A special circuit has been developed for programming and sensing the element. Figure 10 shows a simplified schematic of this circuit. Programming occurs when a logic LO is asserted on the gate of a large PMOS transistor. Since the fuse programs at relatively low bias, logic and programming circuits share a common supply voltage.

The sensing circuit is a novel and well-balanced solution to a stringent set of requirements, the foremost being that the sensing currents must be kept very low. The core of this circuit comprises a pair of matched N-channel transistors, which perform the sensing, and a pair of matched P-channel devices, which act as current-sensing output loads. The N-channel sensing transistors are connected in a current mirror configuration, such that, if the fuse-reference resistance on the left were equal to the unburned fuse resistance on the right, both circuit branches would have equal current. In practice, the reference resistance is set to about 8 times that of the unburned fuse. This ratio of reference to fuse creates a default (unburned) output voltage that is low enough to be interpreted as a logic LO value. Additionally, for a programmed fuse, the resulting output voltage is sufficiently high to be interpreted as logic HI. Therefore, the gain of the circuit is sufficient for single-ended voltage outputs.

In this circuit, the ratio of reference to unburned fuse resistance represents a balanced tradeoff between output high voltage (VOH) and output low voltage (VOL) levels. With a ratio of 8, noise margins for VOH and VOL signals are roughly equal. The resulting noise margin is adequate to guard-band the circuit from expected manufacturing variations in transistor Vt and channel length.

 

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