[time-nuts] Switching regulator 12 > 4 V (3.3)

Attila Kinali attila at kinali.ch
Sun Dec 4 15:32:26 EST 2016

Moin Ulrich,

On Sun, 4 Dec 2016 14:46:34 -0500
KA2WEU--- via time-nuts <time-nuts at febo.com> wrote:

> Who has a good recommendation for a switching power supply circuit, 12V DC  
> in, 4 V/ 500mA out , Exactly 3.3 for IC voltage.

You can either use one of the DC/DC converter modules by Muarata/Cui/..
or run your own. The former is of course easier and faster if you
just need something working. The later is IMHO the recommended approach
if you are doing your own board anyways, as you can tailor the DC/DC
converter to your needs and thus ensure minimal ripple/EMI.

Overall, doing your own DC/DC converter is pretty easy, if you know
a bit of RF design and know where the current flows. Using a modern
DC/DC converter chip like the LMR14006 will get you there with an
easy to solder case. Additionally you will need a Schottky diode that
can withstand some beating (aka high reverse recovery). TI lists the
NXP PMEG6010 in their reference design. As inductor I would probably
go with one of the Würth WE-TPC series (low resistance, high saturation
current and nicely shielded). Add some ceramic capacitors (something
like 10-100µF), and you are done.

For PCB layout, keep the current path from input capacitor to DC/DC
converter ground; the path from switching output over the diode to converter
ground; the path from switching output over inductor output capacitor
to converter ground as short as possible. Add as many ground vias as
possible and at least one complete, uncut ground plane directly underneath
all components. If possible use multiple GND planes. Even though this
thing should not generate much heat (with efficiencies >80% it should
be less than 0.4W for your requirements) it is still quite a bit if it
gets stuck somewhere. Thus the ground planes should be large and also
cover the back side to dissipate the heat.

For additional EMI reduction, put the inductor such, that the start
of the winding points in direction of the DC/DC converter and diode.

			Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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