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LM5104: External gate resistor app note does not mention load

Part Number: LM5104

I'm following the instructions for external gate resistor selection (SLLA385A), and the app note describes a process where oscillations at the gate of the FET are probed to characterize unwanted ringing.

The app note does not say whether this measurement is to be done with the FET loaded with your typical load, or unloaded.

I am currently taking measurements and finding that the ringing is certainly different depending on whether or not the FET is loaded. Ringing is generally worse unloaded (easier to measure!).

How should this measurement be done?

Furthermore, the app note mentions that the gate driver output resistance Rhi and Rlo play a role and are to be considered in calculations as well, but I'm not sure what this would be. Rdson of the gate driver output stage devices?

Even the gate driver used in the app note, UCC5310MC, does not appear to specify any output resistance in its datasheet. What should be used here?

  • Hi Jim,

    Thank you for reaching out! I would recommend taking these measurements when the ringing is worse - with the FETs loaded. Taking this measurement with the FETs loaded will help tune your circuit for better performance when it would normally experience the most adverse ringing.


    Alex Weaver

  • Thanks, I will capture with the FETs loaded. Any suggestions on the gate driver output resistance mentioned in the app note?

  • Hi Jim,

    No problem. I also realized I may have interpreted the question wrong. Ringing is generally worse when FETs are loaded, but if you are experiencing worse ringing with the FETs unloaded, you should design for the worst case.


    The Rhi and Rlo resistors represent the internal resistance during turn on for the high side and low side drivers. This image was taken from another Tech Note discussing power losses in MOSFETs, and I think it might be able to help with your calculations. Here is a link to the article: Link

    Please let me know if you have any other questions, and have a nice weekend!


    Alex Weaver