The only option I have is PDF, but I need PNG.
Should be nice to add all options that we have in LO Draw.
Workaround (okay, not ideal, but it works, right? just think of the length of the bugs list before you add a feature request 
)
Make the formula, like with Insert - Object - Formula, in Draw. Select it, then Export to PNG format, tick Selection and you get exactly what you want.
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