BQ24400_16 Datasheet, PDF (10/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24400
SLUS498A – SEPTEMBER 2001 – REVISED FEBRUARY 2010
www.ti.com
Both top-off and trickle maintenance charge are terminated and the pack never receives any more charge (until a
charge initialization occurs) if the voltage on the BAT pin reaches V_MCV. During trickle maintenance charge,
charging is suspended if V_TS < V_HTF. It resumes when V_TS > V_HTF. The bq24400 is designed to remain in trickle
maintenance charge forever (excluding the two faults just mentioned) in order to keep the Nickel pack full.
Charge Current Control
The bq24400 implements a hysteretic control loop that regulates the current being delivered to the battery pack
to a user programmable value that is set by the value of the R_SNS resistor. A second, outer control loop reduces
the average current delivered to the pack in order to clamp the voltage at the BAT pin to a maximum of V_MCV.
The bq24400 controls the MOD pin to regulate the current and voltage of the pack. The bq24400 monitors
charge current at the SNS input by sensing the voltage drop across a sense-resistor, R_SNS, in series with the
battery pack. See Figure 7 for a typical current-sensing circuit.
BAT-
R_f
R_SNS
1 _SNS
Cf
2 _VSS
Power supply ground
bq24400 ground and BAT-
bq24400
Figure 7. Current-Sensing Circuit
R_SNS is sized to provide the desired fast-charge current (I_MAX).
0.05
I_MAX
=
R_SNS
(4)
If the voltage at the SNS pin is greater than V_SNSLO or less than V_SNSHI, the bq24400 switches the MOD output
high to pass charge current to the battery. When the SNS voltage is less than V_SNSLO or greater than V_SNSHI, the
bq24400 switches the MOD output low to shut off charging current to the battery. A hysteresis capacitor (C_HYS) is
required between the C_MOD pin and the SNS pin to add a healthy amount of hysteresis to the current sense
signal (see Figure 8). Typical hysteresis values are between 5 and 25 mV. The amount of hysteresis can be
calculated by examining the capacitive divider formed by C_HYS and C_f.
Hysteresis (V) = VCC
´
CHYS
(CHYS + Cf )
(5)
10
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