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Using sub-threshold techniques for IC design

Posted: 07 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Sub-threshold? energy consumption? transistors? super-threshold? non-volatile memory?

Low power IC solutions are in high demand with the rapid advancement of handheld devices, wearables, smart cards and the IoT bringing a massive amount of new products to market that all have the same primary need: Powering the device as long as possible between the need to re-charge the batteries.

Low energy consumption has replaced performance as the foremost challenge in electronic design. Performance is important, but it must now accede to the energy capacity of batteries and even the minimal output of energy harvesters. Performance at all costs no longer works; energy consumption is now the dominant requirement. While reducing energy consumption is critically important throughout the electronics industry, the question is: how should that goal be achieved? Ambiq Micro's touts an approach that moves beyond the incremental improvements that other semiconductor companies have taken. They use a different approach to the problem: sub-threshold circuit design.

Energy is consumed in two fundamental ways: as leakage, when a circuit's state isn't changing, and dynamically as internal nodes are charged up and down. For realistic circuits in operation, dynamic power dominates C especially for the higher power supply voltages used in most designs today (figure 1).

Figure 1: Dynamic current dominates with higher operating voltage.

Because dynamic energy varies as the square of the operating voltage, that voltage becomes the biggest lever for reducing dynamic energy consumption (while also having a tangible, but less dramatic, impact on leakage). For example, when compared to a typical circuit operating at 1.8V, a "near-threshold" circuit operating at 0.5V can achieve up to a 13X improvement in dynamic energy. An even more aggressive "sub-threshold" circuit operating at 0.3V can achieve up to a 36X improvement!

Traditional digital designs use the transistor state C "on" or "off" C as a critical concept for implementing logic. Analogue designs likewise assume that a transistor is "on" to some controlled degree, using it for amplification. But sub-threshold operation means that none of the voltages in the chip rise above the threshold voltage (Vth), so the transistors never turn on. Even a logic "high" voltage keeps the transistors "off." This means that completely new design approaches are required.

This whitepaper examines the challenges of sub-threshold design, looking in particular at what's required to overcome the differences from traditional super-threshold design. These considerations drove the development and commercialisation of Ambiq's patented Sub-threshold Power Optimised Technology (SPOT) platform, which the company uses to build circuits.

Sub-threshold was proven decades ago
Sub-threshold design isn't a new concept. As far back as the 1970s, Swiss watchmakers noticed the potential of operating select transistors in the sub-threshold regime. The idea was picked up for pacemakers and RFID tags, but never saw much acceptance beyond that.

After a lull that lasted a couple of decades, the topic regained some academic status in the late 1990s and early 2000s. By that time, the upcoming primacy of energy consumption was evident, and research started into ways that commercial circuit designers could reduce energy consumption. Sub-threshold design techniques were among those ideas.

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