Decoupling of STM32F072CBT6

Question

just a short question while reading the Datasheet. What is the meaning of the Capacitors in the second line (marked red).

Do I need both (100nF AND 4.7uF) connected each of the Supply?

Rohat Kılıç · Answer 1 · 2020-09-21T08:20:42.767

1

For three VDD (VDDIO1) pins: A 100n ceramic capacitor to each, plus a 4u7 ceramic to only one of them is sufficient.

For VDDIO2 pin: a 100n plus a 4u7 ceramic is sufficient.

For VDDA pin: a 10n plus a 1u ceramic is sufficient.

Do I need both (100nF AND 4.7uF) connected each of the Supply?

So yes, each supply rail requires its own 1u/4u7 ceramic capacitor.

edited Sep 21 '20 at 08:20

answered Sep 21 '20 at 08:13

Rohat Kılıç

Yes for the first marked part. But for the others? – vt1111 Sep 21 '20 at 08:15
We use this MCU with only 100nF. We had no problem – Bunyamin Sep 21 '20 at 08:20
@vt1111 see the edit. – Rohat Kılıç Sep 21 '20 at 08:20
@BunyaminTAMAR The specification is made for the MCU to work at maximum clock speed, with all possible peripherals activated and across the entire temperature range. For less demanding configurations you can usually get away with less capacitors. But I wouldn't do it unless you build such a huge quantity that saving a few cents per device outweighs the cost of the additional tests needed to ensure it works reliably. – Codo Sep 21 '20 at 11:01

Nima · Answer 2 · 2020-09-21T10:52:06.060

1

100nf is used for filtering high frequency noises, and higher value capacitors (10uf-47uf) is mostly used for low frequency noises.

If you don't know the range of your noise frequency, it's recommended to use both of them for decupling your IC's power pins.

edited Sep 21 '20 at 10:52

answered Sep 21 '20 at 08:19

Nima

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`100nf is used for filtering low frequency noises, and higher value capacitors (10uf-47uf) is mostly used for high frequency noises.` exactly the opposite. Remember \$Z = 1/jwC\$. – Rohat Kılıç Sep 21 '20 at 08:24
You are right, I figured out my mistake before your comment but I forgot to change both of them :) – Nima Sep 21 '20 at 08:29
It does filter noise, but the main function is to have a local energy storage to keep MCU voltage stable when it needs a pulse of current. If there would be no capacitors, the MCU would be the source of the noise, as it needs a pulse of current at each clock cycle. – Justme Sep 21 '20 at 09:11
@RohatKılıç: Z=1/jwC is only a part of the puzzle, and in fact this single formula gives opposite result: at any frequency a larger cap would give lower impedance and be better for filtering purposes. However, this is not entirely true. Note that the smallest value capacitor should always be placed closest to IC body, as is has lower time constant and actually lower impedance at some higher frequencies, giving overall better circuit performance at high frequencies.(https://electronics.stackexchange.com/questions/116927/decoupling-with-multiple-caps-per-pin-which-should-be-closest). – NoobPointerException Sep 21 '20 at 10:03

2 Answers2