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LDO Document-Slva072

xiangkai zhu
Prodigy 160 points

Document-Slva072 has a derivation of load regulation. Can you get the conclusion Req=1/G*(R1+R2)/R2 from the derivation? If so, then the assumption of equation (23) in the derivation of line regulation in the next section is somewhat problematic. Bringing the expression of Req into the final result (27) should be divided by 2.
In addition, I have doubts about the line regulation. If the saturation zone I is completely independent of Vds, then even if Vin changes, can Vo and I remain unchanged? However, this conclusion cannot be seen from (27). I think the line regulation is related to the unsatisfactory I-V curve. For example, the current in the saturation region is also related to Vds, which is often the case with actual devices.

  • 0
    TI__Prodigy 515 points

    Hi XiangKai:

    I agree on Req=1/G*(R1+R2)/R2. The equations and derivations in the app note look good. If you can provide us with your detailed derivation of your equations, I am glad to compare both. 

    The section 1 Dropout Voltage in the app note give a very good review on how the LDO operates.  Please take time to review in detail. If you can clarify your concern about line regulation, it would help us to provide better answer to you. 

    Regards,

    Min

  • 0 in reply to Min Hudgins
    TI__Prodigy 160 points

    Hi Min:

    The following are the derivation of the document and the derivation when  Req is substituted in the Equ (22).

    Whether the line regulation is related to the undesired I-V curve, for example, the current in the saturation region is also related to Vds, and the actual device is often the same.

    Regards,

    Xiangkai

  • 0 in reply to xiangkai zhu
    TI__Prodigy 515 points

    Hi Xiangkai:

    I am reviewing your derivation. I will get back to you when I finish my analysis.

    Regards,

    Min

  • 0 in reply to Min Hudgins
    TI__Prodigy 160 points

    Hi Min,

    I am sorry to bother you. How do you think of the derived formula?

    Regards,

    Xiangkai

  • 0 in reply to xiangkai zhu
    TI__Prodigy 515 points

    Hello XiangKai:

    I have been thinking about your question. The assumption(small load change causes the output voltage change)  in the load transient can't be applied to the line transient equations. Because in the equation (24), the first part (where you substituted the G with 1/Req*(R1+R2)/R2) )  is the steady state average output voltage, the load current is steady.  

    Hope this helps. 

    Regards,

    Min