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 2005SemtechCorp.

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POWER MANAGEMENT

SC635

PinConfiguration

ApplicationsInformation

GeneralOperation

The SC635 is a powerful 2X charge pump designed to

driveahigh-intensitywhiteLEDwithaconstantcurrent

(Spotlightmode)orwithapulsedcurrentofhigherinten-

sity(Flashmode)usedforcameraflash.

In Flash mode the SC635 output can drive 200mA at

4.25V into an LED for an unlimited duration under all

temperatureandinputconditions.TheFLASHpinisused

totriggerthismode(activehigh).InSpotlightmodethe

SC635regulatestheoutputcurrentto40%oftheFlash

mode current setting when the CTRL pin is pulled high

andtheFLASHpinislow.NotethatFlashmodehasprior-

ityoverSpotlightmode,i.e.,theFLASHpintriggersFlash

moderegardlessofthestateoftheCTRLpin.

OutputcurrentisregulatedbyattachingtheISETpinto

thecathode(s)oftheLEDpackageandalowresistance

sense resistor (typically 1.25Ω).The ISET pin monitors

thevoltageatthecathodeandsignalsthechargepump

toincreaseordecreasetheoutputcurrentuntiltheISET

voltage reaches the programmed setting.The resistor

valueischosentosetthecurrentthroughtheLEDbased

onthisreferencevoltage.

FlashMode

FlashmodeisenabledwhenevertheFLASHpinispulled

highandremainsactiveuntiltheFLASHpinisreleased.

ThismodehashigherprioritythanSpotlightmode,sothe

stateoftheCTRLpinisoverriddenwhenevertheFLASH

pinisactivated.WhileinFlashmodethereferencevolt-

ageontheISETpinissetto250mV.

Outputcurrentsotherthantheratedmaximumof200mA

canbesetbychangingtheR

IOUT=250mV/R

SpotlightMode

SpotlightmodeisenabledbysettingtheCTRLpinhigh

andkeepingtheFLASHpinlow.WheninSpotlightmode,

theSC635canmaintainaconstantcurrentindefinitelyto

driveanLEDorbankofLEDs.TheISETreferencevoltage

issetto00mVsothattheoutputcurrentismaintained

at80mAwhena1.25Ωresistorisused.Spotlightcurrent

isalways40%offlashcurrentforanygivenvalueofR

Inspotlightmode,IOUT=100mV/R

ueshouldnotexceed2Ω,however,toensuretheSC635

doesnotbecomeunstablewhileinSpotlightmodedueto

theoutputcurrentbeingsettoolow.

ProtectionCircuitry

TheSC635alsoprovidesprotectioncircuitrythatprevents

thedevicefromoperatinginanunspecifiedstate.These

functions include Input Over-Voltage Protection (IOVP),

Output Over-Voltage Protection (OVP), Over-Temperature

(OT)Protection,Over-CurrentProtection(OCP),andShort-

CircuitCurrentProtection(SCCP).

Input Over-Voltage Protection

Inputover-voltageprotectionisincludedtopreventopera-

tionathighinputvoltagesthatcoulddamagethedevice.

TheIOVPcircuitsensestheinputvoltageanddetermines

whenthesupplyexceeds6V.Hysteresisisincludedinthis

circuittoavoidchatteringbetweenstates.Whenthevolt-

agerisesabovethisthreshold,thedeviceisdisabledun-

tiltheinputvoltagedropstoalevelwithinthespecified

range.

Output Over-Voltage Protection

Outputover-voltageprotectionisincludedtopreventthe

SC635fromgeneratinganoutputvoltagethatcoulddam-

ageotherdevicesconnectedtoitsuchasloadLEDsand

bypasscapacitors.Whentheoutputvoltageexceeds5.5V,

theOVPcircuitdisablesthechargepumpuntilthevoltage

decreases to a level within the acceptable range.This

circuitallowsthedevicetodriveLEDswithhighforward

voltagesatareducedlevelwithoutexceedingtheoutput

voltagelimitsspecifiedforthedevice.Note,however,that

thiseffectisaconsequenceoftheOVPcircuitandisnot

itsintendedpurpose.

Over-Temperature Protection

The over-temperature circuit helps prevent the device

from overheating and experiencing a catastrophic fail-

ure.Whenthejunctiontemperatureexceeds150°Cthe

deviceisdisabled.Itremainsdisableduntilthejunction

temperaturedropsbelowthisthreshold.AswiththeUVLO

andOVPcircuits,hysteresisisincludedtopreventtoggling

betweenmodes.

Over-Current Protection

When the SC635 is in 2X mode, the input current will

beapproximatelydoubletherequiredoutput.Whenthe

steady-stateloadrequiresthemaximumcurrentavailable

in2Xmode,theOCPcircuitpreventsthedevicefromover-

heatingduetoexcessivepowerdissipation.