BQ24630 Datasheet, PDF (23/33 Pages) Texas Instruments – Stand-Alone Synchronous Switch-Mode Lithium Phosphate Battery Charger with System Power Selector and Low Iq

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SLUS894 – JANUARY 2010
Once the device has powered up, an 8mA discharge current will be 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 (125mA). If the battery voltage gets up above the recharge threshold within
500ms, there is no battery present and the cycle restarts. If either the 500ms or 1 second timer time out before
the respective thresholds are hit, a battery is detected and a charge cycle is initiated.
V_REG
V_RECH
(V_WAKE)
Battery not Detected
Battery
Inserted
V_LOWV
(V_DISH)
Battery Detected
t_{LOWV_DEG} t_{RECH_DEG}
t_WAKE
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:
C_MAX
=
I_DISCH ´ t_DISCH
1.425
´
é
ê¹⁺
R₂
ù
ú
ë ^R1 û
(10)
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 LiFePO₄ 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
(11)
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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