Pulsing input voltage to Buck converter --- unstable?

[NuNDoe]

Hi everyone, this may not be standard use of the device but I'm wondering if it's possible (for reliable operation) to pulse width modulate the input voltage to a buck converter.

Here are the details:
I have a 12V PWM used as power for LED dimming. However, since I only have a single LED, which Vf is at most 4V, I don't simply want to "burn" off the extra voltage through a resistor for efficiency reasons. Can I use a buck converter to efficiently drop down the 12V to 5V and then use a smaller resistor in series with the LED?

It seems like the standard usage for bucks are for stepping down voltage when in the input voltage is constant.... but what if that input voltage is a PWM? Can I get a reliable lower voltage PWM output?

I haven't tested this out but it should work in theory since the switching freq of the buck is on the order of 100KHz where as my input PWM is only about 300Hz... but I don't know if it's bad for the circuit to keep getting pulsed input voltages.

Anyone else have better suggestions for reducing the 12V PWM down to 5V PWM efficienty?
Thanks!

 

[alehman]

By "buck converter" do you mean buck transformer? Or are you asking about a solid-state converter?

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.