11 July 2011

App Note 1

Right out of the gate, this app note has the classic Jim Williams style, with five oscilloscope photos, with three or four traces each.  His application notes can often be identified just by glancing at them because they include scope photos showing real data taken with vintage Tektronix gear. You can see the round bezel in several of the photos, so I'm guessing that the instrument was his favored Tektronix 547 with a 1A4 four-channel plugin.

This app note includes eight pages of tricks using the (now-discontinued) LT1005, a dual-output regulator with a main-output-enable pin. Several input protection schemes are implemented using this enable pin (the clever positive feedback latch, shown in Figure 2C on page AN1-2, is the basis for several variations). I think the best circuit here is actually the 100-pF speed-up capacitor in Figure 3 to get the base charge into Q2. I have a soft spot in my heart for circuit tricks from the days of RTL (resistor-transistor logic, you know, the Apollo technology).

The final two pages explore using negative feedback around the regulator to implement closed-loop control. Hysteresis-loop-based motor-speed controllers are shown on pages AN1-7 and 8. I've always viewed some of his feedback contraptions (like these two) with suspicion. His design approach to feedback loops was always a little too cavalier for my tastes, especially since good models for voltage regulators (particularly good transfer-function models) are hard to come by. I'm pretty sure that the phase margin of these loops has never been determined (as a control-systems aficionado, I prefer a more analytical approach).

He seems to have a never-ending supply of interesting, random, and often hard-to-find hardware for his application circuits. In the various circuits, this note specifies a thermoswitch, a thermistor, a crystal-oscillator oven, and two different motor/tachometer units. Although he gives part numbers, it is sometimes difficult to independently source these parts to duplicate his results. I've had trouble finding his components in the past. Sometimes substitutions are easily found, sometimes not. Some quick online searches show that only one of the specified parts in this app note is still easily sourced (the thermoswitch). Often, the only result returned for an online search for the part numbers is just the same application note.

I wonder: was the primary source of these parts from customers looking specific application assistance, or from his personal junk pile? (That said, I actually have some of the little Canon motor/tach units in my personal junk pile, and I love them for small experiments and lab projects.)

Best quote (page AN1-7): "For example, the small motor listed... is almost unstoppable by the unaided human hand at 150RPM." I imagine a group of engineers gathered around the lab bench, nursing friction burns on their fingers after failing to stop the motor.


Joe Sousa said...

Congratulations, Kent, for starting this series of reviews!

It would be interesting to see a further pursuit of the notions of stability in the histeretic feedback loops you comment about. The contrast you can provide with your classic analytical approach to Jim's approach should give insights to the strengths of these approaches.

Usually, a low open loop phase margin makes itself very apparent in the closed loop circuit in the time domain by excessive ringing to step or pulse stimuli.

Do you see any potential trouble in the stability of the motor control circuits of pages AN1-7,8 where an analysis or measurement of phase margin might have enriched the understanding of the loop behaviour?

Loop gain dependency on speed is also likely in these switched mode histeretic control loops.

Kent Lundberg said...

Low stability margins usually make themselves known in the time domain, but not always! (However, I freely admit, that, as an academic, it's the "edge cases" that are most interesting to me, so I obsess about them.) Bob Pease kind-of-sort-of talks about this issue in his "Troubleshooting Analog Circuits" (see "Pease's Principle" on page 99).

Trouble can arise if there's significant delay (or low-pass behavior) through the regulator. Hysteresis works best when the loop dynamics are integrating or first order. I don't see any obvious sources of trouble in the motor circuits, and to be honest, hysteresis is very forgiving. But without any kind of transfer-function model for the regulator, there's no way to be sure.

Mendelt Siebenga said...

The link to the application note is broken. The application note seems to have moved to http://cds.linear.com/docs/en/application-note/an01f.pdf