BQ24400_16 Datasheet, PDF (8/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24400
SLUS498A – SEPTEMBER 2001 – REVISED FEBRUARY 2010
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disabled. When the voltage at the TS pin is lower than V_TCO, the bq24400 terminates fast charge, moves to the
charge suspended state, and turns off the LED. When V_TS rises above V_HTF, the bq24400 will resume charging in
the trickle maintenance charge state, per Figure 2. In fast charge, when the voltage on the TS pin is higher than
V_LTF, the charger enters the battery conditioning state, as described in the previous section. Fast charge is
resumed when V_TS is less than V_LTF.
Peak Voltage
The bq24400 uses a peak-voltage detection (PVD) scheme to terminate fast charge for NiCd and NiMH
batteries. The bq24400 continuously monitors the voltage on the BAT pin, representing the battery voltage, to
ensure that it never exceeds V_MCV (maximum cell voltage). In addition, it also samples, at a rate of MTO/128, the
voltage on the BAT pin and triggers the peak detection feature if this value falls below the maximum sampled
value by as much as 3.8mV (PVD). In preparation for sampling the BAT pin voltage, the bq24400 briefly turns off
most circuits (the MOD and RC pins will both go low) in order to get the cleanest possible, noise-free
measurement. While the monitoring of the BAT pin voltage is continuous, the sampling of the BAT pin voltage
with the internal ADC only occurs during fast charge. As shown in Figure 5, 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
bq24400
BAT+
R_B1
R_B2
Figure 5. 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
(3)
where N is the number of cells in series. R_B1 + R_B2 should be at least 200kΩ and no more than 1MΩ.
Top-Off and Pulse-Trickle Maintenance Charge
Once constant-current fast charge has ended, the bq24400 measures the value of the C_MTO capacitor and then
proceeds to either top-off or trickle maintenance charge. Top-off is optional and may be desirable on batteries
that have a tendency to terminate charge before reaching full capacity. To enable this option, the capacitance
value of C_MTO connected between the RC pin and V_CC (see Figure 3) should be greater than 0.13µF, and the
value of the resistor connected to this pin should be less than 250kΩ. To disable top-off, the capacitance value
should be less than 0.07µF. The tolerance of the capacitor needs to be taken into account in component
selection.
Once top-off is started, the timer is reset and top-off proceeds until the timer expires, V_MCV is reached, or there is
a temperature fault. During top-off, current is delivered to the battery in pulses that occur each second. The fixed
pulse width allows an average current of 1/16 of the fast charge current to be delivered to the battery every
second. The LED is always off during top-off and trickle maintenance charge.
During top-off, there are three different temperature faults that can occur. If V_TS > V_LTF, top-off is suspended, the
timer is paused, and trickle charge is started. When V_TS falls below V_LTF, top-off is resumed. If V_TS < V_HTF, all
charging stops, but the timer keeps counting. When V_TS > V_HTF, top-off is resumed, if there is still time remaining
on the timer. If there is not time left, trickle maintenance charge is entered. If V_TS < V_TCO, all charging stops. Only
trickle maintenance charge may resume after V_TS > V_HTF.
8
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