SBOS644C February 2013 – March 2018 INA231
An important aspect of the INA231 is that it does not necessarily measure current or power. The INA231 measures both the differential voltage applied between the IN+ and IN– input pins and the voltage applied to the BUS pin. In order for the INA231 to report both current and power values, both the Current register resolution and the value of the shunt resistor present in the application that resulted in the differential voltage being developed must be programmed. The Power register is internally set to be 25 times the programmed least significant bit of the Current register (Current_LSB). Both the Current_LSB and shunt resistor value are used when calculating the Calibration register value. The INA231 uses this value to calculate the corresponding current and power values based on the measured shunt and bus voltages.
The Calibration register is calculated based on Equation 1. This equation includes the term Current_LSB, the programmed value for the LSB for the Current register. This is the value used to convert the value in the Current register to the actual current in amps. The highest resolution for the Current register can be obtained by using the smallest allowable Current_LSB based on the maximum expected current, as shown in Equation 2. While this value yields the highest resolution, it is common to select a value for the Current_LSB to the nearest round number above this value to simplify the conversion of the Current register and Power register to amps and watts, respectively. RSHUNT is the value of the external shunt used to develop the differential voltage across the input pins. The 0.00512 value in Equation 1 is an internal fixed value used to make sure that scaling is properly maintained.
After the Calibration register has been programmed, the Current register and Power register are updated accordingly based on the corresponding shunt voltage and bus voltage measurements. Until the Calibration register is programmed, the Current and Power registers remain at zero.