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Original question:

TPS23755: PoE transformer - alternative part required

TPS23755: TPS23555

Johann Teufelhart MSc
Prodigy 20 points
Part Number: TPS23755
Other Parts Discussed in Thread: TIDA-010034

Hi technical support team

I've a further question belonging the design with the TPS23755

The design guide: TIDA-010034 explains well the steps required for the component selection.

The according schmatic design "TIDA-010034 from 2/3/2019 on page 3 depicts an option for connecting components sot the supply pin VDD.

The optional components - diode and capacitor - are suggested for Low Temperature Conditions fro applications workting near -10°C and less.

My design has to operate properly until near -30°C. So as I understand the explanation written in the TPS23755 datasheet (chapter 8.2.2.20 - Shutdown at Low Temperatures, DVDD and CVDD) these two components are required to eliminate the extra switching cycles during removal of the PoE input.

My questions:

  • What will happen without these two components during removal of the PoE input?
  • How is the behavior of the design affected without these two components during shutdown at low temperature?
  • You only write in the datasheet that "It is acceptable for most applications" but as I understand there can not be a guaranteed monotonic shutdown of the output voltage.
  • Is it possible that the output voltage of the flyback converter rises once again during the sloping condition?
  • Is it correct that in case when I populate these two components (diode and capacitor) for the VDD pin that the capacitor is connected with PGND and not with VSS?

Best regards

Hans

  • 0
    TI__Mastermind 19181 points

    Hi Hans,

    Thanks for reaching out.

    Sorry that the PoE team has traveling and multiple works in list recently. Please expect a delay on the E2E reply. 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 19181 points

    Hi Hans,

    Thanks for your patience. Please find the comments below and hope they help:

    - At -10C or lower, if without D_VDD and C_VDD, the PWM controller may stop switching when PoE shut down and then switching again at a low VDD/VPD voltage. It may cause Vout drop first then rise again for a while (not monotonic shut down). 

    - It may cause Vout drop first then rise again for a while during such shut down. It may influence the load side. 

    - If you see a case that the the monotonic shut down can not be realized by these D_VDD and C_VDD, please let us know the condition. 

    - Yes, Vout may rise again during low temperature shut down if without D_VDD and C_VDD.

    - Yes. It is correct to have C_VDD referring to the PGND.

    Please let me know if you have further questions.

    Best regards,

    Diang