The bottom resistor in the resistive divider is split into two equal-value resistors (RSNS1 and RSNS2) to

accommodate the headroom requirements of the reference voltage (VREF) of the isolated comparator, as

explained in the Detailed Design Procedure section.

8.2.1 Design Requirements

Table 8-1 lists the parameters for this typical application.

Table 8-1. Design Requirements

PARAMETER

VALUE

System input voltage

Single phase, 230 V, 50 Hz

DC link voltage (normal operation)

400 V (maximum)

DC link overvoltage detection level

450 V

Low-side supply voltage

5 V

Voltage drop across the sensing resistor (RSNS1 + RSNS2) for a linear

response

2 V (maximum)

Current through the resistive divider, ICROSS

100 μA (maximum)

Minimum headroom (VDD1 – VREF) for the reference voltage of the

AMC23C11 isolated comparator

1.4 V

Maximum resistor operating voltage

75 V

8.2.2 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 8-1) and the voltage drop across RSNS1 and RSNS2 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 = 667 kΩ and the next closest value from the E96 series is 665 kΩ.

The total sense resistor (RSNS = RSNS1 + RSNS2) 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 AMC3311,

which is 2 V. This resistance 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 split into two equal

resistors (RSNS1 and RSNS2) of 10 kΩ each, a value from the E96 series.

The isolated comparator senses the voltage drop across RSNS2. At the specified DC link overvoltage of 450

V, the voltage drop across RSNS2 is RSNS2 / (RTOP + RSNS1 + RSNS2) × VDC-link and equals 1.12 V. This

value is the trip threshold value, VREF. The trip threshold is set by the resistor RREF and is calculated as (1.12

V – 4 mV) / 100 μA = 11.2 kΩ. A value of 11 kΩ is selected from the E96 series, resulting in a slightly lower

reference voltage of 1.1 V. The 4-mV value subtracted from VREF is related to the hysteresis of the isolated

comparator on the rising edge of VIN. 100 μA is the value of the internal current source connected to the REF pin

of the AMC23C11. For a detailed calculation of the trip threshold, see the AMC23C11 data sheet. The resulting

overvoltage detection threshold is (VREF + 4 mV) / RSNS1 × (RTOP + RSNS1 + RSNS2), or 443 V.

The reference of the AMC23C11 comparator requires a minimum headroom to VDD1 of 1.4 V. The minimum

supply voltage (VDD1) to the isolated comparator is in the minimum output voltage of the AMC3311 high-side

LDO (VHLDO_OUT), which equals 3 V. Therefore, the reference operates at a minimum headroom of 3 V – 1.1 V =

1.9 V and satisfies the design requirements.

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