LT1725
General Purpose
Isolated Flyback Controller
FEATURES
s
s
DESCRIPTIO
s
s
s
s
s
s
s
Drives External Power MOSFET with External
I
SENSE
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 control-
ler 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 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.
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 regu-
lation 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 soft-
start function. A 3V output is available at up to several mA
for powering primary side application circuitry.
APPLICATIO S
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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%
3V
OUT
FB
3.01k
1%
1nF
47pF
SFST
VC
OSCAP
820k
51k
51k
MENAB
51k
2.7k
R
OCMP
R
CMPC
0.1µF
SGND
GATE
I
SENSE
PGND
0.18Ω
1725 TA01a
BAS16
22Ω
V
IN
36V TO 72V
1
18Ω
470pF
47k
V
CC
15µF
1µF
12CWQ06
V
OUT
(V)
+
68Ω
150pF
1.5µF
2
9
V
OUT
= 5V
I
OUT
= 0 to 2A
51Ω
1W
11
10
t
ON
ENDLY
UVLO
100pF
33k
+
150µF
4.75
0
0.5
1.0
I
LOAD
(A)
1.5
2.0
1725 F10b
4
12
IRF620
U
Output Load Regulation
5.25
V
IN
= 48V
V
IN
= 36V
V
IN
= 72V
5.00
U
U
1725f
1
LINER [ LINEAR TECHNOLOGY ]
