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Powering LED and other lighting devices (Part 3)

27 Apr 2015  | Bob Dobkin, John Hamburger

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As shown in figure 2, the MOSFET gate signal is high and the MOSFET is on when the TRIAC is off, bleeding off the leakage current and keeping the input voltage at 0V. As soon as the TRIAC turns on, the MOSFET seamlessly changes back into a normal power delivery device.

Open- and shorted-LED protection
The LED voltage is constantly monitored through the transformer third winding. The third winding voltage is proportional to the output voltage when the main switch is off and the output diode is conducting current. In the event of over-voltage or open-LED, the main switch turns off and the capacitor at the CT pin discharges. The circuit then enters hiccup mode as shown in figure 3.

Figure 3: Output Open-Circuit Event.

In a shorted LED event, the IC runs at minimum frequency before the VIN pin voltage drops below the UVLO threshold as the third winding cannot provide enough power to the IC. It then enters its start-up sequence as shown in figure 4.

Figure 4: Output Short-Circuit Event..

CTRL pins and analogue dimming
The LT3799's output can be adjusted through multiple CTRL pins. For example, the output current would follow a DC control voltage applied to any CTRL pin for analogue dimming. Over-temperature protection and line brownout protection can also be easily implemented using these CTRL pins.

The LT3799 is a complete offline LED driver solution featuring standard TRIAC dimming, active PFC and well-regulated LED current with no opto-coupler. This high performance and feature-rich IC greatly simplifies and shrinks offline LED driver solutions.

About the authors
Bob Dobkin is a founder and Chief Technical Officer of Linear Technology Corporation. Prior to 1999, he was responsible for all new product development at Linear. Before founding Linear Technology in 1981, Dobkin was Director of Advanced Circuit Development at National Semiconductor for eleven years. He has been intimately involved in the development of high performance linear integrated circuits for over 30 years and has generated many industry standard circuits. Dobkin holds over 100 patents pertaining to linear ICs and has authored over 50 articles and papers. He attended the Massachusetts Institute of Technology.

John Hamburger directs global marketing communications programs at Linear Technology, where he was instrumental in developing the Analog Circuit Design book series. Previously with Luminous Networks and Terayon Communication Systems, he helped define marketing strategy from start-up to public company, and held positions with Cypress Semiconductor and AMD. Prior to his career in high tech, he was an editor for Addison-Wesley, Harper & Row, WH Freeman, Harcourt Brace, Stanford University Press, and Runner's World. He holds a degree from the University of Chicago.

Excerpted from Section 21 of "Analog Circuit Design: Volume 3—The Design Note Collection". Edited by Bob Dobkin & John Hamburger, Linear Technology Corporation, Milpitas, CA, USA . Published by Newnes, an imprint of Elsevier. 2015 Linear Technology Corporation. Published by Elsevier Inc. All rights reserved.

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