●Liquid crystal display (LCD) enclosures and cold cathode fluorescent lamps (CCFLs) used in notebook computer and portable electronics displays are becoming increasingly narrow, generating the need for a low profile CCFL power supply. Recent advances in single- and multi-layered piezoelectric ceramic transformers (PZT) have enabled the development of a new generation of efficient, size-reduced backlight converters. The UCC3975/6/7 family of 8-pin PZT controllers integrate the necessary circuitry for operating a PZT-based backlight supply using a flyback, half-bridge, or push-pull topology. The choice of power topology depends on application requirements such as input voltage, lamp voltage, and PZT gain.
●A half-bridge PZT converter, using the UCC3976 is shown in Figure 1. External P- and N-channel MOSFETs are driven out of phase at a fixed 50% duty cycle with anti-cross conduction circuitry provided by the controller. The half-bridge topology uses only a single magnetic component (LRES) reducing board area. As explained in the applications section of this datasheet, regulation of lamp current is achieved by controlling the operating frequency of the system.
●The UCC3977 is designed to control a resonant push-pull topology as shown in Figure 2. This controller alternately drives external N-channel MOSFETs at 50% duty cycle. The push-pull topology requires two external inductors (L1 and L2), but has the advantage of providing increased voltage across the piezoelectric transformer primary. In this case a small overlap is provided to the gate drives, assuring an uninterrupted path for inductor current.
●For piezoelectric transformer applications requiring additional gain, a resonant flyback topology can be implemented using the UCC3975. As shown in Figure 3, a magnetic transformer (T1) provides a stepped up voltage to the piezoelectric transformer primary. When compared to a traditional high-voltage transformer used in a CCFL application, T1 is small and low profile because of its reduced turns ratio and voltage rating. In the resonant flyback application, a single switch is driven at 50% duty cycle producing a half wave rectified sinusoid at the piezoelectric transformer primary.
● 3-V to 13.5-V Operation
● Supports Flyback (UCC3975), Half-Bridge (UCC3976), and Push-Pull (UCC3977) Topologies
● Programmable Voltage Controlled Oscillator
● Open Lamp Protection
● Low Shutdown Current (15-uA Typical)
● Dual MOSFET Drivers
● 8-Pin TSSOP package
● APPLICATIONS
● Notebook Computers
● Portable Electronics Displays
● Portable Instruments
