35.3.2.1 External Resistor Selection

The ZCD requires a current-limiting resistor in series (R_SERIES) with the external voltage source. If the peak amplitude (V_PEAK) of the external voltage source is expected to be stable, the resistor value must be chosen such that an I_{ZCD_MAX}/2 resistor current results in a voltage drop equal to the expected peak voltage. The power rating of the resistor must be at least the mean square voltage divided by the resistor value. (How to handle a peak voltage that varies between a minimum (V_MINPEAK) and maximum (V_MAXPEAK) value is described in the Handling V_PEAK Variations section below).

Equation 35-1. External Resistor

R_{S E R I E S} = \frac{V_{P E A K}}{3 \times 10^{- 4}}