Change the input voltage of the FPGA

[HyperText]

Hi,

I'm trying to change the input voltage of a Xilinx Spartan-3E FPGA (the default one is 1.2V). The board is a Digilent Nexys 2, and of course a tension regulator is involved, hence I have to consider it.
The schematic is the following:

SW1 is the output of the tension regulator and its value is of 0.8V. VCC1V2 is set using the formula: VCC1V2 = 0.8V * [(R78+R79)/R79].
So, by changing the R78 value I can change the input voltage of the FPGA.

However, a user in the Xilinx Forum suggested another solution (http://forums.xilinx.com/t5/Xilinx-...upply-to-core/m-p/116380/highlight/true#M3707) consisting of: "Lift one side or the other of L2. You can connect a variable output bench supply directly to the VCC1V2 net.".
Since I'm not an Electronic, I would like some additional informations:

• I just lift one side of L2 and connect a different power supply (let's say 0.9V, coming from a dedicated external power supply), without any particular precaution?
• how should I calculate the VCC1V2 value? Using the above 0.9V value, VCC1V2 will still follow the above equation? So, VCC1V2 would be equal to: 0.9V * [(R78+R79)/R79]?

Thanks for any hint,
Pierpaolo

 

[Harald Kapp]

These are two entirely different procedures:

1) Use the built-in regulator (LTC3417). It is then clear that by changing R78 (or R79, which will work in the same manner) you can adjust VCC1V2.

2) USe an external power supply. By lifting one of the pins of L2 you disconnect the LTC3417 from the output VCC1V2. You can then use an external power supply (e.g. a benchtop suply as suggested on the other forum) which you connect between GND and VCC1V2 to supply an external voltage. In this case it doesn't matter which values R78 or R79 have because the output SW1 is unused (open circuit from SW1 to VCC1V2 due to lifted pin of L2). When using an external supply you may have to watch out for overcurrent and for correct power sequencing. The FPGA may not tolerate if VCC1V2 and VCC2V5 are not present at the same time (e.g. if you forget to turn on the external supply).

Personally I'd prefer changing one of the resistors in the existing circuit. The datasheet of the LTC3417 doesn't state what happens to the circuit if no feedback from the output is present (as will be the case when you lift one pin f L2). The circuit presently works fine, so there is les risk in changing the output voltage of the existing circuit than adding an external supply with probably unknown and not well suited characteristics.

 

[HyperText]

Thanks for the reply. I just discovered that I was being a bit naive
Now I understand the open circuit mechanism, and all the subsequent implications (and precautions) that you described.

Only one question is left: how (and where) can I connect the external supply? I mean, you said "which you connect between GND and VCC1V2 to supply an external voltage", but how I can do this? How can I physically (on the board) determine this "connection point"? Could you give me any example?


Thanks,
Pierpaolo

 

[Harald Kapp]

To the right side of L2. Very easy, if you lif that end off the PCB, then you have a pad available to solder an external voltage source to.

 

[HyperText]

Thanks for your answers to my noob questions. I'll try it in few days (or next week).

Thanks again,
Pierpaolo