AN2654

Electrical function of the design

9/26

2

Electrical function of the design

2.1

Enable

Enable is the function to turn on/off the VCC of the IC (see Figure 6). A CMOS or TTL logic

signal can be used to drive the L6574 into an on/off state. High enables the chip, low

disables it.

2.2

Operation

In Figure 7, Q1 and Q2 are driven by the L6574 in a half-bridge configuration to power the

CCFL lamps. In order to provide several kV striking voltage for ignition, one LC tank has

been added on the secondary side of the transformer to generate the proper alternative

voltage. After power-on, the MOSFET driving frequency starts with a high frequency and

decreases to a lower operating point following a descending slope. Ignition is accomplished

within this frequency shift. The leakage inductance of the secondary side and the reflected

inductance from the primary side consist of the inductance of the LC tank (refer to Appendix

Prior to ignition the LC resonant circuit provides a striking voltage to the lamp. After ignition,

too. The CCFL becomes an inductive load. This inductive load of half-bridge benefits the

MOSFET by making zero-voltage switching (refer to Figure 8). Thanks to the high-voltage

ZVS topology, the switching and conduction losses are greatly reduced.

Figure 6.

Enable function

Figure 7.

Resonant topology