Datasheet

BAT+
bq24401
2
4
BAT
V
SS
R
B1
R
B2
B1
B2
R
= N 1
R
-
bq24401
SLUS497A –SEPTEMBER 2001–REVISED MARCH 2010 www.ti.com
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.
Figure 10. Battery Voltage Divider and Filter
The resistor values R
B1
and R
B2
are calculated by the following equation:
(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.
Table 3. Temperature-Monitoring Conditions and Actions
TEMPERATURE CONDITION ACTION
During charge qualification, no effect
During fast charge, suspends fast charge and moves into charge
qualification, pauses timer, and flashes LED
V
TS
> V
LTF
Cold battery – checked at all times
During top-off, suspends top-off and moves into trickle maintenance
charge and pauses timer
During trickle maintenance charge, no effect
V
HTF
< V
TS
< V
LTF
Optimal charging range Allows all stages of charging
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