Battery life is a major concern in wireless sensing applications, as it causes a trade-off between system size and power consumption of the electronic circuits connected to it. Even if electronic circuit power consumption is steadily decreasing, the energy density of common energy storage systems is still extremely low in space-constrained applications. In this scenario, energy harvesting is a valuable solution to extend, in theory indefinitely, the autonomy of ubiquitous sensing systems. In particular, vibrational energy harvesters are an excellent solution to power sensors in industrial and automotive applications. This paper presents an electrostatic energy harvester (EEH) interface. Recently, electret-based EEHs have attracted considerable attention because of their capability to generate large powers, even at low accelerations [1]. Unfortunately, these devices are characterized by extremely high internal impedances and their interfacing circuits need to be simultaneously ultra-low-power and capable of working reliably with several tens of Volts applied to the input. To the best of our knowledge, only one solution has been proposed to efficiently interface high-voltage energy harvesters [2]. However, that circuit did not allow fully autonomous battery-less operation and did not work under 25μW available power.The proposed system has been designed in TSMC 0.25 BCD (option 60V) and its block diagram is depicted in Fig. 20.8.1. The system requires 5 discrete components to operate: 1 inductor (L=10mH), 2 capacitors (C IN =100nF and C OUT =2μF-10mF) and 2 resistors (R BIAS =100MΩ and R DC of several GΩ). The system bias current of 10nA is generated by forcing the 1V reference voltage across the resistance R BIAS . Since the input voltage can be as large as 60V and is dependent on the harvested power, the system is powered by its output voltage. For that reason, input and output are shorted in start-up conditions; the Direct-charge circuit has to charge the load capacitance between 0V and a certain programmable threshold before the rest of the system gets activated. When this happens, the main 10nA bias current and the DC-DC buck converter are enabled. In order to maximize the efficiency at low power levels, the inductive converter operates in deep discontinuous conduction mode (DCM) with a new ultra-low-power modulating scheme based on inductor current sensing. The proposed control method optimally controls both on-time, T ON , and off-time of the p-switch, M P , at each cycle, in order to keep peak current and converter equivalent input resistance constant. It can be demonstrated that, in opposition to what happens in conventional PFM and PWM schemes, the proposed simultaneous optimization of T ON and T avoids the increased power consumption (due to the increase of the p-switch duty-cycle T ON /T) when the input voltage gets close to the output voltage.As shown in Fig. 20.8.1, a current sensor is used to generate the copies I LC1 and I LC2 of the inductor current I L, with an attenuation factor of 1000. The first...
