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HT7L4091 数据表(PDF) 9 Page - Holtek Semiconductor Inc |
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HT7L4091 数据表(HTML) 9 Page - Holtek Semiconductor Inc |
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9 / 17 page  Rev. 1.20 9 March 12, 2013 HT7L4091 Current Sense Resistor – RCS Thiscurrentflowsthroughtheexternalsenseresistor RCSandproducesarampvoltageontheCSpin.The comparatorsareconstantlycomparingtheCSpin voltagewithboththevoltageontheLDpinandthe internal250mVreferencevoltage.Oncetheblanking timehaselapsed,theoutputofthesecomparatorscan thenresettheflipflop.Whenoneoutputofthesetwo comparatorsswitcheshigh,theflipflopisresetand thegatedriveoutputswitcheslow.Thegatedrive outputstayslowuntiltheSRflipflopissetbythe oscillator.Inassuminga30%rippleintheinductor, thecurrentsenseresistorRCScanbeobtainedusing thefollowingformula: Ω = × × + = ⋅ × + = = 543 . 0 400 ) 3 . 0 5 . 0 1 ( 25 . 0 2 1 1 25 0 25 0 mA I Ripple) / ( . I . R LEDavg peak CS ChooseRCS=0.54W Input Supply Current Assumethattheinputcurrentdrawnbytheinternal circuitfromtheVINpinisthesumofthecurrent withavalueof1.0mAandthecurrentdrawnbythe gatedriveroftheexternalMOSFET(whichinturn dependsupontheswitchingfrequencyandthegate chargeoftheexternalFET).Assumethatthegate chargeQgateisequalto12nC. IINSP=IIN+Qgate×FPWM =1mA+12nC×50kHz=1.6mA Inaddition,whereIINSPistheinputcurrenttaken fromtheVINpin,FPWMistheswitchingfrequency, QgateisthegatechargeoftheexternalFETandIIN isthecurrenttakenbytheinternalcircuit.FPWMis consideredabouttheminimuminputvoltageandCT hasaminimumvalue. Input Limit Resistor (RIN) Inthisdesign,VAC_min=95Vrms,VUVLO(H)_max=17V Ω = − − × × = − ∆ − × ⋅ = k mA I V V V % R INSP H UVLO DCripple AC in 1 . 48 6 . 1 17 %) 30 1 ( 95 2 ) 1 ( 2 max _ ) ( min _ Choose Rin=60kW Theinputlimitresistorconsiderthehighinputvoltage fromtherectifiedclampvoltageoftheinternalZener diodeandoperatingcurrent.Two30kW/1Wresistors areusedforRin. Output Capacitor – CO Thecapacitor,CO,filtersthecurrentthroughtheLEDs thuslimitingthepeakcurrentoftheLEDstring. Increasingtheinductorripplecurrentcorresponds todecreasingtheinductorvalueandinductorsize. Inordertoreducetheinductorvalueandsizeand obtainasmallerLEDcurrentripple,theaddition ofacapacitorCOisagoodwaytodothis.Usually, aseveralμFoutputcapacitorisaddedinpractical applicationcircuits. AddingCOconnectedacrosstheLEDstringscan reducetheLEDcurrentrippleandwhileincreasing theinductorcurrentripplevariationcandecreasethe inductorvalueandsize. Toassumeinductorcurrentrippleis80%,asmaller inductorvaluecouldbecalculated. mH mA s Ripple I V n T L LED LED off 460 . 1 8 . 0 400 3 . 3 8 7 . 17 max _ = × × × = × × × = µ Choose L=1.4mH and CO=1μF TheactualvaluesofCOandRCSmayneedtobe adjustedtoreducethecurrentrippleandobtainthe targetaverageLEDcurrent.A1μFcapacitorandan RCSasshownintheabovecalculationareagood startpointtoobtainanacceptableresult.Sinceit takessomeeffort,itcanreducetheinductorsize/cost significantly. |
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