Hi all, I'm looking at dc/dcs for running 400mA - 1Amp highpower LEDs=20 with some special requirements, and in thinking about various schemes, I=20 wonder how (or if) something this simple would work. Setting aside the minor additional complexity of making a=20 constant-current vs. constant-voltage dc/dc, what if there were a=20 circuit like: +12V-->PFET-->Cap-->Output(say 5V target). The output would have a voltage divider (i.e. feedback) going to a=20 comparator, which has a reference voltage for its other input. The=20 comparator would turn on the PFET if the feedback voltage were below the=20 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=20 much analog that isn't largely 'cookbook' with tweaks) much less built=20 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=20 value gets larger, it takes more time to charge (thru PFET) and=20 discharge (thru load), and as the comparator hysteresis gets smaller,=20 the PFET switching on-time gets shorter, so it switches more often (less=20 efficient) but you have less output ripple. If the comparator were an opamp, driving the PFET gate directly, you'd=20 have a DIY linear regulator. By using a comparator with hysteresis, you=20 get a dc/dc. I think. For my application, I will know the load (a string of LEDs being=20 high-speed PWM dimmed), the approximate needed output voltage to light=20 them at the designed current, and will be doing a little trickery to=20 handle all that. I can tolerate a moderate (humm... maybe 10%? TBD)=20 output voltage ripple. This seems somehow too simple to work reasonably, but I'm not sure why.=20 C1 would have to handle the ripple current and the inrush current (as=20 would the PFET - something that would need a bit of additional design=20 work, can be handled a few ways), R3 has to be low enough value to turn=20 off the PFET quickly when the NFET turns off, but high enough value to=20 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=20 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 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .