28 November 2011

App Note 65 part 3

Even after the introduction of the LT1182 family of CCFL Switching Regulators on page 36, several of the following circuits in the app note are still borrowed from App Note 55. In other words, in many of these cases, Jim got it right the first time (or is it the third time?).

For example, the low-power CCFL supplies (Figures 40, 41, and 42) are quite similar to App Note 55 Figures 10, 11, and 12. However, both the low-power circuit in Figure 43 and the high-power circuit in Figure 44 are new. Figure 44 produces up to 25W of power!

Floating-lamp circuits are discussed starting on page 40 with Figures 45 and 46. Previously, floating lamps were used to extend the illumination range (without the thermometer effect). Here, floating lamps are explained to be (potentially) more efficient than lamps that are grounded on one end. The circuits in Figures 47 and 48 are mostly the same as App Note 55 Figures 27 and 28.

The following figures are new, utilizing the LT1182 family of CCFL controllers. Using the ROYER pin (discussed previously!) to sense the current in the primary, there is no need for direct feedback from the secondary (lamp) side of the transformer. Thus, the lamp is isolated and floating. Figures 49, 50, and 51 use this approach. Figures 52 and 53 outline a microcontroller interface and the necessary software (not Jim's fault!).

Figure 54 shows a floating-lamp supply that can provide 30W (which is more power than the LT1182 family can deliver). The feedback is implemented with a current-sense transformer.

In the next-to-last section (starting on page 46), Jim outlines the selection criteria for CCFL circuits and discusses the numerous trade-offs involved. He discusses a laundry list of topics on pages 47 through 49. Some of the trade-offs are summarized in the tables in Figures 55 and 56, but clearly, Jim was "asked" to include these summary tables over his objections. Two related quotes betray his true feelings: "There is simply no intellectually responsible way to streamline the selection and design process if optimum results are desired" and the follow-up footnote, "Readers detecting author ambivalence about inclusion of Figures 55 and 56 are not hallucinating." Even the caption is revealing, "Chart makes simplistic assumptions and is intended as a guide only." The word "ambivalence" is clearly the polite version!

The last section, starting with "General Optimization and Measurement Considerations", has a new introduction (and new Figures 57 and 58 displaying the wave shapes of the lamp drive), but the rest of the section comes from App Note 55. Again, the main emphasis is on understanding and optimizing the efficiency of the system design, not just a single piece of it. Figures 60 through 67 (and the accompanying text on "Electrical Efficiency Measurement" and "Feedback Loop Stability Issues") are copied from App Note 55 Figures 19 through 26.

I'll discuss the appendices (64 pages of them!) next time.


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