STEVAL-ILC001V1 STMicroelectronics

AN2654 Application note CCFL backlight half-bridge topology based on L6574 and STD7NS20 Introduction Cold cathode fluorescent lamps (CCFL) are widely used in the backlighting of television and PC monitor applications due to their low cost and high efficiency. A 12 V or 24 V DC voltage is provided for the backlighting inverter to drive the CCFL. In order to improve the efficiency and performance of the total system both for the AC-DC power supply and inverter, STMicroelectronics has introduced a high-voltage ballast-driver IC L6574 that provides the half-bridge solution for CCFL backlighting. Thanks to BCD offline technology, the L6574 can handle high-side voltage rail up to 600 V and consumes less current. Of course, the power loss of the bridge rectifier is reduced. Figure 1 shows the typical LCD TV power section block diagram. Different supply voltages for backlight applications are available on the market. Table 1 shows the required backlight power for various sizes of TV display screens. For the same screen size, there are three different types of supply voltages for CCFL backlight and each consumes a different current. Using a high-voltage backlight CCFL solution allows greatly reducing the power losses of the bridge rectifier and the conduction loss of the primary switch. Figure 1. Typical LCD TV power section block diagram Table 1. Required backlight power and its current consumption vs. supply voltage Current consumption vs. supply voltage (V ) lamp Backlight power TV display screen size (inches) (Watts) 12 V 24 V 100 V 17 24 W 2 A 1 A 0.24 A 19 30 W 2.5 A 1.25 A 0.3 A 27 120 W - 5 A 1.2 A 32 144 W - 6 A 1.44 A 42 264 W - 11 A 2.64 A May 2008 Rev 1 1/26 www.st.comContents AN2654 Contents 1 Board description . 4 1.1 Reference design board and PCB layout 4 1.2 Schematic 5 1.3 Bill of materials 6 2 Electrical function of the design . 9 2.1 Enable 9 2.2 Operation 9 2.3 Soft-start 11 2.4 Dimming 12 2.4.1 Analog dimming . 12 2.4.2 Digital dimming . 12 2.5 Protection . 14 2.5.1 Open lamp protection 14 2.5.2 Overvoltage protection 15 2.5.3 Overcurrent protection 16 3 Description of the half-bridge driver . 17 3.1 Functions and block diagram of the IC . 17 3.2 IC pin description in this reference design 18 3.3 HVG driver, LVG driver and oscillator frequency . 18 3.4 Timing capacitor, oscillator frequency and equations 19 3.5 Figures for F , R and F 20 min min start Appendix A Resonant tank design 22 Revision history 25 2/26