Abstract: A Direct Current/Direct Current (DC/DC) converter for acquiring stabilized low voltage and a telephone power supply circuit based on the DC/DC converter are provided by the present invention. The DC/DC converter includes a power level circuit (1), which receives a dynamic voltage input from an outside circuit; a tank circuit (2), which uses energy-storage elements to store the electric energy outputted from the power level circuit and outputs the electric energy to a load; and a feedback control circuit (3), which receives the voltage from the output and feeds it back to the input of the tank circuit so as to control the connection and disconnection of the input of the tank circuit and further to control output voltage of the tank circuit.