WM8281ECSSLASHR datasheet(306/392 Pages) CIRRUS

306 / 392 page

WM8281

306

Rev 4.0

MICROPHONE BIAS (MICBIAS) CONTROL

There are three MICBIAS generators which provide low noise reference voltages suitable for biasing

electret condenser (ECM) type microphones or powering digital microphones. Refer to the

“Applications Information” section for recommended external components.

The MICBIAS generators are powered from MICVDD, which is generated by an internal Charge Pump

and LDO, as illustrated in Figure 82.

The MICBIAS outputs can be independently enabled using the MICBn_ENA register bits (where n = 1,

2 or 3 for MICBIAS1, 2 or 3 respectively).

When a MICBIAS output is disabled, the output pin can be configured to be floating or to be actively

discharged. This is selected using the MICBn_DISCH register bits.

The MICBIAS generators can each operate as a voltage regulator or in bypass mode. The applicable

mode is selected using the MICBn_BYPASS registers.

In Regulator mode, the output voltage is selected using the MICBn_LVL register bits. In this mode,

MICVDD must be at least 200mV greater than the required MICBIAS output voltages. The MICBIAS

outputs are powered from the MICVDD pin, and use the internal bandgap circuit as a reference.

In Regulator mode, the MICBIAS regulators are designed to operate without external decoupling

capacitors. The regulators can be configured to support a capacitive load if required, using the

MICBn_EXT_CAP register bits. (This may be appropriate for a digital microphone supply.) It is

important that the external capacitance is compatible with the applicable MICBn_EXT_CAP setting.

The compatible load conditions are detailed in the “Electrical Characteristics” section.

In Bypass mode, the output pin (MICBIAS1, MICBIAS2 or MICBIAS3) is connected directly to

MICVDD. This enables a low power operating state. Note that the MICBn_EXT_CAP register settings

are not applicable in Bypass mode; there are no restrictions on the external MICBIAS capacitance in

Bypass mode.

The MICBIAS generators incorporate a pop-free control circuit to ensure smooth transitions when the

MICBIAS outputs are enabled or disabled in Bypass mode; this feature is enabled using the

MICBn_RATE registers.

The MICBIAS generators are illustrated in Figure 82. The MICBIAS control register bits are descrbed

in Table 125.

The maximum output current for each MICBIASn

pin 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. Note that the maximum output current differs between Regulator mode and

Bypass mode.

VOLTAGE REFERENCE CIRCUIT

The WM8281 incorporates a voltage reference circuit, powered by AVDD. This circuit ensures the

accuracy of the LDO Regulator and MICBIAS voltage settings.

WM8281ECS/R Datenblatt(PDF) 306 Page - Cirrus Logic