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Posted on 05 July 2019

Memory LCDs are based on Continuous Grain Silicon Technology.

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Low power application example: Solar reader

The Corporate Publishing division of a large German publisher has received an order to develop a customer magazine on the topic of sustainability for a management consultancy firm. For a small exclusive share of the circulation (approx. 700 copies), which the management consultancy firm will use to approach the top managers of DAX companies, the plan was to integrate an electronic table of contents in the cover of the customer magazine. In line with 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. In order to retain the booklet-like look of the magazine in spite of the electronics, the design was limited to a maximum installation 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.

By Sven Johannsen, Business Development Manager, Sharp Microelectronics Europe and Patrick Delmer, Supplier Business Manager, Arrow Central Europe GmbH

 

Technological approach

With its memory LCD technology and line-up of mini solar panels, Sharp has two key components for creating self-sufficient solutions. Further, in order to operate the system, a processor is required which can handle an extremely low power budget. With the LPC1114, NXP has provided a device that includes one of the most efficient processors currently on the market and is therefore predestined to power self-sufficient solutions.

Memory LCD

Memory LCDs (Figure 1) are a new type of LCD, which are based on Sharp's proprietary Continuous Grain Silicon technology. Thanks to this special coating method, compared to amorphous silicon, large crystalline silicon domains appear on the display glass, the physical properties of which closely resemble those of monocrystalline silicon. This allows relatively complex, slender circuits to be integrated directly on the display glass, thus enabling additional functions to be implemented directly onto it. In the case of memory LCDs, each pixel is allocated its own memory of 1 bit, where the pixel status and thus image information is stored. This means that image information only has to be rewritten to the pixel in instances where there has been a change in content compared to the previous frame. As a reflective display, memory LCDs do not require backlighting either. When combined, the result is that memory LCDs only consume 0.8% of the power consumed by conventional displays of the same size. This is because, in conventional transmissive LCDs, microcontrollers have to rewrite the entire screen contents from frame to frame at a speed of 50 to 60 Hz, even though a large portion of the screen content has not changed. In addition, backlighting represents a major part of the power consumption.

Memory LCDs. Sharp offers its new memory LCDs for low power applications in two technology versions

The LS027B4DH01 2.7" memory LCD 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.

Furthermore, the memory LCDs feature a special type of image representation. Unlike other reflective screens, this new type of LCD does not require polarisers. Thanks to a special polymer network liquid crystal material (PNLC), the image is generated by the status of the pixels changing from transparent to white at reflectiveness of 50%. This gives the display a silver metallic appearance, which is particularly suited to fashionable applications. In a somewhat more conventional version of the memory LCD, polarisers and high reflection (HR) liquid crystals are used. They produce a pure black and white image with excellent readability and a very large viewing angle.

NXP processor

The LPC1114 NXP processor with ARM Cortex-M0 core is a total energy-saving wonder (Figure 2). 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 powered down until processor current consumption is just 240 nA in the standby state. There is also a deep sleep mode, which reduces the current consumption of the LPC1114 to 6 µA.

LPC1114 block diagram

Solar panel

With power output of up to 300 mW and a surface area of just 27.7 cm², the LR0GC02 solar panel (Figure 3) is one of the leading photovoltaic components when it comes to efficiency. 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 (realtime clock (RTC), Flash memory, etc.).

The mini solar panel of the LR0GC02 solar panel has 12.8 percent 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 able to withstand 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.

Circuit

Creating the electronic table of contents as a self-sufficient solution (i.e. the solar reader) from these components was the achievement of Arrow and Hitex. At the heart of the circuit (Figure 4) is the LPC1114 low power processor from NXP. Primarily it controls the display contents, which are 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 using a total of 22 charts, which are stored 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. This provides a dual feature 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 LPC1114 low power processor controls the display contents, which are stored as graphics on the Flash memory. 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 optimise the current consumption of the circuit, the processor goes into deep sleep mode between events (screen refresh), which reduces the processor current consumption to 6 µA. Since deep sleep mode can only be exited by a start logic (external event), the RTC interrupt acts as a "wake-up signal" for the LPC1114. The Screen refresh is set to 0.5 sec. in order to get a blending effect between the pictures. In addition, the Microcontroller toggles the power supply of the serial Flash to reduce overall power consumption. The total setup consumes around 1170 µW in Slideshow Mode.

The challenge

The challenge in realising the solar reader was primarily the inconsistent and generally rather 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, however, the cells only achieve an output power of a few hundred mA at a voltage of 1 - 2 V.

Solution

Two approaches were adopted to ensure constant operation of the solar reader (Figure 5) even under temporarily suboptimum, fluctuating light conditions:

The Thinergy MEC 101 battery cell (bottom right on the PCB) stores power for operation in poor light conditions

Excess energy produced by the solar cell under favourable light conditions must be temporarily stored as a reserve for periods when there is insufficient light. The challenge was to find a storage cell that would meet the requirements of the form factor (maximum total height of 2.5 mm) and also provide sufficient capacity.

The entire circuit would have to be heavily fine-tuned to energy efficiency.

The latest development from US battery specialists, Infinite Power Solutions, emerged as the ideal power storage device. The Thinergy MEC 101 battery cell has a capacity of 1.0 mAh with an output voltage of 4.1 V. This is sufficient to operate the solar reader for almost 19 hours in time mode without the solar cell having to supply additional energy. In addition, the Thinergy cell is just 0.17 mm high, which meant that it could be easily integrated in the circuit.

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

When the solar reader is shut down, only the realtime clock and battery charging electronics continue to be supplied with power. In this mode only 460 nA is required. If the solar reader is exposed to the light, the energy cell recharges itself.

Another particular challenge of this project 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 an LDO controller were also required.

The problem here is that many DC/DC boost converters are very inefficient at low power levels. The LTC3525 from Linear Technology was chosen for this project. With an average load of 140 µA in slideshow mode, this DC/DC converter achieves efficiency of almost 90%. The TPS780033022 from Texas Instruments was chosen as the LDO. Here too the 0.5 µA loss at 0.7 V drop voltage is minimal. In total, the two controllers together require a mere 61 µW.

In addition to generating the supply voltages, charging and monitoring electronics were also required to operate the energy cell. The expertise of Infinite Power Solutions, who contributed a patented solution, was essential here. The advantage of this is that this part of the circuit requires 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 in principle to develop self-sufficient solutions for portable applications that cover their own power requirements using mini solar cells. Since incident light fluctuates greatly in practice, 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, allowing of easy integration in all situations, the Thinergy cells from Infinite Power Solutions show that batteries do not have to have a detrimental impact on the form factor of the application. As an alternative to the MEC cell, Infinite Power Solutions does, however, also offer ready-made modules, which even have the voltage monitoring and charging control electronics pre-integrated.

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

 

 

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