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Be a decoupling capacitor network expert (Part 1)

Posted: 26 Dec 2013     Print Version  Bookmark and Share

Keywords:decoupling capacitor  network  DC supply  Equivalent series resistance  ESR 

Depending upon who is being asked, questions about the design and location of a decoupling capacitor network will return different answers. It's pretty much like asking about the perfect temperature at which beer should be served. The funny thing is that, even though the answers may be very different, each respondent will be sure that only he or she is correct.

Before I discuss my preferred beer temperature and how I design and locate my decoupling capacitors, I think it is important we all understand why we have decoupling capacitor networks in the first place. These networks are intended to perform two functions: to provide a low-impedance path to ground for AC signals and noise signals superimposed on the DC supply voltage, and to act as a local energy store close to the device being decoupled, so that high-frequency demands for current (due to logic gates switching, for example) can be supplied without affecting the voltage rail. Remember that a power supply has a much slower response time to transient demands than the operational speed of the devices it powers. At higher frequencies, on-chip decoupling is required, but that's a story for another day.

Both of these requirements will have bearing on the design of the decoupling capacitor network. We must also understand the parasitic elements and construction of a real-world capacitor, which—along with its capacitive element—will have resistive and inductive elements, as illustrated in figure 1.

Figure 1: Real structure of a capacitor (for decoupling purposes, RP is normally discounted),

Equivalent series resistance (ESR) is defined by the resistance of the leads or pads and losses in the dielectric. This is typically in the range of 0.01 to 0.1Ω for a ceramic capacitor.

Equivalent series inductance (ESL) is defined by internal connections or leads and pads. This is very important in the case of decoupling, because it will dominate over the capacitance above certain frequencies.

From the model above, it is clear that the capacitor C and the ESL will form a series resonance creating a near short (it is not a dead short, due to the ESR). You can calculate the self-resonant frequency (SRF) of a capacitor using the following equation.

Equation for self-resonant frequency (SRF) of a capacitor.

What this means is that, if you have a specific AC frequency you wish to remove, you should ideally select a capacitor with a SRF at the relevant frequency. Another consideration is to ensure a low-impedance profile over a wide frequency band, which will require a range of capacitor values connected in parallel. For example, the network illustrated in figure 2 employs two different value capacitors. Observe that there are more lower-value capacitors than higher-value ones.

Figure 2: An example decoupling capacitor network.

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