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Original question:

LM25037: Oscillator Frequency produced by PSpice model is 1/2 specifications in data sheet

LM25037: PSpice output toggle not reliable with 2MHz oscillator

David Fiori, Jr.
Prodigy 150 points
Part Number: LM25037
Other Parts Discussed in Thread: TINA-TI

1. TI just updated the PSpice LM25037 model so that it's oscillator would perform in accordance with the data sheet; it now runs at 2MHz and the correspondence between programming resistors was just recently corrected.

But while the oscillator as evidenced by the ramp output now runs at 2MHz, the toggle flip flop part of the LM25037 model is not working reliably.

I have found that capacitors used in the toggle flip flop model are too large. While I was able to make the model work by reducing the value of these capacitors. Changing the 1nF capacitors at nodes cqint, cdummy1, and cdummy2 from 1nF to 100pF fixed the problem. However, I do not know to what extent this affects the accuracy of the modeling. It would seem possible, for example, that the propagation time between the oscillator ramp and the output toggling might be effected. It is also possible that failure to operate properly at low voltages might also be effecting giving the false impression the pat works well at voltages lower than is described in the data sheet.

Please let me know what value these capacitors should be if leaving them at 100pF is a problem.

2. I have noticed that the Tina-TI model does not run at 2MHz oscillator frequency either. I am not using Tina-TI, but I would think TI would want that model to conform to it's data sheet too. I tried the Tina-TI model to see if it had the same toggle defect but found I could not get the oscillator to run at 2MHz either.

Thank you very much,

David Fiori, Jr. 

215-584-2214 (cell)

  • 0
    TI__Mastermind 31580 points

    Hi David,

    Thanks for bring these issue on the LM25037 models to out attention.

    I have asked one of our applications engineers to look at this, models are intended to be functional equivalents to the controllers but are obviously not as complex and simulate at higher level to speed up simulation times.

    I don't understand why the flip flop would require 1nF cap value in the simulation lets see what we find out.

    Regards

    Peter

  • 0
    TI__Expert 6080 points

    Hello David,

    I will try to look at the TINA-TI model to replicate your issue. I will update you within 2-3 days.

    Regards,

    Sonal

  • 0 in reply to Peter Meaney
    Prodigy 150 points

    Please understand the problem is not about simulating at a higher level of accuracy. It is about operating at all. 

    Please understand the the problem at hand is that the capacitor used in the model of the the toggle flip flop is too large. It is currently 1nF. It requires a value lower than 1nF to work properly. With 100pF capacitors instead of 1nF values, the toggle function works properly with a 2MHz oscillator/toggle rate.

    Thank you very much for your attention to this matter.

    Sincerely Yours,

    David Fiori, Jr.

    215-584-2214 (cell)