LT1725
General Purpose
Isolated Flyback Controller
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FEATURES
DESCRIPTIO
The LT®1725 is a monolithic switching regulator control-
ler specifically designed for the isolated flyback topology.
It drives the gate of an external MOSFET and is generally
poweredfromathirdtransformerwinding. Thesefeatures
allow for an application input voltage limited only by
external power path components. The third transformer
winding also provides output voltage feedback informa-
tion, 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.
■
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
The LT1725 has a number of features not found on most
other switching regulator ICs. By utilizing current mode
switchingtechniques, itprovidesexcellentACandDCline
regulation. Its unique control circuitry can maintain regu-
lation well into discontinuous mode in most applications.
Optionalloadcompensationcircuitryallowsforimproved
load regulation. An optional undervoltage lockout pin
halts operation when the application input voltage is too
low. An optional external capacitor implements a soft-
start function. A 3V output is available at up to several mA
for powering primary side application circuitry.
■
Optional Undervoltage Lockout
■
Available in 16-Pin SO and SSOP Packages
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APPLICATIO S
■
Telecom Isolated Converters
■
Offline Isolated Power Supplies
■
Instrumentation Power Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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TYPICAL APPLICATIO
48V to Isolated 5V Converter
CTX02-14989
Output Load Regulation
6
470pF
5.25
LT1725
18Ω
V
IN
36V TO 72V
35.7k
1%
BAS16
3V
FB
OUT
1
2
V
= 48V
IN
V
V
= 36V
= 72V
IN
22Ω
47k
V
I
= 5V
= 0 to 2A
3.01k
1%
OUT
OUT
V
SFST
VC
CC
IN
1nF
47pF
+
9
12CWQ06
15µF
1µF
5.00
4.75
51Ω
1W
68Ω
OSCAP
11
10
150pF
820k
33k
UVLO
t
ON
+
51k
1.5µF
100pF
150µF
ENDLY
51k
51k
2.7k
4
12
MENAB
0
0.5
1.5
2.0
1.0
I
(A)
LOAD
GATE
R
OCMP
IRF620
1725 F10b
I
R
CMPC
SENSE
SGND
PGND
0.1µF
0.18Ω
1725 TA01a
1725fa
1