Choosing and Using Bypass Capacitors (Part 2 of 3)

(Editor's Note: Part 1 is available here. Part 3 examines specific application examples.)

Common Types of Capacitors and Trade-offs As described in the previous sections, the materials and structure of a capacitor will dictate its attributes, like parasitics, temperature stability, maximum voltage, linearity, cost and size. A summary of the most popular capacitors available in surface mount packages is given in Table 2 .

Table 2: Common capacitor specifications and trade-offs
(Click to Enlarge Image)

Ceramic capacitors are the most common capacitor type since they are inexpensive, offer a wide range of values, and provide solid performance. Tantalum, Oscon, and Aluminum Electrolytic capacitors are all polarized, specifically to be used as a bypass capacitor. Tantalum found its niche in low-voltage systems. Aluminum electrolytic capacitors are a common choice for low-to-medium frequency systems, but not switching circuits (they hold their charge too well, which doesn't suit them for the rapid cycling of production testing).

Oscon is a special capacitor type developed to provide low parasitics, wide frequency range, full temperature range, giving the best quality available for the highest price tag. If you have the budget, these capacitors will provide quality bypass for any circuit. Mica and Plastic Film capacitors are included for completeness. Their primary use is in filter design instead of bypass.

Since ceramic capacitors are the most widely used bypass capacitors, it is useful to look at the options available in purchasing. As expected, ceramic caps are available in a wide range of values and in a wide variety of packages. Within these parameters, there are further choices which will determine the final price. An example is shown in Table 3 .

Table 3: Classification of Ceramic Capacitor Options
(Click to Enlarge Image)

In a recent bill of materials, the capacitors were labeled as “X7R”. The X and 7 set the widest temperature range. The final letter “R” reports the tolerance of that capacitor over the temperature range. In this case, there would only be a 15% change in capacitor versus temperature. A wider temperature range and tighter tolerance earn higher price tags.

Selecting the Package for the Bypass Capacitor
Once the dielectric material, dielectric quality, temperature range, acceptable leakage and voltage range have been met, the final choice involves package dimensions. Typically, the package size is chosen by “what was used last time” or what is big enough to solder by hand (if a prototype).

What you need to remember is that the equivalent circuit model will change with different packages. The main issue is the equivalent series inductance (ESL). Obviously, a capacitor structure is constant as long as the capacitance value is constant. If that same capacitor is available in a variety of packages, then the connections between the plates and the outer dimensions of the package must change. This appears as additional series resistance and series inductance. The smaller the package is, the smaller the series parasitics.

To demonstrate this trend, see Table 4 . As expected, the effective series inductance decreases monotonically as package size decreases. Pay special attention to the 1206 and 0612 case, Figure 7 .

Table 4: Surface-mount packages and their equivalent series inductances

Figure 7: Example surface-mount packages in the 1206 orientation and the 0612 orientation

Although they have the same footprint, the 1206 has connections on the ends while the 0612 has connections on the longer edges. This simple change in orientation allows the inner package connections to be much smaller. Delightfully, the ESL is reduced by 95%.

In wide-bandwidth circuits, the amount of series inductance sets an upper bound on the ability of the bypass circuit to provide a low-impedance for the power supply pin. This will be further discussed in the following sections.

Sizing Bypass Capacitors
Bypass capacitors are usually sized by convention or typical values. For example, common values are 1 µF and 0.1 µF. In the simplest terms, the larger value handles the lower frequencies and high-current issues while the smaller value handles higher frequencies. The need for multiple capacitors comes from the parasitics associated with real capacitors. Figure 8 plots the impedance of a real capacitor.

Figure 8: Impedance of an actual capacitor (non-ideal)
(Click to Enlarge Image)

The axes are left blank so the values can be scaled to fit any capacitor. The left half of the curve represents the traditional (and ideal) capacitor response: as frequency increases, the impedance of the capacitor decreases. This is desirable, since bypass capacitors provide a low impedance (effectively a short) to AC signals on the power line. The negative slope of the line is constant, but the lateral placement of the line is dependent on the size of the capacitor. For example, a larger capacitor would shift the left half of the curve lower in frequency (farther to the left).

Any inductance in the package of the capacitor will cause a positive slope, as seen in the right half of the plot. In this region of frequencies, the inductance is canceling and then dominating the low impedance provided by the capacitor.

Since the value of the impedance is related to the size and construction of the bypass capacitor, the frequency response is also related, Figure 9 .

Figure 9: Impedance of an actual capacitor (non-ideal) in different surface-mount packages
(Click to Enlarge Image)

Therefore, you want to check the datasheet to ensure that your choice of bypass capacitor is available in a package that will allow you to provide the low impedance necessary for the frequencies present in your system. Remember, the ESLs quoted in Table 4 are in the range of hundreds of picoHenries. Their rising effect on impedance only emerges when system frequencies are greater than 100 MHz.

Bypassing a System with Wide Bandwidth
In some wideband systems, a single capacitor is not sufficient for bypassing. Multiple frequencies couple into the power supply lines and a bypass network must be used to provide a low impedance for a wider range of frequencies. Since the slopes, both negative and positive, of the impedance curves are physical limits, multiple capacitors are connected in parallel.

Of course, care must be taken when selecting the packages of each of the capacitors. A typical bill of materials may dictate that all passive components have the same geometry, such as all 0805 capacitors. The resulting impedance plot versus frequency for three parallel capacitors is plotted in Figure 10 .

Figure 10: Impedance of three capacitors in parallel, in the same surface-mount packages
(Click to Enlarge Image)

Since the same package was used for each of the capacitors, their high frequency responses are the same. Effectively, this negates the use of the smaller capacitors! Instead, the package size should be scaled along with the capacitor size, Figure 11 .

Figure 11: Impedance of three capacitors in parallel, with scaled surface-mount packages
(Click to Enlarge Image)

Of course, this level of detail implies that strict attention has also been paid to the layout of the bypass capacitors. Any additional trace length will also increase the impedance in the bypass path. Each trace also contributes inductance per unit length; longer traces will lower the useful frequency of the bypass path (shifting the curves of Figures 8-11 to the left.) Therefore, bypass capacitors should be placed as close to the power-supply pins as possible. The opposite side of the capacitor needs a via to the ground plane, or a wide ground trace, to keep the impedance low.

(You can read Part 3 by clicking here.)

About the authors
Mike Wong is the VP of application engineering focusing on high-speed analog and mixed-signal applications at Intersil Corp. He has previously worked on power supplies at ASTEC. He has a BSEE from the University of California at Davis.

Tamara Schmitz is a principal application engineer for analog applications at Intersil Corp. She is also a full-time professor of Electrical Engineering at San Jose State University. She has a BSEE, MSEE and PhD in RF CMOS design from Stanford University.

3 comments on “Choosing and Using Bypass Capacitors (Part 2 of 3)

  1. mpkuti
    February 11, 2018

    I am not able to see the figures and the tables.

  2. Steve Taranovich
    February 17, 2018

    Sorry @mpkuti, 

    This 11 year old article is being researched by our IT group. I will alert you when the images are restored

  3. mpkuti
    February 19, 2018

    That would be great, Steve! Thank you!

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