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Posted on 01 September 2019

Active Matrix OLED

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The next generation Display Technology

The latest active matrix OLED (organic light emitting diode) displays have superior picture quality and can be manufactured super thin. Prototypes with a thickness of just 0.25mm are already shown. These types of displays require specific power supplies to achieve best picture quality and smallest solution size. This article explains the OLED display technologies and introduces the TPS65136, a dedicated power supply IC. This IC uses a novel technology generating a positive and negative output voltage from a single inductor with infinite good line regulation.

By Oliver Nachbaur, Texas Instruments Freising

 

The next generation display technology makes its way into the mobile phones and media player market. The OLED display is a self emitting display technology and doesn’t require any backlight. Depending on the display content the power consumption is lower compared to the TFT LC-Display requiring backlight. This is the case especially when most of the picture content is dark or even black. Since most of the user interfaces these days having mostly white picture content this advantage is slightly reduced. However the real appealing factor of the AMOLED (Active Matrix Organic Light Emitting Diode) display is the superior picture quality with its wide viewing angle, fast response time and high contrast ration with excellent color saturation. Since this display can be manufactured with a thickness of less than 1mm it is ideal for the fast growing market of portable devices.

The OLED display entered the market with PMOLED (Passive Matrix Organic Light Emitting Diode) technology mainly used in Sub displays for mobile phones and simple displays for MP3 players. To be used as a main display for mobile phones and for multimedia applications higher resolution, faster response time and higher contrast ration is required. This is possible with the new active matrix OLED displays which are now being used as a main display in mobile phones enabling superior display quality supporting all sorts of multimedia applications.

OLED technology requires a current control driving method

To understand the driving method of am AMOLED display and the associated performance requirements for the OLED power supply a closer look at the OLED architecture is necessary. A simplified circuit, showing one pixel, is shown in Figure 1. The OLED has in principle a similar electrical characteristic to any standard LED (Light Emitting Diode) that brightness depends on the LED current. Therefore a current source, formed with T1 and Cs, is implemented together with an active pixel control switch T2.

Simple AMOLED driving circuit with current source transistor T1 and control switch T2

In Figure 1 transistor T2 is used to turn on and off the pixel. This is fairly similar to any other active matrix display. T1 is used as a current source where the current is given by its gate source voltage. To have a good current source T1 needs to be operated in the saturated region beyond the cut off voltage with sufficient drain source voltage. Cs is the storage capacitor which holds the gate voltage of T1 and thus the current until the pixel is addressed again until the next frame is written. The simple single transistor current source of Figure 1 has a major cost advantage since only one transistor is required, programming the current through the OLED.

The disadvantage of the simple circuit is a variation in current depending on the process variations changing channel width and length and threshold voltage Vth of T1 pixel by pixel. To minimize these effects the OLED backplane process needs to be accurately controlled to achieve same transistor characteristics for each pixel. The second effect influencing the OLED current is the ambient temperature directly influencing the threshold voltage of T1. The voltage between Vdd and the gate of T1 sets the current through the OLED. Because of this any variation in the voltage applied to Vdd directly influences the current through the OLED and thus the brightness. Therefore the power supply providing Vdd needs to be very accurate to achieve best picture quality to avoid any image flicker or pattern. The voltage accuracy of Vss, that is usually a negative voltage, needs to be less accurate because it has almost no effect on the LED current. 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. Especially when the power supply IC is used in the mobile phone the positive supply voltage rail should suppress any input voltage changes caused by battery voltage drops that typically occur during a GSM transmit period. In addition to that, the IC should operate from a wide input voltage Li-Ion battery (2.3V to 4.5V) at highest efficiency, conserving battery life time. To meet all these requirements a novel power supply topology is chosen providing both positive and negative output voltage rails from a Li-Ion battery using just a single inductor.

SIMO Regulator Technology enables superior picture quality

Figure 2 shows the typical application circuit of the TPS65136. The device uses Texas Instruments newly developed SIMO (Single Inductor Multiple Output) regulator technology. The device operates with a four-switch buck boost converter topology. The corresponding inductor current is shown in Figure 3. During the first switch cycle the inductor is charged from the input to ground. In the second switch cycle the inductor is discharged to the positive and negative output terminal simultaneously. Since the inductor is discharged to the positive and negative output simultaneously the fixed peak current control of the IC delivers the same energy, A, to the output and is independent of input voltage. Figure 3 shows two different switch cycles with different input voltage levels where the energy, A, delivered to the output stays the same for both different input voltages.

TPS65136 Buck-Boost Converter topology supporting dual output

The SIMO technology features best in class line transient regulation, buck-boost mode for both outputs and highest efficiency over the entire load current range.

Inductor current of SIMO regulator; Infinite good line transient response

Advanced Power Save Mode enables highest efficiency

As with any battery powered equipment long battery standby time is only achieved when the used converter operates at highest efficiency over the entire load current range. This is especially important when the OLED display is operated in low power mode. This is the case for the main display of a mobile phone, that is fully powered on only during a short period of time operating at nominal and highest operating current. For the most period of time the display is operated in low power mode only showing the time for example. To support that, the device has an advanced power save mode implemented which reduces the converter switching frequency as the load current decreases. Since this is done using a VCO (voltage controlled oscillator) 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.

Future outlook

Since the OLED display technology is just emerging there is still a lot of room to conserve power, increase OLED efficiency and to minimize the total solution size. The future will bring out even more optimized IC solutions supporting various features and OLED applications. In summary, the future is going to be bright.

 

 

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