SSM4321 Data Sheet
Rev. 0 | Page 14 of 24
THEORY OF OPERATION
OVERVIEW
The SSM4321 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 SSM4321 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 SSM4321 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 reduces the amplitude of spectral compo-
nents at high frequencies, thus 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 SSM4321 amplifiers.
The SSM4321 also integrates overcurrent and overtemperature
protection.
POWER-DOWN OPERATION
The SSM4321 contains a clock loss detect circuit that works
with the BCLK input clock. When no BCLK is present, the part
automatically powers down all internal circuitry to its lowest
power state. When a BCLK is returned, the part automatically
powers up.
If BCLK is active but FSYNC or LRCLK is not present, the
amplifier continues to operate, but the ADC, sense blocks, and
digital processing are shut down, reducing quiescent current
when the output sense data is not needed. The ADC shutdown
feature is not available in PDM operating mode.
GAIN SELECTION
The gain of the SSM4321 can be set from 0 dB to 12 dB in 3 dB
steps using the GAIN pin and one (optional) external resistor.
The external resistor is used to select the 9 dB or 12 dB gain
setting (see Table 6).
Table 6. Setting the Gain of the SSM4321 with the GAIN Pin
0 Tie to GND
3 Open
6 Tie to PVDD
9 Tie to GND through a 47 kΩ resistor
12 Tie to PVDD through a 47 kΩ resistor
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.
Pops and clicks can also be classified as undesirable audible
transients generated by the amplifier system and, therefore, as
not coming from the system input signal.
The SSM4321 has a pop-and-click suppression architecture that
reduces these output transients, resulting in noiseless activation
and deactivation.
OUTPUT MODULATION DESCRIPTION
The SSM4321 uses three-level, Σ-Δ output modulation. Each
output can swing from GND to PVDD 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.
When the user wants to send an input signal, an output pulse
(OUT+ and OUT−) is generated to follow the input voltage. The
differential pulse density (V
OUT
) is increased by raising the input
signal level. Figure 38 depicts three-level, Σ-Δ output modulation
with and without input stimulus.
OUTPUT > 0V
+5V
0V
OUT+
+5V
0V
OUT–
+5V
0V
V
OUT
OUTPUT < 0V
+5V
0V
OUT+
+5V
0V
OUT–
0V
–5V
V
OUT
OUTPUT = 0V
OUT+
+5V
0V
+5V
0V
OUT–
+5V
–5V
0V
V
OUT
10752-015
Figure 38. Three-Level, Σ-Δ Output Modulation
With and Without Input Stimulus
