# RE: [SI-LIST] : Variability of Supply voltages

From: Lynne Green ([email protected])
Date: Thu Oct 12 2000 - 11:42:02 PDT

of the power supplies. If one I/O can run at all three voltages
at the same time, you get 9 corners. If only two voltages at
the same time, you get fewer. And include temperature
& process corners, of course.

Where I used to work, we received a dual-voltage I/O cell
design from a contractor that worked ONLY if the power
supplies tracked. And other I/O cells could fall out of spec
at any Vcc1/Vcc2/Temp/Process corner, depending on the
circuit design.

It's always much better to do it right the first time. More
work up front, but it is worth it. The ounce of prevention
vs. the pound of cure, sigh....

- Lynne

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Michael Nudelman
Sent: Tuesday, October 10, 2000 12:57 PM
To: Bill Cohen
Cc: [email protected]
Subject: Re: [SI-LIST] : Variability of Supply voltages

Well, I would (meaning all 9 combinations).

Suppose you use independent DC-DCs to produce all the voltages (we do, for
3.3 and
2.5). It means, that each one of them is totally independent of another. And
the
min-max initial setup point does not really depend on the input voltage, so
you
cannot assume, that if one goes to max, so will another. The initial setup,
due to
very high precision of regulation in todays DC-Dcs is almost solely the
function of
component precision.

Now, suppose, you derive 2.5 from 3.3 by using a linear regulator. Again,
for the
linear, the output is not solely a function of the input; it is rather the
function
of the internal gain and the reference accuracy. So, your output may be at
maximum,
when the input is at minimum and vice versa.

The only case I can think of when everything is at max/min at the same time
is when
you use dual/triple voltage DC-DC; then, since the regulation inside the
DCDC is
done using one voltage, and others are just simply rectified from different
winding
taps of the output transformer, they will follow the voltage, that is
regulated and
will follow it to max or min.

Mike.

Bill Cohen wrote:

> I am a CMOS integrated circuit designer. I need some board level
expertise.
>
> In our I/O circuits we usually have various transistors that run at unique
> maximum voltages. So usually we specify three power supplies in our
circuits.
> The most common are: 3.3V, 2.5V, and 1.8V.
> We then try to perform a worst case analysis using the spice simulator.
> At the present time we don't assume that the three power supplies track.
> So to do a worst case simulation we might run a low 3V supply and a
> high 2V supply. The power supplies would supply a single IC and be
> resident on the same PC board. My question is as follows:
>
> Can we assume that when one power supply is at a maximum the other two
supplies
> will be at a maximum? Should we assume that all three supplies can be
> independent and we need to perform a maxtrix of 9 combinations to do
> a proper worst case analysis?
>
> Bill Cohen
> Circuit Design
> Technolgoy Development Group Toshiba
> Marlborough, MA
> [email protected]
>
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