The quality of the Kelvin connections at the
sense resistor is critical. The sense resistor
must have a temperature coefficient no greater
than 50 ppm to minimize current measurement drift
with temperature. Choose the value of the sense
resistor to correspond to the available
overcurrent and short-circuit ranges of the
BQ27Z746 gas gauge. Select the smallest value
possible to minimize thermal dissipation and still
maintain required measurement accuracy. The value
of the sense resistor impacts the differential
voltage generated across the BQ27Z746 SRP and SRN
nodes during a short circuit. These pins have a
differential voltage should
not
exceed VCC_IN of
± 0.1
V for normal operation. Parallel
sense resistors can be used as long as good Kelvin
sensing is ensured. The device is designed to
support a 1-mΩ to 20-mΩ sense resistor.
BAT should be tied directly to the positive
connection of the battery with a series 1-kΩ
resistor. It should not share a path with the VDD
pin and its 10-Ω series resistor.
In reference to the gas gauge circuit, the
following features require attention for component
placement and layout: VDD bypass capacitor, SRN
and SRP differential low-pass filter, and
I2C communication ESD external
protection.
The BQ27Z746 gas gauge uses an integrating
delta-sigma ADC for current measurements. Add a
100-Ω resistor from the sense resistor to the SRP
and SRN inputs of the device. Place a 0.1-μF
filter capacitor across the SRP and SRN inputs.
Place all filter components as close as possible
to the device. Route the traces from the sense
resistor as differential pairs to the filter
circuit. Adding a ground plane around the filter
network can provide additional noise
immunity.
The BQ27Z746 has an internal LDO that is
internally compensated and does not require an
external decoupling capacitor.
The I2C clock and data pins have
integrated high-voltage ESD protection circuits;
however, adding a Zener diode and series resistor
provides more robust ESD performance. The
I2C clock and data lines have an
internal pulldown. When the gas gauge senses that
both lines are low (such as during removal of the
pack), the device performs auto-offset calibration
and then goes into SLEEP mode to conserve
power.