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highfefciencyfintegratedfdriverfforf 3-stringsfoff100mafleds sc446 powerfmanagement january 28, 2010 1 www.semtech.com features applications description ? wide input voltage range from 4.5v to 27v ? 36v maximum output voltage ? drives up to 30 wleds in 3 strings f ? programmablef ledf currentf forf upf tof 100maf perf stringf f ? +/-f2%fstring-to-stringfcurrentfmatching f ? upftof90%ffefciency ? wide 0.2% to 100% pwm dimming range ? possible analog dimming ? integrated 2.5a power switch ? 700khz switching frequency for small size ? adjustable ovp for cost-efective output cap selection f ? open/shortfledfprotection ? thermal protection with auto-recovery ? thermally enhanced tssop-16 edp package ? pb free, halogen free, andweee/rohs compliant ? medium-sized lcd panel ? notebook display ? automotive car navigation display ? sub-notebook and tablet computer displays ? portable media players the sc446 is a high-efciency multiple string wled driver with an integrated boost converter. it operates over a wide input range from 4.5v to 27v with a maximum out - put voltage of 36v and a 2.5a internal power switch. it can drive up to thirty wleds in 3 strings with current up to 100ma per string. the string-to-string current match - ing is 2% typical, 3% maximum. the overall efciency 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 500f 1. the 700khz switching frequency enables the user to op - timize the external component sizes for efciency. when there are fewer than 10 leds in each string, users can ad - just the output voltage protection yielding an allowable reduction in associated costs, size and voltage ratings of the output capacitor. the sc446 also features comprehensive open and short- circuit led protection functions. it disables the corre - sponding strings with led open or led short conditions while maintaining normal operation of other, unafected led strings. this feature allows lcd panels to remain viewable even under led failure, wire disconnect, or short-circuit conditions. the internal thermal shutdown protects the ic from overheating at abnormal conditions. the sc446 is available in a thermally-enhanced tssop-16 edp package. typicalfapplicationfcircuit 1 3 vout ss pgnd ioset io1-4 comp ovpin agnd vin sw pwm en sc446 3 strings 1 10 vin (4.5v -27v) 2 edp pgnd ioset io1- 3 comp ss agnd vout ovpin vin ovprtn
sc446 2 ? 2010 semtech corporation www.semtech.com pinfconfguration orderingfinformation markingfinformation notesf (1) available in tape and reel only. a reel contains 2,500 devices. (2) available in lead-free package only. this product is fully weee/rohs compliant, pb free and halogen free. (tssop-16 edp) ja ff=f39ofc/w device package SC446TETRT (1,2) tssop-16 edp sc446evb evaluation board 5 5 4 4 3 3 2 2 1 1 d d c c b b a a vout ioset io1 io3 pgnd agnd io2 pwm en comp ss ovprtn ovpin sw vin pgnd 1 9 16 8 marking for the tssop-ep 16 lead packagef nnnnn = part number (examplef sc446) yyww = date (examplef 0952) xxxx = semtech lot # (examplef e901) top mark
sc446 3 ? 2010 semtech corporation www.semtech.com recommendedfoperatingfconditions absolutefmaximumfratings exceeding the above specifcations may result in permanent damage to the device or device malfunction. operation outside of the parameters specifed 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 jesd51 standards. (2) tested according to jedec standard jesd22-a114-b. electricalfcharacteristics thermalfinformation v in pinf supply voltage -0.3 to 30v maximum output power 22w sw, ovpin, ovprtn, vout, io1~io3 voltage -0.3 to 40v ioset voltage -0.3 to 2v ss, comp voltage -0.3 to 4v en, pwm, voltage -0.3 to v in +0.3v pgnd to agnd 0.3v peak ir refow temperature 260c esd protection level (2) 3.5kv input voltage range 4.5v~27v maximum output voltage 36v maximum led current 100ma junction to ambient (1) 39c/w maximum junction temperature 150c storage temperature -65 to +150c lead temperature (soldering) 10 sec 260c unless otherwise specifedf v in =12v, -40c < t a = t j < 105c, r ioset =2.61k w. parameter symbol conditions min typ max units i nputfsupply under-voltage lockout threshold uvlo-th v in rising 4.3 4.45 v uvlo hysteresis uvlo-h 250 mv v in quiescent supply current i in_q no switching 3 ma v in supply current in shutdown i in_s en / pwm = low 1 a oscillator switching frequency f s 0.56 0.7 0.84 mhz internalfpowerfswitcher switch current limit i sw 2.5 3.32 a switch saturation voltage v sat i sw = 1a 200 350 mv switch leakage current i s_leak 0.1 1 a maximum duty cycle d max 88 93 % minimum duty cycle d min 0 % minimum on-time (1) t on_min 100 ns compensation sourcing current i ea_source v comp = 0.5v 5 a sinking current i ea_sink v comp = 2v 6 a
sc446 4 ? 2010 semtech corporation www.semtech.com electricalfcharacteristicsf(continued) notesf (1) guaranteed by design. (2) for achievable pwm dimming minimum pulse-width in applications, see the corresponding curves in typical characteristics. parameter symbol conditions min typ max units controlfsignals en, pwm high voltage v en_h, v pwm_h 2 v en, pwm low voltage v en_l, v pwm_l 0.4 v en, pwm leakage current i en, i pwm v en , v pwm = 5v 0.1 1 a pwm dimming frequency (1) f dimming 50 50k hz pwm dimming minimum duty cycle d min_dimming f dimming = 200hz 0.2 % pwm dimming minimum pulse-width (2) t min_dimming 5 s pwm dimming minimum of time t min_of 200 300 ns fflag voltage v fflag i fflag = 2 ma 0.25 v ss source current i ss_source v ss = 0v 4.5 a ss sink current i ss_sink v ss = 2v at ovp or otp 1 ss switching threshold v ss_switching v in = 12v, t j = 25 c 0.5 0.7 0.85 v ss end value v ss_end v in = 12v 2.5 v over-voltagefprotection ovpin threshold v ovpin_th ovpin - agnd 1.43 1.52 1.58 v ovpin leakage current i ovpin v ovpin = 20v 0.1 1 a ovprtn saturation voltage v ovprtn i ovprtn = 100 a 60 mv ovprtn leakage current i ovprtn v ovprtn = 20v 0.1 a vout internal pull-down current source i ovp v out = v in + 3v 0.9 ma vout leakage current i vout _ leak v out = 40v 0.1 a currentfsourcef(io1f~io3f) current accuracy i o1 ~ i o3 t j = 25 c 93 100 107 ma current matching t j = 25 c +/- 2 +/-3 % maximum led current i omax 150 ma led short-circuit protection v io_scp t j = 25 c, v io1 ~ v io3 2.2 2.35 2.55 v leakage current i io_leak en = 0, v io1 ~ v io3 = v in 0.1 1 a overshoot protection threshold v io1 ~v io3 t j = 25 c, v io1 ~ v io3 0.963 1.07 1.177 v 0.9065 1.07 1.2305 v overshoot protection hysteresis any of i o1 ~ i o3 100 mv over-temperaturefprotection thermal shutdown temperature t otp 150 c thermal shutdown hysteresis t otp_h 30 c
sc446 5 ? 2010 semtech corporation www.semtech.com typicalfcharacteristicsf temperature ( o c) temperature ( o c) uvlo hysteresis vs. temperature vin quiescent supply current vs. temperature temperature ( o c) temperature ( o c) temperature ( o c) sw saturation voltage at 1a vs. temperature temperature ( o c) vout pull down current source vs. temperature temperature ( o c) temperature ( o c) led current source scp threshold vs. temperature comp current source & sink 4.0 4.5 5.0 5.5 6.0 6.5 -40 25 105 comp sink / source current(ua ) source sink comp sink / source current vs. temperature vin = 12v ss current source & sink 1.0 2.0 3.0 4.0 5.0 6.0 -40 25 105 ss sink / source current(ua) source sink vin = 12v ss sink / source current vs. temperature uvlo threshold 4.19 4.20 4.21 4.22 4.23 4.24 -40 25 105 vin uvlo(mv) uvlo threshold vs. temperature uvlo hysteresis 235 245 255 265 275 285 -40 25 105 uvlo hysteresis(v) vin quiescent supply current 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 -40 25 105 vin quiescent supply current(ma) 180 190 200 210 220 230 -40 25 105 sw saturation voltage@1a(mv) sw saturation voltage vin = 4.5v vin = 12v vin = 27v 0.700 0.720 0.740 0.760 0.780 0.800 0.820 -40 25 105 vout pull down current source(ma) vout pull down current vin = 4.5v vin = 12v vin = 27v vout = vin + 3v 2.31 2.32 2.33 2.34 2.35 2.36 -40 25 105 led current source scp threshold(v ) led current source scp threshold vin = 12v led current (ma) led current source saturation voltage vs. led current 0 100 200 300 400 500 600 10 20 30 40 50 60 70 80 90 100 t a = 25 o c led current source saturation voltage (mv)
sc446 6 ? 2010 semtech corporation www.semtech.com typicalfcharacteristicsf(continued) ovpin threshold voltage vs. temperature temperature ( o c) ovprtn saturation voltage vs. temperature efciency ( p boost_output / p input ) boost section output current (ma) conditionf vout = 36v, l1=b1000as-100m 72 74 76 78 80 82 84 86 88 90 92 94 30 60 90 120 150 180 210 240 270 300 efficiency 27 vin 12 vin 5 vin 4.5 vin 21 vin efciency (%) 14 15 10 11 12 13 7 8 9 10 6 7 0.1 0.2 0.5 1.0 2.0 5.0 10.0 20.0 30.0 40.0 50.0 pwm dimming frequency (khz) conditionf vin = 12v, vout = 36v / 10ma x 3 strings pwm dimming minimum pulse width vs. pwm dimming frequency pwm dimming minimum pulse width(us) 9 10 11 6 7 8 9 2 3 4 5 1 2 0.1 0.2 0.5 1.0 2.0 5.0 10.0 20.0 30.0 40.0 50.0 pwm dimming frequency (khz) conditionf vin = 12v, vout = 36v / 100ma x 3 strings pwm dimming minimum pulse width vs. pwm dimming frequency pwm dimming minimum pulse width(us) 0 5 10 15 20 25 30 35 40 45 50 0.1 0.2 0.5 1.0 2.0 5.0 10.0 20.0 30.0 40.0 50.0 pwm dimming frequency (khz) conditionf vin = 12v, vout = 36v / 10ma x 3 strings pwm dimming minimum duty cycle vs. pwm dimming frequency pwm dimming minimum duty cycle (%) 0 2 4 6 8 10 12 0.1 0.2 0.5 1.0 2.0 5.0 10.0 20.0 30.0 40.0 50.0 pwm dimming frequency (khz) conditionf vin = 12v, vout = 36v / 100ma x 3 strings pwm dimming minimum duty cycle vs. pwm dimming frequency pwm dimming minimum duty cycle (%) ovprtn saturation voltage 45 50 55 60 65 70 75 -40 25 105 ovprtn saturation voltage(mv ) i ovprtn = 100ua temperature ( o c) ovpin threshold voltage 1.514 1.518 1.522 1.526 1.530 1.534 -40 25 105 ovpin threshold voltage(v) vin = 12v
sc446 7 ? 2010 semtech corporation www.semtech.com typicalfcharacteristicsf(continued) vin (12v/div) vout (15v/div) vss (1.2v/div) vsw (20v/div) start up time (10ms/div) conditions: 20vin, 36vout / 100ma x 3 led strings conditions: 20v, 36vout / 100ma x 3 led strings vin (12v/div) vout (15v/div) vss (1.2v/div) vsw (20v/div) shut down time (2s/div) vin (12v/div) vout (100mv/div) vsw (12v/div) main power switching time (1us/div) conditions: 12vin, 36vout / 100ma x 3 led strings pwm (5v/div) vout (10v/div) io1 (10v/div) vsw (20v/div) start up by pwm time (5ms/div) conditions: 12vin, 36vout / 100ma x 3 led strings, 200hz pwm dimming, 50% duty cycle vin (12v/div) vout (100mv/div) vsw (12v/div) main power switching time (1us/div) conditions: 20vin, 36vout / 100ma x 3 led strings
sc446 8 ? 2010 semtech corporation www.semtech.com typicalfcharacteristicsf(continued) vin (10v/div) vout (10v/div) vss (1.5v/div) vsw (20v/div) over temperature protection time (100ms/div) conditions: 12vin, 36vout / 100ma x 3 led strings conditions: 12v, 36vout / 100ma x 3 led strings, io1 has one led short circuit vin (10v/div ) vout (10v/div) io1 (10v/div) io2 (1v/div) led short circuit protection time (10ms/div) vin (10v/div) vout (20v/div) io2 (10v/div) led open circuit protection time (50ms/div) conditions: 12vin, 36vout / 100ma x 3 led strings, io1 led string is open circuit pwm dimming time (5ms/div) conditions: 12vin, 36vout / 100ma x 3 led strings, 200hz pwm dimming, 0.2% duty cycle vin (10v/div) vout (1v/div) sw (20v/div) adaptor plug-in time (50us/div) conditions: 12vin to 19vin, 36vout / 100ma x 3 led strings io1 (1v/div) pwm dimming time (5ms/div) conditions: 12vin, 36vout / 100ma x 3 led strings, 200hz pwm dimming, 50% duty cycle io1 (1v/div) sw (20v/div) vout (1v/div) pwm (5v/div) io1 (10v/div) sw (20v/div) vout (1v/div) pwm (5v/div) io1 (10v/div)
sc446 9 ? 2010 semtech corporation www.semtech.com pinfdescriptions pinf# pinfname pinffunction 1 io1 provides constant current source to led string 1. 2 agnd analog ground. 3 ss soft-start pin. 4 comp the output of the internal trans-conductance error amplifer. 5 en enable the device including regulator and led drivers. 6 ioset current source io value set pin. by selecting the resistor connected from this pin to gnd, the corresponding maximum current on all 4 strings are set. 7 pwm pwm dimming control pin for led strings. 8 vout internal pull down current source in over voltage fault. connect this pin directly to boost output. 9 vin power input voltage pin. bypassed with capacitors close to the pin. 10 ovprtn over-voltage protection sense signal return path pin. 11 ovpin over-voltage protection sense signal input. 12 pgnd power ground. 13 sw collector of the internal power switch. 14 pgnd power ground. 15 io3 provides constant current source to led string 3. connect to vin for 2 strings operation. 16 io2 provides constant current source to led string 2. edp solder to the ground plane of the pcb. en status 0 backlight disable 1 backlight enable notef when en = 0; the boost is turned off and disabled. notef any unused io pin should be pulled up to vin.
sc446 10 ? 2010 semtech corporation www.semtech.com blockfdiagram figure 1. sc446 block diagram 5 5 4 4 3 3 2 2 1 1 d d c c b b a a fault-1 tsd ovp fault ss hiccup hiccup 3v3 hiccup uvlo fault control logic vout disable1 en pwm vin - sw agnd comp osc + isense bandgap io1 - ilim pgnd ss one io channel shown + ioset ovprtn hiccup uvlo & tsd + pgnd led open / short circuit protection current source ovpin - led current setting ovp detect s r q 0.9ma 0.9ma 1ua 1ua 4.5ua 4.5ua 4 5 6
sc446 11 ? 2010 semtech corporation www.semtech.com applicationsfinformationf sc446fdetailedfdescription the sc446 contains a high frequency, current-mode boost regulator and four programmable current sources. the led current source value is set using an external resistor while the pwm controller maintains the output voltage at a level keeping the current regulated through the leds. since the sc446 receives feedback from all of the led current sources, all led strings can be turned on at any given time. a typical application would use 3-10 backlight leds for each string, driven up to 100ma. operation the sc446 controls the boost converter set point based on instantaneous requirements of four current sources. therefore, only a single inductor and power switch is needed to provide power to the entire lighting subsystem, increasing efciency and reducing part count. a digital interface to output control is high-bandwidth, supporting digital pwm dimming at 50hz to 50khz dimming frequency, while aggressively shutting the entire supply current down to 3ma (typical), when all led strings are turned of. high frequency switching provides high output power using a tiny 1.0mm high inductor, maximizing efciency for space-constrained and cost-sensitive applications. additionally, both converter and output capacitor are protected from open-led conditions by over-voltage protection. ledfcurrentfprogramming the sc446 features programmable led current regulators. the led current set points are chosen using external resistors tied to the ioset pin. the relationship between the programming resistor value and the output current set point can be described as followsf r ioset = (0.261v) / i led where r ioset is in k?. i led is the led current in amperes. the four output channels have the same output current. start-up during start-up, when the vin pin voltage reaches its uvlo threshold and both the en and pwm signals are set to high, the ss pin begins to source 4.5a as its voltage begins to rise from 0v to its end value of 2.6v. the output voltage of the internal trans-conductance error amplifer (comp), increases and clamps to the ss pin voltage. when the ss pin voltage reaches its switching threshold, output voltage increases. proper decoupling is required on the vin pin, especially for a lower input voltage condition. a 22f, 6.3v rated x5r ceramic capacitor is recommended for a 5v input system. the internal led current source (io1 ~ io3) helps 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 amplifer. the comp pin voltage keeps rising and once the led current reaches its set point, the error information will not be generated by the led current source. the comp pin voltage stays level while keeping the led current in its set point. if the en pin voltage is pulled below 0.4v, the sc446 will stay in shutdown mode drawing less than 1a from its input power supply. during the normal operation, when pwm pin is pulled below 0.4v, the device operates in standby mode, drawing 3ma (typical) current from the input. under this condition, since the en pin is pulled high, soft-start is initiated and the ss pin voltage is raised to its end value. following this, when the pwm signal goes high to enable the sc446, the comp pin voltage will rise quickly since it is not limited by the ss pin. a proper capacitance (10nf ~ 100nf) is required to prevent output voltage overshoot on the comp pin and its external rc network. shutfdownf if the vin pin voltage falls below its uvlo, or the voltage on the en pin goes low, the device will run in shutdown mode as the internal switch and the led current sources will immediately turn of. the ss capacitor is discharged by the internal current source of the ss pin. the ss pin voltage decreases to 0v while the output voltage falls to the same level as the input voltage. if the pwm pin voltage goes low while sc446 is in normal operation, then the sc446 will run in standby mode. the internal switcher and the led current source will immediately turn of. noteC the pwm signal does not afect the ss pin nor its fnal value. mainfpowerfoperation sc446 is a 700khz fxed-frequency, peak current-mode step-up switching regulator with an integrated 2.5a (minimum) power transistor.
sc446 12 ? 2010 semtech corporation www.semtech.com applicationsfinformationf(continued) there are some choices. 1) leave some room between vin range and vo. 2) operate the boost converter at normal load (less ro) 3) increases the boost inductance (l). over-currentfprotection sc446 provides cycle-by-cycle current limiting for its internal switch. if the switch current exceeds 3.32a (the typical current-limit trip point), then the current-limit comparator ilim, will set the latch immediately turning of internal power. all led current sources keep operating in an over-current condition. due to separate pulse-width modulating and current limiting paths, the ocp trip point is not afected by slope compensation (i.e. trip point is not afected by switching duty cycle). over-voltagefprotectionf(ovp) sc446 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 boost converters output voltage is detected at the ovpin pin. if the voltage at the ovpin pin exceeds 1.52v (typical), the boost converter will shut down and a 0.9ma pull- down current will be applied to the vout pin to quickly discharge the output capacitor. this added protection prevents a condition where the output capacitor and schottky diode must endure high voltage for an extended time, which can pose a reliability risk for the users system. refer to evaluation application circuit in page15. the output over voltage trip point can be programmed by r5 and r7 resistor divider. the relationship can be described as followsf where ovpin_th is 1.52v typical. an ovp event causing a fault could disable the boost converter enabling the device to a strong pull-down. this event would cause the soft-start capacitor to discharge. when the soft-start capacitor voltage falls below 0.5v, and the output voltage falls to vin, sc446 enters a soft- start process. the ovp detection circuitry provides a disconnect function during the shutdown state to prevent any leakage from the output. the external ovp resistor divider should be connected between vout and ovprtn with the central referring to the block diagram, figure 1, 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 with the error amplifer 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 amplifer regulates the output voltage to keep the led current at setting point. the double reactive poles of the output lc flter 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. it is well known that, in boost converter, vo is greater than or equal to vin. in normal continuous conduction mode (ccm) operation, where, d is the duty ratio of the pwm power switch. as vin increases, in order to regulate vo to a given constant value, d decreases. when vin approaches vo, d surely leads to 0. in practice, due to the minimum on-time of the pwm power switch, d usually could not approach 0 with infnitely small granularity. at some point, it either produces one pulse with minimum on-time or generates 0 by skipping the pulse. such point could be theoretically calculated for sc446 as follows. for ccmf vin 0.92 vo. for dcm (discontinuous conduction mode)f where, ro is the boost equivalent output resistance (=vo/ io), l is the boost inductor (in uh). in many boost converter designs and operations, pulse skipping is normally allowed at light load conditions. some designers even purposely let the boost power con - verter enter the pulse skipping in order to save power at light load conditions. if some designers do not want pulse skipping mode, based on the conditions provided above, 2 7 7 5 _ _ r r r th ovpin trip ovp + = d v v in o ? = 1 1 o o in v l r v * 10 * 6 . 1 1 1 2 2 ? + +
sc446 13 ? 2010 semtech corporation www.semtech.com applicationsfinformationf(continued) tap connected to ovpin. notef if this disconnect function is not desired, bypass the ovprtn pin and connect the end of the ovp resistor divider directly to gnd. the ovpin pin is sensitive to noise, and a proper decoupling capacitor (1nf ~ 10nf) is required. the combined impedance of the resistor divider for ovpin should be greater than 200k?. ledfshort-circuitfprotection if one or more leds are detected as short-circuit, that string will be latched of. voltage is monitored if it exceeds 2.35v on the internal led current source (io pins). (the io voltage on an abnormal led string will rise earlier than other normal led strings). if the voltage exceeds 2.35v on any io pin, the io current source will latch of. the latch is reset if vin falls below uvlo or it will recycle the en signal. other led strings operate normally. ledfopen-circuitfprotection if any led string is detected as an open-circuit, that string will latch of. if any given string is open, the io current source will go to deep saturation; the comp pins 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 intact strings to reach the shorted led detection level and latch of those strings. because of the led open string vout will continue to rise until it reaches the programmed ovp level. when ovp is reached, the voltage on the io pins are monitored and if any io voltage is less than 0.2v that string will be identifed as open and will latch of. only when vin falls below uvlo, or an en signal is re - cycled, and if thermal shutdown occurs, can this latch be reset. a hiccup cycle is then initiated and the ss pin is discharged slowly with a 1a current source and a 0.9ma discharge path (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 se - quence is initiated to resume normal operation. thermalfshutdownf(tsd) if the thermal shutdown temperature of 150 c is reached, a hiccup sequence is initiated where the boost converter and all io current sources are turned of. ss is discharged by a 1a current source, and a 0.9ma discharge path is turned on to pull down vout. as temperature falls below the tsd trip point, sc446 will retry when ss falls below 0.5v and vout falls to vin. pwmfdimming 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 of 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 C or counter, that varies the pulse-width on a gpio pin. the sc446 allows dimming over two decades in frequency (50hzC50khz), 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 luminescent efciency and color purity. furthermore, advanced lighting efects such as backlight dim-on can be implemented as the sc446 can resolve 10s (minimum), pwm dimming pulse-width. as far as the maximum pwm dimming pulse-width, it depends on the pwm dimming frequency. clearly, it is trivial to get 100% led brightness by pulling pwm pin high constantly. when the user tries to dim the led brightness using pwm signal from 100% down, he or she needs to observe the following. when the pwm dimming signal is actively switching from high to low and to high, there is a minimal off time (t_of_min, 200ns, guaranteed by design) requirement of the pwm dimming signal with this ic. such minimal off time sets the maximum pwm duty ratio before hitting to 100% in the following way. for example, if the pwm dimming frequency f_ pwm=200hz, the d_max=99.996%. if f_pwm=25khz, the d_max=99.5%. with most practical dimming interfaces, the needed dimming steps and resolutions, it is uncommon to run into the above d_max before reaching 100%. while most applications will not run into d_max, the designer should be aware of possible parasitic pwm off f t d min _ max 1 ? =
sc446 14 ? 2010 semtech corporation www.semtech.com applicationsfinformationf(continued) elements from pwm dimming interface to the pwm pin of sc446. usually, simply checking signal d_max at pwm pin of sc446 is sufcient. linearfdimming the linear dimming control is available for sc446 by applying an external control voltage on ioset pin through an external resistor-like circuit (shown below). external environment brightness compensation can also be achieved when the control voltage is generated by a light sensor circuit. the ioset voltage is 0.5v when linear dimming is used and the minimum ioset current must be higher than 27a (i.e. 15ma per led string). the external control voltage slew rate must slow at 1v/10ms. ledfstringsfconnection generally, led strings are connected to io1 ~ io3 pins through a mechanical connector which, generally, cannot support an electrical connection thereby resulting in sig - nifcant noise. consequently, the sc446 led short-circuit protection may false trip when the noise level is large. certain ceramic decoupling capacitor on pins io1 ~ io3 to gnd are useful to prevent the sc446 from noise infuence. as a general guideline, the decoupling capacitance should be limited as follows. where, i_led is the led current per string, vo is the boost output voltage and c_dcple is the suggested decoupling capacitor value. for example, if i_led=10ma, vo=13.5v, the calculated upper bound of c_dcple is about 444pf. one could use 390pf or less in the circuit. if i_led=100ma, vo=13.5v, the calculated upper bound of c_dcple is about 4.44nf. one may use 3.9nf or less in the circuit. in some applica - tions, circuit designers tend to select the decoupling ca - pacitors in the range of (100pf ~ 1nf). 5 5 4 4 3 3 2 2 1 1 d d c c b b a a v_ext r_ext r_ioset ioset o led dcple v us i c 6 . 0 * < for some low led current (e.g. 10ma) applications, it is recommended to add 1m-10mohm resistor from io pin to gnd in order to reduce io pin voltage during pwm dimming. parallelfoperation when two or more sc446s are operating in parallel for a large-sized panel application, audible noise may be ob - served due to non-synchronous switching frequency. the ripple voltage on the input voltage rail will be modulat - ed by the beat frequency resulting in audible noise. this situation can be resolved by adding an input inductor between input voltage rail and the sc446 vin pin. this situation can also be improved by adding more input de - coupling capacitors. inductorfselection the inductance value of the inductor afects the convert - ers steady state operation, transient response, and its loop stability. special attention needs to be paid to three specifcations 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 work - ing conditions. the inductor dc current or input current can be calculated as, 9 , 9 , ,1 287 287 ,1 ? ? i in - input current; i out C output current; v out C boost output voltage; v in C input voltage; C efciency of the boost converter. then the duty ratio is, ' 287 ' ,1 287 9 9 9 9 9 ' v d C 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.
sc446 15 ? 2010 semtech corporation www.semtech.com the inductor peak current is, the converter will work in ccm if l > l boundary . generally the converter has higher efciency under ccm and the inductor peak current is, / ) ' 9 , , 6 ,1 ,1 shdn / ? ? ? for many applications, an inductor with value of 4.7h to 22h should be fne, 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 specifcations. 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 signifcant role for the total efciency since the power transistor is integrated inside the sc446. of course, there is a trade-of between the dcr and inductor size. table 2 lists recommended inductors and their vendors. tablef2.ffrecommendedfinductors outputfcapacitorfselection the next task in sc446 design is targeting the proper amount of ripple voltage due to the constant-current led loads. the two error amplifers 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 fxed 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 simplifed 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 C peak to peak output ripple; i out C output current; v out C boost output voltage; v in C input voltage; f s C 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. tablef3.ffrecommendedfceramicfcapacitor 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 outputfrectifyingfdiodefselection schottky diodes are the ideal choice for sc446 due to their low forward voltage drop and fast switching speed. table 4 shows several diferent schottky diodes that work well with the sc446. 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 of. a diode of 2a will be sufcient for most designs. tablef4.ffrecommendedfrectifyingfdiodes part vendor ss23 ss24 vishay www.vishay.com l f d v i s in peak l ? ? = ? 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 applicationsfinformationf(continued)
sc446 16 ? 2010 semtech corporation www.semtech.com layoutfguidelines the sc446 contains a boost converter and the placements of the power components outside the sc446 should fol - low the layout guidelines of a general boost converter. the evaluation application circuit on page 17 will be used as an example. c2 and c3 are input decoupling capacitor for sc446 vin pin and main power input. c2,c3 should be placed as close as possible to the vin pin to achieve the best decoupling performance. to minimize the switching noise, the switching loop formed by input decoupling capacitors, internal switch, output schottky diode and output capacitors must be minimized. the led current programming resistor(r6), compensation network (r9,c5,c7) and soft start capaci - tor (c6) should be placed as close as possible to sc446. the c14~c17 are decoupling capacitors for led cur - rent source which prevent io pins from noise infuence. c14~c17 should be placed close to each corresponding io pin. use an isolated local agnd plane underneath the con - troller and tie it to the negative side of output capacitor through r14 for better noise immunity. applicationsfinformationf(continued)
sc446 17 ? 2010 semtech corporation www.semtech.com evaluationfapplicationfcircuit item reference quantity description package part vendor 1 c1 1 25v ceramic capacitor, x7r sm_0603 4.7nf panasonic 2 c14, c15, c16 3 50v ceramic capacitor, x7r sm_0603 1nf panasonic 3 c2 1 25v ceramic capacitor, x5r sm_1206 10uf panasonic 4 c8, c9 2 50v ceramic capacitor, x5r sm_1206 4.7uf panasonic 5 c4 1 25v ceramic capacitor, x5r sm_0805 2.2uf panasonic 6 c5 1 6.3v ceramic capacitor, x7r sm_0603 22nf panasonic 7 c6 1 6.3v ceramic capacitor, x7r sm_0603 100nf panasonic 8 d1 1 60v, 2a schottky diode sma b260a diodes or any 9 l1 1 6.8h, 3.67a dr74 6.8h copper or any 10 r1, r14 2 1% smd resistor sm_0603 0r any 11 r2, r3, r4, r7 4 5% smd resistor sm_0603 10k any 12 r5 1 1% smd resistor sm_0603 243k any 13 r6 1 1% smd resistor sm_0603 rioset any 14 r9 1 5% smd resistor sm_0603 1.5k any 15 r8, r10, r12, r13 4 5% smd resistor sm_0603 1r any 16 u1 1 controller edp tssop-16 sc446 semtech 5 5 4 4 3 3 2 2 1 1 d d c c b b a a pwm en io1 io3 io2 vin vout vin vout vout c8 4.7uf/50v c8 4.7uf/50v l1 6.8uh l1 6.8uh r13 1r r13 1r r2 10k r2 10k c9 4.7uf/50v c9 4.7uf/50v r7 10k r7 10k c14 1nf c14 1nf c6 100nf c6 100nf c1 4.7nf c1 4.7nf r4 10k r4 10k r1 0r r1 0r c2 10uf/25v c2 10uf/25v u1 sc446 u1 sc446 io2 16 io1 1 agnd 2 ss 3 comp 4 en 5 ioset 6 pwm 7 vout 8 vin 9 ovprtn 10 ovpin 11 pgnd 12 sw 13 pgnd 14 io3 15 edp 17 r8 1r r8 1r r12 1r r12 1r c7 n.p. c7 n.p. r9 1.5k r9 1.5k r6 rioset r6 rioset c16 1nf c16 1nf c15 1nf c15 1nf d1 d1 c5 22nf c5 22nf r5 243k r5 243k r14 0r r14 0r c3 n.p. c3 n.p. c4 2.2uf/25v c4 2.2uf/25v r10 1r r10 1r evaluationfboardfbillfoffmaterials
sc446 18 ? 2010 semtech corporation www.semtech.com outlinefdrawingf-ftssop-16fedp ? 2010 semtech corporation
sc446 19 ? 2010 semtech corporation www.semtech.com landfpatternf-tssop-16fedp semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phonef (805) 498-2111 faxf (805) 498-3804 www.semtech.com contact information

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