SSM2319

Rev. 0 | Page 14 of 20

THEORY OF OPERATION

OVERVIEW

OUTPUT MODULATION DESCRIPTION

The SSM2319 mono Class-D audio amplifier features a filterless

modulation scheme that greatly reduces the external components

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

The SSM2319 does not require an output filter. Instead, it 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

SSM2319 uses a 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 reduces the amplitude of spectral

components at high frequencies, reducing EMI emission that

may 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 SSM2319 amplifiers.

The SSM2319 uses 3-level Σ-Δ output modulation. Each output

is able to swing from GND to VDD and vice versa. Ideally, when

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 the

constant presence of noise, a differential pulse is generated

in response to this stimulus. A small amount of current flows

into the inductive load when the differential pulse is generated.

However, most of the time, the output differential voltage is 0 V,

due to the Analog Devices, Inc., patented 3-level, Σ-Δ output

modulation feature. 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 36 depicts

3-level Σ-Δ output modulation with and without input stimulus.

OUTPUT > 0V

+5V

0V

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

The SSM2319 also offers protection circuits for overcurrent and

temperature protection.

GAIN

The SSM2319 has a default gain of 12 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 the 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, audible transient sources include system power-up/

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

rate change. The SSM2319 has a pop-and-click suppression

architecture that reduces these output transients, resulting in

noiseless activation and deactivation.

Figure 36. 3-Level Σ-Δ Output Modulation With and Without Input Stimulus