1
LT1425
Isolated Flyback
Switching Regulator
■
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
FEATURES
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 15
µ
A for
standby operation.
DESCRIPTIO
N
U
TYPICAL APPLICATIO
N
U
+
11
ISOLATED
–9V
±
5% AT
20mA TO 200mA
V
–
1425 TA01
*DALE LPE 4841-330MB
12
5V
D1
1N5819
500V
ISOLATION BARRIER
T1*
15
6
4
3
R1
22.6k
1%
5
14
13
7
10
V
IN
LT1425
SGND PGND
V
SW
R
FB
R
REF
R
OCOMP
SHDN
SYNC
V
C
R
CCOMP
R2
3.01k
1%
R3
15k
C4
0.1
µ
F
C3
1000pF
C1
100
µ
F
10V
C2
47
µ
F
16V
+
•
•
F
OUTPUT CURRENT (mA)
0
OUTPUT VOLTAGE (V)
8.5
9.0
8.9
8.8
8.7
8.6
9.5
9.4
9.3
9.2
9.1
50
100
1425 TA02
150
200
Load Regulation
5V to Isolated – 9V
OUT
APPLICATIO
N
S
U
■
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.
