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