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Optoelectronics/Displays??

Implementing analog dimming on the DS39xx CCFL controllers

Posted: 05 Feb 2008 ?? ?Print Version ?Bookmark and Share

Keywords:CCFL? analog dimming? controllers?

There are two common methods to dim cold-cathode fluorescent lamps (CCFLs): burst dimming, which is also called pulse with modulation (PWM) dimming or digital dimming, and analog dimming. This article discusses the advantages and disadvantages of each method.

Burst dimming turns the CCFLs on and off at a certain frequency, which is called the PWM dimming frequency. If the PWM dimming frequency is greater than 60Hz, the human eye cannot detect that the CCFLs are switching on and off. During the high period of the PWM cycle, the CCFLs are turned on and work at the lamp frequency. During the low period of the PWM cycle, the CCFLs are turned off and no current flows through them. By adjusting the duty cycle of the PWM pulses, one can thus increase or decrease the brightness of the CCFLs. The principal advantage of burst dimming is that it can achieve a very large dimming ratio. In some applications, however, the PWM dimming frequency may interfere with the vertical synchronous frequency of the display signals, thereby causing visible effects on the screen. Burst dimming can also cause audible transformer noise.

Instead of turning on the CCFLs in bursts, analog dimming keeps the CCFLs on continuously. Lamp brightness is adjusted by varying the lamp current amplitude. Obviously, a greater current amplitude results in a brighter CCFL, and a lower amplitude renders a dimmer CCFL. Analog dimming has a very narrow dimming range, which is insufficient in some applications. But analog dimming does not cause any audible transformer noise, since the PWM frequency is not present.

The DS3881, DS3882, DS3984, DS3988, DS3992 and DS3994 are controllers for CCFLs that backlight LCDs. To achieve satisfactory visual results or conserve lamp life, dimming is required in most applications. This application note first introduces the two dimming approaches commonly used for CCFLs. It then describes how to implement analog dimming on the DS39xx CCFL controllers.

View the PDF document for more information.




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