WM8990ECS/RV Wolfson Microelectronics, WM8990ECS/RV Datasheet - Page 169

• Manufacturer Part Number: WM8990ECS/RV
• Description: Audio CODECs Stereo CODEC w.Class AB/D speaker driver
• Manufacturer: Wolfson Microelectronics

Specifications of WM8990ECS/RV

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

w
Production Data
SPEAKER SELECTION
For filterless operation, it is important to select a speaker with appropriate internal inductance. The
internal inductance and the speaker's load resistance create a low-pass filter with a cut-off frequency
of:
e.g. for an 8Ω speaker and required cut-off frequency of 20kHz, the speaker should be chosen to
have an inductance of:
8Ω speakers typically have an inductance in the range 20μH to 100μH. Care should be taken to
ensure that the cut-off frequency of the speaker's internal filtering is low enough to prevent speaker
damage. The class D outputs of the WM8990 operate at much higher frequencies than is
recommended for most speakers, and the cut-off frequency of the filter should be low enough to
protect the speaker.
Figure 104 Speaker Equivalent Circuit
PCB LAYOUT CONSIDERATIONS
The efficiency of the speaker drivers is affected by the series resistance between the WM8990 and
the speaker (e.g. inductor ESR) as shown in Figure 105. This resistance should be as low as
possible to maximise efficiency.
Figure 105 Speaker Connection Losses
f
L = R
c
= R
L
L
/ 2πL
/ 2πf
c
= 8Ω / 2π * 20kHz = 64μH
PD, March 2009, Rev 4.0
WM8990
169