Flyback dc/dc Controller Needs No Optoisolator

The LT3837, a synchronous flyback dc/dc controller, employs current-mode operation that regulates the output voltage based on sensing a secondary voltage via a transformer winding. This allows for tight output voltage regulation without the use of an optoisolator, improving dynamic response and system reliability in 10- to 60-W power supplies. The use of synchronous rectification increases conversion efficiency and improves output cross regulation in multiple-output power supplies. For example, the LT3837 can convert a 9- to 36-V input to an isolated output of 3.3 V at 12 A with up to 90% efficiency and with 1% regulation.

Elimination of the optoisolator also cuts design time that would be needed for trimming the power supply, and it improves the output transient response. An adjustable primary gate delay eliminates multiple discrete components from the circuit and optimizes MOSFET switching time and transformer selection. To further simplify the design, the controllers switching frequency can be adjusted from 50 kHz to 250 kHz with a capacitor. The device can also be synchronized to an external oscillator. The LT3837 is rated for -40 deg. C to 125 deg. C operation. It comes in a thermally enhanced 16-lead TSSOP.