SSM2315

Rev. A | Page 12 of 16

THEORY OF OPERATION

OVERVIEW

The SSM2315 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 SSM2315 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

SSM2315 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 provides the benefits of reducing the

amplitude of spectral components at high frequencies, that is,

reducing EMI emission that may otherwise be radiated by

speakers and long cable traces. The SSM2315 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 SSM2315 amplifiers.

The SSM2315 also offers protection circuits for overcurrent and

temperature protection.

GAIN

The SSM2315 has a default gain of 6 dB that can be reduced by

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

External Gain Settings = 160 kΩ/(80 kΩ + REXT)

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 may be sources of audible transients: system power-up

and power-down, mute and unmute, input source change, and

sample rate change. The SSM2315 has a pop-and-click suppression

architecture that reduces these output transients, resulting in

noiseless activation and deactivation.

OUTPUT MODULATION DESCRIPTION

The SSM2315 uses three-level, Σ-Δ output modulation. Each

output can 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 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 differen-

tial pulse is generated.

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

due to the Analog Devices patented, 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.

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

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