Organic light-emitting diode ( OLED ) displays are becoming more common and are most prominent in products such as cell phones, media players and small entry-level TVs. Unlike standard liquid crystal displays, OLED pixels are driven by a current source. To understand how and why OLED power supplies affect display quality, you must first understand OLED display technology and power supply needs. This article will explain the latest OLED display technologies and explore key power supply requirements and solutions, as well as an innovative power supply architecture for OLED power supply needs.
In the market environment, major mobile phone companies are now launching one or more models with OLED displays. Sony is the first to produce OLED TVs, and many other companies have also launched the first sample models. The wide color gamut, high contrast, wide viewing angle and fast response time of OLED displays make these displays ideal for multimedia applications. Self-illuminating OLED technology does not require backlighting, and the power consumption depends on the display content, which consumes much less power than LCDs that use backlights. After the panel size is increased, the high-quality features of the OLED are more obvious. Therefore, the display size of the OLED panel is more than 3 愈, and the future application level will still be dominated by the TV panel. Another OLED display market is the soft display. The current prospects for OLED and electrophoretic display technologies are quite promising, and electrophoretic or bi-stable displays used in e-readers need to improve color quality. On the other hand, in the case of completely soft materials, OLED displays are still not suitable for mass production, depending on the development of backplane technology.
Backplane technology for flexible displays High-resolution color active matrix organic light-emitting diode (AMOLED) displays require an active matrix backplane that uses active switches for switching individual pixels. At present, the liquid crystal (LC) display amorphous silicon process has matured, and it can supply a low-cost active matrix backplane and can be used for OLED. Many companies are currently developing organic thin-film transistor (OTFT) backplane processes for flexible displays, which can also be used in OLED displays to enable the introduction of full-color flexible displays. Both standard and soft OLEDs require the same power supply and drive technology. To understand the OLED technology, its capabilities, and its interaction with the power supply, you must delve into the technology itself. The OLED display is a self-illuminating display technology that does not require any backlight at all. The material used in OLED is an organic material suitable for chemical structure.
OLED technology requires current control drive method. Figure 1 is a simplified circuit diagram showing only one pixel. OLEDs have electrical characteristics that are quite similar to standard organic light-emitting diodes (LEDs), and the brightness is dependent on the LED current. To turn the OLED on and off and control the OLED current, a thin film transistor (TFT) control circuit is required.
Figure 1. Simple Active Matrix OLED Single Pixel Control Example (ITO – Transparent Conductive Film)
In Figure 1, transistor T2 is a pixel control transistor that turns pixels on and off, similar to any other active matrix liquid crystal display technology. T1 is used as the source of current 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, a simple single transistor current source has significant cost advantages because only two transistors are needed. A disadvantage of this type of simple circuit is that the current changes, including changes in the process and Vdd voltage changes. OLED power supply circuits typically provide both Vdd and Vss voltage supply rails. The voltage rail Vdd must be extremely rigorously adjusted 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 does not affect the LED current. Figure 2 shows the effect of Vdd on the voltage fluctuations produced by OLED displays.
Figure 2. Voltage fluctuations in the power rail cause horizontal stripes. As the Vdd of the voltage supply changes, the OLED brightness also changes. The superimposed voltage ripple on Vdd causes horizontal streaks in the image due to differences in brightness. Depending on the display, voltage chain waves greater than 20mV may cause this phenomenon. The degree of horizontal stripe appearance is related to the amplitude and frequency of the superimposed voltage chain. Once the frequency interferes with the frame frequency, streaks will appear. In the general experimental environment, the superimposed voltage chain wave on Vdd is usually less than 20mV. This problem arises when integrating the display with the power supply into a system. Once any subcircuit in the system draws a pulsating current from the system power supply, a voltage chain wave occurs, as is the case with all circuits connected to the system power supply. Subcircuits that typically draw ripple current include GSM power amplifiers, motor drivers, audio power amplifiers, and the like in handsets. In these systems, a voltage chain is applied to the system supply rail. If the AMOLED power supply does not suppress this chain wave, the chain wave will appear at the output and cause the aforementioned image distortion. In order to avoid such problems, the power supply of AMOLED requires a very high power supply rejection ratio and line transient response. For the power supply of AMOLED, the positive voltage supply rail Vdd requires a boost converter, and the negative voltage supply rail Vss requires a buck-boost converter or inverter. This is a challenge for IC manufacturers that offer a suitable power supply because manufacturers need to provide fairly accurate positive voltage rails Vdd and negative voltage rails Vss to achieve the lowest component height and smallest solution size.
In order to meet all of these requirements, a new power supply topology is required to provide a positive and negative output voltage rail from a Li-Ion battery with a single inductor. SIMO regulator technology achieves the best picture quality in its class
Figure 3. TPS65136 buck boost converter topology with dual inputs. Figure 3 shows a general application circuit using the TPS65136, which uses a single inductor multiple output (SIMO) regulator technique and a four-switch buck boost. The converter topology works. SIMO technology achieves the best line transient regulation in its class, buck boost mode for both outputs, and maximum efficiency over the entire load current range. The advanced energy-saving mode achieves the highest efficiency and is the same as any battery-powered device. A long battery standby time can only be achieved when the converter is operating at the highest efficiency of the overall load current range, which is especially important for OLED displays. The OLED display consumes the most power when it is completely white, and the current is relatively small for any other display color, because only white requires all red, green, and blue sub-pixels to be fully lit. For example, a 2.7 å‹ display requires 80 mA of current to render an all-white image, but only 5 mA is required to display other icons or graphics. Therefore, OLED power supplies need to achieve high converter efficiency for all load currents. In order to achieve such efficiency, advanced energy-saving mode technology is needed to reduce the load current to reduce the converter switching frequency. Since this is done through a voltage controlled oscillator (VCO), it is possible to minimize possible EMI problems and to control the lowest switching frequency outside the normal 40 kHz audio range, which avoids noise from ceramic input or output capacitors. . This is especially important when using such devices in mobile applications, and it simplifies the design process.
Conclusion As OLED display technology is still in its infancy, there is still much room for improvement in terms of energy efficiency, OLED efficiency improvement, and minimizing overall solution size. As OLEDs become more mature, OLEDs can be used in architectural lighting or LCD backlights. the use of. Compared to traditional lighting solutions, OLEDs offer lower power consumption and higher design flexibility for both applications, so there are opportunities. For OLED technology, the future is bound to be bright.
In the market environment, major mobile phone companies are now launching one or more models with OLED displays. Sony is the first to produce OLED TVs, and many other companies have also launched the first sample models. The wide color gamut, high contrast, wide viewing angle and fast response time of OLED displays make these displays ideal for multimedia applications. Self-illuminating OLED technology does not require backlighting, and the power consumption depends on the display content, which consumes much less power than LCDs that use backlights. After the panel size is increased, the high-quality features of the OLED are more obvious. Therefore, the display size of the OLED panel is more than 3 愈, and the future application level will still be dominated by the TV panel. Another OLED display market is the soft display. The current prospects for OLED and electrophoretic display technologies are quite promising, and electrophoretic or bi-stable displays used in e-readers need to improve color quality. On the other hand, in the case of completely soft materials, OLED displays are still not suitable for mass production, depending on the development of backplane technology.
Backplane technology for flexible displays High-resolution color active matrix organic light-emitting diode (AMOLED) displays require an active matrix backplane that uses active switches for switching individual pixels. At present, the liquid crystal (LC) display amorphous silicon process has matured, and it can supply a low-cost active matrix backplane and can be used for OLED. Many companies are currently developing organic thin-film transistor (OTFT) backplane processes for flexible displays, which can also be used in OLED displays to enable the introduction of full-color flexible displays. Both standard and soft OLEDs require the same power supply and drive technology. To understand the OLED technology, its capabilities, and its interaction with the power supply, you must delve into the technology itself. The OLED display is a self-illuminating display technology that does not require any backlight at all. The material used in OLED is an organic material suitable for chemical structure.
OLED technology requires current control drive method. Figure 1 is a simplified circuit diagram showing only one pixel. OLEDs have electrical characteristics that are quite similar to standard organic light-emitting diodes (LEDs), and the brightness is dependent on the LED current. To turn the OLED on and off and control the OLED current, a thin film transistor (TFT) control circuit is required.
Figure 1. Simple Active Matrix OLED Single Pixel Control Example (ITO – Transparent Conductive Film)
In Figure 1, transistor T2 is a pixel control transistor that turns pixels on and off, similar to any other active matrix liquid crystal display technology. T1 is used as the source of current 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, a simple single transistor current source has significant cost advantages because only two transistors are needed. A disadvantage of this type of simple circuit is that the current changes, including changes in the process and Vdd voltage changes. OLED power supply circuits typically provide both Vdd and Vss voltage supply rails. The voltage rail Vdd must be extremely rigorously adjusted 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 does not affect the LED current. Figure 2 shows the effect of Vdd on the voltage fluctuations produced by OLED displays.
Figure 2. Voltage fluctuations in the power rail cause horizontal stripes. As the Vdd of the voltage supply changes, the OLED brightness also changes. The superimposed voltage ripple on Vdd causes horizontal streaks in the image due to differences in brightness. Depending on the display, voltage chain waves greater than 20mV may cause this phenomenon. The degree of horizontal stripe appearance is related to the amplitude and frequency of the superimposed voltage chain. Once the frequency interferes with the frame frequency, streaks will appear. In the general experimental environment, the superimposed voltage chain wave on Vdd is usually less than 20mV. This problem arises when integrating the display with the power supply into a system. Once any subcircuit in the system draws a pulsating current from the system power supply, a voltage chain wave occurs, as is the case with all circuits connected to the system power supply. Subcircuits that typically draw ripple current include GSM power amplifiers, motor drivers, audio power amplifiers, and the like in handsets. In these systems, a voltage chain is applied to the system supply rail. If the AMOLED power supply does not suppress this chain wave, the chain wave will appear at the output and cause the aforementioned image distortion. In order to avoid such problems, the power supply of AMOLED requires a very high power supply rejection ratio and line transient response. For the power supply of AMOLED, the positive voltage supply rail Vdd requires a boost converter, and the negative voltage supply rail Vss requires a buck-boost converter or inverter. This is a challenge for IC manufacturers that offer a suitable power supply because manufacturers need to provide fairly accurate positive voltage rails Vdd and negative voltage rails Vss to achieve the lowest component height and smallest solution size.
In order to meet all of these requirements, a new power supply topology is required to provide a positive and negative output voltage rail from a Li-Ion battery with a single inductor. SIMO regulator technology achieves the best picture quality in its class
Figure 3. TPS65136 buck boost converter topology with dual inputs. Figure 3 shows a general application circuit using the TPS65136, which uses a single inductor multiple output (SIMO) regulator technique and a four-switch buck boost. The converter topology works. SIMO technology achieves the best line transient regulation in its class, buck boost mode for both outputs, and maximum efficiency over the entire load current range. The advanced energy-saving mode achieves the highest efficiency and is the same as any battery-powered device. A long battery standby time can only be achieved when the converter is operating at the highest efficiency of the overall load current range, which is especially important for OLED displays. The OLED display consumes the most power when it is completely white, and the current is relatively small for any other display color, because only white requires all red, green, and blue sub-pixels to be fully lit. For example, a 2.7 å‹ display requires 80 mA of current to render an all-white image, but only 5 mA is required to display other icons or graphics. Therefore, OLED power supplies need to achieve high converter efficiency for all load currents. In order to achieve such efficiency, advanced energy-saving mode technology is needed to reduce the load current to reduce the converter switching frequency. Since this is done through a voltage controlled oscillator (VCO), it is possible to minimize possible EMI problems and to control the lowest switching frequency outside the normal 40 kHz audio range, which avoids noise from ceramic input or output capacitors. . This is especially important when using such devices in mobile applications, and it simplifies the design process.
Conclusion As OLED display technology is still in its infancy, there is still much room for improvement in terms of energy efficiency, OLED efficiency improvement, and minimizing overall solution size. As OLEDs become more mature, OLEDs can be used in architectural lighting or LCD backlights. the use of. Compared to traditional lighting solutions, OLEDs offer lower power consumption and higher design flexibility for both applications, so there are opportunities. For OLED technology, the future is bound to be bright.
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