请教一个电路，active capacitance

你的EA out这一点应该是pwm comp的输出吧，然后在这里用R,C做补偿。
这个电路很简单，就是负反馈的opamp，它的闭环TF 近似= 1/s（你可以具体推导）。
那么opamp两端如果有delat_V的压差，就会产生delta_V/(s*R)的电流，这就类似于电容的特性了。(voltage = current/(s*C)，对于电容来说)
那么这里实际上就是R, C串联的补偿。

LS，EA out是EA的输出，不是PWM的输出。EA输出加一个RC是补偿环路的。所示图部分是起到电容作用（含电阻），电路实现原理不是太清楚。

Roughly analysis, the equivalent circuit =Req+1/SCeq = R1+R2+ [R2*R4/(R3+R4) ] * 1/(SAC), Where A is the two stage DC gain.
In this case, this path provides a low frequency Zero for compensation, while this C is active boosting capacitance. This is very important in DC-DC converter design becasue you need a low frequency zero which will consume a lot of area.
If you can give me more details about the value of all bias current of NPN, PNP, resistor and device name(can not see clearly in the picture), it will be helpful to the descriptions.