BQ29209-Q1 Datasheet, PDF (7/23 Pages) Texas Instruments – Voltage Protection with Automatic Cell Balance

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8 Detailed Description
bq29209-Q1
SLUSC62A – JUNE 2015 – REVISED MARCH 2016
8.1 Overview
The bq29209-Q1 provides overvoltage protection and cell balancing for 2-series cell lithium-ion battery packs.
8.1.1 Voltage Protection
Each cell voltage is continuously compared to a factory configured internal reference threshold. If either cell
reaches an overvoltage condition, the bq29209-Q1 device starts a timer that provides a delay proportional to the
capacitance on the CD pin. Upon expiration of the internal timer, the OUT pin changes from a low to high state.
8.1.2 Cell Balancing
If enabled, the bq29209-Q1 performs automatic cell-balance correction where the two cells are automatically
corrected for voltage imbalance by loading the cell with the higher voltage with a small balancing current. When
the cells are measured to be equal within nominally 0 mV, the load current is removed. It will be re-applied if the
imbalance exceeds nominally 30 mV. The cell mismatch correction circuitry is enabled by pulling the CB_EN pin
low, and disabled when CB_EN is pulled to greater than 2.2 V, for example, VDD.
If the internal cell balancing current of up to 15 mA is insufficient, the bq29209-Q1 may be configured via external
circuitry to support much higher external cell balancing current.
8.2 Functional Block Diagram
VDD
VC2
VC1
VC1_CB
GND
CB_EN
CB ^CB2_EN
Logic
CB1 _EN
CTRL
+
Hys.
–
+
–
5-V LDO
and POR
I_CD =
150 nA
CD
0.1 µF
OUT
8.3 Feature Description
8.3.1 Protection (OUT) Timing
Sizing the external capacitor is based on the desired delay time as follows:
td
C_CD = _XDELAY
Where t_d is the desired delay time and X_DELAY is the overvoltage delay time scale factor, expressed in seconds
per microFarad. X_DELAY is nominally 9 s/µF. For example, if a nominal delay of 3 seconds is desired, use a C_CD
capacitor that is 3 s / 9 s/µF = 0.33 µF.
The delay time is calculated as follows:
t_d = C_CD ´ X_DELAY
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