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Original question:

LM5116: LM5116 equation 35 & 37

LM5116: Equation 37 in Datahsheet, why not ripple current for average current limit

Timothy Coulter
Prodigy 90 points
Part Number: LM5116

Tool/software:

I had a similar question to the post below.

It is not clear why the current sense resistor would be calculated from Vcs/(Iout + (Vout * T) / L).

I am solving for the average current limit of my design using the LM5116 with a given sense resistor, so I am left with this equation: Iout = Vcs/Rs - ((Vout*T)/L). The actual current limit I measure is pretty close to this equation, but I do not see why Vout * T / L is correct. I could see subtracting the inductor ripple current, since the inductor ripple current will have an average current value + the ripple current. But the equation of Vout * T / L is not accurate. During the off time of the high-side FET, the equation is Vout *Toff / L, not the whole period T.

Can someone explain why this equation is correct?

LM5116: LM5116 equation 35 & 37 - Power management forum - Power management - TI E2E support forums

  • 0
    TI__Guru 58851 points

    Hello Timothy 

    I cannot find the original derivations of those equations, but I believe the equation 38 was derived assuming VOUT=7.5V, and VIN=7.5V which is absolute minimum input supply voltage in buck topology when VOUT=7.5V.

    If VOUT=7.5V and VIN=7.5V,the equation 35 becomes

    =======> 

    If VOUT=7.5V and VIN=7.5V, the equation 36 becomes

    If VOUT=7.5V and VIN=7.5V, the equation 37 becomes the same as the equation 36

    -EL