HT7466 datasheet(4/9 Pages) HOLTEK

4 / 9 page

Rev. 1.00

September 02, 2011

HT7466

Functional Description

Setting the Output Voltage

The HT7466 is a step down switching DC/DC

converterdevice.Thedeviceswitchingoutput,SW,is

connectedtoastandardconverterLCfiltercircuitfor

buckconversion.Theoutputcurrentissensedusing

anexternalvoltagedividernetworkconnectedtothe

loadoutputvoltageandground,andthesensevoltage

feedbacktothedeviceontheFBpin.Bycomparing

this sense voltage with an internally generated

1.222voltagereference,suitableregulationcanbe

implementedtoachievetherequiredoutputvoltage.

By selectingsuitablevalues of externalvoltage

dividerresistors,thedesiredoutputvoltagecanbe

adjustedtotherequiredlevel.Theoutputvoltagecan

becalculatedusingthefollowingformula:

R 1 + R 2

R 2

V OU T =

´ 1. 222

WhereR2istheresistordividerlowerresistorandR1

istheupperresistor,andtheR1,R2mustbekWlevel.

Compensation Components

Care must be taken when selecting external

components.TheCOMPpinistheoutputofthe

erroramplifierandmustbeproperlyconnectedtoan

externalRCnetworktoensureregulatorloopstability.

Thevaluesofthecompensationcomponentsgiven

inTable1yieldastablecontrolloopfortheoutput

voltageandcapacitorgiven.

Table1CompensationValuesforTypicalOutput

Voltage/CapacitorCombinations.

VOUT

2.5V

15mH 3.9kW 5.6nF 10.5kW 10kW 22mF Ceramic

—

3.3V

15mH 4.7kW 4.7nF 17.0kW 10kW 22mF Ceramic

—

5.0V

15mH 7.5kW 2.7nF 30.9kW 10kW 22mF Ceramic

—

Bootstrap Circuit

Inthisdevicebootstrapcapacitorisconnectedfrom

SW to BS pin, the Step-Down converter is use

N-ChannelMOSFET,andMOSsourcepinisfloating

toground,thisdrivecircuitoftheapplicationis

calledhigh-sidecircuit,andthedrivecircuitused

inthiskindofsituationiscalledbootstrapcircuit.

Largecapacitorvaluescankeepinternalhigh-side

N-ChannelMOSFETgatevoltagegreaterthanzero

whenpowerMOSisturnon;usuallya10nFcapacitor

issufficientformostapplications.

Current Limit Protection

Thedevicehasacycle-by-cyclecurrentlimitto

protecttheinternalpowerMOSFET.Iftheinductor

currentreachesthecurrentlimitthresholdof4.3A,

theMOSFEToscillatorfrequencywillbetransformed

from380kHztoashortcircuitfrequencyof35kHz

andtheoutputcurrentwillremainatitscurrentlimit

value.

Over Temperature Protection

A thermal shutdown is implemented to prevent

damageduetoexcessiveheatandpowerdissipation.

Typicallythethermalshutdownthresholdis150°C.

Thethermalshutdownistriggeredthedevicestops

switchingandwhenthedevicethermalfallsuntilthe

temperaturehysteresiswindowsthedevicewillbe

startsswitchingagain.

Application Information

Inductor

Theinductorisrequiredtosupplyaconstantcurrent

totheoutputloadwhilebeingdrivenbytheswitched

inputvoltage.Thechoiceofinductoraffectssteady

stateoperationaswellastransientbehaviorand

loopstability.Therearethreeimportantelectrical

parameters which need to be considered when

choosinganinductor:

• Theinductorvalue

• DCR-copperwireresistance

• Thesaturationcurrent

Inductor choice is especially important as it is

importanttoensuretheinductordoesnotsaturate

under its peak current conditions, the general

ruleofkeepingtheinductorcurrentpeak-to-peak

rippleapproximately30%ofthenominaloutput

current.Usingthisvalueofripplecurrentisagood

compromisebetweenexcessiveoutputvoltageripple

andexcessivecomponentsizeandcost.Alsoensure

thattheinductorhasalowDCRtominimizepower

losses.

Input/Output Capacitor

Input Capacitor

BecausetheVinpinisconnectedtotheinternalpower

MOSFET,whenthepowerMOSFETisswitchingand

theinputcurrentisdiscontinuous,thereforeaninput

capacitorC1(inPCBlayoutC1andC8isparallels

inputcapacitor)isrequiredtosupplytheACcurrent

tothestep-downconverterwhilemaintainingthe

DCinputvoltage.AlowESRceramiccapacitoris

requiredtokeepnoisetoaminimum.

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