Given the required ripple tolerance and step load amount, a target impedance can be calculated. The impedance of the PDN network on the board should be targeted to be at or below this target impedance at the typical frequency in which decoupling capacitors are most effective (see the PDM tool for frequency ranges). Beyond these frequencies, the internal and mounting inductances of the capacitors reduce their effectiveness.

The formula for target impedance is shown in the following equation.

An example calculation for a 0.80V rail specified with ±3% tolerance (1% DC, 2% AC) and 40A of required step current is as follows:

Recall that ±2% is used as the ripple target because ±1% is assumed to be taken by the tolerance of the VRM.