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WEBENCH® Tools/LM5145: Rlim Value calculation issue for 40V to 60V vin and 3.3V@12A output?

Ashwin S1
Prodigy 90 points
Part Number: LM5145

Tool/software: WEBENCH® Design Tools

40V to 60V vin and 3.3V@12A output,

Below is my conecern, 

1. When i used TI webench i got the value around 5.61K for Rlim.

2. When i used quick start calculator i got around 1.1K.

3. When i did the manual calculation i got the the valure of 561.6ohm. 

Could any please help out on this issue on priority.

WBDesign9_Steady State-6_Fsw400_1p5uH.pdf

  • 0
    TI__Mastermind 23845 points

    Hi Ashwin,

    Let me check the values. Normally i would go with the calculator since the application engineer who is developing this product also develop the calculator

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 90 points

    Hi Arief,

    Thanks for the update. Please let me know ASAP, because of this issue we are holding the design.

    For the calculation, we kept the ripple current ΔIL = 4.8A (40% of the O/P current).

    Regards,

    Ashwin S

  • 0 in reply to Arief Hernadi
    TI__Mastermind 23845 points

    Hi Ashwin,

    I think the answer depends upon what is the Rdson of the Low side MOSFET that you use, inductor selection and the set value for current limit. The value for RILIM can vary.

    From what i see is that you result and the calculator result is in the correct range of resistor. The value of 5.62kohm seems a little large. 

    3173.LM5145 Wide Vin Synchronous Buck Controller Quickstart Calculator r12-7.xlsm

    I would suggest to follow the calculator in this one

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 90 points

    Hi Arief,

    Thanks for the immediate response.

    When we entering the values through the calculator Rlim value is  ~ 1K.

    Below is the manual calculation, Based on the datasheet formula. Please let me know which one is correct, we will be operating the device up to 55°C. 

    RLim  =   (Iout - ΔIL/2)*Rdson(Low side mosfet)/Irdson

               = (12-4.8/2)*11.7mOhm/200*10^-6

    RLim = 561.6 Ohm

     

     

     T = R*C = 1k*15pF = 15nS(which is comparatively higher the datasheet spec)(Calaculator)

    Please advise which is the best one to follow, so that the device would operate at its peak without any malfunction.

     

    Regards,

    Ashwin S

  • 0 in reply to Ashwin S1
    TI__Mastermind 23845 points

    HI Ashwin,

    I think the discrepancy is what method of current sensing that you are planning to use. 

    Are you using the Rdson sensing or are you using Rsense sensing? The Rsense sensing requires an extra sense resistor at the bottom of Low Side MOSFET.

    If you use the Rsense method then the denominator should be 100uA instead of 200uA. 

    Do you enable macros on the excel? Maybe when you choose the drop down box, the forms did not get updated. 

    You can see on the bottom, if i use the RDS on sensing the value of resistor is 562 ohm. This is because the peak to peak ripple current is calculated at the nominal VIN of 54V and the delta IL is 5.5A instead of 4.8A

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 90 points

    Hi Areif,

    I arrived at the same value during the manual calculation for Rdson sensing mode. I am using the RdsON sensing mode.

    Yes the macros is not updated in the excel, i will be using the 562ohm as Rlim and will adjust the Clim value in order to meet the 6ns timing specification.

    Regards,

    Ashwin S

  • 0 in reply to Ashwin S1
    TI__Mastermind 23845 points

    Hi Ashwin,

    Ok let me know if you need further help 

    Thanks

    -Arief