not totally repulsive

[John Larkin]

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

 

[BobW]

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

 

[Jim Granville]

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

 

[Eli Hughes]

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.

 

[Gabor]

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

 

[austin]

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

 

[John Larkin]

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

 

[BobW]

[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

 

[John Larkin]

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

 

[Andy]

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

 

[KJ]

[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.

They did make 5V only FPGAs for years and years...few people want them for
new production designs anymore

KJ

 

[John Larkin]

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?

I think we bought 5 or 6 of the first etch. So far, everything else
works (it's a 4-channel DDS-based waveform generator; a future spin
will be a 4-ch 32 MHz arb.) We plan to add a couple of features and
move one connector, so we won't fab any more of this rev A version.

But based on my measurements and Austin's comments, it sounds reliable
as-is, especially since we can check the actual Vccaux on each unit to
make sure the diode is behaving. So I'd be happy to sell a few of this
version.

We're looking into switchers now. Most of the interesting (small
package, synchronous, 1 amp at least) parts won't accept inputs past
5.5 volts, so I guess we'll switch 12 to 5 with a Simple Switcher +
schottky, use MSOP synchronous switchers from 5 to 3.3 and 1.2, and an
LDO (or even a diode!) for 3.3 to Vccaux.

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.

Why not? We have a reel of 3000 diodes. And we usually check all the
supply voltages anyhow; each has a test point. A switcher or an LDO
could be far more wrong than any diode is likely to ever be.

John

 

[Dave Pollum]

We're looking into switchers now. Most of the interesting (small
package, synchronous, 1 amp at least) parts won't accept inputs past
5.5 volts, so I guess we'll switch 12 to 5 with a Simple Switcher +
schottky, use MSOP synchronous switchers from 5 to 3.3 and 1.2, and an
LDO (or even a diode!) for 3.3 to Vccaux.

John;

A TI TPS75003 power supply controller has 2 3A switchers and a 300mA
LDO. This part is on the Xilinx/Digilent Spartan 3E board. Its max
Vin is 6.5 volts, so as you mentioned you'd have to regulate the
voltage down to 5 volts first.
-Dave Pollum

 

[Jim Granville]

[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.

:-}

Smiley noted, but there is an element of truth in this.

If you look at the microcontroller sector, OnChip regulatros are
very common on newest uC chips, and the Automotive/Industrial sectors
put pressure on 5V compliance at least, and operation preferably.

CPLDs are candidates for OnChip regulators, and those are already done
(just not at great Icc values).

FPGA's could easily REDUCE the supply rail count, but to move all
rails to regulators is a bigger ask, because of the power budgets.
FPGAs are in peril of thermal runaway already, do you want MORE heat
pumped into that tiny area ?!

Also, unlike uC design teams who are very 'analog aware', FPGA
development is rather cocooned in the digital world - Linear stuff !?.

-jg

 

[Andy]

Why not? We have a reel of 3000 diodes. And we usually check all the
supply voltages anyhow; each has a test point. A switcher or an LDO
could be far more wrong than any diode is likely to ever be.

John

....usually check...likely to ever be...

At least the LDO/switcher was tested (or at least a statistically
significant sample was tested) and it's production optimized to meet
published specifications. The diode was neither tested nor optimized
for your "specification". The diode could be completely "right" per
it's specs, and still "wrong" for your implementation. Such is not the
case with the LDO/switcher, so long as your application is within its
published specifications.

Not only do you have to check the diode's performance across parts and
over temperature, input supply variations, with aging, etc.), you also
have to check the FPGA's performance across parts and conditions. What
is the part-part (let alone lot-lot) variance in the fpga supply
current, over temperature, and with aging? Do both components vary
sympathetically or otherwise?

Once you start designing outside datasheet specifications, you lose
the advantages of the manufacturer's testing and process optimization
to meet those specs over the environment, over a large volume of
components. It can be done (and often is), but it takes a lot of
expensive testing and analysis over a large sample base (both
components and conditions) to ensure it works reliably. It should be
the last resort, not the first.

Andy

 

[Andy]

Why not? We have a reel of 3000 diodes. And we usually check all the
supply voltages anyhow; each has a test point. A switcher or an LDO
could be far more wrong than any diode is likely to ever be.

John

....usually...likely to ever be... Those are words I like to hear when
I purchase a product.

A switcher/LDO would have been designed, tested, and optimized to meet
it's specifications (the same ones you would be using) over large
production volumes. Such is not the case with the diode in your
application; it could be completely "right" per its specs, and not
work for your application.

Not only do you have to test a large sample of parts, but over a large
sample of conditions (including component aging). And then there's the
combination of FPGA and diode. How much does the FPGA supply current
change across conditions/environments? Does it vary sympathetically or
otherwise with your diode?

When designing outside of datasheet values, you lose the benefit of
the manufacturer's design, test, and optimization of that part to meet
those specifications over very large production volumes. You can
replace that with your own testing of lots of samples over lots of
conditions, but it is expensive. It should be a last resort, not the
first choice.

Andy

 

[Jon Elson]

...usually...likely to ever be... Those are words I like to hear when
I purchase a product.

A switcher/LDO would have been designed, tested, and optimized to meet
it's specifications (the same ones you would be using) over large
production volumes. Such is not the case with the diode in your
application; it could be completely "right" per its specs, and not
work for your application.

Not only do you have to test a large sample of parts, but over a large
sample of conditions (including component aging). And then there's the
combination of FPGA and diode. How much does the FPGA supply current
change across conditions/environments? Does it vary sympathetically or
otherwise with your diode?

When designing outside of datasheet values, you lose the benefit of
the manufacturer's design, test, and optimization of that part to meet
those specifications over very large production volumes. You can
replace that with your own testing of lots of samples over lots of
conditions, but it is expensive. It should be a last resort, not the
first choice.

Something that nobody has mentioned directly is that running 50 mA through
a Zener diode in the forward-bias mode may (and may not, too) have
effects on the long-term reliability of the diode. You say you used a 1
W Zener, so it should be good to 120 mA reverse-biased. Does the data
sheet give any spec on forward bias limits? I'd want to know the
manufacturer has run some of their typical Zeners forward biased at
significant current for an extended period, as this could be something
they've never tested for long-term effects.

Jon

 

[John Larkin]

John;

A TI TPS75003 power supply controller has 2 3A switchers and a 300mA
LDO. This part is on the Xilinx/Digilent Spartan 3E board. Its max
Vin is 6.5 volts, so as you mentioned you'd have to regulate the
voltage down to 5 volts first.

It seems to need a bazillion external parts. And we've been very
unhappy lately about TI's products and support.

According to the guy I asked to look into this, The Switcher of Choice
seems to be the LTC3411, single msop-packaged synchronous switcher. It
behaves well in simulation with a small 4.7 uH inductor and ceramic
output caps. We'll try some.

John

 

[BobW]

[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.

:-}

Smiley noted, but there is an element of truth in this.

If you look at the microcontroller sector, OnChip regulatros are
very common on newest uC chips, and the Automotive/Industrial sectors put
pressure on 5V compliance at least, and operation preferably.

CPLDs are candidates for OnChip regulators, and those are already done
(just not at great Icc values).

FPGA's could easily REDUCE the supply rail count, but to move all
rails to regulators is a bigger ask, because of the power budgets.
FPGAs are in peril of thermal runaway already, do you want MORE heat
pumped into that tiny area ?!

Also, unlike uC design teams who are very 'analog aware', FPGA development
is rather cocooned in the digital world - Linear stuff !?.

-jg

Jim,

I'm not sure that I totally agree with the "cocooned in the digital world"
statement. If you're working on (modern) FPGAs and you don't know what
Ldi/dt and Cdv/dt are then you are in BIG trouble (imho). The magnitude of
these quantities, now, impact what needs to be done (not to mention
transmission line effects).

It used to be that you could be a "digital" engineer and get by being
ignorant of circuit design. Today, I don't think that a "digital-only"
designer's designs will be successful.

Regarding power, I don't want X and A to add on-chip regulators to their
FPGAs. It's hard enough trying to keep these buggers cool (as you've noted).
I've had my fill of custom heat sinks, heat pipes, and 400lfm turbofans.
I'll take the extra supplies as the lesser-of-two evils.

Bob

 

[Sean Durkin]

FPGA's could easily REDUCE the supply rail count, but to move all
rails to regulators is a bigger ask, because of the power budgets.
FPGAs are in peril of thermal runaway already, do you want MORE heat
pumped into that tiny area ?!

The Lattice XP-family (not XP2) has a built-in LDO for VCCINT (at least
some of the devices, not all of them), and their VCCAUX is 3.3V, so in a
lot of cases you can get away with a single 3.3V supply. Plus they have
their configuration flash onboard (like the new Spartan3AN from Xilinx)
and are available in nice, small packages. Small, non-volatile
configuration, single supply, and usually a lot more logic than any CPLD
can offer, plus embedded SRAM and the likes. Quite nice IMHO.

cu,
Sean