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TPS54560: Issues with variable voltage output 12-42VDC from 48VDC input.

Part Number: TPS54560
Other Parts Discussed in Thread: LMR16020, LM46002, TPS2372, , LMR16030

Ive inherited the attached design from a previous design and im having issues with its operation.

I wonder whether somebody could take a look and comment on the design and offer guidance on how to make it better, more robust or a completely offer a completely different design.

It is meant to take an input of 48VDC (typ) but could see 44 to 60VDC input. The maximum input current available is 600mA. Using the switched resistors, it is meant to supply an output voltage across a range of 12 to 42VDC.

If i set the output to 42V and apply 48VDC with loads of 100 or 200mA, the circuit starts up. Any higher current, 300, 400 or 500mA and it doesnt start up and provide an output voltage. Yet if i start with a load of 100mA and slowly increase in 100mA steps i get a voltage all the way to 500mA.

I see a similar issue if i set the output voltage to 12V, i dont get outputs much after a load of 1400mA

My thoughts are that the design should have been undertaken to achieve 12V at 2A output, 60VDC input. When this was confirmed as working, the feedback resistors could be changed to set the different output voltages. I would expect the efficiencies would drop as you moved further from the 12V output which the components were selected for. Is this a correct assumption?

  • I think that your assumption makes sense.
    You can try using the Webench tool on our website to create the base design.
    And then test the different output voltages.
    You might also look at the LM46002 or the LMR16020
  • Hi Frank,
    I've made a more detailed post at as its a PoE project.

    Ultimately i need to provide a small PCB that will take PoE and provide 12-42VDC depending on switch settings.

    I have noticed that the design i took to PCB wasn't happy providing the power that i needed from switch on.

    I have reverted back to using a TI Eval PCB for the PD interface (TPS2372) and took some measurements when i call for different currents. I can only draw 0.38A by turning the load on, after PD classification. If i try with a higher current the output is removed by the TPS2372. However, if i set the load to draw 0.38A i can then manually increase the current to the maximum levels i need.

    Ultimately i need to identify what is up with my circuit and my understanding, however, if there is a TI part with a Slow Start mechanism that will stage the current draw i would like to know. I realise you offered two parts to look at but im unsure whether the softstart will do what i need.

  • Howard,

    There are lots of possibilities of what's going wrong so I'll throw out some questions to rule things out and we'll go from there:

    1. Please share the layout (not a question but it's important).
    2. Do you have a VIN cap close to the VIN pin and GND of the IC? I don't see one in the schematic.
    3. Do you have any ceramic caps on the output?
    4. How do the pin voltage behave in working and non-working conditions? If you start up into a load (resistive load?) the output doesn't come up at all? Or it starts to, then dies.
    5. Is there ever a point where FB < 0.8V and SW is not switching?

  • Hi Sam,

    Partial progress is being made.

    I have tried loading the TPS2372EVM output with a resistive load as opposed to the E-load and i can draw 600mA on the output (at 48V).

    My next step was to isolate the TPS54560 circuit on my PCB and attach to the TPS2372EVM output. I ran some load tests using both an E-load and resistive load.

    All was well when i selected voltages between 12 and 26V, i was achieving approx 22W of draw across the range. When i selected higher than 26V the available power began to drop.

    I also tried the tps54560 circuit connected directly to my bench PSU and could achieve the required power across all the voltage range 12-42V.

    This is a circuit i inherited in a previous design, so i am open to change. I have run WEBENCH on the part with a requirement of 12V @ 3.0A and the components are significantly different to the ones i am using. I can draw no more than 25.5W on the input (PoE limit) so was aiming to be able to supply 12V @ 2.0A. My thought (rightly or wrongly) was to pick components for 3.0A to provide headroom.

    I can send you (by email) schematics, layout etc but im not allowed to post these in public.

    The output of the TPS2372 is fed via a filter arrangement to the TPS54560. Caps arent close to the VIN pin though. What value do you suggest? I have 3x 2.2uF and 1x 0.1uF in parallel on the feed to the tps54560.

    I have an 220uF electrolytic cap across the output. No ceramic caps.

    Unfortunately, my board has stopped working so i cant make any further measurements. I'm going to make changes to the PCB and then get a new one made, as the layout isn't good and can be bettered.

  • Howard,

    Sorry for my delayed reply.

    If you're interested in a new part I'd recommend LMR16030. This has the specs you require + external soft start. But TPS54560 should work fine for your application if you're redoing the board.

    Feel free to send the board files over to my email so I can take a quick look before you fab.