AN-1213: Powering the AD9788 800 MSPS TxDAC Digital-to-Analog Converter Using the ADP2105 Synchronous Step-Down DC-to-DC Regulator for Increased Efficiency (Rev. A) PDF

AN-1213
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Powering the AD9788 800 MSPS TxDAC Digital-to-Analog Converter Using
the ADP2105 Synchronous Step-Down DC-to-DC Regulator for Increased Efficiency
CIRCUIT FUNCTION AND BENEFITS
This circuit utilizes a pair of ADP2105 synchronous step-down
dc-to-dc regulators to provide the individual power supply rails
required for the AD9788 dual channel, 16-bit, high dynamic
range TxDAC® digital-to-analog converter. The ADP2105 pair
powers the AD9788 at greater than 85% efficiency, which is up
to 50% higher than that obtained using a traditional linear
regulator solution. Total circuit power dissipation using linear
regulators is 1.4 W and using switching regulators only 0.88 W.
This increased efficiency results in lower system level power
consumption, with no measurable degradation in the performance of the AD9788. The ADP2105 is low noise step-down
dc-to-dc converter that uses a fixed frequency, peak current
mode architecture with an integrated high-side switch and
low-side synchronous rectifier. The high 1.2 MHz switching
frequency allows the use of small external inductors and
ceramic capacitors; and the tiny 16-lead, 4 mm × 4 mm
LFCSP_VQ package minimizes PCB area.
The AD9788 supports DAC update rates to 800 MSPS. The low
noise and excellent linearity of the AD9788 enable transmit
architectures from baseband to complex IF frequencies up to
200 MHz. The AD9788 features a rich set of digital signal
processing, including 2×, 4×, and 8× interpolation filter options
and complex digital modulation with a 32-bit NCO resolution,
as well as gain, phase, and offset compensation. The DAC
outputs interface seamlessly with analog quadrature
modulators, such as the ADL537x family.
CIRCUIT DESCRIPTION
Table 1. Devices Connected/Referenced
Product
AD9788
ADP2105
Description
Dual, 16-Bit, 800 MSPS TxDAC® digital-to-analog
converter with low power 32-bit complex NCO
1 A, synchronous, step-down DC-to-DC converter
Figure 1 shows the ADP2105 power supply solution, which
supplies the necessary input power rails to the AD9788 DAC.
The AD9788 has four power domains: two that require 3.3 V
and two that require 1.8 V. The two domains at 3.3 V are
DVDD33, which supplies the I/O circuits, and AVDD33, which
supplies the DAC internal switch core. The two domains at
1.8 V are DVDD18, which supplies the digital processing
circuitry, and CLKVDD18, which supplies the clock receiver
and PLL circuitry. The domains requiring 3.3 V are supplied by
a single ADP2105 (3.3 V) device, and the domains requiring 1.8 V
are supplied by a single ADP2105 (1.8 V) device.
The power domains of each rail are isolated from each other by
use of a ferrite bead, as shown in Figure 1. One bulk capacitor
per domain, plus localized high frequency decoupling capacitors on individual supply pins, provide adequate bypassing to
preserve the dynamic performance of the AD9788. The input
voltage to the ADP2105 impacts the size of the required
inductor and also affects the efficiency of the regulator. Lower
input voltages generally require smaller inductors and improve
the power supply efficiency. The configuration shown uses a
3.3 V input rail to supply the 1.8 V regulator and a 5.0 V input
rail to supply the 3.3 V regulator.
Figure 2 shows a representative comparison of the spectral plots
from the AD9788 when powered from linear regulators versus
the ADP2105 switching regulators. The particular instance
shown is for DAC output frequency, FOUT = 125 MHz, DAC
input data rate, FDATA = 200 MHz, and 4× interpolation. Under
these conditions, the DAC outputs a 125 MHz signal at an
800 MSPS update rate. The output spectrum of the AD9788
shows no measurable increase in noise floor or spurs associated
with the switching frequency under the test cases considered.
The efficiency results in Table 1 compare the overall efficiency
of an LDO regulator design to the ADP2105 based switching
regulator design. The linear regulators and the switching regulators
used the same input voltages. The 1.8 V regulators were supplied
with 3.3 V input rails. The 3.3 V regulators were supplied with
5.0 V input rails. The use of switching regulators resulted in
power savings of 457 mW and an efficiency gain in overall
power consumption of about 50%.
Proper component placement, power and ground plane layout,
and signal routing are critical to a successful design when using
a dc-to-dc regulator, such as the ADP2105. Adhering to the
detailed layout guidelines in the switching regulator data sheet
should lead to a successful application of the device. These
guidelines usually call for switching inductors to be mounted
far away from the DAC and sensitive components in the DAC’s
clock and signal paths—or on the opposite side of the PCB to
help eliminate magnetic flux coupling into sensitive components. High current loops should be kept as short as possible.
Bypass capacitors and compensation networks for the regulators
should be placed close to the device.
Rev. A | Page 1 of 3
AN-1213
Application Note
0.1μF
10Ω
VIN
3.3V INPUT RAIL
4.7μF1
VOUT
ON
1
OFF
16
15
FB
GND IN
14
13
PWIN1
FB: MURATA
BLM18HE601SN1B
FB
1.8V @ ~240mA
LX2 12
EN
2.7μH2
PGND 11
2
GND
3
GND
LX1 10
4
GND
PWIN2 9
22μF1
ADP2105-1.8
COMP
SS
5
6
4.7μF1
8
10μF
X5R 0805
4.7μF: GRM21BR61A475KA73L
22μF: GRM21BR60J226ME39L
2 TOKO D1098AS-DE2812: 2.7µH
39pF
0.1μF
10Ω
0.1μF
CONNECT DIRECTLY TO
PINS 1, 2, 9, 10
FB
1 MURATA
1nF
270kΩ
10μF
VIN
AGND NC
7
22μF1
CONNECT DIRECTLY TO
PINS 33, 43, 60, 16, 23, 53
VIN
0.1μF
5V INPUT RAIL
10μF1
VOUT
14
13
PWIN1
FB: MURATA
BLM18HE601SN1B
2.5μH2 3.3V @ ~109mA
FB
LX2 12
1
EN
2
GND
3
GND
LX1 10
4
GND
PWIN2 9
PGND 11
10μF1
ADP2105-3.3
COMP
SS
5
6
70kΩ
AGND NC
7
1nF
120pF
8
4.7μF1
10μF
CONNECT DIRECTLY TO
PINS 80, 78, 76, 96, 98, 100
0.1μF
VIN
10μF1
CONNECT DIRECTLY TO
PINS 38, 61
FB
X5R 0805
10μF
10μF: GRM21BR61A106KE19L
4.7μF: GRM21BR61A475KA73L
2 SUMIDA CDRH5D28: 2.5µH
1 MURATA
0.1μF
53
23
16
60
43
33
GND IN
10
9
2
1
15
100
98
96
76
78
80
OFF
16
FB
61
38
ON
DVDD33
AVDD33
CVDD18
DVDD18
08756-001
AD9788
Figure 1. Powering the AD9788 DAC from Two ADP2105 Switching Regulators (Simplified Schematic: All Connections and Decoupling Not Shown)
COMMON VARIATIONS
For systems that do not require the full dynamic range of the
16-bit AD9788, the AD9785 and AD9787 offer 12-bit and 14-bit
resolution, respectively, with the same feature set and sample
rates. The lower resolution devices can also be powered by the
ADP2105 without suffering any performance penalty compared
to linear regulators, while realizing similar efficiency gains. The
ADP2105 supplies a maximum of 1 A output current. For
multichannel applications where higher output currents are
required, the ADP2106 offers 1.5 A output current, and the
ADP2107 offers 2 A output current.
With care, both linear voltage regulators and switching regulators
can provide clean power supplies that allow optimal performance
from high dynamic range DACs. Linear regulators provide low
noise voltage outputs but deliver power inefficiently. Switching
solutions show increased efficiency and lower power dissipation
without degradation of DAC performance.
Rev. A | Page 2 of 3
Application Note
AN-1213
ADP2105 SWITCHING REGULATORS
0
200
400
0
200
FREQUENCY (MHz)
400
FREQUENCY (MHz)
08756-002
DAC OUTPUT (dBc)
DAC OUTPUT (dBc)
LINEAR REGULATORS
Figure 2. Output Spectrum for DAC Output Frequency = 125 MHz, DAC Input Data Rate = 200 MHz, DAC Output Rate = 800 MSPS (4× Interpolation)
Table 2. Power and Efficiency for Linear Regulator Supplies vs. ADP2105 Switching Regulator Supplies
Linear Regulator
ADP2105 Switching Regulator
AD9788 Power Supply
Domain Current (mA)
Power (mW)
Efficiency (%)
Power (mW)
Efficiency (%)
DVDD18 + CVDD18
240
792
54
475
91
DVDD33 + AVDD33
109
545
66
405
89
1337
59
880
90
Total
LEARN MORE
Cobb, Michael. Powering High-Speed Analog-to-Digital
Converters with Switching Power Supplies. Technical Paper.
May 2009.
ADIsimPower™ Regulator Interactive Design Tool
Data Sheets and Evaluation Boards
AD9788 Data Sheet
REVISION HISTORY
6/13—Rev. 0 to Rev. A
Changed Document Title from CN-0141 to
AN-1213 .............................................................................. Universal
Changes to Learn More Section ...................................................... 3
1/10—Revision 0: Initial Version
AD9788 Evaluation Board
ADP2105/ADP2106/ADP2107 Data Sheet
ADP2105/ADP2106/ADP2107 Evaluation Board
©2010–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08756-0-6/13(A)
Rev. A | Page 3 of 3
Similar pages