Page 149 - Troubleshooting Analog Circuits

I36 I I. Dealing with References and Regulators

Figure I I. I. When you’re the Czar of Band Gaps, people look up to you.

of 1 or 2 pF; if you use an aluminum electrolytic capacitor, you can get away with 20
to 100 pF, or whatever the data sheet spells out. But in all cases, on all the parts I
know, an electrolytic capacitor will work, and a film or ceramic capacitor won’t
work-its series resistance is just too small. Now, if you put a 1 fl resistor in series
with a 1 pF ceramic capacitor, the filtering will probably be adequate around room
temperature; the loss factor is then similar to a tantalum capacitor. But if you take it
to 40 or +lo0 “C, the ceramic capacitor’s value will shrink badly (refer to Chapter 4
on capacitors) and the regulator will be unhappy again. It may start oscillating, or it
might just start ringing really badly.
Recently one of our senior technicians was helping a customer with some applica-
tions advice. He found that the AC output impedance of an LM3 17 was changing
considerably as a function of the load current coming out of the output transistor. We
had always assumed that the curve in the LM117 data sheet was invariant versus
current load-that was a mistake. Then we found that every other monolithic regu-
lator has the same sliding scale of output inductance. For additional notes on this
phenomenon, I recommend the Errol1 Dietz article (Ref. 1) which I have included as
Appendix C because this tendency of the output inductance to be modulated by the
output current may help to explain why regulators are happy in some cases but
grouchy in other similar situations.
Another regulator problem can occur when you add an external transistor to in-
crease the output current. Since this transistor adds gain at DC, it’s not surprising that
you have to add a big filter capacitor on the regulator’s output to prevent oscillation.
Some of the applications in old National Semiconductor data sheets recommend
specific values for the filter capacitor, and specific types for the boost transistors, but
some of these circuits are quite old. When customers find out that 2N3234s are no
longer available, they’re likely to substitute a more modem transistor that has a faster
response and is likely to oscillate. In this case, a customer might complain about the
DC output’s “bad load regulation” as the regulator is forced into and out of oscilla-
tion. (Whoever said you don’t need an oscilloscope to check out DC problems?)