For future reference this can be done simply using a SOC low end part, PSOC 4M.
Single chip, you need just one external C for bypass power.
Simply drag and drop the PWM, PINs, and Clock resources out of the chip resource
catalog, route them, wire to each other, and double click to config period and desired
pulse width. Can operate as a one shot from an edge signal or just continuous.
One instruction has to be typed into C file, a start instruction for the PWM, done.
Note many other chip resources still available for other stuff, see below. Also see
right hand window for design resources used/left.
Timing accuracy over T and V is +/- 2%. I set it for 400 nS thinking you wanted it no more than
500, but then saw later your note 400 - 600, so simple to change that in settings.
Other stuff on chip....multiple copies of many....
Regards,. Dana.