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You may have less DCAP than you think!

Now that you’ve picked the values for your ceramic decoupling capacitors, how do you decide on the working voltage rating? This is an important factor because unless you derate the dcap working voltage there’ll be less capacitance in the design than you had intended.

It turns out that the actual capacitance value of class II ceramic capacitors such as a 0.1uF X7R or Y5V dcap is influenced by the intensity of the electric field within the capacitor. Class II is the ceramic class from which almost all capacitors used for decoupling on commercial printed circuit boards comes from. Even when the power supply voltage in a circuit is low, the small size of modern SMT capacitors means the electric fields within the dcap can be quite high. And, the fields will be even higher in those capacitors constructed of many plates which are separated by a thin smear of dielectric (this is one of the techniques used by vendors get high values of capacitance in such small packages).

I’ve written extensively on the makeup and characteristics of Class I and Class II ceramic capacitors, but manufacturer’s datasheets and websites also have lots of great information. Some vendors offer software that you can use showing how their capacitors are affected by temperature and voltage. One of my favorites is SpiCap3.0 from AVX, which you can check out here.

If you drill into this you’ll see how strongly the capacitance of class II ceramic capacitors is reduced by voltage. The best way is to look at this is to plot the capacitance as a percentage of the specified working voltage. For instance, you’ll find that, depending on the body style, capacitance value and manufacturer, when exposed to voltages of 50% or more of the working voltage rating, the capacitance of an X7R can be reduced by as much as 20% over its datasheet value. The capacitance can be as low as an astonishing 20% of the specified value for an Y5V, and can be close to that for a Z5U.

The amount of capacitance and the body style determine the number and thickness of the plates within the capacitor, and so greatly influences the severity of these effects. And, since each manufacturer has a different recipe for their ceramic and for determining the number of plates, the effects will vary from manufacture to manufacturer. This means in critical applications you’ll need to carefully test capacitors from second sources.

For example, a 6.3V 100nF X7R capacitor is actually about 80nF if it’s used to decouple a 3.3V power supply, but that’s much better than the 20nF value of a 100nF 6.3V Y5V dcap. Increasing the temperature reduces the capacitance further (especially for the Y5V).

The rule I use is never to operate an X7R type capacitor at more than 75% of its rated working voltage. This holds for timing and for bypass (decoupling) applications. If for some reason Y5V or ZU5 capacitors must be used, never use them at more than 50% of their working voltage. Limiting them to voltages no higher than about 25% of the working voltage spec is even better. Incidentally, I never use them in timing or op-amp applications or to set the loop response in PLL circuits.

The bottom line is to understand types and ratings of the dcaps used in your design, and always account for the effects of voltage and temperature when modeling and simulating your power distribution network.

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