- By Rich Rosen, Field Applications Engineer, National Semiconductor -

The exponential growth of LED lighting has ushered in a vast selection of integrated circuit devices to provide controlled power to LEDs. No longer acceptable to an energy-conscious world, switched-mode LED drivers have long since replaced power-hungry linear current sources as the standard.

Applications from fl ashlights to stadium scoreboards all require precise control of regulated currents. In many instances, real-time changes in LED output intensity are required. Th is function is commonly referred to as dimming control. This article describes some basic LED theory and several techniques used to provide dimming control to switched-mode LED drivers.

LED Brightness and Color Temperature
LED Brightness
The concept of the brightness of visible light from an LED is fairly easy to understand. Assigning a numerical value to the perceived brightness of an LED’s output can simply be measured in units of luminous fl ux density, called candelas (cd). Th e total power output of an LED is a measurement of the amount of Lumens (lm).

It is also important to understand that average forward LED current determines the brightness of an LED. Figure 1 shows the relationship between forward LED current vs. Lumens output for a certain LED. Th e relationship is remarkably linear over useable ranges of IF or forward current. Note the nonlinearity appearing as IF increases. Reduced effi cacy in Lumens per Watt arises as the operating current exceeds the linear range.

High Performance, TRIAC Dimmable LED Driver
National’s industry-leading TRIAC dimmable offl ine LED driver solution is perfect for any application where an LED driver must interface to a standard TRIAC wall dimmer. National’s new TRIAC dimmable LED driver delivers a wide, uniform dimming range free of fl icker, best-in-class dimming performance, and high effi ciency?all while maintaining ENERGY STAR® power factor requirements in a typical application.

Full Range Dimming Capability
National’s TRIAC dimmable LED driver offers 100:1 full range dimming capability, going from full light to nearly imperceptible light in a  continuous range without being extinguished, and maintains a constant current to large strings of LEDs driven in series off of a standard line voltage.

Uniform Dimming Without Flicker
National’s TRIAC dimmable LED driver allows master-slave operation control in multi-chip solutions which enables a single TRIAC dimmer to control multiple strings of LEDs with smooth consistent dimming, free of fl icker. Easy To Use National’s TRIAC dimmable LED driver enables a direct replacement of incandescent or halogen lamp systems that are currently interfaced to a TRIAC dimmer without having to change the original infrastructure or sacrifice performance. In addition, the new TRIAC dimmable LED driver is available in WEBENCH® LED Designer to allow for easy and quick design in.

For details, open the attached file.