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.


27 November 2011

Scope Sunday 17

Sometimes you HAVE to look a gift horse in the mouth.

Not long ago, I bought some oscilloscope plug-ins from a local collector. While I was picking them up, he pointed at a piece of equipment in the corner and asked "You want that? I have no use for it."

It was a Bruel and Kjaer Type 1019 Automatic Vibration Exciter Control. I'd never seen one before, but it looked really cool (since then, I have seen another in a surplus shop). To be honest, I have no use for it either, but I'm not one to say "no" to free vintage equipment, especially such a beautiful piece of art. I just love the giant center knob.

I brought it home and stored it in my basement for a while. When I did finally get around to taking the back panel off and looking at it, this is what I found:

It is a mouse nest the size of a volleyball. There's also significant damage to the rest of the electronics. If I had seen this damage before I brought it home, I would have refused to accept it. (I am certain this nest wasn't built by local rodents because the construction materials are not "native" to my basement.) However, now that I have it, I feel some obligation to clean, fix, and restore it.

Just what I need! Another crazy project!

25 November 2011

IC Pins Named after Persons

This topic deserves its own post!

Jim Williams, in discussing the ROYER pin on the CCFL power-supply chips that Linear Technology produces, commented in a footnote [1] that "Local historians can't be certain, but this may be the only IC pin ever named after a person." After I posed this question in a previous blog post, some faithful correspondents came up with two other examples: the ZENER pin on the CD4046 PLL chip and the KELVIN pin on the CAT2300 switch controller.

Here is the list so far:

1. The ROYER pin on the LT1182 family of CCFL Switching Regulators (and other CCFL chips from Linear Technology):

2. The ZENER pin on the CD4046 Micropower Phase-Locked Loop:

3. The KELVIN pin on the CAT2300 Current Mirror and Switch Controller for SENSEFETs:

Please let me know if you find any more!

[1] Jim Williams, "SMBus controlled CCFL power supply," Linear Technology Magazine, vol. 9, no. 3, p. 35, September 1999.

Footnote: Even with these examples, Jim's statement is still true if you consider that the ROYER pin was probably named directly in honor of George H. Royer, while the other pins were named after functions that were, in turn, named after persons...

23 November 2011

App Note 65 part 2

The discussion of actual power-supply circuits in this app note begins on page 32, with the cautionary introduction:
Choosing an approach for a general purpose CCFL power supply is difficult. A variety of disparate considerations make determining the "best" approach a thoughtful exercise. Above all, the architecture must be extraordinarily flexible. The sheer number and diversity of applications demands this. The considerations take many degrees of freedom.
I think this introduction is partly an inoculation to the "summary" tables that are included later in the app note. See footnote 18 on page 49.

Figures 35 through 38 are quite similar to App Note 55 Figures 6 through 9 (although a newer, higher-frequency switching regulator is used in the last two figures).

Figure 39 is a brand new circuit, using a dedicated CCFL integrated circuit, the LT1183. This single-chip solution is the major advancement in this "fourth generation" treatment of CCFL power supplies, and it includes many of the features that Jim has discussed in previous app notes, such as open-lamp protection and the variable LCD-contrast supply voltage.

Note the name of pin 13 on the LT1183, "Royer". Although he doesn't mention it here, Jim commented in a later publication [1] that "Local historians can't be certain, but this may be the only IC pin ever named after a person."

That's a very good question. Dear loyal readers, is this the only IC pin named after a person?

[1] Jim Williams, "SMBus controlled CCFL power supply," Linear Technology Magazine, vol. 9, no. 3, p. 35, September 1999.

Tutorial footnote (for new readers): The above power-converter topology is a resonant Royer converter. The LT1183 is a controller chip that drives a Royer converter for this specific application: driving the cold-cathode fluorescent lamp (CCFL) for the backlight in a laptop screen.

The so-called ROYER pin connects to the center-tapped primary of the transformer (which is the unregulated-power input in this topology). In the circuit shown, the pin isn't doing much. However, if the BAT pin and the ROYER pin aren't connected together, then all of the input current flows in the BAT pin and out the ROYER pin, and the chip can sense the primary-side input current in the transformer. Thus, the LT1183 can drive the lamp in a floating (isolated) topology, without any direct feedback to the chip from the high-voltage side.

Here's the explanation from the LT1183 datasheet:
ROYER (Pin 13): This pin connects to the center-tapped primary of the Royer converter and is used with the BAT pin in a floating lamp configuration where lamp current is controlled by sensing Royer primary side converter current. This pin is the inverting terminal of a high-side current sense amplifier. The typical quiescent current is 50μA into the pin. If the CCFL regulator is not used in a floating lamp configuration, tie the Royer and BAT pins together. This pin is only available on the LT1182/LT1183/LT1184F.
For more details, see Linear Technology App Note 65: "A fourth generation of LCD backlight technology: Component and measurement improvements refine performance." Appendix K is titled "Who was Royer and what did he design?"

Update: A commenter on another website suggested that an argument could be made for the ZENER pin on the CD4046 Phase-Locked Loop chip. That's a good one. Any others?

Update 2: Further discussion on this topic has been moved to a dedicated post: IC pins named after persons.


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.


18 November 2011

Book 2 Chapter 17

Chapter 17, "There’s no place like home"

Jim's final chapter in the second book discusses the importance of having a home laboratory. "I estimate that about 90% of my work output has occurred in a home lab... A lot of work time is spent on unplanned and parasitic activities. Phone calls, interruptions, meetings, and just plain gossiping eat up obscene amounts of time."

The chapter is filled with great advice. In the following pages, he discusses the requirements for a home lab, including oscilloscopes ("Types 547 and 556 are magnificent machines"), probes ("It's too embarrassing to print how many probes I own."), power supplies, signal sources ("The Hewlett-Packard 200 series sine wave oscillators are excellent, cheap, and easily repaired."), voltmeters (Fluke handhelds and the HP3400A, of course), and other instruments.

The chapter is peppered with glamor shots of his home lab, such as the one below.



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.


16 November 2011

Book 2 Chapter 1

Chapter 1, "The importance of fixing"

In this chapter, Jim tells the story of his start at M.I.T., and how his mentor, Prof. Jerrold Zacharias ("the father of atomic time"), shaped his early career. The key development was Zacharias's refusal to pay for instrument repair and calibration. He simply demanded, "You fix everything." This moment was Jim's introduction to fixing (and stealing good ideas from) Tektronix oscilloscopes, starting with a 1A7 plug-in. As the saying goes, "Good artists copy, great artists steal."

Jim goes on to discuss his educational philosophy with regard to vintage test equipment. "The inside of a broken, but well-designed piece of test equipment is an extraordinarily effective classroom."

He concludes with a quote I love (and have cited before; see Scope Sunday 1): "It just seems sacrilege to let a good piece of equipment die... fixing is simply a lot of fun. I may be the only person at an electronics flea market who will pay more for the busted stuff!"

Best quote (in the caption of Figure 1-1 (above) on page 4): "Oh boy, it's broken! Life doesn't get any better than this."


15 November 2011

Book 2

Another detour for the rest of this week: App Note 61 was published in August 1994. The next app note (App Note 65) was written over a year later in November 1995. In the intervening 15 months, Jim published a second book, "The Art and Science of Analog Circuit Design". So for the rest of this week, I will blog about the book. As I recommended for the first book, if you don't own a copy, you owe it to yourself to buy one.

This book contains twenty chapters, again written by a wide variety of authors, including Barrie Gilbert (again), Greg Kovacs, Carl Battjes, Carl Nelson, and many others. Jim wrote three of the chapters himself. I'll discuss them each in turn. One interesting note at the beginning of the book is in the biographies of the contributors (starting on page xi). In Jim's bio, it says "He lives in Palo Alto, California, with... 28 Tektronix oscilloscopes", twice as many as he claimed last time!


14 November 2011

App Note 61 part 2

Continuing on, Figure 26 shows an improved high-speed pulse generator. Compared to Appendix D in App Note 47, this pulse generator has a distinct advantage in that it is triggered. "This feature permits synchronization to a clock or other event." The 2N2369 is biased just below avalanche breakdown, and the incoming trigger input causes breakdown (due to the difference between Vceo and Vces). Note the comment about the Tektronix 661 with the 4S2 sampling plug-in ("I'm sorry, but 3.9GHz is the fastest 'scope in my house"). Nice upgrade from the 1-GHz 1S2 sampler that he has used previously. "Ground plane type construction with high speed layout, connection and termination techniques are essential for good results from this circuit." Yep. (I think this circuit is the best circuit of the app note.)

Figure 29 shows a special voltage regulator for flash-memory programming. Unfortunately, he does not compare and contrast this design with the designs in App Note 31. Is this circuit just as good? (It probably is, but why?). What are the features of the LT1109 that make it a good fit for this (touchy and sensitive) application?

Figure 31 shows a low-voltage voltage-to-frequency converter (operating from a 3.3-volt supply). The next two circuits are broadband noise sources. Figure 33 uses a Noisecom NC201 noise diode with a selectable filter to produce the noise, and a RMS AGC loop to set the amplitude. Figure 37 uses a standard zener diode as the noise source, but requires a trim to set the initial noise level. Figure 38 is a switchable-output crystal oscillator, which uses diodes to select which crystal is active in the feedback path of the comparator. Cute.

Appendix A includes significant reprints from the manual of an HP3400A True RMS Voltmeter. Jim compares the approach used here to his approach in Figure 22. The instrument he discusses here is a classic, with an impressively creative solutions to several design challenges. The input buffer in Figure A1 uses Nuvistors (because JFETs weren't good enough in 1965). Figure A2 shows a photograph of the input-buffer circuit board. The "video amplifier" in Figures A3 and A4 is an impressive design with DC and AC feedback loops and clever bootstrapping. The chopper amplifier in Figures A5 and A6 uses neon bulbs and photocells for the chopping action! (As Jim says, "Hewlett-Packard has a long and successful history of using lamps for unintended purposes.") Figure A7 shows the circuit board for Jim's RMS-to-DC converter from Figure 22.

The topic of RMS-to-DC conversion was near and dear to Jim's heart. He covered these circuits in detail in App Note 22, and he used them in his CCFL explorations. He helped to design the LT1088 IC and was its main evangelist. In the footnote on page 28, Jim tries to explain the context: "We are all constantly harangued about the advances made in computers since the days of the IBM360. This section gives analog aficionados a stage for their own bragging rights. Of course, an HP3400A was much more interesting than an IBM360 in 1965. Similarly, Figure 22's capabilities are more impressive than any contemporary computer I'm aware of."

Do I detect some frustration in Jim's voice here? Compare all of the circuitry of the HP3400A with the circuit in Figure 22. In effect, I think Jim is saying, "Look how easy I'm making it for you! Why aren't you buying more LT1088 chips?"

Best quote (page AN61-38): "Incidentally, what were you doing in 1965?"

The app note concludes with a great cartoon (perhaps the best cartoon so far).


11 November 2011

App Note 61 part 1

Practical circuitry for measurement and control problems: Circuits designed for a cruel and unyielding world. 40 pages

Another collection of random circuits, like App Note 45 (from June 1991). In fact, the introduction says this circuit collection includes circuits from June 1991 to July 1994. Unfortunately, there is no baby-bottle-rating system, however, there are plenty of great circuits.

Figure 1 shows a switching regulator where the switching frequency is synchronized to the system clock. (Synchronization was also discussed in App Note 55.) This trick is a great one. I once designed a "noise-less" power supply for a sensitive RF receiver with this method... I simply "hid" all of the EMI from the switching regulator underneath the EMI from the digital hardware (I admit that this approach is cheating, but it works).

Some of the circuits were supplied by Steve Pietkiewicz, and many of them were "inspired" by Jim's recent work on CCFL power supplies in App Note 55. Two methods for 1.5V-to-5V conversion are shown in Figure 3 and 6 (high power and low power, respectively). Figure 9 is a low-power CCFL supply (an alternative to App Note 55 Figure 11). Figure 10 is a LCD contrast supply (compare to App Note 55 Figure 14). Figure 11 is a power supply for a HeNe laser (the same as App Note 55 Figure H2), and Figure 12 is an electroluminescent-panel supply.

Jim's interest in instrumentation also appears. Several barometer circuits are shown in Figures 13, 14, 15, and 16, and a quartz-based thermometer appears in Figure 17. A FET-input instrumentation amplifier is shown in Figure 18.

There are also some improvements on circuits from App Note 47. The high-speed adaptive trigger makes another appearance in Figure 21 (compare this circuit to Figure 131 in App Note 47). Another wideband thermally based RMS-to-DC converter is shown in Figure 22 (compare to Figure 137 in App Note 47 and Figure 8 in App Note 22).

The best circuit (so far) is Figure 24, not because it's a great circuit in isolation, but because it is true to his "fixer mentality." Although he doesn't admit it here, I imagine that he was elbow deep into fixing a Tektronix P-6042 current probe when he realized that he couldn't get the replacement parts that he needed (see Figure 25: is M18 still available?). So, he designed his own replacement, and Figure 24 was born.

I'll discuss the rest of the circuits and the appendices next time.

Best quote (page 18, discussing figure 22): "It is worth considering that this circuit performs the same function as instruments costing thousands of dollars."


09 November 2011

App Note 55 part 2

As always, the appendices (up to K!) are great.

Appendices A, B, and G are copied from App Note 49.

The first half of Appendix C, "Achieving meaningful efficiency measurements", is also borrowed from App Note 49, but the second half (starting with the calibration source in Figure C3) shows that considerable effort has gone into making the measurements more meaningful. Jim always devotes great effort to instrumentation, and it shows here. Again, precision RMS-to-DC conversion plays a large role. (Although Figure C6 is captioned "Typical efficiency measurement instrumentation", there's nothing typical about it.) The appendix concludes with a discussion of using calorimetric measures as a efficiency double-check (Figures C7, C8, and C9). As the man says, "Calorimetric measurements are not recommended for readers who are short on time or sanity."

Appendix D discusses more instrumentation, now focusing on photometric measurement. How much light is the CCFL producing? This question raises a key issue, as he explains in the footnote, "It is possible to build highly electrically efficient circuits that emit less light than "less efficient" designs." (See Figure J3.)

The next four appendices are short. Appendix E discusses protection circuitry (important!) for broken lamps. Appendix F discusses shutdown control, and a calibration source (Figure F2) for intensity control. Appendix G is copied from App Note 49, and Appendix H (HeNe laser power supplies) is mostly copied from App Note 49.

Appendix I is a brief (too brief!) discussion of the history and operation of the Royer topology.

Appendix J is my favorite. Titled "A lot of cut-off ears and no Van Goghs", it discusses some not-so-great ideas. On the whole, we engineers don't spend enough time talking about engineering failures, and there is often a lot to learn. Jim relates, "Backlight circuits are one of the deadliest places for theoretically interesting circuits the author has ever encountered." Figures J1, J2, and J3 attempt to increase efficiency by removing the losses in the LT1172. Unfortunately, the resulting lamp drive waveforms are undesirable. Figures J5 through J8 show suboptimal sensing schemes for measuring the output current. It is very instructive to consider why these circuits didn't work: I wish Jim had written more appendices like this one!

Appendix K is a brief discussion of the various sources of inefficiency in a backlight application, including the electrical-to-electrical step, the electrical-to-light step, and the light-to-light step (see Figure K1). Importantly, the electrical-to-electrical efficiency stays high under a variety of operating conditions (see Figure K2), which extends battery runtime.

The app note concludes with a cartoon of his son Michael sitting in front of a 556.


07 November 2011

App Note 55 part 1

"Techniques for 92% efficient LCD illumination: Waste not, want not..." 44 pages.

This app note is Part 3 in the grand saga of cold-cathode fluorescent lamps (CCFL). Part 1 was Figure 36 back in App Note 45, and Part 2 was App Note 49. This app note begins with an unbelievable quote: "One notable aspect of [the publication of App Note 49] is that it generated more response than all previous LTC applications notes combined." It is hard to imagine that this topic is so much more popular than all his other (prolific) work. On the other hand, reading through this app note, you get the sense that CCFLs are poorly understood, and that Jim's efforts were perhaps the first attempt to truly explore this under-appreciated topic.

This app note is effectively a (significant) update to App Note 49. "The partial repetition is a small penalty compared to the benefits of text flow, completeness and time efficient communication." It is dated one year after App Note 49 (August 1992 to August 1993), so it represents a considerable amount of time and effort. "Getting the lamp to light is just the beginning!"

Figures 1 through 4 show some of the additional investigation and research done since App Note 49, summarizing some typical characteristics of CCFLs. The schematic in Figure 6 is mostly copied from Figure 2 in App Note 49 (with new transistors and a new value for C1, which increase the efficiency from 82% to 88%), however, Figures 8 and 9 (with 91% and 92% efficiency, respectively) are new. Low voltage operation (shown in Figure 10) is also new. Figures 11, 12, and 13 are copied from App Note 49, while the more-in-depth discussion of LCD bias (with Figures 14 and 15) is new.

A considerable discussion of mechanical layout begins on page 11 with Figure 16. "Poor layout can easily degrade efficiency by 25%, and higher layout induced losses have been observed."

Feedback-loop stability is discussed starting on page 13. Figures 20 through 26 show some of the problems and cures thereof in the feedback loop. (Footnote 10, "The high priests of feedback..." seems to be mocking me.) The compensation approach used in these circuits is very conservative (see the 2-uF capacitors on the VC pins in Figures 6, 8, 9, and 10). "Isn't a day of loop and layout optimization worth a field recall?" (Is experience speaking here?)

The final two sections discuss dimming ("extending illumination range") and synchronization. Figures 28 and 29 improve the dimming capability of the lamp by using isolated drives (a 40:1 dimming ratio is achieved), maintaining high effeciency. The symmetric electric field around the lamp reduces the occurrence of the "thermometer" effect. Figures 30 through 36 discuss synchronization (a topic that he originally discussed, and dismissed, in Appendix A of App Note 29). "In particular, pen based computers may be especially sensitive to asynchronous components." Figures 32 and 34 abandon the Royer base drive for a flip-flop-based scheme. This approach reduces jitter at the expense of efficiency.

The best quote is the caption of Figure 20: "Destructive high voltage overshoot and ring-off due to poor loop compensation. Transformer failure and field recall are nearly certain. Job loss may also occur."

I'll discuss the appendices next time.