Page 119 - Troubleshooting Analog Circuits

I06 8. Operational Amplifiers-The Supreme Activators

improve reliability. If a heavy overload is applied for a long time, or there is no heat
sink, or the ambient is just too hot, these circuits detect when the chip’s temperature
exceeds 150 “C and then turn off the output. The thermal-limiter circuit in the
LM117 (and other early power ICs) sometimes just decreased the output current to a
safe DC value to hold the die temperature to around 160 “C. In other cases, where the
load is lighter and the thermal gradient transients are different, these thermal limiters
oscillate ON and OFF with a duty cycle that ensures the 160 “C chip temperature. As
I was about to design the LM137, I looked back and decided the latter characteristic
was preferable, so I designed about 5 “C of thermal hysteresis into the thermal-limit
circuit. That way, the circuit makes a strong attempt to restart its heavy load with a
repetition rate of about 100 Hz. If the regulator makes only a feeble attempt, it may
be unable to start some legal loads.
So, we actually designed an oscillation into this thermal-limit circuit, but we never
bothered to mention it on the data sheet. H’mmm ... we shouldn’t be so sloppy. I
apologize. I’ll do better next time. (This situation has a bearing on one of my pet
peeves: Bad data sheets. I get really cross about bad ones, and I really do try hard and
work hard to make good ones. Refer to “How to Read a Data Sheet” (Appendix F and
Ref. 10) because bad data sheets can get the user into trouble.)

Different Methods Uncover Different Errors
Now that you know some op-amp problems to look out for, how do you actually
troubleshoot an op-amp circuit? I usually split my plan along two lines: AC and DC
problems. Examples of AC problems include oscillations and ringing; DC problems
include bad DC output errors and pegged outputs, which are outputs stuck at either
the positive or negative supply rail. Obviously, you need a scope to be sure the circuit
isn’t oscillating. It always makes me nervous when I find out that the customer I’m
trying to help doesn’t even have a scope. I can understand if an engineer only has a
crummy scope, but there are certain problems you cannot expect to solve-nor can
you even verify a design-if you don’t have any scope at all.
If the problem is an AC problem, I first make sure that the input signals are well
behaved and at the values I expect them to be. Then I put my scope probes on all the
pins and nodes of the circuit. Sometimes it’s appropriate to use a 10 X probe, and
other times I shift to 1 X mode. Sometimes I AC-couple the scope: sometimes I DC-
couple it. I check all the pins, especially the power-supply pins. Then-depending on
what clues I see-I poke around and gather symptoms by adding capacitors or RC
boxes to assorted circuit nodes. I try to use two probes to see if the input and output
have an interesting phase relationship, and I simultaneously verify that the output is
still oscillating.
Many of the techniques I use depend on whether the circuit is one I’ve never tried
before or one that I see all the time. Sometimes I find an unbelievable situation, and I
make sure that I understand what’s going on before I just squash the problem and
proceed to the next. After all, if I’m fooling myself, I really ought to find out how or
why, so I won’t do it again.
If the op amp exhibits a DC error or a peg, I first check with my scope to see that
there’s no oscillation. Then I bring in my 5-digit DVM and scribble down a voltage
map on a copy of the schematic. On the first pass, I’m likely to just keep the numbers
in my head to see if I can do a quick diagnosis of a problem that’s obvious, such as a
bad power supply, or a ground wire that fell off, or a missing resistor. Failing that, I
start writing meticulous notes to help look for a more insidious problem. I look at the
numbers on the schematic and try to guess the problem. What failure could cause that