New-Tech Magazine Europe | Dec 2015 Digital edition

bridge rectifier with a high efficiency buck converter that is optimized for high output impedance piezoelectric sources, and maintains a regulated output voltage with a high efficiency synchronous buck regulator. An ultralow quiescent current undervoltage lockout (UVLO) mode with a wide hysteresis window allows charge to accumulate on an input capacitor until the buck converter can efficiently transfer a portion of the stored charge to the output, and the buck converter turns on and off as needed to maintain regulation. Solar power An example of the importance of knowing the lower limit comes with the buck converter, which starts when the input voltage moves above the UVLO rising threshold to transfer charge from the input capacitor to the output capacitor. The 1V UVLO hysteresis window has a lower threshold of around 300 mV above the selected regulated output voltage, and this prevents short cycling during the buck power-up. When the input capacitor voltage is depleted below the UVLO falling threshold, the buck converter is disabled and the extremely low quiescent current of 450nA allows energy to accumulate on the input capacitor from the piezoelectric source. The hysteresis is determined by an algorithm that controls the output through internal feedback from the voltage sense pin. The four output voltages in the LTC3588 show the lower voltages that the energy harvesting source is expected to power, from 1.8V, 2.5V and 3.3V to 3.6V, and these are pin selectable with up to 100mA of

continuous output current. An input protective shunt set at 20V enables greater energy storage for a given amount of input capacitance. The low-loss bridge rectifier has a total drop of about 400mV, with typical piezo generated currents of around 10µA, and the bridge is capable of carrying up to 50mA. All of this allows the charge in the capacitor to smooth out the intermittent nature of the source of power and provide the required voltage to the sensor or controller. Batteries are often used to collect the charge from the energy source, but these also need to be protected from overcharging or undercharging. The MAX17710 from Maxim Integrated can manage the poorly regulated energy harvesting sources with output levels ranging from 1µW to 100mW. For a 0.8V harvest source and a 4.1V cell, the device can deliver over 20 mA (80mW) for as long as the harvest source can support it. To do this, the device includes a boost regulator circuit for charging a lithium battery from a source as low as 0.75V, while an internal regulator to protect the cell from overcharging and an internal voltage protection prevents the cell from over discharging. The selectable output voltages from 1.8V through 2.3V to 3.3V are regulated using a low-dropout (LDO) linear regulator. An increasingly popular source for energy harvesting is a solar cell, and there are several different ways to manage the power coming from such cells. These different approaches are freely available through digital libraries, and can be tested out on a range of evaluation boards. The digital power software libraries

Figure 1: The quiescent current undervoltage lockout (UVLO) uses a hysteresis algorithm to protect the power delivery in a piezoelectric energy harvesting system.

Figure 2: The MAX17710 integrates a low-dropout regulator to protect a capacitor or lithium battery cell from undercharging.

Figure 3a: The basic P&O algorithm (Source: MATHWORKS)

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