LT1725 Datasheet, PDF (1/28 Pages) Linear Technology – General Purpose Isolated Flyback Controller

LT1725
General Purpose
Isolated Flyback Controller
FEATURES
s _{Drives External Power MOSFET with External}
I_SENSE Resistor
s _{Application Input Voltage Limited Only by}
External Power Components
s _{Senses Output Voltage Directly from Primary Side}
Winding—No Optoisolator Required
s _{Accurate Regulation Without User Trims}
s _{Regulation Maintained Well into Discontinuous Mode}
s _{Switching Frequency from 50kHz to 250kHz with}
External Capacitor
s _{Optional Load Compensation}
s _{Optional Undervoltage Lockout}
s _{Available in 16-Pin SO and SSOP Packages}
U
APPLICATIO S
s _{Telecom Isolated Converters}
s _{Offline Isolated Power Supplies}
s _{Instrumentation Power Supplies}
, LTC and LT are registered trademarks of Linear Technology Corporation.
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
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.
TYPICAL APPLICATIO
48V to Isolated 5V Converter
35.7k
1%
3.01k
1%
LT1725
3V_OUT
FB
1nF
47pF
SFST
VC
OSCAP
V_{CC +}
51k
51k
51k
2.7k
0.1µF
t_ON
ENDLY
UVLO
MENAB
R_OCMP GATE
R_CMPC ISENSE
SGND PGND
BAS16 ^{VIN 36V TO 72V}
CTX02-14989
6
1
470pF
18Ω
22Ω
47k
15µF
1µF
2
68Ω
9
12CWQ06
820k
150pF
100pF 33k
1.5µF
11
10
+
150µF
4
12
IRF620
0.18Ω
1725 TA01a
V_OUT = 5V
I_OUT = 0 to 2A
51Ω
1W
Output Load Regulation
5.25
V_IN = 36V ^{VIN = 48V}
V_IN = 72V
5.00
4.75
0
0.5
1.0
1.5
2.0
I_LOAD (A)
1725 F10b
1725f
1