Maximum effective distance of on-chip decoupling capacitors in power distribution grids

Abstract

Decoupling capacitors are widely used to reduce power supply noise. On-chip decoupling capacitors have traditionally been allocated into the available white space on a die. The efficacy of on-chip decoupling capacitors depends upon the impedance of the power/ground lines connecting the capacitors to the current loads and power supplies. A maximum effective radius exists for each on-chip decoupling capacitor. Beyond this effective distance, a decoupling capacitor is completely ineffective. Two effective radii determined by the target impedance (during discharge) and charge time are presented in this paper. Depending upon the parasitic impedance of the power distribution system, the maximum voltage drop as seen at the current load is achieved either at the first droop or at the end of the switching activity (the second droop). Two criteria to estimate the minimum required on-chip decoupling capacitance are developed based on the critical parasitic impedance. To be effective, the decoupling capacitor has to be fully charged before the next switching event. A design space is described that characterizes the tolerable parasitic resistances and inductances, while restoring the charge on the decoupling capacitor within a target charge time. An overall design methodology for placing on-chip decoupling capacitors is presented in this paper. It is shown that for an on-chip decoupling capacitor to be effective, both effective radii criteria should be simultaneously satisfied.