An Active Decoupling Capacitance Circuit for Inductive Noise Suppression in Power Supply Networks

The placement of on-die decoupling capacitors (decap) between the power and ground supply grids has become a common practice in high performance processor designs. In this paper, we present a new active decap circuit that significantly increase the effectiveness of decap in suppressing power supply fluctuations. The proposed circuit senses the supply drop and drives an amplified and inverted voltage fluctuation on the decap. The active decoupling circuit is powered by a separate power supply and we study the optimal allocation of the total C4s/pads between this second power supply and the regular supply as well as the optimal allocation of the total decoupling capacitance between actively switched and traditional static decap. Finally, we demonstrate that the overhead of the proposed method is small compared to the area of the decaps. Using the proposed method, the maximum supply drop is reduced by 45% compared to the use of only traditional decap, corresponding to an increase in the effective decap of approximately 8times.