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TPS2376: TPS2376PWR

Samuel Freal
Prodigy 30 points
Part Number: TPS2376

Tool/software:

Hi,

I'm designing a product on the PD controller side, I want to use the TPS2376 IC.

The product has two objectives :

-> it has to be compliant with the 802.3af standard when it will be connected to a device with a PSE.

-> it can also be connected with a device that is not PoE compliant and this device will provide 22.4V through the correct pins of an Ethernet cable.

In the datasheet of the TPS2376, there is a sentence saying :  "In order to provide flexibility for noncompliant designs, the TPS2376 allows the designer to program the turnon thresholds with a resistor divider. [...] To use the TPS2376, connect a resistor divider between VDD and VSS with the tap connected to the UVLO pin. The total divider resistance appears in parallel with the R(DET), and the combination of the two should equal 24.9 kΩ. The divider ratio should be chosen to obtain 2.5 V at the UVLO pin when V(VDD) is at the desired turnon voltage."

For the non compliant PoE version, the question is : how to choose the correct resistances on the UVLO and the DET pins in order to work with an input voltage at 22.4V ?

For the compliant version, the question is : can we let the UVLO pin floating and put a 24.9kΩ resistance on the DET pin ?

Thank you for your help

  • 0
    TI__Mastermind 37121 points

    Hi Samuel,

    Thanks for reaching out!

    - For the non compliant PoE version, the question is : how to choose the correct resistances on the UVLO and the DET pins in order to work with an input voltage at 22.4V ?

    The UVLO resistance should be significantly higher than the 24.9kΩ. So, you can use, for example, 130k + 1M combination. It is also needed to put a ~4.2V Zener diode in parallel with the 130k resistor to avoid UVLO reach 5V rating.  

    - For the compliant version, the question is : can we let the UVLO pin floating and put a 24.9kΩ resistance on the DET pin ?

    According to the blocking diagram, the UVLO pin needs to connect to a resistor divider to provide the UVLO comparator a valid input voltage. It is not recommended to float it.

    You could also consider to design a product can work with both a 802.3af compliant PSE and a 22.4V passive injector if you use the lower UVLO value. Though it cannot meet the 802.3af's 30V turn off voltage.  

    Best regards,

    Diang

  • +1 in reply to Diang Xing
    Prodigy 30 points

    Hi Diang Xing,

    Thank you for your quick and precise answer.

    Just to be clear and after some calculations from my side : 

     - For the non compliant PoE, we can use on the UVLO pin a 100k with a 18k resistor in order to have 3.4V and be above the upper threshold of 2.5V. Like this, we skip the detection and classification phases and what is after the PD controller will receive 22.4V continuously.

     - For the compliant PoE, we can use on the UVLO pin a 620k with a 40k resistor in order to have 1.81V at Vin=30V (and be under the threshold of 1.87V) and at Vin=42V we have 2.54V (and be above the 2.49V threshold), like this we respect the turn off and turn on phases of figure 18 and we are well below the threshold during the detection and classification phases. And, in order to draw to correct amount of current during the detection phase, we put a 26k resistor on DET pin, like that, 620k+40k in parallel with 26k is 25k and we stay in the range of 23.75k<Rdet<26.25k in order to draw the correct current and be compliant.

    Am I correct ?

    Thank you for your help 

  • 0 in reply to Samuel Freal
    TI__Mastermind 37121 points

    Hi Samuel,

    Thanks for your reply.

    I will take a look on your question and reply you in 5 business days.

    Best regards,

    Diang

  • +1 in reply to Diang Xing
    TI__Mastermind 37121 points

    Hi Samuel,

    Thanks for your reply.

    The non-compliant one looks good to me.

    For the compliant one, you can make the bottom resistor slightly higher (like 41k) to make sure at VIN = 42 V you have higher than 2.57V at UVLO. 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 30 points

    Hi Diang,

    Thanks for having taking the time to answer me.

    Yes you're right I have to make the bottom resistor a little bit higher to be above the upper threshold at 2.57V at UVLO.

    Best regards

    Samuel