“We can see a variety of capacitors in the power filter circuit, such as 100uF, 10uF, 100nF, 10nF different capacitance values. So, how are these parameters determined?

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We can see a variety of capacitors in the power filter circuit, such as 100uF, 10uF, 100nF, 10nF different capacitance values. So, how are these parameters determined?

The digital circuit must run stably and reliably, the power supply must be “clean”, and the energy supply must be timely, that is, the filtering and decoupling must be good. What is filter decoupling? Simply put, it stores energy when the chip does not need current, and can replenish energy in time when current is needed.

Some readers will say that this responsibility is not DC/DC, LDO? Yes, they can handle it at low frequencies, but high-speed digital systems are different.

Next, let’s first look at capacitors.

The function of a capacitor is simply to store charge. We all know that capacitor filtering should be added to the power supply, and a 0.1uF capacitor should be placed on the power supply pin of each chip for decoupling. However, why are the capacitors next to the power pins of some board chips 0.1uF or 0.01uF? Is there any particularity here?

To understand this problem, we must first understand the actual characteristics of capacitors. An ideal capacitor is just a storage of charge, C. The actual manufactured capacitors are not so simple. When analyzing power integrity, our commonly used capacitance model is shown in Figure 1:

figure 1

In Figure 1, ESR is the series equivalent resistance of the capacitor, ESL is the series equivalent inductance of the capacitor, and C is the real ideal capacitor. ESR and ESL are determined by the manufacturing process and material of the capacitor, and there is no way to eliminate it. So, what effect do these two things have on the circuit? ESR affects the ripple of the power supply, and ESL affects the filter frequency characteristics of the capacitor.

**we know:**

Capacitive Reactance of Capacitor:

Zc=1/ωC

Inductive reactance of Inductor:

Zl=ωL, ω=2πf

The complex impedance of a real 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 at work; when the frequency is high to a certain extent, the role of the inductance cannot be ignored; when the frequency is high, the inductance plays a leading role, and the capacitor loses its filtering effect. . So remember, at high frequencies, the capacitor is not a simple capacitor. The filter curve of the actual capacitor is shown in Figure 2:

figure 2

As mentioned above, the equivalent series inductance of a capacitor is determined by the manufacturing process and material of the capacitor. For actual chip ceramic capacitors, the ESL ranges from a few tenths of nH to several nH. The smaller the package, the smaller the ESL.

As can be seen from Figure 2, the filter curve of the capacitor is not flat, it is like a V. That is to say, there is a frequency selection feature. Sometimes we want it to be as flat as possible (pre-board filtering); other times, we want it to be as sharp as possible (filtering or notch).

What affects this characteristic is the quality factor Q of the capacitor:

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.

Usually, tantalum capacitors and aluminum electrolytics have relatively small ESL and large ESR, so tantalum capacitors and aluminum electrolytics have a wide effective frequency range and are very suitable for pre-stage board-level filtering. That is to say, in the input stage of DC/DC or LDO, large-capacity tantalum capacitors are often used for filtering. And in the place close to the chip, put some 10uF and 0.1uF capacitors for decoupling, ceramic capacitors have very low ESR.

Having said so much, in the end place 0.1uF or 0.01uF near the pins of the chip? Listed below for your reference:

Therefore, don’t put 0.1uF capacitors on everything in the future. In some high-speed systems, these 0.1uF capacitors do not work at all.

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