DEMO MANUAL DC1643A LT3575 Isolated Monolithic Flyback Converter Description Demonstration circuit 1643A is an isolated flyback con- The DC1643A can be easily modified to generate different verter featuring LT 3575, a monolithic switching regulator output voltages. Some pre-designed transformers from specifically designed for the isolated flyback converter with vendors such as Wrth Electronics, Sumida, Pulse Engi- an integrated 2.5A, 60V NPN transistor. The DC1643A is neering and Coilcraft, can be used for various applications. designed for 5V output from a 10V to 28V DC input. The LT3575 operates with input supply voltages from The output current capability depends on the input volt- 3V to 40V. The part senses the isolated output voltage age, proper cooling, and the switch voltage stress. The directly from the primary side flyback waveform. No output current is up to 1.4A when the input is higher than third winding or opto-coupler is required for regulation. 10V, and 2A when the input is higher than 20V but less The LT3575 utilizes boundary mode operation to provide than 28V. The output current should not be higher than a high efficiency, small size solution with improved load 2.4A for safe operation. regulation. It can be used in industrial, automotive and medical applications where an isolated output is required. The IC requires minimum load to maintain good output voltage regulation. A Zener diode D4 across the output The LT3575 data sheet gives a complete description of clamps the voltage to ~5.4V. The light load regulation can the part, operation and application information. The data be improved if a preload resistor (R4, optional) is installed. sheet must be read in conjunction with this quick start Depending on input voltage and output regulation, a 40mA guide for DC1643A. minimum load is usually sufficient. Design files for this circuit board are available at DEMO MANUAL DC1643A Quick s tart p D DC1643A is easy to set up to evaluate the performance ing frequency ripple can be reduced by increasing the of the LT3575. Refer to Figure 1 for proper measurement output capacitor C4, or adding a second stage LC filter. equipment setup and follow the procedure below: Please contact factory for more information. 1. With power off, connect the input power supply to V Note: The input EMI filter circuit (L1, FB1, C5, C19, and IN (E1) and GND (E2). C20) can be added by cutting the copper plane along the notch next to C2 and C8 on the top layer of the demo 2. Connect the load to the terminals V + (E5) and OUT circuit. V (E6) on the board. Apply 100mA load to V + OUT OUT and V . Note: For overload conditions, it is recommended to OUT operate the board at a reduced input voltage range 3. Turn on the power at the input. Increase V to 10V IN and/or reduce R8 of the RCD snubber. Please contact slowly. the factory for more information. Note: Make sure that the input voltage does not exceed 5. Once the proper output voltage is established, adjust the 28V. input and load within the operating range and observe 4. Check for the proper output voltages. The output should the output voltage regulation, ripple voltage, efficiency be regulated at 5V (5%). and other parameters. Note: If there is no output, temporarily disconnect the Note: When measuring the input or output voltage power supply. Make sure that the load is connected ripples, care must be taken to avoid a long ground lead with correct polarity. If the output voltage is out of on the oscilloscope probe. Measure the input or output spec, make sure minimum load current is applied to voltage ripple by touching the probe tip directly across the output, the load is not set too high, and the input the V and GND, or V + and V terminals. See IN OUT OUT current does not hit the power supply current limit. Figure 2 for proper scope probe technique. Typical ef- ficiency and regulation curves are plotted in Figures 3 Note: The compensation loop is designed with trade- and 4, respectively. off between transient response and output ripple. To reduce the low frequency output ripple, slow down the 6. Turn off the power supply. Disconnect the load from loop by reducing R15 and increasing C11 (for instance, the demo circuit. change R15 to 6.4k, and C11 to 22nF). The switch- dc1643af 2 ure roce