WM8281ECSSLASHR datasheet(379/392 Pages) CIRRUS

379 / 392 page

WM8281

Rev 4.0

379

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 (‘spikes’) in the power supply voltage can cause malfunctions and unintentional

behaviou

r in other components. A decoupling (‘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 WM8281, these variations can alter the performance of the signal path,

leading to degradation in signal quality. A decoupling 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 WM8281 are detailed below in Table 132.

POWER SUPPLY

DECOUPLING CAPACITOR

LDOVDD, DBVDD1, DBVDD2, DBVDD3

0.1

F ceramic (see Note)

AVDD1, AVDD2

1.0

F ceramic

CPVDD

4.7

F ceramic

MICVDD

4.7

F ceramic

DCVDD

2 x 2.2

F ceramic - one close to each DCVDD pin.

Alternatively, a single 4.7

F ceramic.

If DCVDD is supplied externally (not from LDO1), then

smaller capacitors, eg. 2 x 1

F may be sufficient.

SPKVDDL, SPKVDDR

4.7

F ceramic

Table 132 Power Supply Decoupling Capacitors

Note: 0.1

F is required with 4.7F a guide to the total required power rail capacitance.

All decoupling capacitors should be placed as close as possible to the WM8281 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 balls of the WM8281.

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 application the use of ceramic capacitors with capacitor

dielectric X5R is recommended.

VOLTAGE REFERENCE EXTERNAL COMPONENTS

An external resistor and capacitor must be connected between VREFC and GND, as illustrated in

Figure 92. These components are necessary as part of the Voltage Reference circuit, and should be

positioned as close as possible to the WM8281.

A 220

 (1%) resistor is required for this function. Note that a low-tolerance component must be used.

The 2.2µF X5R ceramic capacitor is recommended.

WM8281

VREFC

2.2

F

220

 (1%)

Figure 92 Voltage Reference External Components

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