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Webench LM5116 low side MOSFET selection bug

Sebastien Saury
Prodigy 30 points
Other Parts Discussed in Thread: LM5116

When running a webench design with the LM5116 if trying to change the low side MOSFET it is the high side one that gets updated.

To replicate the problem go to the LM5116 webpage and open the default webench design. Select M2 (the low side MOSFET) in the schematics or the BOM and select an alternate part for it. M2 doesn't get updated the value selected but M1 does!

  • 0
    Prodigy 30 points
    Just realised the transistors are selected as a pair and not individually so when clicking on selecting a new part for M2 it links to the same menu as selecting a part for M1.
  • 0
    TI__Intellectual 2155 points
    Hi Sebastien,
    The LM5116 FET algorithm chooses the FETs as a high side and low side pair, so when you click on "Select Alternate Part" you will see options for choosing combinations of M1 and M2.  The algorithm should not be able to let you individually select an M2 FET.  
  • 0 in reply to Kevin Ip
    Prodigy 30 points

    Thanks Kevin,

    Realised that just after posting my message...

    Do you know if there is a way of understanding the rational of the Webench tool for suggesting the transistors pairs it does? From the LM5116 data sheet M1 should be selected for fast switching while M2 should be chosen for low Rds in both cases to limit losses. Webench however selects transistors for M2 which have a higher Rds than M1 (design inputs are: Vin = 10.5 to 55V, Vout = 9.5V, I=6A).

    Is there a way of adding transistors that are not in the suggested list?

    Sebastien

  • 0 in reply to Sebastien Saury
    TI__Intellectual 2155 points

    Hi Sebastien,

    The FET selector will rank the FET pairs based on the best impact to the overall system BOM cost, efficiency, and footprint size. The system efficiency is evaluated at VinMax, taking into consideration all the FET losses (switching losses, gate charge loss, conduction loss). BOM cost is also considered, as well as footprint size, although since most packages in the list are about the same size this will have a lesser impact.

    Because of this, you may see the best FET pair chosen by the selector different that what you may be expecting.

    Unfortunately, I don't think that is a way to specify a custom FET for this LM5116 design in WEBENCH right now.