Page 148 - Troubleshooting Analog Circuits

I I. Dealing with References and

Regulators

As the self-appointed Czar of Band Gaps, I am impelled to continue this book with
words of wisdom on voltage references, regulators, and start-up circuits. I also pro-
vide warnings against assumptions about worst-case conditions.
Voltage references and regulators have internal features that make them relatively
immune to problems. But, as with other designs, if you ignore the details, you’ll be
headed for Trouble. Some designs incorporating these parts, such as switching power
supplies, are not for the novice.
Many voltage references are based on band-gap circuitry, but some of the best
references are based on buried zener diodes. If your power supply’s output is in the
8-12 V range, or higher, zener-diode references such as LM329, LM399. or LM369
can provide high stability, low noise, and a low temperature coefficient. If your
power supply is a lower voltage (in the range from 8 down to 1.1 V) you can find
band-gap references that put out a stable voltage anywhere from 0.2 to 5 V with
creditable efficiency and economy. These band-gap references feature as low a tem-
perature coefficient as you’d probably ever be willing to pay for-as good as 20 or
10 ppm /“C. (They also feature enough noise so that a little filtering can make a big
improvement.)
A good buried-zener-diode voltage reference is inherently more stable over the
long term than is a band-gap one-good zener designs change only 5 to 10 pprn per
month. However, if you want the best stability possible, it’s only fair to age, stabilize,
and bum-in the references first. Also, you must screen out the ones that just keep
“walking” away from their initial values by 10 to 20 ppm every week-there are
always a few “sports” that are driftier than the rest. Unfortunately, there’s no quick
and easy test to distinguish between the drifty ones and the stable ones, except for
taking measurements for many hundreds of hours.

Regulators Are Almost Foolproof

In the last 10 years, IC voltage regulators have gotten pretty user-friendly. Many
people use them with no problems at all. Still, my colleagues and I get at least one
call every month about a regulator working poorly. The indignant caller complains,
“It’s getting hot.” We ask, “How big is your heat sink?” The indignant voice
responds, “What do you mean, heat sink?” I credit all of you readers with enough
smarts to recognize that you can’t put a whole lot of power into a little regulator
unless you secure it to a sufficient heat sink or heat fins. Then, there really aren’t too
many things that are likely to go wrong, because voltage regulators have just about
every feature for protection against the world’s assaults.
You’ll have problems with regulators when you don’t provide the required, speci-
fied output bypassing. Most negative regulators and some other types, such as low-
dropout regulators, require an electrolytic bypass capacitor from the output to
ground. If you insert a tantalum capacitor, you may be able to get away with a value

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