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LM25011 feedback resistor calculation WEBENCH / datasheet anomoly

Errol Kowald
Intellectual 540 points
Other Parts Discussed in Thread: LM25011

The LM25011 datasheet states on pg 15, just below equation 8 that "For this example, RFB2/RFB1 = 0.992. RFB1 and RFB2 should be chosen from standard value resistors in the range of 1.0 kΩ – 10 kΩ which satisfy the above ratio." No explanation is given as to why this limited range is necessary.

When WEBENCH is fed the values, 8 < Vin < 36, Vo = 5V at Io = 1.5A, as per the datasheet example, it chooses feedback resistors within the datasheet specified range.


However, when WEBENCH is given the parameters 12 < Vin <14, Vout = 9V at I = 1.3A, it chooses Rfb1 = 9.76k and Rfb2 = 25.5k. The latter is well outside that which the data sheet range.

Is the datasheet wrong, or is WEBENCH wrong?



  • 0
    TI__Mastermind 26140 points
    The wording in the Datasheet is not right. It should be the bottom feedback resistor ONLY should be set to 1kOhm - 10kOhm the top Feedback resistor should be what ever it is calculated to be. Webench is correct, datasheet is to be corrected. Thanks for you feedback!

    David Baba,
  • 0
    TI__Intellectual 2320 points

    Hello Errol,
    You can use a 2k for the bottom resistor and 5.23K for the top resistor in the feedback resistor network in your application. We have seen in some non-sync COT bucks it becomes necessary to have a pre-load more than 500uA to 1mA for proper startup and operation at light load.
    Regards,

    Sourav

  • 0 in reply to Sourav.Sen
    Intellectual 540 points
    Thank you for your replies David and Sourav.

    Sourav,
    your response would imply that WEBENCH has chosen incorrectly.
    Vref / Rfb1 = 2.51 / 9.76k = 257 uA

    The values chosen by WEBENCH, 9.76k and 25.5k have been installed, and the circuit appears to work fine - at least with a 0 to 1A load.
    Perhaps this particular piece of silicone is not one that you refer to ?
  • 0 in reply to Errol Kowald
    TI__Intellectual 2320 points

    Hello Errol,

    It is good to know that it works.

    The resistor values which I had referred to were optimal numbers to ensure a pre-load current. This part however has a pre-charge FET inside which helps during the startup, leaving the pre-load as an extra operating condition. However, it is a helpful when and if the pre-charge FET 'misfires', especially at light load or high Vo startup.

    We will look into the DS and figure out a better way of wording it.

    Regards,

    Sourav