1

LT1425

Isolated Flyback

Switching Regulator

s

No Transformer “Third Winding” or Optoisolator

Required

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±5% Accurate Output Voltage Without User Trims

(See Circuit Below)

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Resistor Programmable Output Voltage

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Regulation Maintained Well Into Discontinuous

Mode (Light Load)

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Optional Load Compensation

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Operating Frequency: 285kHz

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Easily Synchronized to External Clock

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Available in 16-Pin Narrow SO Package

FEATURES

The LT

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

µA for

standby operation.

11

ISOLATED

–9V

±5% AT

20mA TO 200mA

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

710

LT1425

SGND PGND

SHDN

SYNC

R2

3.01k

1%

R3

15k

C4

0.1

µF

C3

1000pF

C1

100

µF

10V

C2

47

µF

16V

+

•

•

F

OUTPUT CURRENT (mA)

0

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

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Isolated Flyback Switching Regulators

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Ethernet Isolated 5V to – 9V Converters

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Medical Instruments

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Isolated Telecom Supplies

, LTC and LT are registered trademarks of Linear Technology Corporation.