What factors are related to parasitic capacitance?

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What factors are related to parasitic capacitance? [Copy link]

When studying the oscillation circuit composed of TLC555, it was found that the parasitic capacitance of the chip cannot be ignored when the frequency is high, so the following experiment was conducted.

The shaded part is the oscillation capacitor outside the chip, which is now removed. CP1, CP2, and CP3 are the chip pin capacitors, and their values given in the manual are approximately 2.1pF.

By changing different RB and testing the output frequency, we can calculate the value of parasitic capacitance by the formula and find that the value of parasitic capacitance is not fixed. The test data is as follows

As RB increases, the equivalent capacitance (parasitic capacitance) will also increase. How to explain this phenomenon?

led2015

The higher the operating frequency of the circuit, the greater the impact of parasitic capacitance on circuit performance

maychang

For bipolar integrated circuits, the parasitic capacitance at the input includes the PN junction capacitance, and the PN junction capacitance is obviously related to the voltage on the PN junction. However, the voltage change on the PN junction can only explain part of the parasitic capacitance change, and there are other factors that also affect the parasitic capacitance of the input pin.

maychang

The capacitance at the chip input also includes the Miller capacitance of the input tube, which is closely related to the working state of the tube and is difficult to calculate.

maychang

[Test the output frequency and calculate the value of parasitic capacitance through the formula]

Your calculation does not take into account the delay of each level (except the input level) behind the integrated circuit. In fact, the delay of each level will make the flip less ideal, the rising and falling edges less steep, and the oscillation period longer, that is, the oscillation frequency is reduced. As a result, the parasitic capacitance calculated based on the oscillation frequency becomes larger.

maychang

For this type of input capacitor, the manual will give the test conditions. Only under the test conditions given in the manual will the input capacitance be the value given in the manual, otherwise it may be very different.

吾妻思萌

Some parasitic capacitance is caused by human's current materials, processes and structures. Just like objects with mass have gravity (I didn't remember the formula at all). It doesn't show up in life because it is so small that it is ignored. The parasitic capacitance in the tube or circuit is divided by the operating frequency, the impact and the τ of charging and discharging. Sometimes the parasitic inductance in the line will oscillate.

吾妻思萌

For this type of input capacitor, the manual will give the test conditions. Only under the test conditions given in the manual, the input capacitance will be the value given in the manual, otherwise it may be very different.

Have you ever made a datasheet? I feel that the quality of datasheets made by different manufacturers varies a lot.

maychang

Azuma Simeng posted on 2023-3-12 13:54 Have you ever made a datasheet? I feel that the quality of datasheets made by different manufacturers is not the same.

I don't produce components, and I haven't made datasheets. It's normal for different manufacturers to have different parameters in their datasheets. Even if it's the same manufacturer and the same model, the parameters will change after improving the process or materials.

吾妻思萌

maychang posted on 2023-3-12 14:39 I don't produce components, nor do I make datasheets. It's normal for different manufacturers to have different parameters in datasheets. Even if it's the same manufacturer and the same model...

Generally speaking, the datasheet will not change much if the material number remains unchanged, but the material number of the same model should change.