Arianna Rajabi

High-fidelity (hi-fi) audio is becoming more and more popular among music fans and movie buffs, from portable speakers to high-end, in-home audio to automotive systems. And from the A to Z of classifications for audio power amplifiers, Class-D amplifiers are known for their nonlinear switching characteristics, which result in much lower power dissipation, excellent efficiency and  reduced number of system components.

Class-D audio amplifiers powered by gallium nitride (GaN) technology, also known for its superior efficiency, are enabling smaller and more efficient Class-D solutions than ever before.

Different than traditional Class A or Class B audio amplifiers, Class-D amplifiers use the audio signal to modulate a pulse-width modulation (PWM) signal that drives the output filtering. These devices constantly switch completely on or off, creating a highly efficient amplifier stage. For portable devices such as headphones or wireless speakers, this efficiency is critical for device battery life.

For lower-power audio amplifiers (<500W), the switching losses of the power transistor will dictate the power dissipation of the system. For a metal-oxide semiconductor field-effect transistor (MOSFET)-based amplifier to achieve hi-fi sound with a total harmonic distortion (THD) >0.1% and a reasonable power-supply rejection ratio (PSRR) of <10dB requires meticulous design work around

Figure 1 below shows the effect of dead-time on THD.

GaN FETs provide superior switching characteristics, which enable even higher efficiency, better thermal sinking reduced size and weight, as well as reduced distortion for Class-D audio solutions.

GaN’s inherent characteristics provide a more ideal model of a small-signal PWM, driving the output filter. Along with this, the absence of a body diode eliminates reverse-recovery charge, which enables increased output linearity. Both characteristics allow GaN to minimize THD and ultimately create higher-quality sound.

TI’s LMG5200 is a 80V integrated GaN power stage combining a half-bridge driver and FETs into one package for an easy-to-use solution in Class-D audio. The LMG5200 offers high- to low-side matching of 2ns, which reduces distortions, and features undervoltage lockout protection (UVLO). Depending on the target of the amplifier, some designs are better suited for discrete solutions.

In order to reduce distortions that result in degraded sound quality, it is critical to minimize dead time while still preventing shoot-through current. Typical MOSFET designs will have >20ns of dead-time, resulting in distortion and power dissipation. TI’s LMG1210 200V half-bridge driver is the fastest GaN driver - capable of 50MHz switching and has resistive capability that offers adjustable dead-time control anywhere from 0-20ns. This integrated dead time control feature simplifies designs and eliminates the need for external implementation or additional software, as well as improving THD, TIMD, and EMI. The effect of dead-time on THD is simulated in the Figure 1, where the y axis is THD%, and the x axis is dead time beginning at 1 ns. The LMG1210 also offers high- to low-side matching of 1.5ns and a rise and fall time of 500ps.

Get started on your design using the LMG5200 evaluation module.