SSM2306

Rev. 0 | Page 12 of 16

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:

•

System power-up/power-down

•

Mute/unmute

•

Input source change

•

Sample rate change

The SSM2306 has a pop-and-click suppression architecture that

reduces these output transients, resulting in noiseless activation and

deactivation.

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.

70

0

0.1

100

FREQUENCY (MHz)

60

50

40

30

20

10

110

= HORIZONTAL

= VERTICAL

= REGULATION VALUE

Figure 34. EMI Emissions from SSM2306

70

0

10

10k

FREQUENCY (MHz)

60

50

40

30

20

10

100

1k

= HORIZONTAL

= VERTICAL

= REGULATION VALUE

Figure 35. EMI Emissions from SSM2306

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.