Power for Spartan 3

Dear all,

I hope this isn't OT but I was wondering if anyone has any experience 
using the TPS75003 to power a Spartan 3?  For my particular application 
I'm having to swap round the output voltages produced by one of the buck 
convertors and the LDO with respect to most of the app notes.  The 
potential dividers which give the reference feedback in the app notes 
come out to give a voltage which is consistently about 40mV above spec 
for all three outputs.  I was wondering if anyone knew why?  Or is it 
just a bit of design headroom?

TIA

-- Peter

Be careful of the resistor values. We have used these parts and from my 
memory the LDO has a different reference voltage and hence resistor ratio to 
the switcher elements. Your 40mV could be resistor or reference tolerance 
have you checked those?

John Adair
Enterpoint Ltd. - Home of Hollybush1. The PC104+ Spartan3 Development Board.
http://www.enterpoint.co.uk

"Peter Mendham" <petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> wrote in 
message news:e3v0va$brk$1@dux.dundee.ac.uk...
> Dear all,
>
> I hope this isn't OT but I was wondering if anyone has any experience 
> using the TPS75003 to power a Spartan 3?  For my particular application 
> I'm having to swap round the output voltages produced by one of the buck 
> convertors and the LDO with respect to most of the app notes.  The 
> potential dividers which give the reference feedback in the app notes come 
> out to give a voltage which is consistently about 40mV above spec for all 
> three outputs.  I was wondering if anyone knew why?  Or is it just a bit 
> of design headroom?
>
> TIA
>
> -- Peter 

I'll second that tolerance concern.  I was surprised to find that the
core voltage on some parts ('C5510 DSP) has a tolerance as low as +-50
mV making it about 3%.  With two resistors and the innate tolerance of
the part, that can be a tough spec to meet.

But I think the OP is saying the values selected provide a voltage 40
mV too high.  I would suggest that you contact Xilinx support about
this.  They put a lot of things in app notes that may or may not be
best for your design.  Don't be afraid to change things, but it
wouldn't hurt to contact them to ask why they picked what they picked.


Or are you talking about the TI app notes?  Actually I don't see how
you can say all three outputs are 40 mV too high when the 1.2 volt
output does not use external resistors!  The values in the TI data
sheet (figure 1) produce about 2.54 volts, 3.29 volts and of course,
1.2 volts.


John Adair wrote:
> Be careful of the resistor values. We have used these parts and from my
> memory the LDO has a different reference voltage and hence resistor ratio to
> the switcher elements. Your 40mV could be resistor or reference tolerance
> have you checked those?
>
> John Adair
> Enterpoint Ltd. - Home of Hollybush1. The PC104+ Spartan3 Development Board.
> http://www.enterpoint.co.uk
>
> "Peter Mendham" <petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> wrote in
> message news:e3v0va$brk$1@dux.dundee.ac.uk...
> > Dear all,
> >
> > I hope this isn't OT but I was wondering if anyone has any experience
> > using the TPS75003 to power a Spartan 3?  For my particular application
> > I'm having to swap round the output voltages produced by one of the buck
> > convertors and the LDO with respect to most of the app notes.  The
> > potential dividers which give the reference feedback in the app notes come
> > out to give a voltage which is consistently about 40mV above spec for all
> > three outputs.  I was wondering if anyone knew why?  Or is it just a bit
> > of design headroom?
> >
> > TIA
> >
> > -- Peter

John,

Thanks for your response.  It think I have the resistor values correct, 
I've attempted to follow the app notes to the letter, anyway.  Resistor 
and Vref tolerances is a good idea, I should have checked that before I 
posted.  Having looked at them, it could possibly be the source of the 
40mV.  If it were required that near the lowest error tolerance bound 
the Vref was still not below the required value, this would seem make 
some sense.  The only niggle is that the error bounds are relative 
(resistor/Vref), and this 40mV over-voltage doesn't change all that much 
from the 1.2V to the 3.3V supplies.  The app notes don't seem to provide 
an explanation for this.

I have come up with two plans, one for a voltage close to spec, another 
for 40mV (approx) over.  Unless I hear any different, I'll probably 
prototype with the former, and only use the latter if things go belly-up.

 From your experience, do you have any other advice about using these 
devices?

Thanks,

-- Peter

John Adair wrote:
> Be careful of the resistor values. We have used these parts and from my 
> memory the LDO has a different reference voltage and hence resistor ratio to 
> the switcher elements. Your 40mV could be resistor or reference tolerance 
> have you checked those?
> 
> John Adair
> Enterpoint Ltd. - Home of Hollybush1. The PC104+ Spartan3 Development Board.
> http://www.enterpoint.co.uk
> 
> "Peter Mendham" <petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> wrote in 
>> I hope this isn't OT but I was wondering if anyone has any experience 
>> using the TPS75003 to power a Spartan 3?  For my particular application 
>> I'm having to swap round the output voltages produced by one of the buck 
>> convertors and the LDO with respect to most of the app notes.  The 
>> potential dividers which give the reference feedback in the app notes come 
>> out to give a voltage which is consistently about 40mV above spec for all 
>> three outputs.  I was wondering if anyone knew why?  Or is it just a bit 
>> of design headroom?

Is your voltmeter calibrated?
Aurash
Peter Mendham wrote:
> John,
> 
> Thanks for your response.  It think I have the resistor values correct, 
> I've attempted to follow the app notes to the letter, anyway.  Resistor 
> and Vref tolerances is a good idea, I should have checked that before I 
> posted.  Having looked at them, it could possibly be the source of the 
> 40mV.  If it were required that near the lowest error tolerance bound 
> the Vref was still not below the required value, this would seem make 
> some sense.  The only niggle is that the error bounds are relative 
> (resistor/Vref), and this 40mV over-voltage doesn't change all that much 
> from the 1.2V to the 3.3V supplies.  The app notes don't seem to provide 
> an explanation for this.
> 
> I have come up with two plans, one for a voltage close to spec, another 
> for 40mV (approx) over.  Unless I hear any different, I'll probably 
> prototype with the former, and only use the latter if things go belly-up.
> 
>  From your experience, do you have any other advice about using these 
> devices?
> 
> Thanks,
> 
> -- Peter
> 
> John Adair wrote:
> 
>> Be careful of the resistor values. We have used these parts and from 
>> my memory the LDO has a different reference voltage and hence resistor 
>> ratio to the switcher elements. Your 40mV could be resistor or 
>> reference tolerance have you checked those?
>>
>> John Adair
>> Enterpoint Ltd. - Home of Hollybush1. The PC104+ Spartan3 Development 
>> Board.
>> http://www.enterpoint.co.uk
>>
>> "Peter Mendham" <petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> 
>> wrote in
>>
>>> I hope this isn't OT but I was wondering if anyone has any experience 
>>> using the TPS75003 to power a Spartan 3?  For my particular 
>>> application I'm having to swap round the output voltages produced by 
>>> one of the buck convertors and the LDO with respect to most of the 
>>> app notes.  The potential dividers which give the reference feedback 
>>> in the app notes come out to give a voltage which is consistently 
>>> about 40mV above spec for all three outputs.  I was wondering if 
>>> anyone knew why?  Or is it just a bit of design headroom?
> 
> 

rickman wrote:
> But I think the OP is saying the values selected provide a voltage 40
> mV too high.

Yes, I am still on the drawing board at this point.

> Or are you talking about the TI app notes?

I am, I haven't found any Xilinx app notes about the part.  Are there any?

> Actually I don't see how
> you can say all three outputs are 40 mV too high when the 1.2 volt
> output does not use external resistors!

Good point, my mistake.  Sorry.  Both Buck 2 and the LDO are approx 40mV 
too high.

> The values in the TI data
> sheet (figure 1) produce about 2.54 volts, 3.29 volts and of course,
> 1.2 volts.

I am indeed.  I am probably being stupid, but using the values in all 
the TI example circuits (I have about 5 variations here) the Buck 2 
potential divider uses values of 61K9 and 36K5 (or 319K and 365K).  By 
equation 15 in the TI datasheet these values give a Vout of 3.343V. 
Similarly, for the LDO the value 2.545V.  These are 43mV and 45mV over 
spec respectively.  Now, it is extremely likely that I am either 
worrying about nothing, or being very stupid.  Is this over-voltage 
specified deliberately, or is it simply a product of using nearest-fit 
resistor values?  Or have I done my calculations badly?  If you can 
clarify that for me I would be very grateful.

Best,

-- Peter

Aurelian Lazarut wrote:
> Is your voltmeter calibrated?
> Aurash

My design-time voltmeter (i.e. my calculator) is well calibrated :)
I'm still on paper at this point.  But thanks for the suggestion 
nevertheless.

-- Peter

Given the effect is on both supplies I would particularly look at anything 
common like the reference voltage or a common resistor value.

John Adair
Enterpoint Ltd. - Home of Broaddown2. The Ultimate Spartan3 Development 
Board.
http://www.enterpoint.co.uk


"Peter Mendham" <petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> wrote in 
message news:e49ql2$pa6$1@dux.dundee.ac.uk...
> John,
>
> Thanks for your response.  It think I have the resistor values correct, 
> I've attempted to follow the app notes to the letter, anyway.  Resistor 
> and Vref tolerances is a good idea, I should have checked that before I 
> posted.  Having looked at them, it could possibly be the source of the 
> 40mV.  If it were required that near the lowest error tolerance bound the 
> Vref was still not below the required value, this would seem make some 
> sense.  The only niggle is that the error bounds are relative 
> (resistor/Vref), and this 40mV over-voltage doesn't change all that much 
> from the 1.2V to the 3.3V supplies.  The app notes don't seem to provide 
> an explanation for this.
>
> I have come up with two plans, one for a voltage close to spec, another 
> for 40mV (approx) over.  Unless I hear any different, I'll probably 
> prototype with the former, and only use the latter if things go belly-up.
>
> From your experience, do you have any other advice about using these 
> devices?
>
> Thanks,
>
> -- Peter
>
> John Adair wrote:
>> Be careful of the resistor values. We have used these parts and from my 
>> memory the LDO has a different reference voltage and hence resistor ratio 
>> to the switcher elements. Your 40mV could be resistor or reference 
>> tolerance have you checked those?
>>
>> John Adair
>> Enterpoint Ltd. - Home of Hollybush1. The PC104+ Spartan3 Development 
>> Board.
>> http://www.enterpoint.co.uk
>>
>> "Peter Mendham" <petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> wrote 
>> in
>>> I hope this isn't OT but I was wondering if anyone has any experience 
>>> using the TPS75003 to power a Spartan 3?  For my particular application 
>>> I'm having to swap round the output voltages produced by one of the buck 
>>> convertors and the LDO with respect to most of the app notes.  The 
>>> potential dividers which give the reference feedback in the app notes 
>>> come out to give a voltage which is consistently about 40mV above spec 
>>> for all three outputs.  I was wondering if anyone knew why?  Or is it 
>>> just a bit of design headroom?
> 

On Mon, 15 May 2006 13:02:43 +0100, Peter Mendham
<petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> wrote:

>John,
>
>Thanks for your response.  It think I have the resistor values correct, 
>I've attempted to follow the app notes to the letter, anyway.  Resistor 
>and Vref tolerances is a good idea, I should have checked that before I 
>posted.  Having looked at them, it could possibly be the source of the 
>40mV.  If it were required that near the lowest error tolerance bound 
>the Vref was still not below the required value, this would seem make 
>some sense.  The only niggle is that the error bounds are relative 
>(resistor/Vref), and this 40mV over-voltage doesn't change all that much 
>from the 1.2V to the 3.3V supplies.  The app notes don't seem to provide 
>an explanation for this.
>
>I have come up with two plans, one for a voltage close to spec, another 
>for 40mV (approx) over.  Unless I hear any different, I'll probably 
>prototype with the former, and only use the latter if things go belly-up.
>
> From your experience, do you have any other advice about using these 
>devices?
>
>Thanks,
>
>-- Peter
>
>John Adair wrote:
>> Be careful of the resistor values. We have used these parts and from my 
>> memory the LDO has a different reference voltage and hence resistor ratio to 
>> the switcher elements. Your 40mV could be resistor or reference tolerance 
>> have you checked those?
>> 
>> John Adair
>> Enterpoint Ltd. - Home of Hollybush1. The PC104+ Spartan3 Development Board.
>> http://www.enterpoint.co.uk
>> 
>> "Peter Mendham" <petermendham@NOCANNEDMEAT.computing.dundee.ac.uk> wrote in 
>>> I hope this isn't OT but I was wondering if anyone has any experience 
>>> using the TPS75003 to power a Spartan 3?  For my particular application 
>>> I'm having to swap round the output voltages produced by one of the buck 
>>> convertors and the LDO with respect to most of the app notes.  The 
>>> potential dividers which give the reference feedback in the app notes come 
>>> out to give a voltage which is consistently about 40mV above spec for all 
>>> three outputs.  I was wondering if anyone knew why?  Or is it just a bit 
>>> of design headroom?

I would say that it is better to err slightly on the high side to
compensate for IR drop between the regulator and the part.  Once you
build the prototype you can measure the actual drop (both ground and
high sides) and tweak your values accordingly.

I have used the TPS75003 in a ceramic capacitor configuration, because
tantalum and small electrolytic capacitors are prohibited in our
applications.  In this case the divider network has to be designed
differently to provide AC feedback from the switch.  I asked TI about
this, and they recommended that I follow the guidance in the TPS64200
datasheet (see figure 27 and associated discussion).  This has worked
well.


================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
greg@guesswhichwordgoeshere.com

Peter Mendham wrote:
>> The values in the TI data
>> sheet (figure 1) produce about 2.54 volts, 3.29 volts and of course,
>> 1.2 volts.
> 
> 
> I am indeed.  I am probably being stupid, but using the values in all 
> the TI example circuits (I have about 5 variations here) the Buck 2 
> potential divider uses values of 61K9 and 36K5 (or 319K and 365K).  By 
> equation 15 in the TI datasheet these values give a Vout of 3.343V. 
> Similarly, for the LDO the value 2.545V.  These are 43mV and 45mV over 
> spec respectively.  Now, it is extremely likely that I am either 
> worrying about nothing, or being very stupid.  Is this over-voltage 
> specified deliberately, or is it simply a product of using nearest-fit 
> resistor values?  Or have I done my calculations badly?  If you can 
> clarify that for me I would be very grateful.

The error on 3.3V is 1.3%, which will be nearest fit resistor values.
Normally, simplest design uses two resistors, and it is hard to nail
a value under 1% with available values.

If this is loosing you sleep, then move to a 3 resistor design, and
also be prepared to pay for resistors under 1% tolerance.

In a real design, you should measure/verify the voltage AT THE DEVICE,
which means a few mV high allows for some trace/choke IR drop.

Also check the dynamic power changes, and output impedance of your
regulators, as that is another error source.

-jg