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TPS2470x Calculator Sheet slvc567

SimonF
Expert 3370 points

Hi Team,


when using the TPS2470x_Design_Calculator_Rev-.xlsx (slvc567) and you type "Yes" in the "Retry?" Column (C66) the sheet throws errors on C78 (#REF). It seems like there is no reference for the variable PLIM.

Can you please have a look?
Thank you for your time!

Best Regards,

Simon

  • 0
    TI__Genius 12621 points

    Hi Simon,

    For reference, the password for the tool can be found on the "intro" tab. It is "TPS2470X" and you can unlock the tool from the "Review" tab on the top of the page.

    It took me a bit but I found the problem with the formula.

    The equation was:

    =MAX(R_dson_125C*Iload_max^2/1000/No_of_FETs,IF(Retry="YES",PLIM*Retry_duty_cycle/(100*No_of_FETs),0))

    But this device does not have a power limit. It believe it should have been:

    =MAX(R_dson_125C*Iload_max^2/1000/No_of_FETs,IF(Retry="YES",25/C55*V_bus_max*Retry_duty_cycle*/(100*No_of_FETs),0))

    which would be the current limit instead, 25mV / C55 (which is in mohm), * the input voltage V_bus_max as that would be the worst case power dissipation during a start into short.

    So with a 25A current limit and 13.2V input and a start into short, the device would limit the total power dissipation of the MOSFETs to 330W. Then it would shut off, and retry later with an effective duty cycle of 4%, so DC would be roughly 13.2W on average. Then you would divide that by the number of FETs. So if it's 2x, then it would be 6.6W on average if there is a start into short circuit condition.

    Note that the TPS2470x family is meant for applications around 10A or less. For this application with a 25A current limit, we would recommend the TPS2471x family instead or for very high performance, the TPS2477x which features analog current monitoring. Both devices have a method to limit the power dissipation of the MOSFET. This helps protect the MOSFET under start-into-short conditions.

    We also have a new calculator tool for both of these devices which is simpler to use. It can be found at www.ti.com/hotswap --> "Tools & Software".

    All the equations and methodology is explained in our application note at www.ti.com/hotswap --> "Technical Documents" --> "Robust Hot Swap Design".

    We also have step-by-step video tutorials at www.ti.com/hotswap --> "Support & Training".

    Finally, attached is a good reference PPT for 12V hot swap design. You can see recommended hot swap + MOSFET combinations based on current level.

    3060.2_4_16_Hot Swap Presentation.pdf

    Thanks!

    Alex