The optocoupler is used alone and delivers some current to a shunt regulator such as implemented bythe MC3337X series or the MC44608. In these devices, the duty-cycle DC is adjusted by injecting a current intoa feedback pin (FB). When the current is low or zero, the duty-cycle is pushed to the max (74% for theMC33370, 80% for the MC44608). If more current is pushed into the pin, the duty-cycle goes toward a fewpercents. The amount of current needed to go from full DC to null DC determines the PWM gain, assuming the
measurement is carried upon a linear portion on the curve DC versus IFB. According to these remarks, there aretwo ways to model the PWM chain but only one is valid to calculate the pole and zeroes created by the primarycompensation network (MC3337X series only). Figure 1 details how the duty-cycle conversion takes place :