TPS23751: 48V TO 12V ISOLATED POWER SUPPLY

Hello Kevin,

Thank you so much for your question. So to reiterate, you don't want to use PoE at all on the front end, you want to inject 48V DC and then convert that voltage to a 12V output (isolated). 

In TI terms, we would call that using an 'adapter input.' If you look at our TPS23751EVM-104 EVM, you will find that it has both the PoE input and an 'adapter input.' You can base your design off of this without the PoE. There are other TI EVMs in this family portfolio that use adapter input if you want to look at more than one implementation.

Additionally, you can leave the PoE portion in the design and use OR-ing techniques to have the adapter be the primary source if you wanted to use PoE in the future. More information on that can be found here: http://www.ti.com/lit/an/slva306a/slva306a.pdf

The VSS/PAD pins are the negative to the PoE power. RTN and ARTN are the board ground. These are separated by an internal FET, so they should not be connected. 

Here, you really have two options:

1. You can just use the RTN and not use the VSS or DEN. This will cause the internal hotswap MOSFET to be turned off. This FET is used in current limiting for the PD, so additional measures should be taken to address this.

1. You can still use the DEN pin by connecting it to VDD. This will force the hotswap MOSFET to always be on. This will require a voltage divider for VDD, DEN and VSS.

CLS is the class pin. This is used specifically for PoE for those operations, so if you are not using PoE then they can be left alone.

Can I ask your reasoning behind wanting this part, but don't want to use the PoE? What draws you to this part over others?

Regards,

Michael Pahl

Hi Michael,

Thank you for the quick reply!

To answer your questions:

Yes, I only wanted to use this part as a power converter from 48V to 12V, with isolation.  There also was an off-the-shelf power transformer, which was a big help.  I'm not a power designer...  This part was affordable and required no exotic parts that I could see.

Yes, want to run this as an adapter input converter.  I have the reference schematic, and see that the adapter input was what I wanted.  The product I'm designing doesn't have Ethernet, so I wasn't concerned about the PoE front end.  I was confused about what to do with Vss and PAD when used this way.  I think I understand this better now.

A few questions for you:

So if I don't connect Vss or PAD, should they be tied together and floated? 

Also, is my assumption that I should tie DEN to Vss to make sure the MOSFET stays off, correct?  What are the ramifications if I resistively tie DEN to VDD as you mentioned, which has the hotswap MOSFET on?  My power draw is pretty low, so I don't anticipate that I need any specific current limiting.  My 48V supply is "infinite" (a really big lithium pack).

And since I don't have a power class associated with the PoE, the CLS pin resister value, I assume, is unimportant as long as it doesn't impose a problem with the 2.5V potential at the pin?  (leave unconnected?)

Also, I would like to see if there is a way to shut off the converter through an opto part.  Any suggestions on a method to do this?

I've attached an updated schematic, where I think I have Vss, PAD, DEN and CLS configured properly.  Your feedback is appreciated

Michael, thanks again for your thoughts on this.

Kevin

(I'm out of the office backpacking through Sunday, so I cannot answer until Monday...)

8877.ADS Controller Main 48V to 12V.pdf

Michael,

Any thoughts on my other questions?  Your help is appreciated!

Kevin

Hello Kevin,

Thank you for your answers and for providing an updated schematic. Here are some thoughts on your questions, thank you for your patience:

So if you do not connect VSS and the PAD, yes tie them together. However, the PAD pin must be connected to a plane. One of the pin's functions is to provide mechanical support, so it must be connected to a plane. But that plane can be floating, and VSS and DEN can be connected to it if PoE is not used.  

The ramifications of connecting DEN to VDD is that it will enable the hotswap MOSFET to turn on. The datasheet explains further how this MOSFET operates. I would suggest it would be simpler to create your own current limiting circuit for power on and power off states. That is the typical time current spikes are seen in  a circuit, and I would suggest either designing protection for that or at least knowing the maximum current draw during startup/shutdown. 

Yes leave CLS unconnected as well. 

As for the optocoupler, you could connect it to CTL and force that pin to RTN. This would stop the PWM controller FET from switching, and cause the converter to be shutoff. 

Let me know if I can clarify anything or if you have any follow up questions!

Regards,

Michael Pahl