The Line Regulation Of DC-DC Converter

When the input voltage is changing in DC-DC converter, how to influence the stability of closed loop? In other words, when the input voltage is varying, the output voltage cann't be stabilized and will also generate small variation. Since the reference voltage is stability, i cann't find what reason result in bad line regualtion.
Pls give me a guideline, thks!

try to have a look at your loop gain and understand it.

You are right, the reference is the stable element in the system. So if it is stable, everything else should be stable.
That is almost true. The problem is that the error amplifier (the one that compares the output to the reference) has a limited gain, it is not infinity. The actual error between the P/S output and the reference is in fact
ε=ΔV/A

Where ε is the final error, ΔV is the difference that would exist if the error amp was not present and A is the gain of the error amp. As you can see, if the gain of the error amp was infinity, then the final error would be zero, in other words the output would faithfully follow the reference and it would not change, except for changes in reference. (We are talking about the DC gain here, or gain at very low frequency).

However, the error amp has a finite gain, which in the end results in a non-zero error. Typical DC gains for the entire chain can be anywhere from 60 to 120dB. 60dB is not that good, it means a gain of 1000, so the error will only be 1/1000 the variation that would occur in open-loop. That is not too great, since the output can change a lot without regulation. So dividing that "a lot" by only 1000 still results in some significant change in the output when the input changes.

Thks VVV for your reminding gain of error amplifer. According to your suggestion, the open gain of error amplifer is somewhat low, but i have a doubt concerning the relation of Vin,Vo and gain of EA,in other words, it has what relation in Vin,Vo and gain of EA. Does your mentioned ΔV equate (Vin-Vo) when the error amp does not present ? I think the ΔV should equate the difference of "+" terminal and "-" terminal of EA.
I have uploaded a buck converter block figure in the attachment. Pls illustrate the relation for me! Thks in advance!

ΔV is the difference in the output voltage without the error amp present and with the input voltage changing by ΔVin.
And if there is a divider from the output to the error amp input, that divider ratio will affect the regulation, too, since the error amp sense a lower difference. Therefore, the real error is even larger, multiplied by the divider ratio. So for a ratio of 2:1, you get twice the error, when the error amp is present.