STM32F446 Vcap value

[mluessi]

Posted on April 09, 2016 at 22:47

I'm designing a product that uses the STM32F446. The reference manual states that Vcap is increased to 4.7uF (from 2.2uF for the F405) on page 15 and later on page 72 (power supply scheme) it lists 2 x 2.2uF caps on the Vcap pins.

My current design uses a 4.7uF, 6.3V ceramic capacitor in a 0402 package. At 1.2V, the capacitance is approximately 3.5uF. Is this sufficient?

I a previous product with a STM32F405, I had uses a 2.2uF, 4V ceramic capacitor in a 0402 package, which has closer to 1uF capacitance at 1.2V. I never had a problem with this design. I could change the capacitor to 10uF, 10V, 0603, which would have approximately 7uF at 1.2V, but due to space constraints I would like to avoid doing so and I'm not sure if using a higher capacitance than recommended could cause problems.

Any ideas?

[John F.]

Posted on April 11, 2016 at 09:18

The STM32F446xC/E DocID027107 Rev 5 Data Sheet tells you to use two capacitors - one on VCAP_1 and one on VCAP_2.

''Two external ceramic capacitors should be connected on VCAP_1 and VCAP_2 pin.''

These should be ceramic (X7R for example) 2.2uF. The 4.7uf value is used where only one VCAP pin is available (on smaller IC package).

''6.3.2 VCAP_1/VCAP_2 external capacitor

Stabilization for the main regulator is achieved by connecting external capacitor CEXT to the VCAP_1 and VCAP_2 pin.

For packages supporting only 1 VCAP pin, the 2 CEXT capacitors are replaced by a single capacitor.

''

CEXT is specified in Table 18. (p78).

The Data Sheet specifies the low ESR required. (Equivalent Series Resistance). The small package capacitor you've chosen (0402) could give you problems. Have a look at -

https://www.maximintegrated.com/en/app-notes/index.mvp/id/5527

[mluessi]

Posted on April 11, 2016 at 16:01

Thanks. I should have been more clear, the package I'm using (QFP64) only has one Vcap pin and I'm using a 4.7uF cap. The

http://www.murata.com/en-us/search/site/searchresult?intcid5=com_xxx_xxx_cmn_hd_xxx&mss_query=LQW2BAN47NJ00L

loses about 20% of its capacitance at 1.2V, so it will effectively be a 3.7uF cap. The question is if this is sufficient or if I should use a 10uF 0603 cap that will be about 7uF at 1.2V.

The 4.7uF cap seems to work fine in prototypes but I want to make sure there are no surprises later on. At the same time, I'm not sure if using 7uF instead of 4.7uF could cause problems.

[John F.]

Posted on April 11, 2016 at 17:08

OK. I don't think there's enough information in the Data Sheet or Reference Manual to work this out. I would use a physically larger capacitor to reduce the variation with voltage (because I wonder if I'd ever get an authoritative answer about suitability of different values).

I'll also offer what's usually Clive's advice - contact your FAE and get technical support from ST. Please post back here if you

do

get a definitive answer.

[re.wolff9]

Posted on April 12, 2016 at 08:20

What do you want? The datasheet specifies 4.7uF, you mount a rediculously small (physycally) capacitor that can be shown to be less than the recommended value.. Will it work? Yes. Will it always work? Nobody can tell you. If you want to go that way, you'll have to do your own qualification. Get a bunch of  boards, and test if they work under all circumstances. Min VCC, max VCC min temp , max temp etc. 

The datasheet recommends using an actual value. It is likely that this is a minimum value. On the other hand, you could argue that if you put a 4.7uF capacitor there, you can say you've followed the datasheet recommendation. The fact that the capacitor loses some of its capacitance is ignored by 99% of the users of that datasheet. If the loss in capacitance was a real factor wouldn't there be a footnote in the datasheet that says: (*) Note the minimum capacitance on VCAP is 4.7uF. As many dielectricums have a drop in capacitance when provided with a DC offset, a 4.7uF capacitor will often not suffice. 

I don't understand why replacing that 4.7uF capacitor with a larger one suddenly requires you to put in a larger value? Why can't you buy a 0603 4.7uF capacitor?

[John F.]

Posted on April 12, 2016 at 08:51

The nominal capacitance required is 4.7uF when VCAP1 and VCAP2 are combined onto one pin. It's likely that the ESR is more critical for stability than the capacitance. I would use an X7R dielectric in a reasonable case size (0603 or 0805) and working voltage (10V) to feel comfortable that the requirements were probably met. There are linear regulators that will operate with ceramic output capacitors - stability is the issue. For more information read,

http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snva167&fileType=pdf&keyMatch=AN-1482&tisearch=Search-EN-Everything

Ask the FAE or use a larger case size (likely to have more constant characterisitcs compared to very small components) and a reasonable dielectric like X7R (best generally available offering required capacitance).