For the INA239-Q1 device to report current values in Ampere units, a constant conversion value must be written in the SHUNT_CAL register that is dependent on the maximum measured current and the shunt resistance used in the application. The SHUNT_CAL register is calculated based on Equation 1. The term CURRENT_LSB is the LSB step size for the CURRENT register where the current in Amperes is stored. The value of CURRENT_LSB is based on the maximum expected current as shown in Equation 2, and it directly defines the resolution of the CURRENT register. While the smallest CURRENT_LSB value yields highest resolution, it is common to select a higher round-number (no higher than 8x) value for the CURRENT_LSB in order to simplify the conversion of the CURRENT.

The R_SHUNT term is the resistance value of the external shunt used to develop the differential voltage across the IN+ and IN– pins. Use Equation 1 for ADCRANGE = 0. For ADCRANGE = 1, the value of SHUNT_CAL must be multiplied by 4.

where

• 819.2 x 10⁶ is an internal fixed value used to ensure scaling is maintained properly.
• the value of SHUNT_CAL must be multiplied by 4 for ADCRANGE = 1.

 

Note that the current is calculated following a shunt voltage measurement based on the value set in the SHUNT_CAL register. If the value loaded into the SHUNT_CAL register is zero, the current value reported through the CURRENT register is also zero.

After programming the SHUNT_CAL register with the calculated value, the measured current in Amperes can be read from the CURRENT register. The final value is scaled by CURRENT_LSB and calculated in Equation 3:

where

• CURRENT is the value read from the CURRENT register

The power value can be read from the POWER register as a 24-bit value and converted to Watts by using Equation 4:

 

where

• POWER is the value read from the POWER register.
• CURRENT_LSB is the lsb size of the current calculation as defined by Equation 2.

 

For a design example using these equations refer to Section 8.2.2.