Re: Ultra low temperature microcontroller ?

On Thu, 12 Jul 2007 20:15:21 GMT, "Donald Harris"
<harrisdw1@xxxxxxxxxxx> wrote:


"Everett M. Greene" <mojaveg@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:20070712.79D3A60.ABA3@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Paul Burke <paul@xxxxxxxxxx> writes:
Morten M Jørgensen wrote:
We are having a hard time finding a suitable microcontroller for our
new
automotive project. The product will have to operate in a very harsh
enviroment including temperatures lower than -40 deg. C.

Put the microcontoller between the driver's thighs. If it drops below
-40 there, nobody's going anywhere.

A little more seriously, can anyone give a simple explanation as to
why low temperatures are a problem for solid-state electronics? Do
the eletrons freeze?

When I talked to a Microchip Rep about using their chips for operating down
to -76 deg C they said that one of the problems is differential
expantion/contraction between the die and the package. At that temperature
the die often will crack.

That is more a storage temperature issue.

For bipolar silicon devices. the junction forward voltage drop has a
temperature coefficient about -2 mV/C so with a junction temperature
from -55 C to +200 C, for instance the transistor Vbe can vary about
500 mV, so extra care is required in designing the biasing
arrangement. This can be problematic in low voltage battery operated
systems, especially as the battery voltage also drops with
temperature.

Also the bipolar current gain drops significantly with temperature and
in oscillators the active device gain must be able to compensate for
the resonator losses i.e. the total loop gain must be at least 1. At
low temperatures the total loop gain may be below 1, in which case
oscillation is not possible. It can take a few seconds or a few
minutes, before the bias current warms the transistor sufficiently, to
increase the current gain and thus making oscillation possible.

There are a large number of ways things can go wrong and if a module
manufacturer has a heat/cold test unit that goes only down to -40 C,
the published temperature range is limited to -40 C, even if the unit
would in practice function reliably at lower temperatures.

Paul

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