SSM2375 Datasheet, PDF (10/13 Pages) Analog Devices – Filterless High Efficiency Mono 3 W Class-D Audio Amplifier

SSM2375
Preliminary Technical Data
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.
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
EMI emission with EDGE = VDD, placing the device in low
emissions mode.
Figure 5. EMI Emissions from SSM2375, 50 cm Cable,
5V VDD, EDGE = GND
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
Figure 6. EMI Emissions from SSM2375, 50cm Cable,
5V VDD, EDGE = VDD
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
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, 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,
Rev. PrC | Page 10 of 13