Energy is a limited resource in wireless sensor networks. In fact, the reduction of power consumption is crucial to increase the lifetime of low power sensor networks. Several approaches on dynamic power management have contributed to reduce the power consumption, but few of them consider the application constraints to optimize it. In this paper, we propose a new application-driven power management approach, where we model the sensor node operation and the application constraints using the hybrid automata framework. We also model a real sensor network application for fire detection and we show the performance of our approach in terms of energy drop, comparing it to an Ideal Model and a Naive approach.