The BQ76905 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 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.

The coulomb counter integrates two hardware digital filters (CC1 and CC2), which each provides a separate digital output. The CC2 digital filter generates a 24-bit raw output and has programmable timing and resolution output, with the output rate also affecting the resolution of the conversion. The effective resolution (defined as the resolution such that the data exhibits 1-sigma variation with ±1-LSB) of the coulomb counter conversions increases with longer timing between output data. The setting options when the coulomb counter is in full power mode are 366 μs (which results in 13-bit effective resolution), 732 μs (14-bit effective resolution), 1.46 ms (15-bit effective resolution), or 2.93 ms (16-bit effective resolution). This output rate is set using data memory configuration bits. In addition, the coulomb counter supports a low power mode, which operates at a 16 times slower rate than the settings listed above, with similar resolution performance at each setting, but drawing only 4 μA rather than about 60 μA at full power.

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 and the coulomb counter in full power mode, or one output every 4 seconds when the device is operating in low power mode.

The integrated passed charge (from integration of the CC1 processed result) is available as a 48-bit value, which includes the upper 16 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. Note that the charge and time are not integrated while in SLEEP mode.

See the BQ76905 Technical Reference Manual for more details on current measurement, charge integration, and their associated programmable settings.