LT1103/LT1105 Offline Switching Regulator THE LT1103 IS OBSOLETE: THE LT1105 IS AVAILABLE, BUT NOT RECOMMENDED FOR NEW DESIGNS FOR INFORMATION PURPOSES ONLY Contact Analog Devices for Potential Replacement FEATURES n n n n n n n n n n n DESCRIPTION 1% Line and Load Regulation with No Optocoupler Switch Frequency Up to 200kHz Internal 2A Switch and Current Sense (LT1103) Internal 1A Totem-Pole Driver (LT1105) Start-Up Mode Draws Only 200A Fully Protected Against Overloads Overvoltage Lockout of Main Supply Protected Against Underdrive or Overdrive to FET Operates in Continuous or Discontinuous Mode Ideal for Flyback and Forward Topologies Isolated Flyback Mode Has Fully Floating Outputs APPLICATIONS n n n n Up to 250W Isolated Mains Converter Up to 50W Isolated Telecom Converter Fully Isolated Multiple Outputs Distributed Power Conversion Networks The LT(R)1105 Offline Switching Regulator is designed for high input voltage applications using an external FET switch. The LT1105 is available and its totem pole output drives the gate of an external FET. Unique design of the LT1105 eliminates the need for an optocoupler while still providing 1% load and line regulation in a magnetic flux-sensed converter. This significantly simplifies the design of offline power supplies and reduces the number of components which must cross the isolation barrier to one, the transformer. The LT1105 current mode switching techniques are well suited to transformer isolated flyback and forward topologies while providing ease of frequency compensation with a minimum of external components. All registered trademarks and trademarks are the property of their respective owners. TYPICAL APPLICATION Fully Isolated Flyback 100kHz 50W Converter with Load Regulation Compensation OPTIONAL OUTPUT FILTER MBR2045 85VAC TO 270VAC + 220F 385V - 220k 1W 499 BRIDGE RECTIFIER + LINE FILTER BAV21 BAV21 13k 1% 1N4148 39F 35V 15V VC Load Regulation 5.25 5.20 5.15 10 18.7k + OSC 0.047F 390pF 1F 25V TRANSFORMER DATA: COILTRONICS CTX110228-3 L(PRI) = 1.6mH NPRI:NSEC = 1:0.05 NBIAS:NSEC = 1:0.27 5.10 220VAC 5.05 5.00 4.95 85VAC 110VAC 270VAC 4.90 330 0.047F 50V 470F WINDINGS FOR OPTIONAL 12VDC OUTPUTS BUK426-800A LT1103 FB GND 4.75k 1% VSW VIN + + *50V 3600F 5V 10A *OUTPUT CAPACITOR IS THREE 1200F, 50V CAPACITORS IN PARALLEL TO ACHIEVE REQUIRED RIPPLE CURRENT RATING AND LOW ESR. 1N4148 1000pF 100 + MUR150 10H VOUT (V) + !! GER ! DAN LTAGE! O V HIGH 1.5KE300A 5W 4.85 0.1F 4.80 LT1103 TA13 Danger!! Lethal Voltages Present - See Text 4.75 0 1 2 3 4 5 IOUT (A) 6 7 8 9 10 LT1103 TA02 Rev. E Document Feedback For more information www.analog.com 1 LT1103/LT1105 DESCRIPTION WARNING 200kHz maximum switching frequency to achieve high power density. Performance at switching frequencies above 100kHz may be degraded due to internal timing constraints associated with fully isolated flyback mode. DANGEROUS AND LETHAL POTENTIALS ARE PRESENT IN OFFLINE CIRCUITS! BEFORE PROCEEDING ANY FURTHER, THE READER IS WARNED THAT CAUTION MUST BE USED IN THE CONSTRUCTION, TESTING AND USE OF OFFLINE CIRCUITS. HIGH VOLTAGE, AC LINE-CONNECTED POTENTIALS ARE PRESENT IN THESE CIRCUITS. EXTREME CAUTION MUST BE USED IN WORKING WITH AND MAKING CONNECTIONS TO THESE CIRCUITS. REPEAT: OFFLINE CIRCUITS CONTAIN DANGEROUS, AC LINE-CONNECTED HIGH VOLTAGE POTENTIALS. USE CAUTION. ALL TESTING PERFORMED ON AN OFFLINE CIRCUIT MUST BE DONE WITH AN ISOLATION TRANSFORMER CONNECTED BETWEEN THE OFFLINE CIRCUIT'S INPUT AND THE AC LINE. USERS AND CONSTRUCTORS OF OFFLINE CIRCUITS MUST OBSERVE THIS PRECAUTION WHEN CONNECTING TEST EQUIPMENT TO THE CIRCUIT TO AVOID ELECTRIC SHOCK. REPEAT: AN ISOLATION TRANSFORMER MUST BE CONNECTED BETWEEN THE CIRCUIT INPUT AND THE AC LINE IF ANY TEST EQUIPMENT IS TO BE CONNECTED. Included are the oscillator, control, and protection circuitry such as current limit and overvoltage lockout. Switch frequency and maximum duty cycle are adjustable. Bootstrap circuitry draws 200A for start-up of isolated topologies. A 5V reference as well as a 15V gate bias are available to power external primary-side circuitry. No external current sense resistor is necessary with LT1103 because it is integrated with the high current switch. The LT1105 brings out the input to the current limit amplifier and requires the use of an external sense resistor. The LT1103/LT1105 have unique features not found on other offline switching regulators. Adaptive antisat switch drive allows wide ranging load currents while maintaining high efficiency. The external FET is protected from insufficient or excessive gate drive voltage with a drive detection circuit. An externally activated shutdown mode reduces total supply current to less than 200A, typical for standby operation. Fully isolated and regulated outputs can be generated in the optional isolated flyback mode without the need for optocouplers or other isolated feedback paths. ABSOLUTE MAXIMUM RATINGS (Note 1) VIN ............................................................................30V VSW Output Voltage (LT1103)....................................50V VSW Output Current (200ns)(LT1105) .................... 1.5A VC, FB, OSC, SS ..........................................................6V ILIM (LT1105)...............................................................3V OVLO Input Current..................................................1mA Lead Temperature (Soldering, 10 sec.).................. 300C Maximum Operating Ambient Temperature Range LT1103C (OBSOLETE).............................. 0C to 70C LT1105C ................................................... 0C to 70C Maximum Operating Temperature Range LT1103C (OBSOLETE)............................ 0C to 100C LT1105C ................................................. 0C to 100C LT1105I .............................................. -40C to 125C Storage Temperature Range .................. -65C to 150C Rev. E 2 For more information www.analog.com LT1103/LT1105 PIN CONFIGURATION LT1105 LT1105 TOP VIEW PWRGND 1 14 VSW OVLO 2 13 NC FB 3 12 NC VC 4 11 15V 5V 5 10 VIN SS 6 9 OSC GND 7 8 ILIM LT1103 FRONT VIEW TOP VIEW GND 1 8 VSW ILIM 2 7 15V FB 3 6 VIN VC 4 5 OSC N8 PACKAGE 8-LEAD PDIP TJMAX = 100C, JA = 130C/W N PACKAGE 14-LEAD PDIP PINS 1 AND 7 MUST BE TIED TOGETHER TJMAX = 100C, JA = 100C/W 7 6 5 4 3 2 1 15V VIN OSC GND VC FB VSW T7 PACKAGE 7-LEAD TO-220 CASE IS CONNECTED TO GROUND. LEADS ARE FORMED TJMAX = 100C, JA = 50C/W OBSOLETE PACKAGE ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT1105CN#PBF LT1105CN#TRPBF LT1105CN 14-Lead PDIP 0C to 100C LT1105IN#PBF LT1105IN#TRPBF LT1105IN 14-Lead PDIP -40C to 125C LT1105CN8#PBF LT1105CN8#TRPBF LT1105CN8 8-Lead PDIP 0C to 100C LT1105IN8#PBF LT1105IN8#TRPBF LT1105IN8 8-Lead PDIP -40C to 125C OBSOLETE PACKAGE LT1103CT7#PBF LT1103CT7#TRPBF LT1103CT7 7-Lead TO-220 0C to 100C Contact the factory for parts specified with wider operating temperature ranges. Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 20V, VC = 0.85V, OVLO = 0V, VSW Open, unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN IQ Supply Current 8V < VIN < 30V, After Device Has Started l ISTART Start-Up Current VIN < VIN Start Threshold Industrial Grade l l VIN Start Threshold VIN Shutdown Threshold VREF Note: Switching Stops When VSW < 10V (LT1103) Note: Switching Stops When VGATE < 10V (LT1105) 5V Reference Voltage 10 TYP MAX UNITS 20 30 mA 200 400 450 A A l 14.5 16.0 17.5 V l 5.0 7.0 8.0 V l 4.80 4.95 5.20 V VREF Line Regulation 10V < VIN < 30V l 0.025 0.1 %V VREF Load Regulation 0mA < IL < 20mA l 0.025 0.05 %mA VREF Short-Circuit Current Commercial Grade Industrial Grade l l 25 20 60 110 120 mA mA 15V Short-Circuit Current Commercial Grade Industrial Grade l l 30 25 130 140 mA mA Rev. E For more information www.analog.com 3 LT1103/LT1105 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 20V, VC = 0.85V, OVLO = 0V, VSW Open, unless otherwise noted. SYMBOL PARAMETER VGATE CONDITIONS 15V Gate Bias Reference 17 < VIN < 30V, 0mA < IL < 30mA l 15V Dropout Voltage VIN = 15V, IL = 30mA l 15V Short-Circuit Current SF DC VFB gm AV Oscillator Scaling Factor FB = 4V, VC = Open, Measured at VSW, ISW = 25mA, OVLO = 5V, fOSC = SF/COSC, 40kHz < fOSC < 200kHz TYP MAX UNITS 13.8 15.0 16.2 V 2.0 2.5 V l 30 70 130 mA l 36 32 40 40 44 48 Hz * F Hz * F Oscillator Valley Voltage 2.0 V Oscillator Peak Voltage 4.5 V Preset Max Switch Duty Cycle (LT1103) FB = 4V, VC = Open, fOSC = 40kHz, ISW = 25mA, Note: Maximum Duty Cycle Can Be Altered at OSC Pin l 58 65 72 % Preset Max Switch Duty Cycle (LT1105) FB = 4V, VC = Open, fOSC = 40kHz, ISW = 25mA, Note: Maximum Duty Cycle Can Be Altered at OSC Pin Industrial Grade l 56 63 70 % l 55 75 % OVLO Threshold Overvoltage Lockout Threshold at Which Switching is Inhibited Industrial Grade l l 2.3 2.2 2.5 2.7 2.8 V V OVLO Input Bias Current OVLO = 2V, Measured Out of Pin (Note 2) l 1.0 3.0 A FB Threshold Voltage I(VC) = 0mA l 4.425 4.400 4.50 4.50 4.575 4.600 V V l l 5 4 10 20 22 A A Industrial Grade l l 8 7 6 11 11 13 14 15 A/V A/V A/V FB Threshold Line Regulation 10V < VIN < 30V l 0.025 0.10 %/V Error Amp Transconductance I(VC) = 50A 17500 20000 24000 mho mho mho FB Input Bias Current FB = VFB (Note 3) Industrial Grade Change in FB Input Bias Current with Change in VC FB = VFB, VC = 1V to 4V (Note 3) l l 9000 6000 5000 12000 12000 Error Amp Voltage Gain 1V < VC < 3V Industrial Grade l l 500 450 1250 VC Switching Threshold Switch Duty Cycle = 0% l 0.85 1.25 1.4 mA Industrial Grade l l 50 50 150 250 300 mV mV l 150 275 Industrial Grade l l 1.5 0.7 3 4.5 4.5 mA mA Error Amp Clamp Voltage FB = 4.75V FB = 4.0V l l 0.3 4.2 0.7 4.4 0.9 4.6 V V Soft-Start Charging Current SS = 0V Industrial Grade l l 25 20 40 60 75 A A Soft-Start Reset Current VIN = 6V, SS = 0.3V l 1 2 Output Switch Leakage (LT1103) VSW = 45V VSW = 15V l l Switch Breakdown Voltage (LT1103) ISW = 5mA l 50 70 VSW Current Limit (LT1103) Duty Cycle = 25% (Note 4) l 2.0 2.5 3.0 A 0.4 0.75 Shutdown Threshold Voltage Error Amp Source Current Error Amp Sink Current BV MIN Output Switch On Resistance (LT1103) l V/V V/V A mA 500 200 A A V Rev. E 4 For more information www.analog.com LT1103/LT1105 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 20V, VC = 0.85V, OVLO = 0V, VSW Open, unless otherwise noted. SYMBOL PARAMETER CONDITIONS IIN ISW IQ Increase During Switch On Time (LT1103) ISW = 0.5A to 1.5A l MIN Switch Output High Level (LT1105) Switch Output High Level Industrial Grade ISW = 200mA, VGATE = 15V ISW = 750mA, VGATE = 15V ISW = 200mA, VGATE = 15V ISW = 750mA, VGATE = 15V l l l l Switch Output Low Level (LT1105) ISW = 200mA ISW = 750mA l l 13.00 12.50 12.75 12.25 TYP MAX UNITS 30 50 mA/A 13.5 13.2 0.25 0.75 V V V V 0.50 1.50 V V Rise Time (LT1105) CL = 1000pF 50 ns Fall Time (LT1105) CL = 1000pF 20 ns ILIM Threshold Voltage (LT1105) Duty Cycle = 25% (Note 5) l 300 375 450 mV Low Switch Drive Lockout Threshold Measured at VSW (LT1103) Measured at 15V Gate Bias Reference (LT1105) l 9.0 9.5 10.5 V High Switch Drive Lockout Threshold Measured at VSW (LT1103) Measured at 15V Gate Bias Reference (LT1105) l 17.0 18.5 20.0 V Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The OVLO pin is clamped with a 5.5V Zener and can sink a maximum input current of 1mA. Note 3: FB input bias current changes as a function of the VC pin voltage. Rate of change of FB input bias current is 11A/V of change on VC. By including a resistor in series with the FB pin, load regulation can be set to zero. Note 4: Current limit on VSW is constant for DC < 35% and decreases for DC > 35% due to internal slope compensation circuity. The LT1103 switch current limit is given by ILIM = 1.76 (1.536 - DC) above 35% duty cycle. Note 5: The current limit threshold voltage is constant for DC < 35% and decreases for DC > 35% due to internal slope compensation circuitry. The LT1105 switch current limit threshold voltage is given by VLIM = 0.225 (1.7 - DC) above 35% duty cycle. TYPICAL PERFORMANCE CHARACTERISTICS Start-Up Supply Current vs Input Voltage Supply Current vs Input Voltage 25C 20 15 10 5 ISHUT 0 0 5 ISTART 10 15 20 INPUT VOLTAGE (V) 25 30 LT1103 G01 500 22 450 21 QUIESCENT SUPPLY CURRENT (mA) START-UP SUPPLY CURRENT (A) SUPPLY CURRENT (mA) 25 Quiescent Supply Current vs Input Voltage 400 350 300 25C 125C 250 200 -55C 150 100 50 0 0 3 9 6 INPUT VOLTAGE (V) 12 15 LT1103 G02 20 125C 19 25C 18 17 16 -55C 15 14 13 12 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40 LT1103 G03 Rev. E For more information www.analog.com 5 LT1103/LT1105 TYPICAL PERFORMANCE CHARACTERISTICS Shutdown Supply Current vs Input Voltage 500 500 21 450 450 400 400 30V 20 19 18 17 8V 16 15 14 13 12 -75 -50 -25 350 300 250 VC = 75mV 200 150 VC = 0 100 50 0 0 25 50 75 100 125 150 175 TEMPERATURE (C) SHUTDOWN CURRENT (A) 22 SHUTDOWN SUPPLY CURRENT (A) QUIESCENT SUPPLY CURRENT (mA) Quiescent Supply Current vs Temperature 250 0 5 20 15 10 25 INPUT VOLTAGE (V) 30 15.5 15.0 45 7.7 7.4 7.1 6.8 LT1103 G07 3.0 100 10 0 25 50 75 100 125 150 TEMPERATURE (C) LT1103 G09 OVERVOLTAGE LOCKOUT THRESHOLD (V) SWITCH FREQUENCY (kHz) LT1103 G10 37 35 -75 -50 -25 1000 60 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 39 Overvoltage Lockout Threshold vs Temperature COSC = 1000pF 63 COSC = 1000pF 41 Switch Oscillator Frequency vs Capacitance 75 66 20 40 60 80 100 120 140 160 180 200 VC (mV) 43 LT1103 G08 Preset Switch Maximum Duty Cycle vs Temperature 69 0 Output Switch Frequency vs Temperature 6.5 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 0 25 50 75 100 125 150 175 TEMPERATURE (C) 72 -55C LT1103 G06 OUTPUT SWITCH FREQUENCY (kHz) VIN SHUTDOWN THRESHOLD (V) VIN START-UP THRESHOLD (V) 16.0 125C 150 100 0 35 8.0 16.5 25C 200 VIN Shutdown Threshold vs Temperature 17.5 PRESET SWITCH DUTY CYCLE (%) 300 LT1103 G05 VIN Start-Up Threshold vs Temperature 14.5 -75 -50 -25 350 50 LT1103 G04 17.0 Shutdown Supply Current vs VC Voltage 100 1000 CAPACITANCE (pF) 10000 LT1103 G11 2.8 2.6 2.4 2.2 2.0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G12 Rev. E 6 For more information www.analog.com LT1103/LT1105 TYPICAL PERFORMANCE CHARACTERISTICS OVLO Input Bias Current vs Temperature Soft-Start Charging Current vs Temperature SOFT-START CHARGING CURRENT (A) OVLO INPUT BIAS CURRENT (A) 5 60 OVLO = 2V -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -75 -50 -25 SOFT-START RESET CURRENT (mA) 0 Soft-Start Reset Current vs Temperature 50 40 30 20 10 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 3 2 1 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G13 4 LT1103 G14 5V Reference Voltage vs Temperature LT1103 G15 5V Load Regulation vs Temperature 5.20 5V Line Regulation vs Temperature 0.025 0.05 0.020 0.04 5.10 5.05 5.00 4.95 4.90 5V LINE REGULATION (%/V) 5V LOAD REGULATION (%/mA) 5V REFERENCE VOLTAGE (V) 5.15 0.015 0.010 0.005 0.03 0.02 0.01 4.85 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 4.80 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G17 LT1103 G16 LT1103 G18 15V Gate Bias Reference vs Temperature 110 15V Gate Bias Dropout Voltage vs Temperature 2.5 100 90 80 70 60 50 40 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G19 15V GATE BIAS DROPOUT VOLTAGE (V) 16.2 15V GATE BIAS REFERENCE (V) 5V REFERENCE SHORT-CIRCUIT CURRENT (mA) 5V Reference Short-Circuit Current vs Temperature 30 -75 -50 -25 0 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 15.8 15.4 15.0 14.6 14.2 13.8 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G20 2.0 1.5 1.0 0.5 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G21 Rev. E For more information www.analog.com 7 LT1103/LT1105 TYPICAL PERFORMANCE CHARACTERISTICS Low Switch Drive Lockout Threshold vs Temperature 110 90 70 50 30 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 10.5 10.2 9.9 9.6 9.3 9.0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G22 19.5 19.0 18.5 18.0 17.5 17.0 -75 -50 -25 LT1103 G24 20 4.56 16 4.52 4.48 4.44 14 CHANGE IN FB INPUT BIAS CURRENT WITH CHANGE IN VC (A/V) 4.60 13 12 12 11 10 8 4 4.40 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G25 5000 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G28 LT1103 G27 0.020 Error Amplifier Voltage Gain vs Temperature 2500 200 PHASE 0.018 0.016 180 160 gm 0.014 140 0.012 120 0.010 100 0.008 80 0.006 60 0.004 40 gm 0.002 0 0.1 20 PHASE 1 0 1000 10 100 FREQUENCY (kHz) LT1103 G29 ERROR AMPLIFIER VOLTAGE GAIN (V/V) 10000 7 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) PHASE (DEGREES) 15000 8 Error Amplifier Transconductance and Phase vs Frequency ERROR AMPLIFIER TRANSCONDUCTANCE (mho) 20000 9 LT1103 G26 Error Amplifier Transconductance vs Temperature 25000 0 25 50 75 100 125 150 175 TEMPERATURE (C) Change in FB Input Bias Current with Change in VC vs Temperature (VC = 1V to 4V) FB Input Bias Current vs Temperature (VC = 1V) FB INPUT BIAS CURRENT (A) FEEDBACK THRESHOLD (V) 20.0 LT1103 G23 Feedback Threshold vs Temperature ERROR AMPLIFIER TRANSCONDUCTANCE (mho) High Switch Drive Lockout Threshold vs Temperature HIGH SWITCH DRIVE LOCKOUT THRESHOLD (V) 130 LOW SWITCH DRIVE LOCKOUT THRESHOLD (V) 15V GATE BIAS SHORT-CIRCUIT CURRENT (mA) 15V Gate Bias Short-Circuit Current vs Temperature 2000 1500 1000 500 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G30 Rev. E 8 For more information www.analog.com LT1103/LT1105 TYPICAL PERFORMANCE CHARACTERISTICS Error Amplifier Source Current vs Temperature Error Amplifier Sink Current vs Temperature 325 300 275 250 225 200 175 4.0 3.5 3.0 2.5 2.0 1.5 -75 -50 -25 150 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) ERROR AMPLIFIER HIGH CLAMP VOLTAGE (V) 4.5 ERROR AMPLIFIER SINK CURRENT (mA) 0 25 50 75 100 125 150 175 TEMPERATURE (C) 0.6 0.5 0.4 1.3 1.1 0.9 0.7 0.5 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G34 4.1 4.0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 3.0 LT1103 VSW CURRENT LIMIT (A) ISW = 1.5A 0.6 ISW = 0.5A 0.2 160 VSW = 45V 120 80 VSW = 15V 40 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G36 LT1103 VSW Current Limit vs Duty Cycle 1.2 1.0 200 LT1103 G35 LT1103 Switch Saturation Voltage vs Temperature LT1103 SWITCH SATURATION VOLTAGE (V) 4.2 LT1103 G33 LT1103 OUTPUT SWITCH LEAKAGE CURRENT (A) VC SWITCHING THRESHOLD (V) ERROR AMPLIFIER LOW CLAMP VOLTAGE (V) 0.7 0.4 4.3 LT1103 Output Switch Leakage Current vs Temperature 1.5 0.8 0.8 4.4 VC Switching Threshold Voltage vs Temperature 0.9 0.3 -75 -50 -25 4.5 LT1103 G32 LT1103 G31 Error Amplifier Low Clamp Voltage vs Temperature (FB = 4.75V) LT1103 VSW Current Limit vs Temperature 3.0 -55C 2.5 25C 2.0 125C 2.9 LT1103 VSW CURRENT LIMIT (A) ERROR AMPLIFIER SOURCE CURRENT (A) 350 Error Amplifier High Clamp Voltage vs Temperature (FB = 4V) 1.5 1.0 0.5 DC = 25% 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G38 0 0 10 20 30 40 50 60 DUTY CYCLE (%) 70 80 LT1103 G39 2.0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G40 Rev. E For more information www.analog.com 9 LT1103/LT1105 TYPICAL PERFORMANCE CHARACTERISTICS 40 30 20 10 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) 3.0 2.5 2.0 1.5 ISW = 750mA 1.0 0.5 ISW = 200mA 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1105 Current Limit Threshold Voltage vs Temperature 2.5 ISW = 750mA 2.0 1.5 ISW = 200mA 1.0 0.5 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G43 LT1105 VSW Rise Time vs Temperature 450 LT1105 VSW Fall Time vs Temperature 100 100 80 80 DC = 25C 425 LT1105 VSW RISE TIME (ns) LT1105 CURRENT LIMIT THRESHOLD VOLTAGE (mV) 3.0 LT1103 G42 LT1103 G41 400 375 350 325 300 -75 -50 -25 60 40 LT1105 VSW FALL TIME (ns) LT1103 DRIVER CURRENT (mA /A) 50 LT1105 VSW High Saturation Voltage vs Temperature LT1105 VSW HIGH SATURATION VOLTAGE (V) LT1105 VSW Low Saturation Voltage vs Temperature LT1105 VSW LOW SATURATION VOLTAGE (V) LT1103 Driver Current vs Temperature CLOAD = 4700pF CLOAD = 1000pF 20 60 CLOAD = 4700pF 40 20 CLOAD = 1000pF 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G46 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G44 0 -75 -50 -25 0 25 50 75 100 125 150 175 TEMPERATURE (C) LT1103 G45 Rev. E 10 For more information www.analog.com LT1103/LT1105 PIN FUNCTIONS LT1103 FB: The Feedback pin is the inverting input to the sampling error amplifier. The noninverting input is tied to a 4.5V reference. The FB pin is used for output voltage sensing. The input bias current is a function of the control pin VC voltage and can be used for load regulation compensation by including a resistor in series with the FB pin. The sampling error amplifier has a typical gm of 0.012 mhos and the output of the sampling error amplifier has asymmetrical slew rate to reduce overshoot during start-up conditions or following the release of an output overload. VC: The VC control pin is used for frequency compensation, current limiting and shutdown. It is the high impedance output of the sampling error amplifier and the input of the current limit comparator. GND: The Ground pin acts as both the negative sense point for the internal sampling error amplifier feedback signal and as the high current path for the 2A switch. Also, the case of the 7-lead TO-220 is connected to ground. Proper connections to ground for signal paths and high current paths must be made in order to insure good load regulation. OSC: The Oscillator pin sets the operating frequency of the regulator with one external capacitor to ground. Maximum duty cycle can also be adjusted by using an external resistor to alter the charge/discharge ratio. VIN: The Input Supply pin is designed to operate with voltages of 12V to 30V. The supply current is typically 200A up to the start-up threshold of 16V. Normal operating supply current is fairly flat at 18mA down to the shutdown threshold of 7V. Switching is inhibited for VIN less than 12V due to the gate drive detection circuit. 15V: A 15V reference is used to bias the gate of an external power FET. The voltage temperature coefficient is typically 3mV/C and the output can source 30mA. Typical dropout voltage is 1.5V for VIN less than 17V and 30mA of load current. VSW: The Switch Output pin is the collector of the internal NPN power switch. This pin has a typical ON resistance of 0.4 and a minimum breakdown voltage of 50V. This pin also ties to the FET gate drive detection circuit. LT1105 All functions on the LT1105 are equivalent to the LT1103 with the exception of the VSW pin and the ILIM pin and the availability of the OVLO, 5V, and SS functions. OVLO: The Overvoltage Lockout pin inhibits switching when the pin is pulled above its threshold voltage of 2.5V. OVLO is implemented with a resistor divider network from the rectified DC line and is used to protect the external FET from an overvoltage condition in the off state. This function is only available on the 14-lead PDIP. 5V: A 5V reference is available to power primary-side circuitry. The temperature coefficient is typically 50ppm/C and the output can source 25mA. This function is only available on the 14-lead PDIP. SS: The Soft-Start pin is used to either program start-up time with a capacitor to ground or to set external current limit with a resistor divider. The SS pin has a 40A pull-up current and is reset to 0V by a 1mA pull-down current during start-up and shutdown. This function is only avail-able on the 14-lead PDIP. VSW: The Switch Output pin is the output of a 1A NPN totem-pole stage. The VSW pin turns the external FET on by pulling its gate high. Break-Before-Make action of 200ns on each switch edge is built in to eliminate cross conduction currents. ILIM: The ILIM pin is the input to the current limit amplifier and requires the use of a noninductive, power sense resistor from ILIM to ground to set current limit. The typical current limit threshold voltage is 350mV. The typical input bias current is 100A out of the pin. Rev. E For more information www.analog.com 11 LT1103/LT1105 BLOCK DIAGRAM LT1103 OSC 15V GATE BIAS 15V 16V START-UP VSW GATE BIAS DETECT OSCILLATOR VIN LOGIC DRIVER 7V SPIKE BLANK ANTISAT + CURRENT LIMIT AMP - SAMPLING ERROR AMP gm = 0.012 AV = 10 6V 0.15 - FB + 5V VREF 5V COMP 4.5V 40A 0VLO SHUT DOWN OVERVOLTAGE LOCKOUT 2.5V RESET 0.15V LT1103 BD VC SS GND LT1105 OSC GATE BIAS DETECT 15V GATE BIAS 15V 16V START-UP OSCILLATOR DRIVER VIN LOGIC VSW 7V DRIVER SPIKE BLANK ANTISAT CURRENT LIMIT AMP - 6V SAMPLING ERROR AMP gm = 0.012 FB ILIM AV = 10 - 4.5V + 5V VREF + 5V COMP 40A 0VLO SHUT DOWN 2.5V OVERVOLTAGE LOCKOUT RESET 0.15V LT1105 BD VC SS GND Rev. E 12 For more information www.analog.com LT1103/LT1105 OPERATION LT1103 The LT1103 is a current mode switcher. Switch duty cycle is controlled by switch current rather than directly by the output voltage. Referring to the block diagram, the switch is turned on at the start of each oscillator cycle. It is turned off when switch current reaches a predetermined level. Control of output voltage is obtained by using the output of a voltage sensing error amplifier to set current trip level. This technique has several advantages. First, it has immediate response to input voltage variations, unlike ordinary switchers which have notoriously poor line transient response. Second, it reduces the 90phase shift at mid frequencies in the transformer. This greatly simplifies closed-loop frequency compensation under widely varying input voltage or output load conditions. Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short-circuit conditions. A start-up loop with hysteresis allows the IC supply voltage to be bootstrapped from an extra primary side winding on the power transformer. From 0V to 16V on VIN, the LT1103 is in a prestart mode and total input current is typically 200A. Above 16V, up to 30V, the 6V regulator that biases the internal circuitry and the externally available 15V regulator is turned on. The internal circuitry remains biased on until VIN drops below 7V and the part returns to the prestart mode. Output switching stops when the VSW drive is less than 10V corresponding to VIN of about 12V. The oscillator provides the basic clock for all internal timing. Frequency is adjustable to 200kHz with one external capacitor from OSC to ground. The oscillator turns on the output switch via the logic and driver circuitry. Adaptive antisat circuitry detects the onset of saturation in the power switch and adjusts driver current instantaneously to limit switch saturation. This minimizes driver dissipation and provides very rapid turn-off of the switch. The LT1103 is designed to drive the source of an external power FET in common gate configuration. The 15V regulator biases the gate to guarantee the FET is on when the switch is on. Special drive detection circuitry senses the gate bias voltage and prevents the output switch from turning on if the gate voltage is less than 10V or greater than 20V, the industry standards for power MOSFET operation. The switch current is sensed internally and amplified to trip the comparator and turn off the switch according to the VC pin control voltage. A blanking circuit suppresses the output of the current limit comparator for 500ns at the beginning of each switch cycle. This prevents false tripping of the comparator due to current spikes caused by external parasitic capacitance and diode stored charge. The 4.5V Zener-based reference biases the positive input of the sampling error amplifier. The negative input (FB) is used for output voltage sensing. The sampling error amplifier allows the LT1103 to operate in fully isolated flyback mode by regulating from the flyback voltage of the bootstrap winding. The leakage inductance spike at the leading edge of the flyback waveform is ignored with a blanking circuit. The flyback waveform is directly proportional to the output voltage in a transformer-coupled flyback topology. Output voltages are fully floating up to the breakdown voltage of the transformer windings. Multiple floating outputs are easily obtained with additional windings. The error signal developed at the comparator input is brought out externally. This VC pin has three functions including frequency compensation, current limit adjustment and total regulator shutdown. During normal operation, this pin sits at a voltage between 1.2V (low output current) and 4.4V (high output current). The error amplifier is a current output (gm) type, so this voltage can be externally clamped for adjusting current limit. Switch duty cycle goes to zero if the VC pin is pulled to ground through a diode, placing the LT1103 in an idle mode. Pulling the VC pin below 0.15V causes total regulator shutdown and places the LT1103 in a prestart mode. LT1105 The LT1105 is a current mode switcher. Switch duty cycle is controlled by switch current rather than directly by output voltage. Referring to the block diagram, the switch is turned on at the start of each oscillator cycle. It is turned off when switch current reaches a predetermined level. Rev. E For more information www.analog.com 13 LT1103/LT1105 Control of output voltage is obtained by using the output of a voltage sensing error amplifier to set current trip level. This technique has several advantages. First, it has immediate response to input voltage variations, unlike ordinary switchers which have notoriously poor line transient response. Second, it reduces the 90phase shift at midfrequencies in the transformer. This greatly simplifies closed-loop frequency compensation under widely varying input voltage or output load conditions. Finally, it allows simple pulse-by-pulse current limiting to provide maximum switch protection under output overload or short-circuit conditions. A start-up loop with hysteresis allows the IC supply voltage to be bootstrapped from an extra primary side winding on the power transformer. From 0V to 16V on VIN, the LT1105 is in prestart mode and total input current is typically 200A. Above 16V, up to 30V, the 6V regulator that biases the internal circuitry and the externally available 5V and 15V regulators are turned on. The internal circuitry remains biased on until VIN drops below 7V and the part returns to prestart mode. Output switching stops when the 15V gate bias reference is less than 10V corresponding to VIN of about 12V. The oscillator provides the basic clock for all internal timing. Frequency is adjustable to 200kHz with one external capacitor from OSC to ground. The oscillator turns on the output switch via the logic and driver circuitry. The LT1105 is designed to drive the gate of an external power FET in common source configuration. The drivers and the 1A maximum totem-pole output stage are biased from the 15V gate bias reference. Special drive detection circuity senses the gate bias reference voltage and prevents the output switch from turning on if this voltage is less than 10V or greater than 20V. Break-Before-Make action of 200ns is built into each switch edge to eliminate cross conduction currents. Switch current is sensed externally through a precision, power resistor. This allows for greater flexibility in switch current and output power than allowed by the LT1103. The voltage across the sense resistor is fed into the ILIM pin and amplified to trip the comparator and turn off the switch according to the VC pin control voltage. A blanking circuit suppresses the output of the current limit comparator for 500ns at the beginning of each switch cycle. This prevents false tripping of the comparator due to current spikes caused by external parasitic capacitance and diode stored charge. A 4.5V Zener-based reference biases the positive input of the sampling error amplifier. The negative input (FB) is used for output voltage sensing. The sampling error amplifier allows the LT1105 to operate in fully isolated flyback mode by regulating the flyback voltage of the bootstrap winding. The leakage inductance spike at the leading edge of the flyback waveform is ignored with a blanking circuit. The flyback waveform is directly proportional to the output voltage in the transformer coupled flyback topology. Output voltages are fully floating up to the breakdown voltage of the transformer windings. Multiple floating outputs are easily obtained with additional windings. The error signal developed at the comparator input is brought out externally. The VC pin has three functions including frequency compensation, current limit adjustment and total regulator shutdown. During normal operation, this pin sits at a voltage between 1.2V (low output current) and 4.4V (high output current). The error amplifier is a current output (gm) type, so this voltage can be externally clamped for adjusting current limit. Switch duty cycle goes to zero if the VC pin is pulled to ground through a diode, placing the LT1105 in an idle mode. Pulling the VC pin below 0.15V causes total regulator shutdown and places the LT1105 in prestart mode. The SS pin implements soft-start with one external capacitor to ground. The internal pull-up current and clamp transistor limit the voltage at VC to one diode drop above the voltage at the SS pin, thereby controlling the rate of rise of switch current in the regulator. The SS pin is reset to 0V when the LT1105 is in prestart mode. A final protection feature includes overvoltage lockout monitoring of the main supply voltage on the OVLO pin. If the OVLO pin is greater than 2.5V, the output switch is prevented from turning on. This function can be disabled by grounding the OVLO pin. Rev. E 14 For more information www.analog.com LT1103/LT1105 APPLICATIONS INFORMATION Bootstrap Start It is inefficient as well as impractical to power a switching regulator control IC from the rectified DC input as this voltage is several hundred volts. Self-biased switching regulator topologies take advantage of a lower voltage auxiliary winding on the power transformer or inductor to power the regulator, but require a start-up cycle to begin regulation. Start-up circuitry with hysteresis built into the LT1103/ LT1105 allows the input voltage to increase from 0V to 16V before the regulator tries to start. During this time the start-up current of the switching regulator is typically 200A and all internal voltage regulators are off. The low quiescent current allows the input voltage to be trickled up with only 500A of current from the rectified DC line voltage, thereby minimizing power dissipation in the start-up resistor. At 16V, the internal voltage regulators are turned on and switching begins. If enough power feeds back through the auxiliary winding to keep the input voltage to the switching regulator above 12V, then switching continues and a bootstrap start is accomplished. If the input voltage drops below 12V, then the FET drive detection circuit locks out switching. The input voltage continues to fall as the VIN bypass capacitor is discharged by the normal quiescent current of the LT1103/LT1105. Once the input voltage falls below 7V, the internal voltage regulators are turned off and the switching regulator returns to the low start-up current state. A continuous "burp start" mode indicates a fault condition or an incomplete power loop. The trickle current required to bootstrap the regulator input voltage is typically generated with a resistor from the rectified DC input voltage. When combined with the regulator input bypass capacitor, the start-up resistor creates a ramp whose slope governs the turn-on time of the regulator as well as the period of the "burp start" mode. The design trade-offs are power dissipated in the trickle resistor, the turn-on time of the regulator, and the hold-up time of the regulator input bypass capacitor. The value of the start-up resistor is set by the minimum rectified DC input voltage to guarantee sufficient start-up current. The recommended minimum trickle current is 500A. The power rating of the start-up resistor is set by the maximum rectified DC input voltage. A final consideration for the start-up resistor is to insure that the maximum voltage rating of the resistor is not exceeded. Typical carbon film resistors have a voltage rating of 250V. The most reliable and economical solution for the start-up resistor is generally provided by placing several 0.25W resistors in series. The LT1103/LT1105 is designed to operate with supply pin voltages up to 30V. However, the auxiliary bias winding should be designed for a typical output voltage of 17V to minimize IC power dissipation and efficiency loss. Allowances must also be made for cross regulation of the bias voltage due to variations in the rectified DC line voltage and output load current. Soft-Start Soft-start refers to the controlled increase of switch current from a start-up or shutdown state. This allows the power supply to come up to voltage in a controlled manner and charge the output capacitor without activating current limit. In general, soft-start is not required on the LT1105 due to the design of the sampling error amplifier gm stage which generates asymmetrical slew capability on the VC pin. This feature exhibits itself as a typical 3mA sink current capability on the VC pin whereas source current is only 275A. The low gm of the error amplifier allows smallvalued compensation capacitors to be used on VC. This allows the sink current to slew the compensation capacitor quickly. Therefore, overshoot of the output voltage on start-up sequences and recovery from overload or shortcircuit conditions is prevented. However, if a longer startup period is required, the soft-start function can be used. Soft-start is implemented with an internal 40A pull-up and a transistor clamp on the VC pin so that a single external capacitor from SS ground can define the linear ramp function. The voltage at VC is limited to one VBE above the soft-start pin (SS). The time to maximum switch current is defined as the capacitance on SS multiplied by the active range in volts of the VC pin divided by the pull-up current: T= C * (3.2V ) 40A Rev. E For more information www.analog.com 15 LT1103/LT1105 APPLICATIONS INFORMATION SS is reset to 0V whenever VIN is less than 7V (prestart mode) or when shutdown is activated by pulling VC below 0.15V. The SS pin has a guaranteed reset sink current of 1mA when either the regulator supply voltage VIN falls below 7V or the regulator is placed in shutdown. Shutdown The LT1103/LT1105 can be put in a low quiescent current shutdown mode by pulling VC below 150mV. In the shutdown mode the internal voltage regulators are turned off, SS is reset to 0V and the part draws less than 200A. To initiate shutdown, about 400A must be pulled out of VC until the internal voltage regulators turn off. Then, less than 50A pull-down current is required to maintain shutdown. The shutdown function has about 60mV of hysteresis on the VC pin before the part returns to normal operation. Soft-start, if used, controls the recovery from shutdown. 5V Reference A 5V reference output is available for the user's convenience to power primary-side circuitry or to generate a clamp voltage for switch current limiting. The output will source 25mA and the voltage temperature coefficient is typically 50ppm/C. If bypassing of the 5V reference is required, a 0.1F is recommended. Values of capacitance greater than 1F may be susceptible to ringing due to decreased phase margin. In such cases, the capacitive load can be isolated from the reference output with a small series resistor at the expense of load regulation performance. Overvoltage Lockout The switching supply and primarily the external power MOSFET can be protected from an extreme surge of the input line voltage with the overvoltage lockout feature implemented on the OVLO pin. If the voltage on OVLO rises above its typical threshold voltage of 2.5V, output switching is inhibited. This feature can be implemented with a resistive divider off of the rectified DC input voltage. This feature is only available on the LT1105 in the 14-lead PDIP and must be tied to ground if left unused. Ground (LT1103) The ground pin of the LT1103 is important because it acts as the negative sense point for the internal error amplifier feedback signal, the negative sense point for the current limit amplifier, and as the high current path for the 2A switch. The tab of the 7-lead TO-220 is internally connected to GND (Pin 4). To avoid degradation of load regulation, the feedback resistor divider string and the reference side of the bias winding should be directly connected to the ground pin on the package. These ground connections should not be mixed with high current carrying ground return paths. The length of the switch current ground path should be as short as possible to the input supply bypass capacitor and low resistance for best performance. The case of the LT1103 package is desirable to use as the high current ground return path as this is a lower resistive and inductive path than that of the actual package pin and will help minimize voltage spikes associated with the high dI/dt switch current. Avoiding long wire runs to the ground pin minimizes load regulation effects and inductive voltages created by the high dI/dt switch current. Ground plane techniques should also be used and will help keep EMI to a minimum. Grounding techniques are illustrated in the Typical Applications section. Ground (LT1105) The ground pin of the LT1105 is important because it acts as the negative sense point for the internal error amplifier feedback signal and as the negative sense point for the current limit amplifier. The LT1105 8-pin PDIP has Pin 1 as its ground. The LT1105 14-pin PDIP has Pin 1 and Pin 7 as grounds and must be tied together for proper operation. To avoid degradation of load regulation, the feedback resistor divider should be directly connected to the package ground pin. These ground connections should not be mixed with high current carrying ground return paths. The length of the switch current ground path should be as short as possible to the input supply bypass capacitor and low resistance for best performance. This will help Rev. E 16 For more information www.analog.com LT1103/LT1105 APPLICATIONS INFORMATION minimize voltage spikes associated with the high dI/dt switch current. Avoiding long wire runs to the ground pin minimizes load regulation effects and inductive voltages created by the high dI/dt switch current. Ground plane techniques should also be used and will help keep EMI to a minimum. Grounding techniques are illustrated in the Typical Applications section. Oscillator The oscillator of the LT1103/LT1105 is a linear ramp type powered from the internal 6V bias line. The charging currents and voltage thresholds are generated internally so that only one external capacitor is required to set the frequency. The 150A pull-up current, which is on all the time, sets the preset maximum on-time of the switch and the 450A pull-down current which is turned on and off, sets the dead time. The threshold voltages are typically 2V and 4.5V, so for a 400pF capacitor the ramp-up time of the voltage on the OSC pin is 6.67s and the ramp-down time is 3.3s, resulting in an operating frequency of 100kHz. Although the oscillator, as well as the rest of the switching regulator, will function at higher frequencies, 200kHz is the practical upper limit that will allow control range for line and load regulation. The lowest operating frequency is limited by the sampling error amplifier to about 10kHz. The frequency temperature coefficient is typically -80ppm/C with a good low T.C. capacitor. This means that with a low temperature coefficient capacitor, the temperature coefficient of the currents and the temperature coefficient of the thresholds sum to -80ppm/C over the commercial temperature range. Bowing in the temperature coefficient of the currents affects the frequency about 3% at the extremes of the military temperature range. The capacitor type chosen will have a direct effect on the frequency tempco. Maximum duty cycle is set internally by the pull-up and pull-down currents, independent of frequency. It can be adjusted externally by modifying the fixed pull-up current with an additional resistor. In practice, one resistor from the OSC pin to the 5V reference or to ground does the job. Note that the capacitor value must change to maintain the same frequency. For example, a 24k resistor from 5V to OSC and a 440pF capacitor from OSC to ground will yield 100kHz with 50% maximum duty cycle. A 56k resistor and a 280pF capacitor from OSC to ground will yield 100kHz with 80% maximum duty cycle. The oscillator can be synchronized to an external clock by coupling a sync pulse into the OSC pin. The width of this pulse should be a minimum of 500ns. The oscillator can only be synchronized up in frequency and the synchronizing frequency must be greater than the maximum possible unsynchronized frequency (for the chosen oscillator capacitor value). The amplitude of the sync pulse must be chosen so that the sum of the oscillator voltage amplitude plus the sync pulse amplitude does not exceed the 6V bias reference. Otherwise, the oscillator pull-up current source will saturate and erroneous operation will result. If the LT1103/LT1105 is positioned on the primary side of the transformer and the external clock on the isolated secondary output side, the sync signal must be coupled into the OSC pin using a pulse transformer. The pulse transformer must meet all safety/isolation requirements as it also crosses the isolation boundary. An example of externally synchronizing the oscillator is shown in the Typical Applications section. Gate Biasing (LT1103) The LT1103 is designed to drive an external power MOSFET in the common gate or cascode connection with the VSW pin. The advantage is that the switch current can be sensed internally, eliminating a low value, power sense resistor. The gate needs to be biased at a voltage high enough to guarantee that the FET is saturated when the open-collector source drive is on. This means 10V as specified in FET data sheets, plus 1V for the typical switch saturation voltage, plus a couple of volts for temperature variations and processing tolerances. This leads to 15V for a practical gate bias voltage. Power MOSFETs are well suited to switching power supplies because their high speed switching characteristics promote high switching efficiency. To achieve high switching speed, a special circuit in the LT1103 senses Rev. E For more information www.analog.com 17 LT1103/LT1105 APPLICATIONS INFORMATION the voltage at VSW prior to turning on the switch. VSW is tied to the source of the FET and should represent the bias voltage on the gate when the switch is off. When the switch first turns off, the drain flies back until it is clamped by a snubber network. The source also flies high due to parasitic capacitive coupling on the FET and parasitic inductance of the leads. An extra diode from the source to the gate or VIN will provide insurance against fault conditions that might otherwise damage the FET. The diode clamps the source to one diode drop above the gate or VIN, thereby limiting the gate source reverse bias. Once the energy in the leakage inductance spike is dissipated and the primary is being regulated to its flyback voltage, the diode shuts off. The source is then floating and its voltage will be close to the gate voltage. If the sensed voltage on VSW is less than 10V or greater than 20V, the circuit prevents the switch from turning on. This protects the FET from dissipating high power in a nonsaturated state or from excessive gate-source voltage. The oscillator continues to run and the net effect is to skip switching cycles until the gate bias voltage is corrected. One consequence of the gate bias detection circuit is that the start-up window is 6V if the gate is biased from VIN and to 4V if the gate is biased from the 15V output. This influences the size of the bypass capacitor on VIN. VSW Output (LT1103) The VSW pin of the LT1103 is the collector of an internal NPN power switch. This NPN has a typical on resistance of 0.4 and a typical breakdown voltage (BVCBO) of 75V. Fast switching times and high efficiency are obtained by using a special driver loop which automatically adapts base drive current to the minimum required to keep the switch in a quasisaturated state. The key element in the loop is an extra emitter on the output power transistor as seen in the block diagram. This emitter carries no current when the NPN output transistor collector is high (unsaturated). In this condition, the driver circuit can deliver very high base drive to the switch for fast turn-on. When the switch saturates, the extra emitter acts as a collector of an NPN operating in inverted mode and pulls base current away from the driver. This linear feedback loop serves itself to keep the switch just at the edge of saturation. Very low switch current results in nearly zero driver current and high switch currents automatically increase driver current as necessary. The ratio of switch current to driver current is approximately 30:1. This ratio is determined by the sizing of the extra emitter and the value of the current source feeding the driver circuitry. The quasisaturation state of the switch permits rapid turn-off without the need for reverse base emitter voltage drive. Gate Biasing (LT1105) The LT1105 is designed to drive an external power MOSFET in the common source configuration with the totem-pole output VSW pin. The advantage is added switch current flexibility (limited only by the choice of external power FET) and higher output power applications than allowed by LT1103. An external, noninductive, power sense resistor must be used in series with the source of the FET to detect switch current and must be tied to the input of the current limit amplifier. The gate needs to be biased at a voltage high enough to guarantee that the FET is saturated when the totem-pole gate drive is on. This means 10V as specified in FET data sheets, plus the totem-pole high side saturation voltage plus a couple of volts for temperature variations and processing tolerances. This leads to 15V for a practical gate bias voltage. Power MOSFETs are well suited to switching power supplies because their high speed switching characteristics promote high switching efficiency. To achieve high switching speed, the gate capacitance must be charged and discharged quickly with high peak currents. In particular, the turn-off current can be as high as the peak switch current. The switching speed is controlled by the impedance seen by the gate capacitance. Practically speaking, zero impedance is not desirable because of the high frequency noise spikes introduced to the system. The gate bias supply which drives the totem-pole output stage should be bypassed with a 1F low ESR capacitor to ground. This capacitor supplies the energy to charge the gate capacitance during gate drive turn-on. The power MOSFET should have a 5 resistor or larger in series with its gate from the VSW pin to define the source impedance. Rev. E 18 For more information www.analog.com LT1103/LT1105 APPLICATIONS INFORMATION The LT1105 provides a 15V regulated output intended for driving the totem-pole output stage. It will source 30mA into a capacitive load with no stability problems. The output voltage temperature coefficient is 3mV/C. If VIN drops below 17V, the 15V output follows about 2.0V below VIN until the part shuts down. If the 15V output is pulled above 17.5V, it will sink 5mA. A special circuit in the LT1105 senses the voltage at the 15V regulated output prior to turning on the switch. The 15V regulator drives the totem-pole output stage and the VSW pin will pull the gate of the FET very close to the value of the 15V output when VSW turns on. Therefore, the 15V output represents what the gate bias voltage on the FET will be when the FET is turned on. If the sensed voltage on the 15V output is less than 10V or greater than 20V, the circuit prevents the switch from turning on. This protects the FET from dissipating high power in a nonsaturated state or from excessive gate-source voltage. The oscillator continues to run and the net effect is to skip switching cycles until the gate bias voltage is corrected. One consequence of the gate bias detection circuit is that the start-up window is 4V. This influences the size of the bypass capacitor on VIN. VSW Output (LT1105) The VSW pin of the LT1105 is the output of a 1A totempole driver stage. This output stage turns an external power MOSFET on by pulling its gate high. Break-BeforeMake action of 200ns is built into each switch edge to eliminate cross-conduction currents. Fast switching times and high efficiency are obtained by using a low loss output stage and a special driver loop which automatically adapts base drive current to the totem-pole low side drive. The key element in the loop is an extra emitter on the output pull-down transistor as seen in the block diagram. This emitter carries no current when the low side transistor collector is high (unsaturated). In this condition, the driver can deliver very high base drive to the output transistor for fast turn-off. When the low side transistor saturates, the extra emitter acts as a collector of an NPN operating in inverted mode and pulls base current away from the driver. This linear feedback loop serves itself to keep the switch just at the edge of saturation. This results in nearly zero driver current. The quasisaturation state of the low side switch permits rapid turn-on of the external FET when VSW pulls high. Fully Isolated Flyback Mode A unique sampling error amplifier included in the control loop of the LT1103/LT1105 eliminates the need for an optoisolator while providing 1% line and load regulation in a magnetic flux-sensed flyback converter. In this mode, the flyback voltage on the primary during "switch off" time is sensed and regulated. It is difficult to derive a feedback signal directly from the primary flyback voltage as this voltage is typically several hundred volts. A dedicated winding is not required because the bias winding for the regulator lends itself to flux-sensing. Flux-sensing made practical simplifies the design of off line power supplies by minimizing the total number of external components and reduces the components which must cross the isolation barrier to one, the transformer. This inherently implies greater safety and reliability. The transformer must be optimized for coupling between the bias winding and the secondary output winding(s) while maintaining the required isolation and minimizing the parasitic leakage inductances. Although magnetic flux-sensing has been used in the past, the technique has exhibited poor output voltage regulation due to the parasitics present in a transformer coupled design. Transformers which provide the safety and isolation as required by various international safety/ regulatory agencies also provide the poorest output voltage regulation. Solutions to these parasitic elements have been achieved with the novel sampling error amplifier of the LT1103/LT1105. A brief review of flyback converter operation and the problems which create a poorly regulated output will provide insight on how the sampling error amplifier of the LT1103/LT1105 addresses the regulation issue of magnetic flux sensed converters. The following figure shows a simplified diagram of a flyback converter using magnetic flux sensing. The major parasitic elements present in the transformer coupled design are indicated. The relationships between the Rev. E For more information www.analog.com 19 LT1103/LT1105 APPLICATIONS INFORMATION primary voltage, the secondary voltage, the bias voltage and the winding currents are indicated in the figures found on the following page for both continuous and discontinuous modes of operation. Simplified Flyback Converter VIN L(IkPRI) 1:N L(lkSEC) R D1 VOUT C1 COMMON S1 VBIAS 1:N1 N = TURNS RATIO FROM SECONDARY TO PRIMARY. N1 = TURNS RATIO FROM SECONDARY TO BIAS. N2 = N/N1 L(lkPRI) = PRIMARY LEAKAGE INDUCTANCE. L(lkSEC) = SECONDARY LEAKAGE INDUCTANCE. R = PARASITIC WINDING, DIODE AND OUTPUT CAPACITOR RESISTANCE. LT1103 AI01 When the switch "turns on," the primary winding sees the input voltage and the secondary and bias windings go to negative voltages as a function of the turns ratio. Current builds in the primary winding as the transformer stores energy. When the switch "turns off," the voltage across the switch flies back to a clamp level as defined by a snubber network until the energy in the leakage inductance of the primary dissipates. Leakage inductance is one of the main parasitic elements in a flux-sensed converter and is modeled as an inductor in series with the primary and secondary of the transformer. These parasitic inductances contribute to changes in the bias winding voltage and thus the output voltage with increasing load current. The energy stored in the transformer transfers through the secondary and bias windings during "switch off" time. Ideally, the voltage across the bias winding is set by the DC output voltage, the forward voltage of the output diode, and the turns ratio of the transformer after the energy in the leakage inductance spike of the primary is dissipated. This relationship holds until the energy in the transformer drops to zero (discontinuous mode) or the switch turns on again (continuous mode). Either case results in the voltage across the secondary and bias windings decreasing to zero or changing polarity. Therefore, the voltage on the bias winding is only valid as a representation of the output voltage while the secondary is delivering current. Although the bias winding flyback voltage is a representation of the output voltage, its voltage is not constant. For a brief period following the leakage inductance spike, the bias winding flyback voltage decreases due to nonlinearities and parasitics present in the transformer. Following this nonlinear behavior is a period where the bias winding flyback voltage decreases linearly. This behavior is easily explained. Current flow in the secondary decreases linearly at a rate determined by the voltage across the secondary and the inductance of the secondary. The parasitic secondary leakage inductance appears as an impedance in series with the secondary winding. In addition, parasitic resistances exist in the secondary winding, the output diode and the output capacitor. These impedances can be combined to form a lumped sum equivalent and which cause a voltage drop as secondary current flows. This voltage drop is coupled from the secondary to the bias winding flyback voltage and becomes more significant as the output is loaded more heavily. This voltage drop is largest at the beginning of "switch off" time and smallest just prior to either all transformer energy being depleted or the switch turning on again. The best representation of the output voltage is just prior to either all transformer energy being used up and the bias winding voltage collapsing to zero or just prior to the switch turning on again and the bias winding going negative. This point in time also represents the smallest forward voltage for the output diode. It is possible to redefine the relationship between the secondary winding voltage and the bias winding voltage as: VBIAS = ( VOUT + Vf + I * RP ) N1 where Vf is the forward voltage of the output diode, I is the current flowing in the secondary, RP is the lumped sum equivalent secondary parasitic impedance and N1 is the transformer turns ratio from the secondary to the bias winding. It is apparent that even though the above point Rev. E 20 For more information www.analog.com LT1103/LT1105 APPLICATIONS INFORMATION Flyback Waveform for Continuous Mode Operation Flyback Waveform for Discontinuous Mode Operation VZENER VZENER PRIMARY SWITCH VOLTAGE PRIMARY SWITCH VOLTAGE [VOUT + Vf + (ISEC * RP)]/N a [VOUT + Vf + (ISEC * RP)]/ N a VIN VIN AREA "a" = AREA "b" TO MAINTAIN ZERO VOLTS ACROSS PRIMARY 0V b AREA "a" = AREA "b" TO MAINTAIN ZERO VOLTS ACROSS PRIMARY b 0V SECONDARY WINDING VOLTAGE SECONDARY WINDING VOLTAGE [VOUT + Vf + (ISEC * RP)] c 0V AREA "c" = AREA "d" TO MAINTAIN ZERO VOLTS ACROSS SECONDARY N * VIN d [VOUT + Vf + (ISEC * RP)] c 0V AREA "c" = AREA "d" TO MAINTAIN ZERO VOLTS ACROSS SECONDARY N * VIN d BIAS WINDING VOLTAGE BIAS WINDING VOLTAGE [VOUT + Vf + (ISEC * RP)]/N1 e 0V e 0V AREA "e" = AREA "f" TO MAINTAIN ZERO VOLTS ACROSS BIAS WINDING N2 * VIN f AREA "e" = AREA "f" TO MAINTAIN ZERO VOLTS ACROSS BIAS WINDING N2 * VIN f IPRI IPRI I PRIMARY CURRENT I [VOUT + Vf + (ISEC * RP)]/ N1 I PRIMARY CURRENT 0A 0A ISEC = IPRI/N ISEC = IPRI/N SECONDARY CURRENT SECONDARY CURRENT 0A 0A IPRI IPRI SWITCH CURRENT I SWITCH CURRENT 0A 0A IPRI IPRI SNUBBER DIODE CURRENT SNUBBER DIODE CURRENT 0A 0A t = (IPRI)[L(lkPRI)]/VSNUB t = (IPRI)[L(lkPRI)]/VSNUB LT1103 WF01 Rev. E For more information www.analog.com 21 LT1103/LT1105 APPLICATIONS INFORMATION in time is the most accurate representation of the output voltage, the answer given by the bias winding voltage is still off from the "true" answer by the amount I * RP/N1. The sampling error amplifier of the LT1103/LT1105 provides solutions to the errors associated with the bias winding flyback voltage. The error amplifier is comprised of a leakage inductance spike blanking circuit, a slew rate limited tracking amplifier, a level detector, a sample-andhold, an output gm stage and load regulation compensation circuitry. This all seems complicated at first glance, but its operation is straightforward and transparent to the user of the IC. When viewed from a system or block level, the sampling error amplifier behaves like a simple transconductance amplifier. Here's how it works. The sampling error amplifier takes advantage of the fact that the voltage across the bias winding during at least a portion of switch off time is proportional to the DC output voltage of the secondary winding. The feedback network used to sense the bias winding voltage is no longer comprised of a traditional peak detector in conjunction with a resistor divider network. The feedback network consists of a diode in series with the bias winding feeding the resistor divider network directly. The resultant error signal is then fed into the input of the error amplifier. The purpose of the diode in series with the bias winding is now not to peak detect, but to prevent the FB pin (input of the error amplifier) from being pulled negative and forward biasing the substrate of the IC when the bias winding changes polarity with "switch turn-on." The primary winding leakage inductance spike effects are first eliminated with an internal blanking circuit in the LT1103/LT1105 which suppresses the input of the FB pin for 1.5s at the start of "switch off" time. This prevents the primary leakage inductance spike from being propagated through the error amplifier and affecting the regulated output voltage. With the effects of the leakage inductance spike eliminated, the effects of decreasing bias winding flyback voltage can be addressed. With the traditional diode/capacitor peak detector circuitry eliminated from the feedback network, the tracking amplifier of the LT1103/LT1105 follows the flyback waveform as it changes with time and amplifies the difference between the flyback signal and the internal 4.5V reference. Tracking is maintained until the point in time where the bias winding voltage collapses as a result of all transformer energy being depleted (discontinuous mode) or the switch turning on again (continuous mode). The level detector circuit senses the fact that the bias winding flyback voltage is no longer a representation of the output voltage and activates an internal peak detector. This effectively saves the most accurate representation of the output voltage which is then buffered to the second stage of the error amplifier. The second stage of the error amplifier consists of a sample-and-hold. When the switch turns on, the sampleand-hold samples the buffered error voltage for 1s and then holds for the remainder of the switch cycle. This held voltage is then processed by the output gm stage and converted into a control signal at the output of the error amplifier, the VC pin. The final adjustment in regulation is provided by the load regulation compensation circuitry. As stated earlier, output regulation degrades with increasing load current (output power). The effect is traced to secondary leakage inductance and parasitic secondary winding, diode and output capacitor resistances. Even though the tracking amplifier has obtained the most accurate representation of the output voltage, its answer is still flawed by the amount of the voltage drop across the secondary parasitic lumped sum equivalent impedance which is coupled to the bias winding voltage. This error increases with increasing load current. Therefore, a technique for sensing load current conditions has been added to the LT1103/LT1105. The switch current is proportional to the load current by the turns ratio of the transformer. A small current proportional to switch current is generated in the LT1103/LT1105 and fed back to the FB pin. This allows the input bias current of the sampling error amplifier to be a function of load current. A resistor in series with the FB pin generates a linear increase in the effective reference voltage with increasing load current. This translates to a linear increase in output voltage with increasing load current. By adjusting the value of the series resistor, the slope of the load Rev. E 22 For more information www.analog.com LT1103/LT1105 APPLICATIONS INFORMATION compensation can be set to cancel the effects of these parasitic voltage drops. The feature can be ignored by eliminating the series resistor and lowering the equivalent divider impedance to swamp out the effects of the input bias current. Frequency Compensation In order to prevent a regulator loop using the LT1103/ LT1105 from oscillating, frequency compensation is required. Although the architecture of the LT1103/LT1105 is simple enough to lend itself to a mathematical approach to frequency compensation, the added complication of input/or output filters, unknown capacitor ESR, and gross operating point changes with input voltage and load current variations all suggest a more practical empirical approach. Many hours spent on breadboards have shown that the simplest way to optimize the frequency compensation of the LT1103/LT1105 is to use transient response techniques and an "RC" box to quickly iterate toward the final compensation network. Additional information on this technique of frequency compensation can be found in Linear Technology's Application Note 19. In general, frequency compensation is accomplished with an RC series network on the VC pin. The error amplifier has a gm (voltage "in" to current "out") of 12000 mhos. Voltage gain is determined by multiplying gm times the total equivalent error amplifier output loading, consisting of the error amplifier output impedance in parallel with the series RC external frequency compensation network. At DC, the external RC can be ignored. The output impedance of the error amplifier is typically 100k resulting in a voltage gain of 1200V/V. At frequencies just above DC, the voltage gain is determined by the external compensation, RC and CC. The gain at mid frequencies is given by: AV = gm 2 * f * C C The gain at high frequencies is given by: AV = gm * RC drops back to 0 (actually 180 since FB is an inverting input) when the reactance of CC is small compared to RC. Thus, this RC series network forms a pole-zero pair. The pole is set by the high impedance output of the error amplifier and the value of CC on the VC pin. The zero is formed by the value of CC and the value of RC in series with CC on the VC pin. The RC series network will have capacitor values in the range of 0.1F to 1.0F and series resistor values in the range of 100 to 1000 . It is noted that the RC network on the VC pin forms the main compensation network for the regulator loop. However, if the load regulation compensation feature is used as explained in the section on fully-isolated flyback mode, additional frequency compensation components are required. The load regulation compensation feature involves the use of local positive feedback from the VC pin to the FB pin. Thus, it is possible to add enough load regulation compensation to make the loop oscillate. In order to prevent oscillation, it is necessary to roll off this local positive feedback at high frequencies. This is accomplished by placing a capacitor in parallel with the compensation resistor which is in series with the FB pin. A value for this capacitor in the range of 0.01F to 0.1F is recommended. The time constant associated with this RC combination will be longer than that associated with the loop bandwidth. Thus, transient response will be affected in that settling time will be increased. However, this is typically not as important as controlling the absolute under or overshoot amplitude of the system in response to load current changes which could cause deleterious system operation. Switching Regulator Topologies Two basic switching regulator topologies are pertinent to the LT1103/LT1105, the flyback and forward converter. The flyback converter employs a transformer to convert one voltage to either a higher or lower output voltage. VOUT in continuous mode is defined as: VOUT = VIN * N * Phase shift from the FB pin to the VC pin is 90 at mid frequencies where the external CC is controlling gain, then DC (1- DC) Rev. E For more information www.analog.com 23 LT1103/LT1105 APPLICATIONS INFORMATION where N is the transformer turns ratio of secondary to primary and DC is the duty cycle. This formula can be rewritten in terms of duty cycle as: DC = VOUT ( VOUT + N * VIN ) L PRI = It is important to define the full range of input voltage, the range of output loading conditions and the regulation requirements for a design. Duty cycle should be calculated for both minimum and maximum input voltage. In many applications, N can vary over a wide range without degrading performance. If maximum output power is desired, N can be optimized: N (OPT ) = VOUT + Vf (V M - VIN(MAX ) - VSNUB ) If maximum output power is not required, then I can be increased which results in lower primary inductance and smaller magnetics. Maximum output power with an isolated flyback converter is defined by the primary flyback voltage and the peak allowed switch current and is limited to: ( VPRI ) I 2 VIN IP (Ip) R E 2 ( VPRI + VIN ) where Vf = Forward voltage of the output diode R = Total "switch" on resistance VM = Maximum switch voltage VSNUB = Snubber clamp level - primary flyback voltage. In the isolated flyback mode, the LT1103/LT1105 sense and regulate the transformer primary voltage VPRI during "switch off" time. The secondary output voltage will be regulated if VPRI is regulated. VPRI is related to VOUT by: ( VOUT + Vf ) N This allows duty cycle for an isolated flyback converter to be rewritten as: DC = Duty Cycle = VIN V (I)(f) 1+ IN VPRI POUT(MAX) = where VPRI = needed, a reasonable starting value is found by assigning I a value of 20% of the peak switch current (2A for the LT1103 and set by the external FET rating used with the LT1105). With this design approach, LPRI is defined as: VPRI ( VPRI + VIN ) An important transformer parameter to be determined is the primary inductance LPRI. The value of this inductance is a trade-off between core size, regulation requirements, leakage inductance effects and magnetizing current I. Magnetizing current is the difference between the primary current at the start of "switch on" time and the current at the end of "switch on" time. If maximum output power is IP = Maximum switch current E = Overall efficiency 75% Peak primary current is used to determine core size for the transformer and is found from: IPRI = ( VOUT ) (IOUT ) ( VPRI + VIN ) + I 2 E ( VPRI ) ( VIN ) A second consideration on primary inductance is the transition point from continuous mode to discontinuous mode. At light loads, the flyback pulse across the primary will drop to zero before the end of "switch off" time. The load current at which this starts to occur can be calculated from: 2 VPRI * VIN ) ( IOUT(TRANSITION) = ( VPRI + VIN )2 ( 2VOUT ) ( f ) (L PRI ) The forward converter as shown below is another transformer-based topology that converts one voltage to either a higher or a lower voltage. Rev. E 24 For more information www.analog.com LT1103/LT1105 APPLICATIONS INFORMATION VOUT in continuous mode is defined as: VOUT = VIN * N * DC The secondary voltage charges up L1 through D1 when S1 is on. When S1 is off, energy in L1 is transferred through free-wheeling diode D2 to C1. The extra transformer winding and diode D3 are needed in a single switch forward converter to define the switch voltage Simplified Forward Converter L1 VIN 1:N VOUT D1 D2 C1 COMMON D3 S1 LT1103 AI02 when S1 is off. This "reset" winding limits the maximum duty cycle allowed for the switch. This topology trades off reduced transformer size for increased complexity and parts count. A separate isolated feedback path is required for full isolation from input to output because voltages on the primary are no longer related to the DC output voltage during switch off time. The isolated feedback path can take several forms. A second transformer in a modulator/demodulator scheme provides the isolation, but with significant complexity. An optoisolator can be substituted for the transformer with a savings in volume to be traded off with component variations and possible aging problems with the optoisolator transfer function. Finally, an extra winding closely coupled to the output inductor L1 can sense the flux in this element and give a representation of the output voltage when S1 is off. TYPICAL APPLICATIONS LT1103 FET Connection 15V LT1103 VSW LT1103 TA03 LT1105 FET Connection 15V LT1105 VSW ILIM LT1103 TA04 Rev. E For more information www.analog.com 25 LT1103/LT1105 TYPICAL APPLICATIONS Setting Oscillator Frequency Setting Overvoltage Lockout OVLOTH R2 LT1105 LT1103/LT1105 OVLO R1 OSC COSC LT1103 TA09 LT1103 TA05 CHOOSE OVLOTH CHOOSE 20kHz fOSC 200kHz LET R1 = 5k 100A I = COSC = SF = fOSC V fOSC 2.5V fOSC ( )( DC 0.66 ) ( )( R2 = ) ( ) OVLOTH -1 R1 2.5V 66% Decreasing Oscillator Maximum Duty Cycle Increasing Oscillator Maximum Duty Cycle Synchronizing Oscillator Frequency to an External Clock 5V LT1105 LT1103/LT1105 LT1103/LT1105 500ns OSC 5V OSC R COSC I1 OSC 1F COSC R 0V I1 1:0.5 LT1103 TA07 COSC LT1103 TA08 LT1103 TA06 CHOOSE 0 DC 0.66 (6 - 9DC) SOLVE FOR X X = 2 0X3 CHOOSE 0.66 DC 1.0 (9DC - 6) SOLVE FOR X X = 2 0 X 1.5 I1 = X * I = X * 100A R = 1.75V I1 COSC = ( I1 = X * I = X * 100A R= ( 2 3X - 2X 100A * 1+ 9 2.5V fOSC )( ) ISOLATION BOUNDARY ) COSC = 3.25V I1 ( ( 2 3X + 2X 100A * 1- 9 2.5V fOSC )( ) ) Rev. E 26 For more information www.analog.com LT1103/LT1105 TYPICAL APPLICATIONS LT1103 Ground Connections 15V VIN OSC GND VC FB VSW SWITCH CURRENT PATH KEEP RESISTANCE LOW LT1103 TA11a TO BIAS WINDING OUTPUT GND SEPARATE GROUND PATH SWITCH CURRENT PATH KEEP RESISTANCE LOW 15V VIN OSC GND VC FB VSW LT1103 TA11b TO BIAS WINDING OUTPUT GND LT1105 Ground Connections HIGH CURRENT GROUND PATH GND VSW ILIM FB VIN VC TO BIAS WINDING OUTPUT LT1103 TA12a Rev. E For more information www.analog.com 27 LT1103/LT1105 PACKAGE DESCRIPTION N Package 8-Lead PDIP (Narrow .300 Inch) (Reference LTC DWG # 05-08-1510 Rev I) .400* (10.160) MAX 8 7 6 5 1 2 3 4 .255 .015* (6.477 0.381) .300 - .325 (7.620 - 8.255) .008 - .015 (0.203 - 0.381) ( +.035 .325 -.015 8.255 +0.889 -0.381 ) .045 - .065 (1.143 - 1.651) .065 (1.651) TYP .100 (2.54) BSC .130 .005 (3.302 0.127) .120 (3.048) .020 MIN (0.508) MIN .018 .003 (0.457 0.076) N8 REV I 0711 NOTE: 1. DIMENSIONS ARE INCHES MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm) Rev. E 28 For more information www.analog.com LT1103/LT1105 PACKAGE DESCRIPTION N Package 14-Lead PDIP (Narrow .300 Inch) (Reference LTC DWG # 05-08-1510 Rev I) .770* (19.558) MAX 14 13 12 11 10 9 8 1 2 3 4 5 6 7 .255 .015* (6.477 0.381) .300 - .325 (7.620 - 8.255) .008 - .015 (0.203 - 0.381) ( +.035 .325 -.015 +0.889 8.255 -0.381 NOTE: 1. DIMENSIONS ARE ) .045 - .065 (1.143 - 1.651) .130 .005 (3.302 0.127) .020 (0.508) MIN .065 (1.651) TYP .120 (3.048) MIN .005 (0.127) .100 MIN (2.54) BSC .018 .003 (0.457 0.076) N14 REV I 0711 INCHES MILLIMETERS *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm) Rev. E For more information www.analog.com 29 LT1103/LT1105 PACKAGE DESCRIPTION T7 Package 7-Lead Plastic TO-220 (Standard) (Reference LTC DWG # 05-08-1422) .390 - .415 (9.906 - 10.541) .165 - .180 (4.191 - 4.572) .147 - .155 (3.734 - 3.937) DIA .045 - .055 (1.143 - 1.397) .230 - .270 (5.842 - 6.858) .460 - .500 (11.684 - 12.700) .570 - .620 (14.478 - 15.748) .330 - .370 (8.382 - 9.398) .620 (15.75) TYP .700 - .728 (17.780 - 18.491) SEATING PLANE .152 - .202 .260 - .320 (3.860 - 5.130) (6.604 - 8.128) BSC .050 (1.27) .026 - .036 (0.660 - 0.914) .135 - .165 (3.429 - 4.191) .095 - .115 (2.413 - 2.921) .155 - .195* (3.937 - 4.953) .013 - .023 (0.330 - 0.584) *MEASURED AT THE SEATING PLANE T7 (TO-220) 0801 OBSOLETE PACKAGE Rev. E 30 For more information www.analog.com LT1103/LT1105 REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION E 11/18 Reflects LT1103 being obsolete and the LT1105 being available. PAGE NUMBER All Rev. E Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license For is granted implication or otherwise under any patent or patent rights of Analog Devices. more by information www.analog.com 31 LT1103/LT1105 TYPICAL APPLICATION Minimum Parts Count Fully-Isolated Flyback 100kHz 50W Converter OPTIONAL OUTPUT FILTER MBR2045 85VAC TO 270VAC + + 220k 1W 220F 385V 499 - BRIDGE RECTIFIER + LINE FILTER !! GER GE!! N A D A OLT V H HIG 1.5KE300A 5W + MUR150 16.2k 1% 39F 25V BUK426-800A VSW VIN + LT1103 10 15V FB GND 5.36k 1% + OSC VC + 50V 470F WINDINGS FOR OPTIONAL 12VDC OUTPUTS BAV21 BAV21 35V 3600F 5V 10A *OUTPUT CAPACITOR IS THREE 1200F, 50V CAPACITORS IN PARALLEL TO ACHIEVE REQUIRED RIPPLE CURRENT RATING AND LOW ESR. 1N4148 1000pF 100 10H 1F 25V 390pF TRANSFORMER DATA: COILTRONICS - CTX110228-3 L(PRI) = 1.6mH NPRI:NSEC = 1:0.05 NBIAS:NSEC = 1:0.27 330 0.1F 0.047F LT1103 TA01 Danger!! Lethal Voltages Present - See Text Load Regulation 5.25 5.20 5.15 270VAC VOUT (V) 5.10 220VAC 5.05 5.00 4.95 110VAC 4.90 85VAC 4.85 4.80 4.75 0 1 2 3 4 5 6 7 9 8 IOUT (A) 10 LT1103 TA14 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1241 High Speed Current Mode Pulse Width Modulators Up to 500kHz Operation LT1246 Off-Line Current Mode PWM 1MHz Operation LT1248 Power Factor Controller Programmable Frequency, 16-Pin SO LT1249 Power Factor Controller 100kHz, SO-8 LT1508 Power Factor and PWM Controller Voltage Mode LT1509 Power Factor and PWM Controller Current Mode Rev. E 32 11/18 www.analog.com ANALOG DEVICES, INC. 2018