BQ2000TS datasheet(7/18 Pages) TI

7 / 18 page

nickel-based pack is below that of the Li-Ion pack. Oth-

erwise, different scaling is required.

Minimum Current (Li-Ion Only)

The bq2000 monitors the charging current during the

voltage-regulation phase of Li-Ion batteries. Fast charge

is terminated when the current is tapered off to 14% of

the maximum charging current.

Initial Hold-Off Period

The values of the external resistor and capacitor

connected to pin RC set the initial hold-off period.

During this period, the bq2000 avoids early termination

due to an initial rise in the battery voltage by disabling

the peak voltage-detection feature. This period is fixed

at the programmed value of the maximum charge time

divided by 32.

hold-off period =

maximum time - out

Top-Off and Pulse-Trickle 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 to pin RC (Figure 4) should be greater

than 0.13

µF, and the value of the resistor connected to

this pin should be less than 15k

Ω. To disable top-off, the

capacitance value should be less than 0.07

µF. The toler-

ance of the capacitor needs to be taken into account in

component selection.

Once enabled, the top-off is performed over a period

equal to the maximum charge time at a rate of 116 that

of fast charge.

Following top-off, the bq2000 trickle-charges the battery

by enabling the MOD to charge at a rate of once every

1.0 second. The trickle pulse-width is user-selectable

and is set by the value of the resistor RMTO, connected to

pin RC. Figure 7 shows the relationship between the

trickle pulse-width and the value of RMTO. The typical

tolerance of the pulsewidth below 150k

Ω is ±10%.

During top-off and trickle-charge, the bq2000 monitors

battery voltage and temperature. These charging func-

tions are suspended if the battery voltage rises above

the maximum cell voltage (VMCV) or if the temperature

exceeds the high-temperature fault threshold (VHTF).

Charge Current Control

The bq2000 controls the charge current through the

MOD output pin. The current-control circuit supports a

switching-current regulator with frequencies up to

500kHz. The bq2000 monitors charge current at the

SNS input by the voltage drop across a sense-resistor,

RSNS, in series with the battery pack. See Figure 9 for a

typical current-sensing circuit. RSNS is sized to provide

the desired fast-charge current (IMAX):

IMAX =

0.05

RSNS

If the voltage at the SNS pin is greater than VSNSLO or

less than VSNSHI, the bq2000 switches the MOD output

high to pass charge current to the battery. When the

bq2000

2468

100

150

200

250

RMTO—k

Ω

100

120

140

160

180

200

2000PNvB3.eps

Shows Tolerance

Figure 7. Relationship Between Trickle Pulse-Width and Value of RMTO

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