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TPS23754: Wrong PoE classification TPS23754 with 802.3at GLOBTEK GT-96300-3656-T3-AP PoE injector

user5877645
Prodigy 20 points
Part Number: TPS23754

Hello,

I have a problem with the 2 event classification of the TPS23754 in combination with the PoE injector GT-96300-3656-T3-AP from Globtek. The injector is a 802.3at injector
but our device which contains the TPS23754 is detected as a 802.3af device.  The indicator LED which is connected to Pin 20 is not illuminated. All other tested power supplies work properly together with our device. And the GT-96300-3656-T3-AP
works properly together with some other tested 802.3at devices.  Threre is some kind of ripple at a frequency of
approximately 3.9kHz on the first classification event. The second one is missing. The evalutation module TPS23754EVM-383 does also not work together with the mentioned poe injector.

The oscillogram of the failed classification is as follows.

Normally the oscillogram should look like this

This is a part of the circuitry around the TPS23754 in our design.

Thanks for any help!

  • 0
    TI__Genius 9230 points

    Hi User,

    The voltage being sent to the PD is generated by the PSE. You have the correct class resistor on the PD side, so I would look closer at the configuration of your injector. It is the injector's job to send the 2nd class voltage waveform if it sense the correct class signature current being output from the CLS pin. To verify that your PD is showing the correct class current, you can apply 18V to VDD-VSS & measure the input current using an ammeter. The class current ranges are shown in section 6.6 of the TPS23754 datasheet.

    If you are measuring the correct class current, then the PSE may either be configured wrong or may be non-complaint to the IEEE802.3at standard. To check your design, feel free to order one of our PSE EVMs to test with a PSE known to be compliant.

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer