VCAP in hardware design

[DElec.1]

I have got a custom PCB using STM32F4. I don't have any capacitor connected to the VCAP Pin and I wonder if this is necessary or not. Which is the use of VCAP and do I need to add it to my design? What can I do to solve my problem of no having one if necessary (I supose it is not a good practice to place one in a bad way due to EMIs and bad behaviour of the capacitor in general)?

Thank you.

[AScha.3 · Accepted solution]

VCAP is decoupling for core VCC.

need SMD cer.caps with short connection to GND.

[DElec.1]

So is it mandatory? I have decoupling capacitors between VDD and VSS but I don't have anything connected to VCAP pin (by the way, in my chip there is only one).

[Peter BENSCH]

Yes, it is mandatory.

The respective reference manuals state:

An embedded linear voltage regulator supplies all the digital circuitries except for the backup domain and the Standby circuitry. The regulator output voltage is around 1.2 V. This voltage regulator requires one or two external capacitors to be connected to one or two dedicated pins, VCAP_1 and for some packages VCAP_2.

...and the data sheets:

Stabilization for the main regulator is achieved by connecting external capacitor CEXT to the VCAP1 and VCAP2 pin. For packages supporting only 1 VCAP pin, the 2 CEXT capacitors are replaced by a single capacitor.

Please note here that this is an additional capacitor(s) that is specifically required for the linear regulator. The other buffer capacitors on VDD/VSS have nothing to do with this, as they cushion the current peaks caused by digital switching operations in the STM32.

Regards

/Peter

[DElec.1]

Thanks! In the case of just one VCAP pin, the capacitor's value would be 2,2+2,2 = 4,4 uF? or just 2,2 uF?

[Peter BENSCH]

The value is given in the data sheet of your device, the exact type of which you have not mentioned so far. However, this is usually a 4.7µF capacitor with an ESR <1 ohm, which you can connect to VCAP_1 in this way.

Regards

/Peter

[Peter BENSCH]

If the problem is solved, please mark this thread as answered by selecting Select as best, as also explained here. This will help other users find that answer faster.

Regards

/Peter