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HT7L4815 データシート(PDF) 6 Page - Holtek Semiconductor Inc |
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HT7L4815 データシート(HTML) 6 Page - Holtek Semiconductor Inc |
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6 / 9 page  Rev. 1.00 6 June 26, 2015 HT7L4815 Preliminary Constant Current Control The HT7L4815 will sense the overall inductor currentandformaclosed-loopwithaninternalerror amplifiertoobtainhighconstantcurrentaccuracy. TheCSvoltageandthe0.2Vreferencevoltageform theinputstoaninternalGmamplifierwhoseoutput isintegratedviaanexternalCOMPcapacitor.The ONtimeoftheMOSFETiscontrolledbytheCOMP voltagetoadjusttheoutputcurrent. LEB on CS – Leading-Edge Blanking EachtimetheinternalpowerMOSFETisswitched on,aturn-onspikewillinevitablyoccuratthesense resistor.Toavoidfaultytriggering,a400nsleading- edgeblanktimeisgenerated.Asthisfunctionis provided conventional RC filtering is therefore unnecessary.Duringthisblankingperiod,thecurrent- limitcomparatorisdisabledandcanthereforenot switchoffthegatedriver. OVP on VCC – Over Voltage Protection InordertopreventPWMcontrollerdamage,the deviceincludesanOVPfunctiononVCC.Should theVCCvoltagebehigherthantheOVPthreshold voltage of 29V, the PWM controller will stop operating immediately.When theVCC voltage decreasesbelowtheUVLOofflevel,thecontroller willreset. LED Open Protection – ZCD OVP The LED voltage is reflected on the ZCD pin througharesistorRZCD.Whenthecurrentonthe resistorRZCDishigherthan300μA,thenZCDOVP protectionwilltakeplace.HerethePWMcontroller willstopoperatingimmediately.WhentheVCC voltagedecreasesbelowtheUVLOofflevel,the controllerwillreset. VOVP-ZCDcanbesetusingthefollowingequation: VOVP-ZCD=VZCDH+IOVP×RZCD VZCDHisthe3VupperclampvoltageontheZCD pin.IOVPrepresentstheOVPcurrentlevelontheZCD pinwhichis300μA.RZCDstandsfortheresistorcon- nectedbetweentheZCDpinandtheLEDpositive terminal. OCP – Over Current Protection The device includes an over current protection functionontheCSpin.Aninternalcircuitdetectsthe currentlevelandwhenthecurrentislargerthanthe overcurrentprotectionthresholdlevel,VOCP/RCS,the gateoutputwillremainatalowlevel. LED Short Protection – SCP TheoutputvoltagedropswhenanumberofLEDs inastringareshortedresultinginavoltagedropat VCC.OncetheVCCdropsbelow10V,thedevice willstopoperating.Undersuchsituations,thestart-up operationwillrechargetheVCCpinthroughthestart- upresistorandthedevicewillentertheUVLOhiccup mode. Thermal Protection Athermalprotectionfeatureisincludedtoprotect thedevicefromexcessiveheatdamage.Whenthe junctiontemperatureexceedsathresholdof150°C, thethermalprotectionfunctionwillturnoffthe DRAIN terminal immediately. When the VCC decreasesbelowtheUVLOofflevel,thecontroller willreset. |
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