SSM2306 AD [Analog Devices], SSM2306 Datasheet - Page 12

• Manufacturer Part Number: SSM2306
• Description: 2 W, Filterless, Class-D Stereo Audio Amplifier
• Manufacturer: AD [Analog Devices]

SSM2306
APPLICATION NOTES
OVERVIEW
The SSM2306 stereo, Class-D, audio amplifier features a filterless
modulation scheme that greatly reduces the external compo-
nents count, conserving board space and, thus, reducing systems
cost. The SSM2306 does not require an output filter; instead, it
relies on the inherent inductance of the speaker coil and the
natural filtering capacity of the speaker and human ear to fully
recover the audio component of the square wave output.
Although most Class-D amplifiers use some variation of pulse-
width modulation (PWM), the SSM2306 uses sigma-delta (Σ-Δ)
modulation to determine the switching pattern of the output
devices. This provides 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 radiate by the use of speakers
and long cable traces. The SSM2306 also offers protection
circuits for overcurrent and overtemperature protection.
GAIN SELECTION
The SSM2306 has a pair of internal resistors that set an 18 dB
default gain for the amplifier. It is possible to adjust the SSM2306
gain by using external resistors at the input. To set a gain lower
than 18 dB, refer to Figure 32 for the differential input configu-
ration and Figure 33 for the single-ended configuration. Calculate
the external gain configuration as
POP-AND-CLICK SUPPRESSION
Voltage transients at the output of audio amplifiers can occur with
the activation or deactivation of shutdown. Furthermore, voltage
transients as low as 10 mV are audible as an audio pop in the
speaker. Likewise, clicks and pops are classified as undesirable
audible transients generated by the amplifier system, and as
such, as not coming from the system input signal. These types
of transients generate when the amplifier system changes its
operating mode. For example, the following can be sources of
audible transients:
•
•
•
•
The SSM2306 has a pop-and-click suppression architecture that
reduces these output transients, resulting in noiseless activation and
deactivation.
External Gain Settings = 344 kΩ/(43 kΩ + R
System power-up/power-down
Mute/unmute
Input source change
Sample rate change
EXT
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EMI NOISE
The SSM2306 uses a proprietary modulation and spread-
spectrum technology to minimize EMI emissions from the
device. Figure 34 shows SSM2306 EMI emission starting from
100 kHz to 30 MHz. Figure 35 shows SSM2306 EMI emission
from 30 kHz to 2 GHz. These figures clearly depict the SSM2306
EMI behavior as being well below the FCC regulation values,
starting from 100 kHz and passing beyond 1 GHz of frequency.
Although the overall EMI noise floor is slightly higher, frequency
spurs from the SSM2306 are greatly reduced.
The measurements for Figure 34 and Figure 35 were taken with
a 1 kHz input signal, producing 0.5 W output power into an 8 Ω
load from a 3.6 V supply. Cable length was approximately 5 cm.
To detect EMI, a magnetic probe was used touching the 2-inch
output trace to the load.
70
60
50
40
30
20
10
70
60
50
40
30
20
10
0
0
0.1
10
= HORIZONTAL
= VERTICAL
= REGULATION VALUE
= HORIZONTAL
= VERTICAL
= REGULATION VALUE
Figure 34. EMI Emissions from SSM2306
Figure 35. EMI Emissions from SSM2306
100
1
FREQUENCY (MHz)
FREQUENCY (MHz)
10
1k
100
10k