Patent ID: 6246272
Filing Date: 2001-06-12
Classification: G01R

Abstract:
A power supply voltage detecting apparatus for detecting a drop in the level of a power supply voltage below a specified level, said power supply voltage detecting apparatus comprising:a voltage divider for dividing said power supply voltage by a specified ratio;an enhancement MOS transistor which has its source connected to its base, and which is fed an output of said voltage divider through its gate; anda depletion MOS transistor having its source and gate connected to its base and operating in saturation as a constant current source providing a constant current to said enhancement MOS transistor, the depletion MOS transistor and the enhancement MOS transistor having same conduction type, conductivity and temperature dependence of threshold voltage;wherein a current flowing through said enhancement MOS transistor is compared to a current flowing through said depletion MOS transistor to detect small variations in the power supply voltage and to provide an output detection signal at a junction between said depletion MOS transistor and said enhancement MOS transistor,wherein the voltage divider comprises a pair of resistors and has no substantial temperature dependence, and the temperature dependence of threshold voltages of said enhancement MOS transistor and depletion MOS transistor are substantially the same and cancel each other, so that a detected voltage does not vary with temperature andwherein the specified ratio is selected so that an output of the voltage divider operates both the depletion MOS transistor and the enhancement MOS transistor in saturation.