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LM5161-Q1: Distortion on SW node waveform

tateo yamashiro
Guru 10890 points
Part Number: LM5161-Q1

Hi,

We got a question from the customer about LM5161-Q1.
Could you help us?

[Question]
They found the distortion at the rising edge of SW node waveform. Please refer to attached file for the detail. We would appreciate it if you could give us your feedback.

LM5161Q1 SW node waveform_03DEC18.pptx

Best Regards,
tateo

  • 0
    TI__Mastermind 25590 points
    Yamashiro-san,

    The slides are correct. This initial wave is due to the reverse recovery of the diode.

    -Sam
  • 0 in reply to Samuel Jaffe
    Guru 10890 points

    Hi, Thank you for your quick support. Let me confirm one thing. The waveform of distortion varies due to the input voltage. I assume this reason is diode capacitance varying due to the input voltage. Is this my understanding correct?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Mastermind 25590 points
    Yamashiro-san,

    I'd like to revise my previous comment. Reverse recovery can cause waveforms similar to this in similar situations but after review I believe I was wrong in my previous statement.

    The team and I believe the part of the waveform marked "1" at the beginning of the switch turn-on is caused by negative inductor current ringing with the SW node capacitance. The low-side FET turns off and the inductor current is negative since this is a fly-buck with a load. This negative current charges the FET capacitances causing this ringing until the high-side FET turns on and pulls the SW node high.

    This can be confirmed with a plot showing the SW node and the inductor current at the rising edge of SW.

    -Sam
  • 0 in reply to Samuel Jaffe
    Guru 10890 points

    Hi, Thank you for your support. I understood your comment.

    The waveform of distortion varies due to the input voltage. I assume this reason is Coss of low side FET variying due to the input voltage. Is this my understanding correct?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Mastermind 25590 points
    Tateo,

    I believe these variations would be more due to the difference in current. A higher VIN would create a higher ripple which would give a more negative inductor current at this condition. That would change the amplitude of the waveform. Other reasons could further explain in more detail exactly what's going on but the conclusion is that this is normal operation.

    -Sam