This paper describes an integrated current-mode buck converter with reliable hysteretic mode controller and modulators for fast dynamic voltage scaling (DVS). The proposed mode controller and modulators enable the DC-DC buck converter to perform its smooth mode transitions at optimal points. The continuous current-limiting and DCM controls drive the DVS to track the reference at the maximum speed in both PWM and PFM modes. The DC-DC buck converter is implemented in a 90-nm 3.3V CMOS process. The maximum efficiency reaches 94.5%. The hysteretic mode controller keeps its efficiency always at more than 75% for any load conditions. During a mode transition, the output voltage has no overshoots, and it can track the reference voltage instantly.