The organic light emitting diode (OLED) display is becoming more and more popular, especially for mobile phones, media player and small entry level TVs. Contrary to a standard liquid crystal display, the OLED pixel is driven by a current source. To understand how and why the OLED power supply impacts the display picture quality, it is key to understand the OLED display technology and power supply requirements. This article explains the latest OLED display technology and discusses the main power supply requirements and solutions. A novel power supply architecture tailored to the OLED power supply requirements is also presented here.
All major mobile phone companies by now offer one or more models featuring an OLED display. Sony has the first OLED TV in mass production and many other companies show first prototypes. The OLED display offers wide color gamut, contrast ratio, viewing angle and fast response time. This makes the display ideal for multimedia applications. The self-emitting OLED technology doesn’t require a backlight and the power consumption depends on the display content. Power consumption can be much lower compared to a LCD using backlight. With a larger panel size the superior image quality of an OLED becomes more noticeable. Therefore, more and more OLED panels being used have a display size >3” and the ultimate application in the future still might be the TV panel. Another market for the OLED display is certainly the flexible display. Currently, the OLED and electrophoretic display technology look most promising. The electrophoretic or bi-stable display being used for electronic reader applications needs to be improved in color quality. On the other hand, currently OLED display is not ready for mass production when using fully-flexible materials. This depends mainly on the backplane technology.
Backplane technology enables flexible displays
High-resolution color active matrix organic light emitting diode (AMOLED) displays require an active matrix backplane using an active switch to turn each pixel on and off. The liquid crystal (LC) display amorphous silicon process is mature and provides a low-cost active matrix backplane, and used for OLEDs as well. For flexible displays companies are working with an organic thin film transistor (OTFT) backplane process. This process also can be used for an OLED display to realize flexible, full color displays. Whether a standard or flexible OLED is being used the same power supply and driving mythology needs to be applied. To understand the OLED technology, capabilities and its interaction with the power supply, a closer look into this technology is given. The OLED display itself is a self-emitting display technology and doesn’t require any backlight. The material for the OLED belongs to the category of organic materials due to its chemical structure.
OLED technology requires a current control driving method
The OLED has electrical characteristics very similar to a standard light emitting diode (LED) where brightness depends on the LED current. To turn the OLED on and off and to control the OLED current a control circuit, thin film transistors (TFTs) are being used.
As the voltage supply Vdd changes, OLED brightness changes as well. Any superimposed voltage ripple on Vdd, can cause horizontal bars on the image due to different brightness levels. Depending on the display, a voltage ripple larger than 20mV already can cause such a phenomena. The visibility of the horizontal bars depends on amplitude and frequency of the superimposed voltage ripple. As soon as the frequency interferes with the frame frequency the bars appear. Under a normal laboratory environment the superimposed voltage ripple on Vdd is usually smaller than 20mV. The problem appears as the display and power supply are integrated into as system. As soon as any sub-circuit in the system draws pulsating current from the system power supply a voltage ripple appears, common to all circuits connected to the system power supply. Typical sub-circuits drawing pulsating current are the GSM power amplifier in a mobile phone, motor driver, audio power amplifier or similar. In such systems, the system supply rail has a superimposed voltage ripple. If the AMOLED power supply doesn’t reject this ripple, it will appear on its output as well causing the discussed visible image distortion. To avoid this, the AMOLED power supply needs to have a very high-power supply rejection ration and line transient response.
For the AMOLED power supply a boost converter is required for the positive voltage rail, Vdd, and a buck-boost or inverter for the negative voltage rail, Vss. This puts the challenge to the IC manufacturer providing a suitable power supply IC providing a very accurate positive voltage rail, Vdd, and negative voltage rail, Vss, achieving minimum component height and smallest solution size.
To meet all these requirements a novel power supply topology is chosen to provide both positive and negative output voltage rails from a Lithium-Ion (Li-Ion) battery using just a single inductor.
SIMO regulator technology enables best-in-class picture quality
Advanced power save mode enables highest efficiency
As with any battery-powered equipment, long battery standby time is only achieved when the converter operates at highest efficiency over the entire load current range. This is especially important for an OLED display. The OLED display consumes its maximum power when the display is fully white, and much lower current for any other display color. This is because only the white color requires all the sub-pixels red, green and blue to be fully turned on. For example, a 2.7 inch display requires 80mA current for a fully white picture and only 5mA current when icons or graphics are displayed. Therefore, the OLED power supply needs to provide high converter efficiency at all load currents. This is achieved by using an advanced power save mode technology reducing the converter switching frequency as the load current decreases. Since this is done using a voltage controlled oscillator (VCO), possible EMI problems are minimized and the minimum switching frequency is controlled to be outside the audio range of typically at 40kHz. This avoids possible audible noise caused by ceramic input or output capacitors. This is especially important when using the device in a mobile phone application and simplifies the design process.
Since OLED display technology is just emerging, there is still a lot of room to conserve power, increase OLED efficiency and minimize the total solution size. As OLED becomes more mature, it is also possible to use OLED for architectural lighting or as backlight for LC Displays. Both opportunities allow lower power consumption and higher design flexibility compared to traditional lighting solutions. For OLED technology, the future seems to be very bright.
To download a datasheet on the TPS65136, visit: www.ti.com/tps65136-ca.
To learn more about this and other power solutions from TI, visit: www.ti.com/power-ca.
Oliver Nachbaur is a member of the Technical Staff at Texas Instrument in Germany where he is a System Engineering Manager for the Display Power Converter group. Oliver has over a decade of experience in the semiconductor industry working as an Applications Engineer and System Engineer on Power Management Products. Oliver received a degree in Electrical Engineering in Ravensburg, Germany. He can be reached at: email@example.com.