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According AN4661

In the STM32F7x2Rx devices, only the VCAP1 pin is available and must be connected to 4.7 µF low ESR between 0.1Ω and 0.2Ω ceramic capacitor.

So how do I go about finding a suitable capacitor?

After looking through many datasheets of various brands of capacitors, some list an "ESR(Ohm) vs frequency(Hz)"-diagram which sounds like that's what is needed. Most graphs looks similar to this: Murata ESR example

Now the ESR value displayed in those diagrams varies wildly over the frequency, which raises the question what frequency do we need to check? I'm guessing the frequency of the HSE clock (8 mhz in my case), but I haven't found anything explicitly stating that.

There is also the Murata LLR-series which is listed as E01 100mΩ (which seems suspiciously consistent), but it's only available in in 1uF.

Can someone point me in the right direction in how to pick a matching capacitor in 0603 package - not sure if that's at all feasible (maybe the above criteria can only be matched in 0402 or 0805)?

Additional info:

  • Using HSE with a crystal of 8Mhz
  • Using PLL
  • Processor running at 168Mhz
  • USB running at 48Mhz

Drawing in the 0.1-0.2R range (red bar) in the graph for the GRM188R61C475ME11, then as I understand it, the green line marks the valid frequencies, which are from ~0.004Mhz to 0.008Mhz and from ~600Mhz to 1600Mhz. enter image description here

At the

  • cpu-clock of 168Mhz the graph shows ~0.05Ohm
  • usb-clock of 42Mhz the graph shows ~0.03Ohm
  • HSE frequency of 8Mhz the graph shows ~0.002Ohm

Neither of the three frequencies is within the demanded range of 0.1Ω and 0.2Ω... So either I'm misinterpreting something or all capacitors from various brands and series I looked at are unsuitable.

Unless the datasheet means that anything < 0.1-0.2R is valid, but in that case for mcu's with both vcap1 and vcap2 it says

When the voltage regulator is enabled, VCAP1 and VCAP2 pins must be connected to 2*2.2 μF low ESR < 2 Ω ceramic capacitor.

So why specifically mention less-than for this case, but mention a very specific range for the only-vcap1 case?

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Summary:

  • The VCAP powers the Core. Hence, the core frequency matters other than system frequency

  • Connecting two 2.2uF capacitors in parallel also helps in reducing ESR because the ESR will be half of the individual parts.

  • A win win situation. you can either mount one 4.7 uF or two 2.2 uF sharing same copper pads.

  • 4.7uF capacitors options are mentioned below


If I have to stick to MLCC only (I also prefer it always over tantalum), here are a few parts which I find best due to

  1. The products are in production
  2. The ESR is well with in 100 m Ohms
  3. Package is 0603 and
  4. Several voltage options are available 10V, 16V, 25V
  5. I can also mount capacitors in parallel which also effectively reduces the ESR

The main frequency range to beseen will be defined by your application. Are you using PLL? Depending on peripheral and core frequency, you can have a look at the graph. I would say, it is fine.

enter image description here

The parts for comparison are of 4.7uF, 0603 package. Choose the capacitors which is suitable based on price and availability. Also have a look at temperature coefficient to see teh acceptable change in the capacitance over temperature range.

Neither of the three frequencies is within the demanded range of 0.1Ω and 0.2Ω... So either I'm misinterpreting something or all capacitors from various brands and series I looked at are unsuitable.

For now, with the lower ESR values, the ceramic capacitors do a great job of having good response and least heat losses due to ripples. But the stability may be a topic (I am not expert in this). ESRs lower than 0.1 Ohms would be okay or may not be. You can write to ST once. Most of the regulators nowadays will be stable even with least ESR capacitors. Only ST should tell.

By the way, the ESR is nearly 0.1 Ohms for the core clock frequency. The VCAP1 is for the core voltage regulator. I would have gone ahead with this.

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  • \$\begingroup\$ I think my confusion comes from not being sure how to interpret the graph. I added more info in the original question, given the range of 0.1-0.2R it seems this is only valid for frequencies of ~0.004Mhz to 0.008Mhz and from ~600Mhz to 1600Mhz. The mcu-clock is 168Mhz, USB on 42Mhz and HSE on 8Mhz... neither of these are within the 'valid' range... but other capacitors seem to be similar (i.e. way lower R then the mcu seems to require). \$\endgroup\$ – kalmiya Aug 20 at 16:53
  • \$\begingroup\$ Anything lower than 100 mohm is not a negative thing. \$\endgroup\$ – Umar Aug 20 at 19:39
  • \$\begingroup\$ Thanks for your advice, really appreciate it - I'll be ordering several of the parts you suggest and just try it. Also tried your suggestion of contacting st via their website and their reply was basically "go away, we only help big companies" (in a more polite way), I have to say that surprised me, didn't think I'd get a reply at all. \$\endgroup\$ – kalmiya Aug 21 at 23:00
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    \$\begingroup\$ Ask in ST community forum. I will also try from my side. \$\endgroup\$ – Umar Aug 22 at 5:51
  • \$\begingroup\$ Was already posted, but no replies so far: community.st.com/s/question/0D50X0000BDmzN2/… \$\endgroup\$ – kalmiya Aug 22 at 12:10
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You can get Tant. to meet the same C=4.7uF and ESR =100 mOhm in a 1206 case while 805 case is twice that ESR. Price increases dramatically near and below 1us = T = ESR*C in any electrolytic while T<10us is low ESR and T>100us is General Purpose.

https://www.digikey.ca/products/en/capacitors/tantalum-polymer-capacitors/70/page/2?k=4.7uf%20tant&ColumnSort=724

... except ceramic may not be so high density but lower ESR*C=Tau , so harder to find large C’s but good in parallel for extending the frequency range>300 kHz.

Normally an expert can model the equivalent RLC values to see if any offending anti resonance with many.

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