Digital-Input Class D amplifiers expand the benefits of traditional Class D and simplify system design

Introduction

A new generation of digital-input Class D audio amplifiers achieves high PSRR performance that is comparable to traditional analog Class D amplifiers. More importantly, digital-input Class D amplifiers provide additional benefits of reduced power, complexity, noise, and system cost.  

Electronics vendors commonly use high-efficiency, filterless, analog-input Class D amplifiers to manage the power requirements of portable audio speakers found in cell phones, tablet computers, and personal navigation devices. These Class D amplifiers allow direct connection to a battery which minimizes losses and reduces component count. The amplifiers also achieve >70dB PSRR performance which is important to avoid audible buzzing with 217Hz demodulated GSM signals. 

Analog-input Class D amplifiers normally require a DAC and line driver amp on the application processor (Figure 1), and this adds die cost, power, and noise to the speaker output. These Class D amplifiers also require careful board design to avoid degradation because of signals coupling onto the analog board routes.

Figure 1. Conventional system with analog-input Class D speaker amps. The DAC and line driver amp on the application processor add die cost, power, and noise to the speaker output.

Digital-input Class D audio amplifiers are immune to most board design issues. Single-channel Class D amplifiers can be placed at remote locations on a board to minimize the routing of the high-current battery and speaker load connections. These amplifiers do not need the DAC and line driver amp of analog-input Class D designs. Thus, space and system costs drop and designs are simpler.

Simplified System Design

The most common type of digital-input for an amplifier is pulse-density modulation (PDM) which requires only two wires: PDM_CLK and PDM_DATA. Single-bit PDM data is created with an oversampled sigma-delta modulator on the application processor (Figure 2).   

Figure 2. System with a PDM-input Class D speaker amp requires only two wires and uses oversampled sigma-delta modulator on the application processor to create single-bit data. 

Figure 3. A system with a PCM-input Class D speaker amp uses three wires but does not require a modulator or upsampling of the data on the application processor.

Older digital-input amplifiers offer adjustable sample rate and/or bit depth that, in some cases, require complex programming of the amplifier. Newer generations of digital-input amplifiers like the MAX98355/MAX98356 automatically detect a wide range of sample rates and bit depths to self-configure without any programming.

In a multichannel implementation the digital-input Class D audio amplifier reduces the number of external capacitors and routed lines on the board. Only PDM_CLK and PDM_DATA lines are needed for PDM inputs to provide stereo data to two Class D amplifiers. The BCLK, LRCLK, and DIN lines are needed for PCM inputs to provide stereo data. As a comparison, a stereo analog-input Class D amplifier will normally require two differential input signals (four wires) to be routed with AC-coupling capacitors. (See Figures 1, 2, and 3.)

Most digital-input amplifiers require both a low digital-supply voltage (1.8V) and a high speaker-supply voltage (2.5V to 5.5V). Now board design and component count can be simplified by using a single-supply Class D amplifier like the MAX98355/MAX98356.