Joel Koltner wrote:
Joerg!

How's it doing weather-wise down in your part of the country? It snowed
like mad Friday night and much of yesterday, although it's been warm enough
that much of it is already melted.


I almost slid down the driveway on my behind this morning :-(

No idea what all those greenies mean with global warming. It sure ain't
happening around here.


"Joerg" wrote in message
...
God fixes a lot of things but AFAICT not ground structures :-)

No harm in asking for his inspiration, though. :-)


Sure, you can ask Him anything. Whether stability is going to be granted
for your design is another matter :-)


I am not a fan of clustered grounds but tend to use full ground planes
instead.

Given your frugality I'm actually a little surprised by this statement! Not
a year ago I attended a National Semiconductor "road show" on their latest
switcher ICs and they made a big deal about layout starting with the premise
that, "we want to show you layouts that actually work on the 2 layer PCB
you're most likely going to be forced to use for cost reasons..."


Sure, some of my designs go the same route and often end up on phenolic.
There I use ground pours and connect them where needed. Making a
switcher work without a ground plane is an art but possible. And by
"work" I mean including passing the EMC test.


Anyway, the "real" circuit this power supply goes into will be a four layer
PCB, so not to worry. I know you're a proponent of *not* cutting up the
ground planes, but that doesn't imply that you'd just drop a via anywhere
you needed "ground" rather than, say, routing the ends of R1/C2/R3/R5 back
to pin 4 of U1 first, does it?


When I have the luxury of a 4-layer I usually drop vias where needed. No
routing ground traces back and stuff. The main reason is that any RF
field that finds its way to that area will cause a home run trace to
pick up some of the field. A good ground plane behaves much better.


It would need a low ESR small cap there. I'd never run a switcher without
some kind of ceramic cap at the output.

You're including the Panasonic "speciality polymer" caps I used in that
statement, right?


Ok, specialty caps can work. But we all know how it goes later. Someone
from purchasing finds a much more "economical" part, a rep sweet-talks
them into it, you are on vacation and someone signs the ECO ... poof. I
try to make sure it all works with jelly-bean parts wherever possible.
The extra cost of a specialty cap can often be more than offset by
specifying a cheaper one plus a ceramic cap. It all depends on where
it's produced. Clients who insist on assembly in high-wage countries
must often eat placement costs of several cents per part and that
requires a whole different design strategy.


(regarding C3)
Even 100pF seems a bit high.

Do you have any advice on how you choose such a cap in general? Are you
aiming for a particularly corner frequency or...?


I never use such caps at all. Then again, I rarely use integrated
switchers that are single-sourced.


That can make it go ballistic ;-)

Yeah, thinking about it that does seem to be more along the lines of how I'd
build an oscillator...

R2 is high. If the worst case leakage of 1uA happens that would put you
above the min threshold.

Good catch, I'll change that.

You need a full ground plane. For example, the ground trace to R2 tunnels
right through the "hot zone" underneath the inductor. As Mark suggested
run a big fat wire from the ground at R5 to pin 4 and another from C2 to
pin 4. Cut the ground trace to R2 off right at that resistor and run a fat
wire to pin 4. It'll be messy but this gets you as close to a good ground
as possible without a relayout.

I have no qualms about messy. :-) I'll try out those modifications tomorrow
and let you know what happens.

Check the source. Does it become soft at the point where jitter starts?

I'll check it.

I am sure you did this but just in case: Make sure your digital scope
definitely does not go into equivalent time sampling. Or just use a real
scope like a Tek 2465.

The scope shots are from an Agilent MSO 6000 which I don't think even has
the option to do equivalent time sampling (it's 1GHz frequency response,
4GSps)... but I'll fire up the 2465 to verify that it's not lying to me.


I wouldn't be surprised if that goes into equivalent time at 5nsec/div
or 10nsec/div. But it is in a much higher price bracket than mine so
maybe it has the option not to switch or at least announces when it does so.


As usual, thanks for the help. Stay warm tonight!


Cranking the wood stove 24/7 right now. Can someone send us a chunk of
that global warming?

--
Regards, Joerg

http://www.analogconsultants.com/