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Maxim > Design Support > Technical Documents > Application Notes > Digital Potentiometers > APP 4135
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 4135
Keywords: CCFL, analog dimming, lamp current amplitude modulation, DS3994, DS3992, DS3988,
DS3991
APPLICATION NOTE 4135
Implementing Analog Dimming on the DS39xx
CCFL Controllers
By: Shoumin Liu
Dec 14, 2007
Abstract: The DS3881, DS3882, DS3988, DS3991, DS3992 and DS3994 are controllers for cold-cathode
fluorescent lamps (CCFLs) that backlight liquid crystal displays (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.
CCFL Dimming Methods
There are two common methods to dim CCFLs: burst dimming, which is also called 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. Analog dimming, moreover, does not
interfere with the vertical synchronous frequency.
Table 1 compares analog dimming and burst dimming. More information can be found in the Application
Note 3997: How to Achieve a 300:1 Dimming Ratio with the DS3881/DS3882 CCFL Controllers.
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