SSM2301

Rev. A | Page 12 of 16

APPLICATIONS INFORMATION

OVERVIEW

The SSM2301 mono Class-D audio amplifier features a filterless

modulation scheme that greatly reduces external component count,

conserving board space and, thus, reducing system cost. The

SSM2301 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 human ear to fully recover the audio

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

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

SSM2301 uses a 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 reduces the amplitude of spectral

components at high frequencies, thereby reducing EMI emission

that might otherwise be radiated by speakers and long cable traces.

The SSM2301 also offers protection circuitry for output short-

circuit and high temperature conditions. When the fault-inducing

condition is removed, the SSM2301 automatically recovers

without the need for a hard reset.

GAIN SELECTION

Pulling the GAIN pin of the SSM2301 high sets the gain of the

speaker amplifier to 12 dB; pulling it low sets the gain of the

speaker amplifier to 6 dB.

It is possible to adjust the SSM2301 gain by using external resistors

at the input. To set a gain lower than 12 dB, see Figure 26 for

differential input configuration and Figure 27 for single-ended

configuration. For external gain configuration from a fixed 12 dB

gain, use the following formula:

External Gain Settings = 20 log[4/(1 + R/150 kΩ)]

To set a gain lower than 6 dB, see Figure 28 for differential input

configuration and Figure 29 for single-ended configuration. For

external gain configuration from a fixed 6 dB gain, use the

following formula:

External Gain Settings = 20 log[2/(1 + R/150 kΩ)]

POP-AND-CLICK SUPPRESSION

Voltage transients at the output of audio amplifiers may 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 may

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 SSM2301 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 must 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.

Make track widths 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.

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 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, 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 that a large-area ground plane be used for

minimum impedances. Good PCB layouts also isolate critical

analog paths from sources of high interference. High frequency

circuits (analog and digital) should be separated from low

frequency circuits. 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-

side 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.

INPUT CAPACITOR SELECTION

The SSM2301 does not require input coupling capacitors if the

are required if the input signal is not biased within this recom-

mended input dc common-mode voltage range, if high-pass

filtering is needed (see Figure 24) or if using a single-ended

source (see Figure 25). If high-pass filtering is needed at the input,

the input capacitor, along with the input resistor of the SSM2301,

forms a high-pass filter whose corner frequency is determined

by the following equation:

The input capacitor can have very important effects on the

circuit performance. Not using input capacitors degrades the

output offset of the amplifier as well as the PSRR performance.