## 3 Design of the Wirelessly Activated Node
### Energy Harvesting and Power Management  
Given the limited amount of energy that can be harvested (down to few tens of \(\upmu\)W), power management needs to be extremely efficient, both in terms of energy harvesting and energy use. As a first remark, the storage elements need to operate with ultra-low leakage currents while storing enough energy to allow node operation. For this reason, we chose to integrate two 470 \(\upmu\)F ultra-low-leakage tantalum capacitors (TMJE477K006 RCQYA, AVX Corporation, Fountain Inn, SC, USA) on the node, capable of limiting leakage currents to less than 2 \(\upmu\)A and coming in a reasonably small form factor (7.3 \(\times\) 4.4 mm\({}^{2}\) footprint; thickness: 4.1 mm). With the aim of minimizing the node's thickness, the capacitors are located on the same side of the PCB allocating the antenna, protruding only 1 mm from the antenna profile. Regarding the harvesting circuitry, the voltage-doubler rectifier is composed of two Schottky diodes (SMS7630-079 LF, Skyworks Solutions, Irvine, CA, USA) and a matching network consisting of one 0.3 pF capacitor and a shorted stub (width: 0.51 mm; length: 5.70 mm). The distributed component choice instead of a lumped one is for losses minimization. The rectifier's measured performance reveals a 25% of RF-to-DC PCE (power conversion efficiency) [23] with \(-\)10 dBm of RF input power, and 34% with \(-\)5 dBm. Figure 4b shows the circuit layout of the power management section. The rectifier output is connected to a buck-boost DC/DC converter (bq25570, Texas Instruments, Dallas, TX, USA) designed explicitly for harvesting energy from low-voltage, low-power sources (down to 100 mV and 15 \(\upmu\)W). According to the literature [24], the maximum RF-to-DC  
Figure 5: Measured results of the scattering parameters of the two-port antenna on board of the wireless sensor node: ports 1 and 2 are shown in Figures 3a and 4c and correspond to the patch and the metal-mounted chip antennas, respectively. efficiency is reached when the rectifier circuit is loaded to \(\sim\)50% of its open-circuit voltage V\({}_{\text{OPEN}}\), i.e., the voltage at the output of the rectifier when the load is absent (open-circuit condition). For this reason, the bq25570 maximum power point tracker (MPPT) sampling network has been programmed in that sense, allowing for measuring the open-circuit input voltage every 16 s, and then transferring the maximum amount of power currently available to the output. Given the application, the node is expected to be unable to receive any input power when the vehicle is turned off for a long time. Therefore, the capacitors will likely be completely discharged when the vehicle is started. The main issue affecting such a scenario is that the bq25570 operates in a very low-efficiency mode (less than 10%) till the output voltage overcomes the cold-start voltage, which is about 2 V. If the DC/DC converter was required to charge the energy storage capacitors (C\({}_{\text{STORE}}\) = 1 mF) directly up to 2 V, with an input power as low as 30 uW, the charging time would be approximately 22 min, unacceptable for several applications where power might be intermittently removed from the system. In order to overcome this problem, a second DC/DC converter, already integrated inside the bq25570, is exploited, which steps-down the voltage at the output of the first DC/DC converter to 3.3 V and is connected to the tantalum capacitors. The step-up DC/DC converter can thus charge only a 22 uF capacitor (C\({}_{\text{BUFFER}}\), whose voltage V\({}_{\text{BUF}}\) is the blue line in Figure 6), thus reducing dramatically the time required to exit the cold-start state and leading to a turn-on time in the range of few seconds, and up to 30 s for the lowest input power level. When V\({}_{\text{BUF}}\) reaches a programmable threshold set to 3.6 V, the bq25570 sets a pin high, signaling that the voltage is sufficient to turn the buck DC/DC converter on.