Page 17 - Issue 2 2023

ELECTRONICS

How to easily achieve superior audio

performance with plug and play

digital-input class D amplifiers

By Matt Felder, distinguished engineer

new generation of plug and play digital-input Class D
audio amplifiers achieves audio performance that is far
A superior to traditional analogue 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,
analogue-input Class D amplifiers to manage the power
requirements of portable audio speakers found in cell phones,
tablet computers, home monitoring and smart speakers. These Figure 1: A conventional system with analogue-input Class D speaker
Class D amplifiers allow direct connection to a battery that amps. The DAC and line driver amp on the application processor add die
minimises losses and reduces component count. The amplifiers cost, power and noise to the speaker output.
also achieve >80 dB PSRR performance, which is important to
avoid audible buzzing with 217 Hz demodulated GSM signals.
Analogue-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
analogue board routes.
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 minimise the routing
of the high current battery and speaker load connections.
These amplifiers do not need the DAC and line driver amp of Figure 2: A system with a PCM-input Class D speaker amp uses
analogue-input Class D designs. Thus, the space and system three wires but does not require a modulator or upsampling of the
costs drop, and designs are simpler. data on the application processor.

Simplified system design rates and bit depths to self-configure without any programming.
Most digital input amplifiers accept pulse-code modulated In a multi-channel implementation, the digital-input Class
2
(PCM) or I S data, which requires three wires: BCLK, LRCLK and D audio amplifier reduces the number of external capacitors
DIN. The PCM data format does not require a modulator or and routed lines on the board. Only BCLK, LRCLK and DIN lines
upsampling of the data on the application processor are needed for PCM inputs to provide stereo or 8-channel
(Figure 2). Some older implementations of PCM-input TDM data. As a comparison, a stereo analogue-input Class D
amplifiers also require a clean master clock (MCLK) to derive a amplifier will normally require two differential input signals
jitter-free sampling clock. Newer PCM input amplifiers like the (four wires) to be routed with AC-coupling capacitors (see
MAX98357, MAX98360 and MAX98365 no longer require the Figures 1 and 2).
MCLK input so the pin count, power consumption and board Most digital-input amplifiers require both a low digital
complexity are all reduced. supply voltage (1.8 V) and a high speaker supply voltage (2.5
Older digital-input amplifiers offer adjustable sample V to 5.5 V). Now the board design and component count can
rate and/or bit depth that, in some cases, require complex be simplified by using a single supply Class D amplifier like the
programming of the amplifier. Newer generations of digital MAX98357 and MAX98360. The MAX98365 can run from a
input amplifiers automatically detect a wide range of sample single 3.0 V to 5.5 V supply or one supply at 1.8 V to 5.5 V and

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