Introduction to OLED Display Technology

To understand how and why OLED power supply affects display image quality, you must first understand OLED display technology and power supply requirements. This article will explain the latest OLED display technology and explore the main power supply requirements and solutions. It will also introduce innovative power supply architectures specifically designed for OLED power supply requirements.

Market environment

All major mobile phone companies have now launched one or more models using OLED displays. Sony has taken the lead in mass-producing OLED TVs, and many other companies have also launched their first prototype models. OLED displays have wide color gamut, high contrast, wide viewing angles, and fast response times, which make them very suitable for multimedia applications. Self-luminous OLED technology does not require backlighting, and its power consumption depends on the display content, which is much lower than that of LCDs using backlighting. As the panel size increases, the high-quality characteristics of OLED become more obvious, so more and more OLED panels have display sizes greater than 3, and the future application level will still be dominated by TV panels. Another OLED display market is flexible displays. At present, the prospects for OLED and electrophoretic display technology are quite promising. Electrophoretic or bistable displays used in e-readers need to improve color quality. On the other hand, when using completely flexible materials, OLED displays are still not suitable for mass production, which mainly depends on the development of backplane technology.

Backplane technology enables flexible displays

High-resolution color active-matrix organic light-emitting diode (AMOLED) displays require an active-matrix backplane that uses active switches to turn each pixel on and off. Liquid crystal (LC) display amorphous silicon processes are now mature enough to provide low-cost active-matrix backplanes that can be used for OLEDs. Many companies are currently developing organic thin-film transistor (OTFT) backplane processes for flexible displays, which can also be used for OLED displays to enable full-color flexible displays. Whether standard or flexible OLED, the same power supply and drive technology are required. To understand OLED technology, its capabilities, and its interaction with the power supply, it is necessary to look deeper into the technology itself. OLED displays are a self-luminous display technology that does not require any backlight at all. The materials used in OLEDs are organic materials with a suitable chemical structure.

OLED technology requires current-controlled driving method

Figure 1 is a simplified circuit diagram showing only one pixel. OLEDs have electrical characteristics quite similar to standard organic light-emitting diodes (LEDs), and the brightness depends on the LED current. To turn the OLED on and off and control the OLED current, a control circuit using a thin-film transistor (TFT) is required.

Figure 1. Simple example of active matrix OLED single pixel control (ITO transparent conductive film)

In Figure 1, transistor T2 is the pixel control transistor that turns the pixel on and off, similar to any other active matrix LCD technology. T1 is treated as a current source, and the current is driven by this gate voltage source. The storage capacitor is Cs, which is used to maintain a stable T1 gate voltage and lock the supply current until the pixel is reconfigured. In Figure 1, the simple single transistor current source has a significant cost advantage because only two transistors are required. The disadvantage of this simple circuit is that the current will vary, which factors include process variations and Vdd voltage variations. OLED power supply circuits usually provide two voltage supply rails, Vdd and Vss. The voltage rail Vdd must achieve very tight regulation to achieve the best picture quality and avoid image flicker. Vss is usually a negative voltage, and its voltage regulation accuracy can be reduced because this voltage is less likely to affect the LED current. Figure 2 shows the effect of voltage fluctuations caused by Vdd on OLED displays.

Figure 2. Voltage fluctuations on the power rails create horizontal streaks

When the voltage supply Vdd changes, the OLED brightness also changes. Superimposed voltage ripples on Vdd can cause horizontal stripes in the image due to different brightness. Depending on the display, voltage ripples greater than 20mV may cause this phenomenon. The degree of appearance of horizontal stripes depends on the amplitude and frequency of the superimposed voltage ripples. Once the frequency interferes with the frame frequency, stripes will appear. In general experimental environments, the superimposed voltage ripples on Vdd are usually less than 20mV. This problem will occur when the display and power supply are integrated into a system. Once any subcircuit in the system draws pulsating current from the system power supply, voltage ripples will appear. This is true for all circuits connected to the system power supply. Subcircuits that typically draw pulsating current include GSM power amplifiers, motor drivers, audio power amplifiers, etc. in mobile phones. In these systems, superimposed voltage ripples will appear on the system supply power rail. If the AMOLED power supply does not suppress this ripple, the ripple will appear at the output and cause the image distortion mentioned above. To avoid such problems, the power supply of AMOLED needs to have extremely high power supply rejection ratio and line transient response.

For the power supply of AMOLED, the positive voltage power rail Vdd requires a boost converter, and the negative voltage power rail Vss requires a buck-boost converter or inverter. This is a big challenge for IC manufacturers who provide suitable power supplies, because manufacturers need to provide quite accurate positive voltage power rail Vdd and negative voltage power rail Vss to achieve the lowest component height and smallest solution size.

To meet all these requirements, a new power supply topology is needed to provide both positive and negative voltage rails from the Li-Ion battery using only a single inductor. SIMO regulator technology enables best-in-class image quality

Figure 3. TPS65136 buck-boost converter topology with dual input support

Figure 3 shows a general application circuit using the TPS65136, which uses single inductor multiple output (SIMO) regulator technology and operates in a four-switch buck-boost converter topology. SIMO technology achieves best-in-class line transient regulation, buck-boost mode for both outputs, and highest efficiency over the entire load current range.

Advanced energy-saving mode for maximum efficiency

As with any battery-powered device, long battery standby times can only be achieved when the converter operates at maximum efficiency over the entire load current range, which is particularly important for OLED displays. OLED displays consume the most power when displaying full white, and less current for any other displayed color, because only white requires all red, green, and blue sub-pixels to be fully illuminated. For example, a 2.7 display requires 80mA to display a full white image, but only 5mA to display other icons or graphics. Therefore, OLED power supplies require high converter efficiency for all load currents. To achieve such efficiency, advanced power-saving mode techniques are used to reduce the load current to reduce the converter switching frequency. Because this is done through a voltage-controlled oscillator (VCO), possible EMI issues are minimized and the minimum switching frequency can be controlled outside the typical 40kHz audio range, which avoids noise generation from ceramic input or output capacitors. This is particularly important when using these devices in mobile phone applications and simplifies the design process.

in conclusion

As OLEDs mature, they can be applied to architectural lighting or LCD display backlighting. Compared with traditional lighting solutions, OLEDs offer lower power consumption and higher design flexibility for both applications.