21 November 2011

App Note 65 part 1

"A fourth generation of LCD backlight technology: Component and measurement improvements refine performance." 124 pages!

This one-hundred-twenty-four-page app note is Part 4 (as the title says, "fourth generation") in the grand saga of cold-cathode fluorescent lamps (CCFLs). Just to remind you:
Being a major, major update to the material published previously, this app note is over twice as long as App Note 55, and it is the second-longest one he ever wrote (App Note 47 is still the king of length). The Preface on page 1 sets the stage and summarizes the issues. The Introduction on page 4 acknowledges the length of this effort, "the longest sustained LTC application engineering effort to date." He also acknowledges the very beginning, "a single circuit in a 1991 publication" (Figure 36 in App Note 45), which he didn't acknowledge in Chapter 11 of his second book.

The organization of this app note, as compared to App Note 55, shows a distinct change in emphasis. "Perspectives on display efficiency" and Figure 1 are first up in the main text. This section is an updated version of Appendix K in App Note 55, but it now takes the stage front-and-center. Clearly, as the circuits got better, display efficiency became a more important issue. Figures 2 through 9 summarize typical characteristics of CCFLs, some of which is above and beyond the material presented in the introduction of App Note 55 (particularly the plots of on-time and lamp emission).

Next up is display losses (illustrated schematically by Figures 10 and 11), and the gallery of "display situations" in Figures 12 through 32 (thirty pages of photos!). "The deleterious effects of display parasitics dominate practical backlight design." The examples shown demonstrate losses from 1.5% (Figure 14) to 31% (Figure 31). I assume that Jim had spent a lot of time talking to customers and saying, "No, don't do that."

The final display-loss-related section is "Considerations for multilamp designs" on page 31. It is funny to examine how his opinion of multilamp designs has changed over time. They were suggested in App Note 49 (page 4), with a little discussion of "different transformer rating" and the "differences in lamp wiring", but, "Practically, these differences are small, and the lamps appear to emit equal amounts of light." In App Note 55 (pages 12 and 13), he writes, "Systems using two lamps have some unique layout problems," but he concludes, "imbalanced illumination causes fewer problems than might be supposed." Here in App Note 65, "The text's tone is intended to convey our distaste for multilamp displays. They are the very soul of heartache."


I'll talk about the circuits next time.


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