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

From: Michael Nudelman ([email protected])
Date: Tue Oct 10 2000 - 12:56:59 PDT

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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