LINEAR TECHNOLOGY LT3825 Specifications and User's Guide
Summary
The LT3825 is an advanced synchronous isolated switching regulator controller perfect for designing high-efficiency medium-power flyback supplies (10W to 60W). This modern controller eliminates the need for traditional optoisolators by sensing output voltage directly from the primary side. The manual details its functionality for applications including medical, telecom equipment, and Power over Ethernet, offering features like wide input range operation, synchronous driving, and precise output regulation without external trimmers. Ideal for engineers building reliable, isolated DC/DC converters.
Page 1 Text Content
LT3825
Isolated No-Opto
Synchronous Flyback Controller with Wide Input Supply Range
FEATURES DESCRIPTIO
Senses Output Voltage Directly from Primary Side The LT®3825 is an isolated switching regulator controller Winding—No Optoisolator Required designed for medium power flyback topologies. A typical Synchronous Driver for High Efficiency application is 10W to 60W with input voltage limited only Input Voltage Limited Only by External by external power path components. A third transformer Power Components winding provides output voltage feedback.
Accurate Output Regulation Without User Trims
The LT3825 is a current mode controller that regulates
Switching Frequency from 50k Hz to 250k Hz
output voltage based on sensing secondary voltage via a
Synchronizable
transformer winding during flyback. This allows for tight
Load Compensation
output regulation without the use of an optoisolator,
Programmable Undervoltage Lockout
improving dynamic response and reliability. Synchronous
Available in a Thermally Enhanced 16-Lead
rectification increases converter efficiency and improves
TSSOP Package
output cross regulation in multiple output converters. APPLICATIO S The LT3825 operates in forced continuous conduction mode which improves cross regulation in multiple winding
Isolated Medium Power (10W to 60W) Supplies
applications. Switching frequency is user programmable
Isolated Telecom, Medical Converters
and can be externally synchronized. The part also has load
Instrumentation Power Supplies
compensation, undervoltage lockout and soft-start circuity.
Isolated Power over Ethernet Supplies
OU TP UT (V EF FI CI EN CY , 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
TYPICAL APPLICATIO
36VIN 48VIN
72VIN
48V to 3.3V at 12A Isolated Supply
36V TO 72V 470µF
47µF SG LOAD CURRENT (A) 3825 TA01b
Regulation
FB VCC SG PG SENSE+
2.2µF SG 3.43
LT3825 15Ω330Ω 3.38
15k SENSE–
36VIN
VC 2.2n F0.1µF
PGDLY 48VIN
t ON SYNC RCMP ENDLY OSC GND SFST CCMP 72VIN
3825 TA01a
LOAD CURRENT (A) 3825 TA01c
Page Summary Contents For LINEAR TECHNOLOGY LT3825 Specifications and User's Guide
Manual Details
| Brand | Linear |
|---|---|
| Pages | 32 |
| File Size | 335.59 KB |
| Published | June 19, 2026 |
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Frequently Asked Questions
Does the LT3825 require an optoisolator for voltage sensing?
No, it uses synchronous transformer winding sensing, eliminating the need for an optoisolator.
What type of power applications is this controller best suited for?
It is ideal for isolated medium power supplies, typically ranging from 10W to 60W.
Can the switching frequency be adjusted or controlled by external signals?
Yes, the switching frequency is user programmable and can also be externally synchronized.
How does the device improve performance and reliability during operation?
It incorporates Programmable Undervoltage Lockout (UVLO) and soft-start circuitry to improve dynamic response.