Couldn't you have Vss=0, Vdd=12, and a potential divider (instead of the inverter) on the output to drop 12V down to 5V?

The problem is, which i am not sure why is that when I change Vss= 0. When Vin is 0 to 2.8 volts the output is no longer a pulse. It is just a staight line because the pulse is so wide. There is no time for the pulse to go low and back high again before the next period.


I change the Vss of my sawtooth generator to 0 volts. but when I change the Vss of the comparator to 0 the sawtooth is offset to about 2.5 volts.

 

Something like this perhaps?
Note that R1 and R2 provide an offset for the sawtooth input.