wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 200

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

WM8400
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POWER SUPPLY DECOUPLING
Electrical coupling exists particularly in digital logic systems where switching in one sub-system
causes fluctuations on the power supply. This effect occurs because the inductance of the power
supply acts in opposition to the changes in current flow that are caused by the logic switching. The
resultant variations (or ‘spikes’) in the power supply voltage can cause malfunctions and
unintentional behaviour in other components. A decoupling (or ‘bypass’) capacitor can be used as an
energy storage component which will provide power to the decoupled circuit for the duration of these
power supply variations, protecting it from malfunctions that could otherwise arise.
Coupling also occurs in a lower frequency form when ripple is present on the power supply rail
caused by changes in the load current or by limitations of the power supply regulation method. In
audio components such as the WM8400, these variations can alter the performance of the signal
path, leading to degradation in signal quality. A decoupling (or ‘bypass’) capacitor can be used to
filter these effects, by presenting the ripple voltage with a low impedance path that does not affect
the circuit to be decoupled.
These coupling effects are addressed by placing a capacitor between the supply rail and the
corresponding ground reference. In the case of systems comprising multiple power supply rails,
decoupling should be provided on each rail. The best protection is achieved by placing the capacitor
as close as possible to the circuit to be decoupled. This is because lengthy interconnect tracks can
result in inductance which will reduce the effectiveness of the capacitor. The decoupling capacitors
should therefore be placed as close as possible to the WM8400.
For the purposes of decoupling from high frequency effects on the power supply rails, the size of the
decoupling/bypass capacitor is dependent upon the frequencies at which decoupling is required. The
higher the frequency of the switching effects, the smaller capacitance that is required to shunt them.
In the case of lower frequencies or for higher current loads, a larger capacitance will be required for
adequate decoupling.
The recommended power supply decoupling capacitors are listed below in Table 106.
Table 106 Power Supply Decoupling Capacitors
A small capacitance is recommended for many of the supplies listed above, as appropriate for the
high frequency switching of the ADCs, DACs and digital core.
A larger capacitance is recommended for AVDD and HPVDD decoupling. In many applications, a
smaller capacitor (eg. 1µF) will be sufficient for the AVDD connection, as the low power design of the
WM8400 results in very low AVDD current. The recommended 4.7µF capacitance will provide a
greater level of immunity to electrical noise on the AVDD and HPVDD power rails.
As noted above, all decoupling capacitors should be as close as possible to the WM8400 device.
The connection between AGND, the AVDD decoupling capacitor and the main system ground should
be made at a single point as close as possible to the AGND ball of the WM8400. Similarly, the
connection between HPGND, the HPVDD decoupling capacitor and the main system ground should
be a single point that is as close as possible to the HPGND ball of the WM8400.
The VMID capacitor is not, technically, a decoupling capacitor. However, it does serve a similar
purpose in filtering noise on the VMID bias voltage to which the microphone and line inputs are
referenced. Using a smaller capacitance is not recommended here as it may lead to an increased
noise level throughout the analogue signal chain.
Similarly, the VINT capacitor is not, technically, a decoupling capacitor. VINT is the output of an
internal linear regulator, therefore the VINT capacitor both acts as a filter for noise and also as the
energy storage device for the regulator.
FLLVDD, DCVDD, I2CVDD, I2S1VDD,
I2S2VDD, MBVDD, HBVDD
AVDD, HPVDD
VMID
VINT
POWER SUPPLY
0.1µF ceramic
4.7µF ceramic
4.7µF ceramic
1uF ceramic
DECOUPLING
CAPACITOR
PP, April 2009, Rev 3.0
Pre-Production
200