STM32H725 Bulk capacitors values

[NirAlon]

Hello all,

Regarding the STM32H725 bulk capacitors:

Question 1:

Is the capacitors required values taken into consideration the capacitance degradation Vs DC bias?

For example:

The SMPS input required is 4.7uF (10uF recommended)

Looking at the STM32H735G-DK (Evaluation board) bill of materials, C(in) of SMPS is a 10uF/4V in a 0402 case.

Under 3.3V, this capacitance drops to ~3uF. to achieve 10uF under 3.3V there in no choice but to use 0603 case size and at least 22uF of capacitance:

In other words, the evaluation board input capacitance does not meet the specs.

Between 0402 and 0603 there is a great difference at tight board design - and this is only one example.

Question 2:

Looking at AN5419 - to achieve best performance for the SMPS input, it is recommended to use 10 μF + 4.7 μF capacitors.

What is the benefit of using two different but so close value capacitors? why don't just use 2x10uF?

Maybe the 4.7uF supposed to be at smaller case capacitor with lower ESL than the 10uF capacitor?

I would appreciate any clarification to the above questions

Thanks

Nir 

[LCE]

my 3 cents:

1) OMG, that's bad design...

a) but it seems that these X7R / X5R from Murata are extra crappy - or was that drop < 50% cap typical for these ceramics?

b) in general, choosing caps with a voltage rating of 4 V for 3.3 V - especially at an SMPS input: bad idea

2) more caps -> lower ESR / ESL, and for sure I would rather use 2x 10u

[NirAlon]

Hi,

Thanks for the reply

There is not much capacitance you can squeeze from a 0402 case, maybe ~3.8uF @ 3.3V at best...

For example: p/n GRM155C80J106ME18 10uF/6.3V

And to think that when you buy an evaluation board, you get an electrical design that should represent the best performance of the component, or at least the optimal performance...
It's a board I wouldn't dare to test for EMI.

Anyway, how should I refer to the capacitance required on the specification sheet?

Should I aim for the required capacitance at the operating DC voltage as mentioned?

Thanks

Nir

[LCE · Accepted solution]

> Should I aim for the required capacitance at the operating DC voltage as mentioned?

That's what I would do, and in case of doubt - if the board space is available - place an extra cap.

[NirAlon]

OK, thanks again

Nir