LTC1698 Isolated Secondary Synchronous Rectifier Controller UFEATURESDESCRIPTIO ■ High Efficiency Over Wide Load Current Range The LTC®1698 is a precision secondary-side forward ■ ± 0.8% Output Voltage Accuracy converter controller that synchronously drives external ■ Dual N-Channel MOSFET Synchronous Drivers N-channel MOSFETs. It is designed for use with the ■ Pulse Transformer Synchronization LT®3781 primary-side synchronous forward converter ■ Optocoupler Feedback Driver controller to create a completely isolated power supply. ■ Programmable Current Limit Protection The LT3781 synchronizes the LTC1698 through a small ■ ±5% Margin Output Voltage Adjustment pulse transformer and the LTC1698 drives a feedback ■ Adjustable Overvoltage Fault Protection optocoupler to close the feedback loop. Output accuracy ■ Power Good Flag of ±0.8% and high efficiency over a wide range of load ■ Auxiliary 3.3V Logic Supply currents are obtained. ■ Available in 16-Lead SSOP and SO Packages The LTC1698 provides accurate secondary-side current UAPPLICATIO S limit using an external current sense resistor. The input voltage at the MARGIN pin provides ±5% output voltage ■ adjustment. A power good flag and overvoltage input are 48V Input Isolated DC/DC Converters ■ provided to ensure proper power supply conditions. An Isolated Telecommunication Power Systems ■ auxiliary 3.3V logic supply is included that supplies up to Distributed Power Step-Down Converters ■ 10mA of output current. Industrial Control Systems ■ Automotive and Heavy Equipment , LTC and LT are registered trademarks of Linear Technology Corporation. UTYPICAL APPLICATIO L1 VIN 36V to 72V + VOUT R2 COUT Q1 R1 VDD BIAS + D1 • • 1 10 T1 V R R5 DD PWRGD C 2 8 Q4 CG VFB CC Q3 16 Q2 D2 FG LTC1698 CFB 6 R 12 PRISEN I VCOMP SNS RSECSEN CCILM RCILM 11 13 ISNSGND ICOMP CSG CSYNC 15 9 TG BG SG SYNC OVPIN • • R4 T2 RSYNC 7 MARGIN VMARGIN LT3781 RK 5 14 VAUX OPTODRV VAUX 3.3V CK 10mA + REF PGND GND O.1µF VFB 3 4 – 1681 F01 VC RE RF CF ISOLATION PLEASE REFER TO FIGURE 12 IN THE TYPICAL APPLICATIONS BOUNDARY SECTION FOR THE COMPLETE 3.3V/15A APPLICATION SCHEMATICFigure 1. Simplified 2-Transistor Isolated Forward Converter 1698f 1
