LM5071 5V output

We would recommend a TPS23753A (or TPS23754/6 based design) over the LM5071. Can you specify your AUX input voltage range?

Can you briefly explain what makes the  TPS23753A preferable over the LM5071?

We have an AUX input voltage range of 24-56VDC. We would like to offer 12VDC but I guess this is asking too much.

Hi Michael,

The reason we push the TPS line of POE PD controllers is mostly support and available reference design/EVM related. New TI reference designs will target the TPS line and not the LM line so you will have many more options with something like the TPS23753.

For the 24-57V range, TPS23753AEVM-003 (5V version of the EVM) can meet this requirement. It is a synchronous design but self driven by a winding on the flyback transformer. Generally at this output power the peak currents in the secondary will prohibit a diode rectifier.

For a 12V adapter, a new transformer would be required. I will search internally to find out if we have the wide VIN, 5V, 2A reference design.

Hi Eric,

we have changed our design to the TPS23754PWP, but we only need 5V @ 7W. We have a height constraint of 9mm and therefore have selected the Würth Elektronik transformer 749 119 250. But maybe there is an alternative with a winding for the synchronous rectifier and a height below 9mm?

Also, I have integrated the TPS2115ADRB for interfacing to a 5V plug power supply. But maybe there is a simpler/cheaper solution for ORing at the 5V output side?

Would it be possible to send you the schematics for approval?

Best regards,

Michael

Hi Michael,

For only 7W, the TPS23753AEVM-001 as a baseline would be a better choice. Several years back, we designed a variant of this using the Wurth 749 119 250 transformer on the TPS23753AEVM-001 and it worked well. I've attached the schematic.

Regarding ORing at the output side, please see "Option 3" in the below referenced application report (SLVA306A)

Regards, Eric

TPS23753A_7W_5V_LowProfile_sch.pdf

Hi Eric,

great, thanks a lot for the TPS23753AEVM-001 with the Würth transformer, which we will use! Two last questions:

1. Can we replace the electrolytic capacitors C2, C10 and C5 against ceramic (X7R) ones, or is there any advantage of the aluminium capacitors I might no be aware of?

2. We simply OR the 5V plug power supply with another MBRS540T3 diode to the 5V output of the TPS23753AEVM-001 or what would be your recommendation?

Regards, Michael

Hi Eric,

could we drop-in replace the TPS23753A against a TPS23754 if we would need PoE+ supply on the 48V rail? (still only using app. 3W at a local 12V output)?

Best regards,

Michael

Hi Michael,

Yes, it is OK to use ceramics for the referenced capacitors. Please check the actual capacitance vs. DC bias voltage as the actual capacitance value will be less than advertised but may still be OK for your end application.

For output ORing, what voltage does your downstream load need? If it can tolerate the ORing diode forward drop (from adapter source to VOUT) then the diode will work fine. If you need closer to 5V then it is possible to use a P-FET to switch in the adapter or a USB power switch.

Does your application have a source preference when both adapter and POE are present? There are some considerations discussed in the application report which can be implemented to set a higher preference to one or the other source when both are present.

Hi Eric,

thanks for your answer. Do you have a reference schematic for using a P-FET to switch the adapter power in? We indeed need as close to +5V downstream as possible.

We would give adapter power priority over POE. It would not matter if the switch would drop POE due to getting below the MPS limit.

Best regards,
Michael

Hi Darwin,

thanks for the pointer to PMP7298. What booster do you recommend going from 3.3V to 11.2V @ 100mA with a tight voltage tolerance of 1.5% ?

Best regards, Michael

Here is a PFET ORing schematic for the adapter.

Hi Eric,

great support - thanks a lot.

BR

Michael