What is the issue here? would have to see a complete schematic of the
circuit and the values of precision for the components. Note that I have
been out of that portion of the electronics business for years but use to
really good at this stuff.
Note for the decoupling to ground was add a capacitor, he might want to add
a resistor (fairly high value to help build the charge on the cap and a
bypass if it gets to "hot"). That is why it is called an R/C decoupling
circuit.



"Joel Koltner" wrote in message
...
These are some 'scope shots of a slightly misbehaving buck converter
whereby the duty cycle of the switch jitters around a bit when the input
voltage is ~3V but doesn't do so at somewhat lower and higher voltages.
(Output voltage is 1.2V and maximum intended output current is around
150mA; the scope shots were taken with a 20 ohm -- 60mA -- load connected.
J6 and J7 are unjumpered and R3 and C1 really are DNIed.)

I'm all ears for suggestions on how to improve this design, both the
schematic part and the layout. The layout is done with the idea that the
four clustered vias are the honest-to-God "ground" reference and that the
controller's ground and passive components' grounds don't see switching
currents as much as possible.

Things I've tried that didn't have a significant effect on the jitter:

-- Dropping 100nF from pin 5 to ground.
-- Increasing C3, up to at least 1nF. Actually, the design doesn't even
seem to need C3 -- I'm thinking it's there to improve the high-frequency
response of the regulator? Linear's datasheet uses a 20pF cap (with a
resistor ~5x larger) in all their examples.
-- Dropping 100nF from the R4/R5 junction to ground.
-- Changed both the input and output capacitors (C4/C5) to 10uF ceramics.
-- Raised C2 up to at least 1nF, which seriously cuts the loop bandwidth.
From doing some load step tests, down to about 2.8V the system is pretty
nicely damped and remains so at higher voltages; below 2.8V you do start
to see a handful of recovery cycles in the step response, but everything
remains stable. Dropping C2 to 100pF will make the loop unstable at
voltage under ~3V.
-- Varied L1 anywhere from ~7.5uH to ~30uH. The size used -- 15uH -- is
larger than typically used, but was chosen based on staying in continuous
conduction mode down to a 20mA load with a 4.2V input.

I very much appreciate the time that people spend looking at stuff like
this on Usenet and thank you all for doing so!

---Joel