This paper presents a fully integrated voltage-controlled oscillator (VCO)-based switched-capacitor (SC) 15-phase dc-dc converter in 65-nm CMOS. We propose two transient-enhancement techniques: segmented frequency modulation (SFM) and multiphase co-work (MCW) control to reduce the latency of the VCO-based control loop and shorten the SC dc-dc converter's transient response time. The SFM can improve the heavy-to-light load transient by dynamically increasing the charge pump discharge current by nine times, while the MCW can enhance the light-to-heavy load recovery by synchronously combining three interleaved flying capacitors together during the transient state. We designed the 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 maintains 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