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Fresuelectronics.com

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2 contributions to Fresuelectronics.com
Decoupling capacitores Values
I have read in many CMOS IC datasheets that the typical values for decoupling capacitors are 100 nF, but on the other hand I have read an article by Eric Bogatin stating that this is a false myth and that this has been going on since the 80's when CMOS IC voltages were much lower. Actually he refers to calculate the decoupling capacitors with respect to the CMOS frequency and their rise times. Could someone clarify me which is the correct way of sizing the decoupling capacitors and the technical reasoning why? I would like to clear this doubt and when I search the internet I see disparity of opinions. thanks. Translated with DeepL.com (free version)
0 likes • Sep '24
Hi Dario, I was watching again the PDN videos and its part related to decoupling capacitor network and I will put here what I have understood to see if it is correct and if not please correct me (in advance I say that I will put certain assumptions in concept although I understand that in general the whole process would be more complicated and would have to look datasheets ... etc.). Steps: 1) Find the operating frequency of the circuit (Assume an ESP32 microcontroller is used = 240MHz operating frequency). Assuming that all the ICs on the PCB is the one with the highest frequency. 2) Look for a capacitor with the lowest possible ESL whose cutoff frequency (Fc=1/(2*PI*SQRT(L+C))) is able to cover the operating frequency (240Mhz) and I have also read that it is recommended to cover up to the seventh harmonic which implies that Fc must be greater than or equal to 7*240MHz = 1680 MHz. So I select a 1pF capacitor and assume ESL (inductance matching) = 15nH. Applying formula of Fc= 1.3 GHz. I give it for good because I cover more than the operating frequency and I approach the 7th harmonic. 3) I calculate the Ztarget =Vdrop(Drop voltage of the most restrictive IC of my PCB)/Imax (of my PCB which will be divided between all my ICs). Typical value of Vdrop=0.5V and I assume an Imax=2.5A. Ztarget = 0.2 Ohm
0 likes • Sep '24
4A) Now I will calculate the number of capacitors that I need to put in parallel to meet the calculated Ztarget. Using the formula N=2*ESL/Ztarget*Tt, the strategy you indicate of putting capacitors of the same value in parallel and assuming that the ESL will be reduced by also being in parallel. Where Tt is the rise time that is calculated as 0.35/BW (BW in this case will be the 240MHz of ESP32) Tt would be approximately 1.5 nS and therefore the N would be approximately 103 capacitors and it doesn't fit me. I haven't understood something well or at some point I'm not doing it right but I don't know where. Could you finally tell me how it is and correct me? Thank you very much in advance
What would you like to learn more about?
Hi everybody! The field of PCB Design is vast, extremely vast! There are so many topics we could cover, but I would like to know more from you: What are the things you would like to learn more about? Write them in the comments section! P.S. Don't be shy about writing in the comments; let's make this a friendly community and a place we can all enjoy ;)
5 likes • Jan '24
I consider myself a beginner in the world of electronic design so, from my humble point of view, I would like to cover all the basics to design an IoT board in a professional way explaining well the basis on which it is based. I mean for example how to choose the power supply, how to define the design rules well, how to correctly define the stack up of the PCB (what types of Stack up configurations are there and which is used for what), what peculiarities should be taken into account if our PCB has an antenna, how to protect our PCB from ESD and how to properly select the protections, general guidelines to take into account the EMC in the design (When you have mixed digital and analog circuits) ... etc... Maybe it is very basic but I would be interested in learning the basics very well and grow as I evolve.
3 likes • Jan '24
@Dario Fresu It would be a good Idea but I think it would be better a project wich imply to select, configure and use little and comercial ARM M3 (STM32 For example). I think so because is really use in professional enviroments and I would like to learn how to integrate the microcontroller into the PCB and how to select the Cristal/Oscillator and how to route it as a proffesional from EMC point of view. As well it woud be wonferful to learn hot to choose the properly prog/debug connector...
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Ivan Mendez alonso
3
38points to level up
@ivan-mendez-alonso-3105
I'm an electronic engineer with experience in several fields like boxlift área.

Active 36d ago
Joined Sep 16, 2025
spain