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NOMINAL CAPACITANCE (%) 1-F 0805 1-F 1206 4.7-F 1210 4.7-F 0805 2 4 6 8 10 12 14 16 DC VOLTAGE (V) 110 100 90 80 70 60 50 40 30 20 10 0 0 4.7-F 1206 1-F 0603 Figure 2 This graph, which plots the voltage performance of 1-μF and 4.7-μF capacitors, shows similar performance for the 1-μF 0603 and the 4.7-μF 0805. had the freedom to increase the values of the resistors involved by about 5× and thereby decrease the capacitance to 1 μF. Figure 2 graphs the voltage behavior of several 16V, 1-μF X7R caps versus that of 16V, 4.7-μF X7Rs. The 0603 1-μF capacitor behaves about the same as the 0805 4.7-μF device. Both the 0805 and 1206 1-μF capacitors perform slightly better than the 1210 4.7-μF size. Thus, by using the 0805 1-μF device, I was able to keep the capacitor size unchanged while getting a capacitor that only dropped to about 85% of nominal, rather than 30%, under bias. But I was still confused. I had been under the impression that all X7R caps should have similar voltage coefficients because the dielectric used was the same, namely X7R. So I contacted a colleague and expert on ceramic capacitors, TDK field applications engineer Chris Burkett, who explained that there are many materials that qualify as “X7R.” In fact, any material that allows a device to meet or exceed the X7R temperature characteristics, ±15% over −55°C to +125°C, can be called X7R. Burkett also explained that there are no voltage-coefficient specifications for X7R or any other ceramic-capacitor type. This is a critical point, so I will repeat it. A vendor can call a capacitor X7R (or X5R, or any other type) as long as the cap meets the temperature-coefficient specs, regardless of how bad the voltage coefficient is. This fact reinforces the old maxim (pun intended) that any experienced applications engineer knows: Read the data sheet! As capacitor vendors have turned out progressively smaller components, they have had to compromise on the materials used. To get the needed volumetric efficiencies in the smaller sizes, they have had to accept poorer voltage coefficients. Of course, the more reputable manufacturers do their best to minimize the adverse effects of this trade-off. Consequently, when using ceramic capacitors in small packages— indeed, when using any component—it is extremely important to read the data sheet. Regrettably, often the commonly available data sheets are abbreviated and will provide little of the information you’ll need to make an informed decision, so you may have to press the manufacturer for more details. What about those Y5Vs that I summarily rejected? For kicks, let’s examine a common Y5V capacitor. I chose a 4.7-μF, 0603-packaged capacitor rated at 6.3V—I won’t mention the vendor, because its Y5V cap is no worse than any other vendor’s Y5V cap—and looked at the specs at 5V and +85°C. At 5V, the typical capacitance is 92.9% below nominal, or 0.33 μF. That’s right. Biasing this 6.3V-rated capacitor with 5V will result in a capacitance that is 14 times smaller than nominal. At +85°C with 0V bias, the capacitance decreases by 68.14%, from 4.7 to 1.5 μF. Now, you might expect this to reduce the capacitance under 5V bias from 0.33 to 0.11 μF. Fortunately, however, those two effects do not combine in this way. In this particular case, the change in capacitance with 5V bias is worse at room temperature than at +85°C. To be clear, with this part under 0V bias, the capacitance drops from 4.7 μF at room temperature to 1.5 μF at +85°C, whereas under 5V bias the capacitance increases with temperature, from 0.33 μF at room temperature to 0.39 μF at +85°C. This result should convince you that you really need to check component specifications carefully. Getting down to spec ifics As a result of this lesson, I no longer just specify an X7R or X5R capacitor to colleagues or customers. Instead, I specify specific parts from specific vendors whose data I have checked. I also warn customers to check data when considering alternative vendors in production to ensure that they do not run into the problems I encountered. The larger lesson here, as you may have surmised, is to read the data sheet, every time, without exception. Ask for detailed data when the data sheet does not contain sufficient information. Remember, too, that the ceramic-capacitor designations X7R, Y5V, and so on imply nothing about voltage coefficients. Engineers must check the data to know, really know, how a specific capacitor will perform under voltage. Finally, keep in mind that, as we continue to drive madly to smaller and smaller sizes, this is becoming more of an issue every day.EDN Acknowledgment The author thanks Chris Burkett, field applications engineer at TDK, for his explanation of why ceramic capacitors of the same nominal type can have widely divergent voltage coefficients. Author’s biography Mark Fortunato is senior principal member of the technical staff in the Communications and Automotive Solutions Group at Maxim Integrated (San Jose, CA). He has spent much of the past 16 years helping customers tame analog circuitry. Before that, Fortunato worked on products ranging from speech-recognition systems to consumer electronics, millimeter-wave instrumentation, and automated teller machines. He regrets that he never got to meet Jim Williams or Bob Pease. w ww.edn-europe.com MARCH 2013 | EDN Europe 25


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