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Newbie klai1
Newbie
7,074 Views
Registered: ‎07-19-2014

PLEASE HELP ME!!! How can I vary the power supply voltage of the Virtex-5 FPGA chip on ML501, ML506, and ML510?

I'm working a large-scale experiment to characterize the affect of supply voltage change on the frequency of an array of ring oscillators in the presence of process variation. I want to do my experiment on FPGA because it is faster than circuit simulation for large sample size and the process variation can be poorly simulated with circuit simulation. I have access to a couple of Xilinx Virtex-5 boards, namely, ML501, ML506, and ML510. I got an online suggestion that if I want to vary the supply voltage of the Virtex-5 FPGA chip on these boards, I should replace the resistor R179 on pin 8 of PTH08T240W (this part handles the VCCINT) with a pot. These boards I have are expensive and the suggested method is quite risky so I am hesitant to try it out. I have a fundametnal understanding of electronics and some experiences in FPGA design. So if anyone has done a similar project before or has a clue about how to safely vary the supply voltage of these boards, please share your incredible knowledge with me :). Thank you in advance for your help!

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Instructor
Instructor
7,060 Views
Registered: ‎08-14-2007

Re: PLEASE HELP ME!!! How can I vary the power supply voltage of the Virtex-5 FPGA chip on ML501, ML506, and ML510?

Just replacing the single resistor with a pot is a bad idea because you could have severe voltage changes while you are varying the pot's resistance.  It's always best to design a resistor network that is limited to safe minimum and maximum values regardless of the pot setting (including momentary open circuit).  On the other hand there are also current sink IC's available for applying small amounts of additional current into the existing feedback node.  These are designed specifically for what you're trying to do and are guaranteed to limit their output current under all conditions.  I don't remember the part number off-hand but I found a device of this type with a 7 bit DAC and a single resistor to adjust full scale current.  Then you just need something like an I2C interface to vary the voltage.  You could do that from the FPGA itself or from a separate I2C master adapter if you feel more comfortable that way.  You can also end up with the same idea using a lab-quality bench current source, but then you need to be careful not to exceed the current that would hurt the circuit.  The best bet is to adjust the supply for tha max allowable Vccint using a precision resistor to replace R179, and then use the current source to always inject more current into the feedback node, which can only cause the Vccint voltage to go down - not up.

 

[Edit]  Here's a link to a current sink/source typically used to digitally alter the output voltage of a switching power supply:

 

http://datasheets.maximintegrated.com/en/ds/DS4402-DS4404.pdf

-- Gabor
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