Re: gEDA-user: Alarm clocks and switching regulators

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

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


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