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Rt value for LM25011

Bharat Patel
Intellectual 405 points

Other Parts Discussed in Thread: LM25011

Hi,

WEBENCH gives me an incorrect value for Rt for the LM25011MY  with the following parameters:

Vin 9.5 to 18 Volts

Vout 8 Volts

Iout 1.9 Amps

The Rt value given is 340Kohms but I calculate it to be about 191Kohms.

Am I doing something wrong?

Regards BP

  • 0
    TI__Intellectual 2360 points
    Hi BP,
    The Rt calculation in the LM25011 WEBENCH design is based on the datasheet equation,
    Rt = (Vout - (Vin x Fs x 15e-9))/(Fs x 4.1e-11) - 500

    When I use Vout=8, Fs = 500kHz, and Vin 9.5 to 18V (exact value doesn't matter much), I get Rt = 341k (ideal value).
    If you are using a different frequency, the value for Rt will be different. For example, 1MHz corresponds to 191k (the value you noted).

    In your WEBENCH design, you can change the target switching frequency using controls on the left side of the design view, in the "Advanced Options" panel. Click the box "User Preferred Frequency", and enter your target frequency in the Frequency box. The practical frequency range is shown in that area for your reference.
  • 0 in reply to Wanda Garrett - WEBENCH Design Center
    Intellectual 405 points
    Thank you Wanda we later noticed the frequency was different on the simulation and could be changed.

    BP