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LM25118: Failure modes

Part Number: LM25118

Hello,

My customer design the power supply board using LM25118.

The LM25118 sometimes gets damaged during operation.(output voltage abnormality)

I have some questions about this device.

Q1.When the HS pin exceeds the rated voltage,is there damage to the HB pin?

I will attach the file(Switching waveform) to this post.

LM25118_Switching waveform.pdf

Q2.Regarding the HS pin pattern layout,what should I be careful about?

Best regards,
Seishin

  • Hello Seishin,

    Thanks for asking your question.

    First, is the customer probing the HS pin correctly? Since this signal is high frequency the ground lead of the probe needs to be short. In the post below it is discussed how the high frequency signal should be probed. IF not probed correctly the voltage can appear higher than it actually is.

    e2e.ti.com/.../2243.understanding-measuring-and-reducing-output-voltage-ripple

    For the layout the capacitor between HS and HB should be as close to the LM25118's pins as possible. I also recommend routing the HO and HS traces side be side to the high-side MOSFET. This will help reduce the loop inductance of the high-side gate driver, reducing voltage spikes. The below article looks at how to minimize these voltage spikes.

    www.ti.com/.../slyt682.pdf


    Is it possible that the external MOSFET that was selected doesn't have a large enough voltage rating?

    Thanks,

    Garrett
  • Dear Garrett,
    Thank you very much for the reply.
    I am sorry for the delay.
    My customer uses the following GND spring for measurement of HS pin.

    Question:
    When voltage over rated voltage is accidentally applied to the pin20(HS),
    Is there a possibility of internal short-circuit between pin 16 (VCC) and pin 18 (HB)?

    There is no problem with the measured waveform of the LM25118 EVAL.

    Best regards,
    Seishin

  • Hi Seishin,

    Please refer to the IC block diagram in the datasheet. There is a high voltage diode link between VCC and HB, and sits at a higher voltage than HS owing to the boot strap capacitor. If over voltage appears at HS, most problem the HB is also overstressed, which may damage the internal diode.

    Thanks,
    Youhao Xi, System and Applications Engineering, APP-BMC-BCS, TI
  • Dear Youhao Xi,

    Thank you for your explanation.
    I understood very well.
    My customer is thinking to modify the PCB pattern layout of the current custom board.

    Best regards,
    Seishin
  • Dear Garrett and Youhao Xi,

    May I ask one more question?
    What is the reverse breakdown voltage of the high voltage diode link between VCC and HB?

    Best regards,
    Seishin

  • Hi Seishin,

    The reverse voltage of the diode between VCC and BST should be the greater than the maximum SW voltage. This will be equivalent to the output voltage.

    Thanks,

    Garrett