The LT®3573 is a monolithic switching regulator specifi - cally designed for the isolated fl yback topology. No third winding or optoisolator is required for regulation. The part senses the isolated output voltage directly from the primary side fl yback waveform. A 1.25A, 60V NPN power switch is integrated along with all control logic into a 16- lead MSOP package. The LT3573 operates with input supply voltages from 3V to 40V, and can deliver output power up to 7W with no external power devices.The LT3573 utilizes boundary mode operation to provide a small magnetic solution with improved load regulation.