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LM61480-Q1: Spread Spectrum

Alexander Feldengut
Prodigy 230 points
Part Number: LM61480-Q1

Hello,

the data sheet contains a formula for calculating RSPSP. It is also mentioned that the resistance value is normally 20 kOhm.


For an application with an input voltage of 13 V, an output voltage of 3.3 V, a switching frequency of 460 kHz and an inductance of 3.3 uH, I get a resistance value of 70 kOhm.

Is the calculation correct? What is the best choice for my application.

Best regards, Alexander

  • 0
    TI__Guru 50120 points

    Hello Alexander,

    The equation in the datasheet shows the below for spread spectrum resistor calculation:

    If you included the 1.22 value in the denominator calculation, the ne Rspsp value would be around 27.7kOhm which is more inline to what is expected here.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 230 points

    Hello Jimmy,

    thank you.

    How exactly should the resistance value be selected? Is a resistance value of 27k adequate for the described application?

    What is the effect for example if the resistance value is 10% lower or higher than the calculated value?

    Best regards

    Alexander

  • +1 in reply to Alexander Feldengut
    TI__Guru 50120 points

    Hi Alexander,

    You should pick the nearest common resistor value based on the equation.

    There should not be too much effect on a resistor that is +/- 10% from nominal calculations.

    Regards,

    Jimmy