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LM20136 Design spreadsheet versus Webench discrepancy

David Wang11
Expert 7400 points
Other Parts Discussed in Thread: LM20136

The LM20136 has two ways of calculating/estimating values for the external components for the device, using the design spreadsheet and webench. I have used both and there is a discrepancy between recommended inductance. Webench shows 2.2uH and the spreadheet shows 1.52uH. Also compensation values are different when I use the same values from webench in the design spreadsheet.

Here are the specifications:

  • Vin: 4.5 to 5.25
  • Iout =5.6A
  • Vout = 3.3V
  • Temp = 85C
  • Soft-Start Time = 7ms
  • Switching frequency = 410Khz (GND SYNC pin)
  • inductor value -> 2.2uH, ESR -> 10.52mOhm

Which method is recommended for this case? And do you know where the discrepancy is coming from?

Thanks

 

  • 0
    TI__Guru 71185 points

    I will pass this to the Webench team

    FD

  • 0
    TI__Intellectual 2930 points

    Hi David

    WEBENCH calculates a a target inductance value, creates an exceptable range around this value and then selects a component based on multiple factors such as proximity to the target, price, size and DCR value. This is why the value of inductor you get from WEBENCH will not be exactly the same as the Speadsheet. 

    The compensation values are different because WEBENCH had a ceramic cap as its output and derated that value of capacitance because of this, it compensated based on this derated value.

    Either inductor value should be fine to use. If you are using the output cap suggested by WEBENCH please use the compensation suggested by WEBENCH as well. Also you should use the Simulations to verify the design is doing what you want.

    Best regards,
    Tommy Jewell