ADAU1977WBCPZ-R7 datasheet(16/64 Pages) AD

16 / 64 page

ADAU1977

Data Sheet

Rev. A | Page 16 of 64

DC-TO-DC BOOST CONVERTER

The boost converter generates a supply voltage for the

microphone bias circuit from a fixed 3.3 V supply. The boost

converter output voltage is programmable using Register 0x03.

The boost converter output voltage is approximately 1 V above

the set microphone bias voltage. The boost converter uses the

clock from the PLL, and the switching frequency is dependent

on the sample rate of the ADC. The FS_RATE bits (Bits[6:5] of

converter switching frequency can be selected to be 1.5 MHz or

3 MHz using Bit 4 of Register 0x02. For the 1.5 MHz switching

frequency, the recommended value for the inductor is 4.7 µH,

whereas for the 3 MHz switching frequency, the recommended

value for the inductor is 2.2 µH.

Table 12 lists the typical switching frequency based on the

sample rates.

Inductor Selection

For the boost converter to operate efficiently, the inductor selection

is critical. The two most important parameters for the inductor

are the saturation current rating and the dc resistance. The recom-

mended saturation rating for the inductor must be >1 A. The dc

resistance affects the efficiency of the boost converter. Assuming

that the board trace resistances are negligible for 80% efficiency,

the dc resistance of the inductor should be less than 50 mΩ.

Table 11 lists some of the recommended inductors for the

application.

Table 11. Recommended Inductors1

Value

Manufacturer

Manufacturer Part Number

2.2 µH

Würth Elektronik

7440430022

4.7 µH

Würth Elektronik

7440530047

Check with the manufacturer for the appropriate temperature ratings for a

given application.

The boost converter has a soft start feature that prevents inrush

current from the input source.

The boost converter has built-in overcurrent and overtemperature

protection. The input current to the boost converter is monitored

and if it exceeds the set current threshold for 1.2 ms, the boost

converter shuts down. The fault condition is recorded into

tion is cleared after reading the BOOST_OV bit (Bit 2) or

the BOOST_OC bit (Bit 0) in Register 0x02. The overcurrent

protection bit, OC_EN (Bit 1), or the overvoltage protection bit,

OV_EN (Bit 3), is on by default, and it is recommended not to

disable the bit.

Each protection circuit has two modes for recovery after a fault

event: autorecovery and manual recovery. The recovery mode

can be selected using Bit 0 of Register 0x03. The autorecovery

mode attempts to enable the boost converter after a set recovery

time, typically 20 ms. The manual recovery mode enables the boost

converter only if the user writes 1 to the MRCV bit (Bit 1). If the

fault persists, the boost converter remains in shutdown mode

until the fault is cleared.

The boost converter is capable of supplying the 42 mA of total

output current at the MICBIAS output. The boost converter has

overcurrent protection at the input; the threshold is around

900 mA peak. Ensure that the 3.3 V power supply feeding the

boost converter has built-in overcurrent protection because there is

no protection internal to ADAU1977 for a short circuit to any of

the ground pins (AGND/DGND/PGND) at the VBOOST_OUT

or VBOOST_IN pin.

By default, the boost converter is disabled on power-up to allow

the flexibility of connecting an external voltage source at the

VBOOST_IN pin to power the microphone bias circuit. The boost

converter can be enabled by using the BOOST_EN bit (Bit 2 of

Capacitor Selection

The boost converter output is available at the VBOOST_OUT pin

(Pin 25) and must be decoupled to PGND using a 10 µF ceramic

capacitor to remove the ripple at the switching frequency. The

capacitor must have low ESR and good temperature stability.

The MLCC X7R/NPO dielectric type with 25 V is recommended.

Care must be taken to place this capacitor as close as possible to

the VBOOST_OUT pin (Pin 25).

Table 12. Typical Switching Frequency Based on the Sample Rates

Boost Converter Switching Frequency

Base Sample Rate (kHz)

Sample Rates (kHz)

Inductor = 2.2 µH

Inductor = 4.7 µH

8/16/32/64

(1024/12) × fS

(1024/22) × fS

44.1

11.025/22.05/44.1/88.2/176.4

(1024/16) × fS

(1024/30) × fS

12/24/48/96/192

(1024/16) × fS

(1024/32) × fS

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