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From: "Mike Engelhardt"
Subject: Re: LT SwitcherCAD Current Source Help
Date: 13 Sep 2002 22:42:51 GMT
References: <3D809F7A.firstname.lastname@example.org> <5Pgg9.email@example.com>
Reply-To: "Mike Engelhardt"
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
> Mike wrote:
> > The implementation of the voltage controlled switch
> > in Berkeley SPICE has a continuous 1st derivative,
> > though it has a discontinuity in value. The
> > continuous 1st derivative helps it converge even
> > when its right at the edge of switching between the
> > two conductivity values.
> > The only improvement is to offer a switch like that
> > in LTspice which is everywhere continuous in value
> > and derivative.
> Weeeeelllll, you could use a VCIS and build up a voltage controled
> resister with a B source and have pretty much any transfer function you
Precisely. In principle you can get continuous value and
derivatives with that method. In practice, however, the
compiler in Berkeley SPICE makes inefficient code and the
execution is slow. Even more serious are the errors in the
Jacobian computed which can defeat the method for
getting continuous value and derivatives. Even though these
issues are fixed in LTspice, you still have to deal with
Newton iteration. Basically, you get poor performance
if you use straight Newton iteration without knowledge of
the non-linear functions you're solving. The integrated
switch(in LTspice) gives the Newton hints for the next
iteration which really picks up the performance and
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