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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
07
55
0-
0
34
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:
External Gain Settings = 160 kΩ/(40 kΩ + R
EXT
)
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