[volt-nuts] LTZ1000 at higher currents
m1k3k1 at hotmail.com
Fri Oct 21 16:04:03 UTC 2011
The cooling comment is because when you reduce the airflow enough to reduce the thermocouple effects on the pcb you may well raise the temp just outside the LTZ1000 can to significantly above 20c.
For a homebrew oil immersion will significantly reduce thermal effects.
Currently I am wondering how much the pickering patent is useful for a homebrew as one then has just transferred the stability issues onto the sample and hold circuit, as that requires sub ppm accuracy there.
We know that at 95/97C that the HP reference is set to gives higher drift. So that sets an upper limit on the current to run through the zener. I am also wondering why they give 5mA as the upper limit? Is it possible that the zener is so small that above 5mA one has current coming out through the shoulder of the end of the zener? By design the diffusions give the lowest breakdown voltage at the planar end portion, and the shoulders have progressively higher breakdown to keep the current away from the surface. BUT, if that was so the pickering patent method would not work. The zener tempco will be higher at higher currents and it will be more sensitive to orientation at higher temperatures too, so that would limit my enthusiasm to run it too hard.
I am still pondering all that while I try and burn in my LTZ sample.
> From: john at devereux.me.uk
> To: volt-nuts at febo.com
> Date: Fri, 21 Oct 2011 12:47:41 +0100
> Subject: Re: [volt-nuts] LTZ1000 at higher currents
> Yes I am assuming a LTZ1000 (not A) and 20C lab environment.
> Could you clarify your remarks about cooling, I can't tell if you mean
> poor cooling (good insulation) or good cooling (heatsink somehow)?
> I was wondering about oil immersion too.
> m k <m1k3k1 at hotmail.com> writes:
> > Hi John,
> > The LTZ1000 can be run much harder, but the LTZ1000a can only be run upto about 40-45 mA before the chip temp is up to 125C where the dift is going to be much higher.
> > I am assuming a 20C lab environment for homebrew kit with decent cooling. But to stop thermal drift you really have to give the chip and pcb poor cooling or dip in parrafin oil.
> > Regards,
> > M K
> >> From: john at devereux.me.uk
> >> To: volt-nuts at febo.com
> >> Date: Wed, 19 Oct 2011 14:14:45 +0100
> >> Subject: [volt-nuts] LTZ1000 at higher currents
> >> Hello everyone,
> >> Is there some reason why LTZ1000s are not run at a lot higher current
> >> than 5mA?
> >> Thermal resistance is 80'C/W, or ~0.5'C/mA. So it would seem OK to run
> >> it at 10, 20, 30mA before seeing serious problems due to raising the
> >> temperature too high.
> >> The datasheet noise graph shows a big fall in noise level between 1 and
> >> 5mA, so this ought to reduce the noise further. I am not sure how far
> >> down in frequency this improvement continues. On the datasheet the
> >> "noise gap" widens more and more with lower frequency, does this extend
> >> to frequencies so low they would be regarded as long term drift?
> >> There is no current limit for the zener that I can see (of course there
> >> will be one eventually).
> >> The "Pickering patent" uses pulsing to achieve a high current in what is
> >> obviously a LTZ1000, the stated reason being to minimise "VLF and long
> >> term instability".
> >> But it seems like even a higher DC current could be of benefit.
> John Devereux
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