POWER AND LINEAR IC SOLUTIONS

Flyback Converter Eliminates Opto

The LT3573 from Linear Technology Corporation is an isolated, monolithic switching regulator designed for use in the flyback topology. It eliminates the need for an optocoupler, external MOSFET, secondary-side reference voltage and extra third winding on the power transformer, while maintaining primary-to-secondary isolation with only one part crossing the isolation barrier.

With an on-chip 1.25A, 60V NPN power switch, the LT3573 can deliver up to 7W of output power from an input voltage that ranges from 3V to 40V. It employs a primary-side sensing scheme that is capable of detecting the output voltage through the flyback primary-side switching node waveform.

During the LT3573 internal switch-off period, the output diode delivers the current to the output and the output voltage is reflected to the primary side of the flyback transformer. The magnitude of the switch node voltage is the sum of the input voltage and reflected output voltage, which the LT3573 is able to reconstruct.

Although most often associated with telecom and datacom uses in which isolation is mandated, isolated outputs are now required in a broad range of DC/DC converter applications. These include noise sensitive devices needing ground separation from a noisy input voltage, such as a car battery, intermediate bus and industrial inputs. A noisy bus voltage can affect displays, programmable logic controllers, GPS systems, hybrid electric vehicles and medical systems.

The LT3573 operates in boundary mode, which is a current-mode scheme that runs at the transition of Continuous Conduction Mode (CCM) and DisContinuous Mode (DCM). Boundary mode simplifies the design, improves load regulation and permits the use of a smaller and less complex transformer when compared to equivalent CCM designs. In many cases, standard off-the-shelf transformers are usable: over 20 suitable versions are listed in the LT3573 data sheet.

The device is available in a small, thermally enhanced MSOP-16 package and a 3mm x 3mm QFN-16, both of which are available in extended and industrial temperature ranges from -40°C to +125°C.

Features

• Isolated flyback switching regulator
• 3V to 40V input voltage range
• 1.25A, 60V integrated NPN power switch
• Up to 7W power capability
• Boundary mode operation
• No transformer third winding required
• No opto-isolator
• Improved primary-side winding feedback load regulation
• Programmable soft start
• Thermally enhanced 16-lead MSOP or QFN-16

Applications

• Industrial
• Automotive
• Medical
• Low-power power over Ethernet

For further information, to register your interest in a Free Demo Board*, or to obtain a copy of the latest Altera/LTC Development Tools Brochure, please click here

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