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LM5155: RT does not work as configured.

Katsuya Ide
Prodigy 50 points
Part Number: LM5155

Hello,

I am designing a flyback boost circuit.
A resistor is connected to the RT pin and the switching frequency is set to about 250 kHz.
However, early in the operation, there is a period of time when the GATE pin oscillates at about 2.2 MHz.

After about 100ms, the oscillation returns to 250kHz as set and the voltage is boosted to the set voltage.

How can this 2.2MHz operation period be eliminated?

We have confirmed that the behavior of the SS and UVLO pins is the problem.

Sorry, I cannot show you the circuit diagram.

Best regards,

Katsuya

  • 0
    TI__Guru 58641 points

    Hi Katsuya-san,

    thank you for using the E2E forum.

    I understand that the schematic can not be shared but may you can provide some details.

    What do you mean with: "We have confirmed that the behavior of the SS and UVLO pins is the problem."

    Can share some parts of the layout (esp. the area around the RT pin)

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 50 points

    Hi Stefan, thanks for your reply.

    I'm sorry. There was an error in the text.
    I wanted to tell you that  We have confirmed that there is "no" problem with the behavior of SS and UVLO.

    We cannot show the full schematic, but we can show the circuits around the device.

    The waveforms are described below.
    In the early phase of the boost, there are periods when it operates at 250 kHz as set, and other periods when it operates at 2.5 MHz.

    I would like to know what causes this behavior.

    Best regards,

    Katsuya

  • 0 in reply to Katsuya Ide
    TI__Guru 58641 points

    Hi Katsuya,

    currently I have no explanation for what can be the issue on that, because i have never seen this before.

    To further investigate on this it would be good to also see the signal on the RT pin during this conditon.

    It would be also good to check the layout if there is anything which can interfere into the RT input or into the Analog Ground. 

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Guru 58641 points

    Hi Katsuya,

    I have not seen an update on this thread for the last 13 days.

    If the issue is solved it would be great to press the is solved button.

    If there is still something open, just let me know with the new details.

    Best regards,

     Stefan