Project Lighting - April 2008 - (Page 6)

CoveR stoRy FIg 1 a 350-ma linear leD DriVeric Such aS the maX16806 eliminateS the neeD for a microcontroller or SWitch-moDe conVerter. circuit to create constant current source is to have a MOSFET in series with an HBLED, measure the HBLED current, compare it to a reference voltage, and feed it back through an operational amplifier to control the gate of the MOSFET. This type of circuit behaves like an ideal current source, which keeps the current constant irrespective of forward-voltage and supplyvoltage variations. Linear HBLED driver ICs such as the MAX16806 (see Figure 1), with integrated MOSFET and a high-accuracy internal reference, provide brightness consistency between each light fixture. The benefits of using a linear driver over a switch-mode driver are that linear drivers are simpler to implement and present no EMI concerns since they have no high-frequency switching. The total solution cost is low due to minimum external component count. The MAX16806 requires an input supply that is only 1 V above the total drop across the LED string. An external sense resistor measures the LED current, thus enabling the MAX16806 to keep the current constant while the input voltage or the LED forward-voltage change. The power dissipation in a linear driver is equal to the LED current times the voltage drop across the internal (or external) series pass device. As the LED current or input voltage increases so does the power loss, thus limiting the use of linear drivers. To limit the power dissipation in the light fixture, MAX16806 measures the input voltage, and if the voltage exceeds its preprogrammed level it will reduce the LED current resulting in reduced power dissipation. This feature can eliminate the need for a switch-mode driver for applications such as automotive dome light or day timer running light (DRL) where the light can Contents Viewpoint Selecting HBLED Drivers for Lighting Apps Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs Lighting News Home Page Product Training Module Online Ordering Suppliers Catalog HBLEDs, and 2.3 to 3.5 V for red and amber HBLEDs. To maintain a constant color spectrum and brightness, the HBLEDs must be driven at their specified current rating. Driving the HBLEDs with a voltage source and limiting the current 6 | Project lighting | aPr 08 with a series resistor will result in unacceptable brightness and emitted spectrum variations. LinEar DrivErs The best approach is to drive the HBLEDs with a constant current source. The simplest http://digikey.com/ http://ad.doubleclick.net/clk;198717884;26055153;t http://ad.doubleclick.net/clk;198717989;26055167;e http://ad.doubleclick.net/clk;198717988;26055168;e http://ad.doubleclick.net/clk;198717986;26055170;v http://ad.doubleclick.net/clk;198717985;26055171;v

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Project Lighting - April 2008 Contents Viewpoint About Project Lighting Sponsor Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs LED Lighting Needs Smarts to be Competitive Lighting News Enter to Win an iPhone Refernce Materials Lighting Quick Links

Project Lighting - April 2008

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