Implantable devices are inserted into the human body for various purposes such as monitoring diverse health conditions and/or assisting physical activities [1, 2]. For implantable devices, the physical volume and energy efficiency are two important factors. First, the physical volume is directly proportional to the burden placed on the human body. Therefore, any additional external passive components should be eliminated in the design of an implantable device. Second, because the implantable devices operate in energy-limited environments, the given energy must be optimally used to minimize power loss. For now, a few types of energy sources, such as batteries, harvested energies, or a combination of them [3–5] are used to power the implantable devices. Though various harvested energies are promising in the future, the batteries are currently reliable and effective. A cion or Li-ion battery voltage of 1.5–5 V usually provides the energy and acts as the energy source. The implantable device is typically powered by sub-1 V supply rail to improve the energy efficiency and prolong its life [6]. Thus, a step-down DC-DC converter with a relatively low voltage conversion ratio (VCR) is highly desirable to act as the power supply of the implantable device, as shown in Fig. 9.1.