Re: Switching power supply question

Jim Thompson wrote:
On Sat, 23 Feb 2008 14:56:31 -0800, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 23 Feb 2008 20:32:37 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Sat, 23 Feb 2008 10:42:48 -0800 (PST), wabehtdieh@xxxxxxxxxx wrote:

A swithing supply is a feedback control system. Therefore why doesn't
it oscillate when you put a capacitive load on it? It should bugger up
the phase margin.

Wab
Probably because some clever person designed the loop so that it
becomes more stable as you add capacitive loads.

Does that mean there can't be many clever persons in groups tasked with LDO design? SCNR ...
Apparently not. A couple of people make opamps that tolerate any
capacitive load. The concept is pretty simple: the output pin is one
end of the compensation cap; adding load capacitance doesn't add a
pole to the loop, it just shifts the dominant pole down.

Not quite as simple as that. There's some feed-around techniques used
to isolate the load.

The guys who
do LDO's ought to be told about this.

John

They can't, without an extra pin, and an external R/C... LDO's can't
afford to waste power with implementing the R on-chip... a small R
forces a large un-integrable capacitor.


First post must have been picked up by a wind gust (it's ugly out here right now).

We discrete designers would not mind an extra pin one bit. We are used to external parts from PWM chips. What's the big deal?


[...]

--
Regards, Joerg

http://www.analogconsultants.com/
.

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