This value depends on the cell chemistry and the
load requirements of the system. For example, to support a minimum battery voltage of
25 V using Li-CO2 type cells with a cell minimum voltage of 2.5 V,
10-series cells should be used.
The BQ76942 device is designed for use with
high-side NFET protection (low-side protection NFETs can be used by leveraging the
DCHG / DDSG signals).
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.
The overdrive level of the BQ76942 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 15 V, then the 11-V overdrive mode within the BQ76942 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 5.5 V can
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 / 20.0 A = 10 mΩ maximum.
However, 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 REG1 is selected to provide the supply for an external host processor, with output voltage selected for 3.3 V.
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 42.5 V. 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 BQ76942 device.
The BJT should support the maximum current
expected from the REG1 (maximum of 45 mA, with short circuit current limit of up to
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 16-1 at D2.
A large resistor (such as 10 MΩ) is
recommended from BREG to VSS to avoid any unintended leakage current that may occur
during SHUTDOWN mode.