The Value Of Adaptive Body Biasing

Power gating solves one problem, but it introduces new design and verification issues.

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By Bhanu Kapoor
Although the use of power gating techniques is essential to manage standby leakage power, it brings in a host of new design and verification issues. This list of new design and verification issues includes putting together a power switch network, incorporating appropriate isolation and retention, addressing x-propagation, dealing with current spikes, and ensuring retention works well. Switching power on and off is a slow event, and more so in a higher-performance design where timing management can become a big issue too.

With the advances in CMOS process technology, scaled down devices needed lower threshold voltages to keep up with performance scaling. The leakage power grew exponentially with the decreasing threshold voltage and has led us to the leakage power problem that we have at hand today.

The adaptive body bias (ABB) technique helps reduce leakage by controlling the device body voltage, leading to an increase in the threshold voltage. Although leakage cannot be eliminated it can be significantly reduced. Because you don’t have to switch power off, it eliminates the need to deal with the host of design and verification issues listed above.

The implementation of ABB has its own challenges, but it is an attractive alternative in many applications and likely to play an increasingly important role in managing leakage power. It also provides a way to deal with the variability issues that are becoming more critical with the advances in the process technology.

And the high-performance requirements for some applications also will make the use of power gating impractical. When you are communicating at 10gbps on Ethernet links, it might take too long to wake up a sleeping link if it was put all the way to sleep as associated packet loss may not be tolerable.

–Bhanu Kapoor is the president of Mimasic, a low-power consultancy.


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