## 6 Practical Realizations of Piezoelectric Power Conversion Circuits
### Examples of Comercially Avaliable Monolithic Micro-Power Electronic AC-DC Converters
The SLEEP comparator reduces switching losses because the step-down converter does not operate continuously but is enabled and disabled when the output voltage is below or above set lower and upper thresholds, respectively. Additionally, internally there is a circuit monitoring the voltage on the input capacitor, which prohibits the operation of the step-down converter if it falls below a limit set at 300 mV above the set voltage at the output. Based on the above-described structure and principle of operation of the LTC3588-1, an experimental study was carried out to demonstrate its effectiveness and practical applicability. More specifically, in the experimental study, a DC1459B-A (from Analog Devices) development system was used. The test AC signal was obtained from a function generator type MFG-2110 with a high internal resistance equal to 10 k\(\Omega\), so as to simulate the typical behavior of a piezoelectric element. The amplitude of the signal is up to 8 V, and the frequency is set equal to 20 Hz. The experimental study was performed in two cases, for an output voltage of 1.8 V and 3.6 V, and for each value of the voltage, a study was performed with four and eight supercapacitors type HV0810-2R7105-R (from Eaton(r) (Dublin, Ireland)) connected in series. For both cases, the signal produced by the piezoelectric element was applied to the input terminals of the built-in voltage rectifier, having an amplitude of up to 10 V. A digital voltmeter type VC820 (from Voltcrat(r) (Hirschau Germany)) was used to measure the \(V_{in}\) input voltage (the voltage on the supercapacitors), the output voltage \(V_{out}\) of the converter, and the Power-Good (PGOOD) voltage, which includes an optoelectronic interface with beam waveguide technology. Since the standard interface cable has an RC232 output, an adapter cable was additionally used that converts the RC232 to USB standard to provide a connection to a personal computer. To visualize the measured voltages, an interface program VC820/840, specially developed by the manufacturer for working with digital voltmeters of the VC820 type, was used. To store the measured voltages, a built-in function was used to save arrays of data in the MS Excel(r) file format. As a result of the experimental study, Figures 32 and 33 present the timing diagrams of the input voltage \(V_{in}\) obtained over several supercapacitors with a nominal capacitance of 1 F, the output voltage \(V_{out}\) of the circuit over a filtering electrolytic capacitor with a capacitance of 10 \(\mu\)F, and the logic state of the PGOOD signal. As can be seen, in Figure 32a,b, the voltage drop across the supercapacitors increases monotonically up to a value approximately equal to the amplitude of the input voltage, while the voltage at the output reaches the set value with certain jumps. Also, as the number of capacitors connected in series increases, the time to reach the set value decreases, as it decreases the equivalent capacitance. The PGOOD signal jumps to its set value when the output voltage reaches approximately 90% of the set value, equal to 1.8 V. Similarly, the time diagrams (Figure 33a,b) were obtained at a set value of 3.6 V. Compared to the timing diagrams in Figure 32, the voltages settle over a longer time interval because the voltage set point is higher. In this case, the established values are reached in approximately the same way. Unlike the first case, the PGOOD signal reaches a value of 3.6 V, depending on the value of the output DC voltage. The basic advantages of the circuit configurations described above with the IC LTC3588-1 are (1) the ability to directly connect a piezoelectric generator that produces an AC voltage, (2) the low self-consumption in the rest mode, which does not exceed 1 \(\mu\)A, (3) in regulation, the LTC3588-1 enters a sleep state in which both input and output quiescent currents are minimal, and (4) the buck converter turns on and off as needed to maintain regulation.