BQ24401_15 Datasheet, PDF (8/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24401
SLUS497A – SEPTEMBER 2001 – REVISED MARCH 2010
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disabled. When the voltage at the TS pin is lower than V_TCO, the bq24401 terminates fast charge, moves to the
charge suspended state, and turns off the LED. When V_TS rises above V_HTF, the bq24401 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.
Rate of Temperature Rise
The bq24401 uses a rate of temperature rise (ΔT/Δt) scheme to terminate fast charge for NiCd and NiMH
batteries. During fast charge, it samples the TS pin voltage every 8 seconds and compares it to the value
measured 2 samples earlier. This feature terminates fast charge if this voltage declines at a rate of V_CC/161
(V/min). Figure 4 shows a typical connection diagram. In preparation for sampling the TS pin voltage, the
bq24401 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 TS pin voltage is continuous, the sampling of the
TS pin voltage with the internal ADC only occurs during fast charge.
Top-Off and Pulse-Trickle Maintenance Charge
Once constant-current fast charge has ended, the bq24401 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.
Following top-off, the bq24401 trickle-charges the battery by enabling the MOD pin to charge at a rate of once
every 1.0 second. The trickle pulse-width is user-selectable and is set by the value of the resistor R_MTO,
connected between the RC pin and V_SS. Figure 5 shows the relationship between the trickle pulse-width and the
value of R_MTO. The typical tolerance of the pulsewidth below 150kΩ is ±10%.
200
180
160
140
120
100
80
60
40
20
4
3
2
1
2 4 6 8 10
Shows Tolerance
50 100 150 200 250
R_MTO - kW
Figure 5. Relationship Between Trickle Pulse-Width and Value of R_MTO
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