ZFF Self-Modulating PWM

You do not adjust the PWM, the 9910 adjusts in order to regulate the current.

You can put in a voltage on the LD (linear dimming- it is meant to drive power LEDs) pin to bring the peak voltage on the current sense down from 0.25V.

When the frequency adjust resistor is tied to the drive pin instead of ground it works in constant off time mode. In this mode the frequency varies as the line and load vary to maintain a constant current output.

That circuit is a whole lot simpler than most PWMs that are found out there on the net. I may use that if I do build my Hydrogen on Demand system eventually.

Using the previously attached Schematic, what could be used to control the duty cycle of that circuit, w/o using a PIC Microcontroller?

Are you asking about the 555 circuit or the HV9910?