The LT1725 is a monolithic switching regulator controller specifically designed for the isolated flyback topology. It drives the gate of an external MOSFET and is generally powered from a third transformer winding. These features allow for an application input voltage limited only by external power path components. The third transformer winding also provides output voltage feedback information, such that an optoisolator is not required. Its gate drive capability coupled with a suitable external MOSFET can deliver load power up to tens of watts. The LT1725 has a number of features not found on most other switching regulator ICs. By utilizing current mode switching techniques, it provides excellent AC and DC line regulation. Its unique control circuitry can maintain regulation well into discontinuous mode in most applications. Optional load compensation circuitry allows for improved load regulation. An optional undervoltage lockout pin halts operation when the application input voltage is too low. An optional external capacitor implements a soft-start.
The LT1725 is a monolithic switching regulator controller designed for isolated flyback topologies. It drives an external MOSFET, provides optional load compensation, undervoltage lockout, and a 3 V reference output, making it suitable for telecom and instrumentation power supplies.
The document includes absolute maximum ratings, electrical characteristics, pin descriptions, timing diagrams, package information, and related part numbers, providing all necessary data for design and implementation.
| Turn‑On Voltage (VEC) | 15.1 V min, 16.0 V typ, 140 V max |
|---|---|
| Supply Current (Ve open) | 6 mA min, 10 mA typ, 15 mA max |
| Feedback Voltage | 1.230 V min, 1.260 V max |
The absolute maximum rating for the supply voltage (VEC) is 140 V.
Soft‑start is provided by connecting an external capacitor to the SFST pin.
Pin 9, labeled 3Vout, supplies a nominal 3 V reference.