BQ24620 Datasheet, PDF (22/30 Pages) Texas Instruments – Stand-Alone Synchronous Switch-Mode Lithium Phosphate Battery Charger with Low Iq

bq24620
SLUS893 – MARCH 2010
V _REG
V _RECH
V _LOWV
Battery not Detected
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Battery
Inserted
Battery Detected
t_{LOWV_ DEG}
t _{RECH_ DEG}
t_WAKE
Figure 16. 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 8:
C_MAX
=
I_DISCH
1.425
´ t_DISCH
´
é
ê¹⁺
R₂
ù
ú
ë ^R1 û
(8)
Where C_MAX is the maximum output capacitance, I_DISCH is the discharge current, t_DISCH is the discharge time, and
R₂ and R₁ 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 Li+ charger, with R2 = 500k, R1 = 100k (giving 10.8V for voltage regulation), I_DISCH = 8mA, t_DISCH = 1
second,
C_MAX
=
8mA ´ 1sec
1.425
´
éêë₁₊
500k
100k
ù
úû
= 930 mF
(9)
Based on these calculations, no more than 930 mF should be allowed on the battery node for proper operation of
the battery detection circuit.
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