This article describes a multilevel hybrid plug-and-play dc–dc converter capable of converting a Li-ion battery input voltage range of 2.7–4.5 V to a system-on-chip voltage range of 0.6–1.3 V with an output spur as low as −53.5 dBm, a switching frequency variation of 0.13%, and peak efficiency of 88.6% at a corresponding power density of 0.061 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The converter, designed in 65-nm CMOS, uses a small 470-nH inductor and is stable with a multilayer ceramic output capacitor with only a 2-m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Omega$</tex-math></inline-formula> series resistance. A fully on-chip adaptive constant-on-time controller is employed, enabling a minimum efficiency of 70% for load currents as low as 30 mA with a response time of 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> s from a rapid 1-A load current step, without any need for external power supplies or control circuits.