adau1442 Analog Devices, Inc., adau1442 Datasheet - Page 23

• Manufacturer Part Number: adau1442
• Description: Sigmadsp Digital Audio Processor With Flexible Audio Routing Matrix
• Manufacturer: Analog Devices, Inc.

VOLTAGE REGULATOR
The ADAU144x digital supply voltage must be set to 1.8 V. The
chip includes an on-board voltage regulator that allows the device
to be used in systems where a 1.8 V supply is not available but a
3.3 V supply is. The only external components needed for this
are a PNP transistor and one resistor. Only one pin, VDRIVE, is
necessary to support the regulator.
The recommended design for the voltage regulator is shown
in Figure 12. The 10 μF and 100 nF capacitors shown in this
schematic are recommended for bypassing but are not necessary
for operation. Each DVDD pin should have its own 100 nF
bypass capacitor, but only one bulk capacitor (10 μF) is needed
for all pins. In this design, 3.3 V is the main system voltage; 1.8 V
is generated at the collector of the transistor, which is connected
to the DVDD pins. VDRIVE is connected to the base of the PNP
transistor. If the regulator is not used in the design, VDRIVE
can be tied to ground.
Figure 12. Voltage Regulator Design
100nF
10µF
+
DVDD
VDRIVE
ADAU144x
3.3V
1kΩ
Rev. PrA| Page 23 of 92
Two specifications must be considered when choosing a
regulator transistor. First, the transistor’s current amplification
factor (hFE or beta) should be at least 200 (TBD). Second, the
collector of the transistor must be able to dissipate the heat
generated when regulating from 3.3 V to 1.8 V. The maximum
digital current draw of the ADAU1442 and ADAU1445, which
use ASRCs, is 445 (TBD) mA. The equation to determine the
transistor’s minimum power dissipation specifications is as
follows:
Many transistors fit these specifications. Analog Devices
recommends the NJT4030P from On Semiconductor. For
projects with stringent size constraints, an FMMT734 from
Zetex can be used.
The ADAU1446, which does not contain ASRCs, has a lower
maximum current draw of approximately 220 (TBD) mA. The
maximum power dissipation of the transistor in this case should
be around 330 (TBD) mW.
(3.3 V − 1.8 V) × 445 (TBD) mA = 667.5 (TBD) mW