You've almost reinvented the hysteretic switcher. Something to think on, for LEDs you don't really need a constant current supply, with its high output impedance and tight regulation. You mostly need a "steady" current supply. Current ripple is tolerable. The human eye can't perceive it. On Wed, Jun 5, 2013 at 5:21 PM, Isaac Marino Bavaresco < isaacbavaresco@yahoo.com.br> wrote: > I had understood that you would put a resistor in series with the > transistor. After looking at your schematic I realized that you are > relying only in the transistor's internal resistance. > > If you replace in my post the word "resistor" with "transistor's > internal resistance" it will make more sense. > > Actually you are dissipating the excess energy in the transistor's and > capacitor's internal resistance. > And you will have REALLY LARGE current spikes. > > > Isaac > > > Em 5/6/2013 19:58, Isaac Marino Bavaresco escreveu: > > Without an inductor your regulator will be linear, it doesn't matter > > whether it is frequency switching the transistor or not. > > If you do the math, you will see that the extra energy will be > > dissipated in the resistor and transistor and will be the same as in th= e > > standard linear regulator. > > All that you have is a linear regulator with a lot of ripple voltage an= d > > current. > > > > Now, if you use an inductor then it may become a completely different > > beast. The inductance acts as an "inertial flywheel" for the current, > > limiting it without converting it in heat as in the resistor and > > transistor (of course it is not an ideal inductor, it has a tiny > > resistance and thus will waste some power). > > The inductor limits the raise and fall of the current, storing energy a= s > > B field while the current is increasing and supplying current while the > > transistor is off. > > > > > > Isaac > > > > > > Em 5/6/2013 17:30, Jesse Lackey escreveu: > >> 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 t= he > >> reference voltage, and turn off the PFET if not. > >> > >> Schematic... > >> > >> > >> 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 (le= ss > >> 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, y= ou > >> 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 (a= s > >> 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 tur= n > >> 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 > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .