Spread spectrum techniques for Class-D audio amplifiers to reduce EMI

For battery powered portable devices it is required to have circuits that combine high efficiency with a small footprint. In the case of Class-D amplifiers the high efficiency can be achieved by applying pulse-width modulation (PWM) to generate the desired output signal. Classical PWM requires an output filter to suppress the idle current. In order to avoid this filter a different modulation scheme is used, which however introduces a larger amount of electromagnetic emissions. Therefore spread-spectrum techniques are applied to distribute the emissions over a wider frequency range. In this article methods are discussed that allow the use of spread-spectrum modulation without introducing audible artifacts due to the modulation.