Practical Design Tips to Avoid PCB Design Limitations on Class D Amplifier Performance

If some basic layout guidelines are not followed, PCB design can limit the performance or reduce the reliability of a Class D amplifier. Some good PC board layout practices for Class D amplifiers are described below. 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 diagram of a stereo BTL Class D power amplifier

Ground Plane

A good ground plane is the key to a quality Class D amplifier layout. If possible, the bottom layer of the board should be used as a dedicated ground plane. A complete ground plane can provide the best performance and the most reliable design. If you have to route signal or power traces on the bottom layer of the board, make them as short as possible. If necessary, in order to make the bottom layer traces short, the traces should be led back to the top layer of the board to avoid long traces on the bottom layer.

Vias are used to connect the top-layer components of the board to the ground plane on the bottom layer of the board. However, vias still block the current from flowing back to the ground plane, so these vias must be used flexibly.

The area directly under the amplifier must be copper-plated. If the amplifier has an exposed pad or plug-in on the bottom of its package, the IC must be soldered to the ground under the amplifier, which can serve as a heat dissipation area for the amplifier. In this case, the ground must be led out from directly under the IC to both sides to ensure that it is exposed. The ground under the amplifier must be punched with many vias to dissipate heat to the bottom layer of the circuit board through the vias, so it can also serve as a heat dissipation area.

It is not recommended to run signal lines directly under the amplifier. Several vias must be connected to the ground plane to ensure that the ground reference points of all devices have a direct and low-impedance path between each other. This is especially important for the output filter. All filter grounds must have a direct path back to the ground plane directly under the amplifier.

Power supply bypass capacitors

To ensure stability and suppress noise and crosstalk, it is very important to add bypass capacitors to the power supply. The output stage of the amplifier absorbs a lot of current and switches quickly. When the output switches, the parasitic inductance between the bypass capacitor and the amplifier power input pin will produce large burrs, so the parasitic inductance must be kept as small as possible. In order to reduce the resonance between the stray inductance and the bypass capacitor at the amplifier power stage, a 100nF capacitor must be used in parallel with a 1uF capacitor at each power input pin.

The 100nF capacitor must be placed as close to the IC as possible (usually no more than 2mm). Also, as shown in Figure 1, the bypass capacitor must be on the same layer as the IC to minimize the total path length (and stray inductance). The 1uF capacitor must be placed in sequence, close to the 100nF capacitor.

Figure 2: Using a 100nF capacitor on the bottom layer of the board for ground connection will significantly increase the total trace length and have a negative impact on board performance.

It is also necessary to use a large volume energy storage capacitor to decouple the power input of the amplifier. The capacitance of the large volume energy storage capacitor depends on the amount of current required by the amplifier. The large volume energy storage capacitor must be connected to the amplifier and the power pin in a star shape and must be as close to the amplifier as possible (ideally less than 30 mm).

Figure 3: The 100nF bypass capacitor must be placed close to the IC.

Figure 4: The 1uF capacitor should be placed after the 100nF capacitor and connected to the ground plane on the bottom of the circuit board through vias.