Maxim > Design Support > Technical Documents > Reference Designs > Medical > APP 5510 Maxim > Design Support > Technical Documents > Reference Designs > Power-Supply Circuits > APP 5510 Maxim > Design Support > Technical Documents > Reference Designs > Protection and Isolation > APP 5510 Keywords: power, pmod, industrial sensors, process control, industrial automation, medical, industrial power, halo electronics, transformer, lakewood, maxrefdes7, industrial, dual supplies, power REFERENCE DESIGN 5510 INCLUDES: Tested Circuit Schematic BOM Description Test Data Layout Lakewood (MAXREFDES7 ): 3.3V Input, 12V (15V) Output Isolated Power Supply Nov 06, 2012 Abstract: This document details the Lakewood (MAXREFDES7 ) subsystem reference design, a 3.3V input, 12V (15V) output, isolated power supply. The Lakewood reference design includes a 3W primary-side transformer H- bridge driver for isolated supplies, and two wide input range and adjustable output low dropout (LDO) linear regulators. Test results and hardware files are included. Introduction The Lakewood design (MAXREFDES7 ) uses an H-bridge transformer driver (MAX256) and a pair of low dropout (LDO) linear regulators (MAX1659 x2) to create a 12V (15V) output isolated power supply from a 3.3V voltage input (Figure 1). This general-purpose power solution can be used in many different types of isolated power applications, but is mainly targeted for industrial sensors, industrial automation, process control, and medical applications. More detailed image (PDF, 1.9MB) Figure 1. The Lakewood subsystem design block diagram. Features Applications Isolated power Industrial sensors 12V (15V) outputs Process control Small printed circuit board (PCB) area Industrial automation Pmod-compatible form factor Medical Detailed Description of Hardware The Lakewood subsystem reference design operates from a 3.3V DC power source. The MAX256 H-bridge transformer driver switches at approximately 475kHz and drives the primary side of the 1:2.6 turns ratio, with the Page 1 of 5 use of a TGM-H281NF transformer from Halo Electronics. The transformer secondary side is connected to voltage doublers that rectify the AC outputs into DC outputs. Two MAX1659 LDOs regulate the voltages to +12V and -12V, respectively. The Zener diodes (D3 and D6) protect the LDOs by keeping their input voltages below 16.1V. The input power can be from the J1 Pmod-compatible connector or from an external power supply connected to the EXT +3.3V and DGND connectors. To change the output voltages of this reference design, simply change the feedback resistors (R2R5) of the LDOs (U2 and U3). The output voltage of the MAX1659 LDOs is set by the following equations: V = V (1 + R2/R3) for U2 OUT SET V = V (1 + R4/R5) for U3 OUT SET Where V = 1.21V. SET For example, for the 15V outputs application, change R2 and R4 to 187k, and change R3 and R5 to 16.2k. This circuit can also be configured for asymmetrical applications. In applications sensitive to output voltage ripple, a lowpass LC pi filter can be added in front of the LDO input. The isolation transformer in this design has an isolation voltage of 1500V . It is recognized by UL 60950 and EN RMS 60950 and falls into thefunctiona insulation class. Quick Start Required equipment: Lakewood (MAXREFDES7 ) board 3.3V 1A power supply Two digital voltmeters Procedure The Lakewood board is fully assembled and tested. Follow the steps below to verify board operation. 1. Place the shunt on jumper JU1 to the 12 position. 2. Connect the positive terminal of the power supply to the EXT +3.3V connector. 3. Connect the negative terminal of the power supply to the DGND connector. 4. Connect the positive terminal of the first voltmeter to the +12V connector. 5. Connect the negative terminal of the first voltmeter to the GND1 connector. 6. Connect the positive terminal of the second voltmeter to the -12V connector. 7. Connect the negative terminal of the second voltmeter to the GND2 connector. 8. Turn on the power supply. 9. Use the respective voltmeters to measure the corresponding positive and negative output voltages. Lab Measurements The Lakewood design was tested for two pairs of output voltage rails: 12V and 15V. Other voltage rails can be achieved by modifying the resistor values of R2, R3, R4, and R5. When set for 12V outputs, the circuit can deliver a maximum load current of approximately 90mA for each of the two rails simultaneously. When set for 15V outputs, the circuit can deliver a maximum load current of approximately 40mA for each of the two rails Page 2 of 5