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LT1725 General Purpose Isolated Flyback Controller
FEATURES
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DESCRIPTIO
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Drives External Power MOSFET with External ISENSE Resistor Application Input Voltage Limited Only by External Power Components Senses Output Voltage Directly from Primary Side Winding—No Optoisolator Required Accurate Regulation Without User Trims Regulation Maintained Well into Discontinuous Mode Switching Frequency from 50kHz to 250kHz with External Capacitor Optional Load Compensation Optional Undervoltage Lockout Available in 16-Pin SO and SSOP Packages
The LT®1725 is a monolithic switching regulator controller specifically designed for the isolated flyback topology. It drives the gate of an external MOSFET and is generally powered from a third transformer winding. These features allow for an application input voltage limited only by external power path components. The third transformer winding also provides output voltage feedback information, such that an optoisolator is not required. Its gate drive capability coupled with a suitable external MOSFET can deliver load power up to tens of watts. The LT1725 has a number of features not found on most other switching regulator ICs. By utilizing current mode switching techniques, it provides excellent AC and DC line regulation. Its unique control circuitry can maintain regulation well into discontinuous mode in most applications. Optional load compensation circuitry allows for improved load regulation. An optional undervoltage lockout pin halts operation when the application input voltage is too low. An optional external capacitor implements a softstart function. A 3V output is available at up to several mA for powering primary side application circuitry.
U APPLICATIO S
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Telecom Isolated Converters Offline Isolated Power Supplies Instrumentation Power Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
48V to Isolated 5V Converter
CTX02-14989 6 LT1725 35.7k 1% 3VOUT FB 3.01k 1% 1nF 47pF SFST VC OSCAP 820k 51k 51k MENAB 51k 2.7k 0.1µF ROCMP RCMPC SGND GATE ISENSE PGND 0.18Ω
1725 TA01a
BAS16 22Ω
VIN 36V TO 72V 1
18Ω
470pF
47k VCC
15µF
1µF
68Ω 150pF 1.5µF 4 11 10
12CWQ06
VOUT (V)
+
2
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VOUT = 5V IOUT = 0 to 2A 51Ω 1W
tON ENDLY
UVLO 100pF 33k
150µF 12 IRF620
+
4.75 0 0.5 1.0 ILOAD (A) 1.5 2.0
1725 F10b
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Output Load Regulation
5.25 VIN = 48V VIN = 36V VIN = 72V 5.00
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1725f
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LT1725
ABSOLUTE
(Note 1)
AXI U
RATI GS
U U W PACKAGE/ORDER I FOR ATIO
TOP VIEW PGND 1 ISENSE 2 SFST 3 ROCMP 4 RCMPC 5 OSCAP 6 VC 7 FB 8 16 GATE 15 VCC 14 tON 13 ENDLY 12 MINENAB 11 SGND 10 UVLO 9 3VOUT
VCC Supply Voltage ................................................. 22V UVLO Pin Voltage .................................................... VCC ISENSE Pin Voltage .................................................... 2V FB Pin Current ..................................................... ± 2.