This paper presents a 400MHz 6-phase buck converter with bond-wire inductors. Peak-and-valley differential sensing, which is realized with a reused on-chip capacitor, corrects inter-inductor current imbalance without power overhead. Owing to dynamic re-allocation of on-chip capacitors, transient response can be accelerated with constrained ripple. The chip fabricated in 28-nm achieved the state-of-the-art power density of 1.23W/mm2 and DVS rate of 75mV/ns. Peak efficiency of 83.7% was measured.