[N8VEM: 18941] Notes on using the LM22677-5 or -adj Vreg

[Tom Lafleur]

John


Here are some notes on using the part...

The trace between output pin to the diode and inductor should be very
short, both parts need to be connected to the GND plane with short
traces... (recommend using 2oz cooper)

​Input caps need to be low ESR type, I typ use 2 to 3 10-20uf ceramics,
​and one .1uf, again make them close to the part as you can... using 1210
parts sizes are easy to solder. even for us old guys..

Output caps again need to be low ESR type, one or two very near the
inductor to GND. 100uf to 200 uf work great.

I alway add room for the two trim resisters.. I always use 1% , if I'm
using the fixed 5.0v part, the R from the FB pin is connect to the output V
via a zero ohms R.

My typical layout is as much GND plane as you can get under the LM22677 and
the diode on both side of the board. Again 2 oz cooper is best. Lots of via
in the gnd plane from top to bottom for better heat transfer and cut down
on R losses.

​In most cases I put the LM22677 on the back side of the board, and some of
the other parts on top.. I used multi-via on all traces to cut down on
losses. I solder the thermal pad on the device​ via a ~ #4 plated hole.

​Ti has a program called webench that can be use to optimize part selection
and design.
I use it to trim my design. I run the device at 500Khz. WEBENCH can be run
from the TI parts page...

Here is the output from Webench for a 6 to 13v in, 5V out at 3.5amps, with
a parts list.. (look like the diodes is missing from the TI schematic when
they converted it to a PDF) ... If using >3.5 amp, the diode will need to
be changed to one that >8amp, and then re run the webench for new plots.


When your ready, I can help with the final part list...

TI Data sheet:
http://www.ti.com/product/lm22677

TI user guide:
http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snva366&fileType=pdf

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Tom Lafleur