Design Idea:
SMBus Controlled CCFL Power Supply

by Jim Williams

Figure 1 shows a cold cathode fluorescent lamp (CCFL) power supply that is controlled via the popular SMBus interface. The LT1786F CCFL switching regulator receives the SMBus instruction. The IC converts this instruction to a current, which appears at the I_OUT pin. This current, routed to the I_CCFL pin, provides a set point for switching regulator operation. The resultant duty cycle at the V_SW pin pulls current through L2. L2, acting as a switched current sink, drives a resonant Royer converter composed of Q1—Q2, C1 and L1. The high voltage sine wave produced at L2's secondary drives the floating lamp.

Current flow into the Royer converter is monitored by the IC at pin 13 ("Royer" in Figure 1).¹ Royer current correlates tightly with lamp current, which, in turn, is proportional to intensity. The IC compares the Royer current to the SMBus-derived current, closing a lamp-intensity control loop. The SMBus permits wide-range regulated lamp-intensity control and allows complete IC shutdown. Optimal display and lamp characteristics permit 90% efficiency. The circuit is calibrated by correlating SMBus instruction codes with attendant RMS lamp current. Detailed information on circuit operation and measurement techniques appears in the references below.    

Figure 1. 90% efficient floating CCFL with 2-wire SMBus lamp-current control

References:

1. Williams, Jim. Linear Technology Application Note 65: A Fourth Generation of LCD Backlight Technology. November 1995.

2. LT1786F Data Sheet. Linear Technology Corp. 1998.