BQ2012SN datasheet(5/20 Pages) TI

5 / 20 page

Gas Gauge Operation

The operational overview diagram in Figure 2 illustrates

the operation of the bq2012. The bq2012 accumulates a

measure of charge and discharge currents, as well as an

estimation of self-discharge. Charge and discharge cur-

rents are temperature and rate compensated, whereas

self-discharge is only temperature compensated.

The main counter, Nominal Available Charge (NAC),

represents the available battery capacity at any given

time.

Battery charging increments the NAC register,

while battery discharging and self-discharge decrement

the NAC register and increment the DCR (Discharge

Count Register).

The Discharge Count Register (DCR) is used to update

the Last Measured Discharge (LMD) register only if a

complete battery discharge from full to empty occurs

without any partial battery charges.

Therefore, the

bq2012 adapts its capacity determination based on the

actual conditions of discharge.

The battery’s initial capacity is equal to the programmed

full count (PFC) shown in Table 2. Until LMD is up-

dated, NAC counts up to but not beyond this threshold

during subsequent charges. This approach allows the

gas gauge to be charger-independent and compatible

with any type of charge regime.

Last Measured Discharge (LMD) or learned

battery capacity:

LMD is the last measured discharge capacity of the

battery. On initialization (application of VCC or bat-

tery replacement), LMD = PFC. During subsequent

discharges, the LMD is updated with the latest

measured capacity in the Discharge Count Register

(DCR) representing a discharge from full to below

EDV1. A qualified discharge is necessary for a ca-

pacity transfer from the DCR to the LMD register.

The LMD also serves as the 100% reference thresh-

old used by the relative display mode.

Programmed Full Count (PFC) or initial bat-

tery capacity:

The initial LMD and gas gauge rate values are pro-

grammed by using PROG1–PROG4. The PFC also

provides the 100% reference for the absolute dis-

play mode. The bq2012 is configured for a given ap-

plication by selecting a PFC value from Table 2.

The correct PFC may be determined by multiplying

the rated battery capacity in mAh by the sense re-

sistor value:

Battery capacity (mAh) * sense resistor (Ω) =

PFC (mVh)

Selecting a PFC slightly less than the rated capac-

ity for absolute mode provides capacity above the

full reference for much of the battery’s life.

bq2012

FG201002.eps

Rate and

Temperature

Compensation

Temperature

Compensation

Charge

Current

Discharge

Current

Self-Discharge

Timer

Temperature

Translation

Nominal

Available

Charge

(NAC)

Last

Measured

Discharged

(LMD)

Discharge

Count

(DCR)

Qualified

Transfer

Rate and

Temperature

Compensation

Rate and

Temperature

Compensation

Temperature Step,

Other Data

Inputs

Main Counters

and Capacity

Reference (LMD)

Outputs

Serial

Port

Chip-Controlled

Available Charge

LED Display

Figure 2. Operational Overview

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