A 0.5-V 1.6-mW 2.4-GHz Fractional-N All-Digital PLL for Bluetooth LE With PVT-Insensitive TDC Using Switched-Capacitor Doubler in 28-nm CMOS

This paper proposes an ultra-low-voltage (ULV) fractional-N all-digital PLL (ADPLL) powered from a single 0.5-V supply. While its digitally controlled oscillator (DCO) runs directly at 0.5 V, an internal switched-capacitor dc-dc converter ``doubles'' the supply voltage to all the digital circuitry and particularly regulates the time-to-digital converter (TDC) supply to stabilize its resolution, thus maintaining fixed in-band phase noise (PN) across process, voltage, and temperature (PVT). The ADPLL supports a two-point modulation and forms a Bluetooth low-energy (BLE) transmitter realized in 28-nm CMOS. It maintains in-band PN of -106 dBc/Hz [figure of merit (FoM) of -239.2 dB] and rms jitter of 0.86 ps while dissipating only 1.6 mW at 40-MHz reference. The power consumption reduces to 0.8 mW during the BLE transmission when the DCO switches to open loop.