FPGA Central

I have tried looking for documentation on the current requirements for
FPGAs.

I am trying to select the most efficient and low power voltage
regulators to provide the 3.3, 2.5 and 1.2 voltages to a Spartan 3
(PQ208). The input power supply is 5V.

I had selected a 100mA Texas Instruments voltage reuglator. But I am
still curious if I am violating any current requirements for the FPGA
pins. The spartan 3 documentation talks about quiescent current
limitations which seem to be pretty low at around maximum of 70mA.

Of course the goal is to have the most low power regulators so that
thermal power generation is at the lowest. Let me know if you have any
expereinces with different voltage regulators.

Thanks

-Yaju N
BYU ECEN Dept

I assume you're going to use switching regulators to generate all voltages directly from 5 volts, this would be the most efficient way to do things. I use the Linear Technologies LTC3411 for this sort of application because it's readily available in small quantities, takes in up to 5.5 volts and outputs down to .8 volts, and it's above 90% efficient for a wide range of output currents, from 10 mA to well over 500 mA. It can supply up to 1.25 amps, though you need big tantalum caps to keep the ripple down when you get up there in current. It does have a few tradeoffs though -- it seems Linear's parts are more expensive than the big manufacturers, and this part will take up more board space than some other solutions and has a bigger BOM, adding to the cost.

Your current draw is going to vary widely depending on your design, and will always be more than the quiescent current. You can easily burn a couple watts in a medium-sized FPGA, and 5-10 watts in a bigger FPGA running at high speed.

Hi


Yaju N wrote:
> I have tried looking for documentation on the current requirements for
> FPGAs.
>
> I am trying to select the most efficient and low power voltage
> regulators to provide the 3.3, 2.5 and 1.2 voltages to a Spartan 3
> (PQ208). The input power supply is 5V.
>
> I had selected a 100mA Texas Instruments voltage reuglator. But I am
> still curious if I am violating any current requirements for the FPGA
> pins. The spartan 3 documentation talks about quiescent current
> limitations which seem to be pretty low at around maximum of 70mA.
>
> Of course the goal is to have the most low power regulators so that
> thermal power generation is at the lowest. Let me know if you have any
> expereinces with different voltage regulators.

Just estimate your power requirement with the on-line xilinx tools. The
power requirements of FPGA vary in function of the design you load in it.

A 64bits 200Mhz DDR interface using DCI is not gonna use the same power
as a a simple slow I2C interface ...


Sylvain

My current(ho ho) favourite switcher is the LTC3414. 4A output. As Chris
says, maybe more expensive than some, but ease of use and time to market is
key in my apps. Don't use Tants, they're made of pure evil! Either ceramic
caps up to 100uF or Panasonic's 5mOhm ESR specialty polymer electrolytics up
to 470uF. I usually defeat the burst mode. One less noise source to worry
about.
All IMHO, YMMV, hope this gives you some ideas!
Best, Syms.
<Chris> wrote in message news:[email protected]..
> I assume you're going to use switching regulators to generate all voltages
directly from 5 volts, this would be the most efficient way to do things. I
use the Linear Technologies LTC3411 for this sort of application because
it's readily available in small quantities, takes in up to 5.5 volts and
outputs down to .8 volts, and it's above 90% efficient for a wide range of
output currents, from 10 mA to well over 500 mA. It can supply up to 1.25
amps, though you need big tantalum caps to keep the ripple down when you get
up there in current. It does have a few tradeoffs though -- it seems
Linear's parts are more expensive than the big manufacturers, and this part
will take up more board space than some other solutions and has a bigger
BOM, adding to the cost.
>
> Your current draw is going to vary widely depending on your design, and
will always be more than the quiescent current. You can easily burn a couple
watts in a medium-sized FPGA, and 5-10 watts in a bigger FPGA running at
high speed.

Besides Linear, check also the upcoming Texas TPS75003... seems good, but
still preliminary announcementl.

>> I am trying to select the most efficient and low power voltage
>> regulators to provide the 3.3, 2.5 and 1.2 voltages to a Spartan 3
>> (PQ208). The input power supply is 5V.
>>

Apart from the current requirements, it seems that I have to worry
about the real estate for heat sinking.
The new TI TPS75003 seems to be good device, but it basically consists
of a linear regulator and not a switching one as I had thought.

I guess I will have to compromise on the real estate for heat sinking.
Maybe I will include a "finned" heat sink.

The search for the best voltage regulator continues.....

> The new TI TPS75003 seems to be good device, but it basically consists
> of a linear regulator and not a switching one as I had thought.

Could you please be more specific on this ?

I was thinking of using it myself. I just quickly read the datasheets.

To me, it seems to have two buck regulators for core and I/O, where you
usually have most of the current, and a linear regulator for VCCAUX.

Given that VCCAUX powers also DCM modules, and its quiescent current is not
so big, perhaps you would have choosen an LDO in any case, to have it less
noisy.

Am I missing something obvious here ?

Yaju N wrote:
> Apart from the current requirements, it seems that I have to worry
> about the real estate for heat sinking.
> The new TI TPS75003 seems to be good device, but it basically consists
> of a linear regulator and not a switching one as I had thought.
>
> I guess I will have to compromise on the real estate for heat sinking.
> Maybe I will include a "finned" heat sink.
>
> The search for the best voltage regulator continues.....

I found a new company that is making a very advanced surface mount
regulator module. They have combined everything onto the chip except
for the capacitors and run it at 5 MHz. This includes the *inductor*!
The module is only 0.5" x 0.32" and can provide up to 3 Amps.

www.enpirion.com

I just took another look at the web site and they now also have 1 Amp
and 6 Amp versions. The 6 Amp version is only 0.67" x 0.32"!



Rick Collins

[email protected]

Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design http://www.arius.com
4 King Ave. 301-682-7772 Voice
Frederick, MD 21701-3110 GNU tools for the ARM http://www.gnuarm.com

I am basicaly trying to find what is the maximum current that will ever
be needed for the FPGA I/0. I know it differs on application, but based
on the maximum current required, I can choose the most efficient
voltage regulators.

The other constraint I have is the real-estate space that would be
needed for heat sinking the voltage regulators For that purpose
swiching regulators would be preferred. In case of TI TPS75003, its
does have buck regulators for the core and I/O but it also has (I
think its "inbuilt") Linear Regulator which would need heat sinking
area.

Maybe I am missing something totally obvious here as well. I dont have
much experience in using Voltage Regulators. My basic constraint is of
course is board space and I am trying to find ways to get around that.