## 6 Practical Realizations of Piezoelectric Power Conversion Circuits
### Examples of Comercially Avaliable Monolithic Micro-Power Electronic AC-DC Converters
A disadvantage of circuit configurations with IC LTC3588 is that the minimum value of the working input voltage is 2.7 V. Moreover, higher stability of the output voltage requires a relatively large value of the inductance \(L_{1}\). The second integrated circuit subject to analysis is the ADP5091 (from Analog Devices) [97], which is intended for harvesting energy from low-power sources. The input of the circuit (Figure 34) is implemented as a high-efficiency step-up converter. It can operate with voltages from 80 mV to 3.3 V during operation, and the minimum cold-start voltage is 380 mV. This allows operation with photovoltaic, thermoelectric, and piezoelectric sources. The ability to temporarily turn off the regulator via an external control signal is added in order to reduce RF emissions. When working with AC sources, it is necessary to add a voltage rectifier externally. There is a built-in MPPT (maximum power point tracking) controller in the circuit, which further increases the conversion factor, due to the operation  
Figure 33: Voltages versus time for the block diagram in Figure 31: (**a**) for output voltage \(V_{out}=3.6\) V (\(D_{0}=\) “1” and \(D_{1}=\) “1”) and four supercapacitors (four capacitors connected in series \(C_{STO}=1\) F/4 = 250 mV; 2.7 V) in series, connected to the output terminal of the voltage rectifier (pin 4); (**b**) for output voltage \(V_{out}=3.6\) V (\(D_{0}=\) “1” and \(D_{1}=\) “1”) and eighth supercapacitors (eighth capacitors in series \(C_{STO}\) = 1 F/8 = 125 mV; 2.7 V) in series, connected to the output terminal of the voltage rectifier (pin 4). Figure 32: Voltages versus time for the block diagram in Figure 31: (**a**) for output voltage \(V_{out}=1.8\) V (\(D_{0}=\) “0” and \(D_{1}=\) “0”) and four supercapacitors (\(C_{STO}=1\) F/4 = 250 mV; 2.7 V) in series, connected to the output terminal of the voltage rectifier (pin 4); (**b**) for the output voltage \(V_{out}=3.6\) V (\(D_{0}=\) “1” and \(D_{1}=\) “1”) and eighth supercapacitors (eighth capacitors in series \(C_{STO}\) = 1 F/8 = 125 mV; 2.7 V) in series, connected to the output terminal of the voltage rectifier (pin 4). at the point of maximum power from the characteristic of the source used. The converted energy can be stored in a supercapacitor or battery connected externally to the circuit to a built-in charge and discharge control unit. In case of insufficient power from the source or insufficient stored energy, a back-up power supply option with an additional battery is provided. The output voltage of the built-in LDO (low drop-out) regulator can be controlled digitally from 1.5 V to 3.6 V. Its maximum output current is 150 mA. The basic highlights of the circuit configuration using the ADP5090 IC are (1) the possibility of operation from 380 mV at cold start-up (after cold start-up, the regulator is functional at an input voltage with amplitude of 0.08 V), (2) that an optional primary cell battery can be connected to certain terminals and can be managed by an integrated power path management control block, and (3) a very small sleeping quiescent current with a typical value of 390 nA. The disadvantages of this circuit configuration are the need for an external voltage rectifier to connect the piezoelectric element, as well as a relatively large number of external discrete RC components.