SSM2302

Rev. 0 | Page 12 of 20

APPLICATION NOTES

OVERVIEW

The SSM2302 stereo 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 SSM2302 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. While most Class-D ampli-

fiers use some variation of pulse-width modulation (PWM), the

SSM2302 uses a Σ-Δ modulation to determine the switching

pattern of the output devices. This provides a number of important

benefits. Σ-Δ modulators do not produces 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 SSM2302 also offers

protection circuits for overcurrent and temperature protection.

GAIN SELECTION

Pulling the GAIN pin high of the SSM2302 sets the gain of the

speaker amplifier to 12 dB; pulling it low sets the gain of the

speaker amplifier to 6 dB.

It is possible to adjust the SSM2302 gain by using external resistors

at the input. To set a gain lower than 12 dB refer to Figure 22 for

differential input configuration and Figure 23 for single-ended

configuration. For external gain configuration from a fixed 12 dB

gain, please use the following formula:

External Gain Settings = 20 log[4/(1 + R/150 kΩ)]

To set a gain lower than 6 dB refer to Figure 24 for differential

input configuration and Figure 25 for single-ended configuration.

For external gain configuration from a fixed 6 dB gain, use the

following formula:

External Gain Settings = 20 log[2/(1 + R/150 kΩ)]

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, therefore as not coming from

the system input signal. Such transients can be generated when

the amplifier system changes its operating mode. For example, the

following can be sources of audible transients: system power-up/

power-down, mute/unmute, input source change, and sample rate

change. The SSM2302 has a pop-and-click suppression architecture

that reduces this output transients, resulting in noiseless activation

and deactivation.

EMI NOISE

The SSM2302 uses a proprietary modulation and spread-

spectrum technology to minimize EMI emissions from the

device. Figure 26 shows SSM2302 EMI emission starting from

100 kHz to 30 MHz. Figure 27 shows SSM2302 EMI emission

from 30 kHz to 2 GHz. These figures clearly describe the SSM2302

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 SSM2302 are greatly reduced.

70

0

0.1

100

FREQUENCY (MHz)

60

50

40

30

20

10

110

= HORIZONTAL

= VERTICAL

= REGULATION VALUE

Figure 26. EMI Emissions from SSM2302

70

0

10

10k

FREQUENCY (MHz)

60

50

40

30

20

10

100

1k

= HORIZONTAL

= VERTICAL

= REGULATION VALUE

Figure 27. EMI Emissions from SSM2302

The measurements for Figure 26 and Figure 27 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.

The EMI was detected using a magnetic probe touching the 2”

output trace to the load.