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TPS2373-4EVM-758: 802.3bt PoE solution using TPS2373-4 and FDMQ8205A

Søren Moldt
Prodigy 10 points
Part Number: TPS2373-4EVM-758
Other Parts Discussed in Thread: TPS2372-4EVM-006, TPS23753

Hi Team,

I'm trying to create a schematic for at hot-swappable PSU that can seamlessly switch between being powered by a battery or PoE (With a preference for the battery)

The design is a bit space constrained so I want to use FDMQ8205A as rectifiers.

So far the rough concept behind the schematic looks like this:

I've looked at the schematic for the TPS2372-4EVM-006 as it came close to what I think could be a solution, but it wouldn't turn off the TPS2373-4 and communicate power requirements to PSE

So therefore I found another solution to my hot-swappable solution.

I found the schematic below in the datasheet for TPS23753, and figured it was the best way to handle turning off the power delivery of the PoE 

To sum it up, my questions are:

  1. What is the purpose of BR_OFF and can the transistor and BR_OFF be left out?
  2. How should the setup be around FDMQ8205A to get the desired result?
  3. Is there a better approach to solving both issues? 

Thanks in advance :-)

Best regards,

Søren M

  • 0
    TI__Mastermind 37081 points

    Hi Søren,

    Thanks for reaching out.

    Before your question, the 12-V adapter may be too low for TPS2373-4 to ensure it can work properly. 

    - If your output voltage is also 12V (or you can add a non-isolated regulator after battery), you can put the adapter on the secondary side to achieve the seamlessly switch (more details can be found in https://www.ti.com/lit/an/slva306a/slva306a.pdf). 

    - You can boost the battery voltage to 24 V - 57 V, then use the APD pin to do the seamlessly switch.

    For your questions:

    1. What is the purpose of BR_OFF and can the transistor and BR_OFF be left out?

    When the adapter is inserted and APD pin is set high, the internal PD hotswap will be turned off to configure the adapter a higher priority than the PoE PD. But in this case, if the adapter voltage is higher than PoE voltage, the adapter voltage could be short to PoE if BR_OFF does not turn off the ideal diode bridge rectifier. 

    2. How should the setup be around FDMQ8205A to get the desired result?

    It should be correct to follow TPS2372-4EVM's schematic. Meanwhile, add the APD circuit based on TPS2373-4EVM.

    Please let us know if you have further concerns or questions.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 10 points

    Hey Diang

    Thanks for the quick and knowledgeable answerGrin

    I have 2 clarifying questions:

    1. Should I use the ADPT from the TPS2373-4EVM schematic as the BR_OFF signal in the TPS2372-4EVM rectifier circuit?
    2. Aren't 12V enough to power the TPS2373-4? I can't find the requirements in the datasheet Sweat smile

    Best Regards,

    Søren

  • 0 in reply to Søren Moldt
    TI__Mastermind 37081 points

    Hi Søren,

    Thanks for your replay. I get busy recently. I will get back to you early next week.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 10 points

    Understandable :-)

    I've stumbled upon another question:

      3. Does the auxiliary power output from  VC_OUT turn off when APD is set high?

    Best regards,

    Søren M

  • 0 in reply to Søren Moldt
    TI__Mastermind 37081 points

    Hi Søren,

    The 3rd question received. Thanks for your understanding  :-)

    Best regards,

    Diang

  • +1 in reply to Diang Xing
    TI__Mastermind 37081 points

    Hi Søren,

    Thanks for your patience. 

    1. Should I use the ADPT from the TPS2373-4EVM schematic as the BR_OFF signal in the TPS2372-4EVM rectifier circuit?

    No, APDT in TPS2373-4EVM is low current one which may not be able to drive the BJT of BR_OFF signal. Besides, APDT could be pulled low by the PN junction in BJT which disable the APD function. It is better to add both APDT signal circuit and BR_OFF signal circuit together. 

    2. Aren't 12V enough to power the TPS2373-4? I can't find the requirements in the datasheet 

    No, 12-V VDD-VSS/RTN is not enough to drive the TPS2373-4. The internal PD circuit normally has ~12.5 V UVLO, and for the adapter at RTN location, you can boost the battery voltage to 24 V - 57 V.  

    3. Does the auxiliary power output from  VC_OUT turn off when APD is set high?

    VC_OUT internal regulator will turn on when PD is trying start up no matter PD's power comes from VDD-RTN (APD is high) or VDD-VSS. Then VC_OUT internal regulator will turn off and VC_OUT will rely on VC_IN voltage. 

    Best regards,

    Diang