Datasheet Linear Technology LT3748

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  Increase Current Capability and Simplify Thermal Design of Flyback Converters with Secondary-Side Synchronous Rectifier Driver in a 5-Pin SOT-23 &mdash LT Journal
  PDF, 376 Kb, File published: Jan 6, 2014
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  Increase Current Capability and Simplify Thermal Design
  of Flyback Converters with Secondary-Side Synchronous
  Rectifier Driver in a 5-Pin SOT-23
  Wei Gu In a flyback converter, current levels in high current applications are constrained
  by heat produced in the output rectifier diode. The clear way to lift this restriction
  is to replace the diode with a much lower voltage drop MOSFET, significantly
  reducing heat produced in the rectifier—reduced heat dissipation increases output
  current capability and efficiency, and simplifies thermal design. The LT8309 is a
  secondary-side synchronous MOSFET driver that replicates the behavior of the
  output diode by sensing the drain-to-source voltage of the MOSFET to determine
  if should be on or off, allowing it to replace a less efficient rectifier diode.
  The LT8309 can be combined with any
  of Linear’s line of no-opto boundarymode flyback ICs (such as the LT3748
  primary side controller) to produce high
  performance isolated power supplies
  with a minimal number of components.
  5V, 8A ISOLATED SUPPLY Figure 1 shows a low voltage, high
  current, low parts count flyback
  supply. The traditional output diode is
  replaced by an ideal diode consisting
  of the LT8309, a MOSFET and a few
  small external components.
  For a MOSFET to act as a diode it must …

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  Isolated Power Supplies Made Easy &mdash LT Journal
  PDF, 1.0 Mb, File published: Jan 18, 2011
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  Isolated Power Supplies Made Easy
  John D. Morris The LT3748 is a switching regulator controller specifically
  designed to simplify the design of isolated power supplies
  using a flyback topology. No third winding or opto-isolator
  is required as the LT3748 senses the isolated output
  voltage directly from the primary-side flyback waveform.
  One challenge in designing a flyback
  converter is that information relating to
  the output voltage on the secondary side
  of the transformer must be fed back to the
  regulator on the primary side in order to
  maintain regulation. Historically, feedback
  across the isolation barrier is achieved
  using opto-isolators or extra transformer
  windings, though both methods present a
  number of design problems. Opto-isolator
  feedback circuits add components, increasing converter size and cost. They also draw
  power, degrading efficiency and complicating thermal design. Opto-isolators also
  make it difficult to accurately regulate
  the output due to their limited dynamic
  response, inherent nonlinearities, typical
  variation from unit-to-unit and variation with age. The usual alternative is
  to add an extra transformer winding, …

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Manufacturer's Classification

• Power Management > Switching Regulator > Flyback, Forward and Isolated Controllers
• Space & Harsh Environment > Extended Temperature Plastic (H & MP) > Extended Temperature (H & MP) Switching Regulators