A 0.3 to 5-MHz Low-Voltage Digitally-Controlled Oscillator for Energy Harvesting Applications

Energy Harvesting is the act of collecting energy from the environment and transforming it into electrical energy. Energy Harvesters in integrated-form are composed of many components, including a DC-DC converter. If this converter is implemented with switched-capacitors, an oscillator is necessary to provide the switching frequency. There is a great demand for ultra-low power and ultra-low voltage oscillators in Energy Harvesting applications, since switching frequency of the DC-DC converter can be adjusted to optimize conversion efficiency. This work proposes a 0.7 V open-loop digitally-controlled oscillator (DCO) in a frequency range of 300 kHz to 5 MHz and maximum power consumption of $5\ \mu \mathrm{W}$ in the TSMC 180 nm technology. It is based on a ring-oscillator with variable capacitive loading for a fine adjustment in frequency and current-starving transistors for frequency calibration. The frequency setting is performed with a 4-bit digital input. The implemented circuit layout occupies an area of 0.0348 mm2. Post-layout simulations have shown monotonic and linear frequency variation with a peak power consumption of $4.9\ \mu \mathrm{W}$ at the maximum frequency of 5 MHz. The calibration capability compensates frequency degradation in post-layout simulations due to significant parasitic capacitances.