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LM5116: Phenomenon that the set frequency increases at startup

Kaji@PAN
Mastermind 7795 points
Part Number: LM5116

Hi,

I'm a FAE for distributors of TI products.
I have a question about a LM5116 from a customer.
Attach the startup waveform of PDF file.
The set oscillation frequency is 100kHz, but it operates at around 500kHz in case of abnormality.
In addition, at the time of abnormality, an excessive current that the inductor current exceeds 90 A flows because the ON width is expanded from the middle.
The effect destroys the LM5116 and the FET.
This phenomenon rarely occurs in an environment of 25°C., but frequently occurs at high temperatures.
Could you tell me why the abnormal behavior happens?
Is there any way to improve abnormal operation?
Best regards,
LM5116data.pdf
  • 0
    TI__Mastermind 34555 points

    Hi Kaji,

    Pls download the circuit calculator for LM5116 on TI website, then modify the circuit design according to calculation result.

    If the circuit still has problem, then pls apply LM5116 EVM and modify the EVM circuit to your application circuit and test it again.

    B R

    Andy

  • 0 in reply to Andy Chen
    Mastermind 7795 points
    Thank you for your reply.
    My customers design on WEBENCH and check their behavior in advance with EVM.
    My customers are asking for information on whether this behavior at startup is a bug in the device or whether it can be improved by changing the constants.
    Best regards,
  • 0 in reply to Kaji@PAN
    Mastermind 7795 points
    additional information.
    My customer also measured the phase margin but it was 60 degrees, so I think the stability of the feedback loop is fine.
    The waveform at the time of abnormality is shown below. Are there any conditions that change the set frequency?
    Also, in the overcurrent state, the on-period is lengthened and the current is going to flow further, so this seems to be performing positive feedback.
    If you need a schematic, I can send it to you personally.
    Best regards,
  • 0 in reply to Kaji@PAN
    TI__Guru 73682 points

    Hi Kaji,

    Please upload the schematic and layout for review. In particular, the DEMB pin sets the operation during a pre-bias startup. The voltage is sensed at SW and the trace from SW at the MOSFETs to the controller should be short and direct.

    Regards,

    Tim