Driving The Backlight: CCFLs Or LEDs?
Bob Arnold, Joe Barnett, Tom Novitsky | ED Online ID #14814 | February 15, 2007
Bob Arnold, Joe Barnett, Tom Novitsky | ED Online ID #14814 | February 15, 2007
The typical LCD backlight can be one or more cold cathode fluorescent lamps (CCFLs) or an array of light-emitting diodes (LEDs). An example of each is shown in Figure 1. The quality of the backlight image depends heavily on the backlight driver. In this article, we will discuss the considerations one can make for CCFLs and LEDs, as well as how to power both kinds of backlight.
GENERAL CCFL CONSIDERATIONS
CCFL backlights are the most common backlight technology and are used in displays ranging from 5.7 to 23 in. or more diagonal. They can have from one to 24 or more lamps mounted along the edge of the LCD or spaced uniformly over the entire back of the display.
Typically, brightness is controlled by modulating the CCFL current or lamp duty cycle. The basic driver is a dc-to-ac inverter powered by 5 to 48 V dc.
GENERAL LED CONSIDERATIONS
LEDs are already used in a wide range of smaller displays. For larger displays, because of their higher power consumption and, in some cases, their mercury content, CCFL backlights are beginning to be replaced by LED backlighting. LEDs may be arranged along the edges of the LCD or as a matrix over the back of the LCD assembly. The LED devices may be arranged in series or parallel. Either configuration will provide uniform LCD lighting. The LED strings can be arranged in parallel using a series resistor in each string to provide string-to-string current balancing as well as lighting redundancy.
GENERAL CCFL CONSIDERATIONS
CCFL backlights are the most common backlight technology and are used in displays ranging from 5.7 to 23 in. or more diagonal. They can have from one to 24 or more lamps mounted along the edge of the LCD or spaced uniformly over the entire back of the display.
Typically, brightness is controlled by modulating the CCFL current or lamp duty cycle. The basic driver is a dc-to-ac inverter powered by 5 to 48 V dc.
GENERAL LED CONSIDERATIONS
LEDs are already used in a wide range of smaller displays. For larger displays, because of their higher power consumption and, in some cases, their mercury content, CCFL backlights are beginning to be replaced by LED backlighting. LEDs may be arranged along the edges of the LCD or as a matrix over the back of the LCD assembly. The LED devices may be arranged in series or parallel. Either configuration will provide uniform LCD lighting. The LED strings can be arranged in parallel using a series resistor in each string to provide string-to-string current balancing as well as lighting redundancy.
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