LT1681 Dual Transistor Synchronous Forward Controller FEATURES DESCRIPTIO n High Voltage: Operation Up to 72V The LT 1681 controller simplifies the design of high power n Synchronizable Operating Frequency and Output synchronous dual transistor forward DC/DC converters. The Switch Phase for Multiple Controller Systems part employs fixed frequency current mode control and n Fixed Frequency Operation to 350kHz supports both isolated and nonisolated topologies. The IC n Adaptive and Adjustable Blanking drives external N-channel power MOSFETs and operates with n Synchronous Rectifier Driver input voltages up to 72V. n Local 1% Voltage Reference The LT1681s operating frequency is programmable and can n Undervoltage Lockout Protection with Hysteresis be synchronized up to 350kHz. Switch phase is also con- n Input Overvoltage Protection trolled during synchronized operation to accommodate mul- n Programmable Start Inhibit tiple converter systems. Internal logic guarantees 50% maxi- n Transformer Primary Saturation Protection mum duty cycle operation to prevent transformer saturation. n Optocoupler Feedback Support The LT1681 incorporates a soft-start feature that provides a n Soft-Start Control controlled increase in supplied current during start-up and U APPLICATIO S after an undervoltage lockout or overvoltage/overcurrent event. n Isolated Telecommunication Systems n The part is available in a 20-lead wide SO package to support Personal Computers and Peripherals n high voltage pin-to-pin clearance. Lead Acid Battery Backup Systems n , LTC and LT are registered trademarks of Linear Technology Corporation. Automotive and Heavy Equipment TYPICAL APPLICATIO 36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint) V = 5V OUT L1 L2 I = 7A OUT 4.7H 4.1H + + 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 8 100V 10 1nF Q6 10 11 100V 4 0.25W C3 Q3 12 0.025 1.5F 1/2W 9 100V 4.7 Q5 T1 V IN V OUT C2:SANYO 100MV22AX ZVN3310F C3, C4: VITRAMON VJ1825Y155MXB BAT54 C5: 4X KEMET T510X337KO10AS L1: COILCRAFT DO1608C-472 L2: PANASONIC ETQP6F4R1LF4 1OV 10k 2k Q1,Q3:100V SILICONIX SUD40N10-25 BIAS 100 1OV 0.22F Q5,Q6: SILICONIX Si4450 0.1F BIAS FZT690 330pF 50V MMBD914LT1 100V T1:COILTRONICS VP5-1200 CMPZ5242B Q10: ON SEMI MMBT3906LTI 73.2k 4.7F 12V 1% 270k 5V 16V BAS21 0.25W OUT 20k 20 17 19 18 16 11 12 15 14 3.3 LTC1693-2 V V BLKSENS TG BSTREF BG SENSE T PGND 6 CC BST MAX 8 13 CMPZ- SG V V 2 CC1 CC2 CMPZ5248B 0.047F OVLO LT1681 5248B 3 5 18V 9 IN2 OUT2 15V 1 V 5V F THERM SYNC SGND SS V FB SHDN REF SET C 1 7 1681 TA01 IN1 OUT1 51 3.01k 5 6 37 4 8 10 4 2 0.1F 1% 10k 56k 52.3k GND2 GND1 100 Q10 + 68F 1.24k 1nF 24k 1F 150pF 4.7nF 1k 1% 20V 0.01F 1% 1681f 1 U ULT1681 ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) ORDER PART Supply Voltages NUMBER TOP VIEW Power Supply (V )............................. 0.3V to 20V CC Topside Supply (V )................... V 0.3V to SHDN 1 20 BST BSTREF V BST LT1681ESW V + 20V (V = 90V) OVLO 2 19 TG BSTREF BST(MAX) LT1681ISW THERM 3 18 Topside Reference Pin (V ).......... 0.6V to 75V BSTREF BSTREF SGND 4 17 BLKSENS Input Voltages 5V 5 16 BG REF SHDN Pin .................................. 0.3V to V + 0.3V CC FSET 6 15 PWRGND All Other Inputs ..................... 0.3V to 5V + 0.3V REF SYNC 7 14 V CC Maximum Currents SS 8 13 SG 5V Pin........................................ 85mA to 10mA REF V 9 12 I FB MAX FSET Pin ............................................. 2mA to 5mA V 10 11 SENSE C All Other Inputs .................................. 2mA to 2mA SW PACKAGE Operating Ambient Temperature Range 20-LEAD PLASTIC SO LT1681E (Note 4) .............................. 40 C to 85 C T = 125 C, q = 85 C/ W JMAX JA LT1681I ............................................. 40 C to 85 C Storage Temperature Range ................ 65 C to 150 C Lead Temperature (Soldering, 10 sec)................. 300 C Consult LTC Marketing for parts specified with wider operating temperature ranges. 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 Operating Supply Voltage Range l 912 18 V CC I DC Active Supply Current (Note 2) 17 22 mA CC l 25 mA DC Active UVL Supply Current V > 1.35V, V = 8V l 800 1200 m A SHDN CC DC Standby Supply Current V < 0.3V 0.5 m A SHDN I DC Active Supply Current TG Logic High (Note 2) l 5 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 l 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 Undervoltage Lockout Threshold Falling Edge l 8.0 8.40 8.60 V CCUVLO Rising Edge l 8.3 8.75 8.95 V Undervoltage Lockout Hysteresis l 0.25 0.35 V 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 1681f 2 U W U U WWW