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What happens if the FX3 chip is powered up with the VBUS pin floating?
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What happens if the FX3 chip is powered up with the VBUS pin floating?

Alessandro posted on 08 Mar 2012 10:05 AM PST
Member
3 Forum Posts

Hello,

I am developing a board with the FX3, powered only with 1.2V and 2.5V, respectively for the core supply and the VIOs supplies.

Since a 5V power supply is not available on my board, I would like to use the USB 5V supply coming from the connector to power the VBUS pin. The other power pins of the CYUSB3014, such as VDD and VIOs, are instead fed by other power supplies, independent from the USB 5V.

This means that when no USB cable is connected, the VBUS pin would be left floating while the other power pins of the chip would be powered with their respective supplies.

Is this condition acceptable without damaging the chip?

Is this condition acceptable also for the VBATT pin (e.g. connecting it to 5V supplied by USB cable)?

 

Kind Regards,

Alessandro

 

Re: What happens if the FX3 chip is powered up with the VBUS pin floating?

sodafarl posted on 08 Mar 2012 03:16 PM PST
Top Contributor
128 Forum Posts

Hi,

I have developed working hardware for the FX3 where the FX3’s VBUS pin is powered only from the USB cable while the rest of the electronics is self powered and it does mean if the USB cable is removed or the host computer powered down then VBUS is floating while the rest of the FX3 is powered. I haven’t seen a problem with this and while the FX3’s data sheet should be clearer  I think this is the way it is meant to work, so that with no VBUS the USB transceivers are disconnected from the bus.  Make sure you put some ESD or over voltage protection on the VBUS signal. I have left the VBATT pin on my hardware unconnected but look at this link to see if it helps.

Sodafarl

http://www.cypress.com/?app=forum&id=167&rID=55972

Re: What happens if the FX3 chip is powered up with the VBUS pin floating?

Alessandro posted on 09 Mar 2012 01:32 AM PST
Member
3 Forum Posts

Hello Sodafarl,

Thank you for your replay. That was what I was searching for. It reassures me a lot!

Do you think that the over voltage protection is necessary even if the board will be always connected to PCs only?

 

Kind Regards,

Alessandro

 

Re: What happens if the FX3 chip is powered up with the VBUS pin floating?

sodafarl posted on 09 Mar 2012 01:44 AM PST
Top Contributor
128 Forum Posts

Hi Alessandro,

I think you will need ESD protection on the VBUS line. I have used  Wurth part 82401646 see this link below for more details.

Sodafarl

 

http://www.we-online.de/web/en/index.php/show/media/07_passive_bauteile_-_standard/toolbox/product_training/Product_Training_USB3.pdf

Re: What happens if the FX3 chip is powered up with the VBUS pin floating?

Alessandro posted on 09 Mar 2012 05:03 AM PST
Member
3 Forum Posts

Thanks Sodafarl. I appreciate your help.

 

Regards,

Alessandro

 



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