LTC3705| Datasheet

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LTC3705 2-Switch Forward Controller and Gate Driver

FEATURES

DESCRIPTIO

High-Speed Top and Bottom Gate Drivers for 2-Switch Forward Converter On-Chip Rectifier and Self-Starting Architecture Eliminate Need for Separate Gate Drive Bias Supply Wide Input Voltage Supply Range: 18V to 80V Tolerant of 100V Input Voltage Transients Linear Regulator Controller for Fast Start-Up Precision UVLO with Adjustable Hysteresis Overcurrent Protection Volt-Second Limit Prevents Transformer Core Saturation Voltage Feedforward for Fast Transient Response Available in 16-Lead Narrow SSOP Package

The LTC 3705 is a controller for a 2-switch forward converter and includes on-chip bottom and top gate drivers that do not require external transformers. For secondary-side control, combine the LTC3705 with the LTC3706 PolyPhase secondary-side synchronous forward controller to create a complete forward converter using a minimum of discrete parts. A proprietary scheme is used to multiplex gate drive signals across the isolation barrier through a tiny pulse transformer. The on-chip rectifier and the same pulse transformer provide gate drive bias power. Alternatively, the LTC3705 can be used as a standalone voltage mode controller in a primary-side control architecture with optoisolator feedback. Voltage feedforward provides excellent line regulation and transient response.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. PolyPhase is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Patent Pending

APPLICATIO S

Isolated 48V Telecommunication Systems Internet Servers and Routers Distributed Power Step-Down Converters Automotive and Heavy Equipment

TYPICAL APPLICATIO
VIN+

36V 72V to 3.3V/20A Isolated 2-Switch Forward Converter
MURS120 T1 Si7852DP 1uF 100V x3 1.2 L1 1.2uH VOUT+

Si7336ADP 2 Si7336ADP CMPSH1-4 330uF 6.3V 3

Si7852DP

MURS120 30m 1W 2m 2W CZT3019 FQT7N10 100k BAS21 0.22uF L1: COILCRAFT SER2010-122 T1: PULSE PA0807 T2: PULSE PA0297 1uF T2 IS IS+ PT + 2.2uF FG SW SG VIN NDRV VCC FS/SYNC FB LTC3706 ITH 680pF 102k 10uF VOUT

VIN

365k

NDRV UVLO VCC 2.2uF SS/FLT

BOOST TG TS BG IS FB/IN+ LTC3705 FS/IN GND PGND VSLMT 162k

15k

PT RUN/SS GND PGND PHASE SLP MODE REGSD 33nF

33nF

U
20k 22.6k
3705 TA01

U

U

3705fb

1

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LTC3705 DATASHEET