BAT+

bq24400

2

4

BAT

B1

B2

R

= N 1

R

-

bq24400

SLUS498A – SEPTEMBER 2001 – REVISED FEBRUARY 2010

www.ti.com

the trickle maintenance charge state, per Figure 2. In fast charge, when the voltage on the TS pin is higher than

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

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

then smooths out this voltage to present a clean signal to the BAT pin.

Figure 5. Battery Voltage Divider and Filter

(3)

Top-Off and Pulse-Trickle Maintenance Charge

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 the resistor connected to this pin should be less than 250k

Ω. 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.

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.

8

Submit Documentation Feedback

Copyright © 2001–2010, Texas Instruments Incorporated

Product Folder Link(s): bq24400