Datasheet

MTO
hold-off period =
32
bq2000
2
RC
7
6
V
SS
V
CC
C
MTO
R
MTO
MTO MTO
MTO = R C 35,988´ ´
bq2000
SLUS138D –JANUARY 2008–REVISED DECEMBER 2009 www.ti.com
(1)
Maximum Charge Time (NiCD, NiMH, and Li-Ion)
The bq2000 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 bq2000 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 R
MTO
and C
MTO
values and the maximum charge time
(MTO) for the bq2000:
(2)
MTO is measured in minutes, R
MTO
in ohms, and C
MTO
in farads. (Note: R
MTO
and C
MTO
values also determine
other features of the device. See Table 4 and Table 5 for details.
If, during fast charge, V
TS
> V
LTF
, then the timer is paused and the IC enters battery conditioning charge until V
TS
< V
LTF
. Since the IC is in the battery conditioning state, the LED flashes at the 1 Hz rate. Once V
TS
<V
LTF
, 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 bq2000 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.
Maximum Temperature (NiCd, NiMH, Li-Ion)
A negative-coefficient thermistor, referenced to V
SS
and placed in thermal contact with the battery, may be used
as a temperature-sensing device. Figure 6 shows a typical temperature-sensing circuit.
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Product Folder Link(s): bq2000