bq2000T

2

7

Battery

Pack

5

TS

N

T

C

VSS

VCC

VCC

RT1

RT2

bq2000T

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SLUS149D – MAY 1999 – REVISED JANUARY 2010

Maximum Temperature (NiCd, NiMH, Li-Ion)

A negative-coefficient thermistor, referenced to VSS and placed in thermal contact with the battery, may be used

as a temperature-sensing device. Figure 6 shows a typical temperature-sensing circuit.

Figure 6. Temperature Monitoring Configuration

During fast charge, the bq2000T compares the battery temperature to an internal high-temperature cutoff

threshold, VTCO, and a low-temperature threshold, VLTF. During fast charge only, the VHTF fault comparator is

disabled. When the voltage at the TS pin is lower than VTCO, the bq2000T terminates fast charge, moves to the

charge suspended state, and turns off the LED. When VTS rises above VHTF, the bq2000T resumes charging in

the trickle maintenance charge state, per Figure 2. In fast charge (either constant current or constant voltage fast

charge), when the voltage on the TS pin is higher than VLTF, the charger enters the battery conditioning state, as

described in the previous section. Fast charge is resumed when VTS is less than VLTF.

Rate of Temperature Rise (NiCd, NiMH)

The bq2000T uses a rate of temperature rise (

ΔT/Δt) scheme to terminate fast charge for NiCd and NiMH

batteries. During fast charge, it samples the TS pin voltage every 8 seconds and compares it to the value

measured 2 samples earlier. This feature terminates fast charge if this voltage declines at a rate of VCC/161

(V/min). Figure 6 shows a typical connection diagram. In preparation for sampling the TS pin voltage, the

bq2000T briefly turns off most circuits (the MOD and RC pins both go low) in order to get the cleanest possible,

noise-free measurement. While the monitoring of the TS pin voltage is continuous, the sampling of the TS pin

voltage with the internal ADC only occurs during the constant current regulation phase of fast charge. If the cell

voltage reaches VMCV, the pack is assumed to be Li-Ion and the TS pin voltage sampling is disabled, as ΔT/Δt is

not a termination criterion for Lithium cells.

Minimum Current (Li-Ion Only)

The bq2000T monitors the charging current during the voltage-regulation phase of Li-Ion batteries. Fast charge is

terminated when the current is tapered off to 7% of the maximum charging current. Note that this threshold is

different for the bq2000.

Once constant-current fast charge has ended, the bq2000T either measures the value of the CMTO capacitor (in

the case of Nickel batteries) and then proceeds to either top-off or trickle maintenance charge or simply

completes the constant-voltage stage of fast charge (in the case of a Li-Ion cell).

Top-Off and Pulse-Trickle Maintenance Charge

An optional top-off charge is available for NiCd or NiMH batteries. Top-off may be desirable on batteries that

have a tendency to terminate charge before reaching full capacity. To enable this option, the capacitance value

of CMTO connected between the RC pin and VCC (Figure 5) should be greater than 0.13 µF , and the value of the

resistor connected to this pin should be less than 250 k

Ω . To disable top-off, the capacitance value should be

less than 0.07 µF. The tolerance of the capacitor needs to be taken into account in component selection.

Copyright © 1999–2010, Texas Instruments Incorporated

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