BQ24400_16 Datasheet, PDF (6/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24400
SLUS498A – SEPTEMBER 2001 – REVISED FEBRUARY 2010
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Initiation and Charge Qualification
The bq24400 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 bq24400 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 bq24400 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 bq24400 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 bq24400 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 bq24400 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.
BATTERY CHEMISTRY
NiCd or NiMH Batteries
(V_BAT < V_MCV always)
Table 1. Charge Algorithm
CHARGE ALGORITHM
1. Charge qualification
2. Trickle charge, if required
3. Fast charge (constant current)
4. Fast charge termination (peak voltage, maximum charge time = 1 MTO)
5. Top-off (optional)
6. Trickle charge
FAST CHARGE TERMINATION
Initial Hold-Off Period
The bq24400 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 bq24400 avoids early
termination due to an initial peak in the battery voltage by disabling the peak voltage-detection (PVD) feature.
This period is fixed at the programmed value of the maximum charge time (MTO) divided by 32.
hold-off period = ^MTO
32
(1)
6
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