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TPS82130: Power supply with multiple outputs

Part Number: TPS82130
Other Parts Discussed in Thread: LM3150

Hello, I have a system powered by a LM3150 circuit that outputs 7V (on a separate power board connected with cables to the main board).

Then on the main board I have to generate several voltages from this 7V input (5V, 4.2V and 3.3V) - I am planning to use TPS82130 and maybe some other similar power modules and I have a few questions:

- do I have to take care about some points regarding the switching frequency of the power modules with respect to the switcing frequency of the LM3150 ?  Maybe to avoid that they are at the same frequency or a multiple of the frequency ?

- is it recommended to use the 7V input to each of the 3 power modules that will output the 3 voltages above ?  Or is it maybe better to convert from 7V to 5V and then from 5V to 4.2V and to 3.3V ?

- are there some recommended practices for such a configuration with multiple voltages to avoid noise, interference and so on ?

Thank you,

Mihai

  • hi Mihai,

    if the switchers are supplied by the same rail then it is best practice to synchronize all of them (best interleave them) to minimize the input filter and avoid beating phenomena...
    in your case, it seems possible to cascade them: this is the best configuration in terms of efficiency, and it is also the recommended one if you don't have specific sequencing constrains...
    for the layout, because you are considering modules, the aspects to take care are very minimal (refer to the layout guidelines session in the device datasheet)...

    hope this helped a bit
    KR
    Vincenzo
  • Hello Vincenzo,

    Many thanks for your response.

    Please let me know what do you mean by interleaving ?

    By cascading you mean to generate 5V then from 5V to generate 4.2V and from 4.2V to convert to 3.3V, OK ?

    In this case (cascaded) - do I have to follow some rules regarding their frequency ?

    Best regards,

    Mihai

  • hi Mihai,

    yes, cascading is something like 12V in --> 5V out --> 3Vout and so on...

    while interleaved is 12Vin --> 5V, 3V , ... out

    in this second scenario, since all the switchers will pollute the VIN but with different noises (FSW) you will need to apply an input filter to any of them; while if you interleave them (all the switchers run at the same FSW with the same clock, just phase shifted) then you need a single input filter (smaller and cheaper)...

    details in the attached app note

     

    let me know if you need more

    KR

    Vincenzo

  • A similar discussion is on this thread: e2e.ti.com/.../576986

    Most customers would make each of the rails from your 7V input. This is ok. I would recommend a ferrite bead between the 7V and each module, but this is not necessarily required. I see many designs with multiple regulators running off of the same input source without issue.
  • Thank you, can you please tell me how I can calculate and choose the ferrite and capacitors for the input ? I know the ferrites are frequently used for filtering but I did not use them until now and I am not sure how to choose the ferrites.
    Thanks,
    Mihai
  • The D/S explains the requirements for the input cap.

    As well, a small case size input cap place closest to the device helps with EMI/noise. This training explains in more detail: training.ti.com/optimizing-emi-behavior-low-power-dcdc-converters

    For the TPS82130, a 470pF in an 0402 is still a good choice. A ferrite which blocks noise at ~400 MHz is best.