This value depends on the
cell chemistry and the load requirements of the system. For example, to
support a minimum battery voltage of 18 V using Li-CO2 type
cells with a cell minimum voltage of 3 V, at least 6-series cells are
required.
The BQ76907 device is
designed for use with low-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 5 V (DC) to 10 V (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.
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 = 200 mV
/ 40.0 A =
5-mΩ
maximum.
Considering a
short circuit discharge current of 80 A, the recommended maximum
SRP, SRN voltage of ≈0.75 V, and the maximum SCD threshold of
500 mV, the sense resistor should be below 500 mV / 80 A=
6.25-mΩ
maximum.
Further tolerance
analysis (value tolerance, temperature variation, and so on) and PCB
design margin should also be considered, so a sense resistor of 1 mΩ is
suitable with a 50-ppm temperature coefficient and power rating of
1 W.
The REGOUT is selected to provide
the supply for an external host processor and the pullup supply for the
I2C bus and ALERT pin, with output voltage selected for 3.3 V.
A 1 μF or larger
capacitor should be placed at the REGOUT pin.
The REGOUT draws its
input current from the REGSRC pin. This pin is connected to PACK+
through a series diode and 10 Ω resistor, with a
1-μF
capacitor to VSS placed at the REGSRC pin.