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 wasnt by harry holt , staff applications e ngineer , 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 w hat 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. its 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 compar ator. 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 i nput and output st ages, see (1) in the references section . h istory 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 o ne 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, t he data sheet showed an application circuit using it as a comparator. tutor ial mt - 083 (3) is a good, general discussion of comparators, covering how comparators are s pecified 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 do es 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 dont! and th en 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. mogh imi (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
ms -2212 technical article www.analog.com ? 2011 analog devices, inc. all rights reserved. november 2011 | page 2 of 5 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 f igure 1. referring to the non - rail - to -r ail o utput, the output ca n only get within about 1 v of the positive supply. to get closer to the r ails, the output stage transistors were changed to a common emi tter configuration as shown in f igure 2. 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 100 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 imped ance. second, the rail - to -r ail output stage is a common emitter stage and , thus , has voltage gain, approximately g m r l . r l is composed of the external load and the output impedance (r o ) of the transistor. with the output operating more than several hundr ed millivolts away from the rail, r o 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. 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 loo p is balanced. 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 a re 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 sym m etr ical. 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 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 non critical, and are selected to ensure that the input to the op amp is
technical article ms -2212 www.analog.com november 2011 | page 3 of 5 ? 2011 analog devices, inc. all rights reserved. within th e specified input voltage r ange (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 usual ly stated in the absolute max imum ta ble as 0.7 v. if there are internal series resistors, they are usually in the 500 ? to 2 k? range. the absolute maximum t able may state that the max imum 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, th e 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 interna l diodes. values were chosen to limit the input current to less than 1 ma. figure 4. comparator, output low figure 5. comparator, output high table 1 lists the maximum supply current specification from the data sheets, the measured supply current with the op amp connected as a follower with v in halfway between the supply pins ( f ig ure 3), and the supply current with the output forced low (f ig ure 4) and forced h igh (f ig ure 5). classic op amp and comparator table 1 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 manufacturers 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 v out is driven high by q5/q3/q4, the supply current will be a function of the values of r4 and r6. these values are sel ected to maximize the op amp performance and minimize die area, not comparator operation. when v out is driven low by q6/r1/q1, the supply current will be determined by r1. again, the values of r1, i1, etc. are chosen for op amp performance, not comparator performance.
ms -2212 technical article www.analog.com ? 2011 analog devices, inc. all rights reserved. november 2011 | page 4 of 5 table 1. competitor type spec (ma) follower (ma) vol (ma) voh (ma) lm358 bipolar 30v 2 0.707 0.506 0.671 lm393 bipolar 36v 2.5 0.548 0.565 0.567 op184 bipolar 30v 2 1.239 1.188 6.683 a bipolar 24v 0.45 0.361 3.442 0.708 b bipolar 30v 3.4 2.785 2.051 3.998 c bipolar 30v 4.5 4.063 5.336 3.786 ad8605 cmos 5v 1.2 0.998 0.544 0.625 a cmos 5v 0.9 0.511 0.361 10.152 b cmos 5v 2.4 1.916 2.759 2.475 c cmos 5v 1.4 1.039 0.822 0.667 red highlight ed values indicate exceeds data sheet limit. figure 6. cmos ail - to - ail op mps 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 r l , 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 o p amps selected. 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 nor mal range and causes the bias circuit to malfunction, then the other op amp will malfunction also. in battery operated systems or when using low current series regulators, the additional supply current should be considered. battery life may be less than ca lculated, or the regulator may not start up under all conditions, especially over temperature. tips for new designs, the easiest solution is dont use op amps as comparators! if you must, or have used one by accident as a comparator: ? check the data sheet to see if the manufacturer has any information on operation as a comparator. some manufacturers are adding this information (9,10) . ? 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. 00293-045 v+ i2 i1 q1 q3 q4 q2 vC q5 v out q6 r6 r3 r2 r1 r4 r5 d1 input from second gain stage
technical article ms -2212 www.analog.com november 2011 | page 5 of 5 ? 2011 analog devices, inc. all rights reserved. summary rail - to- rail output op amps have unique characteristics when operated as comparators. the best solutions to improving battery life and increasing per formance 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 c urrent may greatly exceed the m ax 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. references 1. kester, walt op amp inputs, outputs, single - supply, and rail - to- rail issues mt - 035 tutorial www.analog.com / mt - 035 2. frost, robert mountain interval new york: henry holt & company, 1920. 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 tu torial www.analog.com / mt - 084 7. moghimi, reza amplifiers as comparators? analog dialogue ask the applications engineer 31 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 10. AD8657 data sheet www.analog.com / AD8657 resourc es for resources and information on op am p s, visit www.analog.com / opamps . products mentioned in this a rticle product description op184 single - supply rail-to - rail input/output operational amplifier op284 dua l precision rail -to - rail input/ output operational amplifier ad8605 precision, low noise, cmos, rail -to - rail input/output op erational amp lifier ada4092 -4 micropower, ovp, rail -to - rail input/output operational amplifier a d8657 precision, micropower 18 v cmos rail -to - rail input/output operational amplifier about the author harry holt is a staff applications engineer at analog devices (san jose, ca) in the p recis ion a mplifier s g roup 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 . one technology way ? p.o. box 9106 ? norwood, ma 02062 - 9106, u.s.a . te l: 781.329.4700 ? fax: 781.461.3113 ? www.analog.com trademarks and registered trademarks are the property of their respective owne t a10133 - 0 - 11/11 (0)
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