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LM5116: LM5116 equation 35 & 37

Part Number: LM5116

Hi Team,

My customer has some questions about LM5116 and they would like to know

  1. why does the Vout range affect the Rs calculation equation, comparing equation 33 & 35 & 37. 
  2. what's the Rs equation when Vout >7.5V?
  3. How does equation 37 come from 35? 

  • Hello Hollis,

    The range of Vout will affect the voltage across the inductor during ton and toff during steady state which will in turn affect the inductor current slope as described by:

    VL=L(di/dt).

    Since Rs is the current sense resistor, the value of Vout will change the slope of the current that flows through Rs and should thusly be considered when determining Rs.

    Equation 37 should be used for calculating Rs when Vout>7.5V.

    Equation 37 is the same as equation 35 but it just does not include the inductor current ripple term shown below. Equation 37 is concerned with the changes in the inductor current on a cycle by cycle change while equation 35 accounts for inductor current ripple due to the voltage across the inductor which is established by the difference between Vin and Vout. To put this another way, in equation 37 they've already calculated the optimal slope current  IOS, so the inductor slope current does not need to be accounted for in the calculation of RS, thus they don't include the inductor ripple term.

    I hope this helps.

    Regards,

    Harrison Overturf