This value depends on the cell chemistry and the
load requirements of the system. For example, to support a minimum
battery voltage of 35 V using
Li-CO2
type cells with a cell minimum voltage of 2.5 V, there needs to be at
least 14-series cells.
The BQ769142 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 BQ769142 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 BQ769142 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 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 = 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
analyses
(value tolerance, temperature variation, and so on) and
the
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 an
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 60 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 BQ769142 device.
The BJT should support
the maximum current expected from the REG1 (maximum of 45 mA, with short
circuit current limit of up to ≈80 mA).
A diode can
be
optionally included in the collector circuit of the
BJT
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 16-2 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.