jFET discharge (was: RE: FW: Texas Instruments IC's)

[René Schmitz]

At 11:51 30.05.00 -0700, Tim Ressel wrote:
>Tony et al,
>
>I concidered using a hexfet for that function. Ultra-low ON resistance, real
>fast. OFF leakage?? Not sure. And you have to watch the ripple current on the
>capacitor you are using. You can pull amps out of the cap if the switch
>resistance is too low. A series resistance should fix, but then you limit
>discharge time. Sigh...

In case of the jFET as a discharge switch the Idss is the maximum current
that can flow! Some few mA not more! On-resistance is only interesting in
the region where Vds (or V of your cap) is near or less than Vp. Above that
a jFET acts as a constant current source. I've always seen a linear
starting ramp when testing jFET discharge. 
BTW. The IMO ultimate cure for leakage can be found at my site. One can
combine this with a more leaky switch, and still be faster than with
something else.

Bye,
 René





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