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lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? description microsemi?s lx1994 is a compact, high efficiency, step-up boost controller which is designed to drive a string of white or colored led?s in a backlight or front light system. the lx1994 design is based on a dual mode pfm architecture and provides maximum typical e fficiency greater than 92%. the lx1994 has many unique design features and advantages over competitor solutions. the features included: low quiescent current (100a typical), low shut down current (<1a), dedicate ambient light sensor interface (lx1970), dual dimming modes, low voltage and low offset current sense, and integrated ovp protection. the converter achieves high efficiency, low cost, and flexible design by selection of an external n- channel mosfet, current sense resistors, and integrated ovp protection. the use of external n-channel mosfet allows design to optimize system efficiency. the ovp protection comparator eliminates the need of an external zener diode clamp. the ovp function can be scaled for any output voltage. maximum output current is achievable by selection of the current sense resistor. these features make the controller ideal for pda or digital camera applications to enhance system battery life, the lx1994 provides 2 dimming options and a dedicated ambient light sensor (lx1970) interface. the lx1994 supports a wide range of system battery voltage inputs which ranges from 2.0 to 5.5v. the lx1994 is guaranteed to start up at 2.0v input. the lx1994 is available in miniature 10-pin mlp or msop packages. important: for the most current data, consult microsemi ?s website: http://www.microsemi.com patent pending key features ? efficiency > 92% ? dual pfm architecture to extend battery life ? v in range 2.0v to 5.5v. start up warranty @ 2.0v ? logic control shutdown ? 100a typical quiescent current ? shutdown i q current <1a ? ovp for open string output voltage ? low voltage and offset current sense ? light sensor (lx1970) interface ? dual dimming options (pwm or dc voltage) ? no external zener clamp diode ? 10-pin mlp or msop applications ? pagers ? pda ? cell phone ? portable display ? digital cameras product highlight drv src ovp s/p vin brt v in = 2.0v to 5.5v gnd lx1994 fb 33h fdv303 ups5819 ls snk src vss vdd lx1970 3v cmp auto adjust for ambient light package order info ld plastic mlp 10-pin du plastic msop 10-pin t a ( c) rohs compliant / pb-free rohs compliant / pb-free -40 to 85 lx1994cld lx1994cdu note: available in tape & reel. append the letters ?tr? to the part number. (i.e. LX1994CDU-TR) l l x x 1 1 9 9 9 9 4 4
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 2 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? absolute maximum ratings supply input voltage (in) ...................................................................-0.3v to 7v all input pins ......................................................................................-0.3v to v in src input curr ent ......................................................................................600ma operating temperat ure range .........................................................-40 c to 85 c maximum operating junc tion temperat ure ................................................ 150 c storage temperat ure range...........................................................-65 c to 150 c lead temperature (solde ring 10 seconds) .................................................. 235 c peak package solder reflow temp (40 seconds max. exposur e) ................. 260c (+ 0, -5) note: exceeding these ratings could cause damage to the device. all voltages are with respect to ground. currents are positive into, negative out of specified terminal . thermal data du plastic msop 10-pin thermal resistance - junction to a mbient , ja 113 c/w ld plastic mlp 10-pin thermal resistance - junction to a mbient , ja 49 c/w junction temperature calculation: t j = t a + (p d x ja ). the ja numbers are guidelines for the thermal performan ce of the device/pc-board system. all of the above assume no ambient airflow. package pin out 1 2 56 10 vin s/p gnd drv 8 ovp 9 src 3 brt fb 7 cmp 4 ls du p ackage (top view) 1 2 3 4 56 7 8 9 10 vin s/p brt ls drv fb ovp src gnd cmp connect bottom to power gnd ld p ackage (top view) rohs / pb-free 100% matte tin lead finish functional pin description name description in unregulated ic supply voltage i nput ? input range from 2.0v to 5.5v. bypass with a 1 f or greater capacitor for operation below 2.0v. gnd common terminal for ground reference. brt led current adjustment - accepts a dc analog input. fb led current sense ? connect to current sense resistor. src mosfet current sense input - connects to the external n-channel mosfet source. drv mosfet gate driver ? connects to an external n-channel mosfet gate. ovp over voltage programming pin ? connects to a resistor divider between the output load and gnd to set the maximum output voltage. ls light sensor input ? allows light sensor current i nput to be modulated by the pwm control causing led brightness to be a product of the pw m duty cycle and ambient light level. cmp compensation pin ? apply a 0.1f capacitor for loop compensation. s/p shutdown/pwm pin ? a logic low longer than 100 s causes the ic to enter shutdown mode. applying a pwm signal to this pin and a filter capacitor to the brt pin allows amplitude independent pwm control. p p a a c c k k a a g g e e d d a a t t a a
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 3 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? electrical characteristics unless otherwise specified, the following specifications apply over the operating ambient temperature -40 c t a 85 c except where otherwise noted and the following test conditions: v in = 3.6v, i load = 20ma lx1994 parameter symbol test conditions min typ max units ` operating voltage v in 2.0 5.5 v minimum start-up voltage t a = +25 c 2.0 v start-up voltage temperature coefficient for reference only -2 mv/ c shdn = vin, no external fet 100 200 a quiescent current i q shdn = gnd 0.35 1 a brt full scale bias current ibrt s/p = vin, vbrt = gnd, ils = 0a 7.5 10.5 13.5 a brt light sensor current ibrt s/p = vin, vbrt = gnd, ils = 100 a 110 a s/p logic low voltage v s/p 0.6 v s/p logic high voltage v s/p 1.4 v s/p input dc bias current s/p = vin -1 0.05 1 a s/p pwm frequency 10 1000 khz s/p pulse width 50 ns brt pwm voltage vbrt vs/p = vin (dcs/p = 100%) 270 300 330 mv brt pwm voltage vbrt dcs/p = 50%, fpwm = 100khz 150 mv feedback comparator offset vos vfb ? vbrt, vbrt = 0mv 4 mv scr peak current ipk hyst mode; t a = +25 c 180 240 300 ma efficiency vout = 18v, i load = 20ma, v in = 5.0v 92 % drv sink/source current 140 200 ma maximum switch on-time t on 10 15 20 s minimum switch off-time t off 240 350 460 ns ovp threshold voltage v ovp 1.10 1.22 1.34 v ovp input bias current i ovp v ovp = 1v -50 50 na e e l l e e c c t t r r i i c c a a l l s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 4 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? simplified block diagram fb s/p 10ua switch logic burst i peak drive drv src gnd 0.2 csm 200mv hyst csm 16 bit shift register clear clock out j/k latch j k q csm i peak 100mv i peak vin ref ovp sleep mode control 1.2v x10 0.6v pwm & shutdown detection brt ls cmp 100k 30k 3mv figure 1 ? simplified block diagram e e l l e e c c t t r r i i c c a a l l s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 5 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? theory of operation basic pfm operation the lx1994 dual mode pfm modulator is implemented in two switching modes: the hysteretic and continuous switching mode (csm). in hysteretic switching mode , the basic pfm modulator logic/timing block uses a fixed peak current/ fixed off time where the switch turns on and allows the inductor current to ramp to a finite peak level then shuts off for a fixed duration of time. the basic modulation cycle repeats as long as the converter output voltage is less than the maximum regulation level. when the maximum regulation level is reached, the switch remains off until the output voltage capacitor discharges to a level less than the minimum regulation level. the input signals to the switch logic block are the burst on/off control signal and the peak current detection signals. for low and negligible switch conduction losses the designer may set the peak current comparator at 20mv corresponding to 200ma of output current. in continuous switching mode (csm), the level to the peak current comparator is variable. this current level is developed by integrating the output of the feedback comparator which functions as a high gain bandwidth limited error amplifier. this cu rrent is clamped to the peak switch current limit of 600ma. the integrated capacitor is attached at the cmp pin when the burst on/off control line is forced to the ?on? state. the conversion from hysteretic to csm mode is performed when the burst length exceeds more than 16 switching cycles counting by an internal 16 bits shift register. the internal register is clocked by the switch transitions during each burst period. when the switching cycles exceed 16 cycles, the converter automati cally switches over to csm mode. csm mode switching is latched by a j/k flip-flop. the conversion from csm mode to hysteretic mode is performed when the error amplifier output falls below 10mv (corresponding to 100ma peak current) as determined by a comparator. this resets the j/k flip-flop and converts back to hysteric mode. the lx1994 is a highly effici ent pfm boost converter; its design is based on dual mode pfm for driving a series of white or color leds. the advantage of pfm switching is to minimize system efficiency losses in both heavy and light load operations. the lx1994 does not require an external oscillator due to pfm dual modes switching. in light load operation, the converter minimizes switching losses by delivering more energy than necessary during switching burst period than the inactivity coast period. in heavy load condition, the converter uses the continuous switching current mode (csm) regulation scheme. this minimized p eak switching current and thereby minimizes the conduction losses. losses there are two types of losses in pfm regulator design: the switching loss, and conduction loss; that contribute to system inefficiency. switching loss : energy switching losses are associated with a nfet?s switch changing state (from on to off or vice versa) as a simultaneous high level of voltage and current are at the nfet?s switch during the transition. this switching loss is proportional to the switching frequency. conduction loss : the loss due to current flow in the series resistance of the switch, in ductor, and current sense resistor. conduction loss is proportional to the square of the switch current. output current selection the led output current is regul ated by adjusting of the fb pin voltage. if the fb pin voltage equals the brt pin voltage, the led current is the result of the fb pin voltage divided by the selected current sense resistor. for example: in a 100% duty cycle design, fb pin voltage is 300mv, the current sense resistor is 15 ? . the led current equals: 300mv = 20ma 15 ? a a p p p p l l i i c c a a t t i i o o n n s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 6 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? theory of operation (continued) dimming modes microsemi?s lx1994 provides two dimming options: pwm or dc voltage input. pwm dimming a pwm signal applied to s/p pin (see figure 4). this pwm signal is scaled to the reference such that a n% duty cycle pwm signal will produce an led current of () { } n% ? 10 a+ils ? rbrt rfb if a light sensor (such as microsemi?s lx1970) is used, the light sensor current is applied to the ls pin and adds to the 10a internal current source; in this case the internal current source determines the adjustment range in a pitch black ambient. the pwm signal will scale the light sensor signal allowing the dimming range to increase as the ambient light increases. dc dimming mode in ?dc dimming mode? (see figure 5) the brt pin input voltage can be applied directly to brt pin with the s/p pin pulled high or developed indirectly by applying a pwm signal to the s/p pin and using a scaling resistor and filter capacitor at the brt pin. th e internal current source produces a 10a reference curr ent that is scaled by the resistance applied to the brt pin. protection and ic shutdown ovp : the lx1994 provides ovp protections. if the voltage at the ovp pin exceeds the internal reference voltage (1.2v), the converter will suspend switching. the converter will attempt to regulate the ovp pin to its nominal 1.2v. ic shutdown : to force the ic into shutdown mode, the s/p pin must pull low for a duration longer 100s. in shutdown mode, the switch is off and the led string current typically reduces to a few nano amps of leakage current. a a p p p p l l i i c c a a t t i i o o n n s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 7 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? 6 led efficiency (3.7v input) 14 led efficiency (3.7v input) 50 60 70 80 90 100 0 5 10 15 20 25 led current efficiency 50 60 70 80 90 100 0 5 10 15 20 25 led current efficienc y hysteretic mode waveforms continuous mode waveforms blue = sense voltage green = inductor current blue = sense voltage green = inductor current c c h h a a r r t t s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 8 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? application circuits vdd vss snk src lx1970 auto drv src ovp s/p vin brt v in = 2.0 to 5.5v gnd lx1994 fb 33uh fdv303 ups5819 ls cmp 0.1uf r5 r6 r7 pwm c2 r4 r1 r2 r3 100% duty 80% duty 60% duty 40% duty 20% duty 0% duty auto mode response led current ambient light max 1m 30k 15 2.94k 28.7k 226k 23.2k 1uf c1 22uf figure 2 ?pwm dimming applied to s/p input and light sensor (dimming option 1) 100% duty 80% duty 60% duty 40% duty 20% duty 0% duty auto mode response led current ambient light max vdd vss snk src lx1970 auto drv src ovp s/p vin brt v in = 2.0 to 5.5v gnd lx1994 fb 33uh fdv303 ups5819 ls cmp 0.1uf r5 r6 r7 pwm c2 r4 c1 r1 r2 r3 1m 30k 15 2.94k 28.7k 226k 4.99k 100uf 1uf 1m vcc 110k bss123 + - figure 3 ?pwm dimming applied to s/p input and light sensor (dimming option 2) a a p p p p l l i i c c a a t t i i o o n n s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 9 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? application circuits drv src ovp s/p vin brt v in = 2.0v to 5.5v gnd lx1994 fb 33uh fdv303 ups5819 ls cmp pwm dimming 0.1uf 0.1uf 15 1m 51k figure 4 ? led driver with pwm dimming applied to s/p input off on dimming drv src ovp s/p vin brt v in = 2.0v to 5.5v gnd lx1994 fb 33uh fdv303 ups5819 ls cmp 0.1uf 15 1m 51k figure 5 ? led driver with dc dimming applied to brt input note: the component values shown are only examples for a working system. actual values will vary greatly depending on desired parameters, efficiency , and layout constraints. a a p p p p l l i i c c a a t t i i o o n n s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 10 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? application information ovp p rogramming resistors r6 and r7 of figure 2 program the over voltage clamp level. the value of r6 can be as high (like 1m .) to minimize the quiescent current. the value of r7 can be determined using the following equation where vovp is found in the electrical characteristics t able : ovp out ovp v r7=r6 v-v ?? ?? ?? d esign e xample : let r6 equal 1m and the required clamp voltage is 25v. 1.2 r7 = 1 r = 50.4 ? 25-1.2 m ?? ?? ?? i nductor and c apacitor s election the output filter inductor should be a 1f capacitor with sufficient voltage rating for the ovp setting. inductors in the range of 10h to 47h work best. for the best efficiency a larger value of inductor such as 47h is recommended; larger value in ductors will reduce ripple current which reduces peak currents and improves efficiency. smaller value inductors may be use less board space, so a design trade off is in order. t ransistor and d iode s election a schottky diode should be used with a 1 amp current rating and voltage rating equivalent to the ovp setting. the transistor should be a n-channel mosfet with a logic level gate voltage: good candidates are the fdv303n and the fdn337. for higher voltages, several bss138 can be wired in parallel. l ayout g uidelines the lx1994 requires a tight layout of the cmp pin capacitance. for best resu lts, the 0.1f cmp capacitor should be located directly adjacent to the lx1994 package with etch lengths as short as possible. 1 2 56 10 vin s/p gnd drv 8 ovp 9 src 3 brt fb 7 cmp 4 ls 0.1uf light s ensor i nterface the lx1994 has a ls input pi n to simplify the interface to an lx1970 light sensor. two different circuits are described which provide slightly different response curves. the equations for calculating the component values are also given. for the circuit of figure 2 , the describing equations are: rp30k r4= rp+30k or 1 g4=gp- 30k auto mode: ()() ()()() () ()()() src led cc i+10 a r1r2rp r1r2 + r1rp + r2rp dutycycle i= r5 vr1rp + r1r2 + r1rp + r2rp ? ? ? ? ? ? ? ? ? ? ? ? ? ? cc compliance brt(max) src(max) v-v -v i= r3 manual mode: () ()()() () ()()() led cc 10 a r1r2rp r1r2 + r1rp + r2rp dutycycle i= r5 vr2rp + r1r2 + r1rp + r2rp ? ? ? ? ? ? ? ? ? ? ? ? ? ? example: select r5 = 15 ohms; iled = 20ma max; isrc clamp at 100a; vcc = 3.3; iled in full darkness and 100% duty cycle = 4ma. with r5 = 15 ohms; iled = 20ma max, vbrt(max) = 300mv. with isrc clamp at 100a, vcompliance (lx1970) = 0.68v, vcc = 3.3v, so ( ) 3.3-0.68-0.3 r3= =23.2k 100 a a a p p p p l l i i c c a a t t i i o o n n s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 11 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? application information r3 = 23.2k the level at 100% duty cycle in full darkness is 4ma, which is 20% of the maximum level of 20ma; this implies 80% is attributable to i scr . combining this information with the describing equation for auto mode gives: () ()()() src led(max) i r1r2rp 80%i r5= r1r2 + r1rp + r2rp this implies: () ()()() r1r2rp 0.8.0215 ==2.4k r1r2 + r1rp + r2rp 100 a since the left side is the three resistors in parallel, this can be restated as: -6 111 41610 = + + =g1+g2+gp r1 r2 rp the manual mode equation can be reduced to this assuming 100% duty and 20ma led current (that is 0.3v sense resistor voltage): cc r2rp 0.3r1 0.3r1 == r2+rp (10 ar1)+v -0.3 (10 ar1)+3.0 this can be restated as: -6 11 10 + =3310 + r2 rp r1 or () -6 g2+gp=3310 + 10g1 the auto mode equation can be reduced to this assuming 100% duty , 100a isrc current and 20ma led current (that is 0.3v sense resistor voltage): () () () src cc cc r1rp 0.3r2 = r1+rp (i +10 a)r2 +v -0.3 0.3r2 0.3r2 == (100 +10 a)r2 +v -0.3 110 ar2 +3.0 this can be restated as: -6 11 10 + =36710 + r1 rp r2 or ( ) -6 g1+gp=36710 + 10g2 the equations above can be solved for g1, g2 and gp: -6 g1=34.810 -6 g2=4.4510 -6 gp=37610 knowing gp we can find -6 1 g4=gp- =34310 30k the resistance values are the reciprocal of the conductance?s so: r1 = 28.7k r2 = 225k r4 = 2.91k the value of c1 is selected to give a time constant of ? second and works into r3 (which is 23.2k). 0.5 c1= 23.2k c1 = 21.5f the value of c2 works into rp and the pole should be set at 1/100 of the pwm frequency. 1 c2= 10khz 6.28 2.66k 100 for a 10khz pwm, c2 = 599nf, and a value of 1f works well. circuit of figure 3: the second light sensor interface is very similar to the first; the choice is a matter of user preference. in the second circuit, an active 325mv clamp is used to clamp the maximum led current in auto mode. in this circuit, resistor r3 is reduced to extend the operating ambient light range of the light sensor and filter capacitor c1 must ther efore be increased. a a p p p p l l i i c c a a t t i i o o n n s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 12 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? package dimensions du 10-pin miniature shrink outline package (msop) a1 l e l1 e1 b e s a d c m illimeters i nches dim min max min max a ? 1.10 ? 0.043 a1 0.05 0.15 0.002 0.006 b 0.15 0.30 0.006 0.012 c 0.13 0.23 0.005 0.009 d 2.90 3.10 0.114 0.122 e 0.50 bsc 0.020 bsc e 4.75 5.05 0.187 0.198 e1 2.90 3.10 0.114 0.122 l 0.41 0.70 0.016 0.028 l1 0.95 bsc 0.037 bsc s 0.50 bsc 0.020 0 6 0 6 ld 10-pin plastic micro lead frame package (mlp) d e a a3 d2 e2 b l e a1 top view bottom view pin1 id 12 3 45 6 7 8 9 10 m illimeters i nches dim min max min max a 0.80 1.00 0.0315 0.0394 a1 0 0.05 0 0.0019 a3 0.20 ref 0.0079 ref b 0.18 0.30 0.0071 0.0118 d 3.00 bsc 0.1181 bsc d2 2.23 2.48 0.0878 0.0976 e 0.50 bsc 0.0197 bsc e 3.00 bsc 0.1181 bsc e2 1.49 1.74 0.0587 0.0685 l 0.30 0.50 0.0071 0.0197 note: dimensions do not include mold flash or protrusi ons; these shall not exceed 0.155mm(.006?) on any side. m m e e c c h h a a n n i i c c a a l l s s
lx1994 p roduction d ata s heet microsemi integrated products division 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 13 www. microsemi . com high efficiency led driver copyright ? 2003 rev. 1.0d, 2006-02-09 tm ? notes production data ? information contained 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 microsemi. product processing does not necessarily include testing of all parameters. microsemi reserves th e right to change the configuration and performance of the product and to discontinue product at any time. n n o o t t e e s s

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