Switching power supply system provided with under voltage lock out circuit

A switching power supply system controlling switching operations of switching devices by a control circuit to convert an input voltage into a desired output voltage, the system being provided with an under voltage lock out circuit comprising:

• an input voltage detection unit detecting the input voltage and producing an input voltage digital signal representing the magnitude of the input voltage;
• and
• a voltage level comparison unit carrying out digital comparison of the input voltage digital signal with each of first and second voltage detection level data and outputting the results of the comparisons,
• wherein, when the magnitude of the input voltage represented by the input voltage digital signal is smaller than the magnitude of the voltage represented by each of the first and second voltage detection level data, the switching operations being inhibited, and
• wherein, when the magnitude of the input voltage represented by the input voltage digital signal is larger than the magnitude of the voltage represented by each of the first and second voltage detection level data, the switching operations being permitted.