A 233-MHz 80%-87% efficient four-phase DC-DC converter utilizing air-core inductors on package. - Peter Hazucha; Gerhard Schrom; Jaehong Hahn; Bradley A. Bloechel; Paul Hack; Gregory E. Dermer; Siva G. Narendra; Donald S. Gardner; Tanay Karnik; Vivek De; Shekhar Borkar (IEEE J. Solid State Circuits, 2005)