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Solar reader provides proof for self-sufficient devices

The idea for the solar reader (Figure 1) was initiated by a request of the Corporate Publishing division of a large German publishing house which was asked to develop a customer magazine on the topic of sustainability. For an exclusive share of the circulation (approx. 700 copies) the plan was to integrate an electronic table of contents in the cover of the magazine.


To reflect the idea of sustainability, the electronic table of contents was to be designed as a self-sufficient solution to work under normal room/office light conditions. In addition, there were specific requirements for its aesthetic and functional design. To retain the booklet-like look of the magazine in spite of the electronics, the design was limited to a maximum height of just 2.5 mm. Furthermore, dual use of the concept was required, which would ensure that the recipient would keep returning to the magazine in the long-term.       

Technological approach to self-sufficiency 
Self-sufficiency implies that power supply needs to come from a solar cell and power budget would be quite limited. Therefore all components necessary to realize the electronic table of contents had to be "Ultra Low Power". With its Memory LCD technology and a line-up of mini solar panels, Sharp offers two key components for self-sufficient solutions. Further, with the LPC1114, NXP has provided a device that includes one of the most efficient processors currently on the market.    

Memory LCD operate on 1% of the power of conventional TFT LCDs
Memory LCDs (Figure 2) are a new type of LCD, which are based on Sharp's proprietary Continuous Grain Silicon (CGS) technology. CGS allows integrate relatively complex, slender circuits directly on the display glass, thus adding further functions onto it. With Memory LCDs, each pixel is allocated its own memory of 1 bit to save the pixel status. Therefore image information only has to be rewritten to the pixels which status has changed compared to the previous frame. As a reflective display, memory LCDs do not require backlighting either. The result is that memory LCDs only need 0.8% of the power consumed by conventional displays of the same size.       


The 2.7 inch Memory LCD type LS027B4DH01 used for the electronic table of contents has a power consumption of just 50 µW with constant screen displayed and just 175 µW at a refresh rate of 1 Hz. In addition, the display itself is just 1.53 mm thick and thus meets the specification with regard to the maximum design height. With 5 V supply voltage, the memory LCD can also be supplied directly using solar cells as the voltage source.          

NXP processor: just micro Watts for mega Hertz
The new LPC1114 NXP processor with ARM Cortex-M0 core is a total energy-saving wonder (Figure 3). As the control component for the electronic table of contents, it requires just 500 µW at a clock rate of up to 50 MHz. By comparison, with approx. 10 to 30 mW per MHz, conventional PC processors require 20 to 60 times as much. Furthermore, the CPU has a deep power down mode, where all the processes are stalled until the processor consumes just 240 nA in standby. There is also a deep sleep mode, which reduces the current consumption of the LPC1114 to 6 µA.          

With power output of up to 300 mW and a surface area of just 27.7 cm², the LR0GC02 solar panel (Figure 4) is one of the most efficient photovoltaic components. The 12.8% efficiency of polycrystalline silicon cells is almost double that of conventional amorphous cells. Together the ten cells of the LR0GC02 solar panel deliver an output voltage of 5 V at 60 mA, maximum – in theory, enough to supply the electronic table of contents along with the memory LCD, processor and peripheral units.             

Figure 4:
The mini solar panel of the LR0GC02 solar panel has 12.8% efficiency, enough to supply portable self-sufficient applications with power.

Just 0.8 mm thick, the LR0GC02 is also the thinnest photocell currently on the market, and can therefore be easily integrated into the cover of the booklet. It is also withstands high mechanical loads. The substrate is not made from glass and thanks to double wiring the photocell still delivers full performance even if a cell breaks.        

Unsteady light conditions requires energy storage
The challenge in realising the solar reader was primarily the inconsistent and generally mediocre light conditions present in offices. Although the solar cells supply more than enough power to operate the solar reader in bright daylight even indoors, on dull days, the cells only achieve an output power of a few hundred µA at a voltage of 1 - 2 V.    

To ensure constant operation of the solar reader, storage of excess energy produced by the solar cell under favourable light conditions is indispensable as a reserve for periods when there is insufficient light. The next challenge was to find a storage cell that would meet the requirements of the form factor (maximum total height of 2.5 mm) and still provides sufficient capacity. The solution was to be found at Infinite Power Solutions. With the height of just 0.17 mm their Thinergy MEC 101 battery was easy to integrate in the circuit while its capacity of 1.0 mAh at 4.1 V provides sufficient current to operate the solar reader for almost 19 hours in time mode without additional energy from the solar cell.       

Circuit design driven by low power knowhow
Creating the self-sufficient electronic table of contents from these components was the achievement of Arrow and Hitex.                



The heart of the circuit (Figure 5) is the LPC1114 low power processor from NXP. Primarily it controls the display contents stored as graphics on the Flash memory. The solar reader has two operating modes. In slideshow mode, the details of the magazine contents are represented on 22 charts saved as black and white bitmaps with a resolution of 400 x 240 pixels in the Flash memory. The screen is refreshed every 5 seconds. After three cycles, the solar reader switches to time mode providing a secondary use that induces the recipient of the booklet to keep looking at the cover. In time mode, the corresponding images are clocked by the RTC.    

The built-in button can be used to manually switch back and forth between the modes. It is also used to activate the solar reader from deep power down mode or return it to deep sleep mode.    

 In order to save power, the processor goes into deep sleep mode between events (screen refresh), which reduces the processor current consumption to 6 µA. Since the deep sleep mode can only be exited by a start logic (external event), the RTC interrupt acts as "wake-up signal" for the LPC1114. The Screen refresh is set to 0.5 seconds to achieve a blending effect between the pictures. In addition, the Microcontroller toggles the power supply of the serial Flash to reduce overall power consumption. For the total setup a power consumption of 1170 µW in Slideshow Mode and just 174 µW when showing the clock has been achieved (Figure 6).    

To further optimize the power demand, the solar reader enters deep power down mode after 2.5 hours, which gives the Thinergy cell time to recharge. To protect the circuit, deep power down mode is activated also if the solar reader has been operating in poor light conditions for a long period of time and the battery cells can supply power no longer. For this the microcontroller regularly checks the voltage of the solar cells. If it falls bellow a set threshold for longer than 2 minutes, it shuts down.    

When shut down, only the realtime clock and battery charging electronics continue to operate. In this mode only 460 nA is required allowing the energy cell to recharge when the solar reader is exposed to the light.    

Power management is key for Low Power solutions
Another particular challenge was power management. Since the battery cell supplies 4.1 V, but the display requires 5 V and the microcontroller 3.3 V, a boost function and a LDO controller are necessary. Unfortunately, many DC/DC converters are very inefficient at low power levels. Therefore the LTC3525 from Linear Technology was chosen. This DC/DC converter achieves an efficiency of almost 90% at an average load of 140 µA. For the LDO a similar performance is provided by the TPS780033022 from Texas Instruments with a minimal loss of 0.5 µA at 0.7 V drop voltage. The two controllers together require a mere 61 µW. Further, charging and monitoring electronics are necessary to operate the energy cell. Here the expertise came from Infinite Power Solutions, who contributed a patented solution which demands just 350 nA for its own needs.          

Summary 

With the solar reader Sharp and Arrow have shown that by using suitable low power components (memory LCD, low power processor, etc.) it is possible to develop self-sufficient solutions that cover their power requirements using mini solar cells. Since incident light fluctuates greatly, these types of application require a power storage buffer to build up energy reserves when there is a lot of light so that the applications can also be operated when incident light is insufficient. Thanks to their design height of just 0.17 mm, the Thinergy cells from Infinite Power Solutions show that batteries do not necessarily influence the form factor of applications. As an alternative ready-made modules, with voltage monitoring and charging control electronics pre-integrated can be used.    

The photovoltaic components also provide potential for optimisation. The next generation of Sharp cells with monocrystalline silicon have an efficiency of up to 16.5%. Furthermore, the Japanese company is planning to launch thin-film solar cells for portable applications, achieving enhanced power yield under artificial light conditions.

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