ELECTROLUMINESCENT SUBPIXEL COMPENSATED DRIVE SIGNAL

• US 20100225630A1
• Filed: 03/03/2009
• Published: 09/09/2010
• Est. Priority Date: 03/03/2009
• Status: Active Grant

• Current Assignee: Global OLED Technology LLC (LG Display Co., Ltd.)
• Original Assignee: Global OLED Technology LLC (LG Display Co., Ltd.)
• Inventors: Hamer, John W., Levey, Charles I.
• Granted Patent: US 8,217,928 B2
• Time in Patent Office: Days
• US Class Current: 345/211
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2320/0295 by monitoring each display pixel
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/045 Compensation of drifts in the characteristics of light emitting or modulating elements
• CPC Class Codes: G09G 2360/16 Calculation or use of calculated indices related to luminance levels in display data
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element

Abstract

An electroluminescent (EL) subpixel, such as an organic light-emitting diode (OLED) subpixel, is compensated for aging effects such as threshold voltage V_th shift, EL voltage V_oled shift, and OLED efficiency loss. The drive current of the subpixel is measured at one or more measurement reference gate voltages to form a status signal representing the characteristics of the drive transistor and EL emitter of the subpixel. Current measurements are taken in the linear region of drive transistor operation to improve signal-to-noise ratio in systems such as modern LTPS PMOS OLED displays, which have relatively small V_oled shift over their lifetimes and thus relatively small current change due to channel-length modulation. Various sources of noise are also suppressed to further increase signal-to-noise ratio.

Claims

1. Apparatus for providing a drive transistor control signal to a gate electrode of a drive transistor in an electroluminescent (EL) subpixel, comprising:

(a) the electroluminescent (EL) subpixel having an EL emitter with a first and second electrode, and having the drive transistor with a first supply electrode, a second supply electrode, and the gate electrode, wherein the second supply electrode of the drive transistor is electrically connected to the first electrode of the EL emitter for applying current to the EL emitter;

(b) a first voltage supply electrically connected to the first supply electrode of the drive transistor;

(c) a second voltage supply electrically connected to the second electrode of the EL emitter;

(d) a test voltage source electrically connected to the gate electrode of the drive transistor;

(e) a voltage controller for controlling voltages of the first voltage supply, second voltage supply and test voltage source to operate the drive transistor in a linear region;

(f) a measuring circuit for measuring the current passing through the first and second supply electrodes of the drive transistor at different times to provide a status signal representing variations in the characteristics of the drive transistor and EL emitter caused by operation of the drive transistor and EL emitter over time, wherein the current is measured while the drive transistor is operated in the linear region;

(g) means for providing a linear code value;

(h) a compensator for changing the linear code value in response to the status signal to compensate for the variations in the characteristics of the drive transistor and EL emitter; and

(i) a source driver for producing the drive transistor control signal in response to the changed linear code value for driving the gate electrode of the drive transistor.

2. The apparatus of claim 1, wherein the EL emitter is an OLED emitter.

3. The apparatus of claim 1, wherein the drive transistor is a low temperature polysilicon transistor.

4. The apparatus of claim 1, further including a switch for selectively electrically connecting the measuring circuit to the current flow through the first and second supply electrodes.

5. The apparatus of claim 1, wherein the measuring circuit includes a first current mirror for producing a mirrored current which is a function of the drive current passing through the first and second supply electrodes and a second current mirror for applying a bias current to the first current mirror to reduce impedance of the first current mirror.

6. The apparatus of claim 5, wherein the measuring circuit further includes a current to voltage converter responsive to the mirrored current for producing a voltage signal and means responsive to the voltage signal for providing the status signal to the compensator.

7. The apparatus of claim 1, wherein the drive transistor control signal is a voltage.

8. The apparatus of claim 1, wherein the measured current is less than a selected threshold current.

9. The apparatus of claim 1, wherein the measuring circuit further includes a memory for storing a target signal and a most recent current measurement.

10. The apparatus of claim 1, wherein the compensator further changes the linear code value in response to the linear code value to compensate for the variations in the characteristics of the drive transistor and EL emitter.