Semiconductor integrated circuit device with internal voltage controlling circuit

A semiconductor integrated circuit device having a first internal voltage controlling circuit which lowers an external power source voltage supplied through an external voltage input terminal and produces at an internal voltage output terminal a predetermined internal power source voltage said semiconductor integrated circuit device further having a second internal voltage controlling circuit comprising:

• an internal-voltage drop detection circuit which detects when said internal power source voltage produced by said first internal voltage controlling circuit becomes lower than a predetermined reference voltage, said internal-voltage drop detection circuit including a differential circuit whose output voltage changes when said internal power source voltage becomes lower than a predetermined threshold voltage, an inverter circuit which amplifies changes in the output voltage from said differential circuit to produce a switching output, and a threshold voltage setter for establishing said predetermined threshold voltage from said predetermined reference voltage;
• and
• a switching circuit which is caused to be conductive by said switching output from said inverter circuit so that said external power source voltage is directly supplied to said internal voltage output terminal.