This paper presents a fully integrated VCO-based switched-capacitor (SC) DC-DC converter in 65 nm CMOS. We propose two transient-enhancement techniques: segmented frequency modulation (SFM) and multiphase co-work control (MCW) to reduce the latency of the VCO-based control loop and shorten the SC DC-DC converter response time. We design a 15-phase interleaved converter to support an output voltage of 1 V from a 2.4 V input supply, delivering up to 138 mA of load current, which takes only 25/29 ns for output voltage recovering to the steady state from heavy-to-light/light-to-heavy load transients, respectively. It obtains a peak efficiency of 82.8 % and keeps the efficiency above 80 % from 31 mA to the maximum load current. The SC DC-DC converter chip occupies 0.61 mm, and its output power density is 240 mW/mm.