Why do we need to place a 0.1uF capacitor near the IC chip?

2024-03-17

We can see various capacitors in power filtering circuits, such as 100uF, 10uF, 100nF, and 10nF. How are these parameters determined?

Digital circuits need to operate stably and reliably. The power supply must be "clean," and energy replenishment must be timely, which means that filtering and decoupling must be good. What is filtering and decoupling? Simply put, it is storing energy when the chip does not need current, and replenishing energy promptly when it is needed. Some readers may ask, isn't this the responsibility of DC/DC and LDO?

Yes, they can handle it at low frequencies, but it's different for high-speed digital systems. First, let's look at capacitors. The role of a capacitor is simply to store charge.

We all know that capacitors need to be added for filtering in the power supply. A 0.1uF capacitor is placed at the power pin of each chip for decoupling. However, why do some boards have capacitors of 0.1uF or 0.01uF next to the power pins of the chip? Is there any significance?

To understand this, we need to understand the actual characteristics of capacitors. An ideal capacitor is just a charge storage device, denoted as C, but actual capacitors are not that simple. When analyzing power integrity, we often use the capacitor model shown in Figure 1.

In Figure 1, ESR is the equivalent series resistance of the capacitor, and ESL is the equivalent series inductance of the capacitor. C is the true ideal capacitance. ESR and ESL are determined by the manufacturing process and materials of the capacitor and cannot be eliminated. So, what impact do these two factors have on the circuit? ESR affects the ripple of the power supply, while ESL affects the frequency response characteristics of the capacitor.

We know that: Capacitive reactance:

Zc=1/ωC

Inductive reactance:

Zl=ωL，ω=2πf

The complex impedance of a practical capacitor is:

Z=ESR+jωL-1/jωC
=ESR+j2πf L-1/j2πf C

It can be seen that when the frequency is very low, the capacitor is effective, but as the frequency increases, the role of the inductor cannot be ignored. At higher frequencies, the inductor dominates, and the capacitor loses its filtering effect. Therefore, remember that at high frequencies, a capacitor is not simply a capacitor. The frequency response curve of a practical capacitor is shown in Figure 2.

As mentioned earlier, the equivalent series inductance (ESL) of a capacitor is determined by the manufacturing process and materials. For practical surface-mount ceramic capacitors, ESL can vary from a few tenths of Nh to several Nh, with smaller packages having smaller ESL.

From Figure 2, it can be seen that the filtering curve of a capacitor is not flat; it resembles a 'V', indicating frequency-selective characteristics. Sometimes we want it to be as flat as possible (for pre-stage board-level filtering), while other times we prefer it to be sharper (for filtering or notch filtering).

The quality factor Q affects this characteristic: Q=1/ωCESR

The larger the ESR, the smaller the Q, and the flatter the curve; conversely, the smaller the ESR, the larger the Q, and the sharper the curve.
 

Typically, tantalum and aluminum electrolytic capacitors have relatively small ESL but large ESR, giving them a wide effective frequency range, making them suitable for pre-stage board-level filtering. That is to say, in the input stage of DC/DC or LDO, relatively large-capacity tantalum capacitors are often used for filtering. Meanwhile, near the chip, some 10uF and 0.1uF capacitors are placed for decoupling, as ceramic capacitors have very low ESR.

After discussing this, the question arises: should we place 0.1uF or 0.01uF capacitors near the chip's pins? Below is a list for reference.

So, in the future, don't just automatically assume that you should always use 0.1uF capacitors everywhere. In some high-speed systems, these 0.1uF capacitors might not be effective at all. Please try other value capacitors.

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