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Fixed-frequency CCFL controller

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article image Synchronisation and phase-shift features for multi-lamp applications

MAXIM Integrated Products, represented in Australia by Arrow Electronics , has released the MAX8729, a fully integrated controller optimised to drive multiple cold-cathode fluorescent lamps (CCFL) used in LCD monitor and TV applications. It drives multiple lamps by using a half-bridge inverter consisting of two external n-channel power MOSFETs. Its half-bridge topology minimises the component count while providing near sinusoidal drive waveforms over the entire input range.

It operates in resonant mode during striking and switches over to fixed-frequency operation after the lamps are lit. This ensures reliable striking under all conditions and reduces transformer stress.

It achieves a 10:1 dimming range by turning the CCFL on and off using a digitally adjusted PWM (DPWM) method. CCFL brightness can be set with an analogue voltage on the CNTL pin. Synchronisation and phase-shift features for both the switching and DPWM frequencies permit daisy-chaining multiple MAX8729s. The switching and DPWM frequencies can be easily adjusted with external resistors.

Phase-shift select pins, PS1 and PS2, can be used to program up to four different phase shifts, allowing up to six MAX8759s to be used together. The gate drivers drive large power MOSFETs typically used in applications where one transformer powers four CCFLs in parallel. An internal 5.3V linear regulator powers the MOSFET drivers and most of the internal circuitry.

The MAX8729 is available in a low-cost, 24-pin QSOP, and operates from -40°C to 85°C. A preassembled MAX8729 evaluation kit with recommended external components is available to reduce design time.

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