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Dallas Semi unrolls CCFL controllers for LCDs

Posted: 03 Jan 2007 ?? ?Print Version ?Bookmark and Share

Keywords:CCFL? LCD? controller? Dallas Semi? backlight?

Dallas Semiconductor has introduced the DS3992/DS3994, 2/4-channel controllers for CCFLs used to backlight LCDs in TV and PC monitor applications. The DS3992 supports a one-lamp-per-channel configuration with fully independent lamp control, while the DS3994 supports configurations of one to four channels and allows multiple DS3994 controllers to be cascaded.

These controllers use a push-pull drive scheme to convert a DC voltage (5-24V) to the high-voltage (600-1200Vrms and 300-1400Vrms, respectively) AC waveform that is required to power CCFLs. The push-pull drive scheme uses a minimal number of external components, which reduces component and assembly cost and makes the PCB design easy to implement. The push-pull drive scheme also provides an efficient DC-to-AC conversion and produces near-sinusoidal waveforms.

Accurate current control
Each channel in these controllers drives two logic-level n-channel MOSFETs that are tied between the ends of a step-up transformer and ground. The transformer has a center tap on the primary side that is connected to the DC supply. The DS3992/DS3994 alternately turn on the two MOSFETs to create the high-voltage AC waveform on the secondary side. By varying the duration of the MOSFET turn-on times, the controllers accurately control the amount of current flowing through the CCFL.

A series resistor on the low-voltage side of the CCFL enables current monitoring. The voltage developed across this resistor is fed to the lamp current monitor (LCM) input on each device. Then, the controllers compare the resistor voltage against an internal reference voltage to determine the duty cycle for the MOSFET gates.

Each CCFL receives independent control, which results in equal brightness across all of the lamps and Maximizes lamp life and brightness.

Both devices operate from 4.5V to 5.5V. The DS3992 is available in a 16-pin SO package, while the DS3994 is available in a 28-pin SO package. Pricing starts at 60 cents for the DS3992 and 85 cents for the DS3994 (1,000-up, FOB USA).

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