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LM2673: GM1 and GM2 gains

Mark Pieper
Expert 4660 points
Part Number: LM2673


Hi,

I need to either analyze or simulate the Bode plot to address a possible stability issue. For analysis I would need the transconductances of the two internal amplifiers. I don't see that info in the DS. If there is an average model, or some other way to simulate the frequency response, please let me know.

Thanks,

Mark

  • 0
    TI__Guru* 78675 points

    Hi Mark,

    I can find GM1=6.73E-4, or 673uS.  However, GM2 does not seem to be a constant but a function of the Gain Compensation block.  I am not sure if the info is readily available given the fact that this device was released >20 years ago. I will check with our design team, though, and get back to you ASAP.  

    The engineers who developed this device gave detailed design guidelines to maintain the circuit stability in Section 8.2 of the datasheet, and hopefully the info would be good enough to help the customers in their designs.  

    Thanks,

    Youhao

  • +1 in reply to Youhao Xi
    TI__Guru* 78675 points

    Hi Mark,

    Sorry I have checked and we cannot find GM2 for analytical use. The definer of the part is using the inductor and capacitor selection tables to confine the design to be stable.  Is it possible to run the sim with step load to verify the stability?

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Expert 4660 points

    Hi Youhao,

    Thank you for looking into this question. We will perform experiments to establish the stability and possibly measure the Bode plot. With that information we can make any needed adjustments.

    - Mark

  • 0 in reply to Mark Pieper
    TI__Expert 4660 points

    Hi,

    We;ve verified good phase margin via measurement.

    To complete this thread, I've examined the PSPICE model created for this device. With the caveat that SPICE models are often an approximation of the actual function, this is what the model uses for GM2 gain:

    GM2 gain is described by a piecewise-linear function as follows:

    Vin(V), G(A/V)

    10V, 1.48mS

    20V, 673uS

    40V, 480uS

    So as an example, the gain for 15Vin would be 1.077mS, calculated by interpolation.

  • +1 in reply to Mark Pieper
    TI__Guru* 78675 points

    Hi Mark,

    Thank you for the updates.  This is good info and hopefully the other customer can benefit from this. 

    May I assume the issue is closed so I can close the thread here?  You can always reopen it by adding a new post.

    Best Regards,

    Youhao