below picture is the LM2841 positive +15v input,positive output +3.3v.
How can i use the LM2841 to implement a negative -15v input,positive output +3.3v.
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You can't do this conversion with a buck regulator. To do a negative to positive conversion you will need a boost regulator. You will just need to connect Vin to ground and GND to -15V and set the output voltage to 18.3V.
No, it will vary with the input voltage. If the -15V will vary and you need to keep the 3.3V solid you will need to level shift and ground reference the feedback using a couple PNP transistors. See the LM5002 circuit in AN-2041 for an example. That trick will work with any boost regulator.
Q7B basically forces the base of Q7A to ground reference while R10 sets the current through Q7A at the regulation point. So R8 is sized to that current to give you whatever voltage above ground you want.
In the LM5002 circuit you are forcing 1.26V across R10. So with that set current you program the output voltage by multiplying 1.26V by the multiple of R8/R10. In that case it is 4, so the output is 5.04V. In your case you could change R8 to 13k and the output would be 3.3V.