LT1725

` Drives External Power MOSFET with External I<sub>SENSE</sub> Resistor
` Application Input Voltage Limited Only by External Power Components
` Senses Output Voltage Directly from Primary Side Winding-No Optoisolator Required
` Accurate Regulation Without User Trims
` Regulation Maintained Well into Discontinuous Mode
`Switching Frequency from 50kHz to 250kHz with External Capacitor
` Optional Load Compensation
` Optional Undervoltage Lockout
` Available in 16-Pin SO and SSOP Packages

· Telecom Isolated Converters
· Offline Isolated Power Supplies
·  Instrumentation Power Supplies

V_CC Supply Voltage ............................................... 22V
UVLO Pin Voltage .................................................. V_CC
I_SENSE Pin Voltage .................................................. 2V
FB Pin Current .................................................. ±2mA
Operating Junction
Temperature Range
LT1725C .............................................. 0°C to 100°C
LT1725I ......................................... 40°C TO 125°C
Storage Temperature Range .......... 65°C to 150°C
Lead Temperature (Soldering, 10 sec).............. 300°C

The LT^®1725 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 softstart function. A 3V output is available at up to several mA for powering primary side application circuitry.