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power management jan. 12, 2010 1 high e ciency integrated driver for 6-strings of 30ma leds sc440a www.semtech.com typical application circuit features wide input range 4.5v to 27v 42v maximum output voltage with adjustable ovp for smaller output capacitor drives up to 72 wleds in 6 strings programmable wled current for up to 30ma per string 1% string-to-string current matching 2a integrated power switch up to 91% e ciency 0.2% to 100% pwm dimming analog dimming 800khz switching frequency for small external com- ponent open/short led protection short led protection disable <1ua shut-down current thermal protection 4mm x 4mm x1mm mlpq-24 package (pb-free, halo- gen free, weee & rohs compliant) applications medium-sized lcd panel notebook display white led power supplies sub-notebook and tablet computer displays portable media players lcd monitors digital video cameras ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description the sc440a is a high-e ciency multiple string wled driver with an integrated boost converter. it operates over a wide input range from 4.5v to 27v with a maximum output voltage of 42v and a 2a internal power switch. it can drive up to 42v output voltage in 6 strings with current up to 30ma per string. the string-to-string current matching is 1% typical, 3% maximum and the overall e ciency is greater than 90% due to the low current sense voltage and a low-impedance internal power switch. the wide pwm dimming range boasts a ratio of 500: 1. the 800khz switching frequency enables the user to optimize the external component sizes for high e ciency. when there are fewer leds in each string, users can use a lower output voltage protection level which yields an allowable reduction in associated costs, size and voltage ratings of the output capacitor. the sc440a also features a comprehensive open and short circuit led protection functions. it disables the corresponding strings with led open or led short conditions while keeping other strings under normal operation. this feature allows lcd panels to remain viewable even under led failure, wire disconnect, or short- circuit conditions. the short led protection function can be disabled. the internal thermal shutdown protects the ic from overheating at abnormal conditions. the sc440a is available in a common 4mm x 4mm x1mm mlpq-24 package. 6 pgnd vin sw pwm en ovprtn sw fflag pgnd iognd sc440a 6 strings 1 12 vin (4.5v -27v) 2 scp_en agnd ioset io1-6 vout ovpin comp ss edp 6 pgnd vin sw pwm en ovprtn sw fflag pgnd iognd sc440a 6 strings 1 12 vin (4.5v -27v) 2 scp_en agnd ioset io1-6 vout ovpin comp ss edp
sc440a 2 www.semtech.com ? 2010 semtech corp. pin con guration marking information ordering information device package SC440AMLTRT (1)(2) 4mm x 4mm x1mm mlpq - 24 sc440aevb evaluation board notes: (1) available in tape and reel only. a reel contains 3,000 devices. (2) available in pb-free and halogen free package only. device is weee and rohs compliant. top view 1 2 3 4 t 5 7891011 17 16 15 14 13 19 20 21 22 23 io1 en agnd fflag ss 4mm x 4mm x1mm mlpq - 24 ovprtn vin comp ioset scp_en vout pwm ovpin sw sw n/c pgnd pgnd 18 12 6 24 io6 iognd io5 io4 io3 io2 ja = 29c/w nnnnn = part number (example: 440a) yyww = date c ode (example: 0752) xxxxx = semtech lot no. (example: e9010 xxxxx = 01-10) top view 440a yyww xxxxx xxxxx
sc440a 3 www.semtech.com ? 2010 semtech corp. exceeding the above speci cations may result in permanent damage to the device or device malfunction. operation outside of the parameters speci ed in the electrical characteristics section is not recommended. notes- (1) calculated from package in still air, mounted to 3 x 4.5, 4 layer fr4 pcb with thermal vias under the exposed pad per jes d51 standards. (2) tested according to jedec standard jesd22-a114-b. electrical characteristics unless otherwise noted, v in = 12v, -40c < t a = t j < 85c, r ioset = 2k. absolute maximum ratings v in pin: supply voltage -0.3 to 30v maximum output power 7.5w ioset voltage -0.3 to 2v sw, vout, io1~io6, ovpin, ovprtn voltage -0.3 to 45v ss, comp voltage -0.3 to 3v scp_en voltage -0.3 to 3.5v en, pwm, fflag voltage -0.3 to v in +0.3v pgnd to agnd and i ognd 0.3v peak ir re ow temperature . 260c esd protection level (2) 2000v recommended operating conditions supply input voltage 4.5v to 27v maximum output voltage 42v maximum led current 30ma thermal information junction to ambient (1) 29c/w maximum junction temperature 150c storage temperature -65 to +150c parameter symbol conditions min typ max units input supply under-voltage lockout threshold u vlo-th v in rising 4.2 4.5 v uvlo hysteresis u vlo-h 250 mv v in quiescent supply current i in-q no switching 4.5 ma v in supply current in shutdown i in-s en / pwm = low 1 a oscillator switching frequency (1) f s 0.64 0.8 0.96 mhz maximum duty cycle (1) d max 90 % minimum duty cycle (1) d min 0% minimum on-time t on-min 200 ns minimum o -time t on-min 100 ns internal power switcher switch current limit i sw 2.0 2.5 a switch saturation voltage v sat i sw = 1a 200 500 mv switch leakage current i s-leak v sw = 12v 0.01 1 a compensation sourcing current i o-h v comp = 0.5v 7.3 a sinking current i o-l v comp = 2v 5.5 a
sc440a 4 www.semtech.com ? 2010 semtech corp. parameter symbol conditions min typ max units control signals en, pwm high voltage v en_h v in = 4.5v to 27v 2 v en, pwm low voltage v en_l v in = 4.5v to 27v 0.4 v en, pwm leakage current i en_leak v en = v pwm = 0v to 5.0v 0.01 1 a pwm dimming frequency (1) f pwm 50 50k hz pwm dimming minimum duty cycle (1) d min_pwm f pwm = 200hz 0.4 % pwm dimming minimum pulse width t pwm_min f pwm = 200hz 10 s fflag voltage v fflag i fflag = 2 ma, normal operation 0.25 v ss source current i ss_h vss = 0v 6 a ss sink current i ss_l vss = 2v at ovp or otp 1.6 ss switching threshold v ss_th t j = 25c 0.7 0.9 1.05 v ss end voltage v ss_end 2.6 v over-voltage protection ovpin threshold voltage v ovpin_th 1.475 1.535 1.595 v ovpin leakage current i ovpin_l ovpin = vin = 20v 0.1 1 a ovprtn saturation voltage v ovprtn i ovprtn =100a 60 mv ovprtn leakage current i ovprtn_l ovprtn = vin 1 a vout internal pull-down in over-voltage fault i ovp 0.9 ma vout leakage current i vout_l v out = 40v 0.1 a current source (io1 ~ io6) backlight current accuracy io1~io6 en, pwm = 1; t j = 25c 27 30 32 ma overshoot protection threshold io1~io6 0.8 v overshoot protection hysteresis any of io1~io6 100 mv led current matching (1) pwm =10% to 100%; t j = 25c pwm = 200hz / 500hz +/-1 +/-3 % maximum led current i o_max 35 ma led short circuit protection threshold v io_scp t j = 25c 2.2 2.35 2.55 v i o o leakage current i o_leak en = 0v, v io1 ~ v io6 = 12v 0.1 1 a scp_en bias i scp_en scp_en = 0v -40 a over-temperature protection thermal shutdown temperature t otp 150 c thermal shutdown hysteresis t otp_h 30 c notes: (1) guaranteed by design. electrical characteristics (continued) unless otherwise noted, v in = 12v, -40c < t a = t j < 85c, r ioset = 2k.
sc440a 5 www.semtech.com ? 2010 semtech corp. typical characteristics uvlo threshold vs temperature rising falling 4 4.1 4.2 4.3 4.4 -40-200 20406080 temperature (oc) vin uvlo (v) switching frequency vs temperature vin=4.5v vin = 21v 765 775 785 795 805 -40-20 0 20406080 temperature (oc) switching frequency (khz) io pin se nse voltage v s led curre nt vin=12v, 8 leds per string ta = 25c 100 200 300 400 500 600 700 5 1015202530 led current (ma) io pin sense voltage (mv) ovpin threshold voltage vs temperature vin = 12v 1.5 1.51 1.52 1.53 1.54 1.55 -40-200 20406080 temperature (oc) ovpin threshold voltage (v) ss current vs temperature vin = 12v source sink 1 2 3 4 5 6 -40-20 0 20406080 temperature (oc) ss current (ua) comp current vs temperature vin=12v source sink 4.5 5 5.5 6 6.5 7 7.5 8 8.5 -40-20 0 20406080 temperature (oc) comp current (ua)
sc440a 6 www.semtech.com ? 2010 semtech corp. typical characteristics (continued) vout pull down current vs temperature vin=4.5v vin=21v 0.82 0.84 0.86 0.88 0.9 0.92 -40-20020406080 temperature (oc) vout pull down current (ua led source scp threshold vs temperature 4.5v 21v 2.42 2.425 2.43 2.435 -40-20 0 20406080 temperature (oc) led current source scp threshold (v) led current setting vs temperature r ioset = 2kohm 29.5 30 30.5 31 -40-20020406080 temperature (oc) led current setting (ma) efficiency vs input voltage vout=26.5v, d1=ss14 l1=10u, ihlp-2525cz1 / vishay, co=10uf 0.83 0.84 0.85 0.86 0.87 0.88 0.89 0.90 0.91 5 7 9 111315171921 input voltage (v) efficiency pwm dimming duty cycle vs led current 5ma x 6 string load, 8 leds per string, l1=mos6020-10u, c2=10uf fpwm = 200 hz fpwm = 50khz 0 5 10 15 20 25 30 35 0 20406080100 duty cycle led current (ma) pwm dimming duty cycle vs led current 30ma x 6 string load, 8 leds per string, l1=mos6020-10u, c2=10uf fpwm = 200 hz fpwm = 50khz 0 50 100 150 200 0 20406080100 duty cycle led current (ma)
sc440a 7 www.semtech.com ? 2010 semtech corp. typical characteristics (continued) start up vin 5v/div ss 2v/div vout 20v/div io6 1v/div time ( 10ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string start up vin 5v/div ss 2v/div vout 20v/div io6 1v/div time ( 10ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string shut down vin 5v/div ss 2v/div en 1v/div time ( 200ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string shut down vin 5v/div ss 2v/div en 1v/div time ( 200ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string main power switching waveform sw 10v/div vout(ac) 50mv/div time ( 0.5us/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string main power switching waveform sw 10v/div vout(ac) 50mv/div time ( 0.5us/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string main power switching waveform sw 10v/div vout(ac) 50mv/div time ( 0.5us/div) conditions: vin = 12v, load = 30ma x 6 strings, 8 leds per string main power switching waveform sw 10v/div vout(ac) 50mv/div time ( 0.5us/div) conditions: vin = 12v, load = 30ma x 6 strings, 8 leds per string
sc440a 8 www.semtech.com ? 2010 semtech corp. typical characteristics (continued) top and otp recovery ss 2v/div flag 2v/div vout 10v/div time ( 100ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string top and otp recovery ss 2v/div flag 2v/div vout 10v/div time ( 100ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string ovp ovpin 1v/div ss 2v/div time ( 1s/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string flag 5v/div ovp ovpin 1v/div ss 2v/div time ( 1s/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string flag 5v/div vin adaptor plug-in case vin 10v/div vout 1v/div time ( 50us/div) conditions: vin = 6v~19v, load = 30ma x 6 strings, 8 leds per string io6 1v/div sw 20v/div vin adaptor plug-in case vin 10v/div vout 1v/div time ( 50us/div) conditions: vin = 6v~19v, load = 30ma x 6 strings, 8 leds per string io6 1v/div sw 20v/div pwm dimming time ( 5ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string keep original here pwm 2v/div sw 20v/div io6 5v/div pwm dimming time ( 5ms/div) conditions: vin = 5v, load = 30ma x 6 strings, 8 leds per string keep original here pwm 2v/div sw 20v/div io6 5v/div
sc440a 9 www.semtech.com ? 2010 semtech corp. pin descriptions pin # pin name pin function 1 io1 provides constant sink current to led string 1. 2 agnd analog ground 3 fflag power failure signal output with open collector. 4 en enable the device including requlator and led drivers. 5 ss soft-start pin 6 ovprtn over-voltage return 7 ovpin over-voltage input 8, 12 pgnd power ground 9, 10 sw collector of the internal power transistor C connect to the boost inductor and the rectifying diode. 11 n/c left open. 13 vin input power supply. 14 ioset current source io value set pin C by selecting the resistor connected from this pin to gnd, the corresponding maximum current on all 6 strings are set. 15 scp_en led short-circuit protection pin C leaving this pin open enables the function, tying it to ground disables it. if any io pin connects to vin, scp_en needs to be left open to activate the protection. 16 comp the output of the internal transconductance error ampli er C this pin is used for loop compensation. 17 vout output voltage pin C internal over-voltage connect the output capacitor and the anode of control pin for led backlight strings. 18 pwm pwm control pin for led backlight strings. 19 io6 provides constant sink current to led string 6. 20 io5 provides constant sink current to led string 5. 21 io4 provides constant sink current to led string 4. 22 iognd constant sink current ground, tying to agnd. 23 io3 provides constant sink current to led string 3. 24 io2 provides constant sink current to led string 2. t thermal pad thermal pad for heatsinking purposes connect not connected internally en status 0 backlight disable 1 backlight enable note: when en = 0; the boost is turned off and disabled. table 1. note: any unused io pin should be pulled up to vin with the scp_en left open.
sc440a 10 www.semtech.com ? 2010 semtech corp. block diagram agnd tsd ovpin io6 + io3 5ua 4ua comp - ovp en hiccup ovprtn isense bandgap vin pgnd - io5 ilim one io channel shown ss ioset pwm hiccup uvlo & tsd sw hiccup led open / short circuit protection osc ss iognd pgnd uvlo + - io2 s r q sw io1 i to v current source ovp detect fault fault-1 + 3v3 led current setting vout fflag io4 control logic disable1 hiccup fault scp_en figure 1. sc440a block diagram
sc440a 11 www.semtech.com ? 2010 semtech corp. applications information sc440a detailed description the sc440a contains a high frequency, current-mode boost regulator and six string led current sources. the led current for all strings is programmed by an external resistor and the pwm controller operates to maintain the output voltage at a level which will keep the current of each string regulated. a typical application would use 3-8 backlight leds for each string, driven with approximately 30ma. operation the sc440a regulates the boost converter output voltage based on instantaneous requirement of the six string current sources. therefore, only a single inductor and power switch is needed to provide power to the entire lighting subsystem, increasing e ciency and reducing part count. a logic interface to output control circuit has high-bandwidth, and supports pwm dimming with 50hz to 50khz dimming frequency while the entire supply current is reduced to 4.5ma (typical) when all led strings are o . high frequency switching provides high output power using a 1.0mm height inductor, maximizing e ciency for space-constrained and cost-sensitive applications. additionally, the converter and output capacitor are protected from open-led conditions by programmable over voltage protection. led current programming the sc440a is a led current programmable regulator. the led current set point is chosen using external an resistor connected to the ioset pin. the relationship between the programming resistor value and the led current set point of each string can be described as follows: ioset led r 60 i where, r ioset is in k . i led is the output current of each string in ma. start-up during start-up, when the vin pin voltage reaches its uvlo threshold, and both en and pwm signals are set to high, the ss pin begins to source 6 a to the ss capacitor and its voltage begins to rise from 0v to its end value (2.6v). the output voltage of the internal error ampli er (comp) is increases and clamped by the ss pin voltage. when the ss pin voltage reaches its switching threshold, the sc440a starts to switch and the output voltage increases. each internal led current source (io1 ~ io6) tries to regulate the led current to its set point. while the output voltage increases, a suitable amount of error information will be generated on the internal error ampli er as the comp pin voltage keeps rising. once each led current reaches its set point, the error information is not generated by the led current source. the comp pin voltage stays at a level which keeps the led current at its set point. if the en pin voltage is pulled below 0.4v and vin reaches to its uvlo, sc440a will stay at shutdown mode, drawing less than 1a from the input power supply. if the pwm pin voltage is pulled below 0.4v when the en pin is pulled high and vin reaches its uvlo, the sc440a runs in standby mode, drawing 4.5ma (typical) from the input power supply. under this condition, soft-start is initiated and the ss pin voltage is raised to its end value since the en pin is pulled high. after that, when pwm signal goes high to enable sc440a, the comp pin voltage will rise as quickly as it can since it is not being limited by the ss pin. a proper capacitance (10nf ~ 100nf) is required for the comp pin and its external rc network in order to prevent output voltage overshoot. shut down when the vin pin voltage falls below its uvlo or en pin voltage goes low, the sc440a will run in shutdown mode. the internal switch and led current sources will be immediately turned o . the ss capacitor is discharged by ss pin internal current source and the ss pin voltage decreases to 0v. the output voltage falls to the same level as the input voltage.
sc440a 12 www.semtech.com ? 2010 semtech corp. if pwm pin voltage goes low while sc440a is at normal operation, sc440a will run in standby mode. the internal switcher and the led current sources will be immediately turned o . the ss pin will not be a ected by the pwm signal and remains at its nal value. main power stage operation sc440a is an 800khz xed-frequency, peak current- mode boost switching regulator with an integrated 2.5a (typical), power transistor. referring to the block diagram, the clock from the oscillation section resets the latch and turns on the power transistor. switch current is sensed with an integrated sense resistor. the sensed current is summed with the slope-compensating ramp and fed into the modulating ramp input of the pwm comparator. the latch is set and the power transistor conduction is terminated when the modulating ramp intersects the error ampli er output (comp). the current-mode switching regulator is a dual-loop feedback control system. in the inner current loop, the ea output (comp) controls the peak inductor current. in the outer loop, the error ampli er regulates the output voltage to keep the led current in set point. the double reactive poles of the output lc lter are reduced to a single real pole by the inner current loop, allowing the simple loop compensation network to accommodate a wide range of input and output voltages. over-current protection if the switch current exceeds 2.5a (the typical current- limit trip point), the current-limit comparator i lim will set the latch and immediately turn o the internal power switch. due to separate pulse-width modulating and current limiting paths, the ocp trip point is not a ected by slope compensation (i.e., trip point is not a ected by switching duty cycle). over-voltage protection (ovp) sc440a includes an external programming 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 ovpin pin. if the voltage at the ovpin pin exceeds 1.5v, the boost converter will shut o and a 1ma pull down current source will be applied to the vout pin to quickly applications information (continued) discharge the over-voltage capacitor. this additional level of protection prevents a condition where the output capacitor and schottky diode must endure high voltage for an extended period of time, which can pose a reliability risk for the system. the total resistance of the divider for the ovp protection should be more than 200k. the output over-voltage trip point can be programmed by r2 and r4 resistor divider (see the schematic on page 17). the relationship can be described as follows: r4 r4 r2 ovpin_th ovp_trip u where ovpin_th is 1.535v typical. an ovp event causes a fault which disables the boost converter and enables the strong pulldown and the fflag pin will also go high. meanwhile, the soft-start capacitor is discharged. when the soft-start capacitor voltage falls below 0.5v, sc440a enters a soft-start process. the ovp detection circuitry provides a disconnect feature during the shutdown state to prevent any leakage from the output. the external ovp resistor divider should be connected between vout and ovprtn while its central tap is connected to ovpin. if this disconnect function is not desired, just connect the end of the ovp resistor divider directly to gnd. the ovpin pin is sensitive to noise, a proper decoupling cap (1nf ~ 10nf) is required. led short-circuit protection if one or more leds are detected as shortened, that corresponding string will be latched o if scp_en is oating. the voltages on all internal led current sources (io pins) are monitored to see if any exceeds 2.3v. (the io voltage on abnormal led string will rise earlier than other oating led strings). if any io pin voltage exceeds 2.3v, that io current source will be latched o and the fflag will go high. the latch can be reset if vin falls below uvlo or recycle en signal. other normal led strings remain at their normal operation. the protection will be disabled if scp_en is tied to gnd. if all io pin voltages reach 0.8v then the internal main switch will be o until any of the io voltages is lower than 0.7v. unused strings the sc440a may be operated with less than 6 strings. in this case, all unused strings should be tied to vin and leave the scp_en pin oating.
sc440a 13 www.semtech.com ? 2010 semtech corp. applications information (continued) led open-circuit protection if any led is detected as open circuit, that string will be latched o . then the comp pin will be driven high and the boost converter duty cycle will increase causing vout to rise. at some point vout will rise high enough to cause all the io pin voltages of the remaining strings to reach the shorted led detection level and those strings are latched o . the fflag pin will also go high. because of the open string, vout will continue to rise until it reaches the programmed ovp level. when ovp is reached, the voltages on the io pins are monitored. if any io voltage is less than 0.2v, that string will be identi ed as open and will be latched o . only vin falling below uvlo, recycle en signal and thermal shutdown will reset this latch. a hiccup cycle is initiated, ss is discharged slowly with a 1.6 a current source and a 1ma discharge path is turned on to pull down vout. when ss falls below 0.5v and vout falls below to vin, the shorted led detection latches are reset and a new soft- start sequence is initiated to resume normal operation. thermal shutdown (tsd) if the thermal shutdown temperature of 150c is reached, a hiccup sequence is initiated where the boost converter and all io current sources are turned o , ss is discharged by a 1.6a current source, and a 1ma discharge path is turned on to pull down vout. as temperature falls below tsd trip point, sc440a will retry when ss falls below 0.5v and vout falls to vin. pwm dimming the pwm input needs to be held high for normal operation. pwm dimming can be done by cycling the pwm input at a given frequency where a low on the pwm input turns o all io current sources and a high turns on all io current sources. the short and open detection latches are blanked for approximately 2s as the pwm input transitions from low-to-high to prevent a false fault detection during pwm dimming. the pwm pin can be toggled by external circuitry 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 sc440a allows dimming over two decades in frequency (50hz-50khz) in order to allow compatibility with a wide range of devices, including the newest dimming strategies that avoid the audio band by using high frequency pwm dimming. in this manner, a wide range of illumination can be generated while keeping the instantaneous led current at its peak value for high e ciency and color temperature. furthermore, advanced lighting e ects such as backlight dim-on can be implemented as the sc440a can resolve pwm from 10% to 90% duty at its highest frequency. an additional advantage of pwm dimming comes to customers who prefer to avoid in-rush currents when lling the boost output capacitor. simply apply the pwm signal to the device at 10% duty for a millisecond or two, and in-rush current is reduced to less than 50ma. this dim time will vary based on the number of leds and the size of the output capacitor, but can be easily determined on the bench and programmed into the c rmware. linear dimming the linear dimming control is available for sc440a by applying an external control voltage on ioset pin through an external resistor as shown below. external environment brightness compensation can also be achieved when the control voltage is generated by a light sensing circuit. led string connection generally, led strings are connected to the io1 ~ io6 pins through a mechanical connector which cannot support an electrical connection at times. this connection might cause noise on the io1 ~ io6. if this function is enabled, the sc440a led short-circuit protection may false trip when the noise level is large enough. certain ceramic decoupling capacitors (100pf ~ 8.2nf) on pins io1 ~ io6 to gnd can help prevent the sc440a from entering the r_ext r_ioset ioset v_ext
sc440a 14 www.semtech.com ? 2010 semtech corp. applications information (continued) false protection, or, simply disable this feature by tying pin 15 to gnd. parallel operation when two or more sc440as are operating in parallel for a large-sized panel application, audible noise may be observed due to a non-synchronous switching frequency. the ripple voltage on the input voltage rail will be modulated by the beat frequency resulting in audible noise. this situation can be resolved by adding an input inductor between the input voltage rail and the vin pin. this situation can also be improved by adding more input decoupling capacitors. inductor selection the inductance value of the inductor a ects the converters steady state operation, transient response, and its loop stability. special attention needs to be paid to three speci cations of the inductor, its value, its dc resistance and saturation current. the inductors inductance value also determines the inductor ripple current. the converter can operate in either ccm or dcm depending on its working conditions. the inductor dc current or input current can be calculated as, v i v i in out out in ? ? i in - input current; i out ? output current; v out ? boost output voltage; v in ? input voltage; C e ciency of the boost converter. then the duty ratio is, d out d in out v v v v v d v d ? forward conduction drop of the output rectifying diode when the boost converter runs in dcm ( l < l boundary ), it takes the advantages of small inductance and quick transient response while avoiding the bandwidth- limiting instability of the rhp zero found in ccm boost converters. the inductor peak current is, the converter will work in ccm if l > l boundary . generally the converter has higher e ciency under ccm and the inductor peak current is, l f 2 d v i i s in in peak - l ? ? ? for many applications, an inductor with value of 4.7 h to 22 h should be ne, such as for the typical case shown on page 1. the inductor peak current must be less than its saturation rating. when the inductor current is close to the saturation level, its inductance can decrease 20% to 35% from the 0a value depending on the vendor speci cations. using a small value inductor forces the converter under dcm in which case the inductor current ramps down to zero before the end of each switching cycle. it reduces the boost converters maximum output current, and produces large input voltage ripple. an inductor with larger inductance will reduce the bandwidth of the feedback loop, possibly higher dc resistance (dcr). inductors dcr plays a signi cant role for the total e ciency since the power transistor is integrated inside the sc440a. of course, there is a trade-o between the dcr and inductor size. table 2 lists recommended inductors and their vendors. table 2. recommended inductors inductor website dr74, 4.7h ~ 15h www.cooperet.com ihlp-2525cz-01, 4.7 ~ 10h www.vishay.com ds85lc, 6.8h ~ 10h www.tokoam.com l f d v i s in peak l ? ? = ?
sc440a 15 www.semtech.com ? 2010 semtech corp. applications information (continued) output capacitor selection the next task in sc440a design is targeting the proper amount of ripple voltage due to the constant-current led loads. the two error ampli ers that control the pwm converter sense the delta between requested current and actual current in each output current regulator. on a cycle-by-cycle basis, a small amount of output ripple ensures good sensing and tight regulation, while the output current regulators keep each led current at a xed value. overall, this allows usage of small output caps while ensuring precision led current regulation. although the mechanics of regulation and frequency dependence may be complex, actual selection of output capacitor can be simpli ed because this capacitor is mainly selected for the output ripple of the converter. assume a ceramic capacitor is used. the minimum capacitance needed for a given ripple can be estimated by, ripple s out out n out out v f v i ) i v (v c ? ? x v ripple ? peak to peak output ripple; i out ? output current; v out ? boost output voltage; v in ? input voltage; f s ? switching frequency. during load transient, the output capacitor supplies or absorbs additional current before the inductor current reaches its steady state value. larger capacitance helps with the overshoot and undershoots during load transient, and loop stability. recommended ceramic capacitor manufacturers are listed in table 3. table 3. recommended ceramic capacitor manufacturers vendor phone website kemet 408-986-0424 www.kemet.com murata 814-237-1431 www.murata.com taiyo yuden 408-573-4150 www.t-yuden.com output rectifying diode selection schottky diodes are the ideal choice for sc440a due to their low forward voltage drop and fast switching speed. table 4 shows several di erent schottky diodes that work well with the sc440a. make sure that the diode has a voltage rating greater that the possible maximum ouput voltage. the diode conducts current only when the power switch is turned o . a diode of 1a will be su cient for most designs. layout guidelines the sc440a contains a boost converter and the placements of the power components outside the sc440a should follow the layout guidelines of a general boost converter. the application circuit on page 17 will be used as an example. the layout illustration diagram is shown on page 19. r5 and c7 form a decoupling lter for the sc440a. c7 should be placed as close as possible to the vin and pgnd to achieve the best performance. c6 is the input power ltering capacitor for the boost converter power train. l1 is the boost converter input inductor. d1 is the output rectifying diode and it is recommended that a schottky diode be used for fast reverse recovery. to minimize switching noise for the boost converter, the output capacitor, c2, should be placed at the bottom, as displayed on page 19, so that the loop formed by c2, d1, and the internal switch, is the smallest. the output of the boost converter is used to power up the leds. r6, c9 and c10 (open, not used), form the compensation network for the boost converter. c9 should return to analog ground. table 4. recommended rectifying diodes part vendor ss13 ss14 vishay www.vishay.com c8 determines the soft-start time and should be connected to analog ground. r8 is the output current programming resistor for io1 through io6 and should return to analog ground. iognd should also be connected to agnd. since there is pad at the bottom of the sc440a for heat dissipation, a copper area right underneath the pad is used for better heat spreading. on the bottom layer of the
sc440a 16 www.semtech.com ? 2010 semtech corp. applications information (continued) board another copper area, connected through vias to the top layer, is used for better thermal performance. the pad at the bottom of the sc440a should be tied to the analog ground. the analog ground should be connected to the power ground at one point for better noise immunity.
sc440a 17 www.semtech.com ? 2010 semtech corp. case 1: schematic for 6 strings 8 leds per string application case 2: schematic for 5 strings 8 leds per string application c8 r7 10k c2 10uf c6 2.2u r2 221k led led led led r4 10k led led r5 1 led led led led gnd c7 2.2u r6 1.5k c9 12n gnd d1 l1 6.8u vin=5v ~ 20v r8 2k led led io1 1 agnd 2 fflag 3 en 4 ss 5 ovprtn 6 ovpin 7 pgnd 8 sw 9 sw 10 pgnd 12 vin 13 ioset 14 scp_en 15 comp 16 vout 17 pwm 18 io6 19 io5 20 io4 21 iognd 22 io3 23 io2 24 sink 25 sc440a 47n r3 20k r9 20k cr4 1n 8 leds per channel pwm c8 r7 10k c2 10uf c6 2.2u r2 221k led led led led r4 10k led led r5 1 led led led led gnd c7 2.2u r6 1.5k c9 12n gnd d1 l1 6.8u vin=5v ~ 20v r8 2k led led io1 1 agnd 2 fflag 3 en 4 ss 5 ovprtn 6 ovpin 7 pgnd 8 sw 9 sw 10 pgnd 12 vin 13 ioset 14 scp_en 15 comp 16 vout 17 pwm 18 io6 19 io5 20 io4 21 iognd 22 io3 23 io2 24 sink 25 sc440a 47n r3 20k r9 20k cr4 1n 8 leds per channel pwm input 12v r7 20k r3 20k c1 10uf c6 2.2u 0 cr4 1nf 0 r2 221k r4 10k 0 c8 47nf 0 c7 2.2u r6 1.5k c9 12n p2 d1 ss14 l1 10u p1 0 r8 3k io1 1 agnd 2 fflag 3 en 4 ss 5 ovprtn 6 ovpin 7 pgnd 8 sw 9 sw 10 nc 11 pgnd 12 vin 13 ioset 14 scp_en 15 comp 16 vout 17 pwm 18 io6 19 io5 20 io4 21 iognd 22 io3 23 io2 24 sink 25 u1 sc440a r9 20k 8 leds per channel coilcraft, mos6020 input 12v r7 20k r3 20k c1 10uf c6 2.2u 0 cr4 1nf 0 r2 221k r4 10k 0 c8 47nf 0 c7 2.2u r6 1.5k c9 12n p2 d1 ss14 l1 10u p1 0 r8 3k io1 1 agnd 2 fflag 3 en 4 ss 5 ovprtn 6 ovpin 7 pgnd 8 sw 9 sw 10 nc 11 pgnd 12 vin 13 ioset 14 scp_en 15 comp 16 vout 17 pwm 18 io6 19 io5 20 io4 21 iognd 22 io3 23 io2 24 sink 25 u1 sc440a r9 20k 8 leds per channel coilcraft, mos6020
sc440a 18 www.semtech.com ? 2010 semtech corp. item quantity reference part 1 1 cr4 1nf 2 1 c2 10f, 50v, 1210 3 2 c6, c7 2.2, 25v, 0805 4 1 c8 47nf 5 1 c9 12nf 6 1 d1 ss14 7 1 l1 6.8, ihlp-2525cz1 8 1 r4 10k 9 1 r2 221k 10 2 r3, r9 20k 11 2 r7 10k 12 1 r8 2k 13 1 u1 sc440a, semtech 14 1 r5 1, 0805 15 48 leds sml-lx0603uwd 16 1 r6 1.5k bom for 6 string 8-led application case case 3: schematic for 6 strings 12 low vf leds per string application c8 r7 10k c1 4.7uf c6 2.2u r2 249k led led led led r4 10k led led r5 1 led led led led gnd c7 2.2u r6 6.04k c9 12n gnd d1 l1 10u vin=12v r8 3k led led io1 1 agnd 2 fflag 3 en 4 ss 5 ovprtn 6 ovpin 7 pgnd 8 sw 9 sw 10 pgnd 12 vin 13 ioset 14 scp_en 15 comp 16 vout 17 pwm 18 io6 19 io5 20 io4 21 iognd 22 io3 23 io2 24 sink 25 sc440a 47n r3 20k r9 20k cr4 1n 12 low vf leds per string pwm toko, ds86c ss15 c2 4.7uf c3 4.7uf 50v 50v 50v c8 r7 10k c1 4.7uf c6 2.2u r2 249k led led led led r4 10k led led r5 1 led led led led gnd c7 2.2u r6 6.04k c9 12n gnd d1 l1 10u vin=12v r8 3k led led io1 1 agnd 2 fflag 3 en 4 ss 5 ovprtn 6 ovpin 7 pgnd 8 sw 9 sw 10 pgnd 12 vin 13 ioset 14 scp_en 15 comp 16 vout 17 pwm 18 io6 19 io5 20 io4 21 iognd 22 io3 23 io2 24 sink 25 sc440a 47n r3 20k r9 20k cr4 1n 12 low vf leds per string pwm toko, ds86c ss15 c2 4.7uf c3 4.7uf 50v 50v 50v
sc440a 19 www.semtech.com ? 2010 semtech corp. layout illustration diagrams pcb top pcb b ottom components top components bottom
sc440a 20 www.semtech.com ? 2010 semtech corp. outline drawing - mlpq-24 e1 e bxn d/2 lxn inches .020 bsc b .007 bbb aaa n d1 e l e d .012 .100 dim a1 a2 a min .000 - .031 0.30 0.18 .012 0.25 .010 0.50 2.80 0.30 2.55 .004 .004 24 .016 .157 .106 .020 .110 0.10 0.10 24 0.40 4.00 2.70 0.50 bsc millimeters max 0.05 - 1.00 dimensions min 0.00 - nom (.008) .035 .001 max .002 - .039 nom 0.80 0.02 (0.20) 0.90 controlling dimensions are in millimeters (angles in degrees). coplanarity applies to the exposed pad as well as the terminals. notes: 2. 1. 1 2 n e1 .100 .106 .110 2.55 2.70 2.80 pin 1 indicator 4.15 3.85 4.00 4.15 3.85 .157 .152 .163 .152 .163 aaa c a c (laser mark) d e b a1 a a2 seating plane e/2 bbb cab d1
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 sc440a 21 www.semtech.com ? 2010 semtech corp. land pattern - mlpq-24 company's manufacturing guidelines are met. 4.80 .189 z k g z h (c) x p functional performance of the device. shall be connected to a system ground plane. thermal vias in the land pattern of the exposed pad failure to do so may compromise the thermal and/or 2. this land pattern is for reference purposes only. consult your manufacturing group to ensure your notes: 1. dim x y h k p c g millimeters inches (3.95) .010 .033 .122 .020 .106 .106 (.156) 0.25 0.85 2.70 0.50 2.70 3.10 dimensions

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