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LTC4269-1 for LLDP protocol


According to LTC4269-1 datasheet,

a Type 2 PD must be able to recognize both layers of communications and identify a Type 2 PSE.

According to IEEE802.3at-2009,

Type 2 PDs implement both 2-Event Physical Layer classification (see and Data Link Layer
classification (see 33.6) and advertise a 2-Event class signature of 4.

Will LTC4269-1 recognize PSE through data layer negotiations ?

Or need change PSE register bit through Physical Layer ?

Thanks a lot !

  • Hi,

    PD controllers such as the LTC4269-1, exist on the physical layer. The ethernet transformer effectively splits the DC voltage used for PoE from the differential data. The DC voltage goes to the PD controller and the differential data goes to the PHY. The LTC4269-1 does not directly interface with the data layer, so a PHY is required for Link Layer Discovery Protocol (LLDP) negotiations.

    Best Regards,


  • Hi Eric,

    The PSE devices may support HW negotiation

    and/or SW negotiation (LLDP/CDP). The PD is required to support both.


    Simple POE+ switches usually only perform the HW negotiation and fix the

    power provisioning at 30W.

    More complex switches, like Cisco 9800 and HP Aruba, show the very

    unfortunate phenomenon they negotiate enough power at switch-on of the display.

    (The displays remain around 15.5W). As soon as the LED’s switch on and the display

    start to run the applications in more power demanding modus, the power request is

    of course increasing. Due to the missing LLDP protocol, the display is asking for more

    current than is negotiated and that results in an error message on the switch and

    (in most cases) rebooting displays. Some switches remain functional: they simply

    give an error message, but keep on delivering current, but the more advanced

    switches just close the power provisioning.  

    (Qus) In other word, due to that LTC4269-1 exist on the physical layer,

    hence, it can only communicate with POE+ switches by HW negotiation?


    Our customer challenge us that The IEEE 802.3at standard demands the PD

    to support LLPD (/CDP), but following Analog’s explanation as above,

    LTC4269-1 only exist on the physical layer, hence, it won’t receive/recognize/

    even drop the datalink layer/power provisioning packet sent from POE+ switches ?

    Or this datalink layer/power provisioning should be handled

    by Android OS instead (If the powered device happens to be an Android tablet) ?