LT3781 “Bootstrap” Start Dual Transistor Synchronous Forward Controller UFEATURESDESCRIPTIO ■ High Voltage Operation up to 72V The LT®3781 controller simplifies the design of high ■ Synchronizable Operating Frequency and Output power synchronous dual transistor forward DC/DC con- Switch Phase for Multiple Controller Systems verters. The part employs fixed frequency current mode ■ Synchronous Switch Output control and supports both isolated and nonisolated to- ■ Undervoltage Lockout Protection with 6V Hysteresis pologies. The IC drives external N-channel power MOSFETs for Self-Biased Power and operates with input voltages up to 72V. ■ Fixed Frequency Operation to 350kHz The LT3781 is ideal for output derived power schemes, ■ Local ±1% Voltage Reference through the use of a large undervoltage lockout hysteresis ■ Input Overvoltage Protection range. The part is also equipped with an 18V VCC shunt ■ Low Start-Up Current regulator, which prevents exceeding absolute maximum ■ Programmable Start Inhibit for Power Supply ratings while in trickle start applications. Sequencing and Protection ■ Optocoupler Support The LT3781’s operating frequency is programmable and ■ can be synchronized up to 350kHz. Switch phase is also U Soft-Start Control controlled during synchronized operation to accommo- APPLICATIO S date multiple-converter systems. Internal logic guaran- ■ Isolated Telecommunication Systems tees 50% maximum duty cycle operation to prevent trans- ■ Personal Computers and Peripherals former saturation. ■ Distributed Power Step-Down Converters The LT3781 is available in a 20-lead SSOP package. ■ Lead Acid Battery Backup Systems , LTC and LT are registered trademarks of Linear Technology Corporation. ■ Automotive and Heavy Equipment UTYPICAL APPLICATIO36V-72V to 5V/7A DC/DCSynchronous Forward Converter (Quarter-Brick Footprint) L1 L2 VOUT = 5V T1 4.7µH 4.7µH I • OUT = 7A V + + 6 IN VOUT Q1 1 7 2 10Ω MURS120T3 3 0.25W + + C2 C5 C4 • • MBR- 5 22µF 330µF 1.5µF 1nF 0540T1 MURS120T3 100V 10V 100V 8 100V 10 1nF Q6 10 11 Ω R9 • 100V 4 C3 Q3 0.25W 12 0.025Ω 1.5µF 1/2W 9 100V 4.7Ω Q5 V – – IN VOUT ZVN3310F BAT54 1OV 10k BIAS 1OV 100Ω 2k 0.1µF BIAS FZT690 0.22µF MMBD914LT1 330pF 50V 100V 270k 73.2k CMPZ5242B 4.7µF 0.25W 1% 12V 5V 16V BAS21 20k OUT 20 19 18 15 11 14 13 V LTC1693-2 CC V TG BSTREF BG SENSE PGND 3.3k BST 12 8 6 2 SG VCC1 VCC2 OVLO LT3781 0.047µF 3 5 1 9 IN2 OUT2 SHDN 5VREF FSET THERM SYNC SGND SS VC VFB 1 7 IN1 OUT1 1nF 5 6 3 7 4 8 10 3.01k 51Ω 0.1µF 1.24k 4 2 52.3k 1% 1% 100Ω GND2 GND1 C2:SANYO 100MV22AX L2: PANASONIC ETQP6F4R1LF4 6.8k + 68µF C3, C4: VITRAMON VJ1825Y155MXB Q1,Q3:100V SILICONIX SUD40N10-25 10k 1µF 150pF 20V 4.7nF 1k C5: 4X KEMET T510X337KO10AS Q5,Q6: SILICONIX Si4450 0.01µF 1% L1: COILCRAFT DO1608C-472 T1:COILTRONICS VP5-1200 3781f 1
