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03-26-2012

The realities of the maximum-supply-current specification for op amps. Part 1

Understand what this common, important parameter really means and why it may be much higher than you see on the data sheet.

Most ICs’ data sheets list a maximum supply current, but manufacturers often overlook the measurement conditions. For some rail-to-rail-output op amps, certain operating conditions can result in supply currents two to 10 times higher than the stated maximum. This article examines what to look for when trying to determine whether maximum supply current should be a concern for ICs—whether bipolar or CMOS.

Almost all IC 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 supply currents that increase as clock frequency increases, but what about analog parts, specifically op amps? You cannot always use the supply current plus the current to the load as a maximum.

Op amps operate in closed-loop mode, whereas comparators operate in open-loop mode. Designers seldom think about the ramifications of violating this given fact. A problem frequently arises, however, when you attempt to operate an op amp as a comparator. The approach is tempting because many op amps have low offset and low noise, so designers often press them into service as precision comparators. This scenario worked somewhat when older op amps operated from ±15V of power, and their input signals were within ±10V, especially if the designer had added some positive hysteresis to avoid oscillations and speed the transition through the uncertain region. The problem became serious with the advent of rail-to-rail-output op amps (Reference 1).

History

In the digital world, NAND gates, NOR gates, and other digital functions have distinctive military and ANSI symbols. However, in the analog world, op amps and comparators appear as triangles with two inputs and one output. Op amps have long found use as comparators, and many articles about both comparators and op amps as comparators appear in the literature. For example, as far back as 1967, when National Semiconductor introduced the LM101A, the data sheet showed an application circuit using it as a comparator. Analog Devices’ MT-083 data sheet provides a general discussion of comparators, covering how manufacturers specify them and the need for hysteresis, but it does not discuss using op amps as comparators (Reference 2). Another article discusses the general considerations when using op amps as comparators but does not discuss rail-to-rail-output op amps (Reference 3). The article does warn about the input differences with respect to common-mode input voltage and touches on the differences in differential-mode voltages.

Two different authors of an Analog Devices application note and tutorial advise against using an op amp as a comparator but then go on to conclude that doing so may be a proper engineering decision in some applications (references 4 and 5). According to another author, “the devil is in the details,” and he does an excellent job of covering input-protection diodes, phase reversal, and several other op-amp characteristics but argues that careful attention to these details can pay off (Reference 6). He briefly mentions rail-to-rail-output op amps but not supply current.

As supply voltages decreased, one of the methods engineers used to maintain a large voltage swing was to convert the classic output stage to a rail-to-rail-output stage. Figure 1 shows a classic output stage. Referring to the non-rail-to-rail output, the output can get only within approximately 1V of the positive supply. To get closer to the rails, engineers changed the output-stage transistors to a common-emitter configuration (Figure 2). The so-called rail-to-rail output can get within 50 to 100 mV of the supply, depending on the size of the output transistor and the load current.

The realities of the maximum-supply-current specification for op amps
Figure 1. In a classic output stage, referring to the non-rail-to-rail output, the output can get only within approximately 1V of the positive supply. 

The realities of the maximum-supply-current specification for op amps
Figure 2. To get closer to the rails, engineers changed the output- stage transistors to a common-emitter configuration.

Comparing these two output stages, note that the classic output stage has current gain but also has a voltage gain of less than one and a low output impedance. The rail-to-rail-output stage is a common-emitter stage and thus has voltage gain of approximately transconductance times the load resistance, RL. RL comprises the external load and the output impedance of the transistor. With the output operating more than several hundred millivolts away from the rail, output impedance is large and you can usually neglect it, but not if the output is close to the rail. Also note that you can consider the output as a classic two-transistor current mirror, which turns out to be the crux of the problem.

In normal operation, the middle stage pulls down the base-collector node, driving more current into the load and raising the voltage. With negative feedback, as the output voltage rises, the input and middle stages reduce the drive until the closed loop is balanced.

When you use an op amp as a comparator, the middle stage pulls down the base-collector node, trying to close the loop. With no feedback, however, 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. You can drive the output stage in several ways, and combining the difference in mobility between holes and electrons, the increase in supply current is usually asymmetrical.

References

  1. Kester, Walt, “Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues,” MT-035 Tutorial, Analog Devices, 2008.
  2. “Comparators,” MT-083 Tutorial, Analog Devices, October 2008.
  3. Sylvan, John, “High-speed comparators provide many useful circuit functions when used correctly,” Analog Dialogue, Ask the Applications Engineer-5.
  4. Bryant, James, “Using Op Amps as Comparators,” Application Note AN-849, Analog Devices, 2006.
  5. Kester, Walt, “Using Op Amps As Comparators,” MT-084 Tutorial, Analog Devices, 2008.
  6. Moghimi, Reza, “Amplifiers as Comparators?” Analog Dialogue, Ask the Applications Engineer-31, April 2003.

Part 2

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