MS-2212 [ADI]
Technical Article; 技术文章
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Technical Article
MS-2212
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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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MS-2212
Technical Article
In normal operation, the middle stage will pull the base-
collector node down, driving more current into the load and
raising the voltage. With negative feedback, as the output
voltage rises, the input stage and middle stage will reduce the
drive until the closed loop is balanced.
input protection diodes, phase reversal, and several other op
amp characteristics but argues that careful attention to these
details can pay off. He does briefly mention RRO op amps,
but not supply current.
As supply voltages decreased, one of the methods used to try
to maintain a large voltage swing, was to convert the classic
output stage to a “rail to rail” output stage. A classic output
stage is shown in Figure ±. Referring to the non-rail-to-rail
output, the output can only get within about ± V of the
positive supply.
When used as a comparator, the middle stage will pull the
base-collector node down, trying to close the loop, but with
no feedback, it continues to pull harder and harder. This
additional current finds a path from the positive supply pin
to the negative supply pin and appears as additional supply
current. There are several different ways of driving the
output stage, and combined with the difference in mobility
between holes and electrons, the increase in supply current
is usually not symmetrical.
To get closer to the rails, the output stage transistors were
changed to a common emitter configuration as shown in
Figure 2.
To quantify this effect, a bipolar op amp and a CMOS op
amp were obtained from Analog Devices and three of its
major analog competitors. For comparison purposes, the
venerable LM358 dual op amp (non-RRO) and LM393 dual
comparator were also included. The supply current was
measured as a function of supply voltage using three circuits.
Figure 3 shows the classic method for measuring supply
current. The ammeters are connected as shown so that the
supply current of the resistive divider is not included.
Figure 2. Bipolar rail-to-rail output
The “rail-to-rail” output is not really rail-to-rail, but can get
within 50 mV to ±00 mV of the supply depending on the
size of the output transistor and the load current.
Comparing these two output stages, there are three
important things to note: First, the classic output stage has
current gain, but a voltage gain less than one, and very low
output impedance. Second, the rail-to-rail output stage is a
common emitter stage and, thus, has voltage gain,
approximately gm × RL. RL is composed of the external load
and the output impedance (RO) of the transistor. With the
output operating more than several hundred millivolts away
from the rail, RO is very large and can usually be neglected,
but not if the output is close to the rail. Third, the output can
be considered as a classic two transistor ratioed current
mirror. This is the crux of the problem.
Figure 3.
Two ammeters are used to verify that the supply current is
accurate and does not include any undesired current path
through the input pins. The resistor values are noncritical,
and are selected to ensure that the input to the op amp is
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Technical Article
MS-2212
within the specified input voltage range (IVR) from the data
sheet specification table.
To measure supply current when open loop, such as
operation as a comparator, see Figure 4 and Figure 5. Some
low noise, bipolar op amps have diodes between the inputs
to protect the differential input pair, so the maximum
differential voltage is usually stated in the Absolute
Maximum table as ±0.7 V. If there are internal series
resistors, they are usually in the 500 Ω to 2 kΩ range. The
Absolute Maximum table may state that the maximum
differential voltage is ± supply voltage, but this does not
mean that the part operates. A simplified internal schematic
should be consulted. If one is not provided, a quick call to
the manufacturer can resolve this. In these two
configurations, the choice of resistor values is a little more
critical. The resistor values should be low enough to cause
the differential input voltage to be at least 0.5 V to guarantee
that the output is driven hard into the rail but high enough
not to damage the internal diodes. Values were chosen to
limit the input current to less than ± mA.
Figure 5. Comparator, output high
Table ± lists the maximum supply current specification from
the data sheets, the measured supply current with the op
amp connected as a follower with VIN halfway between the
supply pins (Figure 3), and the supply current with the
output forced low (Figure 4) and forced high (Figure 5).
Classic op amp and comparator
Table ± shows that the classic LM358 and LM393 are well
behaved, as expected.
Bipolar Rail-to-Rail Op Amps
All the bipolar rail-to-rail output op amps have supply
current greater than the “maximum” op amp supply current
in one or both comparator circuits. There are several ways to
drive the output stage, so some methods will result in a
supply current increase when driving to one rail or the other.
Without being privy a manufacturer’s internal schematics,
one cannot comment on the behavior.
For the OP284, the second stage and output stage simplified
schematic is shown on the data sheet. See Figure 6.
If VOUT is driven high by Q5/Q3/Q4, the supply current will
be a function of the values of R4 and R6. These values are
selected to maximize the op amp performance and minimize
die area, not comparator operation. When VOUT is driven
low by Q6/R±/Q±, the supply current will be determined by
R±. Again, the values of R±, I±, etc. are chosen for op amp
performance, not comparator performance.
Figure 4. Comparator, output low
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MS-2212
Technical Article
Table 1.
Competitor
Type
Spec (mA)
Follower (mA)
0.707
Vol (mA)
0.506
0.565
1.188
3.442
2.051
5.336
0.544
0.361
2.759
0.822
Voh (mA)
0.671
0.567
6.683
0.708
3.998
3.786
0.625
10.152
2.475
0.667
LM358
Bipolar
Bipolar
Bipolar
Bipolar
Bipolar
Bipolar
CMOS
CMOS
CMOS
CMOS
30V
36V
30V
24V
30V
30V
5V
2
LM393
2.5
2
0.548
OP184
1.239
A
0.45
3.4
4.5
1.2
0.9
2.4
1.4
0.361
B
2.785
C
4.063
AD8605
0.998
A
B
C
5V
0.511
5V
1.916
5V
1.039
Red highlighted values indicate exceeds data sheet limit.
V+
Finally, in the desire to reduce die size and, therefore, cost,
some circuits, such as bias circuits and the associated startup
circuit, may be shared by both op amps. As mentioned
previously (8), if one op amp operates outside of its normal
range and causes the bias circuit to malfunction, then the
other op amp will malfunction also.
R4
I2
INPUT FROM
SECOND GAIN
STAGE
Q5
Q6
Q3
Q1
V
OUT
R1
In battery operated systems or when using low current series
regulators, the additional supply current should be
considered. Battery life may be less than calculated, or the
regulator may not start up under all conditions, especially
over temperature.
R2
Q4
D1
Q2
R5
I1
R3
R6
V–
Tips
Figure 6.
For new designs, the easiest solution is “Don’t use op amps
as comparators!” If you must, or have used one by accident
as a comparator:
CMOS Rail-to-Rail Op Amps
The CMOS op amps have an interesting behavior. In some
cases, the supply current actually goes down when driven to
a rail. The output stage of a CMOS op amp consists of
common source PMOS and NMOS transistors, and gain is
taken in the output stage. The gain is gm × RL, and to get a
reasonable value of transconductance, the drive circuit is
designed to set the quiescent current to a certain value. As
the output is driven into the rail, the drive circuit will
decrease the drive on the complementary transistor.
Depending on the transfer characteristics from the top
transistor to the bottom transistor, the current will actually
decrease. Note the wide variation in behavior among the
four CMOS op amps selected.
•
Check the data sheet to see if the manufacturer
has any information on operation as a
comparator. Some manufacturers are adding
this information (9,±0).
•
•
If the information is not there, ask the
manufacturer if it is available.
If they cannot provide it, measure several date
codes yourself using the circuits shown previously,
and add 50% for a safety factor.
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Technical Article
MS-2212
Summary
RESOURCES
Rail-to-rail output op amps have unique characteristics
when operated as comparators.
For resources and information on op amps,
visit www.analog.com/opamps.
The best solutions to improving battery life and increasing
performance are to use a low cost comparator when a
comparator function is required, tying off any used op amp
sections as followers with the noninverting input connected
to a stable voltage within the input voltage range of the op
amp, or using singles and duals as appropriate instead of
quads. Supply current may greatly exceed the “Max” stated
on the data sheet. Under carefully considered conditions,
unused op amps can be used as comparators, but using the
proper mix of op amps and comparators will result in lower
supply current and well-defined performance.
Products Mentioned in This Article
Product
Description
Single-Supply Rail-to-Rail Input/Output
Operational Amplifier
OP184
Dual Precision Rail-to-Rail Input/Output
Operational Amplifier
OP284
Precision, Low Noise, CMOS, Rail-to-Rail
Input/Output Operational Amplifier
AD8605
ADA4092-4
AD8657
Micropower, OVP, Rail-to-Rail Input/Output
Operational Amplifier
Precision, Micropower 18 V CMOS Rail-to-Rail
Input/Output Operational Amplifier
REFERENCES
ABOUT THE AUTHOR
±. Kester, Walt “Op Amp Inputs, Outputs, Single-Supply,
and Rail-to-Rail Issues” MT-035 Tutorial
www.analog.com/MT-035
Harry Holt is a staff applications engineer at Analog Devices
(San Jose, CA) in the Precision Amplifiers Group where he
has worked for four years, following 27 years in both field
and factory applications at National Semiconductor for a
variety of products, including data converters, op amps,
references, audio codecs, and FPGAs. He has a BSEE from
San Jose State University and is a life member of Tau Beta Pi
and a Senior Member of the IEEE.
2. Frost, Robert “Mountain Interval” New York: Henry
Holt & Company, ±920.
3. ”Comparators” MT-083 Tutorial
www.analog.com/MT-083
4. Sylvan, John, “High-speed comparators provide many
useful circuit functions when used correctly” Analog
Dialogue, Ask the Applications Engineer— 5
www.analog.com/analogdialogue
5. Bryant, James “Using Op Amps as Comparators” 2006
AN-849 at www.analog.com/AN-849
6. Kester, Walt “Using Op Amps As Comparators”
MT-084 Tutorial www.analog.com/MT-084
7. Moghimi, Reza “Amplifiers as Comparators?” Analog
Dialogue Ask the Applications Engineer—3±
www.analog.com/analogdialogue
8. Holt, Harry ”OP Amps: To Dual or Not to Dual”
www.eetimes.com
9. ADA4092-4 data sheet www.analog.com/ADA4092-4
±0. AD8657 data sheet www.analog.com/AD8657
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Trademarks and registered trademarks are the property of their
respective owne TA10133-0-11/11(0)
• Fax: 781.461.3113 • www.analog.com
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