Joel Koltner wrote:
Hi Joerg,

"Joerg" wrote in message
...
When I have the luxury of a 4-layer I usually drop vias where needed. No
routing ground traces back and stuff.

OK, that makes good sense.

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.

In my case I think there's a greater chance that someone will take the X5R/X7R
ceramic cap and find that... hey... there's this other "ceramaic cap" that's
much cheaper... let's use it! ...and it turns out to be a Y5V.


Remember the late 80's? Z5U was all the rage. "Hey, not only are they
cheaper, we can also use smaller footprints!" Then a chemical plant had
an explosion and it was pretty much the main source for the stuff that
mixes up this ceramic. People were bidding on those caps like in a Texas
style auction until the last one was gone and whole production lines
came to a grinding halt.


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.

Better the money goes to boutique capacitor manufacturers than assemblers?


Well, you have to weigh one against the other and see what is lower in
cost. At the end of the day SMT assembly just isn't competitive at
prices of several cents a pop (for mass production).


(regarding C3)
I never use such caps at all.

Any ideas why the folks at Linear do? Am I correct in thinking that it's a
bit of a "speed up" cap/high-pass filter that's meant to provide improved
transient response? It seems as though usually the transient response is
going to be determined by C2/R3/C1 anyway.


It might be there to compensate for the input capacitance at FB. Just
ask them via email.


Then again, I rarely use integrated switchers that are single-sourced.

Any suggestion on multi-sourced/readily avaialble ICs that are small (I'm sure
the PCB layout guy would already be throwing a fit at SO-8 sized) and fast
(1MHz so that the inductor can be small)?


Unfortunately not. I don't design many buck converters, it's mostly
boost, SEPIC or Forward. There my favorite is the LM3478 but it won't
work here. You could design one around a Schmitt Inverter but it'll cost
extra time upfront. The reward would come years down the road where
you'd be sipping margaritas while others frantically deal with part
obsolescences. I suppose this isn't a mass product. If it were you'd
almost have to do a discrete design.

Do you really need 90% efficiency here?

--
Regards, Joerg

http://www.analogconsultants.com/