Heat dissipation in the PNP transistors
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JochiSt commented
When having a look at the thermal image (sorry for the bad quality) one can observe two hot spots, which are the PNP transistors of the current source. For the first prototype they are SOT23 - quite small and running at a DAC of 768 for a longer time causes them to raise their temperature.
For the next prototype a through hole transistor or a larger package including some cooling area might be a good choice.
JochiSt commented
Replacing the BC856 with a BC558 might be a good choice, the BC558 can withstand 100mA continuos collector current. This should reduce the device temperature further.