system elektronik GmbH: Systemhaus für TFT Displays - New generation: Industrial TFT-LCD with Backlight in LED technology

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New generation: Industrial TFT-LCD with Backlight in LED technology

Toshiba Matsushita Display Technology Co., LTD (TMDisplay) presents a new generation of LDC TFT displays in cooperation with system elektronik GmbH.

The display line-up from 5.7” (144.8 cm) up to 10.4” (26cm) fulfils the standards for use in an industrial environment such as extended temperature range, shock & vibration resistance and long-term availability. The new display line-up with resolutions from VGA (640x480) up to XGA (1024x768) covers all formats that are popular for an industrial environment. The 8.4” display is even available in two resolutions (SVGA and XGA), so the user can switch his system to a higher resolution without any change of mechanics. For this industrial line-up and for the first time ever an EDID-EEPROM is directly integrated on the display electronics. Due to this the display can be automatically detected and adjusted by the driver electronics.
The displays are also available in versions with touch sensor in 4 wire resistive technology. The sensor from company DMC is laminated on the display by TMDisplay on cleanroom conditions. This minimizes logistics and costs for appropriation of a suitable environment of display and sensor for the user. Target group for these displays are users from the industrial measurement technology, medical technology and general control engineering

LED or CCFL?
Compared to CCFL backlight an LED backlight offers essential advantages. Due to the fact that an inverter that supplies the CCFL tube is no longer necessary for LED material costs can be reduced and interference radiation of the whole assembly can be decreased. Construction-conditioned LED backlights can be driven on a larger scale of luminance than CCFL based systems. For use in fields of extended temperature from -20 up to +70°C LED backlights can definitely show their advantages. Especially at a temperature below 0°C there are no restrictions regarding cold start as it is the case with CCFL; a huge advantage for outdoor use.

Mid-Power LED
The special construction of the LED strand chosen by TMDisplay is the structure of the double LEDs themselves on the one hand and the arrangement on the reflector on the other hand. White Mid-Power LEDs are used, e.g. a InGaN-layer on a GaN-structure, which emits blue light with a wave length of 450nm up to 470nm. This structure is, as usual, capped with Cerium doped yttrium-aluminium-garnet and covered with pottant. The blue light of the LED is partly transferred to a wide yellow spectrum of around 580nm. Together with the blue of the LED and due to the fact that the yellow light stimulates the red and green receptors in the human eye the result for the viewer seems to be white light. A fine-tuning with rare earth elements helps to improve the overall impression of the white light and leads to a pleasant colour temperature of the LCD, similar to the light of the CCFL. Process variations during LED production lead to different colour temperatures of the LEDs from warm yellow to cold blue. Due to a clever mix and adjustment of the display internal optical filter very good compromises for the colour temperature and for the industrial environment can be reached.

LED backlight unit
As shown in picture LED_detail the LEDs are soldered up on a flexible PCB. In order to reach a better heat dissipation the copper surface of the circuit path is continued under the LEDs. The flexible PCB is mounted on a massive aluminium carrier. This guarantees the thermotechnical convenient transfer to the backplane of the displays. The connection of the LED unit to the integrated current source is made by a foil connector.

LEC current source integrated
To secure a thermal secure usage of the LEDs the operation by a constant current source is essential. All new LCD types of the product line-up already have an integrated current source for the LED strand, so the user does not have to make any considerations regarding current source. Additionally, a specially designed IC LM3431 from National Semiconductor is used. Depending on display size the LEDs are interconnected in rows at 2x6, 2x8 and 3x7. The IC LM3431 is able to identify errors. The failure of one LED can be balanced by the control circuit. If several LEDs might fail the corresponding LED strand will be switched off. The current source has a supply voltage of 12VDC. A PWM dimming from 1 to 100% at 200Hz PWM frequency is possible.

Long-term available and mounting compatible
Especially in the industrial environment the question regarding replacement of displays from one generation to the next is made with “last” consequence. Here it is definitely not sufficient to keep only diagonal and resolution. Key words here are Form Fit-Function or, in other words, for the user everything remains as it was, except for the fact that the display is much better. If then costs can be reduced, as for example due to loss of inverter it can only be better. These requirements have been realized for the new TMDisplay line-up. Here, a small restriction has to be noted: The housing at the upside of the LTA065B0D2F was scaled up by 3mm compared to the predecessor to reach a better illumination and to avoid halation. The center of screen will remain as before to the side mounting points.

Reduction of pollution
The MTBF of the LED backlight is indicated with typically 70.000 hours and therefore definitely longer than CCFL-systems. Considering also the fact that there is no damage of the light guide by internal ultraviolet radiation as for CCFL technology, the lifetime of the whole display can be even extended by replaceable structure of the LEDs and keep full luminance (after replacement of LED unit). The energy balance is not only far more positive compared to CCFL but also on the “right side” regarding environmental questions as the components do not include any quicksilver (keyword: RoHS)

Advantages of LED backlight at a glance
• longer lifetime
• no high voltage in the display
• less interference radiation
• better dimming
• less power input
• more rugged against shock and vibration