How to express negative semidefinite constraints of matrices in CVX

for example, A is a variable in the problem， then how to express A+B(B is a constant）is a negative semidefinite matrice （A+B<=0,mean negative semidefinite）

-(A+B) == semidefinite(n)

or if in sdp mode
A + B <= 0

Thank you very much for your prompt reply, with your suggestion, the problem has been resolved！