A mixed-signal continuous-time-processing standard CMOS implementation of an asynchronous sigma-delta modulator aimed to drive a switching amplifier operating as an on-chip wideband adaptive power supply is presented in this work. The paper first briefly discusses the fundamental limit tracking capabilities of a two-level switching signal to inband-error-free track a bandlimited signal with minimum average switching frequency. It is argued the adequacy of an adaptive asynchronous sigma-delta modulator (AAΣΔ) to approximate such fundamental characteristics. The second part of the paper presents mixed-signal design details of the various subcircuits implementing a CMOS low-power digitally-programmable AAΣΔ modulator, with 7 MHz average switching frequency operation and 1000 µm × 640 µm area occupancy.