Sharp and Arrow have made a one-off solar-powered e-reader called Solar Reader from off-the-shelf components.
"Together with our distributor Arrow we realised this proof of concept, that in principle it is possible to design portable devices purely powered by photovoltaic components and completely independent from external power supply or replaceable batteries," said Ralf Schafer, a spokesman for Sharp told EW.
Key components are a 'memory LCD' and two thin solar panels from Sharp, and an ARM Cortex-M0 microcontroller from NXP.
The reflective memory LCD uses Sharp's proprietary 'continuous-grain' silicon thin-film transistors to implement not only an active matrix, but also one bit of memory per pixel.
"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," said Sharp.
Memory LCD is Sharp's response to the non-LCD E Ink displays, which use electrophoretic technology, used in most e-readers.
In this case the 1.53mm thick LS027B4DH01 2.7in. LCD has a power consumption of 50µW with a constant image and 175µW at a 1Hz refresh rate.
The LPC1114 processor requires 500µW at up to 50 MHz, said Sharp, and powers down to 6µA in deep sleep.
0.8mm thick polycrystalline LR0GC02 solar panels produce up to 300mW from 27.7 cm² at 12.8% efficiency - converted to 5V at up to 60mA.
"In theory, this is enough to supply the electronic table of contents along with the memory LCD, processor, real time clock, flash memory, and the rest," said Sharp. "The solar cells are 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."
In slideshow mode, the RTC wakes the processor periodically and refreshes the screen every 0.5s, for a total consumption of 1.17mW.
Indoors on dull days the solar cell output drops to under 2V, so a rechargeable battery has been added to store spare energy when conditions are brighter.
The battery is a Thinergy MEC 101 from US firm Infinite Power Solutions which is 0.17mm thick (max. height 2.5mm) and stores 1mAh at 4.1V.
"This is sufficient to operate the solar reader for almost 19 hours without the solar cell having to supply additional energy," said Sharp.
When the reader is shut down, only the real-time clock (110nA) and Infinite Power Solutions' proprietary battery charging electronics (350nA) are drawing power.
Power conversion from 4.1V to 5V for the display and to 3.3V for the MPU comes from a Linear Technology LTC3525 - supplying the 140µA needed for slideshow mode at almost 90% efficiency.
Texas Instruments' TPS780033022, with 0.5µA loss at 0.7V drop, was chosen as an LDO.
Together, the two power chps require 61µW.
