Technical Article
MS-2212
.
History
The Maximum Supply Current
That Wasn’t
In the digital world, NAND gates, NOR gates, etc., had
distinctive MIL/ANSI symbols, but in the analog world, for
some unknown reason, op amps and comparators were
shown as a triangle with two inputs and one output, “and
that has made all the difference”(2). Op amps have been
used as comparators for quite awhile and many articles have
been written about both comparators, and op amps used as
comparators. As far back as ±967, when the LM±0±A was
introduced, the data sheet showed an application circuit
using it as a comparator. Tutorial MT-083 (3) is a good,
general discussion of comparators, covering how
By Harry Holt, Staff Applications Engineer,
Analog Devices, Inc.
IDEA IN BRIEF
For most integrated circuits, a maximum supply current is
listed on the data sheet. Often overlooked are the
measurement conditions. For some rail-to-rail output op
amps, certain operation can result in supply currents two to
ten times higher than the stated maximum. Whether bipolar
or CMOS, some tips are given as to what to look for to see
whether or not this is a concern.
comparators are specified and the need for hysteresis with
comparators, but does not discuss using op amps as
comparators. Sylvan (4) discusses the general considerations
when using op amps as comparators but does not discuss
rail-to-rail output op amps specifically. He does warn about
the input differences with respect to common-mode input
voltage and touches on the differences in differential mode
voltages. Bryant (5) starts by saying “However, the best
advice on using an op amp as comparator is very simple—
don’t!” and then covers a variety of things to consider,
concluding that in some applications, it may be a proper
engineering decision. Kester (6) also warns against using op
amps as comparators, and grudgingly admits there are a few
cases were it might make sense. Moghimi (7) discusses the
differences between op amps and comparators, warning,
“the devil is in the details” and does an excellent job covering
lmost all integrated circuit data sheets have a
guaranteed maximum supply current, but you
cannot always use this number for your worst case
power calculations. It’s well known that CMOS digital parts
have a supply current that increases as clock frequency
increases, but what about analog parts, specifically op amps?
Can you use the supply current plus the current supplied to
the load as a maximum? (Hint: not always…..)
Op amps are designed to be operated closed loop, while
comparators are operated open loop. Although this simple
statement is obvious, seldom do we think about the
ramifications of violating this. The more frequent problem is
when operating an op amp as a comparator. It is tempting,
because many op amps are designed to have very low offset
and very low noise, so they are pressed into service as
precision comparators. When op amps were powered on
±±5 V, and input signals were within ±±0 V, this worked
somewhat, especially if some positive hystersis was added to
avoid oscillations and speed up the transition through the
uncertainty region. The problem became serious with the
advent of rail-to-rail output op amps. For a good explanation
of the input and output stages, see (±) in the References
section.
Figure 1. Classic bipolar output stage
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©2011 Analog Devices, Inc. All rights reserved.
November 2011 | Page 1 of 5
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