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kris_
Newbie
Newbie
6,999 Views
Registered: ‎09-30-2008

LVCMOS25

Hi,

 

If I have a Virtex4 bank configured with Vccio = 2.5 v will I damage the device if driving it with 3.3 v without a series resistor? Or what will the effects be?

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4 Replies
iwalel
Xilinx Employee
Xilinx Employee
6,990 Views
Registered: ‎07-25-2008

Hi Kris,

 

Virtex-4 devices have a clamp diode to Vcco and GND on their inputs.

If the input voltage exceeds the Vcco voltage (as it happens in your case) you will switch on these diodes.

The maximum clamp diode current is around 10mA and 100mA in total for all IOs.

You will have to limit the current through the clamp diodes.

 

 Also have a look at answer record AR#10835.

 

 Hope that helps!

 

 

kris_
Newbie
Newbie
6,974 Views
Registered: ‎09-30-2008

Thanks for the answer.

 

So if connected to a  DS90LV04/A line driver which has a Ios = 9 mA it would probably work?  Sinking that much

current into Vcco is perhaps not very good for the SI of the other pins in the bank ...

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iwalel
Xilinx Employee
Xilinx Employee
6,964 Views
Registered: ‎07-25-2008

Hi Kris,

 

I think the Ios = 9mA is not the absolute maximum current the driver can provide.

But the maximum current the driver can provides while maintaining the minimum guaranteed output voltage.

In other words, if you would short the output to ground you would see a current much higher than 9mA.

Pelase correct me if I am wrong though.

 

The problem with this reverse current is usually with the power supply.

Have a look at this answer record as well: http://www.xilinx.com/support/answers/20496.htm

It should give you a fairly good idea how this works.

 

In my opinion you will have to change the voltage levels on Vcco, use a level shifter (or somthing similar) or use a series resitor to limit the current through the clamp diode.

 

 

Let me know if that makes sense!

 

 

Cheers!

 

kris_
Newbie
Newbie
6,929 Views
Registered: ‎09-30-2008

I'm not sure about the definition of Ios but you are probably right. If anyone has a link to a good definition I would be happy.

 

I think you are right regarding my options, however none of them work for me unfortunately.

 

Thanks for you help! 

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