Post Go back to editing

Can the LT4295, LT4293, LT4275A and LT4276A handle dual-signature in PD applications?

One Ethernet input to the POE controller. Is it can handle  dual-signature in the PD application? If yes, are there any reference design or sample circuit?

Parents
  • Hello,

    IEEE 802.3bt defines two PD topologies: single-signature and dual-signature. Single-signature PDs present a valid detection signature on both pairsets, while dual-signature PDs present separate detection signatures on each of the two pairsets. From the definition of dual-signature, this topology requires two separate PD controllers. You can build a dual-signature PD with 2x LT4295s, or 2x LT4293s, or 2x LT4294s, I would only use LT4275 or LT4276 if you are building a Class 4 PD.

    Why are you interested in a dual-signature PD instead of a single-signature PD? Dual-signature PDs require twice the components for little benefit. What kind of circuitry are you powering with your PD? 

    Best Regards,

    Eric

  • Hi Eric,

    Thank you for your help. I want to have the POE up to 90W, and I can power up the components which one need 24V/1A and other needs 12v/5.5A.  And I curious that Will be the  dual signature high efficiency than  single-signature and more stable?

    Best,

    Sky

  • Hi Sky,

    Let’s clarify that 90W number you’re using for your power budget. IEEE 802.3bt PSEs output between 90W and 99.9W, while IEEE 802.3bt PDs can draw up to 71.3W to account for worst-case operation with a 100-meter cable. To be precise, this is 71.3W for a single-signature PD, or 35.6W for each half of a dual-signature PD.

    This 71.3W is defined at the input of the PD, so it includes all losses going from the cable to your 12V and 24V loads. There's really no benefit of building a dual-signature PD, you could always build a single-signature PD that powers multiple DC-DC converters.

    Let's talk about power. Your 12V / 5.5A rail will consume most of the 71.3W power budget on its own. If you need these high powers, then ADI’s proprietary LTPoE++ standard is an option for you. LTPoE++ allows for up to 90W at the PD input, which gets you closer to your original numbers. Higher powers may be possible in a closed and engineered system.

    Best Regards,

    Eric

Reply
  • Hi Sky,

    Let’s clarify that 90W number you’re using for your power budget. IEEE 802.3bt PSEs output between 90W and 99.9W, while IEEE 802.3bt PDs can draw up to 71.3W to account for worst-case operation with a 100-meter cable. To be precise, this is 71.3W for a single-signature PD, or 35.6W for each half of a dual-signature PD.

    This 71.3W is defined at the input of the PD, so it includes all losses going from the cable to your 12V and 24V loads. There's really no benefit of building a dual-signature PD, you could always build a single-signature PD that powers multiple DC-DC converters.

    Let's talk about power. Your 12V / 5.5A rail will consume most of the 71.3W power budget on its own. If you need these high powers, then ADI’s proprietary LTPoE++ standard is an option for you. LTPoE++ allows for up to 90W at the PD input, which gets you closer to your original numbers. Higher powers may be possible in a closed and engineered system.

    Best Regards,

    Eric

Children