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Derivation of the Rsns Formula in LM5022 Boost Converter

Il Jun Oh
Prodigy 10 points
Part Number: LM5022

Tool/software:

I am designing a Boost converter using the LM5022.

While selecting the Rsns resistor, I referred to the equation provided in the datasheet.

However, I am having difficulty understanding how this formula is derived.

For reference, here are the specifications of my Boost converter:

  • Input Voltage (Vin): 12 V
  • Output Voltage (Vout): 18 V
  • Output Current (Iout): 3 A
  • Switching Frequency (fsw): 200 kHz
  • Inductor Value (L): 30 uH
  • Output Diode Forward Voltage Drop: 1 V
  • Rs1 : 40 ohm
  • Csns : 1 nF

  • 0
    TI__Mastermind 19595 points

    Hello Jun Oh,

    Please note that our datasheets do not contain all details about the internals of our devices.


    The LM5022 is a very old device where you can even find some kind of block diagram of the internal circuitry.
    In case of newer devices this detail is no longer shown.

    The sense resistor is not only used for an overcurrent protection, but - more importantly - to sense the current for the inner cycle-by-cycle peak-current-mode control loop.

    The equation for the overcurrent protection would simply be Vcl / Ilim.
    But for the control loop regulation it is extended with factors for dependence on frequency ( L * fsw ) and for the slope compensation ( (Vo-Vin)*3*D )
    Please understand that I cannot provide more details.

    Best regards
    Harry