Implantable medical devices (IMD) with stimulation system-on-chip (SoC) have been essential techniques for disease treatments and rehabilitations. As neuromuscular stimulation injects a large amount of stimulus energy into the body, its energy efficiency and safety should be carefully considered, which otherwise damages cellular tissues. Conventional current stimulation suffers from large power losses across current sources. Even adopting the adaptive supply voltage, the stimulator efficiency is still limited below 60% . The switched capacitor stimulation (SCS) system charges the capacitor and transfer its charges to the tissue, achieving stimulator efficiency up to 84% -. However, previous SCS systems only operate with AC input voltages directly from wireless power, which can be interrupted in loosely-coupled inductive links. To take advantages of using a rechargeable battery or a supercapacitor for reliable IMD operation, the SCS system that can efficiently operate with both DC and AC inputs is required. Also, more aggressive techniques to further improve stimulator efficiency and efficacy are highly needed.