Patent ID: 6504418
Filing Date: 2003-01-07
Classification: H03K

Abstract:
A high voltage tolerant input/output interface circuit for protecting circuit elements having a first, low operating voltage, the interface circuit comprising:a first terminal for connecting the interface circuit to a source of variable voltage; a fixed voltage source; a second terminal connected to the fixed voltage source; a node terminal for connecting the interface circuit to an integrated circuit including the low voltage circuit elements; a pad connected to the first terminal; means connected to the pad to keep the voltage at the pad within a first defined range; a high voltage tolerant pass gate having an operating voltage that is greater than the first operating voltage and electrically located in series between the pad and the node terminal to maintain the voltage at the node terminal and at said integrated circuit within a second defined range that is within the first defined range, and thereby to protect said integrated circuit and the low voltage circuit elements thereof from voltages outside said second range; said pass gate being electrically connected to said fixed voltage source and using said fixed voltage source to maintain the voltage at the node terminal within said second defined range despite variations in the voltage of the variable voltage source; and wherein the interface circuit further comprises a keeper device connected to the node terminal to apply a given voltage to said node terminal when a low voltage is transmitted to the node terminal through the pass gate.