LT3781

Features:

High Voltage Operation up to 72V
Synchronizable Operating Frequency and Output Switch Phase for Multiple Controller Systems
Synchronous Switch Output
Undervoltage Lockout Protection with 6V Hysteresis for Self-Biased Power
Fixed Frequency Operation to 350kHz
Local 1% Voltage Reference
Input Overvoltage Protection
Low Start-Up Current
Programmable Start Inhibit for Power Supply Sequencing and Protection
Optocoupler Support Soft-Start Control


Application

Isolated Telecommunication Systems
Personal Computers and Peripherals
Distributed Power Step-Down Converters
Lead Acid Battery Backup Systems
Automotive and Heavy Equipment


Specifications

Power Supply (VCC)
Low Impedance Source Voltage ............. 0.3V to 20V
Shutdown Mode:
(Supply Self-Regulates to 18V)
Maximum Input Current ............................... 20mA
Topside Supply (VBST) ....................................................
VBSTREF  0.3V to VBSTREF +20V (VBST(MAX) = 90V)
Topside Reference Pin (VBSTREF) ............... 0.6V to 75V
SHDN Pin Voltage ........................... 0.3V to VCC + 0.3V
All Other Input Voltages .............. 0.3V to 5VREF + 0.3V
5VREF Pin SYNC Current ....................................... 10mA
FSET Pin Current ...................................... 2mA to 5mA
All Other Input Pin Currents ...................... 2mA to 2mA
Operating Ambient Temperature Range ...40°C to 85°C
Operating Junction
Temperature Range (Note 4) .................40°C to 125°C
Storage Temperature Range ..................65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C

Description

The LT®3781 controller simplifies the design of high power synchronous dual transistor forward DC/DC converters. The part employs fixed frequency current mode control and supports both isolated and nonisolated topologies. The IC drives external N-channel power MOSFETs and operates with input voltages up to 72V.

The LT3781 is ideal for output derived power schemes, through the use of a large undervoltage lockout hysteresis range. The part is also equipped with an 18V power supply pin regulator which prevents exceeding absolute maximum ratings while in trickle start applications.

The LT3781's operating frequency is programmable and can be synchronized up to 350kHz. Switch phase is also controlled during synchronized operation to accommodate multiple-converter systems. Internal logic guarantees 50% maximum duty cycle operation to prevent transformer
saturation.

The LT3781 is available in a 20-lead SSOP package.