Circuit board layout guidelines for Class-D amplifiers

1) 100nF power supply bypass capacitors

2) 1µF power supply bypass capacitors

3) Snubber components

4) Electrolytic capacitors

5) Output filter components

• The 100nf and 1µf decoupling capacitors should be as close as possible to the IC. The 100nf capacitor should be less than 2mm from IC. The 100nf capacitor and the traces connecting it to the IC should all be on the same side of the board as the IC to minimize trace length and stray inductance.

• The 100nf capacitor should be an X7R multilayer ceramic chip capacitor (MLC). The 1µf capacitor should be either tantalum or a X7R multilayer ceramic chip capacitor.

• The snubber network should be as close as possible to the IC. Use an X7R ceramic capacitor rated for at least 100V and make sure that the resistor can handle the power dissipation.

• Use star connections for power traces from the electrolytic capacitor to the ICs.

• Power and output traces should be as short and as wide as possible to minimize stray resistance and inductance.

• Route the output signal paths close together to minimize the loop area and keep the filter components as close to the IC as possible.

• Place as much of the circuitry as possible on the top side of the board and try to use the bottom side of the board only for a ground plane. Place signal and power traces on the bottom side of the board only when absolutely necessary and use vias judiciously.

• Keep the low voltage circuitry on the input side of the amplifier away from the power circuitry on the output side of the amplifier.

• Use surface-mount components whenever possible. SMT devices have much lower parasitic inductance (particularly important for bypass capacitor performance).

• class-D amplifier component placement and trace routing needs to be done by hand. Do not use automatic place and route software.

• 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 the output traces also have high voltages and currents so they should be kept far from sensitive signals and components such as clocks and PLLs.

• Use a ground plane.

About the authors:
Simone Ferri is a Strategic Marketing and Application Manager at STMicroelectronics. During his eight years at ST, Simone has focused on Smart Card application first to move then to Audio products. Simone has a PhD in electronics and a MBA at Politecnico of Milan University.

John Widder is a Market Development Manager at STMicroelectronics. During his eight years at ST, John has focused on design and development support for printers and audio products. Before joining ST, John spent 20 years working in printer design and development. John has a BSEE from the University of Portland and a Master's degree in Engineering Management from Washington State University.

Related links:
Understanding output filters for Class-D amplifiers
How Class D audio amplifiers work
Class D Audio Amplifiers: What, Why, and How
Design and analysis of a basic class D amplifier
Designing high-power Class-D audio power amplifiers