HT7L4811 datasheet(9/12 Pages) HOLTEK

9 / 12 page

Rev. 1.30

June 26, 2015

HT7L4811

Boundary Conduction Mode – BCM

ThepowerMOSFETisturnedonbyinductorcurrent

zero-crossingdetection.Thecurrentzero-crossing

canbedetectedbyaZCDvoltage.Whentheinductor

currentisatthezerocrossingpoint,thevoltage

ontheZCDpinwilldroprapidly.TheHT7L4811

thendetectsthefallingedgeandturnsonthePower

MOSFET.Theboundaryconductionmodeprovides

lowturn-onswitchinglossesandhighconversion

efficiency.

Zero Current Detection – ZCD

TheZCDvoltageisdesignedtooperatebetween0V

and3Vfornormaloperation.Ifthevoltageonthe

ZCDpingoeshigherthan1.5V,theZCDcomparator

waitsuntilthevoltagegoesbelow1V.Whenthe

inductorcurrentisatthezerocrossingpoint,the

voltageontheZCDpinwilldroprapidly.Thedevice

willthendetectthe1Vfallingedgeandturnonthe

PowerMOSFET.The0.5Vhysteresisavoidsanyfalse

triggeringactionsduetonoise.

Constant Current Control

The HT7L4811 will sense the overall inductor

currentandformaclosed-loopwithaninternalerror

amplifiertoobtainhighconstantcurrentaccuracy.

TheCSvoltageandthe0.2Vreferencevoltagearethe

inputsofaGmamplifierwhoseoutputisintegrated

viaanexternalCOMPcapacitor.TheONtimeofthe

MOSFETiscontrolledbytheCOMPvoltagetoadjust

theoutputcurrent.

LEB on CS – Leading-Edge Blanking

EachtimetheexternalpowerMOSFETisswitched

on,aturn-onspikewillinevitablyoccuratthesense

resistor.Toavoidfaultytriggering,a400nsleading-

edgeblanktimeisgenerated.Asthisfunctionis

provided conventional RC filtering is therefore

unnecessary.Duringthisblankingperiod,thecurrent-

limitcomparatorisdisabledandcanthereforenot

switchoffthegatedriver.

Gate Driver Clamp

TheDRVpinisconnectedtothegateofexternal

MOSFETtocontrolitsON/OFFfunction.Toprotect

the external power MOSFET from being over-

stressed,thegatedriveroutputisclampedto17V.

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

TheVZCDHistheupperclampvoltage3VontheZCD

pin.TheIOVPrepresentstheOVPcurrentlevelonthe

ZCDpinwhichis300μA.TheRZCDstandsforthe

resistorconnectedbetweentheZCDpinandtheLED

positiveterminal.

OCP – Over Current Protection

TheHT7L4811includesanovercurrentprotection

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,

thethermalprotectionfunctionwillturnofftheDRV

terminal immediately.When theVCC decreases

belowtheUVLOofflevel,thecontrollerwillreset.

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