This paper focuses on a spectrum-sharing based cognitive radio fading broadcast channel (BC) with a singleantenna secondary base station (SBS) and M secondary receivers (SRs) concurrently utilizing the same spectrum band with one delay-sensitive primary user (PU). The quality-of-service requirement in the primary network is given by the primary user's outage probability constraint (POC). We address the optimal power allocation problem for the ergodic sum capacity (ESC) maximization in the secondary BC network subject to a POC and an average transmit power constraint at SBS. Optimality conditions reveal that in each fading block SBS will choose only one SR with the highest channel power gain and allocate the block to that user. Furthermore, if PU's power strategy is assumed to be ON-OFF with constant power when ON, the secondary network throughput scaling for largeM in Rayleigh fading is also investigated. It is shown that the secondary sum throughput in Rayleigh fading BC scales like O(log(logM)). Numerical results support the theoretical results derived in the paper.