Datasheet

BAT-
Powersupplyground
bq24400groundandBAT-
SNS
R
f
C
f
1
2
bq24400
V
SS
R
SNS
MAX
SNS
0.05
I =
R
C
HYS
Hysteresis (V) = V
CC
(C + C )
HYS f
´
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.
Figure 7. Current-Sensing Circuit
R
SNS
is sized to provide the desired fast-charge current (I
MAX
).
(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
.
(5)
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