Datasheet

MTO
hold-off period =
32
bq24401
SLUS497A –SEPTEMBER 2001–REVISED MARCH 2010 www.ti.com
Initiation and Charge Qualification
The bq24401 initiates a charge cycle when it detects
• Application of power to V
CC
• Battery replacement
• Exit from sleep mode
Immediately following initiation, the IC enters a charge-qualification mode. The bq24401 charge qualification is
based on battery voltage and temperature. If the voltage on the BAT pin is less than the internal threshold, V
LBAT
,
the bq24401 enters the battery conditioning state. This condition indicates the possibility 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 Maintenance Charge," the
trickle pulse-width is user-selectable and is set by the value of the resistance connected between the RC pin and
V
SS
.
During charge qualification, the LED pin blinks at a 1Hz rate, indicating the pending status of the charger.
Once battery conditioning (trickle charge) has raised the voltage on the BAT pin above V
LBAT
, the IC enters fast
charge, if the battery temperature is within the V
LTF
to V
HTF
range. The bq24401 will stay in the battery
conditioning state indefinitely and will not progress to fast charge until the voltage on the BAT pin is above V
LBAT
and the temperature is within the V
LTF
and V
HTF
range. No timer is implemented during battery conditioning.
Fast Charge (Current Regulation)
Following charge qualification (which includes trickle charge, if required), the bq24401 begins fast charge fast
using a current-limited algorithm. During the fast-charge period, it monitors charge time, temperature, and voltage
for adherence to the termination criteria. This monitoring is further explained in later sections. While in the fast
charge state, the LED pin is pulled low (the LED is on). 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
the BAT pin remains below V
MCV
.
Table 1 summarizes the charging process.
Table 1. Charge Algorithm
BATTERY CHEMISTRY CHARGE ALGORITHM
1. Charge qualification
2. Trickle charge, if required
3. Fast charge (constant current)
NiCd or NiMH Batteries
(V
BAT
< V
MCV
always)
4. Fast charge termination (rate of temperature rise, maximum charge time = 1 MTO)
5. Top-off (optional)
6. Trickle charge
FAST CHARGE TERMINATION
Initial Hold-Off Period
The bq24401 incorporates a user programmable hold-off period to avoid premature fast charge termination that
can occur with brand new nickel cells at the very beginning of fast charge. The values of the external resistor and
capacitor connected to the RC pin set the initial hold-off period. During this period, the bq24401 avoids early
termination due to an initial rise in the battery temperature by disabling the rate of temperature rise (ΔT/Δt)
feature. This period is fixed at the programmed value of the maximum charge time (MTO) divided by 32.
(1)
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