SSM2377
Rev. 0 | Page 13 of 16
THEORY OF OPERATION
OVERVIEW
The SSM2377 mono Class-D audio amplifier features a filterless
modulation scheme that greatly reduces the external component
count, conserving board space and, thus, reducing system cost.
The SSM2377 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 SSM2377 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 emissions that might otherwise be
radiated by speakers and long cable traces.
• Due to the inherent spread-spectrum nature of Σ-Δ modu-
lation, the need for oscillator synchronization is eliminated
for designs that incorporate multiple SSM2377 amplifiers.
The SSM2377 also integrates overcurrent and overtemperature
protection.
GAIN SELECTION
The preset gain of the SSM2377 can be set to 6 dB or 12 dB
using the GAIN pin, as shown in Table 5.
Table 5. GAIN Pin Function Description
Gain Setting (dB) GAIN Pin Configuration
6 Tie to VDD
12 Tie to GND
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 audible 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.
The SSM2377 has a pop-and-click suppression architecture that
reduces these output transients, resulting in noiseless activation
and deactivation from the
SD
control pin.
EMI NOISE
The SSM2377 uses a proprietary modulation and spread-spectrum
technology to minimize EMI emissions from the device. For
applications that have difficulty passing FCC Class B emission
tests or experience antenna and RF sensitivity problems, the
ultralow EMI architecture of the SSM2377 significantly reduces
the radiated emissions at the Class-D outputs, particularly above
100 MHz. Figure 35 shows the low radiated emissions from the
SSM2377 due to its ultralow EMI architecture.
60
50
40
30
20
10
0
ELECTRIC FIELD STRENGTH (dBµV/m)
30
130
230
330
430
530
FREQUENCY (MHz)
630
730
830
930
1000
+
+
+
+
09824-035
FCC CLASS B LIMIT
HORIZONTAL POLARIZATION
VERTICAL POLARIZATION
Figure 35. EMI Emissions from the SSM2377
The measurements for Figure 35 were taken in an FCC-certified
EMI laboratory with a 1 kHz input signal, producing 1.0 W of
output power into an 8 Ω load from a 5.0 V supply. The SSM2377
passed FCC Class B limits with 50 cm, unshielded twisted pair
speaker cable. Note that reducing the power supply voltage greatly
reduces radiated emissions.
OUTPUT MODULATION DESCRIPTION
The SSM2377 uses three-level, Σ-Δ output modulation. Each
output can swing from GND to V
DD
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, noise sources are always present.
Due to the 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, the output differential voltage is 0 V,
due to the Analog Devices, Inc., three-level, Σ-Δ output modula-
tion. This feature ensures that the current flowing through the
inductive load is small.
