BQ24171_15 Datasheet, PDF (21/39 Pages) Texas Instruments – bq24171 JEITA-Compliant Stand-Alone, Switched-Mode Li-Ion and Li-Polymer Battery Charger With Integrated MOSFETs and Power Path Selector

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bq24171
SLUSAF2C – FEBRUARY 2011 – REVISED APRIL 2015
Feature Description (continued)
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 gets up 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.
VBAT_RE
Battery
Absent
Battery
Absent
VRECH
VLOW
Battery
Present
Figure 16. 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
capacitances can be calculated according to the following equation:
C_MAX
=
I_DISCH ´ t_DISCH
(2.05 V - 1.45 V)
´
é
ê¹⁺
R₂
ù
ú
ë ^R1 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 FB pin.
(7)
9.3.15.1 Example
For a 3-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.
8 mA ´ 1 sec
C_MAX
=
0.6 V
´
é 500 kW ù
êë¹⁺ _{100 kW} úû
= 2.2 mF
(8)
Based on these calculations, no more than 2200 µF should be allowed on the battery node for proper operation
of the battery detection circuit.
9.3.16 Battery Short Protection
When SRN pin voltage is lower than 2 V, it is considered as battery short condition during charging period. The
charger will shut down immediately for 1 ms, then soft start back to the charging current the same as precharge
current. This prevents high current may build in output inductor and cause inductor saturation when battery
terminal is shorted during charging. The converter works in nonsynchronous mode during battery short.
9.3.17 Battery Overvoltage Protection
The converter will not allow the high-side FET to turn on until the battery voltage goes below 102% of the
regulation voltage. This allows 1-cycle response to an overvoltage condition – such as occurs when the load is
removed or the battery is disconnected. A total 6-mA current sink from SRP/SRN to AGND allows discharging
the stored output inductor energy that is transferred to the output capacitors. If battery overvoltage condition lasts
for more than 30 ms, charge is disabled.
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