Technical Article

MS-2212

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November 2011 | Page 1 of 5

©2011 Analog Devices, Inc. All rights reserved.

The Maximum Supply Current

That Wasn’t

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.

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

±15 V, and input signals were within ±10 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 (1) in the References

section.

History

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 1967, when the LM101A 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

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

Figure 1. Classic bipolar output stage

A