A cavity-down ball grid array semiconductor package with a heat spreader is provided, in which a substrate is formed with at least a ground ring, a plurality of ground vias, a ground layer, and at least an opening for receiving at least a chip. The mobile communication device receives the interrogation signal from the stationary unit, and generates and transmitting a respond signal, including a unique identification number of the mobile communication device, to said interrogation signal. When the comparator detects that the current of the motor is increased to the target current value, the holding circuit holds the circuit state such that the switch unit is turned off.