SBASAA7A May 2021 – September 2021 AMC1411
The 300-μA cross-current requirement at the maximum DC-link voltage (1200 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 8-1) 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 1200 V / 75 V = 16. The calculated unit value is 4 MΩ / 16 = 250 kΩ and the next closest value from the E96 series is 249 kΩ.
RSNS is sized such that the voltage drop across the resistor at the maximum DC-link voltage (1200 V) equals the linear full-scale range input voltage (VFSR) of the AMC1411 that is 2 V. This voltage is calculated as RSNS = VFSR / (VDC-link, max – VFSR) x RTOP, where RTOP is the total value of the top resistor string (16 x 249 kΩ = 3984 kΩ). RSNS is calculated as 6.65 kΩ and matches a value from the E96 series.
Table 8-2 summarizes the design of the resistive divider.
|Unit resistor value, RX||249 kΩ|
|Number of unit resistors||16|
|Sense resistor value, RSNS||6.65 kΩ|
|Total resistance value||3990.65 kΩ|
|Resulting current through resistive divider, ICROSS||300.7 µA|
|Resulting full-scale voltage drop across sense resistor RSNS||2.000 V|
|Power dissipated in unit resistor RX||22.5 mW|
|Total power dissipated in resistive divider||361 mW|