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LM25141-Q1: How to calculate the RHO,RHOL,RLO,RLOL resistance value ?

Went Cao
Genius 5550 points
Part Number: LM25141-Q1

Hi, all

      In the LM25141-Q1 datasheet, how to calculate the RHO,RHOL,RLO,RLOL resistance value ? Why need the resistans ?

  • 0
    TI__Mastermind 25590 points
    Went,

    The resistors are there to allow you to slow down the turn-on/off of the FETs (aka slew rate control). A larger resistor will turn on/off the FET slower meaning a softer transient and less ringing. This correlates to better EMI performance in the tens-hundreds of MHz range.

    You can build the circuit with 0-ohm resistors and populate them during testing if you need them to help EMI performance.

    You can calculate the value needed by knowing the capacitance of your FETs and calculating how quickly you'd like them to turn on. Or you can do what I do and populate 10 ohms for RHO, RLO, and RLOL (RHOL=0ohms) and modify as necessary.

    -Sam
  • 0 in reply to Samuel Jaffe
    Genius 5550 points
    Dear Sam
    Could you give me the equation about calculating the value of RHO,RHOL,RLO,RLOL resistans ?
  • 0 in reply to Went Cao
    TI__Mastermind 25590 points

    Went,

    A capacitor through a resistor will charge with the equation:

    Vcapacitor = Vsupply * (1 - e^(-t/(RC))

    Use that equation to calculate the resistor necessary to meet your rise time spec (rise from 10% Vsupply to 90% Vsupply) using the parasitic capacitance of your FET.

    You can also utilize this site for quick calculations.

    -Sam