HT7327-2 datasheet(9/16 Pages) HOLTEK

9 / 16 page

Rev. 1.00

August 26, 2015

HT73xx-2

Application Information

Thedevicesarea3-terminallowdropoutseries

linearvoltageregulators.Itisimportantthefollowing

applicationpointsarenotedifcorrectoperationisto

beachieved.

External Circuit

Itisimportantthatexternalcapacitorsareconnected

toboththeinputandoutputpins.Fortheinputpin

suitablebypasscapacitorsasshownintheapplication

circuitsshouldbeconnectedespeciallyinsituations

whereabatterypowersourceisusedwhichmayhave

ahigherimpedence.Fortheoutputpin,asuitable

capacitorshouldalsobeconnectedespeciallyin

situationswheretheloadisofatransientnature,in

whichcaselargercapacitorvaluesshouldbeselected

tolimitanyoutputtransientvoltages.

Thermal Considerations

Themaximumpowerdissipationdependsonthe

thermalresistanceoftheICpackage,thePCBlayout,

therateofthesurroundingairflowandthedifference

betweenthejunctionandambienttemperature.The

maximumpowerdissipationcanbecalculatedbythe

followingformula:

PD(MAX)=(TJ(MAX)–Ta)/θJA

whereTJ(MAX)isthemaximumjunctiontemperature,

TaistheambienttemperatureandθJAisthejunction-

to-ambientthermalresistanceoftheICpackagein

degreesperwatt.ThefollowingtableshowstheθJA

valuesforvariouspackagetypes.

Package

θJA value oC/W

SOT89-3

200 oC/W

TO-92

200 oC/W

8SOP-EP

125 oC/W

For maximum operating rating conditions, the

maximumjunctiontemperatureis150°C.However,

it is recommended that the maximum junction

temperaturedoesnotexceed125°Cduringnormal

operationtomaintainanadequatemarginfordevice

reliability.Thederatingcurvesofdifferentpackages

formaximumpowerdissipationareasfollows:

100

125

150

0.2

0.4

0.6

0.8

1.0

SOT89-3, TO-92

Ambient Temperature (oC)

8SOP-EP

0.8W

0.5W

Power Dissipation Calculation

Inordertokeepthedevicewithinitsoperatinglimits

andtomaintainaregulatedoutputvoltage,thepower

dissipationofthedevice,givenbyPD,mustnot

exceedtheMaximumPowerDissipation,givenby

PD(MAX).ThereforePD≤PD(MAX).Fromthediagramit

canbeseenthatalmostallofthispowerisgenerated

across the pass transistorwhich is actinglikea

variableresistorinserieswiththeloadtokeepthe

outputvoltageconstant.Thisgeneratedpowerwhich

willappearasheat,mustneverallowthedeviceto

exceeditsmaximumjunctiontemperature.

Vref

VIN

GND

OUT

Vfb

VIN

Common

VOUT

IIN

Common

ILOAD

Inpracticalapplicationstheregulatormaybecalled

upon to provide both steady state and transient

currents due to the transient nature of the load.

Althoughthedevicemaybeworkingwellwithinits

limitswithitssteadystatecurrent,caremustbetaken

withtransientloadswhichmaycausethecurrentto

riseclosetoitsmaximumcurrentvalue.Caremust

betakenwithtransientloadsandcurrentsasthiswill

resultindevicejunctiontemperatureriseswhichmust

notexceedthemaximumjunctiontemperature.With

bothsteadystateandtransientcurrents,theimportant

currenttoconsideristheaverageormoreprecisely

theRMScurrentwhichisthevalueofcurrentthatwill

appearasheatgeneratedinthedevice.Thefollowing

diagramshowshowtheaveragecurrentrelatestothe

transientcurrents.

Time

ILOAD

ILOAD(AVG)

Asthequiescentcurrentofthedeviceisverysmallit

cangenerallybeignoredandasaresulttheinputcur-

rentcanbeassumedtobeequaltotheoutputcurrent.

Thereforethepowerdissipationofthedevice,PD,can

becalculatedasthevoltagedropacrosstheinputand

outputmultipliedbythecurrent,givenbytheequation,

PD=(VIN–VOUT)×IIN.Astheinputcurrentisalso

equaltotheloadcurrentthepowerdissipationPD=

(VIN–VOUT)×ILOAD.However,withtransientloadcur-

rents,PD=(VIN–VOUT)×ILOAD(AVG)asshowninthe

figure.

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