Reduce EMI from Class D Amplifiers Using New Modulation Techniques and Filter Architectures - Application Note - Maxim

Class D amplifiers have always offered better efficiency than Class AB amplifiers for low-power applications. This inherent advantage, however, rarely outweighed the traditional disadvantages of Class D, which were higher cost, reduced audio performance, and the need for output filtering. Recent advances in Class D technology, however, have brought Class D costs and audio performance up to equal Class AB. Some novel Class D output modulation schemes, moreover, now ease the EMI burden in many applications. Introduction The state of the art in Class D amplification has progressed rapidly in the past few years, most noticeably for lower power applications that require less than 50W per channel. For these low-power applications, Class D has an inherent efficiency advantage over the traditional Class AB amplifier, because the Class D output stages are always on or off, with no intermediate bias stage necessary. Historically, however, this efficiency advantage never held widespread appeal for designers, because Class D also had notable disadvantages: higher parts cost, poor audio performance (compared to Class AB), and the need for output filtering. Two major factors have reversed this design trend in recent years, making Class D more appealing in a wade range of applications. First, there is market need. Two rapidly growing end-equipment markets benefit from Class D amplification in different ways; cell phones and LCD flat-panel displays. With cell phones, both the speakerphone and PTT (Push-to-Talk) modes benefit from Class D's higher efficiency, which provides longer battery life. The growth of LCD flat-panel displays generated a need for 'cool running' electronics, as the display color contrast suffers under elevated operating temperatures. As Class D efficiency means that less power is dissipated in the drive electronics, the LCD flat-panel displays operate cooler and with better looking pictures. The second factor to affect use of Class D is the technology itself. Driven by market needs, several manufacturers now offer improved Class D technology with costs more equitable and audio performance on a par with Class AB. Some novel Class D output modulation schemes, moreover, now ease the EMI burden in many applications.