The values of the external resistor and capacitor con-
nected to pin RC set the initial hold-off period. Dur-
ing this period, the bq2000T avoids early termination
by disabling the ∆T/∆t feature. This period is fixed at
the programmed value of the maximum charge time
divided by 32.
hold-off period =
maximum time - out
32
Top-Off and Pulse-Trickle Charge
An optional top-off charge is available for NiCd or
NiMH batteries. Top-off may be desirable on batter-
ies that have a tendency to terminate charge before
reaching full capacity. To enable this option, the ca-
pacitance value of CMTO connected to pin RC (Figure
4) should be greater than 0.13µF, and the value of the
resistor connected to this pin should be less than
15kΩ. To disable top-off, the capacitance value
should be less than 0.07µF. The tolerance of the ca-
pacitor 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 bq2000T 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, which is on pin RC. Figure 7 shows the relation-
ship between the trickle pulse-width and the value of
RMTO. The typical tolerance of the pulsewidth below
150kΩ 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 tem-
perature 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 cur-
rent-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 be-
tween the battery pack’s positive terminal and VSS
scales the battery voltage measured at pin BAT.
7
bq2000T
1
2468
10
50
100
150
200
250
2
3
4
20
40
60
RMTO—k
Ω
80
100
120
140
160
2000PNvB3.eps
Shows Tolerance
Figure 7. Relationship Between Trickle Pulse-Width and Value of RMTO
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