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Highly Integrated 2-/4-Channel CCFL Controllers Reduce the BOM Cost of Multilamp LCD-Backlighting Circuits

DALLAS, TX— Dallas Semiconductor, a wholly owned subsidiary of Maxim Integrated Products, introduces the DS3992/DS3994, 2-/4-channel controllers for cold-cathode fluorescent lamps (CCFLs) that are used to backlight liquid-crystal displays (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.

The DS3992/DS3994 use a push-pull drive scheme to convert a DC voltage (5V to 24V) to the high-voltage (600VRMS to 1200VRMS and 300VRMS to 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.

Each DS3992/DS3994 channel 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 DS3992/DS3994 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 $0.60 for the DS3992 and $0.85 for the DS3994 (1000-up, FOB USA).

Miércoles, 20 Diciembre, 2006 - 07:04
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