3 Countering EMI

To counter EMI, it is essential to include PCB floor planning as part of the circuit design with the electrical engineer, PCB layout engineer, and manufacturing engineer. General PCB guidelines for dealing with EMI include:

• Placement of decoupling capacitors between power and ground where voltage fluctuations are determined to exist. Haphazard placement of caps can exacerbate the EMI problem. References for the use of decoupling capacitors are included at the end of this application report
• Power planes should be backed off from the edges of the PCB
• Avoidance of traces cut within ground or power planes, which can create an unintended aperture
• Adequate termination of all high-frequency clock lines.
• Proper filtering of PCB connectors
• Good PCB design will avoid loop antennas. Loop antennas encompass any route in which both forward and return currents are on a well-defined conducting path.

Optimally, you will stop radiation by suppressing the source of current that is feeding the antenna.

For the audio designer, it is important to consider the following:

• Keep traces from the audio amplifier to the speaker as short as possible. PCB traces and wires act as antennas with significant radiation occurring once the trace length reaches λ/4.
• For filterless Class D systems, the trace and cable length connecting the amplifier’s output to the speakers will likely be the largest source of RF emissions.

The practice of placing ferrite beads in series with the loudspeakers close to the amplifier can be effective. In order to better understand the suppression mechanism of ferrite beads, it is useful to break the ferrite bead down into frequency-dependent resistive and inductive elements - R(f ) and L(f ) - as shown in Figure 3-1.

Regarding EMI suppression purposes, ferrite beads act as resistors, but because R_DC = 0, there is no DC voltage drop. This makes them useful in cases where frequencies of interest are significantly below 1 MHz. Also, as shown in Figure 3-1, it is important to understand that the ferrite bead is effective when considered part of a two-element voltage divider. Both Z₁ and Z₂ are frequency dependent. To achieve the desired function of a low-pass filter, the following relationships should be present: Z₂ >Z₁ at desired frequencies and Z₁>Z₂ at noise frequencies.

Figure 3-1 Resistance and Inductance as a Function of Frequency

The ferrite usually serves as the series element, and the shunt element is a capacitance - either a physical capacitance or a lumped capacitance. The transfer function indicates that Z₁ increases and Z₂ decreases with frequency (1/jωC) and that the system is damped to the extent that resonance effects are significantly diminished. The primary difficulty with periodic square waves inherent in Class D is the concentration of energy at the harmonic intervals.

In the push to create a "quiet" low-EMI class D amplifier, one approach is to dither that frequency back and forth or spread the spectrum of the switching so the energy at any one point in the spectrum is reduced. Compared to traditional Class D amplifiers, the spread-spectrum-modulation scheme has some key advantages. Efficiency and low THD+N are maintained, but radiated noise and EMI can be significantly reduced, as seen in Figure 3-2.

Figure 3-2 Comparison of Fixed-Frequency and Spread-Spectrum Modulation

The LM48511 is a spread-spectrum-modulated class D audio amplifier. It also includes a built-in boost regulator that drives the supply voltage to 7 V, increasing amplifier output power and the audio-sound pressure level compared to an unboosted amplifier. The boost regulator allows the amplifier to maintain a constant output level, even when powered from a decaying voltage source such as a battery.

The LM48511 amplifier features a logic-selectable, spread-spectrum modulator that reduces EMI, eliminating the need for output filters or chokes. As shown in Figure 3-3, the spread-spectrum modulator feeds a standard H-bridge that drives the bridgetied-load speaker. In spread-spectrum mode, the switching frequency varies randomly by 10% around a 330 kHz center frequency, decreasing EMI emissions radiated by the speaker and associated cables and traces. Where a fixed-frequency Class D exhibits large amounts of spectral energy at multiples of the switching frequency, the spread-spectrum architecture of the LM48511 amplifier spreads that energy over a larger bandwidth, reducing peak noise power in the circuit.

Electromagnetic interference is a system-level concern, and it is essential for today’s audio engineer to design with EMI in mind, including the best possible design practices and judicious choice of components and materials.

Figure 3-3 LM48511 - A Spread-Spectrum Modulated Class D Amplifier