The DRV871x-Q1 differential shunt amplifier gain and shunt resistor value are selected based on the dynamic current range, reference voltage supply, shunt resistor power rating, and operating temperature range. In bidirectional operation of the shunt amplifier, the dynamic range at the output is approximately calculated as shown in Equation 11. The output of the amplifier can swing from the midpoint reference (VAREF / 2) to either 0.25 V or VAREF - 0.25V depending on the polarity of the input voltage to the amplifier.
If only unidirectional current sensing is required, the amplifier reference can be modified to expand the dynamic range at the output. The is modified through the CSA_DIV SPI register setting. In this mode, the dynamic range at the output is approximately calculated as shown in Equation 12.
Based on VAREF = 3.3 V, the dynamic out range in both bidirectional or unidirectional sensing can be calculated as shown below:
The external shunt resistor value and shunt amplifier gain setting are selected based on the available dynamic output range, the shunt resistor power rating, and maximum motor current that needs to be measured. This exact values for the shunt resistance and amplifier gain are determine by both Equation 15 and Equation 16.
Based on VSO = 1.4 V, IMAX = 25 A and PSHUNT = 3 W, the values for shunt resistance and amplifier gain can be calculated as shown below:
Based on the results, a shunt resistance of 4 mΩ and an amplifier gain of 10 V/V can be selected.