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Circuit board layout guidelines for Class-D amplifiers

[The first article in this series explains the purpose of the low-pass filter components in Class-

D amplifiers and how to calculate their values.]

Introduction

PCB design can limit the performance or reduce the reliability of a class-D amplifier if

some basic layout guidelines are not followed. The following describes some good PC

board layout practices for class-D amplifiers. The STA517B (175 watts per channel)

digital power amplifier with two BTL outputs is used as an example, but the basic

concepts are the same for all class-D amplifiers.

Figure 1 - Schematic for a stereo BTL class-D power amplifier

Ground plane

A good ground plane is the key to a good class-D amplifier layout. The bottom side

of the circuit board should be used exclusively as a ground plane if possible; a full

ground plane will provide the best performing and most reliable design. If you must

place signal or power traces on the bottom side of the board, keep them as short as

possible. If necessary, bring the trace back to the top side of the board for a short dis-

tance to break up long traces on the bottom side.

Vias are used to connect components on the top side of the board to the ground planeon the bottom side of the board. However, vias also block current flow on the ground

plane so they should be used judiciously.

The area directly beneath the amplifier should be filled with copper. If the amplifier

has an exposed pad or slug on the bottom of the package, then the IC should be sol-

dered to the ground under the amplifier so that it can act as a heat sink. In this case,

the ground should extend out from underneath the IC on both sides so that it is ex-

posed to free air. The ground under the amplifier should have many vias to conductheat to the bottom of the board so it can be used as a heat sink as well.

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Running signals underneath the amplifier is not advised. Several vias should be

stitched into the ground plane to ensure that the ground points of all components

have a direct, low-impedance path to each other. This is particularly important for the

output filters. All of the filter grounds should have a direct path back to the ground

plane underneath the amplifier.

Power Supply Bypass CapacitorsPower supply bypass capacitors are extremely important for insuring reliability and

preventing noise and crosstalk. The amplifier output stages draw large amounts of

current and switch on and off very quickly. The parasitic inductance between the by-

pass capacitors and the amplifier power supply inputs can generate large spikes

when the output switches, therefore the parasitic inductance must be kept as small as

possible. A 100nf capacitor should be used in parallel with a 1f capacitor on each of

the supply inputs for the amplifier power stage to minimize the impact of resonance

between the stray inductance and the bypass capacitors.

The 100nf capacitors should be located as close as possible to the IC (no more than 2

mm away). They should also be on the same layer as the IC to minimize the total path

length (and stray inductance) as shown in figure 1. The 1f capacitors should be

placed next, as close as possible to the 100nf capacitors.

Figure 2 - Using the bottom side of the board for the 100nf capacitor

ground connection can lengthen the total trace length considerably and

have a negative effect on circuit performance.

Large bulk storage capacitors should also be used to decouple the amplifier from the

power supply. The value of the bulk storage capacitors will depend on the amount of

current required by the amplifier. The bulk storage capacitors should have star con-

nections to the amplifier and the power supply and they should be located as close to

the amplifier as possible (ideally less than 30mm).

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Figure 3 - The 100nf bypass caps should be placed closest to the IC.

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Figure 4 - The 1uf caps should be placed after the 100nf caps. Vias connect

the 1uf caps to the ground plane on the bottom of the board.