This paper studies multi-user wireless powered communication networks, where energy constrained users scavenge energy of the radio frequency signals radiated from a hybrid access point (H-AP). The energy is then utilized for the users' uplink information transmission to the H-AP in time division multiple access mode. In this system, we aim to maximize the uplink sum rate performance by jointly optimizing energy and time resource allocation for multiple users. To this end, we first derive the optimal downlink energy transmission policy at the HAP. Based on this result, analytical resource allocation solutions are obtained. Simulation results confirm that the proposed algorithms offer significant sum rate performance gain over conventional schemes.