Forums

LTSpice Simulation Hanging

Started by Tim Wescott February 8, 2016
On Mon, 08 Feb 2016 16:22:31 -0700, Jim Thompson wrote:

> On Mon, 08 Feb 2016 16:15:46 -0700, Jim Thompson > <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: > >>On Mon, 08 Feb 2016 16:24:26 -0600, Tim Wescott >><seemywebsite@myfooter.really> wrote: >> >>>LTSpice files not included because -- it's files, not file. >>> >>>So, I have a fairly complicated circuit in LTSpice. I'm playing some >>>tricks on an LT3757A -- basically I need an externally settable voltage >>>output, so I'm hijacking the FBX line and driving it from an op-amp >>>summing circuit. Everything was fine until I put that op-amp in there, >>>but I suspect that I've just made the circuit too damned complicated, >>>or too weird from LT's point of view. >> >>Post your .ASC >> >>Did "driving it from an op-amp summing circuit" change the loop gain or >>mung the phase? >> > [snip] > > You should be able to set the output voltage _without_ putting the OpAmp > in the feedback loop. > > What range of output voltages are you seeking? > > ...Jim Thompson
0-200, from a 0-4V command. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Mon, 08 Feb 2016 16:24:26 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>LTSpice files not included because -- it's files, not file. > >So, I have a fairly complicated circuit in LTSpice. I'm playing some >tricks on an LT3757A -- basically I need an externally settable voltage >output, so I'm hijacking the FBX line and driving it from an op-amp >summing circuit. Everything was fine until I put that op-amp in there, >but I suspect that I've just made the circuit too damned complicated, or >too weird from LT's point of view. > >The simulation is not quite hanging, but it's getting stuck in a state >where it's progressing at tens of femptosecond/second, on a fairly speedy >machine. > >So my question is -- is there anything I can jigger in the various >tolerances or other "don't touch this unless you know what you're doing >and/or don't mind your simulation being meaningless" settings? > >At the moment I've managed to make it work much better by removing some >circuitry and replacing it with a fixed supply -- but it would be nice if >I knew some more general ways of making all well, and as things stand >I've just blown my overall efficiency calculations out of the water. > >TIA.
If the circuit has radically different time constants, it can run slow. Switchers are especially nasty, and have to run for hundreds or thoudsands of cycles to boot. The example circuts on the LTC web site usually run reasonably fast. Sometimes just changing a part value or two, by a little bit, really speeds up the sim. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Mon, 8 Feb 2016 23:13:10 -0000 (UTC), Frank Miles
<fpm@u.washington.edu> wrote:

>On Mon, 08 Feb 2016 16:24:26 -0600, Tim Wescott wrote: > >> LTSpice files not included because -- it's files, not file. >> >> So, I have a fairly complicated circuit in LTSpice. I'm playing some >> tricks on an LT3757A -- basically I need an externally settable voltage >> output, so I'm hijacking the FBX line and driving it from an op-amp >> summing circuit. Everything was fine until I put that op-amp in there, >> but I suspect that I've just made the circuit too damned complicated, or >> too weird from LT's point of view. >> >> The simulation is not quite hanging, but it's getting stuck in a state >> where it's progressing at tens of femptosecond/second, on a fairly >> speedy machine. >> >> So my question is -- is there anything I can jigger in the various >> tolerances or other "don't touch this unless you know what you're doing >> and/or don't mind your simulation being meaningless" settings? >> >> At the moment I've managed to make it work much better by removing some >> circuitry and replacing it with a fixed supply -- but it would be nice >> if I knew some more general ways of making all well, and as things stand >> I've just blown my overall efficiency calculations out of the water. >> >> TIA. > >Could you replace the added opamp with some kind of infinite-bandwidth >circuit (e.g. controlled source)? Of course this might negate the whole >advantage of doing the simulation, but it might be good as a starting >point. Once that's going you can make it more realistic.
Using e or g, and an RC, to fake an opamp is a good way to speed things up. You can make the GBW anything you like. The Universal Opamp seems pretty good too. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Mon, 08 Feb 2016 18:36:25 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Mon, 08 Feb 2016 16:22:31 -0700, Jim Thompson wrote: > >> On Mon, 08 Feb 2016 16:15:46 -0700, Jim Thompson >> <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >> >>>On Mon, 08 Feb 2016 16:24:26 -0600, Tim Wescott >>><seemywebsite@myfooter.really> wrote: >>> >>>>LTSpice files not included because -- it's files, not file. >>>> >>>>So, I have a fairly complicated circuit in LTSpice. I'm playing some >>>>tricks on an LT3757A -- basically I need an externally settable voltage >>>>output, so I'm hijacking the FBX line and driving it from an op-amp >>>>summing circuit. Everything was fine until I put that op-amp in there, >>>>but I suspect that I've just made the circuit too damned complicated, >>>>or too weird from LT's point of view. >>> >>>Post your .ASC >>> >>>Did "driving it from an op-amp summing circuit" change the loop gain or >>>mung the phase? >>> >> [snip] >> >> You should be able to set the output voltage _without_ putting the OpAmp >> in the feedback loop. >> >> What range of output voltages are you seeking? >> >> ...Jim Thompson > >0-200, from a 0-4V command.
Gack! That'll cost you a tranny >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Mon, 08 Feb 2016 18:21:16 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

Have you tried adding parallel R to the inductores and series R to
those caps? Say 1G ohm.

Cheers
On Mon, 08 Feb 2016 18:21:16 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

[snip].
> >The seeming excess of passive components around the op-amp is for a >reason (good start-up behavior), and doesn't seem to have much of a >bearing on whether the thing locks up or not. > >.asc: > >Version 4 >SHEET 1 4556 1460 >WIRE 1808 -352 1696 -352
[snip asc listing] It runs, see... <http://www.analog-innovations.com/SED/Wescott_Adj_Switcher_2016-02-09_08-30-45.png> doesn't hang, but goes nowhere. Your added OpAmp structuring is just plain weird >:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Mon, 08 Feb 2016 16:24:26 -0600, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>LTSpice files not included because -- it's files, not file. > >So, I have a fairly complicated circuit in LTSpice. I'm playing some >tricks on an LT3757A -- basically I need an externally settable voltage >output, so I'm hijacking the FBX line and driving it from an op-amp >summing circuit. Everything was fine until I put that op-amp in there, >but I suspect that I've just made the circuit too damned complicated, or >too weird from LT's point of view. > >The simulation is not quite hanging, but it's getting stuck in a state >where it's progressing at tens of femptosecond/second, on a fairly speedy >machine. > >So my question is -- is there anything I can jigger in the various >tolerances or other "don't touch this unless you know what you're doing >and/or don't mind your simulation being meaningless" settings? > >At the moment I've managed to make it work much better by removing some >circuitry and replacing it with a fixed supply -- but it would be nice if >I knew some more general ways of making all well, and as things stand >I've just blown my overall efficiency calculations out of the water. > >TIA.
Control the feedback like this... <http://www.analog-innovations.com/Wescott_Adj_Switcher_Control_Method_2016-02-09_09-30-45.png> Keeps the OpAmp out of the feedback path. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Tue, 09 Feb 2016 08:36:41 -0700, Jim Thompson wrote:

> > Your added OpAmp structuring is just plain weird >:-} > > ...Jim Thompson
It comes from doing software for years instead of analog circuits. Lines of code don't cost much, which puts me in the habit of adding stuff to tweak behavior. It turns out that if I turn the command voltage on _after_ all the power supplies have stabilized then I don't need all the extra crap around the op-amp to keep the thing from overshooting. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com