Placing of capacitor close to IC power pin - Logic

[FreshmanNewbie]

I have 2 capacitors.

The first one is 100nF and the other one is 1uF.

While doing the layout, which capacitor should be placed close to the IC power pin and Why?

I have seen in many places that the smaller capacitor (100nF) is placed close to the IC pin.

Can someone tell me why its a good practice and what is the reason behind it?

 

[KlausST]

Hi,

The 100nF one is considered to be better for higher frequencies. Less HF impedance.
Thus it makes sense to place it closer to the IC pin. Else you kill this benefit by adding increased trace impedance.

But the shortest placement of capacitor to the pin is useless if not the return path: capacitor_GND via GND_plane to the IC_GND is equally low impedance. A GND_trace instead of a GND_plane will not work.

Klaus

 

[danadakk]

Capacitor technology, polymer tants best for bulk caps :

The OS-CON's are polymer in this graph.


Regards, Dana.

 

[andre_luis]

Whenever possible I put low impedance (ceramic) capacitors not only close to the integrated circuits, modules, relays, connectors, etc, but also at the regulator input and output stages.

 

[danadakk]

https://www.eeweb.com/properties-and-application-of-bypass-capacitors/

https://www.infineon.com/dgdl/Infineon-AN1032_Using_Decoupling_Capacitors-ApplicationNotes-v05_00-EN.pdf?fileId=8ac78c8c7cdc391c017d073d44f06172

https://www.ti.com/lit/an/scba007a/scba007a.pdf

Above might help.


Regards, Dana.

 

[dick_freebird]

The area of the supply-transient current loop is what you are trying to minimize so as to not add more ESL to the effective C. Trace width / plane goes to ESL and ESR.

 

[stenzer]

Hi,

have a look at the K-SIM capacitor simulator by KEMET [1]. There you can have a look on the different characteristics e.g. impedance & ESR of a single capacitor as well as a combination of several capacitors. There you can spot the frequency where a single capacitor has its lowest impdeance, and where the combination of several capacitors provide low impedance.

[1] https://ksim3.kemet.com/capacitor-simulation

BR

 

[Taihung]

Hi
It is called decoupling capacitor.
As you know, all component on PCB connect to each other by net/trace. They have their length and impedance.
Example 1, when output of opam change, it need more or reduce current. If has no near storage power like decouling cap, the current (trasition current) need to provide from far point on PCB. Make output signal is not like as expected.
Example 2, MCU often run @ Mhz, upto 200Mhz also. As car toy, that only need 27Mhz to remote upto 20m. When clock and peripheral run, they on and off upto 200Mhz, the current through trace also up and down 200Mhz. If decoupling too far, trace too long, or return current impedance too high -> voltage ripple 200Mhz become higher. This will radiate to air like antenna. -> Faile EMC or make noise to other component. Or too bad case is lost logic level.
Example 3, Some net/trace has high impedance (often input pin/net), this is easily effected by noise. So add decoupling cap to absorb noise. Make circuit working properly.
Belong to frequency, decoupling is bulk of value to cover wide range. Higher frequency need smaller capacitance (apply for ceramic cap). 1uf, 100nf, 1nf, 10nf, 33pf, 10pf.