To followup my own post - in thinking about it a bit further, it seems=20
not practical due to very high and hard to predict/control current when=20
charging the cap.  Example PFET/cap:

DMP2540 PFET 25V/4amp, 40 mOhm
EEE-FK1C471 electrolytic cap, 470uF 16V, 160 mOhm, 600mA ripple current

So for 12V supply and 5V output voltage, we get approx current when the=20
PFET turns on:
I=3D V/R =3D (12V-5V)/(0.04+0.160)=3D 35amps.  Ha!

So as long as the PFET and cap are ideal devices, this'll work just=20
fine.  :)  Or if there is a few ohms resistance somewhere to control=20
current, but that lowers efficiency so much that it doesn't make sense=20
to do.  However, if the output voltage is near the input voltage, the=20
current thru the PFET will be much less, and this scheme may work=20
reasonably well.  Or if the power supply has a relatively high output=20
impedance, like some battery types.

J


Jesse Lackey wrote:
> Hi all, I'm looking at dc/dcs for running 400mA - 1Amp highpower LEDs
> with some special requirements, and in thinking about various schemes, I
> wonder how (or if) something this simple would work.
>
> Setting aside the minor additional complexity of making a
> constant-current vs. constant-voltage dc/dc, what if there were a
> circuit like:
>
> +12V-->PFET-->Cap-->Output(say 5V target).
> The output would have a voltage divider (i.e. feedback) going to a
> comparator, which has a reference voltage for its other input.  The
> comparator would turn on the PFET if the feedback voltage were below the
> reference voltage, and turn off the PFET if not.
>
> Schematic...
> <http://www.celestialaudio.com/pfet-comparator-cap-dcdc.png>
>
> Now I haven't even tried a simulation (which I rarely do, I don't do
> much analog that isn't largely 'cookbook' with tweaks) much less built
> it, so I have no 'real data' on how well or if it works.
>
> But it sure seems like it would operate more or less correctly.  As C1's
> value gets larger, it takes more time to charge (thru PFET) and
> discharge (thru load), and as the comparator hysteresis gets smaller,
> the PFET switching on-time gets shorter, so it switches more often (less
> efficient) but you have less output ripple.
>
> If the comparator were an opamp, driving the PFET gate directly, you'd
> have a DIY linear regulator.  By using a comparator with hysteresis, you
> get a dc/dc.  I think.
>
> For my application, I will know the load (a string of LEDs being
> high-speed PWM dimmed), the approximate needed output voltage to light
> them at the designed current, and will be doing a little trickery to
> handle all that.  I can tolerate a moderate (humm... maybe 10%? TBD)
> output voltage ripple.
>
> This seems somehow too simple to work reasonably, but I'm not sure why.
>    C1 would have to handle the ripple current and the inrush current (as
> would the PFET - something that would need a bit of additional design
> work, can be handled a few ways), R3 has to be low enough value to turn
> off the PFET quickly when the NFET turns off, but high enough value to
> not be too wasteful when the PFET is on.
>
> I like that I can set the output ripple by choosing C1's value and the
> comparator hysteresis.
>
> Thoughts?
> What's going to fail first if run 24/7 for years?
> Similar, polished, known-working designs anywhere?
>
> Thanks all!
> J
>
--=20
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