Implantable biomedicai systems usually operate in energy-limited environments and exhibit large variation of power consumption ranging from constant low-power (bio-signal sensing) to sporadic high-power (stimulation and/or burst data transmission) . Thus, energy-efficient switching power converters that can support a wide range of load variation are necessary to meet this requirement. Pulse frequency modulation (PFM) power converters can support this large load variation with a high power-conversion efficiency (PCE) . However, their unpredictable switching noise degrades the power supply integrity and hence deteriorates the signal quality in implantable biomédical systems. Pulse-width modulation (PWM) power converters can provide predictable output spectrum, but their PCE in light loads suffers. Although various techniques have been explored over the past years [3-5], the PCE of the PWM converters is still not adequate for low current levels (<100uA) that most of the implantable biomedicai systems consume.