Target impedance
Beginner
A rule for how steady the power supply must stay when the chip suddenly needs more electricity.
Novice
The most the supply may sag per amp of sudden extra demand: the allowed ripple voltage divided by the largest current step, . Impedance is resistance generalized to currents that change over time.
Expert
Applied at every frequency: the regulator covers slow changes, board and package capacitors the middle range, and on-chip capacitance the fastest. The resonance between on-chip capacitance and package inductance makes an impedance peak that must also stay under the target.
Explained in Power planning (Design Flow).
See also: Power delivery network (PDN), Decap cell.