Patent ID: 8400278

Claim:
A semiconductor device comprising: an antenna circuit; a power supply circuit; and a signal control circuit, wherein the power supply circuit comprises: a rectifier circuit which rectifies signals from the antenna circuit; a control circuit; a battery which is charged by the rectified signals; a low-frequency signal generation circuit which receives the rectified signals; a switching circuit; and a power source circuit, wherein the control circuit is configured to supply a power outputted from the rectifier circuit to the power source circuit through the switching circuit and to charge the battery when a voltage outputted from the rectifier circuit is higher enough than a voltage of the battery, wherein the control circuit is configured to supply the power outputted from the rectifier circuit to the power source circuit through the switching circuit and not to charge the battery when the voltage outputted from the rectifier circuit is not higher enough but still higher than the voltage of the battery, and to supply a power of the battery to the power source circuit through the switching circuit and not to charge the battery when the voltage outputted from the rectifier circuit is lower than the voltage of the battery, wherein the low-frequency signal generation circuit comprises: a ring oscillator that outputs an oscillation signal at a frequency; a divider circuit configured to divide the frequency of the oscillation signal outputted from the ring oscillator and provide an output of plurality of sequentially divided oscillation signals; and an AND circuit configured to generate a low-duty ratio signal from the output of the divider circuit, and wherein the switching circuit controls the power supplied from the battery to the power source circuit and the power outputted from the rectifier circuit to the power source circuit, by the low-duty ratio signal from the low-frequency signal generation circuit.