This article presents a 12-to-1 flying capacitor cross-connected (CCC) converter with an inserted duty cycle ( D) > 0.5 control for fast transient response. As analyzed, CCC and double-step-down (DSD) converters have almost the same power loss and silicon area, while the CCC uses a much smaller input capacitance. Besides, with the proposed D > 0.5 control, the CCC achieves >2× inductor current slew rate and thus a faster transient response, when compared with a conventional DSD converter. Subsequently, we use a primary-secondary synchronized hysteretic (SH) controller that further improves the transient response. We investigate the small-signal model to guide the controller's design. Moreover, we propose a shared bootstrap circuit to save the silicon area. The prototype chip fabricated in a 0.18- μ m HV-SOI process occupies a die area of 2.8 mm2. The measured peak efficiency is 86.8% with a 12-V input, a 1.2-V output, and a 2-MHz switching frequency. With two 0.74- μ H inductors and a 10.6- μ F output capacitor, the measured undershoot is 110 mV with a 4-A load current step. The normalized undershoot is 0.73× the minimum undershoot that DSD can achieve.