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I am not sure if your question refers to LM5156 or LM5160. Anyway, you can always use a flyback topology with primary side regulation to create negative output voltages.
Our Boost controllers/converters do not support the inversion of the FB signal. Therefore, even if you can shift the FB voltage to a positive value, the phase would still be inverted, so that the regulation would not work.
So we recommend using a Buck to create negative output voltages. You can refer to PMP20904 to demonstrate the concept: www.ti.com/.../PMP20904
