This video helps you learn the operation of this circuit in detail..!
In this video, initially operational amplifier output voltage is found by making use of golden rules of operational amplifier along with KCL and ohms law
Later, the voltage across capacitor at gate terminal of NMOS is found using time constant concept of the low pass filter.
Now the voltage of capacitor after 100 micro seconds is found and then check for ON state of NMOS by comparing Gate voltage with Vgs
Later, the comparator output is found and hence the duty cycle.
