Revisiting LT3780 power supply - Part 1 - LT board

Finally, improving the current limiting ended up with a much bigger redesign.
The previous one was flawed : Need a negative power supply to have linear amplification around zero, very noisy choke nearby,... => Did not work very well.


Summary of the new system :

• Removed current sensing on the LT3870 completely, shorting the 7 mOhm shunt to have a common ground with external components.
• High side : Just do voltage sensing, no current sensing.
• Low side :  just do current sensing  + current limiting together
• Rewrite the INA219 lib to match my needs , i.e. simpler ina219 on github
• Use a smaller shunt resistor (20 mOhm) for current sensing
• Do a full rewiring, cleaner/better this time.

Regarding the LT3870 board itself, very little changes :

• Vmin hardwired to Vcc/2
• CC mode => dont care, we dont use it anylonger, it is not connected anyway (diode removed)
• VC mode => external pots 
• The current shunt has been shorted  (under the yellow kampton, on the right side of the pic below)
• Removed the diode and pulled a wire from the left side , to be used later for max current. When all is good, it should be 0.8 v. We'll pull it higher when we exceed max current.