SBOS644C February 2013 – March 2018 INA231
The INA231 performs two measurements on the power-supply bus of interest. The voltage developed from the load current that flows through a shunt resistor creates the shunt voltage signal that is measured at the IN+ and IN– pins. The device can also measure the power supply bus voltage by connecting this voltage to the BUS pin. The differential shunt voltage is measured with respect to the IN– pin whereas the bus voltage is measured with respect to ground.
The INA231 is typically powered by a separate supply that can range from 2.7 V to 5.5 V. The bus that is being monitored can range in voltage from 0 V to 28 V.
Based on the fixed 1.25 mV LSB for the bus voltage register, a full-scale register would result in a 40.96-V value. However, the actual voltage that is applied to the input pins of the INA231 should not exceed 28 V.
There are no special considerations for power-supply sequencing because the common-mode input range and power-supply voltage are independent of each other; therefore, the bus voltage can be present with the supply voltage off, and vice-versa.
As noted, the INA231 takes two measurements, shunt voltage and bus voltage. It then converts these measurements to current, based on the Calibration register value, and then calculates power. See the Configure, Measure, and Calculate Example section for additional information on programming the calibration register.
The INA231 has two operating modes, continuous and triggered, that determine how the ADC operates after these conversions. When the INA231 is in the normal operating mode (that is, the MODE bits of the Configuration register are set to '111'), it continuously converts a shunt voltage reading followed by a bus voltage reading. After the shunt voltage reading, the current value is calculated based on Equation 3. This current value is then used to calculate the power result using Equation 4. These values are subsequently stored in an accumulator, and the measurement and calculation sequence repeats until the number of averages set in the Configuration register is reached. Note that the current and power calculations are based on the value programmed into the Calibration register. If the Calibration register is not programmed, the result of the current and power calculations is zero. Following every sequence, the present set of measured and calculated values are appended to the previously collected values. After all of the averaging has been completed, the final values for shunt voltage, bus voltage, current, and power are updated in the corresponding registers and can then be read. These values remain in the data output registers until they are replaced by the next fully completed conversion results. Reading the data output registers does not affect a conversion in progress.
The mode control bits in the Configuration register also permit selecting specific modes to convert only the shunt voltage or the bus voltage in order to further allow the monitoring function configuration to fit specific application requirements.
All current and power calculations are performed in the background and do not contribute to conversion time.
In triggered mode, writing any of the triggered convert modes into the Configuration register (that is, the MODE bits of the Configuration register are set to 001, 010, or 011) triggers a single-shot conversion. This action produces a single set of measurements. To trigger another single-shot conversion, the Configuration register must be written to again, even if the mode does not change.
In addition to the two operating modes (continuous and triggered), the INA231 also has a power-down mode that reduces the quiescent current and turns off current into the INA231 inputs, which reduces the impact of supply drain when the device is not being used. Full recovery from power-down mode requires 40 ms. The registers of the INA231 can be written to and read from while the device is in power-down mode. The device remains in power-down mode until one of the active modes settings are written into the Configuration register.
Although the INA231 can be read at any time, and the data from the last conversion remain available, the conversion ready flag bit (CVRF bit, Mask/Enable register) is provided to help coordinate single-shot or triggered conversions. The CVRF bit is set after all conversions, averaging, and multiplication operations are complete for a single cycle.
The CVRF bit clears under these conditions: