This article proposes a fully integrated single-crystal oscillator (XO)-based clock management IC (CMIC) with three key features for 5G cellular mobile devices. First, to reduce fabrication cost by eliminating a 32.768-kHz crystal, an on-chip real-time clock (RTC) using a machine-learning (ML)-based RC oscillator (RCO) calibration is proposed for the excellent frequency stability of the RTC; thus, it achieves a 0.68-ppm/°C stability, nearly comparable to that of an RTC crystal, with the power consumption of 48.2 $\mu \text{W}$ . Second, to extend battery life through a fast start-up operation of the XO, the sampling phase-locked loop (S-PLL)-based injection technique reduces the start-up time and energy by 18.2 and 8.8 times, respectively, despite a high swing of 1.2 V. Finally, to mitigate a phase noise (PN) degradation according to a capacitive load ( ${\rm C_{L}}$ )-trimming of XO for the temperature compensation, a ${\rm C_{L}}$ -dependent feedback resistor adjusting a 4kTR noise through a pulsewidth modulation is also proposed; thus, the 76.8-MHz XO accomplishes a PN of less than −158.2 dBc/Hz at 100-kHz offset while consuming the power of 1.08 mW. Consequently, the proposed single-XO-based CMIC meets all reference clock requirements of the system-on-chipset (SoC) and RF chipsets for 5G frequency range (FR)1 and FR2 communication.