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LTspice issues.

Discussion in 'Electronic Design' started by Jamie, Feb 21, 2010.

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  1. Jamie

    Jamie Guest

    I don't use LTspice a lot how ever, recently I have been poking around
    with a simple buck circuit that just does not seem to do in spice as it
    does in real life. I can put those little things aside how ever, I do
    have a problem when changing values and then exec the sim. It pops up
    with an error stating something on the line of
    "Step size to small x.xxxxxx xxxxxx, error at NC_01"

    that may not be the exact message but it's close..

    This can happen just about any where on any component I change the
    value on. To fix the problem, I decreas/increase the value by 1.

    For example:
    If change an inductor to 150uf, it didn't like that. I could
    go larger or smaller, but I found that all I needed to do was to
    add 1 or subtract 1 to make the sim happy.

    This does not happen in just inductors, it happens with R's caps

    Any one have something on that?
  2. pimpom

    pimpom Guest

    I don't have an answer but I get similar errors from time to
    time. I haven't been using LTSpice for very long either. When the
    "error at...." message comes up, it's usually when I forget to
    complete a connection and leave a node open. It may also happen
    when one makes a connection that LTSpice doesn't understand.

    I have even less clue about the "Step size too small" thing.
    Until someone with more insight comes along to enlighten us, I
    surmise that it happens when certain combinations of circuit
    arrangement, component values and signal and measurement
    parameters require more complex calculations than LTSpice wants
    to attempt. Maybe multiple parasitic oscillations. I don't know.
    Just guessing.

  3. Hello Jamie,

    If the simulator stops with "time step too small", you should try some
    options to help the solver.


    ..tran 20m

    Set a small time step in .TRAN , e.g 100n if you have a 100kHz switching

    ..tran 0 20ms 0 100n

    If it alreay fails at the beginning, you should try with the
    option "startup" in the .TRAN command.
    Sometimes additional ".nodeset" will help to get the simulation

    ..tran 0 40ms 0 100n startup

    I prefer to continue as shown below.


    Control Panel -> SPICE -> Reset to default
    Control Panel -> Hacks -> Reset to default

    There are some options which can be helpful. Try either one,
    some or all in combination.
    These are SPICE directives which you place in your schematic.

    ..options gmin=1e-10
    ..options abstol=1e-10
    ..options reltol=0.003


    If that fails, you could try with the Alternate solver.
    Therefore don't use any option from above orr set them to their default

    Control Panel -> SPICE -> Reset to default
    Control Panel -> SPICE -> Solver: Alternate

    The default values:
    ..options gmin=1e-12
    ..options abstol=1e-12
    ..options reltol=0.001


    If it still fails, go back to the normal solver.

    Control Panel -> SPICE -> Solver: Normal

    Use the following only as the last option, because it can have a
    lot of side effects, especially if you have used a larger value
    for cshunt.

    ..options cshunt=1e-15

    This adds a capacitor with 1fF from every node to GND.
    I wouldn't go higher than 1e-14.

    You should also use a combination of these options as in 2) in this case.
    ..options gmin=1e-10
    ..options abstol=1e-10
    ..options reltol=0.003

    Best regards,
  4. Jamie

    Jamie Guest

    I have come to the conclusion there is a bug in it. Because when I
    receive this error, changing a value enough to make it happy allows a
    SIM operation. Then after a SIM run, I can go back and change that same
    component to the value I had that generated the error to which it will
    then exec the SIM with no problems.

    Since I do a lot of coding my self, I see this as an initiation problem
    of variables or something of that sort on start up!.

    I don't recall seeing this problem before the last update that was
    done. But then again, I'm getting old and memory loss maybe an issue :)

  5. Hello Jamie,

    Can you send me one example for trying?

    Best regards,
  6. Hello Jamie,

    Maybe you feel it's different, because LTspice nowadays tries with
    "pseudo transient analysis" to find the operating point when the classic
    methods failed.
    Older versions simply started with the transient simulation even without
    having found an operating point.
    You could suppress this method with this SPICE-directive.
    ..options ptrantau=0

    Best regards,
  7. Jamie

    Jamie Guest

    As I indicated in another message, I found that after I make the SIM
    happy, I run a SIM and if I change the values back to where they were
    that caused the problem, I can rerun SIM with no issues ;/

    Something is wrong with the software. A initiation problem perhaps?

    I do use the options in the control panel to make it nearest to
    real world operation as possible. Nice features btw and still learning
    what most of them do, good thing there are docs :)

    I tried a combination of things and no matter what I did, once the
    error was there, it just wasn't going to start. This error takes place
    at the very beginning. It does not happen once the annalistic data
    starts collecting.

    Who knows, maybe one day I'll track it down..
  8. Jamie

    Jamie Guest


    But toggling the values between SIMS does not explain why I can get it
    to SIM with the values that originally generated the problem.

    Oh well. One day I may find out the issue.
  9. Jamie

    Jamie Guest

    I'll keep that noted, I don't remember seeing this in older versions, It
    may have something to do with it.

  10. Fred Abse

    Fred Abse Guest

    Changing inductors to capacitors can have lots of consequences :)

    Assuming that you didn't actually mean what you wrote, try using the
    "alternative" solver for a start.

    Maybe post the .asc file and people might take a look at it.
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