WM8948ECSSLASHR datasheet(184/189 Pages) WOLFSON

184 / 189 page

WM8948

Production Data

PD, May 2011, Rev 4.1

184

The recommended power supply decoupling capacitors for WM8948 are listed below in Table 73.

POWER SUPPLY

DECOUPLING CAPACITOR

DCVDD, DBVDD, LDOVDD, SPKVDD

4.7

μF ceramic

LDOVOUT

2.2

μF ceramic

VMIDC

4.7

μF ceramic

Table 73 Power Supply Decoupling Capacitors

All decoupling capacitors should be placed as close as possible to the WM8948 device. The

connection between GND, the LDOVOUT decoupling capacitor and the main system ground should

be made at a single point as close as possible to the GND ball of the WM8948.

The VMIDC capacitor is not, technically, a decoupling capacitor. However, it does serve a similar

purpose in filtering noise on the VMID reference. The connection between GND, the VMID

decoupling capacitor and the main system ground should be made at a single point as close as

possible to the GND ball of the WM8948.

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.

MICROPHONE BIAS CIRCUIT

The WM8948 is designed to interface easily with electret microphones. These may be connected in

single-ended or differential configurations. The single-ended method allows greater capability for the

connection of multiple audio sources simultaneously, whilst the differential method provides better

performance due to its rejection of common-mode noise.

In either configuration, the microphone requires a bias current (electret condenser microphones) or

voltage supply (silicon microphones), which can be provided by MICBIAS. This reference is

generated by an output-compensated amplifier, which requires an external capacitor in order to

guarantee accuracy and stability. The recommended capacitance is 4.7

μF, although it may be

possible to reduce this to 1

μF if the analogue supply (LDOVOUT) is not too noisy. 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.

Note that the MICBIAS voltage may be adjusted using register control to suit the requirements of the

microphone. Also note the WM8948 supports a maximum current of 3mA. If more than one

microphone is connected to the MICBIAS, then combined current must not exceed 3mA.

A current-limiting resistor is also required when using an electret condenser microphone (ECM). The

resistance should be chosen according to the minimum operating impedance of the microphone and

MICBIAS voltage so that the maximum bias current of the WM8948 is not exceeded. Wolfson

recommends a 2.2k

Ω current limiting resistor as it provides compatibility with a wide range of

microphone models.

The recommended connections for single-ended and differential microphone modes are illustrated in

Figure 62 and Figure 63.

WM8948ECS/R Datenblatt(PDF) 184 Page - Wolfson Microelectronics plc

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