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HT7991 Datasheet(PDF) 9 Page - Holtek Semiconductor Inc |
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HT7991 Datasheet(HTML) 9 Page - Holtek Semiconductor Inc |
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9 / 13 page  Rev. 1.20 9 June 01, 2017 HT7991 Component Selection Guide Inductor The selected inductor should have a saturation currentthatmeetsthemaximumpeakcurrentofthe converter.Anotherimportantinductorparameteris thedcresistance.Thelowerthedcresistancegains thehighertheefficiencyoftheconverter.Formost applications,theinductorvaluecanbecalculated fromthefollowingequation: L=()× Vin 2 (Vout−Vin) Iripple×fsw Vout Thehighervalueofripplecurrentreducesinductance, buttheconductanceloss,coreloss,andcurrentstress oftheinductorandswitchingdevicesincrease.It’s suggestedthatchoosingtheinductorripplecurrentto be30%ofthemaximumloadcurrent. Diode Thebreakdownvoltageratingofthediodeispreferred tobehigherthanthemaximuminputvoltage.The currentratingforthediodeshouldbeequaltothe maximumoutputcurrentforbestreliabilityinmost applications.Inthiscase,itispossibletouseadiode withaloweraveragecurrent.Howeverthepeak currentshouldbehigherthanthemaximumload current. Input Capacitor AlowESRceramiccapacitorisneededbetweenthe VINpinandGNDpin.Useceramiccapacitorswith X5RorX7RdielectricsforitslowESRandsmall temperaturecoefficients.Formostapplications,the capacitanceintherangeof2.2μFto10μFcapacitoris sufficient. Output Capacitor Theselectionofoutputcapacitorisdrivenbythe maximumallowableoutputvoltageripple.Using ceramiccapacitorswithX5RorX7Rdielectricsforits lowESRcharacteristicissuggested.Thecapacitance intherangeof10μFto100μFissufficient. Recommended Component Values VOUT R3 L1 D1 C1 47µF C2 0.1µF OFF ON VIN R1 R2 VIN EN OC LX FB GND 5 4 6 1 3 2 HT7991 C3 47µF C4 0.1µF VOUT (V) R1 (kΩ) R2 (kΩ) C3 (μF) C4 (μF) L1 (μH) 5.1 75 (1%) 10 (1%) 47 0.1 3.3 12.0 190 (1%) 10 (1%) 47 0.1 6.8 |
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