Investigation of on-chip inductors for fully-integrated DC-DC converters

Abstract

On-silicon inductors using a bulk 0.18μm CMOS process have been designed. By shunting different metal layers in parallel, inductor values and quality factors were simulated. Selected inductors were then employed in two open-loop buck converters for comparison: the first used an off-chip discrete diode, and the second used an on-chip active diode. All inductors and converters were sent for fabrication. The fabricated inductors were then measured to have values more than 250 nH over a wide range of frequency, validating the simulation results. The buckconverters were switched at 30MHz with a fixed duty ratio of 0.5, to generate an output voltage of 1.2V from an input voltage of 2.4V. The peak efficiency was measured to be 69.1% for a light load current of 12.9 mA.