bq2000

2

7

Battery

Pack

5

TS

N

T

C

VSS

VCC

VCC

RT1

RT2

C ELL

B1

B2

MC V

V

R

=

N

1

R

V

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ö

´

-

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÷

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bq2000

www.ti.com

SLUS138D – JANUARY 2008 – REVISED DECEMBER 2009

Figure 6. Temperature Monitoring Configuration

During fast charge, the bq2000 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 bq2000 terminates fast charge, moves to the

charge suspended state, and turns off the LED. When VTS rises above VHTF, the bq2000 will resume 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.

Peak Voltage (NiCd, NiMH)

The bq2000 uses a peak-voltage detection (PVD) scheme to terminate fast charge for NiCd and NiMH batteries.

The bq2000 continuously monitors the voltage on the BAT pin, representing the battery voltage, to ensure that it

never exceeds VMCV (maximum cell voltage). In addition, it also samples, at a rate of MTO/128, the voltage on

the BAT pin and triggers the peak detection feature if this value falls below the maximum sampled value by as

much as 3.8mV (PVD). In preparation for sampling the BAT pin voltage, the bq2000 briefly turns off most circuits

(the MOD and RC pins will both go low) in order to get the cleanest possible, noise-free measurement. While the

monitoring of the BAT pin voltage is continuous, the sampling of the BAT 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 BAT pin voltage sampling is disabled, as PVD is not a termination criterion for

Lithium cells. As shown in Figure 3, a resistor voltage-divider between the battery pack's positive terminal and

VSS scales the battery voltage measured at the BAT pin.

For Li-Ion battery packs, the resistor values RB1 and RB2 are calculated by the following equation:

(3)

where N is the number of cells in series and VCELL is the manufacturer-specified charging voltage. RB1 + RB2

should be at least 200k

Ω and no more than 1MΩ.

A NiCd or NiMH battery pack consisting of N series cells may benefit by the selection of the RB1 value to be N–1

times larger than the RB2 value. This sets the per cell regulation voltage (VCELL) equal to VMCV. It is critical that

VCELL be set high enough that the nickle pack not reach voltage regulation, thus allowing proper termination by

PVD. Typical VCELL for a nickle pack is between 1.7V and 2V.

In a mixed-chemistry design, a common voltage-divider is used as long as the maximum charge voltage of the

nickel-based pack is below that of the Li-Ion pack. Otherwise, different scaling is required. See Figure 7 for an

example.

Copyright © 2008–2009, Texas Instruments Incorporated

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