WM8994 Datasheet, PDF (348/359 Pages) Wolfson Microelectronics plc – Multi-Channel Audio Hub CODEC for Smartphones

WM8994
Production Data
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
behavior 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 WM8994, 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 recommended power supply decoupling capacitors for WM8994 are listed below in Table 148.
POWER SUPPLY
LDO1VDD, LDO2VDD, DBVDD, AVDD2
DECOUPLING CAPACITOR
0.1F ceramic (see Note)
SPKVDD1/SPKVDD2
4.7F ceramic
AVDD1
4.7F ceramic
DCVDD
1F ceramic
CPVDD
4.7F ceramic
VMIDC
4.7F ceramic
VREFC
1F ceramic
Table 148 Power Supply Decoupling Capacitors
Note: 0.1F is required with 4.7F a guide to the total required power rail capacitance, including that
at the regulator output.
All decoupling capacitors should be placed as close as possible to the WM8994 device. The
connection between AGND, the AVDD1 decoupling capacitor and the main system ground should be
made at a single point as close as possible to the AGND ball of the WM8994.
The VMID capacitor is not, technically, a decoupling capacitor. However, it does serve a similar
purpose in filtering noise on the VMID reference. The connection between AGND, the VMID
decoupling capacitor and the main system ground should be made at a single point as close as
possible to the AGND ball of the WM8994.
Due to the wide tolerance of many types of ceramic capacitors, care must be taken to ensure that the
selected components provide the required capacitance across the required temperature and voltage
ranges in the intended application. For most applications, the use of ceramic capacitors with capacitor
dielectric X5R is recommended.
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PD, April 2012, Rev 4.4
348