TFT pixel threshold voltage compensation circuit with light-emitting device initialization

A method of operating a pixel circuit for a display device that is operable in an initialization phase, a compensation phase, a programming phase, and an emission phase;

• the operating method comprising the steps of:
• providing a pixel circuit comprising:
• a drive transistor configured to control an amount of current to the light-emitting device during the emission phase depending upon a voltage applied to a gate of the drive transistor;
• a second transistor connected to the gate of the drive transistor, wherein the drive transistor can be diode-connected between the gate and a second terminal of the drive transistor through the second transistor;
• a light-emitting device that is connected at a first node to the second terminal of the drive transistor and at a second node to a first voltage supply;
• a third transistor that is connected to the first node of the light-emitting device,
• a fourth transistor that is connected between a third terminal of the drive transistor and a data voltage supply;
• a fifth transistor that is connected between the third terminal of the drive transistor and a second voltage supply;
• and
• a capacitor having a first plate that is connected to the gate of the drive transistor and a second plate that is connectable to a reference voltage supply;
• during the initialization phase, reducing memory effects from previous frames by performing the steps of:
• placing the fourth transistor in the off state, and placing the fifth transistor in the off state to disconnect the second voltage supply from the pixel circuit;
• placing the third transistor in the on state to apply a first initialization voltage to the first node of the light-emitting device;
• setting the reference voltage to a first voltage;
• placing the second transistor in an on state, wherein the drive transistor becomes diode-connected through the second transistor such that the first initialization voltage is applied to the gate of the drive transistor and the first plate of the capacitor;
• and
• placing the third transistor in the off state at an end of the initialization phase;
• during the compensation phase, at least partially compensating a threshold voltage of the drive transistor by the steps of:
• placing the fourth transistor in an on state and applying a reference data voltage through the fourth transistor to the third terminal of the drive transistor, wherein the reference data voltage is set such that a voltage across the light-emitting device is below a threshold voltage of the light emitting device;
• during the programming phase applying a data voltage corresponding to a current greyscale value by the steps of:
• setting the reference voltage to a second voltage for programming the full data range;
• and
• changing the data voltage supply from the reference data voltage to a current data voltage corresponding to a required current through the light-emitting device during the emission phase;
• placing the fourth transistor in an off state after applying the current data voltage;
• and
• placing the second transistor in an off state such that the drive transistor is no longer diode-connected, wherein the current data voltage is stored by the storage capacitor;
• and during the emission phase, performing the steps of:
• setting the reference voltage to a third voltage to change the gate voltage of the drive transistor to the operational voltage range, in which the drive transistor controls the amount of current to the light-emitting device;
• placing the fifth transistor in the on state to connect the second power supply to the drive transistor;
• and
• controlling an amount of current to the light-emitting device depending upon a voltage at the gate of a drive transistor.

A pixel circuit for a display device comprising:

• a drive transistor configured to control an amount of current to a light-emitting device depending upon a voltage applied to a gate of the drive transistor;
• a second transistor connected to the gate of the drive transistor and a second terminal of the drive transistor such that, when the second transistor is in an on state the drive transistor becomes diode-connected such that the gate and a second terminal of the drive transistor are connected through the second transistor;
• a light-emitting device that is connected at a first node to the second terminal of the drive transistor and at a second node to a first voltage supply;
• a third transistor that is connected between an initialization voltage supply and the first node of the light-emitting device, wherein a node N1 is a connection of the second terminal of the drive transistor, the first node of the light-emitting device, and the third transistor;
• and
• at least one capacitor having a first plate that is connected to the gate of the drive transistor and a second plate that is connectable to a reference voltage supply;
• wherein the pixel circuit is operable in an initialization phase, and during the initialization phase the second and third transistors are in an on state such that a first initialization voltage is connected to the first plate of the capacitor, the gate of the drive transistor, and the first node of the light-emitting device.