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

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

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

Hi Eric,

Thank you for the clarification. If I use the LTPoE++ standard, I still can use the dual-signature PD, right? But each side the maximum power will be only 45W, right? And It will cost the double of the components compare with the single-signature PD,right?

Best,

Sky

Hi Sky,

No, LTPoE++ only supports single-signature, like IEEE 802.3af and IEEE 802.3at before it. You can always use multiple DC-DC converters inside a single-signature PD, so you can split the 90W between your 12V and 24V rails.

Best Regards,

Eric

Hi Eric,

Thank you so much. You help me a lot. Have a nice holiday!

Hello.

I need dual-signature LT4295.

I need PoE pass through (power 2 devices via 1 cable, 35.6W per device).

With single-signature I need to add PSE (LTC4291-1/LTC4292) into intermediate device.

With dual-signature I do not need PSE in the middle.

Can LT4295 work like dual-signature PD?

Hello,

Can you ask this question in a new thread? Can you also include block diagram showing your idea for "PoE pass through"?

Thank you,

Eric