Page 112 - Analog Circuit Design Art, Science, and Personalities

Milton Wilcox

12. Analog Design Discipline

I would like to use a true story to illustrate what I believe is the fundamental neces-
sity for success in analog circuit design: attention to detail. 1 doubt that analog de-
sign is any different in this respect from any other field of intricate endeavor-
be it digital circuit design or internal combustion engine design or violin design.
Beyond the need for an understanding of basic laws governing solid state circuit
operation, what analog absolutely demands is meticulous attention to detail.
Analog design is about taking the time to anticipate all the possible consequences
of z circuit approach, and about following up every quirk or anomaly you might
notice while evaluating a breadboard or running a simulation. It is about devising
different ways to test for the same result, and about devising test conditions that
might be considered out of bounds for the circuit function, because somewhere,
someday you know a customer is going to. It is about knowing more than anyone
else in the world about your circuit. When a circuit is used because it was written up
in a design magazine, or because it is almost like one which worked the last time. or
because it is a last minute change to meet schedule, disaster is invited. This applies
to analog circuits large and small, from complex to very simple. This applies to the
string of three diodes in my story.
My story takes place during a brief stint at a small, aggressive, and very naive
company trying to break into the analog integrated circuit business. There 1 experi-
enced firsthand the consequences of not adhering to this analog design discipline of
attention to detail. The three diodes were in the thermal detector circuit of a neat
little chip designed to turn discrete power MOSFETs on and off. The project had
been started some 18 months previously with guidance from a major power FET
manufacturer. Full of confidence and with the urgency of management (who were
already counting the revenues), the fledgling design team had set out to create a
chip in as short a time as possible. By the time I arrived, they were on their second
complete mask set and third designer. Finally, when there was no one else left to
work on it, 1 inherited thejob of cleaning up the chip for release to production.
My example circuit shown in Figure 12- 1 is a fairly straightforward arrangement
to achieve thermal sensing on an integrated circuit. The voltage across three series-
connected diodes is compared to that of a temperature independent reference
voltage. When the temperature of the diodes rises to approximately 150 "C, thcir
voltage drops below that of the reference, and thc output of the comparator signals
overtemperature. In the FET driver chip, this function was desired to shut down the
power FET in high ambient temperatures (see Figure 12-1).
While designing an analog circuit., T believe in using every tool available to eval-
uate the operation of a circuit. By using both breadboards and computer simulation
tools. results can be checked against each other. I don't hesitate to use first-order
hand calculations, too, which can be great for keeping SPICE or other circuit simu-
lators honest.

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