Multimode crystal oscillator for power management unit with digitally controlled AGC loop in 0.18μm CMOS technology

In this paper, a novel multimode crystal oscillator approach is proposed for a Power Management Unit (PMU). It consists of a digitally controlled Automatic Gain Control (AGC) loop allowing multiple power modes to meet the system´s low phase noise and power consumption requirements. A new technique with a programmable switched capacitor array controlled by a counter is presented preserving the integrity of the clock during the mode transition. A 32.768 kHz crystal oscillator using the proposed idea was fabricated in a 0.18 μm CMOS technology. Measurement results show that it achieves phase noise of -120 dBc/Hz at 1 kHz offset in normal operating mode and only consumes 860nA current off 3.3V backup battery in RTC-only (low-power) mode including all the digital supporting circuitry. It also shows the seamless transition between the operating modes.