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參數(shù)資料
| 型號： | SC635EVB |
| 廠商： | Semtech Corporation |
| 英文描述： | Charge Pump Flash LED Driver |
| 中文描述： | 電荷泵閃光LED驅(qū)動器 |
| 文件頁數(shù)： | 5/12頁 |
| 文件大?。?/td> | 357K |
| 代理商： | SC635EVB |
5
2005SemtechCorp.
www.semtech.com
POWER MANAGEMENT
ApplicationsInformation
GeneralOperation
TheSC635isapowerful2Xchargepumpdesignedto
driveahigh-intensitywhiteLEDwithaconstantcurrent
(Spotlightmode)orwithapulsedcurrentofhigherinten
-
sity(Flashmode)usedforcameraflash.
SC635
InFlashmodetheSC635outputcandrive200mAat
4.25VintoanLEDforanunlimiteddurationunderall
temperatureandinputconditions.TheFLASHpinisused
totriggerthismode(activehigh).InSpotlightmodethe
SC635regulatestheoutputcurrentto40%oftheFlash
modecurrentsettingwhentheCTRLpinispulledhigh
andtheFLASHpinislow.NotethatFlashmodehasprior
-
ityoverSpotlightmode,i.e.,theFLASHpintriggersFlash
moderegardlessofthestateoftheCTRLpin.
OutputcurrentisregulatedbyattachingtheISETpinto
thecathode(s)oftheLEDpackageandalowresistance
senseresistor(typically1.25Ω).TheISETpinmonitors
thevoltageatthecathodeandsignalsthechargepump
toincreaseordecreasetheoutputcurrentuntiltheISET
voltagereachestheprogrammedsetting.Theresistor
valueischosentosetthecurrentthroughtheLEDbased
onthisreferencevoltage.
FlashMode
FlashmodeisenabledwhenevertheFLASHpinispulled
highandremainsactiveuntiltheFLASHpinisreleased.
ThismodehashigherprioritythanSpotlightmode,sothe
stateoftheCTRLpinisoverriddenwhenevertheFLASH
pinisactivated.WhileinFlashmodethereferencevolt-
ageontheISETpinissetto250mV.
Outputcurrentsotherthantheratedmaximumof200mA
canbesetbychangingtheR
ISET
value.InFlashmode,
IOUT=250mV/R
ISET
.
SpotlightMode
SpotlightmodeisenabledbysettingtheCTRLpinhigh
andkeepingtheFLASHpinlow.WheninSpotlightmode,
theSC635canmaintainaconstantcurrentindefinitelyto
driveanLEDorbankofLEDs.TheISETreferencevoltage
issetto00mVsothattheoutputcurrentismaintained
at80mAwhena1.25Ωresistorisused.Spotlightcurrent
isalways40%offlashcurrentforanygivenvalueofR
.
Inspotlightmode,IOUT=100mV/R
.Theresistorval-
ueshouldnotexceed2,however,toensuretheSC635
doesnotbecomeunstablewhileinSpotlightmodedueto
theoutputcurrentbeingsettoolow.
ProtectionCircuitry
TheSC635alsoprovidesprotectioncircuitrythatprevents
thedevicefromoperatinginanunspecifiedstate.These
functions include Input Over-Voltage Protection (IOVP),
OutputOver-VoltageProtection(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
decreasestoalevelwithintheacceptablerange.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
WhentheSC635isin2Xmode,theinputcurrentwill
beapproximatelydoubletherequiredoutput.Whenthe
steady-stateloadrequiresthemaximumcurrentavailable
in2Xmode,theOCPcircuitpreventsthedevicefromover
-
heatingduetoexcessivepowerdissipation.
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