Buck-boost DC-DC converter plays an important role in battery-powered devices, as Liion battery outputs 4.2V to 2.8V while high-performance analog circuits require a relatively high supply of 3.3V. More importantly, an envelope supply modulator needs a buck-boost converter with wide and fast dynamics. A conventional buck-boost (CBB) converter has two main drawbacks: 1) all its four power switches need to withstand the highest voltage rating to cover the input/output voltage ranges, limiting the power conversion efficiency; 2) its inherent right-half-plane (RHP) zero limits the transient response speed. New topologies with flying capacitor (C_F) [1, 2] have been proposed to reduce the conduction loss by removing one switch in the main current path. However, during the mode transitions in [1, 2], the C_F voltage would fluctuate, leading to significant charging or discharging currents that can create substantial current/voltage spikes and energy losses. Topologies in [3, 4] require only one mode for the entire operating range. Meanwhile, some topologies need LDMOS or stacked transistors for withstanding highvoltage stress, such as V_IN+V_OUT in [1], 2V_OUT in [3], or 2V_IN in [4], which degrades the efficiency and increases fabrication cost. Still, these topologies exhibit slow transient response. To address this, [5, 6] propose buck-based topologies that eliminate the RHP zero. However, their efficiency is compromised by the additional switch in the main current path.