17 November 2011

Book 2 Chapter 11

Chapter 11, "Tripping the light fantastic"

This fifty-five-page chapter discusses "how the best circuit I ever designed came to be". (I'm not sure if I agree with that assessment.) This chapter is Part 3b in the grand saga of cold-cathode fluorescent lamps (CCFLs). Just to remind you:
I'm calling this chapter "Part 3b" because it is nearly identical to Part 3a. The only really difference between this chapter and App Note 55 is the introduction on pages 139 to 153 (and the epilogue on page 174).

The introduction discusses the genesis of this project, starting with his postpartum blues after the publication of App Note 47. He claims that Bob Dobkin asked him to look into backlights around Christmas 1991, and then an engineer from Apple called him a few days later. (This narrative, while it makes a good story, seems to omit the CCFL circuit in App Note 45 from June 1991.) The engineer from Apple, Steve Young, had seen the cartoon in App Note 35 ("Call me and lets discuss your switching regulator requirements"), and was looking for some applications help with the power converters in the Powerbook, including the backlight.

The description of taking apart portable computers ("The Luddite approach to learning", page 142), sets the stage for Jim's significant research and exploration of CCFLs. He explains, "almost all of them utilized a purchased, board-level solution to backlight driving." No vendors were optimizing their designs (likely because nobody really understood the lamps). Jim was traveling into unexplored territory. This fact perhaps explains the quote at the beginning of App Note 55, "One notable aspect of [the publication of App Note 49] is that it generated more response than all previous LTC applications notes combined."

In the next part of the story, Jim explores the high-voltage resonant power supplies in oscilloscopes, including the Tektronix 547 (Figure 11-4) and the Tektronix 453 (Figure 11-6, also shown below).

Finally, he introduces the Royer topology on page 148 and discusses the start of his work on page 152. The best quote: "But there comes this time when your gut tells you to put down the pencil and pick up the soldering iron... Build and raft and start paddling."

The rest of the chapter (pages 154 to 193) is basically a reprint of App Note 55.


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