LT1425 Isolated Flyback Switching Regulator FEATURES DESCRIPTION n No Transformer Third Winding or Optoisolator The LT 1425 is a monolithic high power switching regu- Required lator specifically designed for the isolated flyback topol- n 5% Accurate Output Voltage Without User Trims ogy. No third winding or optoisolator is required the (See Circuit Below) integrated circuit senses the isolated output voltage n Resistor Programmable Output Voltage directly from the primary side flyback waveform. A high n Regulation Maintained Well Into Discontinuous current, high efficiency switch is included on the die along Mode (Light Load) with all oscillator, control and protection circuitry. n Optional Load Compensation The LT1425 operates with input supply voltages from 3V n Operating Frequency: 285kHz to 20V and draws only 7mA quiescent current. It can n Easily Synchronized to External Clock deliver output power up to 6W with no external power n Available in 16-Pin Narrow SO Package devices. By utilizing current mode switching techniques, it U provides excellent AC and DC line regulation. APPLICATIONS The LT1425 has a number of features not found on other n Isolated Flyback Switching Regulators switching regulator ICs. Its unique control circuitry can n Ethernet Isolated 5V to 9V Converters maintain regulation well into discontinuous mode in most n Medical Instruments applications. Optional load compensation circuitry allows n Isolated Telecom Supplies for improved load regulation. An externally activated shut- down mode reduces total supply current to 15A for , LTC and LT are registered trademarks of Linear Technology Corporation. standby operation. TYPICAL APPLICATION 5V to Isolated 9V Load Regulation OUT 500V 9.5 ISOLATION BARRIER 5V 9.4 T1* V 9.3 + C1 C2 ISOLATED D1 100F 12 47F 9.2 9V 5% AT 1N5819 10V 16V 20mA TO 200mA V IN 9.1 11 V SW R1 9.0 F 22.6k LT1425 8.9 1% *DALE LPE 4841-330MB 15 3 R SHDN FB 8.8 R2 6 SYNC 3.01k 8.7 1% 4 5 R V 8.6 REF C C3 14 R 8.5 1000pF OCOMP 13 0 50 100 150 200 R3 R CCOMP C4 15k OUTPUT CURRENT (mA) SGND PGND 0.1F 1425 TA02 710 1425 TA01 1 OUTPUT VOLTAGE (V) + U ULT1425 ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) TOP VIEW ORDER PART Supply Voltage ........................................................ 20V NUMBER GND 1 16 GND Switch Voltage ......................................................... 35V NC 2 15 SHDN SHDN, SYNC Pin Voltage........................................... 7V R 3 14 R FB OCOMP R Pin Current....................................................... 2mA LT1425CS FB V 4 13 R C CCOMP Operating Junction Temperature Range LT1425IS R 5 12 V REF IN Commercial .......................................... 0C to 100C SYNC 6 11 V SW Industrial ......................................... 40C to 100C SGND 7 10 PGND Storage Temperature Range ................. 65C to 150C GND 8 9 GND Lead Temperature (Soldering, 10 sec).................. 300C S PACKAGE 16-LEAD PLASTIC SO T = 145C, = 75C/ W JMAX JA Consult factory for Military grade parts. ELECTRICAL CHARACTERISTICS V = 5V, T = 25C, V open, V = 1.4V, unless otherwise specified. IN J SW C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Feedback Amplifier I Reference Current Measured at R Pin with R = 3.000k 402 408 414 A REF FB REF l 396 420 A I R Pin Input Current 500 nA IN REF g Feedback Amplifier Transconductance I = 10A (Note 2) l 400 1000 1600 mho m C I , I Feedback Amplifier Source or Sink Current l 30 50 80 A SOURCE SINK V Feedback Amplifier Clamp Voltage 1.9 V CL Reference Voltage/Current Line Regulation 5V V 18V l 0.01 0.04 %/V IN Voltage Gain (Note 3) 500 V/V V Sense Error l 10 25 mV IN Output Switch BV Output Switch Breakdown Voltage I = 5mA l 35 50 V C V(V ) Output Switch ON Voltage I = 1A l 0.55 0.85 V SW SW I Switch Current Limit Duty Cycle = 50%, 0C T 100C l 1.35 1.60 1.9 A LIM J Duty Cycle = 50%, 40C T 100C l 1.25 1.60 1.9 A J Duty Cycle = 80% 1.30 A Current Amplifier Control Pin Threshold Duty Cycle = Minimum 0.95 1.2 1.3 V l 0.85 1.4 V Control Voltage to Switch Transconductance 2 A/V Timing f Switching Frequency 260 285 300 kHz l 240 320 kHz t Minimum Switch ON Time 170 210 260 ns ON t Flyback Enable Delay Time 150 ns ED t Minimum Flyback Enable Time 180 ns EN Maximum Switch Duty Cycle l 85 90 95 % 2 U W U W U WW