WM8958 Datasheet, PDF (373/377 Pages) Wolfson Microelectronics plc – Multi-Channel Audio Hub CODEC for Smartphones

Pre-Production
WM8958
PCB LAYOUT CONSIDERATIONS
Poor PCB layout will degrade the performance and be a contributory factor in EMI, ground bounce
and resistive voltage losses. All external components should be placed as close to the WM8958
device as possible, with current loop areas kept as small as possible. Specific factors relating to
Class D loudspeaker connection are detailed below.
CLASS D LOUDSPEAKER CONNECTION
Long, exposed PCB tracks or connection wires will emit EMI. The distance between the WM8958 and
the loudspeaker should therefore be kept as short as possible. Where speakers are connected to the
PCB via a cable form, it is recommended that a shielded twisted pair cable is used. The shield should
be connected to the main system, with care taken to ensure ground loops are avoided.
Further reduction in EMI can be achieved using PCB ground (or VDD) planes and also by using
passive LC components to filter the Class D switching waveform. When passive filtering is used, low
ESR components should be chosen in order to minimise the series resistance between the WM8958
and the speaker, maximising the power efficiency.
LC passive filtering will usually be effective at reducing EMI at frequencies up to around 30MHz. To
reduce emissions at higher frequencies, ferrite beads can also be used. These should be positioned
as close to the device as possible.
These techniques for EMI reduction are illustrated in Figure 98.
SPKP
SPKN
EMI
Long, exposed tracks emit EMI
SPKP
SPKN
Short connection wires will reduce EMI emission
SPKP
SPKN
SPKP LOW ESR
SPKN LOW ESR
SPKP
SPKN
Figure 98 EMI Reduction Techniques
w
Shielding using PCB ground (or VDD)
planes will reduce EMI emission
LC filtering will reduce EMI emission
up to around 30MHz
Ferrite beads will reduce EMI emission
at frequencies above 30MHz.
PP, August 2012, Rev 3.4
373