AS3658 datasheet(61/158 Pages) AMSCO

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AS3658

Data Sheet Confidential - Detail ed Descri ption-Power Management Functions

transferred to the DeltaCharge register. The CalReq bit is cleared automatically after the calibration has completed

successfully and FGOffCal has been written to the register.

Please note that offset calibration is not possible while the charger is active. If the CalReq bit is set while the charger is

active the calibration will start automatically after the charger has been disabled by clearing the ChEn bit or if the

external charge adapter has been removed. If during an offset calibration procedure the charger is enabled the offset

calibration mode is terminated, the CalReq bit is cleared, the current value of the elapsed time counter is transferred to

the ElapsedTime register and the DeltaCharge register is loaded with (FFFF)h.

Calculation of Battery Status

The host system can calculate all the parameters necessary for estimating the remaining battery capacity by

evaluating ElapsedTime, DeltaCharge and FGOffCal.

Calculating Elapsed Time

The host system can evaluate the change in time

Δt by setting the UpdReq bit in the FuelGauge register and reading

ElapsedTime after UpdReq has been automatically cleared. The change in time in seconds is given by:

Δt = ElapsedTime x 3600 / 4096.60 [s]

(EQ 1)

Note that the absolute accuracy of

Δt is directly related to the absolute accuracy of the internal reference oscillator. To

cancel the error associated with the accuracy of the oscillator, a correction factor CV can be introduced. CV can be

evaluated by comparing the change in time calculated by (1) with some reference value

ΔtREF obtained from a RTC or

measured during system calibration. CV is given by:

CV =

ΔtREF / Δt(EQ 2)

By multiplying

Δt and CV the correct value for the change in time can be calculated:

ΔtCORR = CV x Δt [s]

(EQ 3)

Calculating Average Current

The host system can calculate the average current during the last time period by setting the UpdReq bit in the

FuelGauge register and reading DeltaCharge and ElapsedTime after UpdReq has been automatically cleared.

Together with FGOffCal determined during offset calibration mode the average current is given by:

IAVG = DeltaCharge / (Δt x AVFC x Rsense) – FGOffCal x 3.05µV / Rsense [A]

(EQ 4)

Δt is the change in time in seconds calculated by (1), AVFC is the gain of the VFC in Hz/V, Rsense is the value of the

sense resistor in

Ω and FGOffCal is the offset calibration value. As DeltaCharge and Δt both are proportional to the

oscillator frequency, no correction factor needs to be introduced in the formula.

Calculating Accumulated Current

Accumulated current is used to calculate the absolute remaining capacity of the battery. It is given by:

IACC = IAVG x ΔtCORR [A]

(EQ 5)

Calculating the Remaining Capacity

Remaining capacity is the entire goal of fuel gauging. It is given by:

RC = RC + IACC [As]

(EQ 6)

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