LTC4099
4099fd
Recall, however, that in many cases the actual battery
chargecurrent,IBAT,willbelowerthantheprogrammed
current,ICHG,duetolimitedinputpoweravailableand
prioritizationofthesystemloaddrawnfromVOUT.
Constant-Voltage
Oncethebatteryterminalvoltagereachesthepresetfloat
voltage,thebatterychargerwillholdthevoltagesteady
and the charge current will decrease naturally toward
zero.Twovoltagesettings,4.100Vand4.200V,areavail-
ableforfinalfloatvoltageselectionviatheI2Cport.For
applicationsthatrequireasmuchruntimeaspossible,
the4.200Vsettingcanbeselected.Forapplicationsthat
seektoextendbatterylife,theLTC4099’sdefaultsetting
of4.100Vshouldbeused.
Full Capacity Charge Indication (C/x)
SincethePROGpinalwaysrepresentstheactualcharge
current flowing, even in the constant-voltage phase of
charging,thePROGpinvoltagerepresentsthebattery’s
state-of-chargeduringthatphase.TheLTC4099hasafull
capacitychargeindicationcomparatoronthePROGpin
whichreportsitsresultsviatheI2Cport.Selectionlevels
fortheC/xcomparatorof50mV,100mV,200mVand500mV
areavailablebyI2Ccontrol.RecallthatthePROGpinservo
voltagecanbeprogrammedfrom500mVto1.2V.Ifthe1V
servosettingrepresentsthefullchargerateofthebattery
(1C),thenthe100mVC/xsettingwouldbeequivalentto
C/10.Likewisethe200mVC/xsettingwouldrepresent
C/5andthe500mVsettingC/2.
Charge Termination
The battery charger has a built-in termination safety
timer.Whenthevoltageonthebatteryreachestheuser-
programmedfloatvoltageof4.100Vor4.200V,thesafety
timerisstarted.Afterthesafetytimerexpires,chargingof
thebatterywilldiscontinueandnomorecurrentwillbe
delivered.Thesafetytimer’sdefaultendingtimeoffour
hoursmaybealteredfromonetoeighthoursinone-hour
incrementsbyaccessingtheI2Cport.
Automatic Recharge
Afterthebatterychargerterminates,itwillremainoff,
drawingonlymicroamperesofcurrentfromthebattery.
Iftheportableproductremainsinthisstatelongenough,
thebatterywilleventuallyselfdischarge.Toensurethat
thebatteryisalwaystoppedoff,anewchargecyclewill
automaticallybeginwhenthebatteryvoltagefallsbelow
VRECHRG(typically4.100Vforthe4.200Vfloatvoltage
settingand4.000Vforthe4.100Vfloatvoltagesetting).In
theeventthatthesafetytimerisrunningwhenthebattery
voltagefallsbelowVRECHRG,itwillresetbacktozero.To
preventbriefexcursionsbelowVRECHRGfromresettingthe
safetytimer,thebatteryvoltagemustbebelowVRECHRGfor
morethan2.5ms.Thechargecycleandsafetytimerwill
alsorestartiftheVBUSUVLOcyclesLOWandthenHIGH
(e.g.,VBUSorWALLisremovedandthenreplaced)orif
thechargerismomentarilydisabledusingtheI2Cport.
TheflowchartinFigure4representsthebatterycharger’s
algorithm.
Thermistor Measurement
Thebatterytemperatureismeasuredbyplacinganega-
tive temperature coefficient (NTC) thermistor close to
thebatterypack.Thethermistorcircuitryisshowninthe
BlockDiagram.
Tousethisfeature,connectthethermistorbetweentheNTC
pinandgroundandabiasresistorfromNTCBIAStoNTC.
Thebiasresistorshouldbea1%resistorwithavalueequal
tothevalueofthechosenthermistorat25°C(R25).
TheLTC4099willpausechargingwhentheresistanceof
thethermistordropsto0.484timesthevalueofR25or
4.84kfora10kthermistor.ForaVishaycurve2thermis-
tor,thiscorrespondstoapproximately45°C.Ifthebattery
charger is in constant-voltage (float) mode, the safety
timeralsopausesuntilthethermistorindicatesareturn
tovalidtemperature.TheLTC4099isalsodesignedto
pausechargingwhenthevalueofthethermistorincreases
to2.816timesthevalueofR25.ForaVishaycurve2
10kthermistor,thisresistance,28.16k,correspondsto
approximately0°C.Thehotandcoldcomparatorseach
haveapproximately4°Cofhysteresistopreventoscillation
aboutthetrippoint.
Ifthecurve2thermistor’stemperaturerisesabove60°C,
itsvaluewilldropto0.2954timesR25.Whenthishappens,
theLTC4099detectsthiscriticallyhightemperatureand
indicatesitviatheI2Cport(seeTable7).Ifthiscondition
operaTion