The LT3748 is a high input voltage isolated flyback DC/DC controller that significantly simplifies the design of an isolated DC/DC converter. No opto-isolator, third winding or signal transformer is required for feedback since the output voltage is sensed from the primary-side flyback signal during the off time of the power switch. The LT3748 operates over a 5V to 100V input voltage range, drives an external N-channel power MOSFET and can deliver up to about 60 watts. The LT3748 operates in boundary mode, which is a variable frequency current mode control switching scheme, resulting in ±5% typical regulation over the full line, load and temperature range. Boundary mode also permits the use of a smaller transformer compared to equivalent continuous conduction mode designs. The output voltage is easily set by two external resistors and the transformer turns ratio. Twenty five off-the-shelf transformers are identified in the data sheet for ease of use. Additional features include an onboard low drop regulator for IC power, programmable soft start, undervoltage lockout, adjustable current limit and output voltage temperature compensation. The LT3748 is available in a small MSOP-16 package with four pins removed for additional high voltage pin spacing and has an operating junction temperature range of -40°C to 125°C.