WM8994ECSSLASHR datasheet(354/363 Pages) CIRRUS

354 / 363 page

WM8994

354

Rev 4.6

Digital Mic

DMICDAT1

4.7uF

MICBIAS1

AGND

Digital Mic

DMICCLK

VDD

CLK

DATA

VDD

CLK

DATA

CHAN

VDD

Digital

Microphone

Interface

Figure 90 Digital Microphone Connection

The MICBIAS generators are identical output-compensated amplifiers, which require an external capacitor in

order to guarantee accuracy and stability. The recommended capacitance is 4.7

F. A ceramic type is a

suitable choice here, providing that care is taken to choose a component that exhibits this capacitance at the

intended MICBIAS voltage.

The maximum output current is noted in the “Electrical Characteristics”. This limit must be observed

on each MICBIAS output, especially if more than one microphone is connected to a single MICBIAS

pin. The MICBIAS output voltage can be adjusted using register control to suit the requirements of the

microphone.

CLASS D SPEAKER CONNECTIONS

The WM8994 incorporates two Class D/AB 1W speaker drivers. By default, the speaker drivers

operate in Class D mode, which offers high amplifier efficiency at large signal levels. As the Class D

output is a pulse width modulated signal, the choice of speakers and tracking of signals is critical for

ensuring good performance and reducing EMI in this mode.

The efficiency of the speaker drivers is affected by the series resistance between the WM8994 and

the speaker (e.g. PCB track loss and inductor ESR) as shown in Figure 91. This resistance should be

as low as possible to maximise efficiency.

GND

Losses due to resistance between WM8994 and speaker (e.g. inductor ESR)

This resistance must be minimised in order to maximise efficiency.

Switching

Losses

Class D

output

SPKVDD

SPKVDD/2

Figure 91 Speaker Connection Losses

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