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LM3150: Rlim and Cff value calculation

Abhilash G
Prodigy 250 points
Part Number: LM3150

Hi ,

I have the following doubts w.r.t LM3150 design.

1. Rlim is calculated using Rlim = Icl*Rdson/Ilim-th.

Load current :  8A

Rds On= 3.3mOhm

By choosing to go into current limit when the average output load current is 20% higher than the output load current of 8A while the inductor ripple current ratiois 1/3 of the load current will make ICL= 7A. Then the following calculation of RLIM is:

Rlim= 7*3.3/75= 308 ohm.

While in the webench, Rlim for the above parameters shows to be around 1.18K. How is it been caculated ?

2. Cff= VOUT / (VIN-MIN x fS x ZFB)

Vout = 4.2V

Vin min = 9

Fs = 500KHz

Rfbt;73.2K

Rfbb :10K

therefore, Cff= 106 pF

While on webench the value of Cff shows to be about 330pF. Why ?

Thanks and regards,

Abhilash G

106.08378870674
  • 0
    TI__Mastermind 23845 points

    Hi,

    I will forward this to the webench group

    Thanks

    -Arief

  • 0
    TI__Mastermind 26740 points

    Hi Abhilash,

    Would it be possible to share your WEBENCH design using public link option (see screenshot below for reference)? This will help us check calculations for your conditions.

    Regards,

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 250 points

    Hi Amod,

    Please find below the public link of the design.

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=1C02BC96AFF3D30B

    Regards,

    Abhilash G

  • 0 in reply to Abhilash G
    TI__Intellectual 2320 points

    Hi Abhilash,

    Thanks for sharing the design. We are looking into WEBENCH calculation for Rlim and Cff. 

    We shall let you know once we figure out the reason for discrepancy in WEBENCH and data-sheet.

    Best Regards,

    Harish

  • 0 in reply to Harish K R
    TI__Intellectual 2320 points

    Hi Abhilash,

    We have corrected the equation for Rlim & Cff selection in WEBENCH. However the update will be available on ti.com by 2nd week of July.

    Kindly let us know in case you have any queries.

    Below are the details regarding the equations

    1. Rlim: Below is the snapshot of data-sheet for Rlim selection

    Below are the breakup for calculations

    a. Iocl = 1.2 * Iout (20% margin)

           = 1.2 * 8 = 9.6A

    b. Delta IL can be calculated using below equation

         ((VinMax - Vout ) ∗ Vout)/(Fsw ∗ L ∗ VinMax )

          From your shared design, VinMax = 24.2V, Vout = 5V, Fsw = 486KHz and L (Inductor) = 3.3uH 

          Delta IL = 2.47A

    c. Hence Icl can be calculated using equ (6) from data-sheet (see above image)

         Icl = 8.775A

     d. Finally Rlim can be calculated using equ (7)

          i. One thing to note here is WEBENCH uses accurate Rdson of selected Low side MOSFET based on its junction temperature. 

               From your shared design, CSD18531Q5A is getting selected as Low side MOSFET. For now you can use 7.4mohm for your calculation.

               This value will change if the MOSFET selected is different.

          ii. Ilim-th is considered as 75uA based on electrical characteristic table (Pg No 5 from data-sheet under Soft-Start section)

    Rlim comes out to be around 866ohm for your shared design.

          

    2. Cff can be calculated using below equations.

    From your shared design, VinMin = 7V, Vout = 5V, Fsw = 486KHz, Rfb1 = 73.2Kohm and Rfb2 = 10Kohm.

    Cff comes out to be around 167pF for your shared design. 

     

    Thanks & Regards,

    Harish

  • 0 in reply to Harish K R
    Prodigy 250 points

    Thank you Harish.

  • 0 in reply to Abhilash G
    TI__Intellectual 2320 points

    Hi Abhilash,

    The updates are now available on ti.com.

    Thanks & Regards,

    Harish