ST AN3110 Application note

AN3110

Application note

Using the STVM100 to automatically adjust VCOM voltage in e-paper

Introduction

The widespread use of multimedia electronic devices, coupled with environmental concerns over the manufacturing and use of traditional paper products, provide the opportunity for the utilization of electronic paper (e-paper).

Electronic paper is the best replacement for conventional paper. A high-quality image can be displayed for weeks without power. Energy is consumed only during the short refresh time. Unlike its counterpart, liquid crystal display (LCD) devices, e-paper does not require a backlight, which not only saves energy, but also makes reading more comfortable.

Achieving high-quality images on e-paper requires the correct setting of the VCOM voltage on the PC board. STMicroelectronics offers the STVM100 VCOM calibrator which is ideal for e-paper applications and allows the VCOM voltage to be set automatically. In this application note, the e-paper principle is briefly described so that the user can understand the necessity of an accurate VCOM voltage setting and how the STVM100 can be used for this purpose.

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General principle of e-paper	AN3110

1 General principle of e-paper

Using Electrophoretic Display (EPD) technology, pre-charged particles within the capsules can be either driven to the top plate (common electrode) of the e-paper which is white due to the reflection of the light, or driven to the bottom plate (column electrodes) of the paper which is dark. The particles are in a bi-stability state so that the position can be maintained even though there is no power applied to the e-paper.

Similar to liquid crystal display (LCD) devices, some of the EPD devices are also formatted by pixels with the active matrix controlled using TFT (Thin-Film Transistor) technology.

In Figure 1, the gate driver sends a line selection pulse to the top line and turns on the first line TFT. The data (analog voltage) is transferred from the source driver to the pixels in the first line. After the first line data is updated, the gate driver selects the second line and then the third, and so on. Once the last line has been scanned, the entire picture on the screen has been updated (whole e-paper refresh). Then the gate driver begins again from the first line and starts the next refresh if required. The line selection pulse pattern is configurable in the gate driver.

Figure 1. E-paper display layout

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AN3110	General principle of e-paper

Figure 2 shows the detailed pixel schematic. The Cep is the equivalent capacitance of the e- paper. The position of the pre-charged particles in the capsules between the two electrodes depends on the absolute voltage applied on the electrodes.

The common electrodes of all the pixels are connected together and a constant Vcommon voltage (VCOM) is required to drive this electrode. The VCOM voltage should be set correctly panel-by-panel for the best performance. Residual pixels (“ghosting” effect) will appear after content refresh if the VCOM voltage is not set correctly.

After the line is selected by the gate driver, the corresponding column electrode is connected to the source driver by the TFT. When the source driver supplies positive voltage to the column electrodes, the positively-charged particles are repelled by the positive electric field and moved to the top plate of the capsule. The white particles are reflective and reflect the ambient light to the viewer so that this pixel is viewed white. Otherwise, if negative voltage is supplied to the column electrode, the positively-charged particles are attracted to the negative electric field and moved towards the bottom side of the capsule which then appears dark.

Figure 2. Basic e-paper pixel schematic

To source driver

To gate driver

TFT

Column electrode

Cep

Common electrode

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