Page 150 - Troubleshooting Analog Circuits

Regulators Are Almost Foolproof I37

When these customers ask for help, I not only explain how to stop the oscillation. but
I give them Pease’s Principle (see the box, “Pease’s Principle” in Chapter 8). How-
ever, these days most engineers find it’s better to use a bigger regulator (LM350 at 3 A,
LM338 at 5 A) because if you just add on an external transistor, you cannot protect it
from overheating. Consequently the external power transistor has lost favor.

Too Much Voltage Leads to Regulator Death

You can kill any regulator with excessive voltage. So if you’re driving inductive
loads, or if your circuit has an inductive source, make sure to have a place for the
current to go when the normal load path changes. For example, if you’re using the
LM350 as a simple battery charger with only a few microfarads of filter capacitor on
the input, a short between the output and ground is usually disastrous: When the
regulator tries to draw an increasing amount of current from the transformer and then
goes into current limit, the inductance of the transformer will give you marvelous 80-
V transients, which then destroy the LM350. The solution is to put IO00 pF-rather
than just 1 or 10 pF-across the input.
Users get accustomed to seeing regulators with output noise of about 0.01% of the
rated DC output. They get indignant when the noise doubles or triples due to l/f or
popcorn noise. The chances of finding a noisy regulator are quite small, so when
some noisy ones do show up, it’s a shock. Unfortunately, no high-volume manufac-
turer of regulators is in a position to test for those low noise levels, or to guarantee
that you’ll never see a noisy part. Please don’t expect the manufacturer to admit the
parts are bad or unreliable or worthy of being replaced. If you do depend on super-
quiet ICs, or ICs with other specially selected characteristics, it’s wise to keep a spare
stock of selected and tested parts in a safe. Then, you can use them when some of the
ones you just bought happen ro be a little too noisy.

What Is Worst Case?

Once I designed a circuit to drive a 200 fl load (a rather light load) at the far end of a
2OOO-ft RG174U cable. The specifications called for me to test the circuit by driving
the near end of the coaxial cable with a low-impedance square wave. I called the
engineer who wrote the spec and recommended that we perform the test with about a
39 Q source impedance to avoid bad ringing and reflections along the unterminated
cable. He told me that this impedance wasn’t necessary; he had already checked out
the worst-case conditions, with no cable and with 2000 ft of cable. I asked him if he
had checked it with 250 ft of cable. “Why, no,” he said. So I suggested he try that,
Shortly thereafter, he called me back and agreed that the reflections with 250 ft of
cable were intolerable without at least some nominal value of resistance at the source.
He had incorrectly assumed that the worst case occurred with the longest cable. It’s
true that the attenuation was worst with the long length of lossy RG 174U cable. But
it was this attenuation that caused the ringing and reflections to appear damped out.
With the shorter 250-ft cable, a worst-case condition existed at a place he hadn’t
expected to find it.
So, be cautious about where you look for worst-case conditions. An op amp may
exhibit its worst performance at an output voltage other than its maximum negative
or positive swing-or even other than zero volts or zero output current. A regulator’s
worst-case operating conditions may not be at its full-rated load current. When a
regulator’s power source is resistive, the power dissipation may be higher at three-
quarters of its rated current than at full current.