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Texas Instruments TAS5825M Design Considerations for EMC Application notes
Application Report
SLAA856 – July 2018
TAS5825M Design Considerations for EMC
Andy Liu
ABSTRACT
The TAS5825M Class-D audio power amplifier is the latest TI digital input amplifier that uses advanced
PWM switching techniques for reducing electromagnetic interference (EMI) without degrading audio
performance. This application note describes the system design and printed circuit board (PCB) guidelines
used to maximize the technology employed in this device. These techniques include the EMI suppression
without the need for expensive inductor filters and the reduction of external component count.
1
2
3
4
Contents
General Overview ............................................................................................................
Advanced Emission Suppression .........................................................................................
Printed Circuit Board Design for EMC ....................................................................................
TAS5825M EMI Test Results ..............................................................................................
1
Class-D Audio Amplifier..................................................................................................... 2
2
Fixed-Frequency Mode Modulation
3
Spread Spectrum Mode Modulation....................................................................................... 3
6
Comparison of FFM and SSM Modulation ............................................................................... 4
7
Radiated Emission - Horizontal Pre-Scan - BC Filter ................................................................... 6
8
Radiated Emission - Vertical Pre-Scan - BC Filter ...................................................................... 6
9
Radiated Emission - Horizontal Pre-Scan - LC Filter ................................................................... 7
10
Radiated Emission - Vertical Pre-Scan - LC Filter ...................................................................... 7
11
Conducted Emission – Line Pre-Scan – BC Filter
12
Conducted Emission – Neutral Pre-Scan – BC Filter ................................................................... 8
13
Conducted Emission – Line Pre-Scan – LC Filter ....................................................................... 9
14
Conducted Emission – Neutral Pre-Scan – LC Filter .................................................................. 10
2
2
4
5
List of Figures
.......................................................................................
......................................................................
3
8
List of Tables
1
2
3
4
5
6
7
8
.................................................................. 6
Radiated Emission Margins – Vertical – BC Filter ...................................................................... 6
Radiated Emission Margins – Horizontal – LC Filter ................................................................... 7
Radiated Emission Margins – Vertical – LC Filter ...................................................................... 7
Conducted Emission Margins – Line – BC Filter ........................................................................ 8
Conducted Emission Margins – Neutral – BC Filter .................................................................... 9
Conducted Emission Margins – Line – LC Filter ........................................................................ 9
Conducted Emission Margins – Neutral – LC Filter ................................................................... 10
Radiated Emission Margins – Horizontal – BC Filter
Trademarks
All trademarks are the property of their respective owners.
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1
General Overview
1
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General Overview
The emphasis on green technologies and sleek-looking electronics (such as the flat panel TV) has lead
manufacturers to produce space-efficient and attractive products without sacrificing performance. The
TAS5825M Class-D audio power amplifier provides Class AB audio performance using only the PCB as
heat sink due to its high efficiency. In addition, the TAS5825M device has advanced PWM modulation and
switching schemes that help reduce EMI while eliminating the need for the traditional Class-D output filter.
PWM filtering requires only smaller and less expensive RF filter components. No external heat sink and
less RF filtering result directly in PCB size reduction.
Discussions in the following sections explain the PCB layout practice and external components selection
in order to achieve optimal audio performance and pass electromagnetic compatibility (EMC) specification
EN55022.
• Section 2 describes the advanced emission suppression techniques used to combat EMI.
• Section 3 discusses the PCB design guidelines for audio quality and EMC.
• Section 4 shows the EMC results.
2
Advanced Emission Suppression
2.1
Spread Spectrum Modulation
EMI is electromagnetic radiation emitted by electrical systems with fast changing signals that are common
to the outputs of a Class-D audio power amplifier. EMI encompasses two aspects: emission and
susceptibility. Emission refers to the generation of unwanted electromagnetic energy by the equipment.
Susceptibility, by contrast, refers to the degree in which the equipment is affected by the electromagnetic
disturbances. EMC is achieved by addressing both emission and susceptibility issues. The TAS5825M
device has advanced emission suppression technology which enables the device to run without an LC
filter with speaker wires up to one meter long and still meet the EMI regulatory standards such as
EN55022, CISPR 22, or FCC Part 15 Class B.
The TAS5825M device features an advanced spread spectrum modulation mode with low EMI emission to
lower the overall system cost. This reduced system cost is achieved by replacing large expensive LC
output filters with small, low-cost ferrite beads filters. The spread spectrum modulation scheme exhibits
less EMI by flattening the wideband spectral components from the speaker cables and still retains the
high-efficiency feature of a traditional Class-D amplifier.
Figure 1 shows the topology of a conventional (nonspread-spectrum) BD modulation Class-D amplifier.
The BD switching technique uses an internally generated triangular waveform with a fixed frequency and a
complementary signal pair at the input stage. The output PWM changes the duty cycle to generate a
moving average of the signal that correspond to the input analog signal. The advantages of PWM
switching topology is high efficiency, which provides low power consumption and small thermal design.
+
Output A
±
+
Output B
±
Figure 1. Class-D Audio Amplifier
2
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Advanced Emission Suppression
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VIN ±
VIN +
OUTP
OUTN
Figure 2. Fixed-Frequency Mode Modulation
The TAS5825M device features two modulation modes: fixed-frequency modulation mode (FFM) and
spread spectrum modulation (SSM) mode. In the conventional FFM mode (Figure 1) the frequency of the
triangular waveform is fixed as shown in Figure 2. In SSM mode, the frequency of the triangular waveform
frequency varies cycle-to-cycle with a center frequency at switching frequency configured. SSM mode
improves EMI emissions radiated by the speaker wires by spreading the energy over a larger bandwidth
and reducing the wideband spectral content. On the other hand, FFM produces larger amounts of spectral
energy at multiples of the PWM switching frequency. The cycle-to-cycle variation of the switching
frequency does not affect the efficiency of the audio amplifier. Figure 3 shows the effects of the frequency
variation on the triangular waveform.
VIN ±
VIN +
OUTP
OUTN
Figure 3. Spread Spectrum Mode Modulation
Compared to traditional FFM Class-D amplifier, the spread spectrum scheme has reduced the peak
energy of the switching frequency and lessens harmonics. Figure 6 shows a comparison of FFM and SSM
modulation.
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Advanced Emission Suppression
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Figure 4. FFM Modulation
Figure 5. SSM Modulation
Figure 6. Comparison of FFM and SSM Modulation
2.2
Dephase and Multi-Device Phase Synchronization
In addition to the spread spectrum technology, the TAS5825M device employs dephase circuits to further
reduce electromagnetic emission without degrading audio performance.
The dephase circuit improves EMI and noise performance by interleaving the switching timing between the
two audio channels. This improved EMI and noise performance reduces conducted emission on the PVDD
line because the output ripple current of the two audio channels will be out of phase, and the ripple peak
current from the PVDD line will thus be reduced to half value.
Also, TAS5825M supports 45° phase shifts between multiple channels to reduce EMI in multi-device
systems. See more details in the TAS5825M datasheet.
3
Printed Circuit Board Design for EMC
3.1
Printed Circuit Board Layout
It is necessary to follow recommended PCB guidelines for EMC success. Proper PCB floor planning,
component selection, component placement, and routing are all essential to counter EMI. Emissions are
exacerbated by improper layout, components, and output trace length causing antenna effect. Practical
PCB design guidelines for achieving EMC include:
• Place the high-frequency decoupling capacitors as close to the power pin and ground pin of the device
as possible to reduce the parasitic inductance of the trace. To ensure low AC impedance over a wide
frequency range for noise reduction, use good quality, low-ESR, 1-nF ceramic capacitors. For midfrequency noise due to PWM transients, use another good quality 0.1 µF ceramic capacitor placed as
close as possible to the PVDD leads.
• Use a continuous ground plane and avoid voltage offset on the ground planes whenever possible.
• Low impedance routing back to source (return signal).
• Power planes should be away from the edges of the PCB.
• Proper filtering of the PCB connectors.
• Place EMC snubbers and ferrite bead filters as close as possible to the IC. Minimize unfiltered loops
and trace length as well as stray inductance.
• Keep amplifier output traces to the speaker as short as possible. PCB traces and the speaker wire are
the largest sources of emission.
4
TAS5825M Design Considerations for EMC
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3.2
Ferrite Bead Filter
Low-cost ferrite bead filters are used to suppress EMI. They are placed close to the amplifier output to
minimize loop antennas. At low frequencies, ferrite beads act as 0 Ω resistors with no DC drop. However,
the impedance of ferrite bead increases significantly at frequencies above 1 MHz to suppress radiation.
Ferrite beads also play a significant role on the THD+N of the system. Examples of ferrite beads which
have been tested and worked well with the TAS5825M device include the NFZ2MSM series from Murata.
The trade-off of ferrite beads is their impedance and rated current. If the rated current can meet the
system requirements, larger impedance means larger EMI margins for the EMI, especially for the 5 MHz
to 50 MHz frequency range. 300 ohm @ 100 MHz is a typical value, which can pass EMI in most
applications.
The trade-off of capacitors is their capacitance and idle current. Larger capacitance leads to larger idle
current. Using 2.2 nF instead of 1 nF capacitors is helpful in the 5 MHz to 100 MHz range.
3.3
Power Supply and Speaker Wires
When performing the conducted emission test, it is essential to keep the AC power cable away from the
speaker cables. This prevents stray signals from coupling to power source and other potential unintended
radiators or conductors.
When ferrite bead filters are used, an AC to DC adapter with an EMI filter is required to pass conducted
EMI. Most applications (for example TVs, Voice Controlled Speakers, Wireless Speakers and Sound Bars)
that need a 110 V / 220 V AC power supply usually have already included an EMI filter in the AC to DC
adapter. Some applications use DC power supplies. They need a simple EMI filter on the PVDD supply.
Refer to this application note: AN-2162 Simple Success With Conducted EMI From DC to DC Converters
(SNVA489C).
3.4
TAS5825M Device Configurations
If ferrite bead filters are chosen as the output filters, we recommend to choose BD modulation, set the
switching frequency to 384kHz and also have Spread Spectrum enabled. Refer to the TAS5825M
datasheet for the recommended Spread Spectrum configurations.
4
TAS5825M EMI Test Results
The following sections show the EMI test results from a certified third-party vendor. The radiated and
conducted EMI plots below are taken using a TAS58x5M EMI Test Board, which is a 2-layer board and
has been designed specifically for EMI testing. In addition, a 12 V TV power supply is used to power the
board in the conducted emission testing.
4.1
EN55022 Radiated Emission Results
TAS58x5M EMI Test Board, ferrite bead filter, BD modulation, PVDD = 12 V, 8 Ω + 56 uH load, 105 cm
speaker cables, Spread Spectrum enabled, Fsw = 384 kHz, Po = 4 W per channel.
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TAS5825M EMI Test Results
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Figure 7. Radiated Emission - Horizontal Pre-Scan - BC Filter
Table 1. Radiated Emission Margins – Horizontal – BC Filter
NO.
Frequency
[MHz]
Reading
[dBµV/m]
Factor
[dB]
Limit
[dBµV/m]
Margin
[dB]
Height
[cm]
Level
[dBµV/m]
Angle
[°]
1
174.530
41.71
-15.10
40.00
13.39
200
26.61
259
2
222.545
47.31
-16.76
40.00
9.45
100
30.55
243
3
287.050
48.19
-14.52
47.00
13.33
100
33.67
313
Figure 8. Radiated Emission - Vertical Pre-Scan - BC Filter
Table 2. Radiated Emission Margins – Vertical – BC Filter
6
NO.
Frequency
[MHz]
Reading
[dBµV/m]
Factor
[dB]
Limit
[dBµV/m]
Margin
[dB]
Height
[cm]
Level
[dBµV/m]
Angle
[°]
1
30.000
51.99
-16.04
40.00
4.05
100
35.95
252
2
33.880
50.44
-16.05
40.00
5.61
100
34.39
61
3
44.550
44.66
-15.73
40.00
11.07
100
28.93
20
4
49.885
41.32
-15.36
40.00
14.04
100
25.96
233
5
179.380
37.47
-15.52
40.00
18.05
100
21.95
142
6
282.685
40.74
-14.68
47.00
20.94
200
26.06
356
TAS5825M Design Considerations for EMC
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TAS58x5M EMI Test Board, inductor filter, BD modulation, PVDD = 12 V, 8 Ω + 56 uH load, 105 cm
speaker cables, Spread Spectrum disabled, Fsw = 768 kHz, Po = 4 W per channel.
Figure 9. Radiated Emission - Horizontal Pre-Scan - LC Filter
Table 3. Radiated Emission Margins – Horizontal – LC Filter
Frequency
[MHz]
Reading
[dBµV/m]
Factor
[dB]
Limit
[dBµV/m]
Margin
[dB]
Height
[cm]
Level
[dBµV/m]
Angle
[°]
1
67.345
39.94
-16.77
40.00
16.83
200
23.17
352
2
101.295
41.42
-18.36
40.00
16.94
200
23.06
247
3
174.045
40.71
-15.06
40.00
14.35
200
25.65
45
4
225.455
46.07
-16.70
40.00
10.63
100
29.37
254
5
288.990
49.26
-14.46
47.00
12.20
100
34.8
223
NO.
Figure 10. Radiated Emission - Vertical Pre-Scan - LC Filter
Table 4. Radiated Emission Margins – Vertical – LC Filter
NO.
Frequency
[MHz]
Reading
[dBµV/m]
Factor
[dB]
Limit
[dBµV/m]
Margin
[dB]
Height
[cm]
Level
[dBµV/m]
Angle
[°]
1
30.970
42.84
-16.05
40.00
13.21
100
26.79
278
2
37.275
36.85
-16.04
40.00
19.19
100
20.81
281
3
67.345
39.20
-16.77
40.00
17.57
200
22.43
204
4
181.320
37.34
-15.70
40.00
18.36
100
21.64
148
5
294.810
43.37
-14.25
47.00
17.88
100
29.12
278
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TAS5825M EMI Test Results
4.2
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EN55022 Conducted Emission Results
TAS58x5M EMI Test Board, ferrite bead filter, BD modulation, PVDD = 12 V, 8 Ω + 56 uH load, Spread
Spectrum enabled, Fsw = 384 kHz, Po = 4 W per channel.
Figure 11. Conducted Emission – Line Pre-Scan – BC Filter
Table 5. Conducted Emission Margins – Line – BC Filter
NO.
Frequency
[MHz]
Corr. Factor
[dB]
QP Limit
[dBµV]
AV Limit
[dBµV]
AV Reading
[dB]
AV Margin
[dB]
QP Reading
[dBµV]
QP Margin
[dB]
1
0.16564
9.60
65.18
55.18
23.85
-21.73
40.34
-15.24
2
0.43152
9.60
57.22
47.22
27.66
-9.96
34.40
-13.22
3
1.19550
9.60
56.00
46.00
22.95
-13.45
29.89
-16.51
+4
4.82398
9.63
56.00
46.00
27.57
-8.80
34.06
-12.31
Figure 12. Conducted Emission – Neutral Pre-Scan – BC Filter
8
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Table 6. Conducted Emission Margins – Neutral – BC Filter
NO.
Frequency
[MHz]
Corr. Factor
[dB]
QP Limit
[dBµV]
AV Limit
[dBµV]
AV Reading
[dB]
AV Margin
[dB]
QP Reading
[dBµV]
QP Margin
[dB]
1
0.15000
9.60
66.00
56.00
23.32
-23.08
42.96
-13.44
2
0.41979
9.60
57.45
47.45
25.92
-11.93
32.88
-14.97
+3
4.82398
9.63
56.00
46.00
26.94
-9.43
33.55
-12.82
TAS58x5M EMI Test Board, inductor filter, PVDD = 12 V, BD modulation, 8 Ω + 56 uH load, Spread
Spectrum disabled, Fsw = 768 kHz, Po = 4 W per channel.
Figure 13. Conducted Emission – Line Pre-Scan – LC Filter
Table 7. Conducted Emission Margins – Line – LC Filter
Frequency
[MHz]
Corr. Factor
[dB]
QP Limit
[dBµV]
AV Limit
[dBµV]
AV Reading
[dB]
AV Margin
[dB]
QP Reading
[dBµV]
QP Margin
[dB]
+1
0.70522
10.40
56.00
46.00
19.28
-16.32
21.29
-24.31
2
19.02819
10.80
60.00
50.00
19.77
-19.43
26.74
-22.46
NO.
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Figure 14. Conducted Emission – Neutral Pre-Scan – LC Filter
Table 8. Conducted Emission Margins – Neutral – LC Filter
Frequency
[MHz]
Corr. Factor
[dB]
QP Limit
[dBµV]
AV Limit
[ dBµV]
AV Reading
[dB]
AV Margin
[dB]
QP Reading
[dBµV]
QP Margin
[dB]
+1
0.70522
10.40
56.00
46.00
19.68
-15.92
21.69
-23.91
2
19.03210
10.90
60.00
50.00
19.44
-19.66
26.42
-22.68
NO.
4.3
Conclusions
The TAS5825M device has the proven advanced RF emission suppression technology that helps to
design an EMI-compliant audio system without compromising cost and performance. By adhering to the
guidelines discussed in this report, EMI requirements are met and costly PCB rework is avoided.
Please note that we usually recommend inductor filters so that customers can make good use of
TAS5825M features and capabilities.
For further questions and discussions on this topic, go to the TI E2E Forums (http://e2e.ti.com/).
10
TAS5825M Design Considerations for EMC
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