The Replicape has two DC-DC buck step down regulators, one for supplying the 5V to the BeagleBone (Black), a MCP16321 and one 12V so you can use 12V fans or LEDs for keeping your stuff cool. (Pun intended)
When both the Replicape and the Manga Screen were finished and I was finally able to test them together, an issue arose. The screen would suddenly cut out during boot because the power to the USB was cut by the BBB. Only by lowering the brightness significantly and adding an extra electrolytic capacitor across the 5V input was I able to make them work together. Why was this happening?
It turns out that Rev A3A has a transient load issue when powered through the Replicape board. This was traced to the 5V DC-DC step down regulator. But even with slow changes in load, for instance ramping up the brightness on the display, the display would suddenly be cut when the load was high.
The Beaglebone has a power management circuit for the USB, a TPS2051 which has an under voltage lockout of 2V. This is probably what kicked in when a combination of switching noise and a sharp transient occurred.
Switching noise issues are not the easiest to debug. It doesn’t help (much) to add lots of capacitance to the output of the converter, it simply will not help reduce noise in the 200MHz range.
One solution to this problem was to limit the slew rate of the N-MOS on the step down converter by adding a resistor in series with the boost capacitor. This does decrease the efficiency of the converter by something in the region of 1%, but that is a small price to pay for a working USB power.
Below is a picture of a Replicape with a slew rate limiting resistor and one without. It is clear that the switching noise has been dramatically reduced with the slew rate limiting resistor.
Heat management is an important on such a small board, but a 1% reduction in efficiency should be acceptable.
A different solution would be to filter the noise by using an rc/lc filter or similar on the output, an RC snubber as they call it.
Sorry for the few updates lately, I’ve been in limbo trying to get ready for Maker Faire Rome. And the in the end I never did get the printer running in time for the event, haha! Still, it was a good show.
Ok, so this is the only hardware issue I am aware of, so hopefully CircuitCo will start production now.
Also, HipsterBot is dead! It’s now called Thing! You know, ‘cos it prints things? More updates in that later, I’m working on calibrating the printer now, so a few videos in a few days I’m sure!