The quality of the Kelvin connections at the
sense resistor is critical. Verify that the sense resistor has a temperature
coefficient no greater than 50ppm to minimize current measurement drift with
temperature. Choose the value of the sense resistor in correspondence with the
available overcurrent and short-circuit ranges of the BQ27Z561 gas gauge. Select
the smallest value possible to minimize the negative voltage generated on the
BQ27Z561 VSS node during a short circuit. This pin has an absolute minimum of
–0.3V. Use parallel resistors as long as good Kelvin sensing is established. The
design of the device supports a 1mΩ to 3mΩ sense resistor.
Directly tie the BAT_SNS to the positive
connection of the battery. Confirm that the BAT_SNS shows not share a path with
the BAT pin.
In reference to the gas gauge circuit,
differential low-pass filter and I2C communication require attention
for component placement and layout.
The BQ27Z561 gas gauge uses an integrating
delta-sigma ADC for current measurements. Add a 100Ω resistor from the sense
resistor to the SRP and SRN inputs of the device. Place a 0.1μF filter capacitor
across the SRP and SRN inputs. If required for a circuit, add 0.1µF filter
capacitors 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. Add a ground plane
around the filter network to provide additional noise immunity.
The BQ27Z561 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 pull-down. When the gas
gauge senses that both lines are low (such as during removal of the pack), the
device performs auto-offset calibration and enters into SLEEP mode to conserve
power.