LT3825
Isolated No-Opto
Synchronous Flyback Controller
with Wide Input Supply Range
DESCRIPTION
FEATURES
The LT®3825 is an isolated switching regulator controller
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Senses Output Voltage Directly from Primary Side
Winding—No Opto-Isolator Required
Synchronous Driver for High Efficiency
Input Voltage Limited Only by External
Power Components
designed for medium power flyback topologies. A typical
application is 10W to 60W with input voltage limited only
by external power path components. A third transformer
winding provides output voltage feedback.
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Accurate Output Regulation without User Trims
Switching Frequency from 50kHz to 250kHz
Synchronizable
The LT3825 is a current mode controller that regulates
output voltage based on sensing secondary voltage via a
transformer winding during flyback. This allows for tight
output regulation without the use of an opto-isolator,
improvingdynamicresponseandreliability. Synchronous
rectification increases converter efficiency and improves
output cross regulation in multiple output converters.
Load Compensation
Programmable Undervoltage Lockout
Available in a Thermally Enhanced 16-Lead
TSSOP Package
The LT3825 operates in forced continuous conduction
modewhichimprovescrossregulationinmultiplewinding
applications. Switching frequency is user programmable
and can be externally synchronized. The part also has load
compensation,undervoltagelockoutandsoft-startcircuity.
L, LT, LTC, LTM, Burst Mode, SwitcherCAD, Linear Technology and the Linear logo are registered
APPLICATIONS
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Isolated Medium Power (10W to 60W) Supplies
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Isolated Telecom, Medical Converters
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Instrumentation Power Supplies
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Isolated Power over Ethernet Supplies
trademarks and No R
, ThinSOT are trademarks of Linear Technology Corporation. All other
SENSE
trademarks are the property of their respective owners. Protected by U.S. Patents, including
6948466, 5841643.
Efficiency
92
TYPICAL APPLICATION
90
36V
IN
48V
IN
88
72V
IN
86
84
82
80
48V to 3.3V at 12A Isolated Supply
+
V
3.3V
12A
+
OUT
V
IN
36V TO 72V
470µF
•
100pF
20Ω
×4
T1
28.7k
3.01k
•
T1
20Ω
47µF
47k
78
•
47Ω
2
5
7
8
9
10 11 12
3
4
6
+
3825 TA01b
LOAD CURRENT (A)
SG
SG
402k
15k
Regulation
×2
+
–
FB
V
PG
3.43
3.38
3.33
3.28
3.23
3.18
3.13
SENSE
CC
2.2µF
SG
1µF
0.02Ω
UVLO
330Ω
15Ω
LT3825
SENSE
V
36V
48V
72V
IN
IN
IN
2.2nF
PGDLY
0.1µF
C
t
SYNC
R
ENDLY
OSC GND SFST
C
CMP
ON
CMP
10nF
100k
•
•
0.22µF
0.1µF
10k
12k 100k
2.1k 150k
47pF
3825 TA01a
2
5
7
8
9
10 11 12
3
4
6
3825 TA01c
LOAD CURRENT (A)
3825fe
1