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LM5045 / slope compensation resistor

K.Yaita
Guru 29720 points
Hi Team,
 
In datasheet Page.22, there is the following formula(5).
 
 
 
Is the constant of "Rcs" common for both "Transformer Current Sense(Figure.28)" and Resister Current Sense(Figure.29)?
 
 
I think Rcs of Figure.29(Resister Current Sense) is generally more smaller than that of Figure.28(Transformer Current Sense), so the required value of Rslope is different between Figure.28 and Figure.29.
Is my understanding correct?
 
Best Regards,
Yaita / Japan disty
  • 0
    TI__Mastermind 34465 points

    Yes you are correct. Figure 28 and figure 29 are different implementations of the same overall circuit . It is perhaps unfortunate that the same circuit designators were used in both figures. But when you go through the calculation, you will get the correct result.

     

    Regards,

    John

  • 0 in reply to John Griffin1
    Guru 29720 points
    John-san,
     
    I appreciate for your support.
    I understood.
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    Guru 29720 points

    Hi,

    Could you tell me derivation process of the following formula(5)?
    Why the slope compensation resistor required to implement dead-beat control can be calculated as formula(5)?



    My customer wants to know it.
    Your support would be appreciated.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Intellectual 1550 points
    Yaita,

    1. The compensation slope should be at least half the downslope of the inductor current transformed to the primary side in order to effectively eliminate the subharmonic oscillation seen when duty>0.5.

    The downslope of the inductor current is Vout/Lfilter on the secondary side, then transforming it to the primary side yields Vout*Rcs/(Lfilter*NTR). The compensation slope is Rslope*Islope*Fosc. So Rslope*Islope*Fosc>0.5*Vout*Rcs/(Lfilter*NTR).

    2. 0.5 is the minimum slope requirement. If you want to eliminate the subharmonic oscillation in one switching cycle, the slope needs to be 1. So Rslope*Islope*Fosc>Vout*Rcs/(Lfilter*NTR) which is the equation you had in your question.

    Let me know if you need further clarification.

    Regards,
    Wangxin