Antifuse memory IP fuels low-power designs

Embedded nonvolatile memory is becoming more prevalent in a wide range of chips, particularly for power-sensitive applications. Memory IP for such apps requires the design of both the basic memory bit and the memory macro architecture to minimize power demands. An appropriate one-time programmable (OTP) memory macro can meet nonvolatile-memory requirements while offering low-power operation.

Many applications that require nonvolatile memory (NVM) do not need hundreds or thousands of rewrite cycles. Code storage, calibration tables, setup parameters and the like seldom, if ever, need changing once programmed. For cases in which occasional change is required, an appropriate memory management algorithm can skip over outdated information and use previously empty memory to hold the updates. Such management lets a low-cost and secure antifuse-based OTP memory serve as a design's embedded memory just as effectively as a rewritable NVM.

The cost advantages of antifuse-based OTP memory stem from the cell size and process complexity. The antifuse memory's design can be as small as one transistor using technology such as Sidense's 1T-Fuse memory IP. The result is a memory cell area that is much smaller than floating-gate multi-time-programmable (MTP) memories. The small bit cell size results in a smaller memory array footprint, which in turn reduces the area-related cost of the die.

The reliability of antifuse-based OTP stems from the simplicity of its operation. When programmed, the antifuse is a permanent and desired short circuit that cannot be accidentally formed under normal memory read operations. Floating-gate NVM, though rewritable, can wear out because of the need to tunnel electrons on and off the gate during programming and erasure. The tunneling operation will eventually break down the oxide layer isolating the floating gate, creating a permanent, undesired short-circuit in the memory cell.

The simplicity of operation also makes OTP memory an inherently lower-power design than other NVMs. By transistor count alone, for example, antifuse-based memory would be expected to draw less power. In addition, its compact cell size means that arrays are physically smaller. That lowers the capacitance of the bit and word lines, reducing both precharge and switching power consumption.

Although OTP bit-cell design determines the minimum power needed to read a memory cell, of equal importance are many other design factors that go into the arrays of bit cells that constitute the full memory macro. Current sensing, for instance, should be avoided, because it requires the presence of direct current through the cell and through a reference. That dc runs all the time, burning power even when the memory is idle. By contrast, a low-power charge-sensing scheme collects all the charge leaking through the cells to create a voltage signal for the sense amplifier. The consistent programmed-state characteristics of the single transistor split-channel cell make such charge sensing practical and reliable.

Another memory cell design factor that can reduce power demands is to make the sense amplifiers not consume any dc power. Sidense SLP macros use a cross-coupled latch-type sense amplifier that doesn't turn on until there is sufficient voltage on an input to be correctly read. Once a sense amp turns on, positive feedback drives it to one of two zero-current states. Because everything else in the design follows simple static CMOS logic, the only dc is leakage.

The small single-transistor OTP bit cell minimizes array die area and cost while cutting read power consumption