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TPS23753A: Vout drop when Power source change to POE

Gary Teng
Genius 5900 points
Part Number: TPS23753A
Other Parts Discussed in Thread: TPS23753, , TPS2372, TPS2373

Hi Sir,

 Customer is testing TPS23753's power source changing function. They set up their board by following application note:

At their testing, when plugging POE source, there's a Vout drop.

Ch1: Vout 5V,

Ch2: VDD-VSS

Ch1: GATE

Ch2: VC

Ch3: Vout 5V

Using option2 at following link has similar result.

https://www.ti.com/lit/an/slva306a/slva306a.pdf

 Customer want to confirm is the output drop is always happened? Or how to modify TPS23753A circuit to prevent the voltage drop? Any other chip can change power source from adaptor to POE smoothly? Thanks for your kindly help.

Best regards,

Gary Teng

  • 0
    TI__Genius 12257 points

    Hello Gary,

    The voltage drop may occur when the MPS is not satisfied.
    They could try implementing the following circuit,
    with the D_p diode & series resistor may help to provide the needed current to satisfy MPS:


    Section 3.6 of the document you linked can provide additional information on this.

    This smooth transition is easier done with devices that have an automatic MPS feature and in-rush delay like TPS2372,
    but can be achieved manually with this TPS23754A as well.

    Hope this helps!
    Best,
    Sarah

  • 0 in reply to Sarah Lao
    Prodigy 40 points

    Hi Sarah,

    slva306a.pdf data sheet

    Option 2 Limitations and 3.2 Option 2, PoE Preference

    #PoE preference will cause a restart as PoE takes over

    pls confirm the TP23753A POE Preference , Is there a restart mechanism?

    k.c

  • 0 in reply to k chen
    TI__Genius 12257 points

    Hi K.C.,
    Yes, the restart can occur when switching to PoE for Option 2, PoE preference.

    Best,
    Sarah

  • 0 in reply to Sarah Lao
    Genius 5900 points

    Hi Sarach,

    KC had ever test adding Dp and 1.5K resistor at their test circuit. We see the gate stop its switching at GATE (that means PD restart?) and output is drop (adaptor can't take over as power source...?).

    1. Is there other solution to keep the translation smoothly after adding the MPS circuit? KC didn't remove adaptor power currently.. He just want to translate power source from adaptor to POE smoothly. Or there's always a restart if using option 2 circuit?

    2. Any other solution can swith POE/Adaptor power smoothly? TPS2373 or?

    https://e2e.ti.com/blogs_/b/powerhouse/posts/delivering-a-smooth-transition-from-ac-to-poe-power

    Thanks for your response.

    Regards,

    Gary

  • 0 in reply to Sarah Lao
    Prodigy 40 points

    Hi Sarah,

    Thank very mach ,Any other IC solution support  PoE preference?.

    condition:not restart and adapter +12v input and PoE preference

    k.c

  • 0 in reply to k chen
    TI__Genius 12257 points

    Hi Gary and KC,

    Sorry, yes there will always be a restart for the option 2 circuit with PoE priority, if the V_POE < V_ADAPTER.

    May I know the voltage of your PoE?
    If you want to achieve the seamless transition, then the voltage of your PSE may need to be higher than 12V .

    Currently it is not possible to have a seamless solution this way.

    Thanks, 
    Sarah

  • 0 in reply to Sarah Lao
    Prodigy 40 points

    Hi Sarah,

    V_POE ---> 48V+-20% ,V_ADAPTER.---> 12V+-20%

    Thank-K.C

  • 0 in reply to k chen
    TI__Genius 12257 points

    Hi KC and Gary,
    The problem with the option 2 layout is there are many opportunities for the PoE to shut off.
    To possibly avoid this, you would need the hotswap fet always on, since turning on the internal hotswap will make it starts from inrush mode.
    In inrush mode, the PWM CTL can be stopped to cause Vout drop. Also need to     implement a high side FET for poe priority,
    and then need to limit the current on this FET, which can be very difficult.

    In this case, it is advised to instead use option 3, where you have a better chance of achieving the seamless transition.
    But you will need to lower V_adapter to satisfy V_adapter ≤ V_out if you want to keep PoE priority.

    Best, 
    Sarah

  • 0 in reply to Sarah Lao
    Prodigy 40 points

    Hi Sarah,

    Thank very mach

    use option 3 is ok ,I do not have any problem at the time being, if there is any i will ask again.

    k.c