Offline flyback controller for isolated supplies
The LT3798IMSE#PBF is an isolated flyback switching controller from Analog Devices, designed for offline and DC-DC converters where galvanic isolation is required. It drives an external MOSFET (no internal switch) and operates from a 10V to 38V supply, covering the rectified output of universal AC mains and many industrial bus voltages. The switching frequency can be set anywhere from 500Hz to 150kHz, giving you flexibility to optimize transformer size and EMI filtering for the application. The -40°C to 125°C operating range means it handles under-hood automotive, outdoor telecom cabinets, and factory-floor enclosures without a temperature derating headache. Packaged in a 16-MSOP-EP with exposed pad, it needs a solid thermal via pattern under the pad to keep the junction cool at higher loads.
What the switching frequency range means for your design
The 500Hz to 150kHz range is unusually wide. At the low end, you can run a large transformer with low core loss for noise-sensitive audio or instrumentation supplies. At the high end, you shrink the magnetics for a compact board, but you'll need to watch the switching losses in the external MOSFET and the transformer's interwinding capacitance. The controller doesn't have a fixed frequency — you set it with a resistor, so you can tune it during development to dodge a noisy band or meet a specific EMI limit. No internal switch means the MOSFET choice is yours: pick one with low Rds(on) for efficiency or a fast body diode for hard-switching topologies.
Isolated output — why it matters
The LT3798 senses the output voltage from the primary-side flyback waveform, eliminating the need for an optocoupler or separate feedback winding.