LT3781 Bootstrap Start Dual Transistor Synchronous Forward Controller FEATURES DESCRIPTIO n High Voltage Operation up to 72V The LT 3781 controller simplifies the design of high n 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 n Synchronous Switch Output control and supports both isolated and nonisolated to- n 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. n Fixed Frequency Operation to 350kHz The LT3781 is ideal for output derived power schemes, n Local 1% Voltage Reference through the use of a large undervoltage lockout hysteresis n Input Overvoltage Protection range. The part is also equipped with an 18V V shunt CC n Low Start-Up Current regulator, which prevents exceeding absolute maximum n Programmable Start Inhibit for Power Supply ratings while in trickle start applications. Sequencing and Protection n The LT3781s operating frequency is programmable and Optocoupler Support n can be synchronized up to 350kHz. Switch phase is also Soft-Start Control U controlled during synchronized operation to accommo- APPLICATIO S date multiple-converter systems. Internal logic guaran- n Isolated Telecommunication Systems tees 50% maximum duty cycle operation to prevent trans- n Personal Computers and Peripherals former saturation. n Distributed Power Step-Down Converters The LT3781 is available in a 20-lead SSOP package. n Lead Acid Battery Backup Systems , LTC and LT are registered trademarks of Linear Technology Corporation. n Automotive and Heavy Equipment TYPICAL APPLICATIO 36V-72V to 5V/7A DC/DC Synchronous Forward Converter (Quarter-Brick Footprint) L2 V = 5V L1 OUT T1 4.7H I = 7A OUT 4.7H + + 6 V V IN OUT Q1 1 7 10 2 MURS120T3 0.25W 3 + C5 MBR- + C2 C4 5 330F 0540T1 22F 1.5F 1nF MURS120T3 10V 100V 100V 100V 8 10 1nF Q6 10 11 100V R9 4 0.25W C3 Q3 12 0.025 1.5F 1/2W 9 100V 4.7 Q5 V IN V OUT ZVN3310F BAT54 1OV 10k 2k BIAS 100 1OV 0.22F 0.1F BIAS FZT690 50V MMBD914LT1 100V 330pF CMPZ5242B 270k 73.2k 4.7F 12V 0.25W 1% 5V 16V BAS21 OUT 20k 20 19 18 15 11 14 13 3.3k LTC1693-2 V V TG BSTREF BG SENSE PGND 6 CC BST 8 12 SG V V 2 CC1 CC2 0.047F OVLO LT3781 3 5 9 IN2 OUT2 1 V 5V FSET THERM SYNC SGND SS V FB SHDN REF C 1 7 IN1 OUT1 51 3.01k 5 6 37 4 8 10 1nF 1.24k 2 4 0.1F 1% 52.3k GND2 GND1 1% 100 C2:SANYO 100MV22AX L2: PANASONIC ETQP6F4R1LF4 6.8k + C3, C4: VITRAMON VJ1825Y155MXB Q1,Q3:100V SILICONIX SUD40N10-25 68F 10k 4.7nF 1k 1F 150pF C5: 4X KEMET T510X337KO10AS Q5,Q6: SILICONIX Si4450 20V 0.01F 1% L1: COILCRAFT DO1608C-472 T1:COILTRONICS VP5-1200 3781f 1 U ULT3781 UUW ABSOLUTE AXI U RATI GS PACKAGE/ORDER I FOR ATIO (Note 1) ORDER PART TOP VIEW Power Supply (V ) CC NUMBER Low Impedance Source Voltage ............. 0.3V to 20V 1 20 V SHDN BST Shutdown Mode: 2 19 TG OVLO LT3781EG (Supply Self-Regulates to 18V) 3 18 BSTREF THERM LT3781IG Maximum Input Current ............................... 20mA 4 17 NC SGND Topside Supply (V ) .................................................... 5 16 NC 5V BST REF 6 15 BG V 0.3V to V +20V (V = 90V) FSET BSTREF BSTREF BST(MAX) 7 14 PWRGND Topside Reference Pin (V ) ............... 0.6V to 75V SYNC BSTREF 8 13 V SS SHDN Pin Voltage ........................... 0.3V to V + 0.3V CC CC 9 12 SG V All Other Input Voltages .............. 0.3V to 5V + 0.3V FB REF 10 11 SENSE V C 5V Pin Sink Current ......................................... 10mA REF FSET Pin Current ...................................... 2mA to 5mA G PACKAGE 20-LEAD PLASTIC SSOP All Other Input Pin Currents...................... 2mA to 2mA T = 125 C, q = 90 C/W JMAX JA Operating Ambient Temperature Range (Note 4) ...............40 C to 85 C Consult LTC Marketing for parts specified with wider operating temperature ranges. Operating Junction Temperature Range ...............................40 C to 125 C Storage Temperature Range ..................65 C to 150 C Lead Temperature (Soldering, 10 sec).................. 300 C ELECTRICAL CHARACTERISTICS The l denotes specifications which apply over the full operating temperature range, otherwise specifications are T = 25 C. A V = V = 12V, V = 0V, V = 2V, V = V = 1.25V, C = C = C = 1000pF. CC BST BSTREF VC FB REF TG BG SG SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Supply and Protection V Undervoltage Lockout Threshold Falling Edge l 8.0 8.4 8.6 V CCUVLO Rising Edge l 13 14.5 16 V V Shutdown Mode Shunt Regulator 100m A < I 10mA l 16.5 18 19.9 V CCSHDN VCC I DC Active Supply Current (Note 2) 17 22 mA CC l 25 mA DC Active UVLO Supply Current V = 1.35V, V = 8V l 800 1200 m A SHDN CC DC Standby Supply Current V < 0.3V l 16 30 m A SHDN V DC Active Supply Current TG Logic High (Note 2) l 5.0 8.5 mA BST DC Standby Supply Current V < 0.3V 0.1 m A SHDN V Shutdown Rising Threshold l 1.15 1.25 1.35 V SHDN Shutdown Threshold Hysteresis 100 150 200 mV I Soft-Start Charge Current V = 2V l 14 10 6 m A SS SS V Soft-Start Reset Threshold 225 mV SS V Boost Undervoltage Lockout Falling Edge l 5.7 6.4 7.1 V BSTUVLO (V -BSTREF) Rising Edge l 6.5 7.0 7.5 V BST Boost UVLO Hysteresis l 0.3 0.6 V 5V External Reference V 5V Reference Voltage 0 (I I ) < 20mA 4.85 5.0 5.10 V 5VREF 5VREF VC l 4.80 5.15 V I Short-Circuit Current Source, I = 0 l 20 45 mA 5VREFSC VC R Output Impedance 0 (I I ) < 20mA 1 W 5VREF 5VREF VC 3781f 2 U WWW