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Passive matrix OLEDs enable cheaper mobile phone displays

12 Feb 2008

Low-power driver technology developed by Dialog Semiconductor will enable low-cost, high-resolution passive matrix OLEDs to be used in mobile phone displays.

“The technology is particularly suitable for use in these displays because of their low power, superb colours and image quality.”

Dialog Semiconductor has announced a new technology that it claims extends the lifetime and reduces the power consumption of high-resolution passive matrix OLEDs. According to the company, its new SmartXtend technology offers the lower cost of passive matrix technology, but without the lifetime disadvantages normally associated with this technology.

"Passive matrix OLEDs are much simpler to manufacture than active matrix OLEDs because there is no thin-film transistor (TFT) substrate," Manoj Thanigasalam, general manager of Dialog Semiconductor's system business unit, told optics.org. "This means that there are fewer processing steps required in the manufacturing line."

Active matrix OLEDs are primarily being used for large-area TV and monitor displays, while SmartXtend was specifically developed for use in today's mobile phones. "SmartXtend has been developed for mobile phones, which vary in size from 1.6 to 4 inches across the diagonal and with resolutions of 320x240 pixels," commented Thanigasalam. "The technology is particularly suitable for use in these displays because of their low power, superb colours and image quality when compared with LCD TFT."

Although passive matrix OLEDs are cheaper than active matrix OLEDs, their uptake in high-resolution displays has been limited because of their poor operational lifetime. "As resolution is increased, a higher current must be injected into the anode diode over a shorter period of time to achieve the desired brightness," explained Thanigasalam. "It is this high current that accelerates the aging process inside the diode, reducing the intensity of its light output over time."

In an attempt to extend the lifetime, the company focused on reducing the peak current by driving more than one row at a time. This is done by decomposing each video frame into two sub-frames. During the first of these sub-frames the rows are driven two at a time with the same data, which means that a lower peak current is applied over a longer period of time.

"During the second sub-frame, corrective data is added to each row to give each pixel its individual colour," explained Thanigasalam. "A passive matrix OLED panel driven with conventional schemes requires about 150 µA per anode, whereas only 100 µA is required to drive the same panel when SmartXtend is used."

Although such driving schemes have been studied in the past, the company claims to be the first to implement a cost-effective solution that is suitable for low power, mobile devices. What's more, it also claims that battery life will be extended by reducing the average power of the technology.

"SmartXtend reduces the power of the whole display subsystem by up to 30% by reducing the number of pre-charge cycles, the current and voltage," explained Thanigasalam. "For example, a display that consumes about 600 mW when driven with conventional passive matrix schemes consumes about 400 mW when SmartXtend is used."

The company currently has a prototype under evaluation with a number of customers and expects the product to sample by the end of 2008 and to be available for production early in 2009.

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