ZHCSLH4 April 2021 AMC3336
PRODUCTION DATA
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 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 linear full-scale range input voltage (VFSR) of the AMC3336 that is +1 V. This voltage is calculated as RL1SNS = VFSR / (VPeak – VFSR) x (RTOP + RBOT), where (RTOP + RBOT) is the total value of the RL11 and RL12 resistor 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.
| PARAMETER | 120-VRMS LINE VOLTAGE | 230-VRMS LINE VOLTAGE |
|---|---|---|
| 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 |
| Resulting current through resistive divider, ICROSS | 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 Section 7.3.5 section.