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Calculation of iMOTION™ ADC offset voltage – KBA235410

Calculation of iMOTION™ ADC offset voltage – KBA235410

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Community Translation: iMOTION™ ADCオフセット電圧の計算 – KBA235410

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Choosing the ADC offset voltage for leg shunt or single shunt configuration

The ADC offset voltage is measured in millivolts (mV), at the current sense pin when the shunt current is zero. The voltage drops across the shunt resistor due to current, which will help to determine the current at each instance. This is essential for sensorless estimation of the rotor position in the FOC algorithm. If the offset voltage is not configured properly, this may lead to the following:

  • False GateKill fault.
  • Improper current measurement.
  • Overcurrent protection (OVC threshold = (Imax_peak × Input current scaling + ADC offset voltage)) may fail if the ADC offset voltage is not chosen properly. This results in overcurrent detection not beeing triggered and may lead to component damage.

 Calculating the offset voltage to the ADC

= (R1 /(R1+R2)) × VDD

In general,  = (VDD/2) /

  • : Internal gain of more than 1 helps to measure the current from the lower shunt resistor, and reduces the shunt power loss.
  • : Offset voltage
  • : Operating voltage of the controller as shown in Figure 1
  • R1, R2: Resistors in offset and gain circuit as shown in Figure 1

 BinduPriya_G_0-1655719538716.pngFigure 1    Current feedback offset and gain

Calculating current input scaling

 = R2 / (R1+R2)

Current input scaling =  × Rs

  • : External gain
  • Rs: External shunt resistor in the power board
  • R1, R2: Resistors in the offset and gain circuit

If an external opamp is used based on the shunt resistor and the maximum voltage drop across the resistor, select the opamp gain.

Example:

If Rs = 0.05 W, maximum current = 25 A, the voltage drop across the shunt resistor is 1.25 V.
If the maximum ADC value of the controller is 3.3 V or 5 V, the gain of the external opamp must be 2.64 or 4 respectively.

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