BQ24618_15 Datasheet, PDF (25/43 Pages) Texas Instruments – System Power Selector and Low

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V_REG
V_RECH
(V_WAKE)
V_LOWV
(V_DISH)
Battery not detected
bq24618
SLUSA55B – OCTOBER 2010 – REVISED APRIL 2015
Battery
inserted
Battery detected
t_{LOWV_DEG}
t_{RECH_DEG}
t_WAKE
Figure 17. Battery-Detect Timing Diagram
Ensure 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 follows:
C_MAX
=
I_DISCH ´ t_DISCH
0.5
´
é
ê¹⁺
ë
R₂
R₁
ù
ú
û
where
• C_MAX is the maximum output capacitance.
• I_DISCH is the discharge current.
• t_DISCH is the discharge time.
• R₂ and R₁ are the voltage feedback resistors from the battery to the VFB pin.
(10)
The 0.5 factor is the difference between the RECHARGE and the LOWV thresholds at the VFB pin.
Example
For a three-cell Li+ charger, with R2 = 500 kΩ, R1 = 100 kΩ (giving 12.6 V for voltage regulation), I_DISCH = 8 mA,
t_DISCH = 1 second,
8mA ´ 1sec
C_MAX
=
0.5
´
é
êë¹⁺
500k
100k
ù
úû
= 2.7 mF
(11)
Based on these calculations, no more than 2.7 mF should be allowed on the battery node for proper operation of
the battery detection circuit.
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