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These functions are not physically disconnected from the battery when the car is presumably off. Instead, they remain connected but in a quiescent state, and the milliamps of current drain each of the many modules—the count easily reaches one hundred—which adds up to continual "vampire" drain on the car's battery. No one wants to find that the battery is drained after the car has not been started for a few weeks, especially in cold weather where its capacity is greatly reduced. Therefore, minimal quiescent current is not just an IoT concern, but an automotive one as well. Data sheets for basic analog components such as op-amps and comparators have always seemed to be somewhat of a contradiction. On one hand, these are fairly simple analog functions, serving as basic building blocks, and there shouldn't be much to say about them, at least in principle. Yet the reality is that their data sheets are comprehensive, often running tens of pages, with many detailed specifications and numerous graphs defining their performance under nominal conditions as well as under variations in rail voltage, temperature, load characteristics, and other perspectives. For older op-amps and comparators, the quiescent current was often only a small note on the basic specification table. Now the quiescent current is usually listed in large type on the first page of the data sheet: It's that important. Literally thousands of unique op-amp/comparator devices are available from dozens of reputable vendors, and each one has an array of specifications for top-, second-, and third-tier parameters. With the growth of IoT (and auto applications), there has been a rearrangement of which parameters go into which tier, and quiescent current has moved to the top in many cases. It's About Priorities and Tradeoffs Any assessment of IoT-focused op-amp/comparator parameters begins with the application specifics, of course. Temperature is the most measured physical variable and doesn't change quickly, due to the inherent thermal lag; similarly, factors such as rotational speed (rpm) and chemical- reaction results don't change instantaneously Figure 4. This is good news for the power-constrained IoT world, as high-speed operation generally requires more power to quickly source and sink current within the IC, for example. Similarly, applications such as automotive infotainment may demand high linearity, low distortion, and moderate bandwidth to ensure audio quality, but these may be of less concern in a modest IoT data- acquisition situation (although exceptions exist), and enhanced fidelity generally requires increased operating power. The question that engineers attempt to answer is simple to state: Among the thousands of available op-amps/ comparators, which one is best for the application? The answer is also simple: There is no best one, even after you Figure 4: The range of physical variables with which an IoT design needs to interface is large and diverse; many of them have fairly low bandwidth and speeds. (Source: Texas Instruments) eliminate those that are not a good fit or are optimized for other application classes. Instead, developing the answer requires understanding and then balancing the application priorities and tradeoffs. The first step to do is obvious: Develop a list of the most important parameters and minimum acceptable specifications for each. The second step is harder, as it requires a definition of how much of parameter X you would give up for an improvement of a certain amount in parameter Y; for example, determining how much is worth paying in additional power dissipation to get a device that offers additional needed bandwidth. A basic calculation demonstrates the impact of low quiescent current versus active operating current. Consider an IoT application using an op-amp with 1mA operating current and 1µA quiescent current, with a 1 percent duty cycle. The total current used by this op-amp is (99 × 1µA) + (1 × 1mA) or approximately 1.01mA. As the ratio between active and quiescent current-mode changes, or the duty cycle changes, the total current required will change as well. If the quiescent current drops by a factor of ten, down to 100nA, the total current needed will drop even closer to the operating current alone. Why the emphasis on active and quiescent current as a measure rather than power? The reason is that the important capacity figure-of-merit for most sources, including batteries and supercapacitors, is measured in amp-hours (A-hrs) or milliamp-hours (mA-hours), and the integral of current versus time is what determines the length of time that the source is viable. Power, in contrast, defines the rate at which this energy is used, but being able to deliver the power at a high-enough rate from the energy source is rarely a constraining issue. Of course, there's more to low-current design than using low-current, micropower components, although they do play a major role. Other design techniques are also used, such as 13

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