BQ28400PWR Datasheet, PDF (18/24 Pages) Texas Instruments – Tablet PC and Netbook 2-Series Cell Li-Ion Battery Gas Gauge and Protection

bq28400
SLUSA61A – OCTOBER 2010 – REVISED DECEMBER 2010
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Cell Balancing
Cell balancing increases the useful life of battery packs. Cell-to-cell differences in self-discharge, capacity, and
impedance can lead to different charge states among the cells; however, the charger terminates the charge
based on the summed voltage only, which may leave some cells undercharged and others overcharged. To
remedy this imbalance and to achieve the goal of having all cells reach 100% state-of-charge at charge
termination, it is necessary to reduce the charge added to the overcharged cells by creating a current bypass
during charging.
Cell balancing in the bq28400 is accomplished by connecting an external parallel bypass load to each cell and
enabling the bypass load depending on each individual cell's charge state. The bypass load is typically formed by
a P-CH MOSFET and a resistor. The series resistors that connect the cell tabs to VC1~VC2 pins of the bq28400
are required to be 1 kΩ. The bq28400 balances the cells during charge by discharging those cells above the
threshold set in Cell Balance Threshold, if the maximum difference in cell voltages exceeds the value
programmed in Cell Balance Min. During cell balancing, the bq28400 measures the cell voltages at an interval
set in Cell Balance Interval. On the basis of the cell voltages, the bq28400 either selects the appropriate cell to
discharge or adjusts the cell balance threshold up by the value programmed in Cell Balance Window when all
cells exceed the cell balance threshold or the highest cell exceeds the cell balance threshold by the cell balance
window.
Cell balancing only occurs when charging current is detected and the cell balance threshold is reset to the value
in Cell Balance Threshold at the start of every charge cycle. The threshold is only adjusted once during any
balance interval.
Sense Time
t_S
Cell Balance
Interval
VREG
Cell Balance
Threshold
Higher Cell
Voltage
Bypass is Active
Cell Balance
Min
Cell Balance
Window
Middle Cell
Voltage
Lowest Cell
Voltage
Figure 2. Cell Balance
The bq28400 supports cell balancing using an external MOSFET, as illustrated in Figure 3.
Figure 3 shows an example of a cell-balancing circuit for a 2-series cell application. In this circuit, Q1 and Q2 are
the external MOSFETs—specifically, Si1023 P-channel MOSFETs. These FETs were chosen because of its low
gate-to-source threshold voltage.
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