Re: gEDA-user: Alarm clocks and switching regulators

[John Griessen <john@xxxxxxxxxxxxxx>]

Randall Nortman wrote:
> 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,
[jg] Can you feed that one from a switcher output or intermediate voltage...





  since it is supplying only
> very low current circuits, I could even put
> in a diode and then a special bulk capacitor just for the linear
> regulator, 
[jg]Now you're at it.   Experiment with some of those.

> 
>> 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

[jg]no, just the noisy set.


  in one corner of the
> board, and carve moats 

Yes, guard voltage 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,

[jg]That will limit fields.  For your speeds and frequencies you will do best
to connect the switching inductors and chip zone to the supplied quiet zones with chains of RC or  RLC filters
in between.  The current conductors might wibble some and break the incantation's rules
and still be just fine...if you have an AC damping transmission-line-like chain of R and Cs, maybe some L.

> It's all black magic to me. 

Oh...noooh... be sure and observe while experimenting, don't just follow incantations...

not supposed to let any traces cross the
> moat.)
> 
That last sounds like a spell...skip it.

It has to have  a reason -- like keep the application circuit traces out of the moat...
Not all...

John G
-- 
Ecosensory   Austin TX


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