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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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HT7945 데이터시트(PDF) 6 Page - Holtek Semiconductor Inc |
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HT7945 데이터시트(HTML) 6 Page - Holtek Semiconductor Inc |
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6 / 11 page  Rev. 1.00 6 April 13, 2012 HT7945 LED Open Detector Protection ThedeviceincludesanLEDopenprotectionfunction. IfanyoneofISENpinsisdisconnectedfromthe LEDload,thedevicewillstopdrivingtheISEN pin,automaticallyignoringtheopenpin.TheLED currentoftheotherISENpinswillnotbeinfluenced byanyopenISENpin.WhentheopenISENpins arere-connectedtotheLEDload,therewillbeno current.TheseISENpinswillremaindisableduntil thepowerisrecycled. LED Short Detector Protection ThedeviceincludesanLEDshortcircuitprotection function.Ifmorethan2~3LEDsareshortcircuited onanyISENpinorthevoltageleveloftheISENpin isgreaterthan5.6V,thedevicewillturnoffthatISEN pinandautomaticallyignoretheshortedpin.The LEDcurrentofotherISENpinswillnotbeinfluenced byanyshortedISENpins.IfevenonlyoneISENpin remainsoperationalduetoshortsonotherpins,itwill stillmaintainnormaloperation.TheshortedISEN pinsremaindisableduntilthepowerisrecycled. Over-Temperature protection – OTP Aninternalthermalshutdownfunctionisincluded topreventdevicedamageduetoexcessiveheatand powerdissipation.Typically,thethermalshutdown thresholdofis140°C.Whenthethermalshutdown functionisactivated,thedevicestopsswitchinguntil thetemperaturefallstobelow90°Ctypically.When thisoccursthedeviceresumesswitchingonceagain. Soft Start Function Converteroperationstartsimmediatelyafterpower on.Inordertoavoidthepossibilityoflargein-rush currentstotheloadduringthispoweronperiod,a soft-startfunctionisimplementedtopreventthis problemfromoccurring. Application Information Inductor Selection Theinductorchoiceaffectssteadystateoperationas wellastransientbehaviorandloopstability.Thereare threeimportantelectricalparameterswhichneedtobe consideredwhenchoosinganinductor: • Theinductorvalue • DCR–copperwireresistance • Thesaturationcurrent Inductor choice is especially important as it is requiredtoensuretheinductordoesnotsaturate underitspeakcurrentconditions.Thegeneralrule istokeeptheinductorcurrentpeak-to-peakrippleat approximately30%ofthenominaloutputcurrent. Asatypicalexample,whenusingtheHT7943boost converter, operating in both discontinuous and continuousconductionmodes,thetypicalapplication circuitvalueoftheinductor,L1,wouldbearound 10μH. Input/Output Capacitor Output Capacitor Theoutputcapacitordeterminesthesteadystate outputvoltageripple.Inthecompensationparameters, theoutputcapacitorisoneoftheparameters,andif thecapacitanceistoobigortoosmall,itcancause systeminstability.Itsvaluemustbebasedonthe applicationcircuitrecommendedoutputcapacitor value.AlowESRceramiccapacitorisrequiredto keepnoisetoaminimum.A4.7μFceramiccapacitor issuitablefortypicalapplications. Input Capacitor Aninputcapacitorisrequiredtosupplytheripple currenttotheinductorandisalsousedtolimitthe inputnoise,allowingthedevicetoobtainastable DCpowersupply.Astheinputcapacitanceisnot a compensation parameter there are no stability problems, however a capacitor must always be connectedalongwithaninputpowersupply.For typicalapplications,a4.7μFceramiccapacitoris sufficient.Thiscapacitormustbeconnectedvery closetotheVINpinandinductor,withshorttraces forgoodnoiseperformance. |
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