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EDNE MARCH 2013

Not al X7Rs are created equal Since my RC time-constant problem was far greater than would be explained by the specified temperature variation, I had to dig deeper. Looking at the data for capacitance variation versus applied voltage for my capacitor, I was surprised to see how much the capacitance changed with the conditions I had set. I had chosen a 16V capacitor to operate with a 12V bias. The data sheet indicated that my 4.7-μF capacitor would typically provide 1.5 μF of capacitance under those conditions. Now, that explained the problem my RC circuit was having. The data sheet then showed that if I just increased the size of my capacitor from the 0805 to the 1206 package size, the typical capacitance under the specified conditions would be 3.4 μF. This called for more investigation. I discovered that Murata Manufacturing Co (www.murata. com) and TDK Corp (www.tdk.com) offer nifty tools on their Web sites that let you plot the variations of capacitors over different environmental conditions. I investigated 4.7-μF capacitors of various sizes and voltage ratings. Figure 1 graphs the data that I extracted from the Murata tool for several different 4.7-μF ceramic capacitors. I looked at both X5R and X7R types, in package sizes from 0603 to 1812 and with voltage ratings from 6.3 to 25V dc. Note, first, that as the package size increases, the capacitance variation with applied dc voltage decreases—and does so substantially. A second interesting point is that, for a given package size and ceramic type, the capacitor voltage rating seems often to have no effect. I would have expected that using a 25V-rated capacitor at 12V would result in less variation than using a 16V-rated capacitor under the same bias. Looking at the traces for X5Rs in the 1206 package, it’s clear that the 6.3V-rated part does indeed perform better than its siblings with higher voltage TA BLE 2 CAPACITAN CE OF X7R CAPS WITH A 12V BIAS Size C (μF) % of Nominal 0805 1.53 32.6 1206 3.43 73 1210 4.16 88.5 1812 4.18 88.9 Nominal 4.7 100 ratings. If we were to examine a broader range of capacitors, we would find this behavior to be common. The sample set of capacitors that I considered in my investigation did not exhibit the behavior to the same extent as the general population of ceramic capacitors would. A third observation is that, for the same package, X7Rs have better temperature sensitivity than do X5Rs. I do not know whether this holds true universally, but it did seem so in my investigation. Using the data from this graph, Table 2 shows how much the X7R capacitances decreased with a 12V bias. Note that there is a steady improvement with progressively larger capacitor sizes until the 1210 size; going beyond that size yields no real improvement. Choosing the right capacitor In my case, I had chosen the smallest available package for a 4.7-μF X7R because size was a concern for my project. In my ignorance, I had assumed that any X7R was as effective as any other X7R; clearly, this is not the case. To get the proper performance for my application, I had to use a larger package. I really did not want to go to a 1210 package. Fortunately, I 1812-X7R-25V 1210-X7R-25V 1210-X5R-25V 1210-X5R-16V 1206-X7R-25V 1206-X7R-16V 1206-X7R-10V 1206-X5R-25V 1206-X5R-16V 1206-X5R-10V 1206-X5R-6.3V 0805-X7R-16V 0805-X7R-10V 0805-X5R-16V 0805-X5R-10V 0805-X5R-6.3V 0603-X5R-6.3V CAPACITANCE (F) X5R 1210 X7R 1210 X5R 1206 X7R 1812 X7R 1206 X7R 0805 5 10 15 20 25 DC VOLTAGE (V) 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0 X5R 0603 X5R 0805 Figure 1 As this graphic representation of temperature variation versus dc voltage for select 4.7-μF capacitors shows, as the package size increases, the capacitance variation with applied voltage substantially decreases. 24 EDN Europe | MARCH 2013 www.edn-europe.com


EDNE MARCH 2013
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