Sort of like the common jelly bean MC34063 switcher, but capable of fully 100% duty cycle, does it exist ?
I'm aware there are many choices from different IC makers like Linear, TI and etc, its just most of them are exotic and not that easy to source like the MC34063 or those common National Simple Switcher family ics.
It doesn't need to be highly efficient, heck, even 70% is more than enough and switching freq at < 500Khz is fine too, and if it has thru hole version, that is even better.
Targeted circuit is just a simple adjustable step down from 12 volt source to drive a DC motor (a drill) ranging from 5 volt up to full 12 volt as the source, with 2 or 3 Amp max.
Since its driving a motor, output voltage ripple and the regulation performance like transient response are not that critical.
I doubt there are any such devices, not least because although 100% is easy enough, duty cycles very close to 100% are impossible because of the finite amount of time it takes to turn a transistor on or off. Since most power supply control ICs are built around analogue control loops, they'll always max out at some point below (say) 90% duty cycle.
It doesn't sound like you actually need a regulated supply like that, though. Can you not build a circuit to generate a rectangular wave with variable duty cycle, and use that to turn the 12V supply to the motor on and off?
I built a circuit to control power to an electric heater last year, which doesn't sound like too dissimilar a problem. It consists of a relaxation oscillator and a comparator; the oscillator provides a voltage that goes up and down, and the comparator generates an output which indicates whether the oscillator voltage is above or below some variable threshold. Set the threshold outside the oscillator's range and you get 100% output duty cycle. Gradually lower the threshold voltage and you can get a continuously variable duty cycle all the way down to 0.
