
SSM2335
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THEORY OF OPERATION
OVERVIEW
The SSM2335 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 SSM2335 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
SSM2335 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 emission that might otherwise be radiated by
speakers and long cable traces. The SSM2335 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 SSM2335 amplifiers.
The SSM2335 also offers protection circuits for overcurrent and
temperature protection.
GAIN
The SSM2335 has a default gain of 18 dB that can be reduced by
using a pair of external resistors with a value calculated as follows:
External Gain Settings = 160 kΩ/(20 kΩ + REXT)
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 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 SSM2335 has
a pop-and-click suppression architecture that reduces these out-
put transients, resulting in noiseless activation and deactivation.
OUTPUT MODULATION DESCRIPTION
The SSM2335 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 differ-
ential pulse is generated.
Most of the time, however, output differential voltage is 0 V, due
to the Analog Devices patent pending, 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 33depicts three-level, Σ-Δ output modulation with and without
input stimulus.
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Figure 33. Three-Level, Σ-Δ Output Modulation With and Without Input Stimulus