The quality of the Kelvin
connections at the sense resistor is critical. The sense resistor should have a
temperature coefficient no greater than 50 ppm in order 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 BQ76907
device. Parallel resistors can be used as long as good Kelvin sensing is
ensured.
In reference to the system
circuitry, the following features require attention for component placement and
layout:
differential
low-pass
filter
and I2C communication.
The BQ76907 device uses an
integrating delta-sigma coulomb counter ADC for current measurements. For best
performance, 100-Ω resistors should be included from the sense resistor
terminals to the SRP and SRN inputs of the device, with a 0.1-μF filter
capacitor placed across the SRP and SRN pins. Optional 0.1-µF filter capacitors
can be added for additional noise filtering at each sense input pin to ground.
All filter components should be placed as close as possible to the device,
rather than close to the sense resistor, and the traces from the sense resistor
routed in parallel to the filter circuit. A ground plane can also be included
around the filter network to add additional noise immunity.
These filter components between
the sense resistor and the SRP and SRN terminals provide filtering of noise
components, but they also introduce an RC time constant delay, nominally 20 µs
using the two 100-Ω and single differential 0.1-μF components. If this delay
introduces too much additional time into the response of the device to short
circuit events, the filter time constant can be reduced, with the tradeoff of
providing less filtering.
The I2C clock and data
pins have integrated ESD protection circuits; however, adding a Zener diode and
series resistor on each pin provides more robust ESD performance.