5.3 A 95µW 24MHz digitally controlled crystal oscillator for IoT applications with 36nJ start-up energy and >13× start-up time reduction using a fully-autonomous dynamically-adjusted load

Wireless sensor nodes (WSN) in IoT applications (e.g., Bluetooth Low Energy, BLE) rely on heavily duty-cycling the wireless transceivers to reduce the overall system power consumption [1]. This requires swift start-up behavior of the transceiver. The crystal oscillator (XO) generates a stable reference clock for the PLL to synthesize a carrier and to derive clocks for all other parts of the transceiver SoC, e.g., ADC and the digital baseband. The typical start-up time (T s ) of an XO is relatively long (∼ms) due to a high quality factor of the crystal quartz. This leads to a significant (up to 30%) power overhead for a highly duty-cycled transceiver with a short packet format, e.g., the packet length is as short as 128µs in BLE (Fig. 5.3.1). A reduction of T s of the XO is necessary, at the same time, the power overhead to enable a fast start-up should be minimized in order to reduce the overall energy consumption (Fig. 5.3.1).