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OP284 Scheda tecnica(PDF) 1 Page - Analog Devices |
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OP284 Scheda tecnica(HTML) 1 Page - Analog Devices |
1 / 5 page Technical Article MS-2212 . www.analog.com 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 |
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