Fuse sensing scheme with auto current reduction

Abstract

An improved circuit for sensing and programming fuses in integrated circuits. The circuit is broadly comprised of a fuse cell, a reference circuit, a sense amplifier and a level detector. In one embodiment of the present invention, a two-stage sensing scheme is implemented. The improved fuse sensing circuit uses current-mode sensing and implements an auto-read current reduction scheme. Using a level-detect circuit, the virtual ground is raised automatically if the high-voltage power supply exceeds core supply (Vdd) by a fixed dc voltage. This reduces effective sensing voltage and the read current and thus helps preserve unblown fuse integrity. In one embodiment of the invention, four modes of operation are implemented: 'Normal Read,' 'Unblown_Read,' “Blown_Read_1” and “Blown_Read_2.” The default read mode is the “normal read” while the “Unblown” and “Blown” read modes are for fuse calibration purposes. In the “Unblown_Read” read mode, the circuit is operable to compare the fuse resistance against a lower reference resistance, closer to an unblown fuse resistance value, in order to make the comparison more stringent. Similarly, the “Blown_Read_1” and “Blown_Read_2” modes allow a more stringent comparison for a blown fuse resistance.