Vcap(vddcore) on pic32 device problem ???

[nick703]

hello friends i have a pic32 device which communicate to USB cdc protocol. but my problem is VCAP(CDDCORE) capacitor value . i have try 10uf/50v , 47uf/25v , 22uf/25v 0.1uf and 47uf/50v different type of capacitor on this pin to ground. but i cannot communicate my device. in some 47uf/50v capacitor works like charm but in datasheet say this pin value between -0.3v to 2.0v and when i measure this pin voltage i get 1.82 v . so every time when i get 1.82v then my communication failed and some capacitor 47uf/50v added i get 1.83v or 1.84v then communication completed. so which type of capacitor value i put on my device to work properly . please help me!!!:|

 

[bigdogguru]

What is the specific model number of your PIC32?

BigDog

 

[nick703]

hello bigdoggure my pic is PIC32MX575F256H

 

[nick703]

when i search another forum that all says capacitor value as well as it's ESR (Equivalent series resistance) value. then finally i read all pic32 datasheet an datasheet say's capacitor value which ESR is <1 ohms. ok

so my question is is that any way how to check capacitor ESR value ???

 

[Tahmid]

You can use an ESR meter to check capacitor ESR value.

http://en.wikipedia.org/wiki/ESR_meter

You can also do simple ESR measurements without a dedicated ESR meter. Take a look at the following:

http://geoffg.net/Measuring_ESR.html
**broken link removed**

Hope this helps.
Tahmid.

 

[FvM]

then finally i read all pic32 datasheet an datasheet say's capacitor value which ESR is <1 ohms.

Yes. This is one of the points you better read before starting a design.

To quote the full datasheet note:

It is important that the low-ESR capacitor is placed as close as possible to the VCAP/VCORE pin.

The list of capacitors in your original post suggests that they won't be placed close to the pin due to their large size. A 10 µF/6.3V ceramic capacitor is the first choice, coming in 0805 or 0603 package (even 0402 is already available these days).

Needless to say that a low inductance ground layout with short return path to VSS pins is essential as well. Also good VDD to VSS bypassing is generally required. I'm not sure what's the primary reason for USB communication failure. If you manage to get unstable Vcore regulator by bad bypassing, it may be USBPLL loss of lock.

In any case, you don't need an ESR meter when using a known low ESR capacitor.