LM5071: LM5071 multiple output reference circuit, and (generally) adjusting 5V to 5.5V on POE-PD Flyback designs

Michael Wohl

Part Number: LM5071
Other Parts Discussed in Thread: PMP11107, TPS23753, TPS23730, TPS23753A, TPS23731, TPS23734, TPS23758, TPS23758EVM-080, TPS23731EVM-095

Hello,

We are designing POE (as a PD) onto several circuit cards of a new machine design. FYI: We are intending to design one circuit to meet the needs of three applications (different PCBA's).

There are two possible (isolated) topologies:

1) A two-output POE-PD circuit: 3.3V at <2A and 5.5V at <0.6A

2) A single output POE-PD 5.5V at <3A followed by an LDO to create the 3.3V down stream.

In todays supply chain we unfortunately start with a search for "what's available" rather than "what we need".

In that context we came across the LM5071 and its eval kit design (see figure 17 in the LM5071 datasheet). I know the kit is no longer available, but it does contain a design for the specific dual output (#1 above) we are looking for...and amazingly all the main components (like the transformer are also available).

Questions:

1. The LM5071 shows active life cycle. Is it recommended for new designs? its been a round a long time it seems....

2. If using the LM5071 presents a problem, do you have a suggestion for replacing the LM5071 in that specific reference circuit or can you point to another example 2-output circuit that fits this requirement?

Also, In the case of topology #2 above (single 5.5V output)....can we use any available POE-PD controller chip with a 5V reference design and modify its R2R ladder to it's FB pin in order to tweak it (ie: just a 10% change) from a 5V design to a 5.5V output? Or is there more to it than that? Do you have a recommendation for this?

Thank you for your support.

Mike

over 2 years ago

0 Michael Pahl over 2 years ago

TI__Mastermind 21320 points

Hello Mike,

There is something like PMP11107 you could consider.

https://www.ti.com/tool/PMP11107

You will probably need a transformer redesign to get more power out of the 3.3V rail, but magnetic vendors are able to do this.

The LM5701 is a PoE PD + PWM Controller for Type 1 applications. Noe that only 13W can be provided at the input, then you have to account for efficiency drops.

We have had multiple generations of new parts in this power space. I would recommend the TPS23753A, or the TPS23731 (for the newest features, and if you truly need 5.5V / 3A). The TPS23753a can also be substituted for TPS23753. The TPS23731 can be substituted with the TPS23734 and TPS23730.

The TPS23758 integrates the primary MOSFET which provides the highest level of integration.

The TPS23758EVM-080 is 5V/2.3A, but we have tested the transformer up to 3A. There is also a 3.3V variant of this design.

The TPS23731EVM-095 is 5V/5A if you needed more power.

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Michael Wohl over 2 years ago in reply to Michael Pahl

Prodigy 240 points

Hello Michael,

Thank you for the quick reply.

Unfortunately, after looking at TI and normal distribution channels, NONE of the parts you recommend are available. That is why I originally reached out to ask whether the LM5071 is recommended for new designs. I'm sort of gathering from your feedback probably not.

- Is the LM5071 recommended for new designs?

Also, in regard to my question about modifying a 5V flyback design to be 5.5V.....In your recommended TPS23731EVM-095, can resistors R35 and/or R38 be adjusted to simply change the output from 5V to 5.5V or even 6V.

- That is, can we modify the resistor ladder in the feedback loop of a flyback design to move a supply rail 10% or 20% without a magnetics change?

Thank you

Mike

+1 Michael Pahl over 2 years ago in reply to Michael Wohl

TI__Mastermind 21320 points

Hello Michael,

So long as a part is active on TI.com it can be used in new designs. However, as you discovered, if a part/EVM was made a decade or two ago, not all of those specific parts are going to be currently available. Our designs do not use any special magnetics or passives, so I am sure there are replacement parts if you can do a search for them.

You could modify a 5V design to 5.5V with a resistor change. At 6V, I would strongly consider a re-design. I recently changed a design from 5V to 4.5V and the feedback circuit still had to be adjusted, so you should check that in your design testing.