Design Considerations for a Digital Input MEMS Speaker Audio Amplifier with Energy Recovery

The advances in MEMS technology offer now the possibility to manufacture loudspeakers, which need special drivers to achieve optimum performance. Driving capacitive loudspeakers (transducers) introduces a number of interesting challenges, which are addressed in this work.A digital amplifier, comparable to a class-D amplifier for electrodynamic loudspeakers is presented. The amplifier has a digital input and therefore also includes the A/D-conversion required for producing an analog output. In order to achieve a high efficiency the A/D-conversion is combined with a buck-boost converter and uses energy recovery to increase the efficiency. Using charge transfer efficient strategies for the design of such an amplifier will be given. The results are valid for capacitive loads in general and therefore useful for a wide range of problems, like MEMS drivers or switching control of power transistors.This article describes fundamental advantages of MEMS speaker based audio systems and the most important parameters for the required audio amplifier. A newly developed concept based on a non-inverting bidirectional buck boost converter will be used to derive the parameters as well as to explain the most important design criteria for an audio amplifier development for capacitive loads.