9.2.1 Design Requirements

Table 9-1 lists the parameters for this typical application.

Table 9-1. Design Requirements

PARAMETER

VALUE

System input voltage

Single phase, 230 V, 50 Hz

Maximum DC link voltage

400 V

High-side supply voltage

3.3 V or 5 V

Low-side supply voltage

3.3 V or 5 V

Maximum resistor operating voltage

75 V

Voltage drop across the sense resistor (RSNS) for a linear response

2 V (maximum)

Current through the resistive divider, ICROSS

100 μA (maximum)

9.2.2 Detailed Design Procedure

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.

Table 9-2. Resistor Value Example

PARAMETER

VALUE

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

AMC1311

SBAS786C – DECEMBER 2017 – REVISED JUNE 2022

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