[time-nuts] Switching regulator replacement for 7805
attila at kinali.ch
Sun Dec 4 16:09:50 EST 2016
On Sun, 4 Dec 2016 15:45:55 -0500
Bob Camp <kb8tq at n1k.org> wrote:
> Given that the parts to build one are fairly easy to get and that we likely have “nutty”
> EMI requirements. Maybe a small board that drops into a 78xx footprint is the
> better solution.
I don't fully agree. To go full "nutty" you would want to use the chips
in the QFN packages, as those have shorter leads and a large ground pad
to keep EMI down. But soldering an 0.5mm QFN by hand is impossible
(believe me, I tried) and not everyone has an oven. Also a lot of the
EMI performance depend on what output current you expect. Using a 1A
design for 10mA is a good way to get poor EMI performance as the
chip will go from PWM (aka fixed frequency) to PFM (aka fixed on time)
mode under low load to increase efficiency. The same goes for input
voltage. So we would need to do multiple designs for different
output current and input voltage ranges... which kind of defeats
the purpose of "a small board that drops into....".
What could be done, though, is a small board with all components,
but the inductor on it. So then one could select the right inductor
for the application and solder it on. Using an inductor that is easy
to solder (like e.g. WE-PD3) with a wide selection of values should
do the trick.
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