BQ2004 datasheet(3/16 Pages) TI

3 / 16 page

Functional Description

Figure 3 shows a block diagram and Figure 4 shows a

state diagram of the bq2004.

Battery Voltage and Temperature

Measurements

Battery voltage and temperature are monitored for

maximum allowable values. The voltage presented on

the battery sense input, BAT, should represent a

two-cell potential for the battery under charge.

resistor-divider ratio of:

RB1

RB2

- 1

is recommended to maintain the battery voltage within

the valid range, where N is the number of cells, RB1 is

the resistor connected to the positive battery terminal,

and RB2 is the resistor connected to the negative bat-

tery terminal. See Figure 1.

Note: This resistor-divider network input impedance to

end-to-end should be at least 200k

Ω and less than 1MΩ.

A ground-referenced negative temperature coefficient ther-

mistor placed in proximity to the battery may be used as a

low-cost temperature-to-voltage transducer. The tempera-

ture sense voltage input at TS is developed using a

resistor-thermistor network between VCC and VSS. See

Figure 1. Both the BAT and TS inputs are referenced to

SNS, so the signals used inside the IC are:

VBAT -VSNS =VCELL

and

VTS -VSNS =VTEMP

Discharge-Before-Charge

The DCMD input is used to command discharge-before-

charge via the DIS output. Once activated, DIS becomes

active (high) until VCELL falls below VEDV, at which time

DIS goes low and a new fast charge cycle begins.

The DCMD input is internally pulled up to VCC (its inac-

tive state). Leaving the input unconnected, therefore,

results in disabling discharge-before-charge. A negative

going pulse on DCMD initiates discharge-before-charge

at any time regardless of the current state of the

bq2004. If DCMD is tied to VSS, discharge-before-charge

will be the first step in all newly started charge cycles.

Starting a Charge Cycle

A new charge cycle (see Figure 2) is started by:

VCC rising above 4.5V

VCELL falling through the maximum cell voltage,

VMCV where:

VMCV = 0.8

∗ VCC ± 30mV

A transition on the INH input from low to high.

If DCMD is tied low, a discharge-before-charge is exe-

cuted as the first step of the new charge cycle. Other-

wise, pre-charge qualification testing is the first step.

The battery must be within the configured temperature

and voltage limits before fast charging begins.

The valid battery voltage range is VEDV <VBAT <VMCV

where:

VEDV = 0.4

∗ VCC ± 30mV

The valid temperature range is VHTF <VTEMP <VLTF,

where:

bq2004

Fg2004a.eps

bq2004

VCC

PACK +

PACK -

SNS

RT1

RT2

RB2

RB1

bq2004

Negative Temperature

Coefficient Thermister

PACK+

PACK-

BAT

SNS

Figure 1. Voltage and Temperature Monitoring

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