LTC4015 datasheet(39/76 Pages) LINER

39 / 76 page

LTC4015

4015fb

For more information www.linear.com/LTC4015

OPERATION

Where KQC = 8333.33Hz/V

fTICKisthendividedbyaprescaler(QCOUNT_PRESCALE_

FACTOR)thatisprogrammablefrom1to65535(defaultof

512). The prescaler effectively increases integration time

by a factor equal to QCOUNT_PRESCALE_FACTOR. At

each under or overflow of the prescaler, the accumulated

voltageregister(QCOUNT)isincrementedordecremented

one count. The value of accumulated voltage is read via

the I2C interface.

To achieve the specified precision of the Coulomb counter

the differential voltage VCSP–CSN must stay within ±50mV.

For differential input signals up to ±300mV the Coulomb

counter will remain functional but the precision of the

Coulomb counter is not guaranteed.

The value of external sense resistor, RSNSB, influences

the gain of the Coulomb counter. A larger sense resistor

gives a larger differential voltage for the same current

which results in more precise Coulomb counting. Thus

the amount of charge represented by the least significant

bit (qLSB) of the accumulated charge is given by:

qLSB=

QCOUNT_PRESCALE_FACTOR

KQC•RSNSB

qLSBunits arein coulombs(Amp-seconds)

where:

KQC = 8333.33Hz/V

QCOUNT_PRESCAL_FACTOR = value of the

prescaler

RSNSB is in Ω

If the value of QCOUNT reaches 0 or 65535, the value

saturates. QCOUNT does not wrap.

As long as input power is applied and the battery charger

is enabled, the LTC4015 will allow lead-acid batteries to

remain in a CV state indefinitely. As the battery slowly

discharges itself internally, the LTC4015 replenishes the

lostcharge,andthischargeisaccumulatedbytheCoulomb

counter. After a long period of time, the Coulomb counter

accumulated charge due to battery self discharge current

while the charger is in CV mode could cause significant

errors relative to the actual SOC of the battery.

Coulomb Counter Applications

Thefollowingexamplesdemonstratesimpleapplicationsof

theLTC4015Coulombcounterfunction.Formoreadvanced

applications,includingcalibrationofstate-of-chargebased

ontemperature,pleasecontactLinearTechnologyApplica-

tions Engineering for more information.

EXAMPLE: Setting QCOUNT_PRESCALE_FACTOR and

initializingandcalibratingQCOUNT:ChargingtwoLi-Ion

3.5Ah batteries in parallel at 6.4A total

Total capacity = 7.0 Ah = 7.0A • 3600sec = 25200C

Constant-current charging at 6.4A requires RSNSB =

32mV/6.4A = 5mΩ

The maximum value of the QCOUNT register is 65535.

To prevent over ranging QCOUNT, the highest allowed

value of qLSB is 25200C/65535 = 0.385C

Using:

qLSB=

QCOUNT_PRESCALE_FACTOR

KQC•RSNSB

RSNSB

CSP

CCREFM

4015 F10

CCREFP

CONTROL

LOGIC

POLARITY

DETECTION

CHARGER

BATTERY

CSN

GND

IBAT

–

PRESCALAR

LTC4015

LOGIC/

SERIAL PORT

Figure 10. Coulomb Counter Section of the LTC4015

LTC4015 Datasheet(PDF) 39 Page - Linear Technology

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