SLUSE14A December 2020 – February 2021 BQ76942
The BQ76942 device monitors pack current using a low-side sense resistor that connects to the SRP and SRN pins through an external RC filter, which should be connected such that a charging current will create a positive voltage on SRP relative to SRN. The differential voltage between SRP and SRN is digitized by an integrated coulomb counter ADC, which can digitize voltages over a ±200 mV range and uses multiple digital filters to provide optimized measurement of the instantaneous, averaged, and integrated current. The device supports a wide range of sense resistor values, with a larger value providing better resolution for the digitized result. The maximum value of sense resistor should be limited to ensure the differential voltage remains within the ±200-mV range for system operation when current measurement is desired. For example, a system with maximum discharge current of 200 A during normal operation (not a fault condition) should limit the sense resistor to 1 mΩ or below.
The SRP and SRN pins can also support higher positive voltages relative to VSS, such as may occur during overcurrent or short circuit in discharge conditions, without damage to the device, although the current is not accurately digitized in this case. For example, a system with a 1-mΩ sense resistor and the Short Circuit in Discharge protection threshold programmed to a 500 mV level would trigger an SCD protection fault when a discharge current of 500 A was detected.
Multiple digitized current values are available for readout over the serial communications interface, including two using separate hardware digital filters, CC1 and CC2, as well as a firmware filter CC3.
The CC1 filter generates a 16-bit current measurement that is used for charge integration and other decision purposes, with one output generated every 250 ms when the device is operating in NORMAL mode.
The CC2 filter generates a 24-bit current measurement that is used for current reporting, with one output every 3 ms when the device is operating in NORMAL mode (which can be reduced to one output every 1.5 ms based on setting, with reduced measurement resolution). It is reported in 16-bit format, and the 24-bit CC2 data is also available as raw coulomb counter ADC counts, provided in 32-bit format (with the data contained in the lower 24 bits and the upper 8 bits sign-extended).
The CC3 filter output is an average of a programmable number of CC2 current samples (up to 255), based on configuration setting. The CC3 output is reported in 32-bit format.
The integrated passed charge is available as a 64-bit value, which includes the upper 32 bits of accumulated charge as the integer portion, the lower 32 bits of accumulated charge as the fractional portion, and a 32-bit accumulated time over which the charge has been integrated in units of seconds. The accumulated charge integration and timer can be reset by a command from the host over the digital communications interface.