On Wed, 30 Apr 2008 14:37:29 -0700, "Joel Koltner"
wrote:

For those of you who read the "slightly misbehaving switchers" thread (which
has long since scrolled off my news server), I've had the chance to take a
look at the behavior of the switcher in question now that it's on an 8-layer
board with a ground plane. I've attached the results in the same format as
the original document. The Reader's Digest version of the results seems to
be... "nothing much has changed" -- there's still noticeable jitter in the
duty cycle with input voltages around 3V.

In any case, since the original (2-layer PCB) ended up working OK, I'm not
particularly worried about this (even though I wish the jitter weren't there
and don't really understand what causes it), I just wanted to share the
results with everyone helped on with the original problem.

---Joel

I'm surprised that you got clean switching at all! That via carrying
all the high current spikes on pin 4 is modulating your feedback
voltage and compensation cap C84. C86 and C87 should have been
connected to pin 4 on the top layer with a copper pour. Plus, C87 and
L3 could have been placed more optimally to reduce the current loop
path. Switcher layout is critical if you want them to work cleanly.

I would have done the layout per the attached image.

---
Mark