LT3753 Active Clamp Synchronous Forward Controller FEATURES DESCRIPTION n Input Voltage Range: 8.5V to 100V The LT 3753 is a current mode PWM controller optimized n Programmable Volt-Second Clamp for an active clamp forward converter topology, allowing n High Efficiency Control: Active Clamp, up to 100V input operation. Synchronous Rectification, Programmable Delays A programmable volt-second clamp allows primary switch n Short-Circuit (Hiccup Mode) Overcurrent Protection duty cycles above 50% for high switch, transformer and n Programmable Soft-Start/Stop rectifier utilization. Active clamp control reduces switch n Programmable OVLO and UVLO with Hysteresis voltage stress and increases efficiency. A synchronous n Programmable Frequency (100kHz to 500kHz) output is available for controlling secondary side syn- n Synchronizable to an External Clock chronous rectification. The LT3753 is available in a 38-lead plastic TSSOP pack- APPLICATIONS age with missing pins for high voltage spacings. All registered trademarks and trademarks are the property of their respective owners. n Industrial, Automotive and Military Systems n 48V Telecommunication Isolated Power Supplies TYPICAL APPLICATION 36V to 72V, 5V/20A Active Clamp Isolated Forward Converter V OUT V 9:2 IN 5V 36V TO 72V 3.3H 20A + 4.7F 250V 5 100V 0.22F 560F 3 200 5 10V BSC0902NSI 68nF 47F 250V IRF6217 100nF 10V 10k BSC0902NSI AOUT BSC190N15NS3 V OUT IN OC SYNC I 105k SENSEP 1k UVLO V SEC 0.012 100 1.96k V I OUT SENSEN LT3753 4.7nF OVLO 1k 1.87k SOUT 22F INTV CC GND 1F + 10V V 10V 100 LT1431 PS2801-1 137k 4.7F GND-F 25V REF 1F GND-S 137k 30.1k 22nF 14.7k COLL 100k 44.2k 1F 57.6k 1k 100k 34.8k 2.2nF 3.3nF 3753 TA01 250V Rev. C 1 Document Feedback For more information www.analog.com T AO T AS T OS T BLNK IVSEC RT SS1 SS2 FB COMPLT3753 TABLE OF CONTENTS Features ..................................................... 1 D Open Loop Control: No Opto-Coupler, VSEC Applications ................................................ 1 Error Amplifier or Reference ............................... 19 Typical Application ........................................ 1 R : Open Pin Detection Provides Safety ....... 19 IVSEC Description.................................................. 1 Transformer Reset: Active Clamp Technique .....20 Table of Contents .......................................... 2 LO Side Active Clamp Topology (LT3753)...........20 Absolute Maximum Ratings .............................. 3 HI Side Active Clamp Topology (LT3752-1) .........22 Order Information .......................................... 3 Active Clamp Capacitor Value and Voltage Pin Configuration .......................................... 3 Ripple .................................................................22 Electrical Characteristics ................................. 4 Active Clamp MOSFET Selection ........................23 Typical Performance Characteristics ................... 7 Programming Active Clamp Switch Timing: Pin Functions .............................................. 10 AOUT to OUT (t ) and OUT to AOUT (t ) AO OA Block Diagram ............................................. 11 Delays ................................................................. 24 Timing Diagrams ......................................... 12 Programming Synchronous Rectifier Timing: Operation................................................... 14 SOUT to OUT (t ) and OUT to SOUT (t ) SO OS Introduction ....................................................... 14 Delays ................................................................. 24 Part Start-Up ...................................................... 14 Soft-Start (SS1, SS2) .........................................25 Applications Information ................................ 15 Soft-Stop (SS1) ..................................................25 Programming System Input Undervoltage Hard-Stop (SS1, SS2) .........................................26 Lockout (UVLO) Threshold and Hysteresis ......... 15 OUT, AOUT, SOUT Pulse-Skipping Mode ............27 Soft-Stop Shutdown ........................................... 15 AOUT Timeout ....................................................27 Micropower Shutdown ....................................... 15 Main Transformer Selection ...............................27 Programming System Input Overvoltage Generating Auxiliary Supplies .............................28 Lockout (OVLO) Threshold ................................. 15 Primary-Side Auxiliary Supply ............................29 Programming Switching Frequency .................... 16 Secondary-Side Auxiliary Supply .......................29 Synchronizing to an External Clock .................... 16 Primary-Side Power MOSFET Selection .............30 INTV Regulator Bypassing and Operation ...... 16 Synchronous Control (SOUT) .............................31 CC Adaptive Leading Edge Blanking Plus Output Inductor Value .........................................32 Programmable Extended Blanking...................... 17 Output Capacitor Selection .................................32 Current Sensing and Programmable Slope Input Capacitor Selection ...................................32 Compensation .................................................... 18 PCB Layout/Thermal Guidelines ........................33 Overcurrent: Hiccup Mode .................................. 18 Package Description ..................................... 35 Programming Maximum Duty Cycle Clamp: Revision History .......................................... 35 D (Volt-Second Clamp) ................................ 18 Typical Application ....................................... 36 VSEC Related Parts .............................................. 36 Rev. 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