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 detection drift
with temperature. Choose the value of the sense resistor to correspond to the
available overcurrent and short-circuit ranges of the BQ77307 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.
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.