If the peak amplitude of the external voltage is expected to vary, the series resistor (RSERIES) must be selected to keep the ZCD source and sink currents below the absolute maximum rating of ±IZCD_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 ±IZCD_MAX and the minimum is at least IZCD_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.