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TINA/Spice/OPA237: OPA237/Oscillation during transition

Kaveh Mohammad
Intellectual 585 points
Part Number: OPA237
Other Parts Discussed in Thread: TINA-TI, , LT1013, LMC6001, TLV2262AM

Tool/software: TINA-TI or Spice Models

My circuit gets unstable when Pin 3 of the OPAMP OPA237 make a step from 3V to 0 V. See the attachment.

I am trying to justify using simulations. So I added L1, C4, and Vin to plot overall loop gain.

I have plotted Vo (loop gain) when VS3 (voltage on pin 3 of OPA237) is 3 V and when VS3 is 0 V.

This does not get me anywhere because when VS3 is 3V T1 transistor is saturated and when VS3 is 0V T1 transistor is off. In both cases VO is close to 0 (-50db).

The actual oscillation happens when T1 is coming off of the saturation (VS3=3) and enters the active region before it enters off mode (VS3=0).

I do not know how to simulate and justify VO loop gain phase margin becomes 0 when this transition happens.

Please advise.

Thanks

  • 0
    TI__Intellectual 820 points

    Hi ,

    Please attach hereby and share your TINA circuit schematic (TSC file) in proper set-up, where you are facing instability issue, so that we can look into it.

    Standalone OPA237 wise I don't see any instability issues [see attached] and hence issue can be coming in from rest of the loop elements/configuration.e2e_237.TSCe2e_237_2.TSC

  • 0 in reply to Ahana Ghosh
    Intellectual 585 points

    In practice we use LT1013 instead of OPA237 but because the model for LT1013 is old previously someone in LT1013 forum suggested to use the model for a similar OPAMP such as TLV2262AM, OPA237, LMC6001 for simulation.

    This circuit oscillates at -45 degree C. The oscillation is captured at the negative pin and output of the OPAMP.  

    Simulation Explanation.TSC

  • 0 in reply to Kaveh Mohammad
    Intellectual 585 points
    The problem here is that, in practice we measure oscillation at -45 but I can not simulate it not because of temperature.
    As I said, the oscillation happens when pin 3 of OPAMP make step transition from 3V to 0V. When it is 3V T1 is saturated so overall loop gain simulated is almost 0. When it is 0 V T1 is off and again overall loop gain is 0. I am going to find out why the overall loop gain becomes -1 during the transition.
  • 0 in reply to Kaveh Mohammad
    TI__Intellectual 820 points

    Hi,

    1. In the circuit you had shared as attachment, res R2 has didn't have a proper connection with Cap C1 and op-amp output(vo).

    2. Temperature for any required simulation in TINA can be set in from Analysis --> Set Analysis Parameters

    [Type in sim temperature and click ok]

    3. Thirdly I applied a 3 to 0 V transition step in the Pin3 (+ pin) of the op-amp as you have mentioned, in the given circuit after ensuring point 1 & 2. I didn't observe any oscillation at the feedback or Vo node due to transition.

    Please see schematic and result.

    Lastly to avoid any kind of tool issue, I would suggest you to use the latest version of TINA-TI [9.3.150.328 SF-TI] for model simulations.e2e_8mar.tsc

  • 0 in reply to Ahana Ghosh
    Intellectual 585 points

    I am using v9.3.200.277 SF-TI from

    My point is this circuit oscillates in practice at -45 but I can not see any sign of oscillation in simulation.

    Besides, when I use LT1013, VO (OPAMP) settles about 1 V eventually but in practice it settles on 0 V and this is when oscillation happens. With OPA237 VO (OPAMP) settles on 0. Is this because the model for LT1013 is old?

  • 0 in reply to Kaveh Mohammad
    TI__Intellectual 820 points

    Hi,

    Understood your problem now.

    Yes, LT1013 is a much old model and can have problems.

    Regarding the problem you are saying, I am not exactly sure how closely our models replicate the temperature dependency of all major parameters which can also lead to difference in behavior from real circuit. Because models are usually, mainly for accurate Typical (TT) corner.

    We will get back to you on this.

    Also I will request you to ensure that the real circuit you are working on is correct.

  • 0 in reply to Ahana Ghosh
    TI__Genius 9775 points
    Hi Kaveh,

    FYI, Generally speaking, we don't model variation vs. temperature. This is especially true for loop stability analysis. Occasionally for certain parameters we will, but this will be rare. For instance in the case of OPA237, if you open the reference design (.TSC) on its TI product folder, it will show on the right side of the screen the text description of which parameters are modeled vs. temperature. In this case, Input offset voltage vs. temp is modeled, but majority of the other parameters do not have any temperature variation modeled in it.

    Herman
  • 0 in reply to Herman Theodorus
    Intellectual 585 points

    How can I open OPA ref design on TI product folder to see which paramters are modelled? Can you explain it again in detail?

    Thanks

  • 0 in reply to Kaveh Mohammad
    TI__Intellectual 820 points

    Hi,

    The parameters which are captured are mentioned here in the ref design,

    Whatever parameters are captured w.r.t temperature will also be mentioned here in description.

  • 0 in reply to Ahana Ghosh
    Intellectual 585 points

    How can I open a component ref des in TINA to find out what parameters are modeled?

  • 0 in reply to Kaveh Mohammad
    TI__Intellectual 820 points

    Hi,

    You can go to the following product folder link,

    Then click the following(shown in image), it will download automatically.

    Open the downloaded TSC file in TI NA-TI to view as I have shown in the last message.

  • 0 in reply to Ahana Ghosh
    Intellectual 585 points
    Thanks. Is there a reference design TSC file for LT1013 as well to see what parameters are modelled?
  • 0 in reply to Kaveh Mohammad
    TI__Intellectual 820 points
    Hi,

    In the product folder of LT1013, I can only see the PSpice package, no TINA ref design is there.
    In the net-list file also nothing is mentioned in description. It is much old model from 2002 and hence the release templates might be different.So hard to mention for this one. One needs to simulate and check .