SSM2375

Preliminary Technical Data

Rev. PrC | Page 10 of 13

THEORY OF OPERATION

OVERVIEW

The SSM2375 mono 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 SSM2375 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 SSM2375 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 SSM2375 amplifiers. The SSM2375 also

integrates overcurrent and temperature protection.

GAIN SELECTION

In addition, preset gain of SSM2375 can be set from 0dB to

12 dB in 3dB steps with one external resistor (optional). The

external resistor is used to select 9dB and 12dB gain settings, as

shown below in Table 5.

Table 5. Gain Function Descriptions

Gain Setting

GAIN Pin Configuration

12dB

Tie to VDD through 47k Ω

9dB

Tie to GND through 47k Ω

6dB

Tie to VDD

3dB

Open

0dB

Tie to GND

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.

The SSM2375 has a pop-and-click suppression architecture that

reduces these output transients, resulting in noiseless activation and

deactivation from the SD control pin.

EMI NOISE

The SSM2375 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 SSM2375 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 5 shows SSM2375 EMI emission tests per-

formed in a certified FCC Class-B laboratory in normal

emissions mode (EDGE = GND). Figure 6 shows SSM2375

emissions mode.

Figure 5. EMI Emissions from SSM2375, 50 cm Cable,

Figure 6. EMI Emissions from SSM2375, 50cm Cable,

The measurements for Figure 5 and Figure 6 were taken in

an FCC-certified EMI laboratory with a 1 kHz input signal,

producing 0.5 W output power into an 8 Ω load from a 5 V

supply. Cable length was 50 cm, unshielded twisted pair

speaker cable. Note that reducing the supply voltage greatly

reduces radiated emissions.

OUTPUT MODULATION DESCRIPTION

The SSM2375 uses three-level, Σ-Δ output modulation. Each

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, there are always noise sources present.

Due to this 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. However, most of the time, output

differential voltage is 0 V, due to the Analog Devices three-level,