Regarding putting a constraint in RooAddPdf object

Dear experts,
I want to put a constraint s.t my nsig can go to negative. But the total pdf cannot go to negative. Now, for large negative yields my pdf is going to negative in some toy cases. I want to avoid such a situation without applying a direct constraint on nsig (i.e without directly constraining the nsig s.t the nsig cannot go to negative).

I want to put a constraint on the total pdf

RooAddPdf final_pdf(“final_pdf”,“final pdf”,RooArgList(final_signal,final_background),RooArgList(nsig,nbkg));
RooFitResult* fitresult = final_pdf.fitTo(*data, Minimizer(“Minuit2”), Extended(kTRUE), Minos(kTRUE), Save());//,PrintLevel(-1));

I want that final_pdf doesnot go to negative at any point by putting final_pdf minimum to 0 if such a case happens. Please tell me whether that can be done at RooFit and how ? Please provide me some idea how to fix the final_pdf minimum to some value.

With regards,