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LM5085: EMI issues with LM5085!!

David Sanchez20
Prodigy 150 points
Part Number: LM5085

Dear all,

I'm trying to reduce the size of a previous buck converter designed unisg the LM5085. My design parameters are:

24V < Vin < 65V

Iout_nom = 10A

Iout_min = 0.5A

Vout = 12.5V

By following the schematic attached, I've drawn the layout also attached. During the validation (I've stooped in IOUT = 3A), I've noticed and erratic behaviour of my converter in Vin = 30V  because VOUT raises 2.5V approximately and from here comences to decay to 12V (see attached table). in different VIn, the buck converter works fine!

Do you think I've EMI problems? There's one think I hate the most about my layout, PGATE and ISEN cross each other in different layers. Do you think thi's is a big issue?? 

Many thanks,

David

  • 0
    TI__Mastermind 23845 points

    Hi,

    From the schematic, R4, C7 and C8 need to be populated if you don't have enough ripple at the output .

    I also tried to put in your design parameter into the calculator and it gives me warning that design over 8A is not recommended. 

    I attached the calculator in this document as your reference, you can try to play around with the resistor injection value and see if you can get a higher current. 

    Are you able to get the steady switching waveform up to 2A? Meaning Frequency is constant and no oscillation was seen?

    We can also provide you with the EVM if you like so you can test out your design values with the recommended layout

    7801.LM(2)5085_Quick_Start.xls

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 150 points

    Hi,

    How could I know which type of ripple filter configuration is needed??

    Thanks!

    DAvid

  • 0 in reply to David Sanchez20
    TI__Mastermind 23845 points

    Hi,

    Type 1 is the lowest cost but higher ripple at the output

    Type 2 is lower output voltage ripple compared to type 1 with the addition of CFF

    Type 3 is minimum output voltage ripple at the output. 

    Therefore the selection of type 1,2,or 3 is dependent upon how much ripple requirement at the output. If you can tolerate a large ripple then you can use type 1. 


    The idea of this control scheme is you have to have a ripple in the FB node to make sure that the IC works properly.

    You can cross check this value with webench design also but you need to scale the output current down for webench because the part is not recommended for 8A or higher current

    Thanks 

    -Arief 

  • 0 in reply to Arief Hernadi
    TI__Mastermind 23845 points

    Hi

    I hope you have resolved your issue. For now i will close the thread feel free to open it again if you still have issue

    Thanks

    -Arief