BQ2011 datasheet(5/20 Pages) TI

5 / 20 page

Gas Gauge Operation

The operational overview diagram in Figure 2 illustrates

the operation of the bq2011. The bq2011 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

bq2011 adapts its capacity determination based on the

actual conditions of discharge.

The battery's initial capacity is equal to the Pro-

grammed Full Count (PFC) shown in Table 1.

Until

LMD is updated, NAC counts up to but not beyond this

threshold during subsequent charges. This approach al-

lows the gas gauge to be charger-independent and com-

patible 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

EDV. 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

battery capacity:

The initial LMD and gas gauge rate values are pro-

grammed by using PFC. The PFC also provides the

100% reference for the absolute display mode. The

bq2011 is configured for a given application by se-

lecting a PFC value from Table 1. The correct PFC

may be determined by multiplying the rated bat-

tery capacity in mAh by the sense resistor 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.

bq2011

FG201104.eps

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

Rate and

Temperature

Compensation

Figure 2. Operational Overview

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