Decoupling Capacitor Placement for Flat PDN Impedance with Rogue Wave Consideration

Abstract

Target impedance has been used to guarantee the performance of the power delivery network (PDN) for many years, but it assumes the PDN impedance profile is flat without resonant peaks. However, the PDN impedance profile is never flat, so that the step response of the PDN can comprise components of multiple resonant frequencies with corresponding impedance peaks in the frequency domain. The superposition of resonant voltage waveforms excited by transitions of current load can result in worst-case scenario and then violates maximum specified voltage. A strategy of placing decoupling capacitor to flat the PDN impedance is proposed in this paper. The defined impedance deviation of the PDN impedance with the proposed method is 7.93 percent which is about two times better than 16.25 percent with commercial tools.