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BQ24401 Datasheet(PDF) 4 Page - Texas Instruments

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Part No. BQ24401
Description  Programmable NiCd/NiMH Fast-Charge Management Device
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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BQ24401 Datasheet(HTML) 4 Page - Texas Instruments

 
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Initiation and Charge Qualification
The bq24401 initiates a charge cycle when it detects
l
Application of power to VCC
l
Battery replacement
l
Exit from sleep mode
Immediately following initiation, the IC enters a
charge-qualification mode. The bq24401 charge qualifica-
tion is based on battery voltage and temperature. If
voltage on pin BAT is less than the internal threshold,
VLBAT, the bq24401 enters the charge-pending state.
This condition indicates the possiblility of a defective or
shorted battery pack. In an attempt to revive a fully
depleted pack, the bq24401 enables the MOD pin to
trickle-charge at a rate of once every 1.0s. As explained in
the section “Top-Off and Pulse-Trickle Charge,” the
trickle pulse-width is user-selectable and is set by the
value of the resistance connected to pin RC.
During this period, the LED pin blinks at a 1Hz rate,
indicating the pending status of the charger.
Similarly, the bq24401 suspends fast charge if the battery
temperature is outside the VLTF to VHTF range. (See Ta-
ble 4.) For safety reasons, however, it disables the pulse
trickle, in the case of a battery over-temperature condi-
tion (i.e., VTS <VHTF). Fast charge begins when the bat-
tery temperature and voltage are valid.
NiCd and NiMH Batteries
Following qualification, the bq24401 fast-charges NiCd or
NiMH batteries using a current-limited algorithm. Dur-
ing the fast-charge period, it monitors charge time, tem-
perature, and voltage for adherence to the termination
criteria. This monitoring is further explained in later sec-
tions. Following fast charge, the battery is topped off, if
top-off is selected. The charging cycle ends with a trickle
maintenance-charge that continues as long as the voltage
on pin BAT remains below VMCV.
Charge Termination
Maximum Charge Time
The bq24401 sets the maximum charge-time through pin
RC. With the proper selection of external resistor and ca-
pacitor, various time-out values may be achieved. Figure
3 shows a typical connection.
The following equation shows the relationship between
the RMTO and CMTO values and the maximum charge
time (MTO) for the bq24401:
MTO=RMTO ∗ CMTO ∗ 35,988
MTO is measured in minutes, RMTO in ohms, and CMTO
in farads.
(Note: RMTO and CMTO values also determine other fea-
tures of the device. See Tables 2 and 3 for details.)
Maximum Temperature
A negative-coefficient thermistor, referenced to VSS and
placed in thermal contact with the battery, may be used
as a temperature-sensing device. Figure 4 shows a typical
temperature-sensing circuit.
During fast charge, the bq24401 compares the battery
temperature to an internal high-temperature cutoff
threshold, VTCO. As shown in Table 4, high-temperature
termination occurs when voltage at pin TS is less than
this threshold.
DT/Dt
When fast charging, the bq24401 monitors the voltage at
pin TS for rate of temperature change detection, ∆T/∆t.
The bq24401 samples the voltage at the TS pin every 16s
and compares it to the value measured 2 samples earlier.
This feature terminates fast charge if this voltage de-
clines at a rate of
V
161
V
Min
CC
Figure 4 shows a typical connection diagram.
4
bq24401
Battery Chemistry
Charge Algorithm
NiCd or NiMH
1. Charge qualification
2. Trickle charge, if required
3. Fast charge (constant current)
4. Charge termination (∆T/∆t, time)
5. Top-off (optional)
6. Trickle charge
Table 1. Charge Algorithm


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