not totally repulsive

I usually have +5 volts available in VME modules, so I generally
linear-regulate down from +5 to 3.3, 2.5, and 1.2 for Spartan3 fpga's.
VME has lots of power and lots of air flow. My favorite trick is to
use an LM1117 regulator with its ADJ pin grounded, to make 1.25 volts.
A second LM1117 has its ADJ pin riding on the 1.25, so I get 2.5, all
with no resistors.

Now this new gadget: it's an uncooled small box powered from a 12 volt
wart. I don't need much 5 volts, so I switched directly to 3.3 and did
the same LM1117 thing. Oops. The 1117 has about a 1.1 volt dropout, so
I'm getting about +2.2 for Vccaux, sort of marginal.

So I'm thinking, why not yank the regulator and put a diode from 3.3
to make 2.5? So I pulled all the MELF diodes we have in stock. The
current draw on +2.5 is about 40 mA, increasing to 50 mA after
configuration (XC3S400, running mostly at 64 MHz). So I'm looking for
a diode with 0.8 volts drop at 50 mA.

As expected, big 1 amp, low-voltage (100v) diodes have the least drop,
around 0.64 volts. Higher voltage diodes, 600 and 1000 volts increase,
to about 0.70. So I tried some 1-watt zeners in the forward direction.
Bingo. A 5.1 volt Zetex part is 0.84, and an 8.2 volt zener is 0.805.

I wonder what might be the trend of zener forward voltage versus zener
reverse voltage. Doping and stuff.

http://img141.imageshack.us/my.php?image=diodeklugeqo3.jpg

I'm going to spin the board after this batch is used up, for other
reasons, so I suppose I'll do it right next pass. Probably go to
switchers for most everything.

John

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message 
news:eiqvi3d3sfklij5lig4l258751bsopj0as@4ax.com...
>
>
> I usually have +5 volts available in VME modules, so I generally
> linear-regulate down from +5 to 3.3, 2.5, and 1.2 for Spartan3 fpga's.
> VME has lots of power and lots of air flow. My favorite trick is to
> use an LM1117 regulator with its ADJ pin grounded, to make 1.25 volts.
> A second LM1117 has its ADJ pin riding on the 1.25, so I get 2.5, all
> with no resistors.
>
> Now this new gadget: it's an uncooled small box powered from a 12 volt
> wart. I don't need much 5 volts, so I switched directly to 3.3 and did
> the same LM1117 thing. Oops. The 1117 has about a 1.1 volt dropout, so
> I'm getting about +2.2 for Vccaux, sort of marginal.
>
> So I'm thinking, why not yank the regulator and put a diode from 3.3
> to make 2.5? So I pulled all the MELF diodes we have in stock. The
> current draw on +2.5 is about 40 mA, increasing to 50 mA after
> configuration (XC3S400, running mostly at 64 MHz). So I'm looking for
> a diode with 0.8 volts drop at 50 mA.
>
> As expected, big 1 amp, low-voltage (100v) diodes have the least drop,
> around 0.64 volts. Higher voltage diodes, 600 and 1000 volts increase,
> to about 0.70. So I tried some 1-watt zeners in the forward direction.
> Bingo. A 5.1 volt Zetex part is 0.84, and an 8.2 volt zener is 0.805.
>
> I wonder what might be the trend of zener forward voltage versus zener
> reverse voltage. Doping and stuff.
>
> http://img141.imageshack.us/my.php?image=diodeklugeqo3.jpg
>
> I'm going to spin the board after this batch is used up, for other
> reasons, so I suppose I'll do it right next pass. Probably go to
> switchers for most everything.
>
> John
>

Keep in mind that VCCAUX is used to power all sorts of stuff, and I don't 
think it's all documented. So, if you change the FPGA design you may get 
more/less VCCAUX current. That along with temperature effects may push the 
voltage beyond its published limits.

If it were mine, I would bite the bullet and put in a proper ldo.

Maybe some wonderful Xilinx guy (e.g. Austin) will drop his $0.02 regarding 
the constancy of VCCAUX current on S3.

Bob

John Larkin wrote:

> 
> I usually have +5 volts available in VME modules, so I generally
> linear-regulate down from +5 to 3.3, 2.5, and 1.2 for Spartan3 fpga's.
> VME has lots of power and lots of air flow. My favorite trick is to
> use an LM1117 regulator with its ADJ pin grounded, to make 1.25 volts.
> A second LM1117 has its ADJ pin riding on the 1.25, so I get 2.5, all
> with no resistors.
> 
> Now this new gadget: it's an uncooled small box powered from a 12 volt
> wart. I don't need much 5 volts, so I switched directly to 3.3 and did
> the same LM1117 thing. Oops. The 1117 has about a 1.1 volt dropout, so
> I'm getting about +2.2 for Vccaux, sort of marginal.
> 
> So I'm thinking, why not yank the regulator and put a diode from 3.3
> to make 2.5? So I pulled all the MELF diodes we have in stock. The
> current draw on +2.5 is about 40 mA, increasing to 50 mA after
> configuration (XC3S400, running mostly at 64 MHz). So I'm looking for
> a diode with 0.8 volts drop at 50 mA.
> 
> As expected, big 1 amp, low-voltage (100v) diodes have the least drop,
> around 0.64 volts. Higher voltage diodes, 600 and 1000 volts increase,
> to about 0.70. So I tried some 1-watt zeners in the forward direction.
> Bingo. A 5.1 volt Zetex part is 0.84, and an 8.2 volt zener is 0.805.
> 
> I wonder what might be the trend of zener forward voltage versus zener
> reverse voltage. Doping and stuff.
> 
> http://img141.imageshack.us/my.php?image=diodeklugeqo3.jpg
> 
> I'm going to spin the board after this batch is used up, for other
> reasons, so I suppose I'll do it right next pass. Probably go to
> switchers for most everything.
> 
> John

There will be a temperature variantion on this, and you should verify 
the drop in operating conditions (it is a diode, and any ringing will be
rectified - so a slow one like a Zener is probably a good choice )

Also note that the % Vcc variation is amplified on this.
3.3C +/- 10% or 3.0..3.6 , now becomes 2.2-2.8V, and a 20% window,
is now 27.3% - so you'll need tighter starting Vcc levels.

But it will work. I've also looked at using Yellow LEDs as low-cost
1.8V shunt regulators for CPLDs :)

-jg

Jim Granville wrote:
> John Larkin wrote:
> 
>>
>> I usually have +5 volts available in VME modules, so I generally
>> linear-regulate down from +5 to 3.3, 2.5, and 1.2 for Spartan3 fpga's.
>> VME has lots of power and lots of air flow. My favorite trick is to
>> use an LM1117 regulator with its ADJ pin grounded, to make 1.25 volts.
>> A second LM1117 has its ADJ pin riding on the 1.25, so I get 2.5, all
>> with no resistors.
>>
>> Now this new gadget: it's an uncooled small box powered from a 12 volt
>> wart. I don't need much 5 volts, so I switched directly to 3.3 and did
>> the same LM1117 thing. Oops. The 1117 has about a 1.1 volt dropout, so
>> I'm getting about +2.2 for Vccaux, sort of marginal.
>>
>> So I'm thinking, why not yank the regulator and put a diode from 3.3
>> to make 2.5? So I pulled all the MELF diodes we have in stock. The
>> current draw on +2.5 is about 40 mA, increasing to 50 mA after
>> configuration (XC3S400, running mostly at 64 MHz). So I'm looking for
>> a diode with 0.8 volts drop at 50 mA.
>>
>> As expected, big 1 amp, low-voltage (100v) diodes have the least drop,
>> around 0.64 volts. Higher voltage diodes, 600 and 1000 volts increase,
>> to about 0.70. So I tried some 1-watt zeners in the forward direction.
>> Bingo. A 5.1 volt Zetex part is 0.84, and an 8.2 volt zener is 0.805.
>>
>> I wonder what might be the trend of zener forward voltage versus zener
>> reverse voltage. Doping and stuff.
>>
>> http://img141.imageshack.us/my.php?image=diodeklugeqo3.jpg
>>
>> I'm going to spin the board after this batch is used up, for other
>> reasons, so I suppose I'll do it right next pass. Probably go to
>> switchers for most everything.
>>
>> John
> 
> There will be a temperature variantion on this, and you should verify 
> the drop in operating conditions (it is a diode, and any ringing will be
> rectified - so a slow one like a Zener is probably a good choice )
> 
> Also note that the % Vcc variation is amplified on this.
> 3.3C +/- 10% or 3.0..3.6 , now becomes 2.2-2.8V, and a 20% window,
> is now 27.3% - so you'll need tighter starting Vcc levels.
> 
> But it will work. I've also looked at using Yellow LEDs as low-cost
> 1.8V shunt regulators for CPLDs :)
> 
> -jg
> 
> 
> 
> 
> 


You realize that there ire tiny switching regulators that are about the 
same size as the monolithic linear ones?


National semi was any easy tool that will generate a reference design.

On Nov 6, 10:31 am, Eli Hughes <emh...@psu.edu> wrote:
> Jim Granville wrote:
> > John Larkin wrote:
>
> >> I usually have +5 volts available in VME modules, so I generally
> >> linear-regulate down from +5 to 3.3, 2.5, and 1.2 for Spartan3 fpga's.
> >> VME has lots of power and lots of air flow. My favorite trick is to
> >> use an LM1117 regulator with its ADJ pin grounded, to make 1.25 volts.
> >> A second LM1117 has its ADJ pin riding on the 1.25, so I get 2.5, all
> >> with no resistors.
>
> >> Now this new gadget: it's an uncooled small box powered from a 12 volt
> >> wart. I don't need much 5 volts, so I switched directly to 3.3 and did
> >> the same LM1117 thing. Oops. The 1117 has about a 1.1 volt dropout, so
> >> I'm getting about +2.2 for Vccaux, sort of marginal.
>
> >> So I'm thinking, why not yank the regulator and put a diode from 3.3
> >> to make 2.5? So I pulled all the MELF diodes we have in stock. The
> >> current draw on +2.5 is about 40 mA, increasing to 50 mA after
> >> configuration (XC3S400, running mostly at 64 MHz). So I'm looking for
> >> a diode with 0.8 volts drop at 50 mA.
>
> >> As expected, big 1 amp, low-voltage (100v) diodes have the least drop,
> >> around 0.64 volts. Higher voltage diodes, 600 and 1000 volts increase,
> >> to about 0.70. So I tried some 1-watt zeners in the forward direction.
> >> Bingo. A 5.1 volt Zetex part is 0.84, and an 8.2 volt zener is 0.805.
>
> >> I wonder what might be the trend of zener forward voltage versus zener
> >> reverse voltage. Doping and stuff.
>
> >>http://img141.imageshack.us/my.php?image=diodeklugeqo3.jpg
>
> >> I'm going to spin the board after this batch is used up, for other
> >> reasons, so I suppose I'll do it right next pass. Probably go to
> >> switchers for most everything.
>
> >> John
>
> > There will be a temperature variantion on this, and you should verify
> > the drop in operating conditions (it is a diode, and any ringing will be
> > rectified - so a slow one like a Zener is probably a good choice )
>
> > Also note that the % Vcc variation is amplified on this.
> > 3.3C +/- 10% or 3.0..3.6 , now becomes 2.2-2.8V, and a 20% window,
> > is now 27.3% - so you'll need tighter starting Vcc levels.
>
> > But it will work. I've also looked at using Yellow LEDs as low-cost
> > 1.8V shunt regulators for CPLDs :)
>
> > -jg
>
> You realize that there ire tiny switching regulators that are about the
> same size as the monolithic linear ones?
>
> National semi was any easy tool that will generate a reference design.


I use them and they work great, but you won't find them easy to cobble
onto an existing PC board that expected a 3 terminal regulator.  Most
come in packages with no leads and require a large solder pad for the
heat slug in the middle.  For very low currents you could use a 5-pin
SOT23 package like the LM3674MF-ADJ.  It's a nice device but doesn't
come in a 2.5V version so you need the external resistor divider to
get 2.5V.

For the prototypes you are probably best off looking for a very
low drop-out linear device, which are available in 2.5V versions
and require very few external components.  IIRC NXP makes some that
are stable with little or no capacitance.

Regards,
Gabor

John,

Dropping the 3.3v to 2.5v with a pn junction is just so easy to do, that 
I am sure you are not the first one to do this (in fact, we did with a 
SCR in the lab for a Spartan 2 app note - 2.5v core).

Other than the obvious, that the voltage is likely to vary over 
temperature beyond the recommended Vccaux range, if you have verified 
that your design works over the temperature range you need, then you are 
"done."  You need to be sure no one changes the recipe for your zener 
diode (perhaps just buy all you will need, and then the next revision 
just use a LDO regulator that operates off of a lower voltage).

Vccaux is used for bandgap reference voltage generators, which will 
start working at 1.8 volts, and work fine up to beyond 3.0 volts.  There 
are a few more circuits also using Vccaux, but generally, it is not as 
fussy as say the Vccint.  It will affect output timing, as Vccaux is 
used for the predrivers, and it will also affect LVDS inputs (on some 
parts where the diff-amp is powered by Vccaux, the latest parts use Vcco 
for that however).

The reason for the recommended 5% specification, is that we have to 
specify a lot, and it is far easier to specify all supplies at 5%, 
rather than have each supply have its own rated range, and then have to 
characterize everything over all of the ranges.

At less than 100 mA, it is not easy to get ~2.5V from 3.3V any easier 
than what you have described.  An efficient switcher with 250 mW 
capacity is also not easy to find (switchers are inefficient if used at 
a power much much less than what they are designed for).

Austin

On Tue, 06 Nov 2007 09:31:23 -0500, Eli Hughes <emh203@psu.edu> wrote:

>Jim Granville wrote:
>> John Larkin wrote:
>> 
>>>
>>> I usually have +5 volts available in VME modules, so I generally
>>> linear-regulate down from +5 to 3.3, 2.5, and 1.2 for Spartan3 fpga's.
>>> VME has lots of power and lots of air flow. My favorite trick is to
>>> use an LM1117 regulator with its ADJ pin grounded, to make 1.25 volts.
>>> A second LM1117 has its ADJ pin riding on the 1.25, so I get 2.5, all
>>> with no resistors.
>>>
>>> Now this new gadget: it's an uncooled small box powered from a 12 volt
>>> wart. I don't need much 5 volts, so I switched directly to 3.3 and did
>>> the same LM1117 thing. Oops. The 1117 has about a 1.1 volt dropout, so
>>> I'm getting about +2.2 for Vccaux, sort of marginal.
>>>
>>> So I'm thinking, why not yank the regulator and put a diode from 3.3
>>> to make 2.5? So I pulled all the MELF diodes we have in stock. The
>>> current draw on +2.5 is about 40 mA, increasing to 50 mA after
>>> configuration (XC3S400, running mostly at 64 MHz). So I'm looking for
>>> a diode with 0.8 volts drop at 50 mA.
>>>
>>> As expected, big 1 amp, low-voltage (100v) diodes have the least drop,
>>> around 0.64 volts. Higher voltage diodes, 600 and 1000 volts increase,
>>> to about 0.70. So I tried some 1-watt zeners in the forward direction.
>>> Bingo. A 5.1 volt Zetex part is 0.84, and an 8.2 volt zener is 0.805.
>>>
>>> I wonder what might be the trend of zener forward voltage versus zener
>>> reverse voltage. Doping and stuff.
>>>
>>> http://img141.imageshack.us/my.php?image=diodeklugeqo3.jpg
>>>
>>> I'm going to spin the board after this batch is used up, for other
>>> reasons, so I suppose I'll do it right next pass. Probably go to
>>> switchers for most everything.
>>>
>>> John
>> 
>> There will be a temperature variantion on this, and you should verify 
>> the drop in operating conditions (it is a diode, and any ringing will be
>> rectified - so a slow one like a Zener is probably a good choice )
>> 
>> Also note that the % Vcc variation is amplified on this.
>> 3.3C +/- 10% or 3.0..3.6 , now becomes 2.2-2.8V, and a 20% window,
>> is now 27.3% - so you'll need tighter starting Vcc levels.
>> 
>> But it will work. I've also looked at using Yellow LEDs as low-cost
>> 1.8V shunt regulators for CPLDs :)
>> 
>> -jg
>> 
>> 
>> 
>> 
>> 
>
>
>You realize that there ire tiny switching regulators that are about the 
>same size as the monolithic linear ones?

Certainly. But they need inductors and probably secondary lc filtering
before we get to the analog stuff. I believe I mentioned going to
switchers next rev, although making Vccaux with an ldo from +3.3 ain't
bad. Even if we upgrade to an XC3S1500 at 128 MHz, which we may do,
the current will still only be about 100 mA, which is only 80 mW loss
in a linear reg.

Dang, the power supplies are more trouble than the FPGAs.

John

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

[snip]

>
> Dang, the power supplies are more trouble than the FPGAs.
>
> John
>

If they wanted to, these FPGA manufacturers *could* make a FIVE-VOLT-only 
FPGA.

I think they're just being stubborn.

:-}

Bob

On Tue, 06 Nov 2007 07:15:46 -0800, austin <austin@xilinx.com> wrote:

>John,
>
>Dropping the 3.3v to 2.5v with a pn junction is just so easy to do, that 
>I am sure you are not the first one to do this (in fact, we did with a 
>SCR in the lab for a Spartan 2 app note - 2.5v core).
>
>Other than the obvious, that the voltage is likely to vary over 
>temperature beyond the recommended Vccaux range, if you have verified 
>that your design works over the temperature range you need, then you are 
>"done."  You need to be sure no one changes the recipe for your zener 
>diode (perhaps just buy all you will need, and then the next revision 
>just use a LDO regulator that operates off of a lower voltage).


Thanks for the insight on Vccaux internals. Hell, sounds like I can
use the diode forever!

As far as tc goes, even a "full diode" tc of 2.5 mV/K won't affect
things much. But when a silicon diode has this much forward drop, a
good chunk of the drop is ohmic, which has the opposite tc from the pn
junction itself. I wouldn't be surprised if this diode were running
near its zero-tc point. I'll measure it if I get a chance.

John

On Nov 6, 10:31 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Tue, 06 Nov 2007 07:15:46 -0800, austin <aus...@xilinx.com> wrote:
> >John,
>
> >Dropping the 3.3v to 2.5v with a pn junction is just so easy to do, that
> >I am sure you are not the first one to do this (in fact, we did with a
> >SCR in the lab for a Spartan 2 app note - 2.5v core).
>
> >Other than the obvious, that the voltage is likely to vary over
> >temperature beyond the recommended Vccaux range, if you have verified
> >that your design works over the temperature range you need, then you are
> >"done."  You need to be sure no one changes the recipe for your zener
> >diode (perhaps just buy all you will need, and then the next revision
> >just use a LDO regulator that operates off of a lower voltage).
>
> Thanks for the insight on Vccaux internals. Hell, sounds like I can
> use the diode forever!
>
> As far as tc goes, even a "full diode" tc of 2.5 mV/K won't affect
> things much. But when a silicon diode has this much forward drop, a
> good chunk of the drop is ohmic, which has the opposite tc from the pn
> junction itself. I wouldn't be surprised if this diode were running
> near its zero-tc point. I'll measure it if I get a chance.
>
> John

Do the right thing after the spin: LDO from 3.3V or switcher from 12V.

Prior to that, how many existing boards do you need to use up, and how
many customers can you afford to loose when they do not work
reliably?

Even after you test one or two samples over temperature extremes, a
sample of one or two is no basis to design a production run on.

Andy