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Webench/LM25116: LM25516 Webench design

rbifly
Prodigy 30 points
Part Number: LM25116

Tool/software: Webench

I am implementing a design using the LM25116.  Vin = 7V-20V, output 5v @ 5 amps.  I previously used Webench to validate/verify and all verified okay.  But today I went to create the  same  design using Webbench with these same parameters and I get the message 'Design cannot be created with this device.  Please press OK and select a different device from the list'.  Looking closer it says that ' This design is not stable due to phase margin either smaller than 35 or greater than 120.'  I don't think this is correct.  I still have my original design accessible, and that still simulates and shows adequate margins at 20 volts ( phase margin 72.89, gain -26.32).  If I use the Webench tool off the LM25116 page, it will only create a design for input parameters 7-19.8 volts.  Is there a problem with the tool or current LM25116 model?  Want to make sure there are no issues going forward.

  • 0
    TI__Intellectual 2155 points

    Hi rbifly,

    The calculations on the original design should remain valid, as there have not been any updates to the LM25116 model for the last few months. How long ago did you create your original design?

    Also, I am able to create a design with your specifications if I set the optimization knob to setting '4'.

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=6621DFA1C4EF5221

  • 0 in reply to Kevin Ip
    Prodigy 30 points

    I created the design on 12/21/16.  I also was able to create another LM25116 design using optimization level 4.  Interesting that a lot of the part/value recommendations are different compared to the original?  Wondering why the design can't be created at 20 volts and optimization 3? 

  • 0 in reply to rbifly
    Prodigy 30 points
    I may have created the original LM25116 design using 7-19volts, and later changed that design to 7-20 volts. But I fail to see why Webench would consider that 20 volts is an issue? At least according to the Webench simulations. In that design, L1 is a 4uH inductor. Ccomp, Rcomp, and Ccomp2 are 3900pF, 5.23K,and 18pF respectively.
  • 0 in reply to rbifly
    TI__Intellectual 2155 points

    Hi rbifly,

    Perhaps the design at optimization 3 needed more capacitance at the output and therefore not passing the phase margin checker. Your design requirements all well within the specification of the part. On the simulation, if there is low phase margin, adding more output capacitance may help. It can be further tuned by adjusting the compensation components.