Joel Koltner wrote:
Hi Joerg,

"Joerg" wrote in message
. ..
Certainly doesn't look normal. Did you ever contact LTC about it?

No, but I suppose I should for the sake of completness here. As I've
mentioned, this isn't the first time I've seen a buck regulator behave like
this, and I'd love to nail down the cause and find a solution.


I have never seen one do that. But then again I rarely use PWM chips for
switchers. Have a hard time trusting them and their long term
availability ...

In the past there was more than one occasion where a chat with app
engineering at various mfgs ended up with them saying something like "Oh
drat!".


C86 is a bit sub-optimal, should be between inductor and chip so it's closer
to pin 6.

Good point.

I assume GND is a full plane.

Yes... the stack-up is...

Layer 1: Top - Routing
Layer 2: Full ground plane
Layer 3: Split plane, VBat in the area of the board shown, +3.3V (comes from a
linear regulator) over most of the board


Good, then you could place that via so the PWM chip has a much shorter
connection to VBAT. IIRC the trace was quite wide so you could probably
do that with a quick Gerber edit if your ECO procedure allows it.


Layer 4: Routing
Layer 5: Full plane, output of this regulator (1.2V)
Layer 6: Ground
Layer 7: Routing
Layer 8: Bottom - Routing

Thanks for your help,
---Joel




--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.