This paper explores the large-signal and small-signal dynamics of a series-capacitor (SC) buck-type converter and introduces an optimal closed-loop control scheme to accommodate both the steady-state and transient modes. As opposed to a conventional buck converter, where time-optimal control is realized by a single on-off cycle, in the SC-buck topology there is a need to distribute the switching phases to satisfy the charge-balance of the flying capacitor. The new control method hybrids a voltage-mode small-signal controller for steady-state operation and a non-linear, state-plane based transient-mode control scheme for load transients. A detailed principle of operation of the SC-buck converter is provided and explained through an average behavioral model and state-plane analysis. The operation of the controller is experimentally verified on a 12W 12V-to-1.5V converter, demonstrating voltage-mode control operation as well as time-optimal response for load transients.