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[FAQ] INA233: How to use the power accumulator

Part Number: INA233

How to use the power accumulator

  • The INA233 has a power accumulator built into the device, which records the sum of the power measurements, number of measurements, and number of times the sum register has rolled over.

    Here's how the part works, from single measurement to accumulated power:

    1. The voltage across the current sense resistor is measured and stored in the MFR_READ_VSHUNT
      1. Current is calculated based on the MFR_ADC_CONFIG and MFR_CALIBRATION registers and stored in the READ_IOUT register
    2. The bus voltage is measured and stored in READ_VOUT
    3. Power is calculated from the current and bus voltage measurements in the 16-bit READ_POUT register
    4. The 16-bit power number is added to the 16-bit power accumulator.
      1. If the sum is greater than 0xFFFFh, then the power accumulator register rolls over, the remainder of the power measurement is added to the accumulator, and the rollover count is increased by 1.  (Personally I like to think of the rollover byte as another byte of the accumulator, so it is more like a 24-bit accumulator than a 16-bit accumulator with 8-bit rollover counter).
      2. The 24-bit sample count register is increased by 1.
      3. These 48 bits of data are stored in the READ_EIN register.

    Dividing the power accumulator by the sample count gives an average energy used for the monitored time period.  A common question about the power accumulator is about how much time can power measurements be taken before the energy accumulator overflows?  The answer depends on a number of factors -

    • The sense resistor
    • The peak current
    • The CURRENT_LSB (set in the calibration register)
    • The ADC conversion time for current measurement (set in the ADC config register)
    • The ADC conversion time for bus voltage measurement (set in the ADC config register)
    • The number of ADC measurements taken per average when averaging is set to > 1.

    The following excel worksheet can help calculate how long the INA233 can sample before overflow occurs in the Power Accumulator.  Understand that it is based on average expected power and not maximum power.  If your system operates at maximum power for a long period of time, it can fill the accumulator much more quickly than anticipated.  If you are concerned with this, make the Expected Average Current and Peak Current fields the same.

    INA233 Overflow Calculator.xlsx

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