- 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.
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