Decreased Effectiveness of On-Chip Decoupling Capacitance in High-Frequency Operation

This paper shows the decreased effectiveness of on-chip decoupling capacitance in high-frequency operation. On-chip decoupling capacitance is often used to decrease the variation of the propagation delay caused by power/ground noise, i.e., dynamic IR-drop and/or delta-I noise. However, it is shown in this paper that decoupling capacitance is only effective for coping with dynamic IR-drop if the recharging time between switching events is sufficient. In other words, the effectiveness of decoupling capacitance for dynamic IR-drop in high-frequency operation is less than that of a fully-charged decoupling capacitor. The recharging time and the effectiveness of a decoupling capacitor depend on the propagation delay of the average circuit path which is used to determine the total switching current of a given macro/chip and clock cycle time. If the propagation delay of the critical paths is approximately equal to that of the average circuit path, then it is shown in this paper that adding decoupling capacitance never improves the maximum frequency of the system due to dynamic IR-drop limitations. On the other hand, if the propagation delay of the critical paths is larger than that of the average circuit path, then the maximum frequency is improved by adding decoupling capacitance. In both cases, a new metric, called the apparent capacitance, can be used to help make correct decisions about decoupling capacitance planning.