BQ24401_15 Datasheet, PDF (12/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24401
SLUS497A – SEPTEMBER 2001 – REVISED MARCH 2010
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Battery Voltage Input
As shown in Figure 10, a resistor voltage-divider between the battery pack's positive terminal and V_SS scales the
battery voltage measured at the BAT pin. A low-pass filter then smooths out this voltage to present a clean signal
to the BAT pin.
2
V_SS
4
BAT
bq24401
BAT+
R_B1
R_B2
Figure 10. Battery Voltage Divider and Filter
The resistor values R_B1 and R_B2 are calculated by the following equation:
R_B1
R_B2
=
N
-1
(5)
where N is the number of cells in series. R_B1 + R_B2 should be at least 200kΩ and no more than 1MΩ.
TEMPERATURE MONITORING
The bq24401 measures the temperature by the voltage at the TS pin. This voltage is typically generated by a
negative-temperature-coefficient thermistor. The bq24401 compares this voltage against its internal threshold
voltages to determine if charging is safe. These thresholds are the following:
• High-temperature cutoff voltage: V_TCO = 0.225 × V_CC. This voltage corresponds to the maximum temperature
(TCO) at which any charging is allowed. The bq24401 terminates charging if the voltage on the TS pin falls
below V_TCO.
• High-temperature fault voltage: V_HTF = 0.25 × V_CC. This voltage corresponds to a maximum allowed pack
temperature (HTF) in all states except for fast charge. During fast charge, HTF faults are disabled to allow for
a normal increase in pack temperature.
• Low-temperature fault voltage: V_LTF = 0.5 × V_CC. This voltage corresponds to the minimum temperature (LTF)
at which fast charging or top-off is allowed. If the voltage on the TS pin rises above V_LTF, the bq24401
suspends either fast charge or top-off and begins a trickle charge. When the voltage falls back below V_LTF,
fast charge or top-off resumes from the point where suspended. If V_TS > V_LTF, the charger will always be in
trickle charge.
Table 3 summarizes these various conditions.
TEMPERATURE
V_TS > V_LTF
V_HTF < V_TS < V_LTF
Table 3. Temperature-Monitoring Conditions and Actions
CONDITION
Cold battery – checked at all times
Optimal charging range
ACTION
During charge qualification, no effect
During fast charge, suspends fast charge and moves into charge
qualification, pauses timer, and flashes LED
During top-off, suspends top-off and moves into trickle maintenance
charge and pauses timer
During trickle maintenance charge, no effect
Allows all stages of charging
12
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