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.

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 2-1 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.

Figure 2-1 Class-D Audio Amplifier

Figure 2-2 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 2-1) the frequency of the triangular waveform is fixed as shown in Figure 2-2. 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 2-3 shows the effects of the frequency variation on the triangular waveform.

Figure 2-3 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. FFM Modulationshows a comparison of FFM and SSM modulation.

FFM Modulation

SSM Modulation

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.