MTO

hold-off period =

32

bq2000

2

RC

7

6

VSS

VCC

CMTO

RMTO

MTO

MTO

MTO = R

C

35,988

´

´

bq2000

SLUS138D – JANUARY 2008 – REVISED DECEMBER 2009

www.ti.com

(1)

Maximum Charge Time (NiCD, NiMH, and Li-Ion)

The bq2000 sets the maximum charge-time through the RC pin. With the proper selection of external resistor and

capacitor values, various time-out values may be achieved. If the timer expires while still in constant-current

charging, the bq2000 assumes a Nickel chemistry and proceeds to top-off charge (if top-off is enabled) or trickle

maintenance charge. Figure 5 shows a typical connection.

Figure 5. Typical Connection for the RC Input

The following equation shows the relationship between the RMTO and CMTO values and the maximum charge time

(MTO) for the bq2000:

(2)

MTO is measured in minutes, RMTO in ohms, and CMTO in farads. (Note: RMTO and CMTO values also determine

other features of the device. See Table 4 and Table 5 for details.

If, during fast charge, VTS > VLTF, then the timer is paused and the IC enters battery conditioning charge until VTS

< VLTF. Since the IC is in the battery conditioning state, the LED flashes at the 1 Hz rate. Once VTS<VLTF, fast

charge restarts and the timer resumes from where it left off with no change in total fast charge time.

For Li-Ion cells, when the battery reaches the constant-voltage phase of fast charge, the bq2000 adds an

additional MTO of time to whatever time was left over from the constant current fast charge timer. Thus, the pack

could spend longer than 1 MTO in constant-voltage fast charge, but is always limited to 1 MTO in

constant-current fast charge. This feature provides the additional charge time required for Li-Ion cells.

For Nickel cells, if top-off is enabled, the timer is reset on the completion of fast charge before beginning top-off

charge.

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.

8

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