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.