Line drivers

Peter Johansen

Other Parts Discussed in Thread: LM5072

I am going to design a PoE injector box using the Rx/Tx wire pair in the RJ45. I will use traditional magnetics.

Question is if a complete PHY is needed (like the DP83849) as a media converter RJ45 to RJ45 or if it is possible to use some differential receiver driver IC's (like SN65LVDx10x) ?

over 8 years ago

0 Ross Pimentel over 8 years ago

TI__Mastermind 23675 points

Hi Peter,

Do you mind showing me a diagram of what you are trying to do?
If you are only doing an injector, you should not need to use additional PHY or transceiver re-drivers.
Also, please make sure your magnetic is rated for PoE, or ensure current levels do not saturate your magnetic.

Kind regards,
Ross

0 Peter Johansen over 8 years ago in reply to Ross Pimentel

Intellectual 935 points

The diagram is not finished yet, but the idea is to use the transformer as used in the examples for LM5072 (in the datasheet), DA2257. The product that require the supply is having the LM5072 in an isolated configuration as shown in the datasheet. The power consumption is max 4W. The cable length from the Switch to the PSE is max 20 meters and from PSE to device max 10 meters, so no long transmission lines.

0 Ross Pimentel over 8 years ago in reply to Peter Johansen

TI__Mastermind 23675 points

Hi Peter,

Please send me the diagram when you have it.
The reason why is that if you are doing a PoE midspan then you do not need a transceiver. It greatly simplifies what you would be doing.

You would essentially need only two RJ45s that bridge. Here is an example of how a midspan works:
www.ti.com/.../slvu301.pdf

Figure #1 shows the schematic and you can see that the power is taped but the actual data is passed through using J2 and J1.

Kind regards,
Ross

0 Peter Johansen over 8 years ago in reply to Ross Pimentel

Intellectual 935 points

Just to make my application more clear: I need the pins 4-5 and 7-8 on my device for another purpose (Relay signal) so I need to supply the PoE through the signal lines. So traditional Midspan is not the solution. But by looking into your diagram it seems that you simply supply the the injected voltage to both options, 4-5+7-8 and 1-2+3-6. Question is: do I not need to worry about damping through the transformer for the signal wires ?

0 Ross Pimentel over 8 years ago in reply to Peter Johansen

TI__Mastermind 23675 points

Hi Peter,

Midspan has nothing to do with what pairs you use. You can use either PoE alternative A or alternative B.
Midspan is concerned with taping and not the pairs used. There should be no issue with having a midspan because insertion loss is usually around 1.1dB and the PHYs are designed to exceed 125m cable. With 30m being your typical application length, you should have no issues.

Kind regards,
Ross

0 Peter Johansen over 8 years ago in reply to Ross Pimentel

Intellectual 935 points

Thank you for your fine explanation and assistance.
/Peter

0 Peter Johansen over 8 years ago in reply to Peter Johansen

Intellectual 935 points

My schematic now looks like this, including some TVS' for protection. Feel free to comment.Ethernet PoE Schematic.pdf

0 Ross Pimentel over 8 years ago in reply to Peter Johansen

TI__Mastermind 23675 points

Hi Peter,

One question regarding your application is why did you include 49.9 ohm termination resistors and a 3.3V rail?
The midspan is isolated. There should not be a need for biasing.

Kind regards,
Ross

0 Peter Johansen over 8 years ago in reply to Ross Pimentel

Intellectual 935 points

I have now figured out, that those 49,9 ohm resistors are simply a copy/paste error from another circuit. Thank you for the good comment.
/PeterJ

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