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HT7L4091 Datasheet(PDF) 8 Page - Holtek Semiconductor Inc |
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HT7L4091 Datasheet(HTML) 8 Page - Holtek Semiconductor Inc |
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8 / 17 page  Rev. 1.20 8 March 12, 2013 HT7L4091 Then,theminimumcapacitorvaluecanbecalculated as: ( ( ) ) ( ) ( ) F µ ms mA V V V T I V n C DC AC e disch LED LED η 9 . 15 7 . 0 95 2 95 2 9 . 0 2232 . 6 ) 400 3 . 3 8 2 ( 2 ) 2 ( 1 2 2 2 _min 2 _min arg _max = × × − × × × × × × = − × × × × × × ≥ Choose C1=22μF Considering±20%capacitancevariation,theworst caselowervalueofthecapacitanceis17.6μF,which ismuchlargerthan14.3μF.Itcanbecalculatedthat theinputDCrippleis24.5%whentheinputBuck capacitoris17.6μF. Therefore,iftherealcapacitorvalueislessthan thecalculatedvalue,thevoltageripplewillexceed themaximumrangeof30%whichisthespecified assumptioninthecalculation. Switching Frequency and Duty Cycle Frequencyinterferenceshouldbetakenintoaccountto minimiseinterferencewithotherelectricalappliances. Heresettheminimumswitchingfrequencytoavalue of40kHzforsafety.IfEMIsuppressionisgood,the switchingfrequencycanbedecreasedto30kHzto obtainbetterefficiency. SincetheHT7L4091operatesinconstantofftime,the switchingfrequencywouldbechangedbytheinput andoutputvoltage.Theslowestswitchingfrequency occurswhenthedutycycleisatamaximumvalue. Themaximumdutycyclecanbecalculatedas, ( ) ( ) 2731 . 0 % 5 . 24 1 95 2 3 . 1 8 3 . 3 1 2 _min 1 , _max min _ max _ max = − × × + × = − − ⋅ + ⋅ = = ripple AC D F LED DC O V V V V n V V D Turn-off Time s k f D T PWM off 173 . 18 40 2731 . 0 1 1 min _ max = − = − = µ Off-time Resistor – RT AresistorconnectedtotheRTpindeterminestheoff- timewhichhasavariationrangefrom-10%to+10%. Sincetheworkingfrequencyhasaminimumtarget, theCTisconsideredtocalculatedtheRT: Ω = ≤ → × = k pF s R R C T T T T off 03 . 413 44 173 . 18 µ Choose390kWand13kWforRTareused Theofftimeis: Toff_typ=CT_typ×RT=40p×403K=16.1μs Toff_max=CT_max×RT=44p×403K=17.7μs Toff_min=CT_max×RT=36p×403K=14.5μs Theactualminimumfrequencycanbecalculatedas: T D K f off PWM 07 . 41 7 . 17 2731 . 0 1 1 max _ max min _ = − = − = µ fPWM_TYP@Vac_min=45.2kHz,fPWM_MAX@Vac_min=50.1kHz Inductor Design The ripple current is selected to be 30% of the nominalLEDcurrent.IftheLEDaveragecurrent ILEDis400mA,theLEDstringVoltage=n×VLED, max =8×3.3VwhereVLED,maxistheLEDmaximum forwardvoltage,thentheinductorcanbecalculated bythefollowingformula. mH mA s Ripple I V n µ T L LED LED off 894 . 3 3 . 0 400 3 . 3 8 7 . 17 max _ = × × × = × × × = Choose L=3.8 mH |
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