Body Bias Control for Renewable Energy Source with a High Inner Resistance

Sensor nodes used in Internet of Things (IoT) are required to work an extremely long time without replacing the battery. Natural renewable energy such as a solar battery is a hopeful candidate for such nodes. Here, a power model for operating an Silicon on Insulator (SOI) device with a solar battery including a large inner resistance is proposed, and applied to a micro-controller V850E-star and an accelerator CMA-SOTB2. Unlike the ideal case, the maximum operational frequency was achieved with reverse biasing by suppressing the leakage current which decreases the supply voltage. Under the room light with a large inner resistance, the strong reverse bias is effective, while a relatively weak reverse bias is advantageous under the bright light. The proposed model is appeared to be useful to estimate the appropriate body bias voltage both for V850E-star and CMA-SOTB2. In the V850E-star, the estimated operational frequencies were different from the real chip, while they were relatively matched when CMA-SOTB2 was used under the low illuminance.