Single port PSE design using the TPS23841

Attached is a reference design schematic for a single port mid-span PSE design using TPS2384. TPS23841 is P2P compatible with TPS2384 and has the lower UVLO for industrial type applications. TPS23841 defaults the port UV and OV fault to disable and as such is a bit more accommodating of the wider input range.

The photo below shows the top side of the reference design. TPS2384 is on the bottom side (underneath the array of vias to the left of T2).

                                                                                                        TPS2384

Thanks Eric,

We have decided to use this device, but have a few questions...

-          Can we change the 0805 components to 0603 or smaller?  The fewer new parts we have to add the better.

-         We don't need the AC section of the reference design, what can be removed?  How do we reconnect?

-          Can all the jumpers be removed?  Anything else that is just in there for test?

-          What do they do the with unconnected ports?  Leave floating?

-          Do they have to use the I2C port?

Thanks again,

J

 -Can we change the 0805 components to 0603 or smaller?  The fewer new parts we have to add the better. Sure, but make sure to preserve the voltage ratings. C17, R15, C22 types and ratings are critical to performance. Choose according to the guidelines in the TPS2384 datasheet. R13 and R14 can be omitted by connecting pin 21 and 22 directly to pin 24.

-We don't need the AC section of the reference design, what can be removed?  How do we reconnect? The basic single port PSE is on sheet 2 only. You will have to furnish 48V power to the PSE at the V48 node.

-          Can all the jumpers be removed?  Anything else that is just in there for test? What jumpers? J4 and J5 are RJ45 ethernet connectors. R18/D7/D18 are for port ON indication so if you do not need that it can be removed.

-          What do they do the with unconnected ports?  Leave floating? As drawn, the unused port pins for U3 are connected properly. Pins 11, 38, and 43 are left open.

-          Do they have to use the I2C port? No, the I2C port is not required in auto mode operation.

Sorry,

We were looking at the EVM, not the PMP5480.  Is there a BOM associated with the attached reference design?

Thanks, this is very helpful.

Also, I'm not seeing where in the datasheet to look for component selection guidelines.  We've decided to use the TPS23481, but I don't see it in either datasheet.

Thanks

General guidelines for the CINT, CT, and RBIAS are in the terminal functions (pin description) table starting on sheet 7. I have attached the BOM for PMP5480 and this will contain a better description of the actual component values used in this design.

Thanks Eric,

This has been extremely helpful for us to make a decision.  We are starting the schematic now, and have a few more questions, sorry...

- Is there an easy way of shutting off power to the load?  Can we add a FET?  The EVM has a hotswap controller on the front end, but I think that would shutoff power to the entire device.  I'm thinking we still want to have the TPS23841 on to detect a load, but may want to shut power off to the load.  Can we position FET's somewhere else to accomplish this?

- We assume we need the 3.3V circuit?  We have 3.3V available elsewhere on the board, but probably will just use the 3.3V off of the TPS23841.

- This circuit doesn't have the I2C connected, as it is in auto mode.  However, we are considering bringing out the I2C just in case we need it in the future.  However, the EVM has optocouplers isolating those signals.  If those are necessary, we will just leave it in auto mode.  Do we have to use isolation?

- We are getting rid of the AC piece as we have 48V available.  Is D6 still necessary?  It might be protecting the circuit from the load, but we weren't sure.

- Are there any other changes to be aware of when going from the 2384 to the 23841?

- Finally, there is some text in this schematic that says "Alternative B" near the magnetics.  We are curious what that means, as we are considering just hooking up 4 lines instead of all 8.  We just wanted to know if there might have been a different version that was connected that way. 

Thanks again for the fast responses! 

- Is there an easy way of shutting off power to the load?  Can we add a FET?  The EVM has a hotswap controller on the front end, but I think that would shutoff power to the entire device.  I'm thinking we still want to have the TPS23841 on to detect a load, but may want to shut power off to the load.  Can we position FET's somewhere else to accomplish this? TPS23841 "probes" for a load using the Px/Nx pin pairs (P1/N1 for this single port design). Putting a "switch" between P1/N1 and the load (at the ethernet connector J5 on PMP5480) will disable detection by TPS23841. If you simply want to disable the internal FET (between N1 and 1RTN inside the device), you can use the POR signal (use a pullup to V3D3 and then pull POR low to disable the TPS23841 and any load which is connected).

- We assume we need the 3.3V circuit?  We have 3.3V available elsewhere on the board, but probably will just use the 3.3V off of the TPS23841. V3.3 is a 3.3V output from the TPS23841. It is also used for internal digital circuitry and can be used externally for very low power things like pullups. I wouldn't load V3.3 by any more than ~ 2mA externally.

- This circuit doesn't have the I2C connected, as it is in auto mode.  However, we are considering bringing out the I2C just in case we need it in the future.  However, the EVM has optocouplers isolating those signals.  If those are necessary, we will just leave it in auto mode.  Do we have to use isolation? The isolation aspect is really a function of the system. The IEEE standard requires that PSE output (from RJ45 connector) is isolated from any earth ground. If the circuitry used to communicate with the TPS23841 was referenced to earth, then the opto-isolators would be required between the micro and TPS23841. If you simply want to have the I2C interface available for diagnostics (manufacturing for example), then you could interface directly with the TPS23841 w/o isolation. But, it is really a system level consideration.

- We are getting rid of the AC piece as we have 48V available.  Is D6 still necessary?  It might be protecting the circuit from the load, but we weren't sure. D6 protects the TPS23841 port from ESD and other surges that may be present on the RJ45. D6 is a "dual" line-line device so if you had something else rated for surges it could be used. D6 is a pretty small package though.

- Are there any other changes to be aware of when going from the 2384 to the 23841? Heatsinking (via PCB) is more stringent with a higher power device. TPS23841 has a wider input voltage range for use in industrial applications and by default, the port UV and OV flags are disabled (to allow the wider input range). Other than that, they are quite similar except for the current limit set point. For the higher power levels, T2 will have to be changed (search for a POE equipped ethernet transformer rated for type 2 PSE current levels (600mA).

- Finally, there is some text in this schematic that says "Alternative B" near the magnetics.  We are curious what that means, as we are considering just hooking up 4 lines instead of all 8.  We just wanted to know if there might have been a different version that was connected that way. PMP5480 was designed as a "POE Midspan" which will typically inject voltage on the second ethernet pair set (referred to alternative B in the IEEE standard). And PMP5480 used a giga-bit ethernet transformer but you can just use a simple 10/100 ethernet transformer rated for the possible load current (higher for TPS23841).

The different alternatives are explained well in the IEEE standard which is available for download: http://standards.ieee.org/about/get/802/802.3.html  Download version 802.3at 

Thank you very much,

Just for clarification on the shutdown question.  We are in disagreement on our end.  If we leave it as is, it won't shut down, right.  We could control the POR low to turn it off, but we don't necessarily know if a load is connected or not.

We can't monitor the resistance without I2C connected, and power to the TPS23841.  However, if we turn on power to detect the resistance, do we risk damaging a non-poe device that is attached.

Is that a correct understanding?

Please read the Auto Mode section of the TPS23841 datasheet starting on pg 12 through pg 20.

Discovery (or detection) mode starts on pg 14. TPS23841 probes the port for a POE compliant device (with a low voltage, current limited circuit) and if that device meets the IEEE requirements, it will furnish power to the load. TPS23841 does this automatically in AM mode with no need for I2C intervention.

When a compliant device is found and power is applied, TPS23841 then monitors the port and will know when the PD is disconnected (see pg 17 for powered mode operation). When TPS23841 detects that the load was disconnected then it will remove the voltage and go back to detection mode.

So, POR and I2C management are not neccesary in auto mode operation.

Hi Eric,

We have the reference design you provided into our design now, but now we feeling some scope creep.  Management would like us to DNP another pair for future expansion. 

The Ethernet PHY that we are using only uses half of the transformer, and I was asked to investigate using the other 2 pairs for this future expansion. However, I believe only the bottom pair is being used for power-over-ethernet in the ref design.  So, my question is what do I need to add to allow us to use the other half of the transformer for power-over-ethernet only? No data will be sent on that pair.  Is this possible?

Thanks,

J