260- $\mu$ W DCO With Constant Current Over PVT Variations Using FLL and Adjustable LDO

In this brief, the design of an ultralow power digitally controlled oscillator (DCO) for Bluetooth low-energy applications is proposed. The DCO core is designed to operate in the subthreshold region. The DCO power consumption is reduced mainly by maintaining the constant current over process, voltage, and temperature (PVT) variations in the oscillator core via an adjustable low-dropout (LDO) regulator. The power consumption is also reduced by employing <inline-formula> <tex-math notation="LaTeX">${g_{m}/I_{D}}$ </tex-math></inline-formula> optimization methodology and by lowering the supply voltage. A very small capacitance of about <inline-formula> <tex-math notation="LaTeX">${\Delta }\text{C}{\approx }16$ </tex-math></inline-formula> aF through a customized lateral metal-oxide-metal capacitor bank is utilized to attain small switchable capacitance in the fine capacitor bank tuning. The parasitic inductance due to routing is eliminated by using an aluminum metal with a very large width. The proposed design is fabricated with 55-nm CMOS process. The experimental results show that the power consumption of the proposed DCO including both LDO and bandgap voltage reference is <inline-formula> <tex-math notation="LaTeX">$260~{\mu }\text{W}$ </tex-math></inline-formula> with the supply voltage of 0.55 V, and the constant current consumption is <inline-formula> <tex-math notation="LaTeX">$480~{\mu }\text{A}$ </tex-math></inline-formula> over PVT variations. The tuning range of 2.23–2.7 GHz and frequency resolution of 2.44 kHz are achieved. At 1-MHz offset frequency from the carrier frequency of 2.44 GHz, the DCO phase noise and the figure of merit (FOM<inline-formula> <tex-math notation="LaTeX">$_{{T}}$ </tex-math></inline-formula>) considering the frequency tuning range are −119.05 and 199.12 dBc/Hz, respectively.