LT1425
Isolated Flyback
Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1425 is a monolithic high power switching regu-
lator specifically designed for the isolated flyback topol-
ogy. No “third winding” or optoisolator is required; the
integrated circuit senses the isolated output voltage
directly from the primary side flyback waveform. A high
current, high efficiency switch is included on the die along
with all oscillator, control and protection circuitry.
The LT1425 operates with input supply voltages from 3V
to 20V and draws only 7mA quiescent current. It can
deliver output power up to 6W with no external power
devices. By utilizing current mode switching techniques, it
provides excellent AC and DC line regulation.
The LT1425 has a number of features not found on other
switching regulator ICs. Its unique control circuitry can
maintain regulation well into discontinuous mode in most
applications. Optional load compensation circuitry allows
for improved load regulation. An externally activated shut-
down mode reduces total supply current to 15µA for
standby operation.
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No Transformer “Third Winding” or Optoisolator
Required
±5%
Accurate Output Voltage Without User Trims
(See Circuit Below)
Resistor Programmable Output Voltage
Regulation Maintained Well Into Discontinuous
Mode (Light Load)
Optional Load Compensation
Operating Frequency: 285kHz
Easily Synchronized to External Clock
Available in 16-Pin Narrow SO Package
APPLICATIONS
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Isolated Flyback Switching Regulators
Ethernet Isolated 5V to – 9V Converters
Medical Instruments
Isolated Telecom Supplies
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
5V to Isolated – 9V
OUT
500V
ISOLATION BARRIER
5V
T1*
9.5
9.4
+
V
C2
47µF
16V
–
V
IN
V
SW
11
R1
22.6k
3 1%
•
•
LT1425
15
6
4
C3
1000pF
SHDN
SYNC
V
C
R
REF
R
OCOMP
R
CCOMP
SGND PGND
7
10
R
FB
*DALE LPE 4841-330MB
R2
3.01k
1%
5
14
13
C4
0.1µF
R3
15k
+
12
OUTPUT VOLTAGE (V)
C1
100µF
10V
D1
1N5819
ISOLATED
–9V
±
5% AT
20mA TO 200mA
F
9.3
9.2
9.1
9.0
8.9
8.8
8.7
8.6
8.5
0
50
100
150
OUTPUT CURRENT (mA)
200
1425 TA02
1425 TA01
U
U
U
Load Regulation
1
LINER [ LINEAR TECHNOLOGY ]
