This paper presents the first monolithically integrated two-phase series capacitor buck converter. This converter achieves over 60 A/cm3 current density. The series capacitor buck converter topology enables high frequency (HF) operation up to 5 MHz per phase without special magnetics or compound semiconductors. An adaptive constant on-time controller provides fast load transient response and fixed frequency operation in steady state. The series capacitor is precharged before switching and monitored to protect against faults. Experimental results for a 12 V input, 1.2V/10A output application demonstrate higher peak efficiency than a conventional buck converter while operating at 4 times higher switching frequency.