Two INA213B devices are used for current sensing in this design. The INA213 is a current-shunt monitor design for both high- and low-side current measurements. Both of these devices are supplied by 3.3 V, referenced to ground, and provide a gain of 50 V/V. U2 measures the current across the 2-mΩ sense resistor R1, which provides an input current measurement for both the system rail and battery charge currents. U4 measures the current across the 2-mΩ sense resistor R2, which provides a measurement of the current on the system output rail.

Figure 2-7 Current Sensing Schematic

There are also two current-sense buffers that are used to filter the system current rail. The buffer including U6 includes a low-pass filter with a corner frequency of 159 Hz. This provides the MCU with an accurate measurement of the system current while mitigating high-frequency noise. The buffer including U8 includes a low-pass filter with a corner frequency of 0.159 Hz. This provides the MCU with an average current signal over about 6 seconds.

The MCU only has one ADC pin available for current sensing so a 0-Ω resistor was added in series with each of these outputs to provide the ability to manually select between which current sense output is delivered to the MCU.