Circuit Note CN-0508 Devices Connected/Referenced 1.5 A Single Resistor Rugged Linear LT3081 Regulator with Monitors 42 V, 6 A Synchronous Step-Down Regulator LT8612 with 3 A Quiescent Current 42 V, 3 A Synchronous Step-Down LT8609 Regulator with 2.5 A Quiescent Current Circuits from the Lab reference designs are engineered 100 mA Regulated Charge Pump Inverters LTC1983-5 and tested for quick and easy system integration to help in ThinSOT solve todays analog, mixed-signal, and RF design 16-Bit SPI nanoDAC+ with 2 LSB INL and challenges. For more information and/or support, visit AD5683R www.analog.com/CN0508. 2 ppm/C Reference 3.2 MHz, 0.8 V/s Low Power Over-the-Top LT6015 Precision Op Amp Dual Comparator with Known Power-Up ADCMP392 State 4-Channel, Low Noise, Low Power, 24-Bit, AD7124-4 Sigma-Delta ADC with PGA and Reference 75 Watt, Single-Output Benchtop Power Supply of 0 V to 27.5 V and current limiting/constant current operation EVALUATION AND DESIGN SUPPORT (up to 3 A). Circuit Evaluation Boards With a Raspberry Pi-compatible expansion header, electronic CN-0508 Circuit Evaluation Board (EVAL-CN0508-RPIZ) control through either a local touchscreen or over a wireless or Design and Integration Files Schematics, Layout Files, Bill of Materials, Software, wired network connection is possible. Output voltage can be Mechanical Enclosures controlled manually or by software, and a manual current-limit control sets the transition from constant voltage to constant CIRCUIT FUNCTION AND BENEFITS current operation. A high-quality benchtop power supply is an essential piece of The benchtop power supply features a hybrid buck/linear equipment in any electronics or science laboratory because architecture, providing low output ripple and low output when power integrity is compromised, sensitive circuits can fail capacitance, excellent transient response, regulation to 0 V and in unexpected ways. However, most of the power supplies on 0 A, and low power dissipation that eliminates the need for heat the market today offer competitive specifications at the expense sinks. The complete solution is low cost, compact, and easily of high cost, size, and heat dissipation. configured for standalone operation or integration into other The circuit shown in Figure 1 is a single-channel, 75 W benchtop equipment. power supply featuring an adjustable wide output voltage range Rev. 0 Circuits from the Lab reference designs 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages Tel: 781.329.4700 www.analog.com due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2020 Analog Devices, Inc. All rights reserved. CN-0508 Circuit Note ILIM ILIM FB VIN LT3081 LT8612 VOUT + 1.7V (BUCK REGULATOR) VOUT SET VREF TEMP IMON IMON 11 TEMP 1 5V AT 2A LT8609 VOUT IMIN (RPI POWER) MINIMUM VREF ILIM 11 SPI AD5683 LT3081 5V 5V (OP AMP LTC1983 VOUT POWER I MIN SUPPLY) SET TEMP IMON IMON TEMP 2 Figure 1. CN-0508 Block Diagram 100 8 CIRCUIT DESCRIPTION EFFICIENCY 90 The CN-0508 provides a low cost, adjustable power solution 7 with performance comparable to high-performance commercial 80 6 power supplies. 70 5 The design uses a hybrid circuit topology of a buck converter preregulator and two parallel linear regulators. This topology 60 4 combines the high power efficiency of a buck converter and the 50 3 low output noise, low ripple, and adjustable current limiting of the linear regulators. 40 2 V = 24V POWER LOSS OUT V = 18V OUT No heat sink other than the circuit board itself is required, in 30 V = 12V 1 OUT V = 5V V = 36V OUT IN contrast to linear bench supplies in which pass devices (either V = 3.3V OUT 20 0 discrete transistors or integrated circuit regulators) require 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 I (A) external heat sinks to sufficiently dissipate the heat. LOAD Figure 2. Efficiency and Power Loss of DC Bench Supply at 36 V Input and POWER SUPPLY CORE Various Output Conditions LT8612 Buck Converter Figure 2 shows the efficiency and power loss of the composite The first stage in the CN-0508 design is the LT8612 synchro- regulator, showing that the worst-case total power loss is 7 W, nous buck converter. A buck converter, or step-down switch allowing free convection cooling when the board is in open air mode power supply, efficiently reduces a higher dc voltage to and requiring only a small fan when placed in an enclosure. a lower voltage. A buck converter also provides low power dissi- Typically, a buck converter regulates its output voltage based on pation and high power density in a small package, relative to a a feedback divider such that: linear regulator of similar current capability. The dc input to the system is filtered and bypassed by a total of R HIGH VV +1 OUT FB 32 F before entering the input of the buck converter. The R LOW LT8612 efficiently steps down the 30 V nominal input to a Where VFB is 970 mV in the case of the LT8612. CN-0508 voltage approximately 1.7 V above the power supply output, features a modified feedback path that regulates the output of slightly above the maximum dropout voltage of 1.5 V of the the buck stage to a fixed 1.7 V above the output of the following LT3081. Keeping the voltage drop across the LT3081 regulators stage as shown in Figure 3. just above dropout minimizes power dissipation and eliminates the need for additional heat sinks. Rev. 0 Page 2 of 8 EFFICIENCY (%) 20198-001 POWER LOSS (W) 20198-002 =