Page 128 - Troubleshooting Analog Circuits

Common-Mode Excursions Unpredictable I15

perfectly valid response; for other input ranges, you can get screwy response but cause
no harm to the comparator; and for others, you’ll instantly destroy the comparator.
For example, for an LM339-type comparator running on a single 5-V supply, if
one of its inputs is in the 0-3.5 V range, then the other input can range from 0-36 V
without causing any false outputs or causing any harm to the comparator. In fact, at
room temperature, the out-of-range input can go to -0.1 V and still produce the cor-
rect output.
But, heaven help you if you pull one of the inputs below the 4.1 V level, say to
-0.5 or 4.7 V. In this case, if you limit the comparator’s input current to less than 5
or 10 mA, you won’t damage or destroy the comparator. But even if no damage occurs,
the outputs of any or all of the comparators in the IC package could respond falsely.
Current can flow almost anywhere within the IC’s circuitry when the substrate diode
(which is inherent in the device’s input transistor) is forward biassed. It is this current
that causes these false outputs.
We’ll try to be more clear about V,-, specs in the future. Maybe next time at
National Semiconductor, we’ll phrase the spec sheet’s cautions a little more vigor-
ously. In fact-Ta-da-here is the correct phrase from the LM612 data sheet : “The
guaranteed common-mode input voltage range for this comparator is V- S V,, 5
(V+-2.0 V), over the entire temperature range. This is the voltage range in which the
comparisons must be made. If both inputs are within this range, of course the output
will be at the correct state. If one input is within this range, and the other input is less
than (V- + 32 V), even if this is greater than V+, the output will be at the correct state.
If, however, either or both inputs are driven below V-, and either input current ex-
ceeds 10 PA, the output state is not guaranteed to be correct.” And, this definition
applies nominally to the LM339, LM393, and also to the LM324 and LM358 ampli-
tiers if you are applying them as a comparator. So, you cannot say we are not trying
to make our data sheets more clear and precise4ven if it does sometimes take 20
years to get it just right.
Still, if you stay within their rated common-mode range, comparators are not that
hard to put to work. Of course, some people disdain reading data sheets, so they get
unhappy when we tell them that differential signals larger than f5V will damage the
inputs of some fast comparators. But this possibility has existed since the existence ot
the pA710, so you’ll have to clamp, clip, or attenuate the input signaldifferential or
otherwise-so the fast comparators can survive.

An Unspoken Problem
Something else that does not usually get mentioned in a data sheet is common-mode
slew problems. The good old LM3 1 1 is one part that is otherwise very well-behaved,
but causes some confusion when common-mode slew problems arise. But to some
extent all comparators can have these troubles. If one input suddenly slews up to
exceed the other’s level, you may see an unexpected, extra delay before the com-
parator’s output changes state. This delay arises because the comparator’s internal
nodes do not slew fast enough for its outputs to respond. For example, a 10-V step
can accrue an extra 100-ns delay compared with the delay for a 100-mV step. And if
both inputs slew together, the output can spew out indeterminate glitches or false
pulses even if the differential inputs don’t cross over. Be careful if your circuit has
comparator inputs of this sort, yet cannot tolerate such glitches.
Come to think of it, I get occasional complaints from engineers along the lines of,
“I’ve been using this comparator for years without any trouble, but suddenly it doesn’t
work right. How come?” When we inquire, we find that the comparators have been
operating very close to the edge of the “typical” common-mode range, well beyond