dimming mode -1.3 1.3 % regulated cs pin voltage at the lowest cs pin, i cs = 20ma 0.5 0.75 v led current on rise time t cs_rise 200 ns led current off fall time t cs_fall 400 ns csx current to prg current ratio i cs /i prg 50 prg reference voltage v prg 2.0 v led current temperature profiler led over temperature shut- down voltage threshold v ot 1.47 v ot recovery voltage threshold v ot 1.8 v ot input bias current 1 a v prg v ot = 2.48v 2.0 v v prg v ot = 1.8v 1.2 v v ot input vs. prg pin reference voltage (note 1, 2) v prg v ot = 1.47v 0.8 v vldo regulator v in 6.5v; 0ma i vldo 6.0ma 4.75 5.0 5.25 v vldo output voltage v ldo v in < 6.5v; 0ma i vldo 6.0ma 4.5 5.25 v vldo source current i vldo external available current 1 ma uvlo v uvlo vldo rising 4.0 4.25 4.5 v uvlo hysteresis 0.25 v boost converter drv rise time t rise c l = 1000pf; 10% to 90% of v ldo 25 nsec drv fall time t fall c l = 1000pf; 90% to 10% of v ldo 14 ns obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 7 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 elect ri cal ch a ract eri st i cs unless otherwise specified, the following specificati ons apply over the operating am bient temperature of -40 0 c < temp <85 0 c, and the following test conditions: vin=12v, v ot = 3v, en = vin , r prg = 4.99 k ? , l=6.8 h, r src = 0.075 ? , c output = 4.7uf ultrasonic pulse skipping (usps) maximum switching interval r t =100k ? , ultrasonic mode 23 40 50 s thermal protection over temperature shutdown (note 2) 140 c thermal shutdown recovery (note 2) 100 c notes 1. with a 100k ntc thermistor, bc onstant = 4250, and rbias = 22.1k, v ot threshold values equate to the following temperature points: a. ot shut off temperature = 85c b. ot derating threshold = 60c c. ot recovery temperature = 75c 2. this parameter is bench char acterized but not ate tested. 3. based on % variation from (i cs max+i cs min)/2. obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 8 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 typical characteristics @ 25c (refer to figure 1) sy st em e ffi ci en cy v s load sleep cu rren t v s i n pu t v olt age efficiency vs output load analog dimming mode 40% 50% 60% 70% 80% 90% 100% 0 20 40 60 80 100 120 total led current (ma) efficiency 6v-eff 12v-ef f 18v-ef f system efficiency vs output load analog dimming mode sleep current vs input voltage 3 4 5 6 7 8 9 10 11 6 8 10 12 14 16 18 20 22 24 26 28 vin (v) i_sleep (ua) su pply cu rren t v s i n pu t v ol t age ( n o load) st ri n g cu rren t m at ch i n g v s i nput v olt a ge quiescent current vs input voltage 1 1.2 1.4 1.6 1.8 2 6 8 10 12 14 16 18 20 22 24 26 28 vin (v) i_q (ma) led string current matching vs. input voltage i set = 20.04ma -0.5% -0.4% -0.3% -0.2% -0.1% 0.0% 0.1% 0.2% 0.3% 0.4% 0.5% 3 6 9 1 21 51 82 1 input voltage (v) percent variation from range midpoint variation iled1 variation iled2 variation iled3 variation iled4 variation iled5 variation iled6 an alog i n pu t v s i n t ern al di gi t al di m m i n g an alog i n pu t v s i cs(x ) pwm output vs analog input 0.4 0.598 0.795 0.99 1.2 1.379 1.59 1.78 1.97 2.2 2.36 0 10 20 30 40 50 60 70 80 90 100 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 analog input (v) pwm output (% duty cycle ) . led current vs analog input 2 4.9 10.4 15.6 20.1 0 4 8 12 16 20 24 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 analog input (v) led current (ma) obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 9 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 st art u p wav eform s analog dimming. full load swi t ch i n g wav eform s analog dimming. 50% load led cu r r en t wa v eform s d = 50% at 17.5khz, digital dimming led cu r r en t wa v eform s d = 5% at 17.5khz, digital dimming led cu r r en t wa v eform s d = 1% at 200hz, digital dimming led cu r r en t wa v eform s usps operation; led current = 0.5ma v led en +5v 6x led current vcs 6x led current inductor current pwm input vcs 6x led current inductor current pwm input boost fet drain vcs 6x led current inductor current inductor current boost output led current inductor current boost output vcs 6x led current obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 10 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 led cu r r en t wa v eform s line transient response; vin 10v to 20v step led cu r r en t wa v eform s line transient response; vin 20v to 10v step led prot ect i on wav eform s shorted led response C 1 to 5 shorted leds in a single string led prot ect i on wav eform s led open response boost current boost voltage v cs cs current 5 led short C vcs incremented by shorting one led at a time up to vcs short threshold of 15 v v ovp v boost v cs ovp threshold boost output led current vin boost output led current vin obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 11 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 t y pi cal appli cat i on 6v to 28v 6.8uh 1m 78.7k 0.075 b160 large lcd backlight 5v lx1972 dpst pwm ot ovp gnd di pwm vldo vin drv src n/cct dsel1 cs1 cs2 cs3 cs4 cs5 cs6 prg 4.99k 24.9k clf 22.1k ntc 100k@25c b=4250 scl scd gnd adr vdd ref aout ain lx1800 smbus 2324 32 57 9 10 12 1314 15 16 17 18 6 19 21 22 11 4.7uf en pgnd dsel2 18 20 4.7uf/50v x5r 1210 rt 4 5v si3458dv 3 amp 4.7uf/50v x5r, 1210 4.7uf/6.3v,x7r figure 1 C typical application C this circuit demonstrat es the lx1996 and lx1972 interfacing to the lx1800, which is an smbus adc/dac. this application also uses the display power saving technology (dpst) signal. key components - bill of materials LX1996ILQ = multi-string led back-light controller, microsemi lx1800ild = smbus to analog interface, microsemi lx1972ibc = ambient light detector, microsemi grm188r60j475ke19 = 4.7uf/6.3v, x7r, 0603, murata grm32er71h475ka88 = 4.7uf/50v, x5r, 1210, murata slf6028t-6r8m1r5-pf, 6.8 h, 1.5a, tdk si3458dv = n channel mosfet, 60v/1.2a, tsop-6,vishay ncp18wb104j03ra = ntc, 100k at 25c, b = 4250, murata b160 = schottky diode 1a/60v, sma, vishay obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 12 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 operat i on t h eory b oost c onverter the lx1996 uses a variable peak current variable off time hysteretic pfm converter topology where inductor peak current is digitally controlled. it is a current mode step up converter in pfm burst mode. the lx1996 controls the inductor peak and the off time to minimize the switching frequency variation over a wide input power supply range. it regulates the boost output voltage based on the lowest voltage among the cs pins. this converter stays in burst pfm mode for the entire load range and has extremely fast load response. it automatically increases or decreases the inductor peak current depending on the output load to minimize the output voltage ripple while maintaining the output in regulation. maximum coil peak current can be up to 1.55a with a 75m ? sense resistor. for a higher power system, a user can use a smaller resistance current sense resistor. conversely, a lower power system can use a larger resistance current sense resistor. typical switching frequency is over 1mhz. a 6.8uh inductor is recommended for systems that require direct digital dimming. a larger inductor may be used for a higher efficiency if the system runs only in analog dimming mode. vldo r egulator regulated 5v is generated from vin. it provides power to the internal 5v circuits, an external nfet, and an optional external light sensor (lx1972) or smbus chip (lx1800). the total maximum output current is limited to 1ma from the vldo pin. a capacitor must be closely connected across the vldo pin and gnd for noise decoupling and phase compensation. a value of 4.7 f is required. the vldo regulator shuts off while the chip is in sleep mode (en=low) to minimize the sleep mode current. the vldo output voltage is monitored by the under voltage lockout circuit. led c urrent s inks cs1 through cs6 are the led current sinks. voltage information on these pins is used to regulate the boost converter output. boost converter output regulation is based on the lowest cs pin voltage. this regulation scheme insures that the led string with the largest total v f has ample headroom for regulation, while maximizing the total efficiency for all other strings. cs pin voltage (at the lowest cs pin) is linearly proportional to cs current. for example, typical v cs will be 800mv at i cs = 20ma, and 80mv at i cs = 2ma. the lx1996 uses active current sources for th e best string current matching between strings. the high impedance outputs have excellent voltage compliances, therefore, v f matching of the leds is less critical. the current sources can be paralleled for higher current rating. led current can be set with an external resistor using the following formula: 50 r v i prg prg led = v prg = 2v for either direct dig ital dimming or direct analog dimming. in combined analog and digital dimming, v prg = 2v with di 2v. when di < 2v, then v prg = di. d imming c ontrol the lx1996 offers 6 methods of dimming using only 2 pins (see table 1). d irect d igital d imming (pwm) the led current source outputs are directly turned on and off by the pwm signal. a high on pwm input pin turns on the output. the current pulse magnitude is independent of pwm duty cycle and is set by the pr g resistor with a fixed 2v reference at the prg pin. c ombined a nalog and d igital d imming (pwm) the led current source outputs are directly turned on and off by the pwm signal. a high on pwm input pin turns on the output. the current pulse magnitude is dependent on the voltage input at the di pin. current is set by the prg resistor, with a reference voltage at the prg pin that varies with the voltage input at the di pin in a 1:1 ratio. the minimum and maximum reference voltage at the prg pin is clamped at 0.2 and 2 volts, respectively. a nalog to d igital d imming analog to digital dimming mode converts a dc voltage on the di input into a pwm output. di input will be in the range of 0.4 to 2.4v. an internal pwm signal is generated to control the led output, just as in direct digital dimming. two external components are required to make this mode work; c t and r t . r t is a reference current setting resistor for the internal dimming oscillator, and the ultrasonic pulse skipping timer. r t must be a fixed 100k ? . the internal oscillator frequency is set by the value of the c t capacitor , and is found by the following equation: f = 5e -6 /c t , c t in farad, f in hertz. obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 13 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 operat i on t h eory a nalog to d igital d imming with external sync the internal dimming oscillator can be synchronized to an external clock on the pwm input. a pll loop filter capacitor must be connected between clf and gnd pins for synchronization. use 0.1uf for 200hz, 0.001uf for 20khz. the rt, ct and clf pins can be left open if both the internal dimming and ultrasonic pulse skipping modes are not used. d irect a nalog d imming in this mode, a dc voltage on the di pin is used to vary the reference voltage at the prg pin. the prg pin reference voltage, in conjunction with the resistor connected between the prg pin and ground, sets the led current. the ratio of di input voltage to prg pin voltage is 1:1, up to 2v input. the di input voltage range is 0v to 5v, however, the voltage at prg pin is limited to 0.2v to 2v with an internal voltage clamp. thus the dimming ratio is limited to 10:1. a higher dimming ratio is obtained in the combined analog and digital dimming mode. the table below summarizes the dimming modes. table 1 - dimming modes dsel1 dsel2 dimming method low low direct digital dimming low hi combined analog and digital dimming modes hi low direct analog dimming hi hi direct analog dimming with ultrasonic pulse skipping (usps) open low analog to digital dimming with internal clock open hi analog to digital dimming with external clock synchronization. obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 14 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 operat i on t h eory digital dimming modes of operation direct digital dimming freq. range controller performance pwm 100hz to 25khz ? led current will be turned on/off directly by pwm. ? the maximum led current is set by a programmable resistor. ? dimming ratio is 10:1 at internal frequency = 25khz. it will be proportionally higher at lower frequency. the minimum pulse width is 4 sec combined analog and digital dimming freq. range controller performance pwm 100hz to 25khz ? signal into di pin will be either from an ambient light sensor (als) or a variable voltage source, v di ? the led current is linearly controlled by a programmable resistor and v di ? valid v di range is 0.2v to 2.0v ? led current will be turned on/off directly by pwm. ? dimming ratio is 100:1 at pwm = 25khz. it will be proportionally higher at lower frequency. the minimum pulse width is 4 sec analog dimming modes of operation direct analog dimming freq. range controller performance n/a ? signal into di pin will be either from ambient light sensor (als) or the adjustable voltage source, v di ? the led current is linearly controlled by a programmable resistor and v di ? valid v di range is 0.2v to 2.0v ? the dimming ratio can go up to 10:1. direct analog dimming with ultra sonic pulse skipping (usps) freq. range controller performance n/a ? enables usps in analog dimming mode to reduce boost inductor noise by moving the switching frequency out of the audible range. analog to digital dimming freq. range controller performance internal frequency 100hz C 25khz ? pwm duty cycle controlled by the signal into the di pin ? signal into di pin will be either from an ambient light sensor (als) or a variable voltage source, v di ? v di will be in the range of 0.4v to 2.4v ? pwm signal is generated by an internal oscillator. dimming frequency is set by a capacitor connected to the c t pin ? the maximum led current is set by a programmable resistor connected to the rprg pin. ? led current will be turned on/off directly by the internal pwm. ? dimming ratio is 10:1 at internal frequency = 25khz. it will be proportionally higher at lower frequency. the minimum pulse width is 4 sec analog to digital dimming wi th external synchronization clock freq. range controller performance internal frequency 100hz C 25khz ? pwm duty cycle controlled by the signal into the di pin ? signal into di pin will be either from an ambient light sensor (als) or a variable voltage source, v di ? v di will be in the range of 0.4v to 2.4v ? pwm signal is generated by an internal oscillator. dimming frequency is set by a capacitor connected to the c t pin ? the maximum led current is set by a programmable resistor connected to the rprg pin. ? led current will be turned on/off directly by the internal pwm ? the internal pwm synchronizes to a clock on the pwm input pin ? dimming ratio is 10:1 at internal frequency = 25khz. it will be proportionally higher at lower frequency. the minimum pulse width is 4 sec obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 5 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 operat i on t h eory ultra sonic pulse skipping (usps) at low led current level, th e boost frequency may go to audible range. with an usps option (analog dimming mode only), the audible noise is completely eliminated. when this mode is enabled, maximum switching interval is limited to 40 s, thus keeping the switching frequency beyond audible range. usps is an option for analog dimming only. protection modes t hermal s hutdown the lx1996 includes a thermal pr otection circuit. when the ic senses temperature over +140c (typ), the lx1996 turns off the led output current and other circuits. the output recovers when the die temperature drops down to 100c. the thermal protection does not latch off the ic. o ver v oltage p rotection a simple resistor divider between the boost output and ground is used to program the lx1996s ovp limit. the over-voltage threshold is set to 2v at the ovp pin. when ovp limit threshold is met, the lx1996 will regulate the boost output to the ovp 2v threshold. ovp limit should be set to a voltage approximately 2 volts above the maximum total v f of the individual diode strings. led o pen / short protection in case of an open string, the open string cs voltage will drop to zero and the boost voltage will rise up to its programmed ovp limit or the cs pin maximum voltage limit. when the boost output reaches the ovp limit or the voltage of any cs pin reaches 25v, the open string is latched off from the regulation loop. at the moment the open string latches off, the boost converter will stop switching until the lowest cs pin voltage gets down to 0.8v. the change in boost output voltage should have no impact to the remaining active led string currents since they are provided by high impedance current sinks. once the open condition is corrected, cycling power or the en input will reset the latched off cs string. in case of the led short condition, lx1996 output works normally until the cs voltage rises to 15v (up to four shorted leds, typical). when the cs pin voltage reaches 15v, the shorted string is latched off and is disconnected from the boost converters regulation loop. if the latched off string cs pin voltage reaches 25v, the circuit will hold the cs pin voltage at this level. led c urrent t emperature p rofiler for led bar temperature sensin g, the led current profiler circuit receives a voltage on the ot pin from the output of a voltage divider network. this network consists of a thermistor and a bias resistor connected between +5v and gnd. the led current profiler circuit will re duce the led current setting by reducing the prg pin voltage. with our recommended external components, derating begins at 60c (recommended by most led manufacturers). the led current will reduce proportional to the temperature, and shut o ff when the sensing temperature reaches 85c. the led current will recover when the sensing temperature drops to 75c. s ee example chart below (recovery not shown). led current vs. temperature 20ma 15ma 10ma 5ma 20c 40c 60c 80c 100c b = 4500 b = 4000 b = 3000 25ma 0c obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 16 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 appli cat i on i n form at i on i nductor c urrent s ense r esistor s election (r src ) peak inductor current is determined by the resistor connected between src pin and gnd, r src. its value is determined by the following equation: ma l 20ns vcc r 112.5mv il boost src pk + = typically, peak current is determined by the size of the fet desired. peak current will also determine the amount of energy stored in the inductor during a single switching cycle. the v cs detect signal is fed into a 4 bit up/down counter to determine the peak current level for each boost condition (v in , v out , and i out ). there are 15 total current steps available. the dc maximum threshold, v scr , is equal to 112.5mv, and the constant voltage increment is equal to 112.5mv/15 = 7.5mv. if we select r src = 75m ? , then step current: i step .= 7.5mv/75m ? = 100ma maximum inductor peak current: i peak (max) = 100ma x 15 +50ma = 1.55a note: 50ma term is due to propagation delay and is based on 4.7uh inductor and v in = 12v. if higher output power is needed, the value of r sense must be lowered accordingly. for example: if 50m ? is used: each current step will be: i step = 7.5mv/50m ? = 150ma maximum inductor peak current: i peak (max) = 150ma x 15 + 50ma = 2.3a the current sense signal is very sensitive; therefore, the lx1996 src pin must be connected directly to the junction of the sense resistor, r src , and the nfet source, as close to r src as practical. the return side of the sense resistor must tie directly to the pgnd pin, and then tie the pgnd pin to the ground plane. t hermal p rofiler c omponent s election the following circuit is a typical application using the ot pin for temperature profiling: 5v r1 ntc ot use ntc thermistor with resistance of 100k at 25c. b constant can be from 3000 to 4500. 1. find out the ntc resistance at 85c (r 85 ) by manufacturing chart or formula: 2. calculate r1 value to set th e shut off temperature at 85c by: r1 = 2.4 x r 85. this will set vot = 1.47v when ntc is at 85c. thermal profile is set by the internal circuit with the following equation: v prg =(v ot -0.8)*1.2, v ot =(r therm /(r therm + r1))*5; where r therm is the thermistor resistor at prg pin, 2v is the 100% value and 0.8v is the shut off point (40%). obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 17 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 appli cat i on i n form at i on b oost c onverter i nductor s election (l boost ) the lx1996 boost converter operates always in pfm mode, and in conjunction with the current sink circuit to support digital dimming mode with a minimum pulse width of 4 s. therefore, a boost inductor must be selected to satisfied digital dimming mode minimum pulse width requirements, with enough energy to supply the output during this period. the lx1996 controls the off time and peak current for hysteretic pfm converter to pology. the off time (t off ) will vary proportionally with input voltage per the following formula: t off = 150 + (22 x v in ) (nsec) (1) and pfm frequency: f pfm = v in /(v out x t off ) (hz) (2) for any instant, the stored energy must be large enough to supply the output load and ou tput capacitor. therefore: ? l min *i2f pwm v out x i out (3) from equation 1, 2, and 3, the minimum boost inductor value can be calculated. with a specified minimum pulse-width of 4 sec, the maximum boost inductor value must allow the inductor to reach the maximum peak current in less than 4 sec. otherwise, the led current can not get to the maximum. l max = v in dt/di (4) to calculate the maximum inductor value, v in will be at minimum, di will be at maximum, and dt is set at 2us which is ? of the minimum pulse width. for example: with 10 white led in series for 6 strings with i-led = 20ma, total i out = 120ma. minimum input voltage is 6v. per nichia v f (avg) = 3.15v, and 3 = 0.15v, then the worst case for the peak led voltage is 31.7v. adding 0.8v for the output diode forward drop, the boost voltage will be: v out = 31.7v + 0.8v = 32.5v the maximum peak inductor current is 1.55a with 75m ? current sense resistor. substitute values into equations 1, 2, and 3: t off = 282nsec f pwm = 654.7khz l min = 4.96uh from equation 4, the maximum value of the boost inductor is: l max = 7.7uh the inductor value will be in the range of 4.96uh to 7.7uh. lower inductance provides faster transient response; higher inductance reduces ripple curr ent and improves efficiency. when selecting the inductor, peak current rating, series resistance, and physical size should be all considered. the right candidate for this part is 6.8uh, 1.6a (6x6mm2) from tdk or coil craft. ovp s etting the worst case for white led forward voltage v f = 3.5v. therefore, the boost voltage that is set for 10 leds will be: v led = 10xv f +2v = 37v to minimize power loss through the ovp resistor network, select a top resistor r top = 1m ? . the bottom resistor is selected: r bottom = v ovp x r top /( v led C v ovp ); r bottom = (2 x 1m ? )/(37-2) = 57.1k ? select the closest resistor value: r bottom = 56.2k ? . obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 8 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 appli cat i on i n form at i on o utput c apacitor s election the output capacitor is preferre d to be ceramic for low esr, small size and relatively lower co st. the capacitor value needs to be large enough to minimize the ripple voltage, which is sensed at cs pins for the feedback current sink regulation. for up to 42 leds, we recommend a minimum of 4.7uf; for larger number of leds, use larger capacitor such as 6.8uf or 10uf. o utput d iode s election a high switching frequency demands a high-speed rectifier. a schottky diode is recommended with a rating of 1a/60v. 1a rating is more than adequate to meet the peak and average current required. 60v rating is to cover up to ovp limit when using 10leds per string. s witching m osfet s election the switching mosfet should be an n-channel mosfet with a low rds on rated at v gs = 4.5v, and a low gate charge. selection of a fet with the best combined low rds on and gate charge (q g ) will generally improve efficiency. the mosfet breakdown rating must be higher than the ovp setting. good candidates are si3458ds or am2358n. obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 9 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 pcb lay ou t gu i de pcb l ayout r ecommendations (s ee figure 2 schematic ) it is recommended that decouplin g capacitors c7 and c4 be placed as close as po ssible to the lx1996. th e inductor current sens e resistor r1 must tie directly to pgnd pin, and then tie to the power ground plane at that pin . for a good practical layout of switching boost circuit, keep loops as short as possible for the boo st switching components ( q1 , l1, d1, & c3). connect a separate signal ground plane to gnd pin, a separate power ground plane to pgnd pin, and then tie them together at gnd pin. it is important that the lx1996 have a good thermal path to ambient to dissipate heat. the simplest way to do this is to heat s ink the lx1996 bottom pad directly to the ground plane by placing 4 or more vias in the gro und pad directly under the lx1996 footprint. place the ldo filter capacitor (c5), r5 (r t resistor), c2 (c t capacitor), c1 (c lf capacitor), and r4 (r prg resistor) as close to the ic as practical. pcb layout guide C t op l ayer actual pcb size (h x w): 10mm x 40mm obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 20 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 pcb lay ou t gu i de pcb layout guide C b ottom l ayer pcb layout guide C t op s ilkscreen obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 21 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 pcb lay ou t gu i de gnd c3 4.7uf/50v r5 100k r6 22.1k r3 68.1k c7 .1uf/50v c2 2.2nf c1 0.1uf r4 4.99k thermistor r21m l1 slf6028t-4r7m1r6-pf u1 lx1996 vcc 24 en 1 +5v 2 dsel1 3 dsel2 4 di 5 ot 6 agnd 12 prg 11 rt 8 ct 9 clf 10 pwm 7 cs2 14 n/c 23 cs1 13 cs6 18 cs5 17 cs4 16 cs3 15 ovp 19 pgnd 20 src 21 drv 22 q1 si2308ds r1 0.075 j2 fh12f-10s-0.5sh 12 3 4 5 6 7 8 9 10 d1 1n5819 c5 4.7uf r7 n/u c4 4.7uf/50v j1 sm07b-srss-tb 12 3 4 5 6 7 f1 3.15 a 5v 5v 5v sot23 1206 1210 powermite unless otherwise specified: - pcb dimension = 10mm x 40mm - all resistors are in ohm, and 0603 size - all capacitors size are 0603 to leds assembly vin (6 - 28v) en di pwm gnd 6 x 6 mm 6.4 x 9.1mm 4.95 x 8mm 5v dsel1 100k r8 figure 2 C lx1996 example schematic obsolete downloaded from: http:///
lx1996 p roduction d atasheet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 22 copyright ? 2007 rev. 1.0, 2007-12-14 mult i -strin g led back-li g ht controller tm ? www. microsemi . com l l x x 1 1 9 9 9 9 6 6 pack age di m en si on s m illimeters i nches dim m in m ax m in m ax a 0.80 1.00 0.031 0.039 a1 0 0.05 0 0.002 a3 0.20 ref 0.008 ref b 0.18 0.30 0.007 0.011 d 4.00 bsc 0.157 bsc e 4.00 bsc 0.157 bsc e 0.50 bsc 0.019 bsc d2 2.30 2.55 0.090 0.100 e2 2.30 2.55 0.090 0.100 l 0.30 0.50 0.012 0.020 e a d e b e2 d2 l a3 a1 notes: 1. controlled dimensions are in mm, inches are for reference only. 2. dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm (.006) on any side. lead dimension shall not include solder coverage. production data C information c ontained in this document is proprietary to microsemi and is current as of publication date. this document may not be modified in any way without the express written consent of micro semi. product processing does not necessarily include testing of all parameters. microsemi reserves the right to c hange the configuration and performance of the product and to discontinue product at any time. obsolete downloaded from: http:///

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