AMC1350 datasheet(24/37 Pages) TI

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8.2.1 Design Requirements

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

Table 8-1. Design Requirements

PARAMETER

120-VRMS LINE VOLTAGE

230-VRMS LINE VOLTAGE

System input voltage

120 V ±10%, 60 Hz

230 V ±10%, 50 Hz

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

±5 V (maximum)

Current through the resistive divider, ICROSS

100 μA

8.2.2 Detailed Design Procedure

This discussion covers the 230-VRMS example. The procedure for calculating the resistive divider for the 120-

VRMS use case is identical.

The 100-μA, cross-current requirement at peak input voltage (360 V) determines that the total impedance of

the resistive divider is 3.6 MΩ. The impedance of the resistive divider is dominated by the top resistors (shown

exemplary as R1 and R2 in Figure 8-1) and the voltage drop across RSNS can be neglected for a short time.

The maximum allowed voltage drop per unit resistor is specified as 75 V; therefore, the total minimum number of

unit resistors in the top portion of the resistive divider is 360 V / 75 V = 5. The calculated unit value is 3.6 MΩ / 5

= 720 kΩ and the next closest value from the E96 series is 715 kΩ.

The effective sense resistor value RSNSEFF is the parallel combination of the external resistor RSNS and the

input impedance of the AMC1350-Q1, RIN. RSNSEFF is sized such that the voltage drop across the impedance

at maximum input voltage (360 V) equals the linear full-scale input voltage (VFSR) of the AMC1350-Q1 (that is,

+5 V). RSNSEFF is calculated as RSNSEFF = VFSR / (VPeak – VFSR) × RTOP where RTOP is the total value of the

top resistor string (5 × 715 kΩ = 3575 kΩ). The resulting value for RSNSEFF is 9.96 kΩ. In a final step, RSNS is

calculated as RSNS = RIN × RSNSEFF / (RIN – RSNSEFF). With RIN = 1.25 MΩ (typical), RSNS equals 52.47 kΩ

and the next closest value from the E96 series is 52.3 kΩ.

Table 8-2 summarizes the design of the resistive divider.

Table 8-2. Resistor Value Examples

PARAMETER

120-VRMS LINE VOLTAGE

230-VRMS LINE VOLTAGE

Peak voltage

190 V

360 V

Unit resistor value, RTOP

634 kΩ

715 kΩ

Number of unit resistors in RTOP

Sense resistor value, RSNS

53.6 kΩ

52.3 kΩ

Total resistance value (RTOP + RSNS)

1953.4 kΩ

3625.2 kΩ

Resulting current through resistive divider, ICROSS

97.3 μA

99.3 μA

Resulting full-scale voltage drop across sense resistor RSNS

4.993 V

4.982 V

Peak power dissipated in RTOP unit resistor

6 mW

7.1 mW

Total peak power dissipated in resistive divider

18.5 mW

35.7 mW

AMC1350-Q1

SBASAB0 – DECEMBER 2021

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