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

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