The quality of the Kelvin connections at the
sense resistor is critical. The sense resistor should have a temperature
coefficient no greater than 50ppm 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 BQ78709-Q1 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 BQ78709-Q1 device
uses an integrating delta-sigma coulomb counter ADC for current measurements.
For best performance, 100Ω resistors must be included from the sense resistor
terminals to the SRP and SRN inputs of the device, with a 0.1μF filter capacitor
placed across the SRP and SRN pins. Optional 0.1µF filter capacitors can be
added for additional noise filtering at each sense input pin to ground. All
filter components must 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 20µs using the two 100Ω and single differential 0.1μF 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.