SSM2804CBZ-R7 datasheet(13/36 Pages) AD

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SSM2804

Rev. 0 | Page 13 of 36

THEORY OF OPERATION

The SSM2804 audio subsystem features a filterless modulation

scheme that greatly reduces the external component count, con-

serving board space and, thus, reducing system cost. The SSM2804

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 SSM2804 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 emissions that might otherwise

be radiated by speakers and long cable traces.

•

The SSM2804 does not require external EMI filtering for

twisted speaker cable lengths shorter than 10 cm. If longer

speaker cables are used, the SSM2804 has emission limiting

circuitry that allows significantly longer speaker cable.

•

Due to the inherent spread-spectrum nature of Σ-Δ modu-

lation, the need for modulator synchronization is eliminated

for designs that incorporate multiple SSM2804 amplifiers.

Using the I2C control interface, the gain of the SSM2804 can

be selected from a range of +12 dB to −63 dB in 32 steps. Other

features accessed from the I2C interface include the following:

•

Independent left/right channel shutdown

•

Variable ultralow EMI emission limiting circuitry

•

Automatic level control (ALC) for high quality speaker

protection

•

Stereo-to-mono mixing operation

The SSM2804 also offers protection circuits for overcurrent and

overtemperature protection.

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 SSM2804 has a pop-and-click suppression architecture that

reduces these output transients, resulting in noiseless activation

and deactivation.

OUTPUT MODULATION DESCRIPTION

The SSM2804 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, noise sources are always present.

Due to the 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, the output differential voltage is 0 V,

due to the Analog Devices, Inc., three-level, Σ-Δ output modu-

lation. 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

(OUT+ and OUT−) is generated to follow the input voltage.

The differential pulse density (VOUT) is increased by raising the

input signal level. Figure 28 depicts three-level, Σ-Δ output

modulation with and without input stimulus.

OUTPUT > 0V

+5V

OUT+

+5V

OUT–

+5V

VOUT

OUTPUT < 0V

+5V

OUT+

+5V

OUT–

–5V

VOUT

OUTPUT = 0V

OUT+

+5V

OUT–

+5V

–5V

VOUT

Figure 28. Three-Level, Σ-Δ Output Modulation

With and Without Input Stimulus

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