Page 198 - Troubleshooting Analog Circuits

Final Floobydust I85

Comments on Table for LM3524 Switching Regulator:
Obviously, if a circuit of this complexity gives problems, you will need to be almost
as wise as the person who designed it. (Or maybe even wiser?) And, you will need to
memorize what all the correct waveforms ought to look like, so you can detect errors
and deviations. You will need a compact breadboard so you can evaluate the critical
components in sockets. There may be, in any given batch of badly running parts.
several different modes of unhappiness. If you have to troubleshoot a lot of these,
you will be a good troubleshooter when you are done.
NOTE: This circuit is based on Figure 15 from the 1989 National Semiconductor
LM3524 data sheet, which did not work because the diodes to pins 12 and 13 were
drawn backwards . . . sorry about that. I noticed this problem even before I started to
build this. Then I did build this. It ran. Anyhow, now you know why the LM2575 is
called a “Simple Switcher”- because of comparison to old circuits like this.

Final Floobydust
I almost forgot this, but when you make up these little breadboards for evaluating
circuits and their components, you may need some tiny connectors for diodes or
transistors or small capacitors. You cannot use those nylon breadboarding panels, as
mentioned in Chapter 13, because the capacitances and inductances will be hope-
lessly bad. You just about have to use the same basic PC board that is used for the
real circuit, and then install tiny component jacks that accept 0.01 8-inch leads. such
as Amp’ type 50462, or Interconnection Products2 type 450-2598-0 1-03-00.
If it’s inconvenient to go out and shop for these component jacks, you can “roll
your own”: get some of those strips of sockets, the ones that are 25 in a line, such as
Digikey’s Catalog No. A208. A209, or ~imilar.~ If strays are not important, just snip
off a group of jacks-as many sockets as you need in a row. But if you need low
capacitance and very low leakage, just use your diagonal nippers to snip away the
plastic, and use the little jacks one by one. You will want to avoid beating these up,
as they are a little delicate, but they are excellent sockets for diodes, Rs, Cs, Qs, and
other small components with thin leads. If you need a little component jack for
0.040-inch diameter pins, the Interconnection Products type 450-3729-01 -03-00 is
good for that, and a similar part from Amp is 645-508- 1. The main point is, you want
to affect the stray capacitances and inductances of the real circuit as little as possible,
so when you stuff it full of good parts, it works OK. Then when you swap in a bad
part, it should be obvious what’s to blame.
Here’s afinal list of some of the most common problems in circuits:
Swapped resistors-installed in the wrong place
Resistors of the wrong value-wrong code

Diodes installed backwards
Electrolytic capacitors installed backwards
Broken wires
Links installed in the wrong place, or, missing
Flakey connector

I. AMP, P. 0. Box 3608, Harrisburg, PA. 17105. (717) 5640100.
2. Interconnection Products Inc., 2601 South Gamsey. Santa Ana, CA 92797 (7 14) 540-9256
3. Digikey. P. 0. Box 677, Thief River Falls, Minnesota 56701-0677, (800) 344-4539.