SMBus Controlled CCFL Power Supply

1999-09-01

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.

关于作者

Jim Williams

James M. Williams（1948年4月14日-2011年6月12日）是一名模拟电路设计人员兼技术文章作者，先后就职于麻省理工学院(1968–1979)、Philbrick、National Semiconductor (1979–1982)和凌力尔特公司(LTC) (1982–2011)。[1]他撰写了350多篇有关模拟电路设计的论文[2]，包括5本书、21篇National Semiconductor应用笔记、62篇凌力尔特应...