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Drive high-power LED strings from just about any input

15 Jun 2015  | Bob Dobkin, John Hamburger

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PWM dimming requires a level-shift from the PWMOUT pin to the high-side LED string. The max PWM dimming ratio increases with higher switching frequency, lower PWM dimming frequency, higher VIN and lower LED power. In this case, a 100:1 dimming ratio is possible with a 100Hz dimming frequency and a 48V input. Although higher switching frequency is possible, the duty cycle has its limits. Generous minimum on-time and minimum off-time restrictions require a frequency on the lower end of its range (150kHz) to meet both the harsh high-VIN-to-low-VLED (80VIN to one 3.5V LED) and low-VIN-dropout requirements (10VIN to 7VLED).

OVP of the buck mode LED driver has a level shift as well. Without the level-shifted OVP network tied to FB, an open LED string would result in the output capacitor charging up to VIN. Although the buck mode components will survive this scenario, the LEDs may not survive being plugged into a potential equal to VIN.

Buck-boost mode
A common LED driver requirement is that the ranges of both the LED string voltage and the input voltage are wide and overlapping. In fact, some designers prefer to use the same LED driver circuit for several different battery sources and several different LED strings. Such a versatile configuration trades some efficiency, component cost, and board space for design simplicity, and time-to-market.

The buck-boost mode driver in Figure 286.3 uses a single inductor. It accepts inputs from 9V to 36V to drive 10V–50V LED strings at 400mA.

Figure 3: An A Buck-Boost Mode LED Driver with Wide-Ranging VIN and VLED

The inductor current is the sum of the input current and the LED string current; the peak inductor current is equal to the peak switching current. Below 9V input, CTRL analogue dimming scales back the LED current to keep the inductor current under control. UVLO turns off the LEDs below 6VIN. COUT, DI and MI can see voltages as high as 95V here.

The LT3756 controller is a versatile high power LED driver. It has all the features required for large (and small) strings of high power LEDs. Its high voltage rating, optimised LED driver architecture, high performance PWM dimming, host of protection features and accurate high side current sensing make the LT3756 a single-IC choice for a variety of lighting systems.

Data sheet for LT3756

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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