Circuit Note CN-0193 Devices Connected/Referenced Circuits from the Lab™ tested circuit designs address common design challenges and are engineered for quick and easy system integration. For more information and/or support, visit www.analog.com/CN0193. AD5504 Quad, 12-Bit, 30 V/60 V DAC ADP1613 650 kHz /1.3 MHz Step-Up PWM DC-to-DC Switching Converter High Voltage (30 V) DAC Powered from a Low Voltage (3 V) Supply Generates Tuning Signals for Antennas and Filters EVALUATION AND DESIGN SUPPORT in being able to tune these applications—for example, compensating for component tolerance errors, fine tuning the cutoff frequency of a filter, or network impedance matching for tunable antennas. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 generates a high voltage signal that controls the capacitance of a BST (barium strontium titanate) capacitor. The capacitance of a BST capacitor can be altered by applying a voltage of between 0 V and 30 V to the correct terminal. This changes the thickness of the dielectric and, hence, the capacitance. BSTs are often used for tuning antenna arrays or tunable filters. Clearly, there is an advantage VDD 1.5µH 22µF 6.3V 1 10pF 10nF 97.6kΩ VDD COMP D SS 8 2 FB FREQ 7 3 EN VIN 6 4 GND SW 5 1µF The boost circuit can also be used in LED driver applications and to supply receiver bias voltages in optical communications systems. 110nH 1N5819HW-7-F 10Ω ADP1613 This application requires a convenient, compact, and low cost circuit to generate the high voltage power supply because it is often not practical to add a separate supply simply for this function alone. The circuit in Figure 1 meets these requirements using the ADP1613 boost converter and the AD5504 30 V/60 V DAC. Total board area for the boost regulator circuit is only 43 mm2. The ADP1613 is available in an 8-lead MSOP package and the AD5504 in a 16-lead TSSOP. S SI4346DY-T1-E3 G 10nF 140kΩ 5.62kΩ 1µF 50V 2.2µF 50V VDD 0.1µF 50V 16 VDD TO MICROCONTROLLER 14 0.1µF 50V VLOGIC VDD 13 R_SEL 1 CLR 2 SYNC 3 SCLK 4 SDI 5 SDO 6 LDAC VOUTA 12 0V TO 30V VOUTB 11 0V TO 30V VOUTC 10 0V TO 30V VOUTD 9 0V TO 30V DGND AGND 6 7 AD5504 09608-001 Circuit Evaluation Boards CN-0193 Circuit Evaluation Board AD5504 Evaluation Board (EVAL-AD5504EBZ) Design and Integration Files Schematics, Layout Files, Bill of Materials Figure 1. Boost Supply and High Voltage DAC Provide Tuning Signal for BST Capacitors (Simplified Schematic: All Connections Not Shown) Rev. 0 Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2011 Analog Devices, Inc. All rights reserved. Circuit Note CN-0193 CIRCUIT DESCRIPTION changed by writing to the appropriate DAC registers via the serial interface. The DACs can be updated simultaneously by pulsing the Load DAC (LDAC) pin low, thus allowing for all four BST capacitors to be changed at the same time. The circuit has a 3 V (VDD) power supply available, and the BST capacitors require voltages in excess of 20 V for full control. The two main circuit blocks are the ADP1613 step-up switching converter and the AD5504 high voltage DAC. The circuit diagram is shown in Figure 1. Using the circuit shown in Figure 1, it is possible to generate DAC output voltages up to 30 V. The output voltages are used to set the bias voltage for BST capacitors, which, in turn, adjust the antenna response. Figure 2 shows the equivalent circuit of a BST capacitor used as a tunable matching network, and Figure 3 shows the transfer function of the BST capacitance as a function of bias voltage and the resulting antenna response. More information on BST capacitors can be found at www.agilerf.com. The ADP1613 is a step-up dc-to-dc switching converter with an integrated power switch capable of providing an output as high as 20 V. Using additional external components, higher voltages can be achieved. The ADP1613 has an adjustable soft start function to prevent inrush current when the device is enabled. The pin-selectable switching frequency and PWM current mode architecture allows easy noise filtering and yields excellent transient response. The components connected around the ADP1613 generate a 32 V output from a 3 V input. AD5504 OUTPUT (CONTROL VOLTAGE) BST CAP 09608-002 + BST CAP In any circuit where accuracy is important, it is important to consider the power supply and ground return layout on the board. The printed circuit board (PCB) containing the circuit should have separate analog and digital sections. If the circuit is used in a system where other devices require an AGND-toDGND connection, the connection should be made at one point only. This ground point should be as close as possible to the AD5504. The circuit should be constructed on a multilayer VOLTAGE VARIABLE CAPACITANCE ANTENNA RESPONSE 0 1 –4 –8 0.8 3 –12 S11 (dB) 2 0.6 –16 1 –20 3 0.4 2 –24 0.2 0 SEE ANTENNA RESPONSE #1 SEE ANTENNA RESPONSE #2 SEE ANTENNA RESPONSE #3 0 5 10 15 ANTENNA RESPONSE #1 ANTENNA RESPONSE #2 ANTENNA RESPONSE #3 –28 20 –32 780 BIAS VOLTAGE (V) 800 820 840 860 880 900 FREQUENCY (MHz) Figure 3. Bias Voltage vs. BST Capacitance, and Resulting Antenna Response Rev. 0 | Page 2 of 3 920 940 960 980 09608-003 NORMALIZED CAPACITANCE DC BLOCK 4.62nH Figure 2. BST Capacitor Equivalent Circuit The 32 V output from the ADP1613 is used as the power supply for the AD5504. The AD5504 is a quad channel, 12 bit DAC capable of an output voltage up to 60 V. The full-scale output of the AD5504 is determined by the state of the R_SEL pin. In this application, R_SEL is connected to VDD, selecting a full-scale output of 30 V. The AD5504 is controlled by a serial interface, which is compatible with 3 V logic. The DAC outputs are 1.0 DC BLOCK RF SIGNAL The ADIsimPower design file includes the bill of material, detailed schematic, bode plots, efficiency plots, transient response, as well as a suggested board layout. 1.2 ANTENNA + The ADIsimPower™ design tool provides an easy way for designers to determine the appropriate components based on input and output requirements. The design of the ADP1613 circuit shown in Figure 1 uses the "lowest cost" option of ADIsimPower with an input voltage of 3 V, output voltage of 32 V, and a load current of 40 mA. The design can be downloaded at www.analog.com/CN0193-PowerDesign. Circuit Note CN-0193 CIRCUIT EVALUATION AND TEST PCB with a large area ground plane layer and a power plane layer. See MT-031 Tutorial for more discussion on layout and grounding. The power supply to the AD5504 should be bypassed with 10 μF and 0.1 μF capacitors. The capacitors should be as physically close as possible to the device, with the 0.1 μF capacitor ideally right up against the device. The 10 μF capacitor should be either the tantalum bead type or ceramic type. It is important that the 0.1 μF capacitor have low effective series resistance (ESR) and low effective series inductance (ESL), such as is typical of common ceramic types of capacitors. This 0.1 μF capacitor provides a low impedance path to ground for high frequencies caused by transient currents due to internal logic switching. See MT-101 Tutorial for more details on proper decoupling techniques. The power supply line should have as large a trace as possible to provide a low impedance path and reduce glitch effects on the supply line. Clocks and other fast switching digital signals should be shielded from other parts of the board by digital ground. The circuit of Figure 1 is tested by applying a 3 V power supply to VDD. This creates a 32 V supply for the AD5504 (measurable on Pin 14) and also supplies the VLOGIC supply for the AD5504. A microcontroller, DSP, or FPGA is used to provide the appropriate digital interface signals to the AD5504. For normal operation CLR should be high. The SYNC , SCLK, and SDATA lines should be operated as described in the AD5504 data sheet to write data to the various registers of the AD5504. When data is written to a DAC register with LDAC low, the appropriate output will update immediately. When data is written to a DAC register with LDAC high, the DAC output will remain at its current value until LDAC is pulsed low. LEARN MORE CN0193 Design Support Package: http://www.analog.com/CN0193-DesignSupport ADIsimPower Design File for CN0193: http://www.analog.com/CN0193-PowerDesign The ADIsimPower design file shows a recommended layout for the ADP1613 portion of the circuit. The file can be downloaded at www.analog.com/CN0193-PowerDesign. A complete design support package for this circuit note can be found at www.analog.com/CN0193-DesignSupport. Kavanagh, Ken. "Boost Supply and High-Voltage DAC Provide Tuning Signal for Antennas and Filters," Analog Dialogue, 44-12 Back Burner, December 2010. COMMON VARIATIONS MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Analog Devices. Other boost regulators can be substituted, depending on system requirements. Refer to the ADIsimPower™ design tool for details. The AD5501 is a single channel version of the AD5504. BST Capacitors: www.agilerf.com ADIsimPower™ Design Tool, Analog Devices. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Data Sheets and Evaluation Boards AD5504 Data Sheet ADP1613 Data Sheet AD5504 Evaluation Board ADP1613 Evaluation Board REVISION HISTORY 1/11—Revision 0: Initial Version (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09608-0-1/11(0) Rev. 0 | Page 3 of 3