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LM5121: gate resistor for LO

Guru 29720 points
Part Number: LM5121
Other Parts Discussed in Thread: CSD18543Q3A, LM5122

Hi Team,

My customer saw overshoot and oscillation at LO pin so considers to put gate resistor between LO pin and Nch FET(CSD18543Q3A).
He taked into account the following guidelines however I heard the issue couldn't be avoided.
>LO and PGND should be connected to the gate and source of the low-side N-channel MOSFET through short and low inductance paths.
 
Could you tell me how to adjust the gate resistor value?



Best Regards,
Yaita

  • Hi Yaita,

    Can you please post a screen capture of the overshoot and oscillation of the LO pin?

    Adding the gate resistor in series with the LO pin and gate of the MOSFET will increase the turn-on and turn-off time of the MOSFET. This will decease the effective dead time of the LM5122. The way to get around this is to add a diode in parallel to the gate resistor with the ANODE on the gate of the MOSFET and the Cathode on the LO pin. This will increase the turn on time but keep the turn off time fast, helping to ensure there is enough dead time.

    Typically the the gate resistance is 1ohm to 10ohm. This can be changed depending on the needs of the application.

    -Garrett
  • Hi Garrett-san,

    Thank you for your support.

    I attached waveform of LO pin.
    I heard the surge voltage is proportional to outout current and 10ohm gate resistor couldn't improve the surge.
    *The folowing capture is at full load (3.2A).




    Best Regards,
    Yaita

  • May I have your comments from waveform above?

    Best Regards,
    Yaita

  • Hi Yaita-san,

    It seems you are measuring at the LO not at the MOSFET gate. It appears your layout may have large trace inductrance along the gate drive signal path, and the 10 Ohm gate resistor should help reduce the MOSFET gate to source voltage ringing. The MOSFET gate-source matters.

    Thanks,
    Youhao Xi, Applications Engineering