4 Current Sensing Circuit Diagnostic

The shunt is directly connected to BQ76972 SRP and SRN pins with a filtering network. The BQ76972 can not distinguish the open-wire and short circuit event of the current sensing circuit. For functional safety consideration, a simple external circuit is required to diagnose these abnormal events and the working condition of BQ76972 current sensing function,shown in Figure 4-1.

Figure 4-1 Current Sensing Circuit Diagnostic

Series resistors R₃ and R₄ are added between BQ76972 SRN/SRP pins and mictrocontroller GPIOs or DAC. R₃ and R₄ form an resistor divider between the filter resistors R₁ and R₂, and ground. In normal operation, the MCU pins are in high impedance state which has small enough leakage current so that the normal current sensing accuracy is not affected. In the diagnostic operation, MCU can drive pins high and then make a divided voltage between SRP and SRN, so by reading the BQ76972 current measurement, MCU can distinguish the SRP/SRN pins are not shorted together or to VSS. Also, this approach can be used to diagnose the BQ76972 SCD, OCC and OCD functions. The series resistance can be sized to trigger the current related protections alert or fault.

The effect of MCU pin leakage current can be ignored because the current measurement is differential. If only R3 or R4 is added, the leakage current of the MCU pins need be as low as enough in case of affecting the current sensing accuracy. The resolution of columb counter is 7.6uV so that the leakage current is recommended to be lower than 70nA to make sure the voltage drop on R1 or R2 can not cause LSB error.