Patent ID: 7309976

Claim:
A DC/DC converter for acquiring a predetermined DC output voltage by dropping an external power supply voltage to be input, comprising: a switching transistor arranged to pass the external power supply voltage intermittently by turning ON/OFF according to a switching control signal; a smoothing circuit arranged to acquire the DC output voltage by smoothing the output of the switching transistor; a switching control circuit to which an internal power supply voltage is supplied and which is arranged to output the switching control signal based on the feedback-input DC output voltage; a comparison circuit arranged to compare the DC output voltage with a first reference voltage and output a selecting signal in a first or second state; an error amplification circuit arranged to compare the feedback-input internal power supply voltage with a second reference voltage, to amplify an error thereof and to output an internal power supply control voltage; a first linear transistor which turns ON when the selecting signal is in the first status and which inputs the external power supply voltage and outputs a voltage according to the internal power supply control voltage as the internal power supply voltage; and a second linear transistor which turns ON when the selecting signal is in the second status and which inputs the DC output voltage and outputs a voltage according to the internal power supply control voltage as the internal power supply voltage.