LT3573
Isolated Flyback Converter
Without an Opto-Coupler
FEATURES
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DESCRIPTION
The LT
®
3573 is a monolithic switching regulator specifi-
cally designed for the isolated flyback topology. No third
winding or optoisolator is required for regulation. The
part senses the isolated output voltage directly from the
primary side flyback waveform. A 1.25A, 60V NPN power
switch is integrated along with all control logic into a 16-
lead MSOP package.
The LT3573 operates with input supply voltages from
3V to 40V, and can deliver output power up to 7W with
no external power devices.The LT3573 utilizes boundary
mode operation to provide a small magnetic solution with
improved load regulation.
LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Patents pending.
3V to 40V Input Voltage Range
1.25A, 60V Integrated NPN Power Switch
Boundary Mode Operation
No Transformer Third Winding or
Optoisolator Required for Regulation
Improved Primary-Side Winding Feedback
Load Regulation
V
OUT
Set with Two External Resistors
BIAS Pin for Internal Bias Supply and Power
NPN Driver
Programmable Soft-Start
Programmable Power Switch Current Limit
Thermally Enhanced 16-Lead MSOP
APPLICATIONS
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Industrial, Automotive, Medical
Low Power PoE
TYPICAL APPLICATION
5V Isolated Flyback Converter
V
IN
12V TO 24V
B340A
V
IN
SHDN/UVLO
51.1k
R
FB
R
REF
TC
R
ILIM
SS
VC
28.7k
10k
10nF
20k
1nF
3573 TA01
Load Regulation
3
24μH
1N4148
80.6k
2.6μH
47μF
V
OUT–
OUTPUT VOLTAGE ERROR (%)
10μF
357k
3:1
0.22μF
2k
V
OUT+
5V, 0.7A
2
1
0
–1
–2
–3
0
200
400
600
800 1000 1200 1400
3573 TA01b
V
IN
= 24V
V
IN
= 12V
LT3573
6.04k
SW
GND
TEST BIAS
I
OUT
(mA)
4.7μF
3573f
1
LINER [ LINEAR TECHNOLOGY ]
