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I am confused about 2 different articles about decoupling capacitors and usb filtering.

First one: https://www.autodesk.com/products/eagle/blog/what-are-decoupling-capacitors/

this says i should use 2 different decoupling capacitors for each ic. One for high frequency(0.1 uf) and one for low frequency(10uF) noise. This makes great sense.

The second one is about usb voltage filtering. http://andybrown.me.uk/2015/07/24/usb-filtering/

This article says "It’s a little known fact that the USB 2.0 standard mandates a maximum of 10µF in parallel with VUSB to limit the inrush current"

So the problem is, i have 3 ics. with each has 10 uF in parallel i get way more impedance between 5V and gnd since they're all parallel between 5v-gnd.

Is using only 1 piece of 10uF at the end of filter instead of placing 3 pieces 10uF for each ic enough?

Edit: One ic is pca9555 nxp.com/docs/en/data-sheet/PCA9555.pdf? (could not seea cap value on this) other is mcp2221A ww1.microchip.com/downloads/en/DeviceDoc/20005565B.pdf

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    \$\begingroup\$ What ICs would those three ICs be? Usually 10uF capacitor per chip is not used. \$\endgroup\$ – Justme Oct 9 at 6:26
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Can you reference(or link) the component datasheets to see what the manufacturer recommends for decoupling capacitors? It is likely that the components dont really need their own 10uF cap. Often times just a 0.1uF or a 0.1uf + 1-4.7uF will be recommended.

I would expect that an individual 0.1uF plus a shared 4.7 or 10uF would work just fine.

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  • \$\begingroup\$ Thank you for answer one is pca9555 nxp.com/docs/en/data-sheet/PCA9555.pdf? (could not seea cap value on this) other is mcp2221A ww1.microchip.com/downloads/en/DeviceDoc/20005565B.pdf \$\endgroup\$ – Doğuhan PALA Oct 9 at 6:35
  • \$\begingroup\$ Ok I see. Yeah I think a 0.1uF as close as possible to the Vdd and Gnd pins for each device and a single 10uF as close to the devices as possible (ideally your components are close together to allow the 10uF to be relatively close to all of the components power inputs). Unless your components are spread out by several inches with very small traces to their power pins then you should be just fine. \$\endgroup\$ – Ben Watson Oct 9 at 6:56
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Do read my answer in Output capacitors placement in PCB

Is using only 1 piece of 10uF at the end of filter instead of placing 3 pieces 10uF for each ic enough?

Yes, 1 capacitor of 10uF will serve all other IC's.
I wonder if you really need 10uF and think you could use 2.2uF or 4.7uF instead. But that depends on cable length and whether the input current of those IC's is high frequent etc.

EDIT based on new comments
You need this 10uF capacitor to filter fluctuations of the voltage of the USB port as well as act as local buffer for the electronics 'behind' it. When there is a sudden increase in demanded current, this current will be initially drawn from this buffer (because the (USB and other) wiring has inductance which makes it too slow to respond to this sudden increase).
The buffer capacitor will make sure the voltage doesn't drop too much.

The microprocessor and the PCA9555 itself hardly draw current.
The total current sourced by all I/Os of the PCA9555 must be limited to 160 mA.
So, if you decide to turn on all ports at once drawing in total 160mA, you may need 4.7uF to prevent the 5V rails drops too much.
But I expect the capacitor value can be reduced.

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  • \$\begingroup\$ thank you so much! The cap value(10uF) on autodesk got me confused. I will use 1 piece 10uF on output of filter circuit. \$\endgroup\$ – Doğuhan PALA Oct 9 at 6:38
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    \$\begingroup\$ @DoğuhanPALA, follow Huisman's (upvoted) recommendation to use 4.7 uF instead of 10 uF. Don't forget that your 10 uF has a tolerance that takes it over the USB maximum. \$\endgroup\$ – TonyM Oct 9 at 7:28
  • \$\begingroup\$ Oh, ok i will do that. Thank you so much for your help! \$\endgroup\$ – Doğuhan PALA Oct 9 at 7:49
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These two questions are nearly unrelated to each other. In normal situation the internal ICs usually are powered with 3.3V, so typically a 5V-to-3.3V regulator is required. And therefore the bypass caps are de-coupled from the 5-V input.

Now, each IC needs de-coupling caps, and their values are usually specified in respective datasheets, with all this spectrum-spreading set of values, 100pF + 0.1uf + whatever. This is a separate issue for IC operating stability and suppression of internally-produced consumption spikes of current.

The USB "stability", however, requires that every USB device upstream-facing port (UFP) to have no more than 10 uF. More specifically, it is not the 10uF limit, but rather a limit for total charge (50 u-Coulombs) that flow into the device's port at plug-in event. It should be less than 50 uC integrated over 100 ms. It is required to avoid voltage "droops" and "drops" when the devices are hot-plugged into USB host, for not to interrupt functionality of already connected and working devices. Therefore a USB-compliant device (or UFP of a hub) MUST NOT have more than 10uF (5V x 10uF) at the input DIRECTLY. (actually, this cap should be no less than 1.0uF for different reasons).

So there is a design dilemma if the set of ICs inside the USB device needs more than 10 uF each, which is however a rare case. This problem must be solved by careful engineering of VBUS power path input, which might include ramp limiting and time delays in consumption.

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