As it was, it wasted around 85 mW whenever the output was turned on at
20 V. That's a silly waste of power. A 100 kΩ pullup works just fine,
so I changed it. Now there's only 4 mW wasted, so I'm happy.
It's a USB Type-C connector just like the one used in v0.1 and v0.2, but
with only SMT pins (except the fixing pins, of course). They're
slightly cheaper and probably nicer for automated assembly, but
impossible to hand-solder. That's okay though: where we're going, we
don't need hand-solderability.
Since it really seems like a good idea to include the flyback diode by
default, I decided to change it to a surface mount device. After that,
I noticed that it would be easy to shave another 0.5 mm off the end of
the PCB, so I did.
Now there's a filled zone connecting the MOSFET output to the connector,
and the + side of the output is close to the MOSFET. Both connector
leads have no thermal relief, for minimum resistance.
0 Ω resistors are cheap, and switches are expensive. It would be nice
to be able to save the price of a switch, since it's not really
necessary anyway. I can program these by SWD, after all.
I added a 0 Ω resistor, R11, to pull the BOOT pin to ground in lieu of
having SW1 installed to do the same job.
Boy, I'm good at messing up transistors! This time it was just the pin
numbers for the collector and emitter reversed, because I didn't copy
the symbol from the datasheet but rather assumed another SMT 2222 would
have the same pinout. Interestingly, it actually was able to control
the MOSFET at VBUS = 5 V and at VBUS = 9 V, but at VBUS = 15 V the
reversed transistor couldn't pull the MOSFET gate down enough.
I had the source and drain reversed—whoops. After fixing it with an
X-Acto knife and some point-to-point wiring, I've updated the schematic
and PCB files accordingly.
THIS ALSO MARKS THE BEGINNING OF DEVELOPMENT OF THE NEXT BOARD REVISION.
Clara Hobbs