Audible Noise Suppression - Renesas P9235A-RB Layout Manual

P9235A-RB Layout Guide

4. Audible Noise Suppression

Wireless power receiver solutions have been observed to produce audible noise. If sound is detected, there are several steps that can be
taken to reduce or eliminate the noise. The first priority is identifying the source (i.e., the rectifier capacitors, the Tx coil ferrite, communication
capacitors). Typically, the rectifier capacitors are the components that generate the audible noise. The reason the noise is present and
associated with the rectifier capacitors is due to the WPC communication signals being generated in the audible frequency range and the use
of small-form factor ceramic capacitors. The noise occurs due to the piezoelectric effect of ceramic capacitors. The capacitors constrict and
expand while providing the communication pulses, and this noise is amplified as it flexes the PCB.
The primary solution to this issue is to use low-acoustic noise capacitors. Alternatively, higher voltage rated components can have superior
piezoelectric properties that can reduce the audible noise. Additionally, placing the capacitors on both sides of the PCB (directly above and
below each other) counters the piezoelectric forces applied to the PCB (cancels the force by each capacitor). Another method is to add slots
through the PCB on both outer sides of the capacitors or directly under each capacitor. One additional approach is to place additional lower
capacitance value components in parallel to reduce the mechanical force of the piezoelectric effect per component.
For any additional questions, contact IDT technical support (see last page for contact information.
© 2019 Integrated Device Technology, Inc 15 March 19, 2019