8.2.1.1 Design Requirements

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

Table 8-1. Design Requirements

PARAMETER

System input voltage

120 V ±10%, 60 Hz

230 V ±10%, 50 Hz

Low-side supply voltage

3.3 V or 5 V

3.3 V or 5 V

Maximum resistor operating voltage

75 V

75 V

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

±1 V (maximum)

±1 V (maximum)

100 μA

100 μA

8.2.1.2 Detailed Design Procedure

The 100-μA crosscurrent 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 and bottom resistors

(shown exemplary as RL11 and RL12 in Figure 8-1) and the voltage drop across RL1SNS can be neglected for

a moment. The maximum allowed voltage drop per unit resistor is specified as 75 V, therefore the total, minimum

number of unit resistors in the top and bottom 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.

RL1SNS is sized such that the voltage drop across the resistor at maximum input voltage (360 V) equals the

strings (5 x 715 kΩ = 3575 kΩ). RL1SNS is calculated as 9.96 kΩ and the next closest value from the E96-series

is 10 kΩ.

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

Table 8-2. Resistor Value Examples

PARAMETER

Peak voltage

190 V

360 V

Unit resistor value, RL11Xand RL12X

634 kΩ

715 kΩ

Number of unit resistors in RL11

1

2

Number of unit resistors in RL12

2

3

Sense resistor value, RL1SNS

10 kΩ

10 kΩ

Total resistance value

1912 kΩ

3585 kΩ

99.4 μA

100.4 μA

Resulting full-scale voltage drop across sense resistor RL1SNS

0.994 V

1.004 V

Peak power dissipated in unit resistor RL11X and RL12X

6.3mW

7.2 mW

Peak power dissipated in resistive divider

18.9 mW

36.2 mW

The AMC3336 requires a single 3.3-V or 5-V supply on its low-side. The high-side supply is internally generated

by an integrated DC/DC converter, as explained in the Isolated DC/DC Converter section.

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