JAJSE67C December 2017 – June 2022 AMC1311
The 100-μA, cross-current requirement at the maximum DC link voltage (400 V) determines that the total impedance of the resistive divider is 4 MΩ. The impedance of the resistive divider is dominated by the top portion (shown exemplary as RX1 and RX2 in Figure 9-1) and the voltage drop across RSNS can be neglected for a moment. The maximum allowed voltage drop per unit resistor is specified as 75 V; therefore, the minimum number of unit resistors in the top portion of the resistive divider is 400 V / 75 V = 6. The calculated unit value is 4 MΩ / 6 = 666 kΩ and the next closest value from the E96 series is 665 kΩ.
RSNS is sized such that the voltage drop across the resistor at the maximum DC link voltage (400 V) equals the linear full-scale range input voltage (VFSR) of the AMC1311, which is 2 V. This voltage is calculated as RSNS = VFSR / (VDC-link, max – VFSR) × RTOP, where RTOP is the total value of the top resistor string (6 × 665 kΩ = 3990 kΩ). RSNS is calculated as 20.05 kΩ and matches a value from the E96 series.
|Unit resistor RX||665 kΩ|
|Number of unit resistors||6|
|Sense resistor RSNS||20.05 kΩ|
|Resulting current through resistive divider ICROSS||99.7 μA|
|Resulting voltage drop across sense resistor||2.000 V|
|Power dissipated in unit resistor RX||6.6 mW|
|Total power dissipated in resistive divider||39.9 mW|