Data Sheet

SSM2356

Rev. A | Page 13 of 16

APPLICATIONS INFORMATION

OVERVIEW

The SSM2356 stereo 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 SSM2356 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

SSM2356 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. Due to the inherent spread-spectrum nature of Σ-Δ

modulation, the need for oscillator synchronization is eliminated

for designs incorporating multiple SSM2356 amplifiers.

The SSM2356 also integrates overcurrent and temperature

protection.

GAIN SELECTION

The preset gain of SSM2356 can be selected between 6 dB and

18 dB with no external components and no change to the input

impedance. A major benefit of fixed input impedance is that

there is no need to recalculate input corner frequency (Fc)

when gain is adjusted. The same input coupling components

can be used for both gain settings.

It is possible to adjust the SSM2356 gain by using external resistors

at the input. To set a gain lower than 18 dB (or 6 dB when

GAIN = GND), refer to Figure 34 for the differential input

configuration and Figure 35 for the single-ended configuration.

Calculate the external gain configuration as follows:

When GAIN = GND (6 dB default gain setting)

Please note that when using external resistors to adjust the gain

performance, it is not recommended to use external series resistors

larger than 10 kΩ due to increased noise floor and reduced

THD+N performance.

POP-AND-CLICK SUPPRESSION

Voltage transients at the output of audio amplifiers may 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 can be

sources of audible transients:

•

System power-up/power-down

•

Mute/unmute

•

Input source change

•

Sample rate change

The SSM2356 has a pop-and-click suppression architecture that

reduces these output transients, resulting in noiseless activation and

deactivation.

EMI NOISE

The SSM2356 uses a proprietary modulation and spread-

spectrum technology to minimize EMI emissions from the

device. For applications having difficulty passing FCC Class B

emission tests, the SSM2356 includes a modulation select pin

(ultralow EMI emission mode) that significantly reduces the

radiated emissions at the Class-D outputs, particularly above

100 MHz. Figure 36 shows SSM2356 EMI emission tests per-

formed in a certified FCC Class-B laboratory in normal

emissions mode (EDGE = GND). Figure 37 shows SSM2356

emissions mode.

60

0

10

20

30

40

50

30

130

230

330

430

530

630

730

830

930 1000

FREQUENCY (MHz)

[1] HORIZONTAL

[2] VERTICAL

FCC CLASS-B LIMIT

Figure 36. EMI Emissions from SSM2356, 1-Channel, 12 cm Cable,

EDGE = GND

60

0

10

20

30

50

30

130

230

330

430

530

630

730

830

930 1000

FREQUENCY (MHz)

[1] HORIZONTAL

[2] VERTICAL

FCC CLASS-B LIMIT

40

Figure 37. EMI Emissions from SSM2356, 1-Channel, 12 cm Cable,