LTC3705 2-Switch Forward Controller and Gate Driver FEATURES DESCRIPTIO High-Speed Top and Bottom Gate Drivers for The LTC 3705 is a controller for a 2-switch forward 2-Switch Forward Converter converter and includes on-chip bottom and top gate On-Chip Rectifier and Self-Starting Architecture drivers that do not require external transformers. Eliminate Need for Separate Gate Drive Bias For secondary-side control, combine the LTC3705 with Supply the LTC3706 PolyPhase secondary-side synchronous Wide Input Voltage Supply Range: 18V to 80V forward controller to create a complete forward converter Tolerant of 100V Input Voltage Transients using a minimum of discrete parts. A proprietary scheme Linear Regulator Controller for Fast Start-Up is used to multiplex gate drive signals across the isolation Precision UVLO with Adjustable Hysteresis barrier through a tiny pulse transformer. The on-chip Overcurrent Protection rectifier and the same pulse transformer provide gate drive Volt-Second Limit Prevents Transformer Core bias power. Saturation Alternatively, the LTC3705 can be used as a standalone Voltage Feedforward for Fast Transient Response voltage mode controller in a primary-side control architec- Available in 16-Lead Narrow SSOP Package U ture with optoisolator feedback. Voltage feedforward pro- APPLICATIO S vides excellent line regulation and transient response. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Isolated 48V Telecommunication Systems PolyPhase is a registered trademark of Linear Technology Corporation. All other Internet Servers and Routers trademarks are the property of their respective owners. Patent Pending Distributed Power Step-Down Converters Automotive and Heavy Equipment TYPICAL APPLICATIO 36V 72V to 3.3V/20A Isolated 2-Switch Forward Converter + + V V IN OUT L1 MURS120 1.2H T1 1.2 Si7852DP 330F 1F Si7336ADP 6.3V 100V 2 Si7852DP 3 x3 Si7336ADP CMPSH1-4 MURS120 2m 30m 10F 2W 1W V V IN OUT CZT3019 100k 2.2F 102k FQT7N10 L1: COILCRAFT SER2010-122 0.22F T1: PULSE PA0807 BAS21 T2: PULSE PA0297 FG SW SG V NDRV V IN CC I S FS/SYNC 365k 1F + I NDRV BOOST TG TS BG IS S T2 + + FB UVLO FB/IN PT LTC3706 ITH 680pF V LTC3705 CC 2.2F FS/IN PT SS/FLT RUN/SS GND PGND PHASE SLP MODE REGSD GND PGND VSLMT 20k 22.6k 15k 162k 33nF 33nF 3705 TA01 3705fb 1 U ULTC3705 UUW ABSOLUTE AXI U RATI GS PACKAGE/ORDER I FOR ATIO (Note 1) TOP VIEW ORDER PART Power Supply (V ) ...................................0.3V to 15V CC NUMBER GND 1 16 TS External NMOS Drive (NDRV) ....................0.3V to 20V I 2 15 TG S NDRV to V ........................................................... 0.3V to 5V CC LTC3705EGN V 3 14 BOOST SLMT Bootstrap Supply (BOOST) ......................0.3V to 115V LTC3705IGN UVLO 4 13 NC Top Source (TS) .......................................... -5V to 100V SSFLT 5 12 NC BOOST to TS .............................................0.3V to 15V NDRV 6 11 V CC GN PART Soft-Start Fault, Feedback, + FB/IN 7 10 BG MARKING Frequency Set, Transformer FS/IN 8 9 PGND + Inputs (SSFLT, FB/IN , FS/IN ) ..................0.3V to 15V 3705 GN PACKAGE All Other Pins (V , I , UVLO) .................0.3V to 5V SLMT S 16-LEAD NARROW PLASTIC SSOP 3705I T = 125C, = 110C/W Peak Output Current <1s (TG, BG)........................... 2A JMAX JA Operating Ambient Temperature Range .. 40C to 85C Order Options Tape and Reel: Add TR Operating Junction Temperature (Note 2) ............ 125C Lead Free: Add PBF Lead Free Tape and Reel: Add TRPBF Storage Temperature Range ................. 65C to 150C Lead Free Part Marking: