LT3825
Isolated No-Opto
Synchronous Flyback Controller
with Wide Input Supply Range
DESCRIPTION
FEATURES
The LT®3825 is an isolated switching regulator controller
n
Senses Output Voltage Directly from Primary Side
Winding—No Optoisolator 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.
n
n
n
n
n
n
n
n
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 optoisolator,
improving dynamic response and reliability. 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
TheLT3825operatesinforcedcontinuousconductionmode
which improves cross regulation in multiple winding ap-
plications.Switchingfrequencyisuserprogrammableand
canbeexternallysynchronized.Thepartalsohasloadcom-
pensation, undervoltage lockout and soft-start circuity.
APPLICATIONS
n
Isolated Medium Power (10W to 60W) Supplies
n
Isolated Telecom, Medical Converters
n
Instrumentation Power Supplies
n
Isolated Power over Ethernet Supplies
L, LTC and LT are registered trademarks of Linear Technology Corporation.
All other 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 12V 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
CMP
ENDLY
OSC GND SFST
C
CMP
ON
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)
3825fa
1