Search This Blog

How to select Bypass Capacitors ?

High-speed switching environments generate noise on power lines (or planes) due to the charging and discharging of internal and external capacitors of an integrated circuit. The instantaneous current generated with the rising and falling edges of the outputs causes the power line (or plane) to ring. This behavior can violate the VCC recommended operating conditions or generate false signals, creating serious problems. A simple and easy solution must be considered to prevent such a problem from occurring. This solution is the bypass capacitor.

A bypass capacitor stores an electrical charge that is released to the power line whenever a transient voltage spike occurs. It provides a low-impedance supply, thereby minimizing the noise generated by the switching outputs of the device.

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. 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 their niche in low-voltage systems. Aluminum electrolytic capacitors are a common choice for lowto-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 and full temperature range (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 set forth in Table 3. 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 vs 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, Table 4 is included. As expected, the effective series inductance decreases monotonically as package size decreases. Special attention should be paid to the 1206 and 0612 case. 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.


Join us:

No comments