BQ24400_16 Datasheet, PDF (7/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24400
www.ti.com
SLUS498A – SEPTEMBER 2001 – REVISED FEBRUARY 2010
Maximum Charge Time
The bq24400 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 fast charge, the
bq24400 proceeds to top-off charge (if top-off is enabled) or trickle maintenance charge. If top-off is enabled, the
timer is reset on the completion of fast charge before beginning top-off charge. Figure 3 shows a typical
connection.
2 _VSS
V_CC ⁷
bq24400
6
RC
C_MTO
R_MTO
Figure 3. 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 bq24400:
MTO = R_MTO ´ C_MTO ´ 35,988
(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 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.
Maximum Temperature
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 4 shows a typical temperature-sensing circuit.
V_CC
2 _VSS
V_CC ⁷
bq24400
5
TS
R_T1
R_T2
N
T
Battery
Pack
C
Figure 4. Temperature Monitoring Configuration
During fast charge, the bq24400 compares the battery temperature to an internal high-temperature cutoff
threshold, V_TCO, and a low-temperature threshold, V_LTF. During fast charge only, the V_HTF fault comparator is
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