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LM5175: Is the gate resister of external FET not mandatory?

Kazuya Nakai59
Guru 20620 points
Part Number: LM5175

 Hello guys,

 One of my customers is designing a schematic of their own board for LM5175 evaluation.

 They have a few questions as the follows. Could you please give me your reply for them?

 Q1. Any gate resister is not used on the schematic of Figure 24 in 9.2 typical application section on page 20 of LM5175 datasheet (Version SNVSA37A).

       I think that high current will be sunk or sourced to gate driver of LM5175 at H/L transient timing.

       Is it no problem if switching noise level is lower than our criteria? Is the gate resister of external FET not mandatory?

 Q2. If the gate resister is mandatory, what is a appropriate gate resistor value? Is it needed to adjust the resistor value with cut and try on the own board?

 Best regards,

 Kazuya.

  • 0
    TI__Guru 60245 points

    Hello Kazuya,

    Q1: Gate resistors are not mandatory but good design practice. The device operates well without gate resistors and the converter as well, but EMI and parasitics might cause issues, so without gate resistors the layout is in any converter even more critical.

    Q2: They are not mandatory and only dependent on the system needs, e.g. EMI.

    BTW, please add 7.8V or 8.2V Zener diodes from the switch nodes to the BOOTx pins as these are not allowed to go more than 8.5V above the switch nodes.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Guru 20620 points

    Hello Brigitte,

    Thank you very much for your reply.

    The customer questions were solved because of your support.

    Thank you again and best regards,

    Kazuya.

  • 0 in reply to Kazuya Nakai59
    Guru 20620 points

    Sorry. I forgot to check the check box.

    Thank you and best regards,

    Kazuya.