the protection circuit of the present invention addresses the problem of indeterminate logic levels caused by loss of one of the power supplies in a two - power - supply cmos integrated circuit . the circuit of the present invention replaces the typical scheme of power supply sequencing to fix the problem . the circuit disclosed herein detects the state of the core voltage and disables the output drivers when the core voltage is detected as being off . the disabled drivers are put into a high impedance state , thereby eliminating the potential for damage and eliminating the need for power supply sequencing . the invention also protects against the sudden loss of the integrated circuit core voltage , vdd , power supply during normal operation .