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Original question:

TPS84259: Looking for a -15V/3A performance device

TPSM63610: -15V/8A output Rfbt/Rfbb Value?

Rice,Lee
Intellectual 370 points
Part Number: TPSM63610
Other Parts Discussed in Thread: LM25148, TPS62918, TPSM63606

Tool/software:

What values ​​of Rfbt/Rfbb should I use to get an output of -15V/8A?

  • +1
    TI__Genius 11250 points

    Hi,

    You can use RFBT = 100 kΩ and RFBB = 7.14 kΩ, from table 7-1 of the datasheet. Any other values in the same ratio should also be ok (as long as they are in the same range of few kOhms to few hundreds of kOhms)

    Can you specify the maximum load current in the application? Is it 8A or 3A (in the original post)?

    Regards,

    Niranjan

  • +1 in reply to Niranjan Nair
    Intellectual 370 points

    The load was higher than expected. So 3A was not enough, so a part with sufficient capacity was needed. 8A should be fine.

  • 0 in reply to Rice,Lee
    TI__Genius 11250 points

    That might be a problem. When using a buck converter in IBB configuration to get negative output, the actual current flowing through the inductor is higher than the output current, IL = Iout x (1 + |VOUT|/(efficiency x VIN)). So for converting 15V to -15V, you will need a current rating of more than twice the actual output current, leaving some margin for efficiency. This is why I suggested the TPSM63610, an 8A part in the original thread where current was mentioned as 3A.

    For more information you can refer this app-note : https://www.ti.com/lit/an/snva856b/snva856b.pdf

    So if the output current needed is 8A you will need a part of around 20A (assuming 70% efficiency). You can have a look at controllers using external FETs for that, like the LM25148

    Regards,

    Niranjan

  • 0 in reply to Niranjan Nair
    Intellectual 370 points

    I am thinking of considering TPS62918 Negative Power rather than LM25148.

    Thank.

  • 0 in reply to Rice,Lee
    Intellectual 370 points

    I don't think I can use TPS62918 because of its low output, so I'm planning to use TPSM63606SRDLR

    .

  • 0 in reply to Rice,Lee
    TI__Genius 11250 points

    If the output current is 8A, TPSM63606SRDLR will also not work. For converting 15V to -15V you will need a bit over twice the output current according to the equation I gave above. With TPSM63606 you might get around 2.5A output current.

    For 15V to -15V at 8A, you would need a device with current around 20A. This will be difficult in a single phase even with controllers. I think the best solution would be use a multi-phase converter. Using 2x706A0 would work for this case, providing 2x10A current and good margin for VIN.

    Regards,

    Niranjan