wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 205

• Manufacturer Part Number: wm8400
• Description: Wolfson Audioplustm Hi-fi Audio Codec And Power Management Unit For Mobile Multimedia
• Manufacturer: Wolfson Microelectronics plc

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Pre-Production
PCB LAYOUT CONSIDERATIONS
Wolfson recommends the following external components for use with LDO Regulators 1, 2, 3 and 4.
Note that larger capacitors will improve load transient response and power supply rejection. A
maximum of 10µF is possible at the output; a maximum of 1µF is possible at the input.
Table 109 Recommended External Components - LDO1, LDO2, LDO3 and LDO4
CDELAY COMPONENT SELECTION
A capacitor between CDELAY and GND determines the Reset Timer duration. This timer sets the
delay between the RSTTRIG input being set high and the NRST output being set high.
The minimum capacitance 10nF results in a Reset Timer duration of 240µs.
The maximum capacitance 10µF results in a Reset Timer duration of 240ms.
When selecting a suitable CDELAY capacitor, this should be compatible with the typical charging
current of 50µA.
The ground connection for the capacitor should be returned directly to the main system ground
(GND).
RIREF COMPONENT SELECTION
A 100kΩ resistor is required between RIREF and AGND. The value of this component impacts on
the device power consumption and also on the internal oscillator operation and CDELAY time. It is
recommended that only a 100kΩ resistor is used.
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 WM8400
device as possible, with current loop areas kept as small as possible. Specific factors relating to
Class D loudspeaker connection and the Power Management (DC-DC Converters and LDO
Regulators) are detailed below.
CLASS D LOUDSPEAKER CONNECTION
Long, exposed PCB tracks or connection wires will emit EMI. The distance between the WM8400
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, with a
minimum of 90% shield coverage. 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 WM8400
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 108.
COMPONENT
C
C
OUT
IN
VALUE
4.7µF
1µF
Murata GRM155R60J105KE19D
Murata GRM188R60J47KE19D
PART NUMBER
PP, April 2009, Rev 3.0
WM8400
0603
0402
SIZE
205