BQ2000 Datasheet, PDF (7/18 Pages) Texas Instruments – Programmable Multi-Chemistry Fast-Charge Management IC

bq2000
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.
maximum time - out
hold-off period =
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 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 0.13µF, 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 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 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 150kΩ 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):
0.05
IMAX = _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
200
180
160
140
120
100
80
60
40
20
4
3
2
1
2 4 6 8 10
Shows Tolerance
50
100
RMTO—kΩ
150
200
250
2000PNvB3.eps
Figure 7. Relationship Between Trickle Pulse-Width and Value of R_MTO
7