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LM25149-Q1: LO output of controller behaves strangely when grounding bootstrap circuit

Rauf Pere
Prodigy 20 points
Part Number: LM25149-Q1
Other Parts Discussed in Thread: LMG1205, LM25149,

Tool/software:

I want to simulate a design with LM25149-Q1 using PSpice for TI. I start of using the project provided with the spice model of LM25149-Q1. For my simulation I want to follow the LM25149 controller with a GaN FET driver LMG1205. For this I do not need the bootstrap circuitry of the controller. Ho should be around 4-5V when pulled high. I've been able to do this by grounding SW pin. When I do this however the LO output of the controller becomes very strange. It is a pulsed signal with very thin pulses. See images attached. Of course I changed the high side mosfet for a voltage controlled switch, the HO can't drive the mosfet without bootstrap. My idea is to ultimately have the GaN driver here in between. But the pulses get messed up when grounding SW. Any assistance is highly appreciated.

Kind regards,
Rauf

  • 0
    TI__Intellectual 1550 points

    Hi Rauf,

    Could you send me the picture of the entire schematic so I can get a better idea of how you configured the device?

    Thanks,

    Dino Maslic

  • 0 in reply to Dino Maslic
    Prodigy 20 points

    Hi Dino,

    Thanks for your response, here is the entire schematic:

  • 0 in reply to Rauf Pere
    TI__Intellectual 1550 points

    Hi Rauf, 

    Grounding SW node can cause some problems since you are constantly having the boot charging circuit try to charge up the CBOOT cap. I don't think you will be able to simulate this properly since there is circuitry that uses CBOOT-SW as its voltage domain. You may want to try a different controller for GaN specific FETs.

    Thanks,

    Dino Maslic

  • 0 in reply to Dino Maslic
    Prodigy 20 points

    Hi Dino,

    I understand. I proceeded by connecting the bootstrap circuitry of the LM25149 as recommended. Then I followed it up with the LMG1205Y GaN driver. I connected two mosfets behind both, so the LO and HO of the controller and driver, as you would in a synchronous buck converter. This works great. But another issue arises when I then try to connect the output inductor to the GaN driver side. The output circuit for just the controller works great but when I add just the inductor to the driver side I get convergence errors. It's strange because I seem to be able to add anything except for the inductor. I've tried auto convergence but that yields a division by zero error. In some other forum I read that the DIODERS setting can help with this. This makes the simulation run but the results are incorrect. This way the simulation also runs too fast which I have learned is usually a bad sign in PSpice. Could you help me with this?

    In the images, the inductor L4 is the problem. Removing this runs the simulation just fine. But any inductor gives errors. I've also provided my full project should you require it.

    Thank you kindly,

    Rauf

    TI-Controller-Driver.zip

  • 0 in reply to Rauf Pere
    TI__Intellectual 1550 points

    Hi Rauf,

    Not too sure on how to fix the convergence problem, let me get back to you after conferring with my team.

    Thanks,

    Dino Maslic