LTC4011 datasheet(18/26 Pages) LINER

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LTC4011

4011fb

applicaTions inForMaTion

operations.However,thispracticeisnotrecommended

forNiMHcellschargedwellaboveorbelowtheir1Crate,

becausefastchargeterminationbasedsolelyonvoltage

inflectionmaynotbeadequatetoprotectthebatteryfrom

asevereovercharge.Aresistorbetween10kand20kmay

beusedtoconnectVTEMPtoVRTifthepausefunctionis

stilldesired.

INTVDDRegulatorOutput

IfBGATEisleftopen,theINTVDDpinoftheLTC4011can

beusedasanadditionalsourceofregulatedvoltageinthe

hostsystemanytimeREADYisactive.Switchingloads

onINTVDDmayreducetheaccuracyofinternalanalog

circuits used to monitor and terminate fast charging.

Inaddition,DCcurrentdrawnfromtheINTVDDpincan

greatlyincreaseinternalpowerdissipationatelevatedVCC

voltages.Aminimumceramicbypasscapacitorof0.1µF

isrecommended.

CalculatingAveragePowerDissipation

TheusershouldensurethatthemaximumratedICjunction

temperatureisnotexceededunderalloperatingconditions.

The thermal resistance of the LTC4011 package (

θJA)

is38°C/W,providedtheexposedmetalpadisproperly

solderedtothePCB.Theactualthermalresistanceinthe

applicationwilldependontheamountofPCBcopperto

whichthepackageissoldered.Feedthroughviasdirectly

belowthepackagethatconnecttoinnercopperlayers

arehelpfulinloweringthermalresistance.Thefollowing

formulamaybeusedtoestimatethemaximumaverage

power dissipation PD(inwatts)oftheLTC4011under

normaloperatingconditions.

where:

IDD=AverageexternalINTVDDloadcurrent,ifany

IVRT=Loadcurrentdrawnbytheexternalthermistor

networkfromVRT,ifany

QTGATE=GatechargeofexternalP-channelMOSFET

incoulombs

QBGATE=GatechargeofexternalN-channelMOSFET

(ifused)incoulombs

VLED=MaximumexternalLEDforwardvoltage

RLED=ExternalLEDcurrent-limitingresistorusedin

theapplication

n=NumberofLEDsdrivenbytheLTC4011

SampleApplications

Figures6through9detailsamplechargerapplications

ofvariouscomplexities.CombinedwiththeTypicalAp-

plicationonthefirstpageofthisdatasheet,theseFigures

demonstrate some of the proper configurations of the

LTC4011.MOSFETbodydiodesareshowninthesefigures

strictlyforreferenceonly.

Figure6showsaminimumapplication,whichmightbe

encounteredinlowcostNiCdfastchargeapplications.

FET-basedPowerPathcontrolallowsformaximuminput

voltagerangefromtheDCadapter.TheLTC4011uses

–∆V to terminate the fast charge state, as no external

temperature information is available. Nonsynchronous

PWMswitchingisemployedtoreduceexternalcomponent

cost.AsingleLEDindicateschargingstatus.

A3ANiMHapplicationofmediumcomplexityisshownin

Figure7.PowerPathcontrolthatiscompletelyFET-based

allowsforbothminimuminputvoltageoverheadandmini-

mumswitchoverlosswhenoperatingfromthebattery.

P-channelMOSFETQ4functionsasaswitchtoconnectthe

batterytothesystemloadwhenevertheDCinputadapter

isremoved.Ifthemaximumbatteryvoltageislessthan

themaximumratedVGSofQ4,diodeD1andresistorR5

are not required. Otherwise choose the Zener voltage

ofD1tobelessthanthemaximumratedVGSofQ4.R5

providesabiascurrentof(VBAT–VZENER)/(R5+20k)for

D1whentheinputadapterisremoved.ChooseR5tomake

thiscurrent,whichisdrawnfromthebattery,justlarge

enoughtodevelopthedesiredVGSacrossD1.

Precharge,fastchargeandtop-offstatesareindicatedby

externalLEDs.TheVTEMPthermistornetworkallowsthe

LTC4011toaccuratelyterminatefastchargeunderavariety

ofappliedchargerates.UseofasynchronousPWMtopol-

ogyimprovesefficiencyandlowerspowerdissipation.

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