VCCAUX too high on a Spartan 3 design

Using linear adjustable regulators for VCCINT (1.25v), VCCIO (3.3v), and 
VCCAUX (2.5v).  VCCINT and VCCIO are dead on, but VCCAUX is 2.72v, 2.88v, 
and 2.92v on the 3 boards I grabbed and measured.  All boards seem to work 
just fine.

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 
measure current yet.  Another thought was to put a shotkey diode between the 
regulator output and the load to see if the Xilinx really is powering 
VCCAUX, but I thought I'd post and see if anyone else has come across this 
issue.  Half the I/O banks are 2.5v and half are 3.3v if that makes any 
difference.

Thanks - Dan 

"Dan K" <danielgkNOSPAM@visi.com> wrote in message 
news:K5%Di.100073$Iu.95108@fe182.usenetserver.com...
> Using linear adjustable regulators for VCCINT (1.25v), VCCIO (3.3v), and 
> VCCAUX (2.5v).  VCCINT and VCCIO are dead on, but VCCAUX is 2.72v, 2.88v, 
> and 2.92v on the 3 boards I grabbed and measured.  All boards seem to work 
> just fine.
>
> 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 
> measure current yet.  Another thought was to put a shotkey diode between 
> the regulator output and the load to see if the Xilinx really is powering 
> VCCAUX, but I thought I'd post and see if anyone else has come across this 
> issue.  Half the I/O banks are 2.5v and half are 3.3v if that makes any 
> difference.
>
> Thanks - Dan
>
>

It sounds like you're driving one of the dedicated configuration pins with a 
3V3 signal.  They should all be 2V5.

There's a Xilinx app note about 3V3 configration which suggests series 
resistors to limit current through the ESD diodes, and provision of 
current-sink capability on the 2V5 supply!

On Sep 6, 4:03 pm, "Andrew Holme" <and...@nospam.com> wrote:
> "Dan K" <danielgkNOS...@visi.com> wrote in message
>
> news:K5%Di.100073$Iu.95108@fe182.usenetserver.com...
>
>
>
> > Using linear adjustable regulators for VCCINT (1.25v), VCCIO (3.3v), and
> > VCCAUX (2.5v).  VCCINT and VCCIO are dead on, but VCCAUX is 2.72v, 2.88v,
> > and 2.92v on the 3 boards I grabbed and measured.  All boards seem to work
> > just fine.
>
> > 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
> > 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.
Peter Alfke

> > the regulator output and the load to see if the Xilinx really is powering
> > VCCAUX, but I thought I'd post and see if anyone else has come across this
> > issue.  Half the I/O banks are 2.5v and half are 3.3v if that makes any
> > difference.
>
> > Thanks - Dan
>
> It sounds like you're driving one of the dedicated configuration pins with a
> 3V3 signal.  They should all be 2V5.
>
> There's a Xilinx app note about 3V3 configration which suggests series
> resistors to limit current through the ESD diodes, and provision of
> current-sink capability on the 2V5 supply!

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
>>>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

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

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

On Thu, 6 Sep 2007 17:23:39 -0500, "Dan K" <danielgkNOSPAM@visi.com>
wrote:

>Using linear adjustable regulators for VCCINT (1.25v), VCCIO (3.3v), and 
>VCCAUX (2.5v).  VCCINT and VCCIO are dead on, but VCCAUX is 2.72v, 2.88v, 
>and 2.92v on the 3 boards I grabbed and measured.  All boards seem to work 
>just fine.
>
>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 
>measure current yet.  Another thought was to put a shotkey diode between the 
>regulator output and the load to see if the Xilinx really is powering 
>VCCAUX, but I thought I'd post and see if anyone else has come across this 
>issue.  Half the I/O banks are 2.5v and half are 3.3v if that makes any 
>difference.
>
>Thanks - Dan 
>
>

Try adding a "dump" resistor from Vccaux to ground, and see if that
pulls the voltage down.

John

Hi John.
That's what I suggested in my posting yesterday.
"Great minds think the same way"  :-)
Peter Alfke

On Sep 7, 9:29 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 6 Sep 2007 17:23:39 -0500, "Dan K" <danielgkNOS...@visi.com>
> wrote:
>
>
>
> >Using linear adjustable regulators for VCCINT (1.25v), VCCIO (3.3v), and
> >VCCAUX (2.5v).  VCCINT and VCCIO are dead on, but VCCAUX is 2.72v, 2.88v,
> >and 2.92v on the 3 boards I grabbed and measured.  All boards seem to work
> >just fine.
>
> >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
> >measure current yet.  Another thought was to put a shotkey diode between the
> >regulator output and the load to see if the Xilinx really is powering
> >VCCAUX, but I thought I'd post and see if anyone else has come across this
> >issue.  Half the I/O banks are 2.5v and half are 3.3v if that makes any
> >difference.
>
> >Thanks - Dan
>
> Try adding a "dump" resistor from Vccaux to ground, and see if that
> pulls the voltage down.
>
> John

On Sep 7, 3:01 am, vasile <picli...@gmail.com> wrote:
> 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


You should make sure that no VCCAUX relative pins are hooked to VCCIO
without a resistor otherwise any fix can blow up the esd diodes. Then,
if you really need to save a couple of milliwatts, power the VCCCORE
regulator from VCCAUX, this will suck the backrush of current right
into another power supply instead of into a resistor. It is no
coincidence that 1.2v LDO regulators are designed to run on 2.5v.

Cheers
Jon Pry

On Sep 7, 7:54 pm, Peter Alfke <pe...@xilinx.com> wrote:
> Hi John.
> That's what I suggested in my posting yesterday.
> "Great minds think the same way"  :-)
> Peter Alfke
>
> On Sep 7, 9:29 am, John Larkin
>
>
>
> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> > On Thu, 6 Sep 2007 17:23:39 -0500, "Dan K" <danielgkNOS...@visi.com>
> > wrote:
>
> > >Using linear adjustable regulators for VCCINT (1.25v), VCCIO (3.3v), and
> > >VCCAUX (2.5v).  VCCINT and VCCIO are dead on, but VCCAUX is 2.72v, 2.88v,
> > >and 2.92v on the 3 boards I grabbed and measured.  All boards seem to work
> > >just fine.
>
> > >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
> > >measure current yet.  Another thought was to put a shotkey diode between the
> > >regulator output and the load to see if the Xilinx really is powering
> > >VCCAUX, but I thought I'd post and see if anyone else has come across this
> > >issue.  Half the I/O banks are 2.5v and half are 3.3v if that makes any
> > >difference.
>
> > >Thanks - Dan
>
> > Try adding a "dump" resistor from Vccaux to ground, and see if that
> > pulls the voltage down.
>
> > John- Hide quoted text -
>
> - Show quoted text -

I'm curious if the original poster has solved the problem in the
indicated way above.
I have a design with Stratix II which has the same behaviour.

thx,
Vasile