Datasheet

Battery
Inserted
BatterynotDetected
BatteryDetected
t
LOWV_DEG
t
RECH_DEG
t
WAKE
V
REG
V
RECH
V
LOWV
(V )
WAKE
(V )
DISH
´
é ù
´
ê ú
ë û
DISCH DISCH
MAX
2
1
I t
C =
R
1.425 1+
R
´
m
é ù
´
ê ú
ë û
MAX
8mA 1sec
C = = 930 F
500k
1.425 1+
100k
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SLUS894A –JANUARY 2010– REVISED OCTOBER 2011 www.ti.com
Once the device has powered up, an 8-mA discharge current is applied to the SRN terminal. If the battery
voltage falls below the LOWV threshold within 1 second, the discharge source is turned off, and the charger is
turned on at low charge current (125 mA). If the battery voltage rises above the recharge threshold within 500
ms, there is no battery present and the cycle restarts. If either the 500-ms or 1-second timer times out before the
respective thresholds are hit, a battery is detected and a charge cycle is initiated.
Figure 22. Battery Detect Timing Diagram
Care must be taken that the total output capacitance at the battery node is not so large that the discharge current
source cannot pull the voltage below the LOWV threshold during the 1-second discharge time. The maximum
output capacitance can be calculated as seen in Equation 10:
(10)
Where C
MAX
is the maximum output capacitance, I
DISCH
is the discharge current, t
DISCH
is the discharge time, and
R
2
and R
1
are the voltage feedback resistors from the battery to the VFB pin. The 1.425 factor is the difference
between the RECHARGE and the LOWV thresholds at the VFB pin.
EXAMPLE
For a 3-cell LiFePO
4
charger, with R2 = 500 kΩ, R1 = 100 Ωk (giving 10.8 V for voltage regulation), I
DISCH
= 8
mA, t
DISCH
= 1 second,
(11)
Based on these calculations, no more than 930 μF should be allowed on the battery node for proper operation of
the battery detection circuit.
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