SSM2311

Rev. 0 | Page 15 of 20

APPLICATION NOTES

OVERVIEW

The SSM2311 mono Class-D audio amplifier features a filterless

modulation scheme that greatly reduces the external components

count, conserving board space and thus reducing the system’s cost.

The SSM2311 does not require an output filter, but instead relies

on the inherent inductance of the speaker coil and the natural

filtering of the speaker and the human ear to fully recover the audio

component of the square-wave output. While many Class-D ampli-

fiers use some variation of pulse-width modulation (PWM), the

SSM2311 uses Σ-Δ modulation to determine the switching

pattern of the output devices. This provides a number of important

benefits. Σ-Δ modulators do not produce a sharp peak with

many harmonics in the AM frequency band, as pulse-width

modulators often do. Σ-Δ modulation provides the benefits of

reducing the amplitude of spectral components at high frequencies;

that is, reducing EMI emission that might otherwise be radiated

by speakers and long cable traces. Due to the inherent spread-

spectrum nature of Σ-Δ modulation, the need for oscillator

synchronization is eliminated for designs incorporating

multiple SSM2311 amplifiers.

The SSM2311 also offers protection circuits for overcurrent and

temperature protection.

GAIN

The SSM2311 has a default gain of 18 dB, but can be reduced by

using a pair of external resistors with a value calculated as follows:

External Gain Settings = 300k/(37.5k + Rext)

POP-AND-CLICK SUPPRESSION

Voltage transients at the output of audio amplifiers can occur when

shutdown is activated or deactivated. Voltage transients as low

as 10 mV can be heard as an audio pop in the speaker. Clicks

and pops can also be classified as undesirable audible transients

generated by the amplifier system and therefore as not coming

from the system input signal. Such transients can be generated

when the amplifier system changes its operating mode. For example,

the following can be sources of audible transients: system power-up/

power-down, mute/unmute, input source change, and sample rate

change. The SSM2311 has a pop-and-click suppression architecture

that reduces these output transients, resulting in noiseless activation

and deactivation.

LAYOUT

As output power continues to increase, care needs to be taken to

lay out PCB traces and wires properly between the amplifier,

load, and power supply. A good practice is to use short, wide

PCB tracks to decrease voltage drops and minimize inductance.

Ensure that track widths are at least 200 mil for every inch of

track length for lowest DCR, and use 1 oz or 2 oz of copper PCB

traces to further reduce IR drops and inductance. A poor layout

increases voltage drops, consequently affecting efficiency. Use

large traces for the power supply inputs and amplifier outputs to

minimize losses due to parasitic trace resistance.

Proper grounding guidelines help to improve audio

performance, minimize crosstalk between channels, and

prevent switching noise from coupling into the audio signal. To

maintain high output swing and high peak output power, the

PCB traces that connect the output pins to the load and supply

pins should be as wide as possible to maintain the minimum

trace resistances. It is also recommended to use a large-area

ground plane for minimum impedances.

In addition, good PCB layouts isolate critical analog paths from

sources of high interference. High frequency circuits (analog

and digital) should be separated from low frequency ones.

Properly designed multilayer printed circuit boards can reduce

EMI emission and increase immunity to the RF field by a factor of

10 or more compared with double-sided boards. A multilayer

board allows a complete layer to be used for the ground plane,

whereas the ground plane side of a double-sided board is often

disrupted with signal crossover. If the system has separate analog

and digital ground and power planes, the analog ground plane

should be underneath the analog power plane, and, similarly, the

digital ground plane should be underneath the digital power

plane. There should be no overlap between analog and digital

ground planes or analog and digital power planes.

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FREQUENCY (MHz)

Figure 47. EMI Emissions from SSM2311