In this example, shown in Figure 9-3, a nominal 10-A load creates a differential voltage of 20 mV across a 2-mΩ shunt resistor. The bus voltage for the INA231 is measured at the external BUS input pin; in this example, BUS is connected to the IN– pin to measure the voltage level delivered to the load. For this example, the BUS pin measures less than 12 V because the voltage at the IN– pin is 11.98 V as a result of the voltage drop across the shunt resistor.

For this example, assuming a maximum expected current of 15 A, the Current_LSB is calculated to be 457.7 μA/bit using Equation 2. Using a value of 500 μA/bit or 1 mA/bit for the Current_LSB significantly simplifies the conversion from the Current register and Power register to amps and watts, respectively. For this example, a value of 1 mA/bit was chosen for the Current register LSB. Using this value for the Current_LSB trades a small amount of resolution for a simpler conversion process on the processor side. Using Equation 1 in this example with a current LSB of 1 mA/bit and a shunt resistor of 2 mΩ results in a Calibration register value of 2560, or A00h.

The Current register (04h) is then calculated by multiplying the decimal value of the Shunt Voltage register contents by the decimal value of the Calibration register and then dividing by 2048, as shown in Equation 3. For this example, the Shunt Voltage register value of 8000 is multiplied by the Calibration register value of 2560 and then divided by 2048 to yield a decimal value for the Current register of 10000, or 2710h. Multiplying this value by 1 mA/bit results in the original 10-A level stated in the example.

Equation 3.

The LSB for the Bus Voltage register (02h) is a fixed 1.25 mV/bit. This fixed value means that the 11.98 V present at the BUS pin results in a register value of 2570h, or a decimal equivalent of 9584. Note that the MSB of the Bus Voltage register is always zero because the BUS pin is only able to measure positive voltages.

The Power register (03h) is then calculated by multiplying the decimal value of the Current register, 10000, by the decimal value of the Bus Voltage register, 9584, and then dividing by 20,000, as defined in Equation 4. For this example, the result for the Power register is 12B8h, or a decimal equivalent of 4792. Multiplying this result by the power LSB (25 times the [1 × 10^–3 Current_LSB]) results in a power calculation of (4792 × 25 mW/bit), or 119.8 W. The Power register LSB has a fixed ratio to the Current register LSB of 25 W/bit to 1 A/bit. For this example, a programmed Current register LSB of 1 mA/bit results in a Power register LSB of 25 mW/bit. This ratio is internally programmed to make sure that the scaling of the power calculation is within an acceptable range. A manual calculation for the power being delivered to the load would use a bus voltage of 11.98 V (12V_CM – 20 mV shunt drop) multiplied by the load current of 10 A to give a result of 119.8 W.

Equation 4.

Table 8-1 shows the steps for configuring, measuring, and calculating the values for current and power for this device.