+ Post New Thread
Results 1 to 4 of 4
  1. #1
    Newbie level 6
    Points: 258, Level: 3

    Join Date
    Jun 2019
    Posts
    14
    Helped
    0 / 0
    Points
    258
    Level
    3

    Setting Ron or Rds of LDO

    I am modeling LDO in PSpice, here i have selected spec as ZXCL280 in dropout vs output current I have understood this as resistance of the MOSFET from dropout graph. That is minimum resistance required for the MOSFET, 2 OHMS (Page No 4). I will be using Ron=1/kp(Vgs-vt)

    https://www.digchip.com/datasheets/p...XCL280-pdf.php

    How will i get to know the right Vgs to select ron, Is this the right method or any other alternative way you can suggest?

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 41,414, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,672
    Helped
    1971 / 1971
    Points
    41,414
    Level
    49

    Re: Setting Ron or Rds of LDO

    You probably want a substantially lower Rout, else as you
    approach the dropout voltage you will "wind up" the error
    amplifier and have no authority (small signal) over the
    output.

    The Vgs you can impose, depends on the pass FET gate
    drive and pass FET species. A PMOS LDO can impose the
    supply difference to the gate (but again you need "room
    to move", besides what you'd calculate as a bare ohmic
    minimum). A NMOS LDO will have an AUX supply and/or
    a charge pump and these are design degrees-of-freedom
    to some extent.

    If this LDo happens to have a functional schematic in the
    datasheet it may offer you these sorts of clues. If not the
    presence or absence of an AUX supply may tell you things,
    but I have also seen LDOs which "keep it all in house" with
    charge pumps inside that you don't see pinned out.



    •   AltAdvertisement

        
       

  3. #3
    Newbie level 6
    Points: 258, Level: 3

    Join Date
    Jun 2019
    Posts
    14
    Helped
    0 / 0
    Points
    258
    Level
    3

    Re: Setting Ron or Rds of LDO

    What is better approach to set Ron for simple LDO loop?



    •   AltAdvertisement

        
       

  4. #4
    Super Moderator
    Points: 83,926, Level: 70
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    17,063
    Helped
    3856 / 3856
    Points
    83,926
    Level
    70

    Re: Setting Ron or Rds of LDO

    Hi,

    feel free to use the internet search on your own:
    * "LDO basics"
    * *How to design an LDO"
    * "LDO design tutorial"
    .. or similar.

    Many useful hits, even videos.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



--[[ ]]--