BQ2000TPN datasheet(7/16 Pages) TI

7 / 16 page

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 1

16 that

of fast charge.

Following top-off, the bq2000T trickle-charges the bat-

tery by enabling the MOD to charge at a rate of once ev-

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

and is set by the value of the resistor RMTO, which is on

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 bq2000T monitors

battery voltage and temperature. These functions 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 bq2000T controls the charge current through the

MOD output pin. The current-control circuit supports a

switching-current regulator with frequencies up to

500kHz. The bq2000T 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 bq2000T switches the MOD output

high to pass charge current to the battery. When the

SNS voltage is less than VSNSLO or greater than VSNSHI,

the bq2000T switches the MOD output low to shut off

charging current to the battery. Figure 8 shows a typical

multi-chemistry charge circuit.

Voltage Input

As shown in Figure 6, a resistor voltage-divider between

the battery pack’s positive terminal and VSS scales the

battery voltage measured at pin BAT.

For Li-Ion battery packs, the resistor values RB1 and

RB2 are calculated by the following equation:

CELL

MCV

=∗









− 1

where N is the number of cells in series and VCELL is the

manufacturer-specified charging voltage. The end-to-end

input impedance of this resistive divider network should

be at least 200k

Ω and no more than 1MΩ.

bq2000T

2468

100

150

200

250

RMTO—k

Ω

100

120

140

160

2000PNvB3.eps

Shows Tolerance

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

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