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HT7939A Datasheet(PDF) 5 Page - Holtek Semiconductor Inc |
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HT7939A Datasheet(HTML) 5 Page - Holtek Semiconductor Inc |
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5 / 13 page  Rev. 1.00 5 April 19, 2012 HT7939A Input and Output Capacitor Selection Theoutputcapacitordeterminesthesteadystate outputvoltageripple.Thevoltagerippleisrelated tothecapacitor’scapacitanceanditsESRwhichis theEquivalentSeriesResistance.Ceramiccapacitors withlowESRvalueswillresultinthelowestripple voltagevaluesandaretherecommendedtype.Due totheirlowESRvalues,thecapacitancevaluecanbe calculatedusingthefollowingequation: ri ppl e SW OU T OU T IN O ou t V F V I V V C ´ ´ ´ - = ) ( Intheequationabove,Vripple=peaktopeakoutput ripple,FSWistheswitchingfrequency. Theoutputcapacitorisrecommendedtobeinarange of1μFto10μF.Theinputcapacitorisrequiredtobe inarangeof2.2μFto10.0μF.Theoutputcapacitor affectstheloopstabilityoftheboostregulator.Ifthe outputcapacitorislowerthanthisrange,theboost regulatorcouldpotentiallybecomeunstable. Schottky Diode Selection Theoutputrectifierdiodesuppliescurrenttothe inductor whentheinternal MOSFETisoff.The averageandpeakcurrentratingsofthediodemustbe greaterthanthemaximumoutputcurrentandpeak inductorcurrent.Thereversebreakdownvoltage mustbegreaterthanthemaximumoutputvoltage.It isrecommendedtouseaschottkydiodewithalow forwardvoltagetominimisepowerdissipationand thereforetomaximisetheefficiencyoftheconverter. A1N5819diodetypeisrecommendedforHT7939A applications. LED Current Selection TheLEDcurrentiscontrolledbythecurrentsense feedbackresistorRfb,Thecurrentsensefeedback referenceis200mV.InordertoensureaccurateLED currents,precisionresistorswitha1%tolerancearethe preferredtypes.TheLEDcurrentcanbecalculated usingthefollowingformula: fb FB LE D fb R R V I mV 20 0 = = WhereILEDistheoutputLEDcurrent,VFB=feedback voltage,Rfb=currentsenseresistor. Digital and Analog Dimming Control TheDigitaldimmingmethodusesaPWMsignal applied to the EN pin and the EN Pin dimming frequencyupto200kHz.Thisisshowninfig.19.The averageLEDcurrentincreasesproportionallywiththe PWMsignaldutycycle.A0%dutycyclecorresponds tozeroLEDcurrent.A100%dutycyclecorresponds toafullLEDcurrent. TherearetwomethodstocontroltheLEDbrightness foranalogdimming.ThefirstmethodusesaDC voltagetocontrolthefeedbackvoltage.IftheDC voltagerangeisfrom0Vto3.3V,theselectionof resistorsprovidedinfig.20controlstheLEDcurrent from20mAto0mA.Otherapplicationsafiltered PWMsignal,whichisshowninfig.21.Thefiltered PWMsignalapplicationactsthesamewayastheDC voltagedimmingcontrol. |
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