Circuit board layout guidelines for Class-D amplifiers

Figure 5 - The snubber components should be placed next. This figure shows the layout for a differential snubber, where the snubber components are connected directly between the two outputs of a BTL amplifier without any connection to ground.

Figure 6 - This figure shows the layout for a common-mode snubber, where snubbers are connected from each output to ground. Common-mode snubbers generally provide better performance than differential snubbers.

Power Supply Electrolytic Capacitor
The electrolytic capacitor and the filter inductors should be placed after the snubbers. The electrolytic capacitor should be as close as possible to all of the high voltage power inputs to the IC.

Figure 7 - Electrolytic capacitor placement

The connections between the electrolytic capacitor and the amplifier should be "starred" to minimize the impact of voltage drops caused by one amplifier on the other amplifiers connected to the same capacitor. Daisy-chaining power between multiple amplifiers can cause noise, crosstalk and reliability problems, even with wide traces.

If there is more than one class-D amplifier IC, each IC should have its own electrolytic capacitor. If there is more than one electrolytic capacitor then the power connections from the capacitors to the power connector should also be starred.

In general, traces that handle high current, such as VCC and the output signal path, should be as wide and as short as possible to minimize their resistance and inductance. VCC and output traces have high voltages and currents so they should be kept far from sensitive signals and components such as clocks and PLLs.

Output Filters
The output filter components should be placed next. The path from the amplifier output to the inductor and from the inductor to the film capacitor carry high currents with a lot of high-frequency content, so it should be as wide and short as possible to minimize stray resistance and inductance.

The inductors should be as close to the amplifier as possible, while maintaining some distance between adjacent inductors. If unshielded inductors are used they should be at least 7.5 mm away from each other to prevent magnetic interference, especially between the inductors of different channels.

Figure 8 - Placement of the output filter components and the high frequency current path

The location of the output filter components and traces is important for minimizing EMI. The loop area between the traces for the low pass filter should be as small as possible. For amplifiers with single-ended outputs, the return path to the amplifier is ground so the loop area should be small - as long as there is a good ground plane on the board.

For amplifiers with BTL outputs, the filter loop area is the area between the traces connecting the IC, the filter inductors, and the film capacitor (see figure 5). The traces in a BTL output filter should run parallel to each other with as little space between them as possible in order to minimize the loop area. However, output traces for separate channels should not run next to each other.

The low-pass filter capacitor and the common-mode filter components should be as close to the inductors as possible. The DC blocking capacitor for amplifiers with single-ended outputs should be placed next. The output connector to the speaker should be as close as possible to the filters.

Figure 9 - Sample output layout for a four channel single-ended amplifier