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 BQ27Z758 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 BQ27Z758 SRP and SRN nodes during a
short circuit. These pins have a differential voltage
shouldnot
exceed VCC_IN of
±
0.1V for normal operation. Parallel sense resistors can be used as long as good
Kelvin sensing is
maintained.
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 BQ27Z758 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 BQ27Z758 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.