SMBus Controlled CCFL Power Supply

Sep 1 1999

143.59 K

Figure 1 shows a cold cathode fluorescent lamp (CCFL) power supply that is controlled via the popular SMBus interface. The LT1786 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.

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

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.

Note:

¹ Local historians can’t be certain, but this may be the only IC pin ever named after a person.

About The Authors

Jim Williams

James M. Williams (April 14, 1948 – June 12, 2011) was an analog circuit designer and technical author who worked for the Massachusetts Institute of Technology (1968–1979), Philbrick, National Semiconductor (1979–1982) and...