This value depends on the cell chemistry and the
load requirements of the system. For example, to support a minimum
battery voltage of 12.5V using Li-CO2 type cells with a cell minimum
voltage of 2.5V, there needs to be at least 5-series cells.
The BQ76922 device is designed for use with
high-side NFET protection
The configuration should be selected for series versus parallel FETs, which may lead to different FET selection for charge versus discharge direction.
These FETs should be rated for the maximum:
Voltage, which should be approximately 5V (DC) to
10V (peak) per series cell.
Current, which should be calculated based on both the maximum DC current and the maximum transient current with some margin.
Power dissipation, which can be a factor of the
RDS(ON) rating of the FET, the FET package, and the PCB
design.
The overdrive level of the BQ76922 device charge
pump should be selected based on RDS(ON) requirements for the protection
FETs and their voltage handling requirements. If the FETs are selected
with a maximum gate-to-source voltage of 15V, then the 11V overdrive
mode within the BQ76922 device can be used. If the FETs are not
specified to withstand this level, or there is a concern over gate
leakage current on the FETs, the lower overdrive level of 5.5V can be
selected.
Sense resistor selection
The resistance value should be
selected to maximize the input range of the coulomb counter but not exceed the
absolute maximum ratings, and avoid excessive heat generation within the resistor.
Using the normal maximum charge or discharge
current, the sense resistor = 200mV / 20.0A = 10 mΩ
maximum.
However, considering a short circuit discharge
current of 80 A, the recommended maximum SRP, SRN voltage of
approximately 0.75V, and the maximum SCD threshold of 500mV, the
sense resistor should be below 500mV / 80A= 6.25mΩ maximum.
Further tolerance analysis (value tolerance,
temperature variation, and so on) and PCB design margin should also be
considered, so a sense resistor of 1mΩ is suitable with a 50ppm
temperature coefficient and power rating of 1W.
The REG1 is selected to provide the supply for an
external host processor, with output voltage selected for 3.3V.
The NPN BJT used for the
REG0 preregulator should be selected to support the maximum
collector-to-emitter voltage of the maximum charging voltage of 22V. The
gain of the BJT should be chosen so it can provide the required maximum
output current with a base current level that can be provided from the
BQ76922 device.
The BJT should support
the maximum current expected from the REG1 (maximum of 45mA, with short
circuit current limit of up to approximately 80mA).
A diode can optionally be
included in the collector circuit of the BJT, in order to avoid reverse
current flow from BREG through the base-collector junction of the BJT to
PACK+ during a pack short circuit event. This diode can be seen in Figure 8-2 at D2.
A large resistor (such as
10mΩ) is recommended from BREG to VSS, to avoid any unintended leakage
current that may occur during SHUTDOWN mode.