MTO

hold-off period =

32

bq2000T

2

RC

7

6

VSS

VCC

CMTO

RMTO

MTO

MTO

MTO = R

C

35,988

´

´

bq2000T

SLUS149D – MAY 1999 – REVISED JANUARY 2010

www.ti.com

FAST CHARGE TERMINATION

Initial Hold-Off Period

The bq2000T incorporates a user programmable hold-off period to avoid premature fast charge termination that

can occur with brand new nickel cells at the very beginning of fast charge. The values of the external resistor and

capacitor connected to the RC pin set the initial hold-off period. During this period, the bq2000T avoids early

termination due to an initial rise in the battery temperature by disabling the rate of temperature rise (

ΔT/Δt)

feature. This period is fixed at the programmed value of the maximum charge time (MTO) divided by 32.

(1)

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

The bq2000T 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 bq2000T 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 bq2000T:

(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 bq2000T 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.

8

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