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2.4GHz VCO features dynamic supply voltage controller

29 Sep 2014

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Toshiba Corp. revealed that it has developed a 2.4GHz voltage controlled oscillator (VCO) for low-power wireless systems, including Bluetooth LE. The device combines the features of a class-D VCO and dynamic control technique of the supply voltage, resulting in a low-voltage operation below the threshold voltage of the transistor.

Recently, applications of wireless communication have widened to include sport, fitness, healthcare, watches and more. As these applications often assume long-life operation with low-capacity batteries such as coin cell battery and energy harvester, it is a requirement to use an extremely low-power wireless IC. This has spurred research into low-power wireless ICs and individual building blocks.

Among building blocks, reducing VCO power consumption is particularly challenging, as superior noise performance depends on high power consumption. Many techniques for low-noise and low-power consumption of VCO have been researched, mainly focused on reducing current consumption. However, for extremely low-power, reduction in the voltage domain is necessary.

To address such concern, Toshiba integrated a class-D VCO developed using advanced CMOS technology, which uses transistors as switches (not as transconductors, like conventional VCOs). Although the class-D VCO can achieve superior noise performance at low supply voltage, the supply voltage also has to be as low as possible for extremely low-power.

The VCO features a technique that uses a low-drop out circuit as the dynamic supply voltage controller. At start-up of the VCO, the supply voltage is boosted to ensure fast, reliable start-up. In the steady-state after start-up, the supply voltage is controlled below the threshold voltage, which keeps the oscillation even in the supply voltage below the threshold voltage of the transistor.

The test chip is fabricated in 28nm CMOS technology with high threshold voltage. While high threshold voltage can reduce leakage current in sleep-mode, it increases power consumption in conventional class-D VCOs, because higher supply voltage is required. However, since the proposed VCO can oscillate at lower voltage than the threshold voltage, low-power oscillation can be available even with high threshold voltage. Extremely low-power consumption of only 171µW is achieved, along with phase noise performance required for low-power wireless systems.

Dynamic supply voltage control can solve the trade-off between leakage current in sleep-mode and active power consumption in active-mode. Furthermore, efficient wireless system can be made available when this low-voltage VCO is used with energy sources such as a DC/DC converter and an energy harvester.

Toshiba's next step is to further reduce power in total wireless systems, as well as VCO building blocks, aiming to realise an extreme low-power wireless IC in the next three years.




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