Reduce EMI and Maintain High Efficiency with Class D Amplifiers in Portable Applications - Application Note - Maxim

Class D amplifiers are typically very efficient, making them ideal candidates for portable applications that require long battery life and low thermal dissipation. However, electromagnetic interference (EMI) is an issue that commonly accompanies the Class D switching topology. Activeemissions limiting reduces radiated emissions and enables "filterless" operation, allowing designers to create small, efficient portable applications with low EMI. A similar version of this article appears on Hearst Electronic Products, March 28, 2012. The rapid adoption of battery-operated consumer electronics such as smartphones, tablets, and portable speakers is driving the steadily rising demand for efficient audio amplifiers. High-efficiency audio amps are needed to extend battery life and lower power consumption while still maintaining low electromagnetic interference (EMI) levels. Class D amps are now widely used for these applications because they offer approximately 90% peak efficiency compared to the 50% for a typical Class AB amplifier. This added efficiency is important because it helps the designer to lower power consumption, extend battery life, use a smaller form factor, and lower thermal dissipation. Class D Amps and EMI High efficiency does, admittedly, come with its own design challenge. A Class D audio amp's switching topology produces EMI, which a designer must mitigate. EMI creates radiated and conducted noise that increases with the signal level and interferes with other subsystems in the device. Engineers must meet the U.S. Federal Communications Commission's (FCC) EMI standards for consumer products produced and sold in the U.S. Similar EMI specifications exist worldwide. Managing EMI is even more challenging because device form factors are constantly shrinking. Engineers of portable applications thus have the added pressured of designing into smaller and smaller form factors. Limited space can force designers to eliminate the output filters, enclosures, and system-level shielding