As others have said, anything without an inductor must dissipate the energy
not used by the LED from the V x I input. It's a "linear" regulator when
it's regulating, whether the linear happens in bursts or continually.

Adding an inductor and making it a boost converter is easy, moderate cost
and works well.

Richard Prosser's relay driver that led to Roman Black's 2 transistor
"Black Regulator" is an elegant way of achieving what you want with two
transistors.

For hardly more (if any) cost an olde steam driven MC34-63 steam driven
SMPS IC will do what you want - probably without an external FET. You can
buy LED drivers on ebay that do what you want and that often cost less than
you could sensibly build them for. They often use MC34-63s.

Example. ebay
Uses LM2596.
0-4A adj.
5-35V in
1.25V-30V adj out.
$5.45 buy now
Free shipping.
http://www.ebay.com/itm/LED-driver-with-constant-current-power-supply-step-=
down-/170988298335?pt=3DLH_DefaultDomain_0&hash=3Ditem27cfb2605f


    R



On 6 June 2013 08:30, Jesse Lackey <jsl-ml@celestialaudio.com> 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
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