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

> >>>measure current yet. Another thought was to put a shotkey diode between
>
> > I would load the Vccaux pin with a 100 Ohm resistor to ground. That
> > adds 25 mA to the regulated current.
> > Normall, this should hardly change the voltage, but if the regulator
> > is back-fed, there will be a big voltage drop.
> > Parallel resistors to ground are nice and easy, and do not cuase any
> > irrepairable tdamage.
>
> You can also drop the values of the two SET resistors, then you do not
> need to change the PCB design.
>
> -jg