Single-Inductor Bipolar-Outputs Converter for the Supply of Audio Amplifiers in Mobile Platforms

Listening music with a headset is now a classical feature of mobile phones. A linear amplifier is still employed to drive the headset as the audio quality must challenge dedicated audio players with somewhat 100 dB of signal-to-noise ratio. This feature must also have the lowest impact on the platform autonomy, size, and cost. A symmetrical power supply is required by such audio amplifiers due to the standard jack connector of headsets. Using an inductive switch mode power supply (SMPS) is the most energy efficient solution to supply audio amplifiers from a battery. Classically, two dc/dc converters are employed to generate the required two symmetrical supply rails from a standard Lithium-Ion battery. It is at the cost of numerous bulky and expensive external components. A solution to reduce the number of external passive components is to consider a single-inductor, bipolar-outputs, (SIBO) dc/dc converter. An early prototype of such a converter has been realized on a 130-nm silicon process and offers primary results, nonoptimal but promising. The feedback control structure is studied based on a small signal model that serves to define the pairing of the loops for designing a decentralized controller. A set of controllers is designed by a pole placement technique and transient performances are demonstrated on audio patterns thanks to a piecewise linear model of the converter. Eighty percent peak efficiency is measured with the primary demonstrator but latter simulation results yield an expected improvement to 90%. The audio specifications appear very constraintful and the proposed SMPS does not meet entirely the latter figures. Limitations are detailed. However, many other applications can benefit from the proposed SIBO dc/dc converter.