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Posted on 01 September 2019

PWM Controller Simplifies High Power LED Designs

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Dimming ratios of 3000:1

High brightness (HB) and super HB LEDs are found in LCD TFT backlighting in highend TVs, industrial lighting, and projectors. One popular area is for instrument panel backlighting, interior lighting, and the brake lights of many cars and trucks.

By Tony Armstrong, Product Marketing Manager, Linear Technology Corporation

 

Luxury automobile manufacturers are increasingly taking advantage of the latest technologies in solid-state LED lighting to enhance the aesthetics of their 2007-2008 model vehicles by relying on these lighter, smaller, and more durable devices for interior and exterior illumination. LEDs promise lower long-term cost and longer life which are among many advantages over incandescent light bulbs for interior lighting, for example, and HID (high-intensity discharge) and halogen lamps for headlights.

Driving LEDs at high current requires a DC/DC converter to accurately regulate the current to ensure uniform light intensity and color integrity as well as to protect the LEDs. Furthermore, a significant challenge is to power one or several strings of LEDs from a battery voltage that can be less than, equal to, or higher than its load voltage. Yet another concern is to efficiently dim the LEDs over a large dimming ratio while preserving their chromatic characteristics at both low and high brightness levels. Furthermore, efficient operation of the DC/DC driver is a crucial requirement, especially in driving HB (high brightness) LEDs, since all the power not emitted as light is dissipated as heat.

The new LTC3783 from Linear Technology is a current-mode multitopology converter with constant-current PWM dimming for driving high-power LED strings and clusters. Proprietary techniques provide extremely fast, true PWM load switching with no transient undervoltage or overvoltage issues: dimming ratios of 3000:1 (at 100Hz) can be achieved digitally as True Color PWM dimming guarantees color integrity of white and RGB LEDs. The LTC3783 allows an additional 100:1 dimming ratio using analog control. This is an important criterion since the human eye is extremely sensitive to minor changes in ambient light. This versatile controller can be used as a boost, buck, buck-boost, SEPIC, or flyback converter, and as a constantcurrent/ constant-voltage regulator. No RSENSE operation uses a MOSFET’s on-resistance to eliminate the current-sense resistor and increase efficiency. Applications for the LTC3783 include high-voltage LED arrays and LED backlighting, as well as voltage regulators in telecom, automotive and industrial control systems. Figure 1 details a typical LTC3783 circuit schematic in a boost configuration to drive a string of high power LEDs. Its key performance features are defined in areas one through five.

The LTC3783 integrates powerful MOSFET drivers to deliver high output power to a cluster of LEDs

Features and Benefits of the LTC3783

High Current: To deliver high current (³1.5A), the LTC3783 drives an external N-channel MOSFET to power high brightness (HB) and Super HB LEDs.

High Voltage: The LTC3783’s 3V to 36V input operation and an output voltage that is scalable depending on the choices of the external power components, easily drives strings (series) or clusters (series + parallel) of LEDs.

Protection: The IC incorporates accurate current and output voltage regulation necessary to protect HB LEDs. Additional protections include overvoltage, overcurrent, and soft start.

Dimming: With True Colorä PWM 3000:1 digital dimming the LED’s constant color is preserved over a wide dimming ratio. Also, the LTC3783 is capable of additional analog 100:1 dimming.

Dimming has three uses in HB LED applications: Adjust brightness of LED; Protect the LED by dimming it when LED becomes too hot; create multiple colors by separately dimming red, green, and blue LEDs.

Multi-Topology: The LTC3783 can be specified for many LED arrangements as buck, boost, buck-boost, or flyback. One IC can be qualified for a variety of system requirements independent of input-output voltage ratio.

Conclusion

To reliably drive a string or cluster of high brightness or super high brightness LEDs the driver controller must have accurate current and voltage regulation, protection circuitry and dimming capability. These LEDs are found in large LCD TFT backlighting in automobile, notebook, TV and monitors, industrial lighting, projectors as well as automotive interior and exterior lighting. These systems may require 16V at 5A. The LTC3783 has special circuitry that makes it ideal for driving LEDs in variety of topologies. One major advantage of the LTC3783 is its simplicity in the single-inductor buck-boost topology (see data sheet). Moreover, the LTC3783’s digital PWM input can be used to digitally dim LEDs. The IC also has a PWM driver for driving a second MOSFET for dimming.

 

 

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