A control circuit for a DC-DC converter has an adaptive voltage position (AVP) control circuit and a switching control circuit. The AVP control circuit generates a position signal based on an output voltage, an output current, a voltage identification code and a set of adaptive voltage control commands. The switching control circuit generates a switching control signal based on the position signal to control the DC-DC converter. When the output current is smaller than a current threshold, the control circuit chooses one of a first voltage position curve and a second voltage position curve as a load line according to the set of adaptive voltage control commands, and when the output current is larger than the current threshold, the control circuit chooses the remaining voltage position curve as the load line according to the set of adaptive voltage control commands.