HT7938A_13 datasheet(4/13 Pages) HOLTEK

4 / 13 page

Rev. 1.10

February 01, 2013

HT7938A

Function Description

VIN Under-Voltage Lockout -- UVLO

ThedevicecontainsanInputUnderVoltageLockout

(UVLO)circuit.ThepurposeoftheUVLOcircuit

istoensurethattheinputvoltageishighenoughfor

reliableoperation.Whentheinputvoltagefallsbelow

theundervoltagethreshold,theinternalFETswitch

isturnedoff.Iftheinputvoltagerisesbytheunder

voltagelockouthysteresis,thedevicewillrestart.

TheUVLOthresholdissetbelowtheminimuminput

voltageof2.6VtoavoidanytransientVINdrops

undertheUVLOthresholdandcausingtheconverter

toturnoff.

Current Limit Protection

Thedevicehasacycle-by-cyclecurrentlimitto

protecttheinternalpowerMOSFET.Iftheinductor

current reaches the current limit threshold, the

MOSFETwillbeturnedoff.Itisimportanttonote

thatthiscurrentlimitwillnotprotecttheoutputfrom

excessivecurrentduringanoutputshortcircuit.Ifan

outputshortcircuithasoccurred,excessivecurrent

candamageboththeinductoranddiode.

Over-Voltage Protection -- OVP

The device provides an over-voltage protection

function.IftheFBpinisshortedtogroundoranLED

isdisconnectedfromthecircuit,theFBpinvoltage

willfalltozeroandtheinternalpowerMOSFETwill

switchwithitsfulldutycycle.Thismaycausethe

outputvoltagetoexceeditsmaximumvoltagerating,

possiblydamagingtheICandexternalcomponents.

Internalover-voltageprotectioncircuitryturnsoffthe

powerMOSFETandshutsdowntheICassoonasthe

outputvoltageexceedstheOVPthreshold.Asaresult,

theoutputvoltagefallstotheleveloftheinputsupply

voltage.Thedeviceremainsinshutdownmodeuntil

thepowerisrestarted.

Over-Temperature Protection -- OTP

A thermal shutdown is implemented to prevent

damagesduetoexcessiveheatandpowerdissipation.

Typicallythethermalshutdownthresholdis150°C.

Whenthethermalshutdownistriggeredthedevice

stopsswitchinguntilthetemperaturefallsbelow

typically125°C.Thenthedevicestartsswitching

again.

Application Information

Inductor Selection

Theselectionoftheinductoraffectssteadystate

operationaswellastransientbehaviorandloop

stability. There are three important electrical

parameters which need to be considered when

choosinganinductor:thevalueofinductor,DCR

(copperwireresistance)andthesaturationcurrent.

Chooseaninductorthatcanhandlethenecessary

peakcurrentwithoutsaturating,andensurethatthe

inductorhasalowDCRtominimisepowerlosses.

A10mH/22mHinductorshouldbeagoodchoicefor

mostHT7938Aapplications.However,amoreexact

inductancevaluecanbecalculated.Agoodrulefor

choosinganinductorvalueistoallowthepeak-to-

peakripplecurrenttobeapproximately30~50%of

themaximuminputcurrent.Calculatetherequired

inductancevalueusingthefollowingequation:

OU T

)

(

I =

(

´ IN (M AX )

IN (M AX )

L( PEAK )

´h

OU T

´ OU T( MA X)

IN (M AX )

Intheequation above,IOUT(MAX)isthemaximum

loadcurrent,DIListhepeak-to-peakinductorripple

current, ηistheconverter efficiency, FSWisthe

converterfrequencyandIL(PEAK)isthepeakinductor

current.

Output Capacitor Selection

Theoutputcapacitordeterminesthesteadystate

outputvoltageripple.Thevoltagerippleisrelatedto

thecapacitor'scapacitanceanditsESR(Equivalent

SeriesResistance).Aceramiccapacitorwithalow

ESRvaluewillprovidethelowestvoltagerippleand

arethereforerecommended.DuetoitslowESR,the

capacitancevaluecanbecalculatedbytheequation:

out

ri pp le

OU T

)

(

Intheequationabove,Vripple=peaktopeakoutput

ripple,FSWistheswitchingfrequency.

A2.2mF~10mFceramiccapacitorissuitableformost

application.

Input Capacitor Selection

Aninputcapacitorisrequiredtosupplytheripple

currenttotheinductor,whilelimitingnoiseatthe

input source.A low ESR ceramic capacitors is

requiredtokeepthenoiseattheICtoaminimum.

A1mF~10mFceramiccapacitorissuitableformost

application.Thiscapacitormustbeconnectedvery

closetotheVINpinandinductor,withshorttraces

forgoodnoiseperformance.

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