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power management 1 high e ciency boost converter for up to 10 leds typical application circuit july 11, 2007 out lx bl c in 2.8 to 5.5v in en iset gnd comp enable and pwm dimming r iset c comp l1 d c out r comp SC4538 applications white led display backlight driver smartphones and pdas cellular handsets mp3 and portable media players dsc, gps, satellite radio, game players ? ? ? ? ? SC4538 features input voltage range 2.8v to 5.5v current-mode pwm control 800khz programmable led current up to 30ma soft-start to reduce in-rush current pwm dimming 100hz to 50khz analog and ltered-pwm dimming at iset over-voltage protection 38v (min.) under-voltage lockout (uvlo) thermal shutdown shutdown current <1a ultra-thin package 2mm x 2mm x 0.6mm lead-free package, weee and rohs compliant ? ? ? ? ? ? ? ? ? ? ? ? description the SC4538 is designed to regulate current for a series chain of white leds in lcd backlighting applications where small size and high e ciency are priorities. this device integrates an 800khz current-mode pwm boost converter and a 30ma programmable low dropout current sink regulator. an external resistor sets the led current up to 30ma. pwm dimming (100hz to 50khz) may be applied directly to the enable (en) pin, or dimming can be controlled by applying an analog signal to the iset circuit. the SC4538 drives a string of up to 10 white leds with a forward voltage of 3.8v or less. the current regulator protects against shorts between the bl and out pins and also eliminates backlight glow during shutdown when using leds with high leakage. under-voltage lockout and thermal shutdown provide additional protection. a small external capacitor and series resistor control soft-start and loop compensation. over-voltage detection protects the SC4538 if the led pin is oating or shorted to ground. the SC4538 is available in an mlpd-8 pin 2mmx 2xmm 0.6mm package, with a rated temperature range of -40c to +85c.
2 pin con guration marking information ordering information note: (1) available in tape and reel only. a reel contains 3,000 devices. (2) lead-free package only. device is weee and rohs compliant. t 1 2 4 5 6 3 7 8 gnd in bl out lx en iset comp mlpd: 2mm x 2mm 8 lead ja = 74 c/w fe0 yw fe0 = marking code for SC4538 yw = date code SC4538 device package SC4538ultrt (1) (2) mlpd-ut-8 2x2 SC4538evb evaluation board
3 exceeding the above speci cations may result in permanent damage to the devic e or device malfunction. operation outside of the parameters speci ed in the electrical characteristics section is not recommended. notes: (1) tested according to jedec standard jesd22-a114-b. (2) calculated from package in still air, mounted to 3 x 4.5, 4 layer fr4 pcb with thermal vias under the exposed pad per je sd51 standards. unless otherwise noted: v in = 3.6v, c in = 2.2f, c out = 1f, c comp = 47nf, r comp = 1.27k, r iset = 5.76k, t a = -40 to 85oc, typical values are at t a = 25oc. electrical characteristics absolute maximum ratings supply voltage (v) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +20.0 lx voltage, output voltage (v) . . . . . . . . . . . . . -0.3 to +45 current sink voltage (v) . . . . . . . . . . . . . . . . . . . . -0.3 to +45 enable voltage (v) . . . . . . . . . . . . . . . . . . . . . . -0.3 to (v in + 0.3) compensation voltage (v) . . . . . . . . . . . . . . . . . . -0.3 to +2.0 current set voltage (v) . . . . . . . . . . . . . . . . . . . . . . -0.3 to +2.0 esd protection level (1) (kv) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 recommended operating conditions ambient temperature range (c) . . . . . . . . . . . . . -40 to +85 supply voltage (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 to 5.5 output voltage (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 to 40 thermal information thermal resistance, junction to ambient (2) (c/w) . . . . . 74 maximum junction temperature (c) . . . . . . . . . . . . . . +150 storage temperature range (c) . . . . . . . . . . . . -65 to +150 peak ir reflow temperature (10s to 30s) (c) . . . . . . . +260 SC4538 parameter symbol conditions min typ max units supply input voltage v in 2.8 5.5 v uvlo threshold v uvlo v in rising 2.40 2.60 2.79 v uvlo hysteresis v uvlo-hys 120 mv quiescent supply current i q not switching 1.8 ma shutdown supply current i shdn en tied to gnd 0.1 1 a en logic high voltage v in 1.80 v en logic low voltage v il 0.8 v en logic input current i il, i ih v in = 5.5v, v en = 0v or 5.5v 0.01 1 a thermal shutdown temperature t sd t j rising 155 c thermal shutdown hysteresis 20 c boost converter characteristics switching frequency f sw 680 800 920 khz maximum duty cycle d max v in = 3.2v, t a = 25c 92 % minimum on-time t on(min) 100 ns
4 electrical characteristics (continued) SC4538 parameter symbol conditions min typ max units boost converter characteristics (continued) switch over-current protection i ocp 550 850 ma switch leakage current i l(lx) v lx = 5.5v 0.01 1 a switch saturation voltage v sat i lx = 0.3a 250 450 mv comp sourcing current i comp v comp = 0.9v, t a = 25oc 6 a comp sinking current v comp = 0.9v, t a = 25oc 7 out over-voltage protection v ovp 38 43 v out internal pull-down current i ovp during ovp condition 1 ma out bias current i out v en = v in , v out = 20v 40 60 a out leakage current v en = 0v, v out = v in = 5.5v 0.01 1 a pwm dimming frequency range (1) f en applied to en pin 100 50k hz pwm dimming duty cycle range (1) d en 200hz on en pin 0 100 % current sink characteristics bl current setting range i bl 130ma bl current setting accuracy t a = 25c -3.5 3.5 % bl leakage current i l(bl) v en = 0v, v bl = 2v 0.01 0.1 a bl current line regulation i bl /v in v in = 3.0 to 5.5v 0.05 ma/v bl voltage v bl 0.35 v iset bias voltage v iset 0.5 v iset-to-i bl gain a iset 230 a/a start-up time t start-up 1.3 s notes: (1) guaranteed by design.
5 typical characteristics all data taken with v out = 20v, r iset = 5.76k, v in = 3.6v, and e ciency () = p led /p in unless otherwise noted. e ciency vs. i bl 0 20 40 60 80 100 0.0 6.0 12.0 18.0 24.0 30.0 5.5v in 3.6v in 2.8v in e ciency vs. input voltage normalized i bl vs. output voltage -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 0.0 8.0 16.0 24.0 32.0 40.0 e ciency vs. input voltage 70 74 78 82 86 90 2.8 3.3 3.9 4.4 5.0 5.5 10ma 20ma 30ma e ciency vs. output voltage e ciency (%) i bl (ma) e ciency (%) v in (v) normalized i bl (%) v out (v) 70 74 78 82 86 90 0.0 8.0 16.0 24.0 32.0 40.0 3.6v in 5.5v in 2.8v in e ciency (%) v out (v) 70 74 78 82 86 90 2.8 3.3 3.9 4.4 5.0 5.5 v out efficiency led efficiency e ciency (%) v in (v) 70 74 78 82 86 90 2.8 3.3 3.9 4.4 5.0 5.5 10 led 6 led 4 led e ciency (%) v in (v) e ciency vs. input voltage SC4538
6 i bl error vs. pwm frequency at 50% duty cycle i bl vs. duty cycle at 200 hz typical characteristics (continued) pwm operation at 50khz and 15% duty cycle 4s/div pwm operation at 200hz and 1% duty cycle pwm operation at 32khz and 50% duty cycle 10s/div pwm operation at 32khz and 10% duty cycle 0 20 40 60 80 100 0.0 20.0 40.0 60.0 80.0 100.0 percentage of maximum i bl (%) pwm duty cycle (%) -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 100.0 1000.0 10000.0 100000.0 i bl error (%) pwm frequency (hz) 2ms/div v lx (20v/div) i out (20ma/div) v out (20v/div) v en (2v/div) v lx (20v/div) i out (20ma/div) v out (20v/div) v en (2v/div) v lx (20v/div) i out (20ma/div) v out (10v/div) v en (2v/div) v lx (20v/div) i out (20ma/div) v out (20v/div) v en (2v/div) 10s/div SC4538
7 typical characteristics (continued) waveforms during over-voltage protection 400s/div typical waveforms at v in = 2.8v 1s/div typical waveforms at v in = 3.6v 1s/div typical waveforms at v in = 5.5v 1s/div start-up response 100s/div v lx (50v/div) i out (20ma/div) v out (20v/div) v en (2v/div) v lx (20v/div) v in (50mv/div) v out (50v/div) v lx (20v/div) v in (50mv/div) v out (50v/div) v lx (20v/div) v in (50mv/div) v out (50mv/div) v lx (20v/div) v out (10v/div) i out (20ma/div) v en (2v/div) SC4538
8 pin descriptions pin # pin name pin function 1lx collector of the internal power transistor connect to the boost inductor and rectifying schottky diode. 2 en enable and brightness control pin for led string 3 iset output current set pin connect a resistor from this pin to gnd to set the maximum current. 4 comp output of the internal transconductance error ampli er this pin is used for loop compensation and soft-start. connect a 1.27 ohm resistor and 47nf capacitor in series to gnd. 5 out boost output voltage monitor pin internal over-voltage protection monitors the voltage at this pin. connect this pin to the output capacitor and the anode of the led string. 6 bl led constant current sink connect this pin to the cathode of the led 7 gnd ground 8 in power supply pin bypass this pin with a capacitor close to the pin t thermal pad pad for heatsinking purposes connect to the ground plane using multiple vias. not connected internally. SC4538
9 block diagram ovp r rsense q osc s error signal selection and summation adj reference and internal regulator uvlo thermal shutdown sense amp comparator out lx gnd comp iset bl en in en adj led setpoint 5 8 2 3 6 1 7 4 SC4538 figure 1
10 SC4538 applications information general description the SC4538 contains an 800khz xed-frequency current- mode boost converter and an independent led current regulator. the led current set point is chosen using an external resistor, while the pwm controller operates inde- pendently to keep the current in regulation. the SC4538 receives information from the internal led current regula- tor and drives the output to the proper voltage with no user intervention. the current owing through the led string is indepen- dently controlled by an internal current regulator, unlike the ballasting resistor scheme that many led current reg- ulators use. the internal current regulator can be shut o entirely without leaking current from a charged output capacitor or causing false-lighting with low led count and high v in . the backlight current (i bl ) is programmed using an external resistor. the path from the en pin to the output control is a high bandwidth control loop. this feature allows the pwm dimming frequency to range between 100hz and 50khz. in shutdown mode, leakage through the current regulator output is less than 1a, keeping the output capacitor charged and ready for instant activation of the led strand. the 800khz switching speed provides high output power while allowing the use of a 1.0mm-high inductor, maxi- mizing e ciency for space constrained and cost-sensitive applications. in addition, the converter and output capaci- tor are protected from open-led conditions by over-voltage protection. pwm dimming the enable pin can be toggled to allow pwm dimming. in a typical application, a microcontroller sets a register or counter that varies the pulse width on a gpio pin. the SC4538 provides dimming between 100hz and 50khz, allowing for compatibility with a wide range of devices, including dimming strategies that avoid the audio band by using high frequency pwm dimming. a wide range of illumination can be generated while keeping the instanta- neous led current at its peak value for luminescent e ciency and color purity. the SC4538 can acomodate any pwm duty cycle between 0 and 100%. a low duty cycle pwm signal used for a few milliseconds provides the additional advantage of reduced in-rush at start up. the start-up delay time between the enable signal going high and the internal current regulator activating causes a small o set dependent upon the pwm frequency seen by the en pin. as the pwm signal frequency goes higher, the e ect of the delay becomes noticeable. zero duty cycle mode zero duty cycle mode is activated when the voltage on the bl pin exceeds 1.3v. in this mode, the comp pin voltage is pulled low, suspending all switching. this allows the v out and v bl voltages to fall. the comp voltage is held low until the v bl falls below 1v, allowing comp to return to its normal operating voltage and switching to resume. protection features the SC4538 provides several protection features to safe- guard the device from catastrophic failures. these features include: over-voltage protection (ovp) soft-start thermal shutdown current limit over-voltage protection (ovp) the SC4538 includes a built-in over-voltage protection circuit to prevent damage to the ic and output capacitor in the event of an open-circuit condition. the output voltage of the boost converter is detected at the out pin and divided internally. if the voltage at the out pin exceeds the ovp limit, the boost converter is shut down, and a strong pull down is applied to the out pin to quickly discharge the output capacitor. this additional level of protection prevents a condition where the output capaci- tor and schottky diode must endure high voltage for an extended period of time. soft-start the soft-start mode reduces in-rush current by utilizing the external compensation network. as the error ampli er ? ? ? ?
11 applications information (continued) SC4538 d is the duty cycle for continuous operation. e ciency () can be approximated by using the curves provided in the typical characteristics section. table 1 has inductors that have been proven to work with SC4538. table 1 -- recommended inductors part number value (h) dcr () rated current (a) toler- ance dimensions (l x w x h) (mm) coilcraft lps4018-223ml 22 0.360 0.70 20% 3.9 x 3.9 x 1.7 murata lqh43cn150k03 15 0.320 0.570 10% 4.5 x 3.2 x 2.6 murata lqh32cn150k03 15 0.580 0.300 20% 3.9 x 2.5 x 1.55 capacitor selection the input capacitor should be at least 2.2f. a larger capacitor will reduce the voltage ripple on the input. the output capacitor values can range from 0.22f to 1f. the compensation capacitor value should be 47nf. capacitors of x5r type material or better can be used for many of the capacitors mentioned above. see table 2 for capacitors that can be used. table 2 -- recommended capacitors part number value (f) rated voltage (v) type case size input capacitor murata grm188c70j22ke20 2.2 6.3 x7s 0603 output capacitor murata grm21br71h105ka12l 1.0 50 x7r 0805 compensation capacitor taiyo yuden emk105bj473kv-f 0.047 16 x7r 0402 diode selection a schottky diode with a forward voltage of 40v and a forward current rating of 1a should be used with this device for optimum performance. the central semiconductor corporation cmosh-4e is a suitable diode for this application. slowly charges the comp node voltage, the duty cycle of the boost switch ramps from 0% to its nal value once in regulation. the gradual increase of the duty cycle slowly charges the output capacitor and limits in-rush current during start up. soft-start is implemented only when the power is cycled on the part. thermal shutdown a thermal shutdown system is included for protection in the event the junction temperature exceeds 155c. in thermal shutdown, the on-chip power switch is disabled. switching and sinking resumes when the temperature drops by 20c. current limit the power switch of the boost is protected by an internal current limit function that opens the switch when the current exceeds the maximum current switch value up to 850ma. inductor selection the inductor value should be within the range of 4.7h to 22h. the dcr needs to be considered when picking the inductor to ensure decent e ciency. the largest inductor package that can be tolerated in the circuit area should be used since the dcr generally decreases with package size. the saturation current of the inductor should be much higher than the peak current of the internal boost switch to ensure that the inductor never enters saturation during normal operation of the part. the equation to calculate the peak inductor current is: 2 ? i i i l in l(peak) where osc in l f l d v ? i u u out in v v 1 d in out out in v i v i u u
12 applications information (continued) pcb layout considerations poor layout can degrade the performance of the dc-dc converter and can be a contributory factor in emi prob- lems, ground bounce, thermal issues, and resistive voltage losses. poor regulation and instability can result. a typical layout is shown in figure 3. the following design rules are recommended: place the inductor and lter capacitors as close to the device as possible and use short, wide traces between the power components. route the output voltage feedback path away from the inductor and lx node to minimize noise and magnetic interference. use a ground plane to further reduce noise interference on sensitive circuit nodes. figure 3C layout ? ? ? SC4538 selection of other components r iset sets the maximum load current for the SC4538. use the following equation to select the proper value: r iset = 230v iset /i load where v iset = 0.5v (typ). refer to figure 2 for selecting values for other current settings. 0 13 27 40 53 67 80 0.0 5.0 10.0 15.0 20.0 25.0 30.0 r iset vs. i bl figure 2 C set resistor value selection graph r iset [k] i bl [ma]
13 outline drawing C mlpd-ut-8 2x2 inches .020 bsc b .007 bbb aaa n l e d .012 dim a1 a2 a min .000 .018 0.30 0.18 .012 0.25 .010 0.40 0.30 .004 .003 8 .014 .079 .016 0.08 0.10 8 0.35 2.00 0.50 bsc millimeters max 0.05 0.60 dimensions min 0.00 nom (.006) - - max .002 .024 nom 0.45 - (0.1524) - controlling dimensions are in millimeters (angles in degrees). notes: 1. a2 d e/2 e bxn bbb c a b coplanarity applies to the exposed pad as well as the terminals. 2. d1 .061 .067 .071 1.55 1.70 1.80 e1 .026 .031 .035 0.65 0.80 0.90 d/2 e/2 e lxn d1 e1 2.00 .079 pin 1 indicator (laser mark) a1 seating plane c b a aaa c 1 n e 2.10 2.10 1.90 1.90 .083 .083 .075 .075 a 2 SC4538
semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805) 498-2111 fax: (805) 498-3804 www.semtech.com contact information 14 land pattern C mlpd-ut-8 2x2 k inches dimensions p z x y c g dim millimeters h .067 1.70 k .031 0.80 r .006 0.15 y r g z p x (c) 1. controlling dimensions are in millimeters (angles in degrees). h .030 .106 (.077) .047 0.75 2.70 (1.95) 1.20 0.30 0.50 .020 .012 this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 2. failure to do so may compromise the thermal and/or functional performance of the device. shall be connected to a system ground plane. thermal vias in the land pattern of the exposed pad 3. SC4538

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