On Sep 7, 8:00 am, Peter Alfke <al...@sbcglobal.net> wrote:
> On Sep 6, 9:03 pm, Jim Granville <no.s...@designtools.maps.co.nz>
> wrote:> Peter Alfke wrote:
>
> > >>>The regulator output voltage is controlled by just 2 resistors. When I
> > >>>changed one of the resistors to lover the voltage a bit, VCCAUX did not
> > >>>change. This leads me to believe that VCCAUX is somehow being "back"
> > >>>powered from the Xilinx chip. These voltages are present like this before
> > >>>the Xilinx chip has been programmed. I have not removed the regulator to
>
> I should have expressed this better: Use the extra pull-down resistor
> just as an experiment to see whether the problem really is reverse
> current going into the regulator, or is something else. Just
> temporarily, just attach the resistor lightly, by hand if possible...
> Peter Alfke

Unfortunately increasing the power dissipation with sinking/sourcing
LDO on VCCAUX or external loads can't be the final solution. If there
is a leackage current path from VCCIO to VCCAUX that must be founded
and supressed somehow.
Using series schottky between VCCAUX and FPGA will drop the output
voltage to VCCAUX - (0.3V...0.7V) depending on load current and diode
type.

Vasile