open loop buck converter: 12V PWM to 4V PWM

[NuNDoe]

My ultimate goal is to efficiently step down a 12V PWM to a 4V PWM without having to use voltage dividers as it's essentially "burning off" the extra voltage. I know for constant voltage, this calls for a buck mode DC-DC converter IC with a feedback loop that controls the output voltage to 4V. But what if the input voltage is a 12V PWM that ranges from 0% to 100%? I doubt the buck mode IC will work in this case when the duty cycle is near 0% because it won't have enough juice to turn on its switching circuits...

so the question is, is there an OPEN LOOP switching circuit that can be made to switch the buck converter even near 0% PWM?

I'm thinking of using a crystal to operate a transistor that can act as the switch for the buck circuit... any ideas on this?

thanks!

 

[leo_o2]

Of course, the duty cycle can be close to 0.
Imagine the period is very big and on time is very small.
And many controller can work under skipping mode that means very low duty cycle.

 

[atripathi]

yes. that can be done. For light load and high input voltage your supply may work in skip mode. on-off for few cycle and then remain off for next several cycle. For example Look at Lt1308 details. also 4-12v input range can be easily covered by controller.