WM8994ECSSLASHR datasheet(352/363 Pages) CIRRUS

352 / 363 page

WM8994

352

Rev 4.6

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

DECOUPLING CAPACITOR

LDO1VDD, LDO2VDD, DBVDD, AVDD2

0.1

F ceramic (see Note)

SPKVDD1/SPKVDD2

4.7

F ceramic

AVDD1

4.7

F ceramic

DCVDD

F ceramic

CPVDD

4.7

F ceramic

VMIDC

4.7

F ceramic

VREFC

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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