The power generated by a solar cell varies significantly with lighting conditions.So a rechargeable storage device, such a supercapacitor, is required to provide continuous power when the solar cell is insufficiently illuminated. Although it has much less charge storage capacity compared to a battery, a supercapacitor requires significantly less maintenance, is easy to charge and its cycle lifetime is orders of magnitude longer than a battery.
The storage device can be combined with the LTC3536 buck-boost DC/DC converter, which is designed to simplify the task of harvesting and managing energy from low input voltage such as photovoltaic cells. LTC3536 can work down to input voltages as low as 1.8V, and provides very high efficiency over a wide range of input voltages above or below the output voltage.
The circuit above shows an application for an LED driver supplied with a solar cell for an emergency LED torch. When the torch is off, the LTC3536 is in shutdown. The quiescent current of less than 1μA minimizes supercapacitor drain when the ambient light is no longer available.
When the LED torch is switched on, the LTC3536 is turned on via the SHDN pin to supply 105mA constant load current to the LED. Figure below shows the high efficiency of this supply, enabling a slow discharge of the two series 60F capacitors down to 1.8V. The LTC3536 regulates the output and the LED current, guaranteeing light for 14 minutes if the supercap is charged up to 5V. It is possible to extend this time by increasing the supercap value or using a battery with an adequate charge control.
Tools & Simulations
LTspice® is a powerful, fast and free simulation software, schematic capture and waveform viewer with enhancements and models for improving the simulation of analog circuits.
To launch ready-to-run LTspice demonstration circuits for this part:
Step 1: Download and install LTspice on your computer.
Step 2: Click on the link in the section below to download a demonstration circuit.
Step 3: If LTspice does not automatically open after clicking the link below, you can instead run the simulation by right clicking on the link and selecting “Save Target As.” After saving the file to your computer, start LTspice and open the demonstration circuit by selecting ‘Open’ from the ‘File’ menu.