DEMO MANUAL DC1694B LT3748 100V Isolated Flyback Converter DESCRIPTION Demonstration circuit 1694B is an isolated flyback con- The LT3748 is a high input voltage isolated flyback con- verter featuring the LT 3748. The demo circuit is designed troller operating with input supply voltages from 5V to for a 12V output from a 22V to 75V DC input. The output 100V, and 1.9A average gate drive source and sink current. current is up to 2.5A. The part senses the isolated output The LT3748 utilizes boundary mode operation to provide voltage directly from the primary side flyback waveform a high efficiency, small size solution with improved load during the off time of the power switch. No third winding, regulation of 5% over the full line, load and temperature opto-coupler, or signal transformer is required for regula- ranges. The LT3748 can be used in a wide variety of indus- tion. A minimum load of approximately 2% of maximum trial, automotive, medical, telecom, data-com applications, load is required on the output to maintain regulation. where regulated isolated output is required. The demo circuit can be easily modified for applications The LT3748 data sheet gives a complete description of of different input, and output voltage/current, with some the part, operation and application information. The data pre-designed transformers from vendors such as Wrth sheet must be read in conjunction with this quick start Electronics, Sumida, Pulse Engineering and Coilcraft. Both guide for Demo Circuit 1694B. EP13 and EFD20 core transformers can be assembled on Design files for this circuit board are available at the board. A two-stage filter is optional for applications DEMO MANUAL DC1694B QUICK START PROCEDURE Demo circuit 1694B is easy to set up to evaluate the NOTE: If higher operating voltage is required, power performance of the LT3748. Refer to Figure 1 for proper components with higher voltage ratings should be used. measurement equipment setup and follow the procedure NOTE: The LT3748 requires minimum load to maintain below. good regulation. A Zener diode D4 is placed between 1. With power off, connect the input power supply to VIN VOUT+ and VOUT to clamp the output voltage to ~13V and GND. if minimum load requirement is not met. Depending on input voltage and output regulation, a 100mA minimum 2. Connect the load to the terminals VOUT+ and VOUT on load is usually sufficient. the board. Apply a load between 100mA to 2.5A to the output. 5. Once the proper output voltage is established, adjust the input and load within the operating range and observe 3. Turn on the power at the input. Increase VIN to 22V. the output voltage regulation, ripple voltage, efficiency NOTE: Make sure that the input voltage does not exceed and other parameters. 75V. NOTE: When measuring the input or output voltage 4. Check for the proper output voltages. The output should ripples, care must be taken to avoid a long ground lead be regulated at 12V (5%). on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across NOTE: If there is no output, temporarily disconnect the VIN and GND, or VOUT+ and VOUT terminals. See the power supply. Make sure that the load is properly Figure 2 for proper scope probe technique. connected. If the output voltage is out of spec, make sure minimum load current is applied to the output, the load is not set too high, and the DC power supply does not hit the current limit. dc1694bfa 2