Joel Koltner wrote:
Hi Joerg,

"Joerg" wrote in message
t...
The distance to C87 isn't so bad since you wrote that this node is now on a
full plane. So it has lots of local capacitance to the ground plane, hoping
they are adjacent. Plus a really low "trace" impedance.

This is all true, but I'd much rather have the capacitor that's supposed to
knock the edges off of those switching spikes snuggled right up next to the
inductor connected with some rather fat traces instead of going through two
smallish vias (inductor to the plane) and then two more (plane back to the
capacitor).

The Vcore net powers a DSP that contains an internal DLL to crank a reference
8MHz up to 100MHz or thereabouts. We've found that the jitter on that DLL's
output is quite sensitive to the output capacitance of this swittcher.
Changing the 47uF output to 10uF will cause the DLL to unlock pretty regularly
(this is a Bad Thing!), and even going to 22uF causes an occasional loss of
lock and noticeably more jitter.


This is not good at all. Something on that DSP seems to be quite
marginal if a wee ripple can knock it off the rocker. Even if you'd
remove the ripple, what if Hubble calls the home star where the Great
Dane lives from the lot next door and his radio makes the power plane
weave a bit?


Just fire it up and see how it performs.

We won't be getting stuffed boards back for something like a month, but I'll
let you know how it goes then!

As usual, thanks for your help.


I'd still call up the app engineers of the DSP manufacturer to see
what's going on in that thing in terms of loop stability.

--
Regards, Joerg

http://www.analogconsultants.com/