Bringing up power on the quad pi3 hat

The first thing I needed to get working on the new quad’s raspberry pi3 hat, was the input DC/DC power converter.  One of the main functions of this board is to take the main DC bus voltage of around 20V, and provide the raspberry pi with 5V power.

In the previous iteration of this board, it was limited to an recommended maximum voltage of around 24V.  As with all the components in my hardware revisions I aimed to support a higher input voltage.  Here I switched parts to the Diodes AP64351 so that I could get to a recommended maximum voltage of 32V (the part’s absolute max is 40V).

Normally, bringing up power isn’t all that interesting.  Either it works, or some pin is obviously connected incorrectly and it doesn’t.  However here, I had different behavior.  When first powering on the device, it kinda flickered the output to 5V or less maybe once every second or so.  While probing with the multimeter, I found that when I probed the soft start selection pin all of a sudden it started to seemingly work!  Assuming the probe’s input impedance must be enough to do something, I soldered on an SMD resistor in parallel with the soft start selection capacitor (after trying more capacitance) and got it to work a little bit, but it was still flaky, cutting out whenever the raspberry pi started to draw significant current during the boot process.


The second instance of the board exhibited similar symptoms, except there I managed to accidentally short the soft start selection pin to 18V, likely toasting it.  However, surprisingly, that seemed to get the chip into a working state!

After much thought (and I should have noticed it in the picture above), I discovered I had managed to populate the incorrect part, the AP64350, not the AP64351.  The x50 version is mostly pin compatible, but has a frequency selection pin where the x51 has a soft start selection pin.

Replacing that chip with the desired part got everything working as expected!