LM3670MiniatureStep-DownDC-DCConverterforUltraLowVoltageCircuitsJanuary2006
LM3670
MiniatureStep-DownDC-DCConverterforUltraLowVoltageCircuits
GeneralDescription
TheLM3670step-downDC-DCconverterisoptimizedforpoweringultra-lowvoltagecircuitsfromasingleLi-Ioncellor3cellNiMH/NiCdbatteries.Itprovidesupto350mAloadcurrent,overaninputvoltagerangefrom2.5Vto5.5V.Thereareseveraldifferentfixedvoltageoutputoptionsavailableaswellasanadjustableoutputvoltageversion(seeorderinginformation).
Thedeviceofferssuperiorfeaturesandperformanceformobilephonesandsimilarportableapplicationswithcom-plexpowermanagementsystems.AutomaticintelligentswitchingbetweenPWMlow-noiseandPFMlow-currentmodeoffersimprovedsystemcontrol.Duringfull-powerop-eration,afixed-frequency1MHz(typ).PWMmodedrivesloadsfrom∼70mAto350mAmax,withupto95%efficiency.HystereticPFMmodeextendsthebatterylifethroughreduc-tionofthequiescentcurrentto15µA(typ)duringlightcurrentloadsandsystemstandby.Internalsynchronousrec-tificationprovideshighefficiency(90to95%typ.atloadsbetween1mAand100mA).Inshutdownmode(Enablepinpulledlow)thedeviceturnsoffandreducesbatterycon-sumptionto0.1µA(typ.).
TheLM3670isavailableinaSOT23-5package.Ahighswitchingfrequency-1MHz(typ)-allowsuseoftinysurface-mountcomponents.Onlythreeexternalsurface-mountcomponents,aninductorandtwoceramiccapacitors,arerequired.
Features
nVOUT=Adj(0.7Vmin),1.2,1.5,1.6,1.8,1.875,2.5,3.3V
n2.5V≤VIN≤5.5V
n15µAtypicalquiescentcurrentn350mAmaximumloadcapability
n1MHzPWMfixedswitchingfrequency(typ.)nAutomaticPFM/PWMmodeswitching
nAvailableinfixedoutputvoltagesaswellasanadjustableversionnSOT23-5package
nLowdropoutoperation-100%dutycyclemodenInternalsynchronousrectificationforhighefficiencynInternalsoftstart
n0.1µAtypicalshutdowncurrent
nOperatesfromasingleLi-Ioncellor3cellNiMH/NiCdbatteries
nOnlythreetinysurface-mountexternalcomponentsrequired(oneinductor,twoceramiccapacitors)nCurrentoverloadprotection
Applications
nnnnnn
MobilephonesHandHeldPDAs
Palm-topPCs
PortableInstruments
BatteryPoweredDevices
TypicalApplication
20075801
FIGURE1.FixedOutputVoltage-TypicalApplicationCircuit
©2006NationalSemiconductorCorporationDS200758www.national.com
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LM3670TypicalApplication
(Continued)
20075830
FIGURE2.AdjustableOutputVoltage-TypicalApplicationCircuit
ConnectionDiagramandPackageMarkInformation
SOT23-5Package
NSPackageNumberMF05A
20075802
Note:Theactualphysicalplacementofthepackagemarkingwillvaryfromparttopart.
FIGURE3.TopView
PinDescriptions
Pin#12345
NameVINGNDENFBSW
Description
Powersupplyinput.Connecttotheinputfiltercapacitor(Figure1).Groundpin.Enableinput.
Feedbackanaloginput.Connecttotheoutputfiltercapacitor(Figure1).SwitchingnodeconnectiontotheinternalPFETswitchandNFETsynchronousrectifier.Connecttoaninductorwithasaturationcurrentratingthatexceedsthe750mAmax.SwitchPeakCurrentLimitspecification.
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LM3670OrderingInformation
VoltageOption
(V)
3.3
OrderNumber(Level95)LM3670MF-3.3LM3670MFX-3.3LM3670MF-3.3LM3670MFX-3.3
2.5
LM3670MF-2.5LM3670MFX-2.5LM3670MF-2.5LM3670MFX-2.5
1.875
LM3670MF-1.875LM3670MFX-1.875LM3670MF-1.875LM3670MFX-1.875
1.8
LM3670MF-1.8LM3670MFX-1.8LM3670MF-1.8LM3670MFX-1.8
1.6
LM3670MF-1.6LM3670MFX-1.6LM3670MF-1.6LM3670MFX-1.6
1.5
LM3670MF-1.5LM3670MFX-1.5LM3670MF-1.5LM3670MFX-1.5
1.2
LM3670MF-1.2LM3670MFX-1.2LM3670MF-1.2LM3670MFX-1.2
Adjustable
LM3670MF-ADJLM3670MFX-ADJLM3670MF-ADJLM3670MFX-ADJ
NOPBNOPB
SDFB
NOPBNOPB
SCZB
NOPBNOPB
S82B
NOPBNOPB
SDBB
NOPBNOPB
SDCB
NOPBNOPB
SEFB
NOPBNOPB
SDDB
SPECNOPBNOPB
PackageMarking
SDEB
SuppliedAs(#/reel)
10003000100030001000300010003000100030001000300010003000100030001000300010003000100030001000300010003000100030001000300010003000
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LM3670AbsoluteMaximumRatings(Note1)
IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.VINPin:VoltagetoGNDENPin:VoltagetoGNDFB,SWPin:
JunctionTemperature(TJ-MAX)StorageTemperatureRangeMaximumLeadTemperature(Soldering,10sec.)
−0.2Vto6.0V−0.2Vto6.0V(GND−0.2V)to(VIN+0.2V)−45˚Cto+125˚C−45˚Cto+150˚C
260˚C
ESDRating(Note3)HumanBodyModel:VIN,SW,FB,EN,GNDMachineModel:
2.0kV200V
OperatingRatings(Notes1,2)
InputVoltageRangeRecommendedLoadCurrentJunctionTemperature(TJ)RangeAmbientTemperature(TA)Range
2.5Vto5.5V0Ato350mA−40˚Cto+125˚C−40˚Cto+85˚C
ThermalProperties
Junction-to-Ambient
ThermalResistance(θJA)(SOT23-5)
250˚C/W
LimitsinstandardtypefaceareforTJ=25˚C.Limitsinboldfacetypeapplyover
thefulloperatingjunctiontemperaturerange(−40˚C≤TJ≤+125˚C).UnlessotherwisenotedVIN=3.6V,VOUT=1.8V,IO=150mA,EN=VINSymbolVINVOUTParameter
InputVoltageRangeFixedOutputVoltage:1.2V
(Note5)
2.5V≤VIN≤5.5VIO=10mA
2.5V≤VIN≤5.5V0mA≤IO≤150mA
FixedOutputVoltage:1.5V
2.5V≤VIN≤5.5VIO=10mA
2.5V≤VIN≤5.5V0mA≤IO≤350mA
FixedOutputVoltage:1.6V,1.875V
2.5V≤VIN≤5.5VIO=10mA
2.5V≤VIN≤5.5V0mA≤IO≤350mA
FixedOutputVoltage:1.8V
2.5V≤VIN≤5.5VIO=10mA
2.5V≤VIN≤5.5V0mA≤IO≤350mA
FixedOutputVoltage:2.5V,3.3V
3.6V≤VIN≤5.5VIO=10mA
3.6V≤VIN≤5.5V0mA≤IO≤350mA
AdjustableOutputVoltage(Note4)
2.5V≤VIN≤5.5VIO=10mA
2.5V≤VIN≤5.5V0mA≤IO≤150mA
Line_regLoad_regVREFIQ_SHDNLineRegulationLoadRegulation
InternalReferenceVoltageShutdownSupplyCurrent
TA=85oC
2.5V≤VIN≤5.5VIO=10mA
150mA≤IO≤350mA
Condition
Min2.5-2.0-4.5-2.5-5.0-2.5-5.5-1.5−4.5-2.0-6.0-2.5-4.0
0.260.00140.50.1
1
Typ
Max5.5+4.0+4.0+4.0+4.0+4.0+4.0+3.0+3.0+4.0+4.0+4.5+4.5
%/V%/mAVµA%%%%%UnitsV%
ElectricalCharacteristics
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LM3670ElectricalCharacteristicsLimitsinstandardtypefaceareforTJ=25˚C.Limitsinboldfacetypeapplyover
thefulloperatingjunctiontemperaturerange(−40˚C≤TJ≤+125˚C).UnlessotherwisenotedVIN=3.6V,VOUT=1.8V,IO=150mA,EN=VIN(Continued)SymbolIQParameter
DCBiasCurrentintoVINCondition
Noload,deviceisnotswitching(VOUTforcedhigherthanprogrammedoutputvoltage)
Min
Typ15
Max30
UnitsµA
VUVLORDSON(P)RDSON(N)ILKG(P)ILKG(N)ILIMη
MinimumVINbelowwhichVOUTwillbedisabled
Pin-PinResistanceforPFETPin-PinResistanceforNFETPChannelLeakageCurrentNChannelLeakageCurrentSwitchPeakCurrentLimitEfficiency
(VIN=3.6V,VOUT=1.8V)
ILOAD=1mAILOAD=10mAILOAD=100mAILOAD=200mAILOAD=300mAILOAD=350mAVIN=VGS=3.6VVIN=VGS=3.6VVDS=5.5VVDS=5.5V
400
2.43602500.10.1620919494949290
1.3
0.4
0.01
PWMMode
550
1000
1130069066011.5750
VmΩmΩµAµAmA
%
VIHVILIENFOSCLogicHighInputLogicLowInput
Enable(EN)InputCurrentInternalOscillatorFrequency
VVµAkHz
Note1:AbsoluteMaximumRatingsindicatelimitsbeyondwhichdamagetothecomponentmayoccur.OperatingRatingsareconditionsunderwhichoperationofthedeviceisguaranteed.OperatingRatingsdonotimplyguaranteedperformancelimits.Forguaranteedperformancelimitsandassociatedtestconditions,seetheElectricalCharacteristicstables.
Note2:AllvoltagesarewithrespecttothepotentialattheGNDpin.
Note3:TheHumanbodymodelisa100pFcapacitordischargedthrougha1.5kΩresistorintoeachpin.Themachinemodelisa200pFcapacitordischargeddirectlyintoeachpin.MIL-STD-8833015.7
Note4:Outputvoltagespecificationfortheadjustableversionincludestoleranceoftheexternalresistordivider.Note5:Theinputvoltagerangerecommendedforthespecifiedoutputvoltagesaregivenbelow:VIN=2.5Vto5.5Vfor0.7V≤VOUT<1.875V
VIN=(VOUT+VDROPOUT)to5.5Vfor1.875≤VOUT≤3.3VWhereVDROPOUT=ILOAD*(RDSON(P)+RINDUCTOR)
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LM367020075832
FIGURE4.SimplifiedFunctionalDiagram
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LM3670TypicalPerformanceCharacteristics
IQ(Non-switching)vs.VIN(unlessotherwisestated:VIN=3.6V,VOUT=1.8V)
IQvs.Temp
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20075804
VOUTvs.VINVOUTvs.IOUT20075806
20075807
Efficiencyvs.IOUTEfficiencyvs.VIN2007580820075809
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LM3670TypicalPerformanceCharacteristics(unlessotherwisestated:VIN=3.6V,VOUT=1.8V)
Frequencyvs.Temperature
RDSONvs.VINP&NChannel
(Continued)
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(VINLineTransient
=2.6Vto3.6V,ILOAD=100mA)
(VINLineTransient
=3.6Vto4.6V,ILOAD=100mA)
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20075813
LoadTransientILOAD=3mAto280mALoadTransientILOAD=0mAto70mA
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LM3670TypicalPerformanceCharacteristics(unlessotherwisestated:VIN=3.6V,VOUT=1.8V)
LoadTransientILOAD=0mAto280mA
LoadTransientILOAD=0mAto350mA
(Continued)
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LoadTransient
ILOAD=50mAto350mALoadTransient
ILOAD=100mAto300mA
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PFMMode
VSW,VOUT,IINDUCTORvs.TimePWMMode
VSW,VOUT,IINDUCTORvs.Time
2007582220075823
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LM3670TypicalPerformanceCharacteristics(unlessotherwisestated:VIN=3.6V,VOUT=1.8V)
SoftStart
VIN,VOUT,IINDUCTORvs.Time
(ILOAD=350mA)
(Continued)
20075824
OperationDescription
DEVICEINFORMATION
TheLM3670,ahighefficiencystepdownDC-DCswitchingbuckconverter,deliversaconstantvoltagefromeitherasingleLi-IonorthreecellNiMH/NiCdbatterytoportabledevicessuchascellphonesandPDAs.Usingavoltagemodearchitecturewithsynchronousrectification,theLM3670hastheabilitytodeliverupto350mAdependingontheinputvoltageandoutputvoltage(voltageheadroom),andtheinductorchosen(maximumcurrentcapability).
Therearethreemodesofoperationdependingonthecur-rentrequired-PWM(PulseWidthModulation),PFM(PulseFrequencyModulation),andshutdown.PWMmodehandlescurrentloadsofapproximately70mAorhigher.LighteroutputcurrentloadscausethedevicetoautomaticallyswitchintoPFMforreducedcurrentconsumption(IQ=15µAtyp)andalongerbatterylife.Shutdownmodeturnsoffthede-vice,offeringthelowestcurrentconsumption(ISHUTDOWN=0.1µAtyp).
TheLM3670canoperateuptoa100%dutycycle(PMOSswitchalwayson)forlowdropoutcontroloftheoutputvoltage.Inthiswaytheoutputvoltagewillbecontrolleddowntothelowestpossibleinputvoltage.
Additionalfeaturesincludesoft-start,undervoltagelockout,currentoverloadprotection,andthermaloverloadprotection.AsshowninFigure1,onlythreeexternalpowercomponentsarerequiredforimplementation.
CIRCUITOPERATION
TheLM3670operatesasfollows.Duringthefirstportionofeachswitchingcycle,thecontrolblockintheLM3670turnsontheinternalPFETswitch.Thisallowscurrenttoflowfromtheinputthroughtheinductortotheoutputfiltercapacitorandload.Theinductorlimitsthecurrenttoarampwithaslopeof
bystoringenergyinamagneticfield.Duringthesecondportionofeachcycle,thecontrollerturnsthePFETswitchoff,blockingcurrentflowfromtheinput,andthenturnstheNFETsynchronousrectifieron.TheinductordrawscurrentfromgroundthroughtheNFETtotheoutputfiltercapacitorandload,whichrampstheinductorcurrentdownwithaslopeof
Theoutputfilterstoreschargewhentheinductorcurrentishigh,andreleasesitwhenlow,smoothingthevoltageacrosstheload.
PWMOPERATION
DuringPWMoperationtheconverteroperatesasavoltage-modecontrollerwithinputvoltagefeedforward.Thisallowstheconvertertoachieveexcellentloadandlineregulation.TheDCgainofthepowerstageisproportionaltotheinputvoltage.Toeliminatethisdependence,feedforwardin-verselyproportionaltotheinputvoltageisintroduced.InternalSynchronousRectification
WhileinPWMmode,theLM3670usesaninternalNFETasasynchronousrectifiertoreducerectifierforwardvoltagedropandassociatedpowerloss.Synchronousrectificationprovidesasignificantimprovementinefficiencywhenevertheoutputvoltageisrelativelylowcomparedtothevoltagedropacrossanordinaryrectifierdiode.
CurrentLimiting
AcurrentlimitfeatureallowstheLM3670toprotectitselfandexternalcomponentsduringoverloadconditionsPWMmodeimplementscycle-by-cyclecurrentlimitingusinganinternalcomparatorthattripsat620mA(typ).
PFMOPERATION
Atverylightload,theconverterentersPFMmodeandoperateswithreducedswitchingfrequencyandsupplycur-renttomaintainhighefficiency.
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LM3670OperationDescription
(Continued)
ThepartautomaticallytransitionintoPFMmodewheneitheroftwoconditionsoccursforadurationof32ormoreclockcycles:
A.TheinductorcurrentbecomesdiscontinuousB.ThepeakPMOSswitchcurrentdropsbelowtheIMODElevel:
DuringPFMoperation,theconverterpositionstheoutputvoltageslightlyhigherthanthenominaloutputvoltageinPWMoperation,allowingadditionalheadroomforvoltagedropduringaloadtransientfromlighttoheavyload.ThePFMcomparatorsensestheoutputvoltageviathefeedbackpinandcontroltheswitchingoftheoutputFETssuchthattheoutputvoltagerampsbetween0.8%and1.6%(typ)abovethenominalPWMoutputvoltage.Iftheoutputvoltageisbelowthe‘high’PFMcomparatorthreshold,thePMOSpowerswitchisturnedon.Itremainsonuntiltheoutputvoltageexceedsthe‘high’PFMthresholdorthepeakcurrentexceedstheIPFMlevelsetforPFMmode.ThepeakcurrentinPFMmodeis:
OncethePMOSpowerswitchisturnedoff,theNMOSpowerswitchisturnedonuntiltheinductorcurrentrampstozero.WhentheNMOSzero-currentconditionisdetected,theNMOSpowerswitchisturnedoff.Iftheoutputvoltageisbelowthe‘high’PFMcomparatorthreshold(seeFigure5),thePMOSswitchisagainturnedonandthecycleisre-peateduntiltheoutputreachesthedesiredlevel.Oncetheoutputreachesthe‘high’PFMthreshold,theNMOSswitchisturnedonbrieflytoramptheinductorcurrenttozeroandthenbothoutputswitchesareturnedoffandthepartentersanextremelylowpowermode.Quiescentsupplycurrentduringthis‘sleep’modeislessthan30µA,whichallowstheparttoachievehighefficienciesunderextremelylightloadconditions.Whentheoutputdropsbelowthe‘low’PFMthreshold,thecyclerepeatstorestoretheoutputvoltageto∼1.6%abovethenominalPWMoutputvoltage.
IftheloadcurrentshouldincreaseduringPFMmode(seeFigure5)causingtheoutputvoltagetofallbelowthe‘low2’PFMthreshold,thepartautomaticallytransitionsintofixed-frequencyPWMmode.
20075803
FIGURE5.OperationinPFMModeandTransitiontoPWMMode
Soft-Start
TheLM3670hasasoft-startcircuitthatlimitsin-rushcurrentduringstart-up.Typicalstart-uptimeswitha10µFoutputcapacitorand350mAloadis400µs:InrushCurrent(mA)
070140
Duration(µSec)
32224256
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InrushCurrent(mA)
280620
Duration(µSec)
256
untilsoftstartends
Note6:Thefirst32µSaretoallowthebiascurrentstostabilize
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LM3670OperationDescription
LDO-LowDropOutOperation
(Continued)
TheLM3670canoperateat100%dutycycle(noswitching,PMOSswitchiscompletelyon)forlowdropoutsupportoftheoutputvoltage.Inthiswaytheoutputvoltageiscon-trolleddowntothelowestpossibleinputvoltage.
Theminimuminputvoltageneededtosupporttheoutputvoltageis
Apolecanalsobeusedathigheroutputvoltages.Forexample,inthetableTable3,thereisanentryfor1.24Vwithbothapoleandzeroatapproximately10kHzfornoiserejection.
INDUCTORSELECTION
Therearetwomainconsiderationswhenchoosinganinduc-tor;theinductorcurrentshouldnotsaturate,andtheinductorcurrentrippleissmallenoughtoachievethedesiredoutputvoltageripple.
Therearetwomethodstochoosetheinductorcurrentrating.Method1:
Thetotalcurrentisthesumoftheloadandtheinductorripplecurrent.Thiscanbewrittenas
•ILOAD•RDSON,PFETLoadcurrent
DraintosourceresistanceofPFETswitchinthetrioderegion
•RINDUCTORInductorresistance
ApplicationInformation
OUTPUTVOLTAGESELECTIONFORADJUSTABLELM3670
Theoutputvoltageoftheadjustablepartscanbepro-grammedthroughtheresistornetworkconnectedfromVOUTtoVFBthentoGND.VOUTisadjustedtomakeVFBequalto0.5V.TheresistorfromVFBtoGND(R2)shouldbeatleast100KΩtokeepthecurrentsunkthroughthisnetworkwellbelowthe15µAquiescentcurrentlevel(PFMmodewithnoswitching)butlargeenoughthatitisnotsusceptibletonoise.IfR2is200KΩ,andVFBis0.5V,thenthecurrentthroughtheresistorfeedbacknetworkis2.5µA(IFB=0.5V/R2).Theoutputvoltageformulais:
•VOUTOutputVoltage(V)
•VFBFeedbackVoltage(0.5Vtyp)•R1ResistorfromVOUTtoVFB(Ω)•R2ResistorfromVOUTtoGND(Ω)
Foranyoutputvoltagegreaterthanorequalto0.7Vafrequencyzeromustbeaddedat10kHzforstability.Theformulais:
•ILOADloadcurrent•VINinputvoltage•Linductor
•fswitchingfrequency•IRIPPLEpeak-to-peakMethod2:
Amoreconservativeapproachistochooseaninductorthatcanhandlethecurrentlimitof700mA.
Givenapeak-to-peakcurrentripple(IPP)theinductorneedstobeatleast
Foranyoutputvoltagesbelow0.7andaboveorequalto2.5V,apolemustalsobeplacedat10kHzaswell.Thelowestoutputvoltagepossibleis0.7V.Atlowoutputvoltagesthedutycycleisverysmalland,astheinputvoltagein-creases,thedutycycledecreasesevenfurther.Sincethedutycycleissolowanychangeduetonoiseisanappre-ciablepercentage.Inotherwords,itissusceptibletonoise.CapacitorsC1andC2actasnoisefiltersratherthanfre-quencypolesandzeros.Ifthepoleandzeroareatthesamefrequencytheformulais:
A10µHinductorwithasaturationcurrentratingofatleast800mAisrecommendedformostapplications.Theinduc-tor’sresistanceshouldbelessthanaround0.3Ωforgoodefficiency.Table1listssuggestedinductorsandsuppliers.Forlow-costapplications,anunshieldedbobbininductorissuggested.Fornoisecriticalapplications,atoroidalorshielded-bobbininductorshouldbeused.Agoodpracticeistolayouttheboardwithoverlappingfootprintsofbothtypesfordesignflexibility.Thisallowssubstitutionofalow-noisetoroidalinductor,intheeventthatnoisefromlow-costbobbinmodelsisunacceptable.
INPUTCAPACITORSELECTION
Aceramicinputcapacitorof4.7µFissufficientformostapplications.Alargervaluemaybeusedforimprovedinputvoltagefiltering.TheinputfiltercapacitorsuppliescurrenttothePFETswitchoftheLM3670inthefirsthalfofeachcycleandreducesvoltagerippleimposedontheinputpowersource.Aceramiccapcitor’slowESRprovidesthebest
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LM3670ApplicationInformation
(Continued)
noisefilteringoftheinputvoltagespikesduetothisrapidlychangingcurrent.Selectaninputfiltercapacitorwithasurgecurrentratingsufficientforthepower-upsurgefromtheinputpowersource.Thepower-upsurgecurrentisapproximatelythecapacitor’svalue(µF)timesthevoltageriserate(V/µs).Theinputcurrentripplecanbecalculatedas:
TABLE1.SuggestedInductorsandTheirSuppliers
ModelIDC2512NB100MDO1608C-103ELL6RH100MCDRH5D18-100
VendorVishayCoilcraftPanasonicSumida
Phone408-727-2500847-639-6400714-373-7366847-956-0666
FAX408-330-4098847-639-1469714-373-7323847-956-0702
OUTPUTCAPACITORSELECTION
Theoutputfiltercapacitorsmoothesoutcurrentflowfromtheinductortotheload,maintainingasteadyoutputvoltageduringtransientloadchangesandreducesoutputvoltageripple.ThesecapacitorsmustbeselectedwithsufficientcapacitanceandsufficientlylowESRtoperformthesefunc-tions.
Theoutputripplecurrentcanbecalculatedas:Voltagepeak-to-peakrippleduetocapacitance=
Voltagepeak-to-peakripple,rootmeansquared=
Voltagepeak-to-peakrippleduetoESR=
Notethattheoutputrippleisdependentonthecurrentrippleandtheequivalentseriesresistanceoftheoutputcapacitor(RESR).
Becausethesetwocomponentsareoutofphasethermsvalueisused.TheRESRisfrequencydependent(aswellastemperaturedependent);makesurethefrequencyoftheRESRgivenisthesameorderofmagnitudeastheswitchingfrequency.
TABLE2.SuggestedCapacitorsandTheirSuppliers
Model
10µFforCOUTVJ1812V106MXJATLMK432BJ106MMJMK325BJ106MM4.7µFforCINVJ1812V475MXJATEMK325BJ475MNC3216X5R0J475M
CeramicCeramicCeramic
VishayTaiyo-Yuden
TDK
408-727-2500847-925-0888847-803-6100
408-330-4098847-925-0899847-803-6296
CeramicCeramicCeramic
VishayTaiyo-YudenTaiyo-Yuden
408-727-2500847-925-0888847-925-0888
408-330-4098847-925-0899847-925-0899
Type
Vendor
Phone
FAX
TABLE3.AdjustableLM3670ConfigurationsforVariousVOUTVOUT(V)
0.70.80.91.0
R1(KΩ)80.6120160200
R2(KΩ)200200200200
C1(pF)20013010082
C2(pF)150nonenonenone
L(µH)4.74.74.74.7
CIN(µF)4.74.74.74.7
COUT(µF)
10101010
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LM3670ApplicationInformation
VOUT(V)
1.11.21.241.241.51.61.71.875
R1(KΩ)240280300221402442487549
(Continued)
TABLE3.AdjustableLM3670ConfigurationsforVariousVOUT(Continued)
R2(KΩ)200200200150200200200200
C1(pF)6856567539393330
C2(pF)nonenonenone120nonenonenonenone
L(µH)4.74.74.74.710101010
CIN(µF)4.74.74.74.74.74.74.74.7
COUT(µF)
1010101010101014.7
Note:(10||
4.7)
2.58062002282104.722
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LM3670ApplicationInformation
BOARDLAYOUTCONSIDERATIONS
(Continued)
PCboardlayoutisanimportantpartofDC-DCconverterdesign.Poorboardlayoutcandisrupttheperformanceofa
DC-DCconverterandsurroundingcircuitrybycontributingtoEMI,groundbounce,andresistivevoltagelossinthetraces.ThesecansenderroneoussignalstotheDC-DCconverterIC,resultinginpoorregulationorinstability.
20075831
FIGURE6.BoardLayoutDesignRulesfortheLM3670
GoodlayoutfortheLM3670canbeimplementedbyfollow-ingafewsimpledesignrules,asillustratedin.
1.PlacetheLM3670,inductorandfiltercapacitorsclose
togetherandmakethetracesshort.Thetracesbetweenthesecomponentscarryrelativelyhighswitchingcur-rentsandactasantennas.Followingthisrulereducesradiatednoise.Placethecapacitorsandinductorwithin0.2in.(5mm)oftheLM3670.
2.Arrangethecomponentssothattheswitchingcurrent
loopscurlinthesamedirection.Duringthefirsthalfofeachcycle,currentflowsfromtheinputfiltercapacitor,throughtheLM3670andinductortotheoutputfiltercapacitorandbackthroughground,formingacurrentloop.Inthesecondhalfofeachcycle,currentispulledupfromground,throughtheLM3670bytheinductor,totheoutputfiltercapacitorandthenbackthroughground,formingasecondcurrentloop.Routingtheseloopssothecurrentcurlsinthesamedirectionpreventsmag-neticfieldreversalbetweenthetwohalf-cyclesandre-ducesradiatednoise.
3.ConnectthegroundpinsoftheLM3670,andfilterca-pacitorstogetherusinggenerouscomponent-sidecop-perfillasapseudo-groundplane.Then,connectthisto
15
theground-plane(ifoneisused)withseveralvias.Thisreducesground-planenoisebypreventingtheswitchingcurrentsfromcirculatingthroughthegroundplane.ItalsoreducesgroundbounceattheLM3670bygivingitalow-impedancegroundconnection.
4.Usewidetracesbetweenthepowercomponentsandfor
powerconnectionstotheDC-DCconvertercircuit.Thisreducesvoltageerrorscausedbyresistivelossesacrossthetraces.
5.Routenoisesensitivetraces,suchasthevoltagefeed-backpath,awayfromnoisytracesbetweenthepowercomponents.ThevoltagefeedbacktracemustremainclosetotheLM3670circuitandshouldbedirectbutshouldberoutedoppositetonoisycomponents.ThisreducesEMIradiatedontotheDC-DCconverter’sownvoltagefeedbacktrace.
6.Placenoisesensitivecircuitry,suchasradioIFblocks,
awayfromtheDC-DCconverter,CMOSdigitalblocksandothernoisycircuitry.Interferencewithnoise-sensitivecircuitryinthesystemcanbereducedthroughdistance.
Inmobilephones,forexample,acommonpracticeistoplacetheDC-DCconverterononecorneroftheboard,
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LM3670ApplicationInformation
(Continued)
arrangetheCMOSdigitalcircuitryaroundit(sincethisalsogeneratesnoise),andthenplacesensitivepreamplifiersand
IFstagesonthediagonallyopposingcorner.Often,thesensitivecircuitryisshieldedwithametalpanandpowertoitispost-regulatedtoreduceconductednoise,usinglow-dropoutlinearregulators.
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LM3670MiniatureStep-DownDC-DCConverterforUltraLowVoltageCircuitsPhysicalDimensions
inches(millimeters)unlessotherwisenoted
5-LeadSOT23-5PackageNSPackageNumberMF05A
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2.Acriticalcomponentisanycomponentofalifesupportdeviceorsystemwhosefailuretoperformcanbereasonablyexpectedtocausethefailureofthelifesupportdeviceorsystem,ortoaffectitssafetyoreffectiveness.
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