This paper proposes an analysis of the active charge injection for a resonant power supply noise by controlling the number of canceling capacitors so that the core supply voltage does not become lower than the voltage at the beginning of the canceling charge injection. Based on this analysis, we propose a circuit that injects adequate amount of charge in accordance with the injection time and the amount of the voltage drop whose code comes from an on-chip voltage drop detector. The proposed circuit is composed of a voltage drop detector, a finite state machine (FSM) that controls the injection and canceling capacitor circuit. The injection controller FSM enables the robust operation against the variation of the current consumed in the core circuit. The post-layout simulation indicates that our proposed circuit reduces the resonant power supply noise by about 30%, and also reduce 71% silicon area compared to the area of passive decoupling capacitors for the same amount of the noise reduction.