On Mon, Sep 24, 2007 at 06:27:19PM -0500, John Griessen wrote: > Randall Nortman wrote: > > > I am dropping anywhere from 20V to 42V (peak) down to 3.3V. > > > it has occured to me to just accept a bigger 120Hz ripple > > and use smaller bulk caps. > > Sure. That's a good direction to explore. Figure the absolute value you need > and keep it above that by knowing how much your current draws are. > Let the switcher input see 5 to 42 Volts and its constant inductor current > mode > will have to exercise some, and you can help it filter out ripple with > attention > to grounds and guard voltage zones in your layout.
Make that 6-42V for my switcher - it needs at least 6V to provide its own internal Vcc. And I like 8-42V for margin. But after I wrote that, I realized that I'm also driving a low-current 5V linear regulator off the same bulk caps, and it probably won't like huge ripple. I'll check the specs and think about how much I can reasonably get away with. I guess that since it is supplying only very low current circuits, I could help it out with a relatively fat capacitor on its output (but still way smaller than big bulk caps). That should help it smooth out line ripple, no? Or I could even put in a diode and then a special bulk capacitor just for the linear regulator, which would not need to be nearly as large to deal with the ripple due to the low current draw. > Keep all that primary side switching off to one side. I'm doing this on a 4-layer board, so I have both ground and 3.3v planes. My plan was to put all the power stuff in one corner of the board, and carve moats around it in the power planes, with just one fairly narrow channel in each plane to the rest of the board. I would take care to line up the moats/channels in the planes, so that the return current flows right underneath the supply current, which I gather is a good thing. (That's something I read online somewhere about split power planes and making sure that there is a return current path immediately underneath each circuit trace to minimize impedances, prevent ground loops, EMI, something wacky like that. It's all black magic to me. I just remember that if you put moats in your ground plane, you're not supposed to let any traces cross the moat.) -- Randall _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user