DC power converter and mode-switching method

A DC power converter, used in an electronic apparatus having a subsystem circuit, comprising:

• a first voltage converting circuit electrically connected to the subsystem circuit via an output end, receiving a system voltage and a first reference voltage, and converting the system voltage to a first output voltage based on the first reference voltage;
• a second voltage converting circuit electrically connected to the subsystem circuit through the output end, receiving the system voltage and a second reference voltage, and converting the system voltage to a second output voltage based on the second reference voltage;
• and
• a reference voltage generating circuit for generating the first reference voltage and the second reference voltage for the first voltage converting circuit and the second voltage converting circuit respectively, wherein the first reference voltage is greater than the second reference voltage by a fixed value;
• wherein the second voltage converting circuit outputs the second output voltage to the subsystem circuit and the reference voltage generating circuit pulls down the second reference voltage by a default value per one time cycle until the second output voltage is a predetermined voltage when the first output voltage at the output end is smaller than a threshold.