If the peak amplitude of the external voltage is expected to
vary, the series resistor (R_{SERIES}) must be selected to keep the ZCD source
and sink currents below the absolute maximum rating of ±I_{ZCD_MAX} and above a
reasonable minimum range. A general rule of thumb for the ZCD is that the maximum peak
voltage should be no more than six times the minimum peak voltage. To ensure that the
maximum current does not exceed ±I_{ZCD_MAX} and the minimum is at least
I_{ZCD_MAX}/6, compute the series resistance, as shown in the equation below.
The compensating pull-up or pull-down for this series resistance can be determined using
the **ZCD Pull-up/Pull-down Resistor** equations shown earlier, as the
pull-up/pull-down resistor value is independent of the peak voltage.

Figure 1. Series Resistor for External
Voltage Range