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 BQ27Z561-R2 gas gauge. Select the smallest value possible to minimize the negative voltage generated on the BQ27Z561-R2 VSS node during a short circuit. This pin has an absolute minimum of –0.3 V. Parallel resistors can be used as long as good Kelvin sensing is ensured. The device is designed to support a 1-mΩ to 3-mΩ sense resistor.
BAT_SNS should be tied directly to the positive connection of the battery. It should not share a path with the BAT pin.
In reference to the gas gauge circuit the following features require attention for component placement and layout: differential low-pass filter and I2C communication.
The BQ27Z561-R2 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. If required for a circuit, 0.1-µF filter capacitors can be added for additional noise filtering for each sense input pin to ground. Place all filter components as close as possible to the device. Route the traces from the sense resistor in parallel to the filter circuit. Adding a ground plane around the filter network can provide additional noise immunity.
The BQ27Z561-R2 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.