Page 131 - Troubleshooting Analog Circuits

I18 9. Quashing Spurious Oscillations

with zero frequency. Although latched-up circuits demand troubleshooting, the good
thing about them is that they sit right there and let you walk up to them and touch
them. And you can measure every thing with a voltmeter to find out how they are
latched. This state of affairs doesn’t mean that troubleshooting them is easy, because
sometimes you can’t tell how the latched-up circuit got into its present state. And in
an integrated circuit, there can be paths of carriers through the substrate that you
can’t “put your finger on.”
The worst aspect of latched-up circuits is that some are destructive, so you can’t
just sit there and let them remain latched up forever. Two approaches for attacking
destructive latches are:
Turn off the power quickly, so the latched-up circuit cannot destroy anything. Try
turning on power for short pulses and watching the circuit as it approaches the de-
structive latch condition. (See Chapter 2.)
Use an adjustable current-limited supply with zero or small output capacitance, (such
as the example in Chapter 2), so when the circuit starts to latch, the fault condition
can easily pull the current-limited power supply’s voltage down.
Another way to inadvertently generate a latched-up condition is to turn on the
outputs of your multiple-output power supply in the “wrong” sequence. Some ampli-
fiers and circuits get quite unhappy when one supply (usually the positive one) turns
on first. Automatic power-supply sequencers can help you avoid this problem. An
antireversal rectifier across each supply can help, too, and is always a good idea for
preventing damage from inadvertently crossed-up power-supply leads or supply
short circuits.
I used to get calls every few months from people who asked me if it was okay to
ship (or launch) products that contained LM108s that may have had +15 V on their
-15 V pins and vice versa. It was always painful for me to tell them, “Don’t ship it-
junk it. And, next time put antireversal diodes on each supply.” Specifically, you
should add these antireversal rectifiers across each bus in your system to protect the
loads and circuits. Also add an antireversal rectifier across each power supply’s
output to protect the supplies (Figure 9.6). Some people think that leaving parts out is
a good way to improve a circuit’s reliability, but I’ve found that putting in the right
parts in the right places works a lot better. Refer also to a running commentary and
debate on this topic in Chapter 13.
If you have any doubt that your anti-oscillation fixes are working, try heating or
cooling the suspected semiconductor device. In rare cases, passive components may
be sufficiently temperature-sensitive to be at blame, so think about them, too. Even if
a circuit doesn’t get better when heated, it can get worse when cooled, so also take a
peek at the circuit while applying some freeze mist.
My point is that merely stopping an oscillation is not enough. You must apply a
tough stimulus to the circuit and see whether your circuit is close to oscillation, or
safely removed from any tendency to oscillate. This stricture applies not only to
regulators but also to all other devices that need oscillation-curing procedures.
For example, if a 474 resistor in the base of a transistor cures an oscillation, but
24 R doesn’t, and 33 Q doesn’t, and 39 R still doesn’t, then 47 R is a lot more
marginal than it seems. Maybe a 754 resistor would be a better idea-just so long as
100- or 120- or 1504 resistors are still safe.
In other words, even though wild guesses and dumb luck can sometimes cure an
oscillation, you cannot cure oscillations safely and surely without some thoughtful
procedures. Furthermore, somebody who has an appreciation for the “old art” will
probably have the best results.