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Negative current, and the DAC should work in sink mode. But yes, the opamp could limit the lowest voltage since the DAC is rail to rail.
Thank you for your response. I'm not sure I get what you are saying. If the DAC worked like it's supposed to, the functionality of the XL6009 shouldn't change, right? But since there is a problem I figured there would have to be a problem with the dac in combination with the feedback. Would a voltage follower help here to isolate the DAC from the feedback circuit?
The DAC does produce the wanted voltage, but I think I should've checked it while it was connected to the XL6009 when it misbehaved. The voltage at the feedback pin is 1.263v because there is a third resistor in the network, changing the topology. There will actually be a negative current flowing through the DAC at some point. I'm curious to see what will happen to the converter when you change its ground. In my case it's probably not usable since the potential between Vin and Gnd would become less than 5v.
I really like your power supply, I'd love to build one from scratch one day and your instructable is very informative. And thanks!
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