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Obtaining a faster dynamic response is a challenge for future microprocessor voltage regulators (VR). The hysteretic control method is one of the fastest control methods, but it is difficult to use in multiphase VRs because of the difficulty in achieving a constant switching frequency and interleaving operation among the phases. To resolve this, a hysteretic control method with a bandwidth-changing loop is proposed. By slowly adjusting the hysteretic bandwidth through the phase-locked loop, the duty cycle is synchronized with an external fixed-frequency clock reference. By phase-shifting this external clock reference for each phase, the interleaving among the paralleled phases can be easily implemented. On the other hand, slow hysteretic bandwidth changing will make this controller perform as though it has conventional hysteretic control during the transient for the fast dynamic response. After the transient, each phase will be pulled back to interleaving status through its bandwidth adjustment. Analysis and design guidelines for the proposed control are provided, and they are verified with hardware.