Active decap design considerations for optimal supply noise reduction

Active decoupling capacitors (decaps) are more effective than passive decaps at reducing local IR-drop problems in the power distribution network. In the basic active decap, two parallel decaps are stacked in series whenever the voltage drop exceeds a given budget to boost the local supply voltage. However, the effectiveness of stacking three or more decaps has not been previously explored. In this paper, we investigate these higher stack height configurations to assess the degree of improvement in supply noise reduction. Extensive simulation results correlated with a test chip indicate that an active decap with a stack height of three provides the best noise reduction if the supply noise level is between 7% -14%, but a stack height of two is best if the noise level is between 14% -16%.