LM13700 strange simulation problem on LTSpice

[luca_zzz]

Hi Everybody
this is my first thread on this forum. Im looking for some suggestion for understanding why my simulation is going wrong.
Im designing my own synthetizer , and Im using LM13700 by Texas Instrument for designing all the different block.
Im using a Spice Model (downloaded from TI's website) to simulate everything before solding, and the first simulation of a VCA (voltage controlled amplifier) was perfect. For this first simulation I used the schematic from the application note of Datasheet (so im quite sure it'll work). I did the same for the second simulation of a VCO (voltage controlled oscillator triango/square) but in this case anything look like working. I've double checked the second schematic (from http://www.ti.com/lit/ds/symlink/lm13700.pdf datasheet) , what should I do?
Can anyone help me to find the problem?

Here there are the two simulation: the first is perfectly working ( triangular control of the amplification)
in the second case the output remain all the time constant.

why the first is working and the second one don't?

 

[Harald Kapp]

Welcome to electronicspoint, Luca.

Your simulation may be stuck in an equilibrium due to idealized behavior of the components. Try to distort the startup by adding ".IC" statements setting one or a few nodes deliberately off the DC operating point (the voltages they have at t=0 of the simulation).

To help us compare your circuit to the application, please tell us which figure in the datasheet you're trying to reproduce.

 

[luca_zzz]

The figure is number 36 in the PDF file.
I've tried to simulate starding Power supply from 0 V, and looks like working but I've now big problem for simulating because after some usec of simulation appear the error " time step too slow" and some number. What can I do?
Is It due to a wrong simulation option?

 

[Harald Kapp]

You're experiencing a problem of convergence. Here are some tips (including a link to further references).

 

[luca_zzz]

I dont know anything about Spice statement and how to use it.
I think maybe two LM13700 in the same schematic is too much transistor for LT. I will need to make schematic with several of this chip, so maybe I can just solve this problem now but I think this will come back soon. Do you think is possible to simulate a schematic with several of this CHIP without problem with statement? In any case how can I use it?
Is there a more powerfull free simulator which i can try ?

 

[Harald Kapp]

I think maybe two LM13700 in the same schematic is too much transistor for LT

Not at all.

Is there a more powerfull free simulator which i can try ?

None that I know of. They all are SPICE based. On the contrary, Imho LTSPICE is one of the most powerful free simulators. YOu will experience the same problem even with a simple 2-transistor astable multivibrator: since all components are ideally equal (when given the same value), an equilibrium state will develop instead of the expected oscillation - and you have only 2 transistors here.

I know, the LTSPICE help isn't one of the coolest help files, but look at ".IC". You can insert a SPICE directive into a schematic by simply pressing the 'S' key or using the menu EDIT -> SPICE directive.
Use the window that pops up to enter a line like
".IC V(n001)=0V V(n002=5V" etc. (do not enter the apostrophes, though.
n001, n001 are the nodes whose voltages you want to set. Pick for example a node that you expect to show an oscillation, measure the DC-voltage in your non-functioning simulation and add a small DC offset.
Example V(n003) = 2.500V in your simulation -> .IC V(n003)=2.510V
Try this method for a few nodes, one after the other.

You can also use the control panel (menu: Simulate -> Control Panel), go to the tab "SPICE", increase Gmin from 10-12 (default) to 10^-11 or more. Be careful as this may change the simulation result.

 

[luca_zzz]

the .IC statement make all the circuit oscillates and this make really happy!
But i've still problem running all the simulation, I also tried to increase Gmin and the other value but the simulation always stop around some tens of usec.

 

[luca_zzz]

 

[luca_zzz]

 

[luca_zzz]

I've tried this two simulation changing in different way the value, but problem isnt solved

 

[Harald Kapp]

The circuit looks o.k, I don't have any other idea than playing around with parameters.

You may want to try TINA-TI, that's another SPICE based simulator by Texas Instruments. As the LM13700 is a TI component, the model you use may be better fitted to TINA-TI's demands.

By the way, once the simulations runs but stops, what's the error message?

 

[luca_zzz]

always the same errore: "time step too slow" and some number that change. maybe I can decrease number of iteration. How can i set this number?

 

[Harald Kapp]

Use the control panel (menu: Simulate -> Control Panel), go to the tab "SPICE". Play with the parameters, but look at the description of the parameters in the help file and try to understand what they do. Otherwise you end with a running simulation that gives menaingless results due to meaningless parameters.

 

[luca_zzz]

HI! I want to say that im currently using TINA-Ti as simulator and I can say is the best platform for analog simulation i've ever tried,
Thank you for your suggestion! My work with synth desinign is going perfectly , will post some result in the future!!