Virtex® Family FPGAs

[milescdpekala]

I am designing a board where the lowest voltage is 3.3 volts. I've looked over the DC switching characteristics and have noticed that the reccommended voltages for VCCINT is ~1.2 V and for VCCAUX is ~2.5V. Is there an easy way to input 3.3V or am I stuck with making a voltage regulator?

Thanks

[gszakacs]

I'm afraid you need to add regulators to your design.  The Virtex 4 will not operate at 3.3V internally.  See the section "Absolute Maximum Ratings" in the datasheet.  Exceeding the absolute maximum ratings WILL damage the part.  However depending on the current required by your design you may be able to use very small voltage regulators to generate the required voltages.

[milescdpekala]

Thanks, thats what I figured. I am going to make a simple zener regulator which should be enough to regulate the voltage.

Thanks again.

[milescdpekala]

Actually, that makes me wonder about the decoupling capacitors. Should I put capacitors on the 1.2V line after it comes out of the regulator or should I simply attach them to the 3.3V source? I am uncertain of what sort of bounce/dip can occur in the VCCINT supply and it doesn't appear to be in the configuration guide (where PDS is discussed). Any ideas?

[gszakacs]

The best voltage regulators still have too much impedance at high frequencies to get away with no capacitors on the load side.  The Vccint supply, 1.2V, is especially important to place high-speed bypass caps on.  The Vccint current is dominated by dynamic switching current, so it has a lot of high-frequency components dominated by the clock frequency of your design and submultiples of this frequency.  Your zener regulator will not stay within the required tolerance without sufficient bypassing.  I suggest to follow the guidelines in the the user guides, or if your code design is complete look at the report files which recommend the necessary bypassing.