I have revised the design again so that the current limiting doesn't tend to oscillate as badly as before. The current limiting is now done with a dedicated op amp that linearly controls the SET voltage of the LT3083 instead of just pulling it to ground like a comparator would. This is inspired by Dave Jones' uSupply.
The power supply itself works quite good after ironing out some bugs in the software debouncing routines and the rotary encoder decoding,
Some of the changes include:
- the LCD LED backlight is driven by a transistor based constant current source for higher efficiency
- the encoder outputs are now connected to the two external interrupt pins of the ATmega8
- fixed current limiting to work better and allow a proper constant current output mode
Also, I have slightly changed the specifications of the power supply. The LT3083 has a maximum input voltage of 18V, so I have changed the maximum output voltage of the supply to 15V so that the maximum input voltage to the LDO from the pre regulator is around 17V.