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Original question:

TPS23861: Design of PoE Injector

TPS23861: PSE Design Review and few queries

Kapil K
Prodigy 190 points
Part Number: TPS23861
Other Parts Discussed in Thread: CSD19538Q3A

Tool/software:

Hello TI Forum Team,

I have created a schematic of PSE controller. It would be highly appreciable if you could review the design. Along with that I have a few queries. 

...

Queries: 

  1. I have added D14 diode along with F2 fuse for protection. Is it fine? Or No need of this D14 diode?
  2. May I know the Threshold voltage of Gate Pin (1-4)?
  3. Can I use this gate pin for LED logic implementation? Making RJ45 Link led terminal to ground.

  • 0
    TI__Mastermind 37121 points

    Hi Kapil,

    Thanks for reaching out!

    1) Please short D14. The port output voltage control will be influenced by the D14's forward voltage drop.

    2) Please add a RC delay for the /RESET pin. 

    3) It is recommended to have not used DRAIN pins to +48V. 

    4) R29 needs to use 47Ohm

    5) For MOSFET T2, please make sure

    • Vth(gs) min > 2 V
    • Rds (on) : normally 50mohm ~ 150mohm at Vgs =10V
    • Ciss : less then 2000pF
    • Qg : less then 50nc
    • BVdss : larger then 100V

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37121 points

    Hi Kapil,

    1. I have added D14 diode along with F2 fuse for protection. Is it fine? Or No need of this D14 diode?

    Please do not use D14 as reason of last post.


    2. May I know the Threshold voltage of Gate Pin (1-4)?

    Gate voltage driver can output 10 -12.5V when fully turn on. Recommended to use NMOS with min Vgs(th) > 2V, typ Vgs(th) > 2.5V 


    3. Can I use this gate pin for LED logic implementation? Making RJ45 Link led terminal to ground.

    It is not recommended, since the TPS23861 has current limiting and foldback protection which will reduce the Vgs when overcurrent. Adding additional load may influence them. Or you need to make the LED current very low.

    Best regards,

    Diang 

  • 0 in reply to Diang Xing
    Prodigy 190 points

    Hi Diang,

    Thanks for the review comments.

    I have few questions for your comments.

    3) It is recommended to have not used DRAIN pins to +48V. 

    Do I understand correctly that I need to connect the unused drain to 48V? What will happen if I connect to the ground?

    Can I keep these pins to float without being connected (Not connected)?

    4) R29 needs to use 47 Ohm

    What will be impact if I keep 49.9-ohm resistor in place of 47 ohm?

    5) For MOSFET T2, please make sure

    I have selected a few parts for T2 MOSFET

    1. BSC070N10NS5
    2. FDMS86182

            What will happen if the (Ciss : less then 2000pF) value will be more than 2000pF?

    Regards

    Kapil

  • 0 in reply to Diang Xing
    Prodigy 190 points

    Hi Diang,

    Thanks for your quick response for my queries. 

    3) As I mentioned that I use this gate pin for LED logic implementation. Making RJ45 Link led terminal to ground. So This PSE gate pin is used to connect led terminal to GND. The LED is powering from 3.3V not from this Gate terminal. So I am using 2 Link LEDs for ground connection. So current consumption will be very less.

    What is your comment? 

  • 0 in reply to Kapil K
    TI__Mastermind 37121 points

    Hi Kapil,

    It is not common to use gate control both MOSFET and control LED flashing.

    As well as you only draw very low current from gate to control LED. And it does not cause foldback/current limiting failure, it should be fine to use. 

    Best regards,

    Diang

  • 0 in reply to Kapil K
    Prodigy 190 points

    Hi Diang,

    May I know the range of current values that is safe for PSE Gate terminal for LED drive?
    I have also asked a few queries for the PSE design review. Can you elaborate on those points as well?

  • 0 in reply to Kapil K
    TI__Mastermind 37121 points

    Hi Kapil,

    I am working on validation and will get back to you by next Wednesday. Thanks for patience. 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37121 points

    Hi Kapil,

    Thanks for your patience.

    About LED and gate driving: the gate souring current could be as low as 18uA. Though no test had been tired to use this pin to drive a LED, it may work if you use 2uA or less current from the gate pin.

    For your other questions:

    3) Just checked the datasheet. Drain can be set open to tie to AGND when not used. Sorry for wrong comment. 

    4) The resistance may be related to the impedance match for voltage measurement. 47 Ohm +-5% is needed for better voltage sensing. 49.9 Ohm may cause voltage sensing error, but should not be very large error. 

    5) FDMS86182 and BSC070N10NS5 are not recommended. They have too low on-resistance (<10 mOhm) and it could become hard for foldback current limiting. Too high Ciss could cause foldback protection response too slow (may be slower than standard requirement).

    CSD19538Q3A is normally recommended for this application. Or other MOSFET with below conditions:

    • Vth(gs) min > 2 V
    • Rds (on) : normally 50mohm ~ 150mohm at Vgs =10V
    • Ciss : less then 2000pF
    • Qg : less then 50nc
    • BVdss : larger then 100V

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 190 points

    Hi Diang,

    Thank you so much for the clarification and valuable inputs. 

  • 0 in reply to Kapil K
    TI__Mastermind 37121 points

    Hi Kapil,

    No problem! I will close this thread for now. Please reply or open a new thread if you have further questions.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 190 points

    Hi Diang,

    Since TPS23861 has 4 gate terminals, I am using one of them for the PSE PoE port. What will happen if I short all the gate terminals or if I use the unused pair for LED driving, which means using Gate 2 and Gate 3 terminals for LED driving.

    Regards

    Kapil

  • 0 in reply to Kapil K
    TI__Mastermind 37121 points

    Hi Kapil,

    You need to floating the unused gate in most of cases. The unused gate may keep output low since there is no valid PSE channels. You may consider to use a voltage derived current booster IC, then use the gate as input and current booster IC to output an enough current for the LED.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 190 points

    Hi Diang,

    Things are clear to me now for PSE gate pint. As you mentioned about voltage derived current booster IC, any reference or part suggestion in your mind?

  • 0 in reply to Kapil K
    TI__Mastermind 37121 points

    Hi Kapil,

    PoE team is doing internal tests and the reply on E2E may be delayed for 3 - 5 business days. Sorry for the delay and thanks for your patience.

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    TI__Mastermind 37121 points

    Hi Kapil,

    Thanks for your patience. 

    The current booster could be a push-pull structure. You may find some if you search "gate driver current booster". We may want one with very low input current and sufficient output current to drive the LED. 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 190 points

    Hi Diang,

    Noted.

  • 0 in reply to Kapil K
    TI__Mastermind 37121 points

    Hi Kapil,

    No problem! I will turn off this thread of now. If you have further questions please reply or open a new thread.

    Best regards,

    Diang