SSM2335

Rev. 0 | Page 12 of 1

6

OUTPUT > 0V

+5V

0V

THEORY OF OPERATION

OVERVIEW

The SSM2335 mono Class-D audio amplifier features a filterless

modulation scheme that greatly reduces the external component

count, conserving board space and, thus, reducing systems cost.

The SSM2335 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. Most Class-D amplifiers

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

SSM2335 uses Σ-Δ modulation to determine the switching

pattern of the output devices, resulting in 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. The SSM2335 does not require

external EMI filtering for twisted speaker cable lengths shorter

than 10 cm. Due to the inherent spread-spectrum nature of Σ-Δ

modulation, the need for oscillator synchronization is eliminated

for designs incorporating multiple SSM2335 amplifiers.

The SSM2335 also offers protection circuits for overcurrent and

temperature protection.

GAIN

The SSM2335 has a default gain of 18 dB that can be reduced by

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

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 may

be generated when the amplifier system changes its operating

mode. For example, the following may be sources of audible

transients: system power-up and power-down, mute and unmute,

input source change, and sample rate change. The SSM2335 has

a pop-and-click suppression architecture that reduces these out-

put transients, resulting in noiseless activation and deactivation.

OUTPUT MODULATION DESCRIPTION

The SSM2335 uses three-level, Σ-Δ output modulation. Each

no input signal is present, the output differential voltage is 0 V

because there is no need to generate a pulse. In a real-world

situation, there are always noise sources present.

Due to this constant presence of noise, a differential pulse is

generated, when required, in response to this stimulus. A small

amount of current flows into the inductive load when the differ-

ential pulse is generated.

Most of the time, however, output differential voltage is 0 V, due

to the Analog Devices patent pending, three-level, Σ-Δ output

modulation. This feature ensures that the current flowing through

the inductive load is small.

When the user wants to send an input signal, an output pulse is

generated to follow the input voltage. The differential pulse

density is increased by raising the input signal level. Figure 33

depicts three-level, Σ-Δ output modulation with and without

input stimulus.

OUT+

+5V

0V

OUT–

+5V

0V

VOUT

OUTPUT < 0V

+5V

0V

OUT+

+5V

0V

OUT–

0V

–5V

VOUT

OUTPUT = 0V

OUT+

+5V

0V

+5V

0V

OUT–

+5V

–5V

0V

VOUT

Figure 33. Three-Level, Σ-Δ Output Modulation With and Without Input Stimulus