Method and apparatus for gray-scale gamma correction for electroluminescent displays

• US 9,311,845 B2
• Filed: 11/04/2013
• Issued: 04/12/2016
• Est. Priority Date: 11/04/2002
• Status: Expired due to Fees

• Current Assignee: iFire IP Corporation
• Original Assignee: iFire IP Corporation
• Inventors: Cheng, Chun-Fai
• Primary Examiner(s): Sharifi-Tafreshi, Koosha
• Application Number:
  US14/071,169
• Publication Number:
  US 20140210874A1
• Time in Patent Office: 890 Days
• Field of Search: 345/55, 345/76, 345/77, 345/78, 345/79, 345/80, 345/82, 345/83, 345/204, 345/205, 345/206, 345/208, 345/209, 345/210, 345/690, 345/691, 345/692, 345/693, 345/694, 315/169.3
• US Class Current: 1/1
• CPC Class Codes: G09G 2310/0259 with use of an analog or digital ramp generator in the column driver or in the pixel circuit
• CPC Class Codes: G09G 2310/027 Details of drivers for data electrodes, the drivers handling digital grey scale data, e.g. use of D/A converters
• CPC Class Codes: G09G 2320/0276 for the purpose of adaptation to the characteristics of a display device, i.e. gamma correction
• CPC Class Codes: G09G 3/30 using electroluminescent panels
• CPC Class Codes: G09G 3/32 semiconductive, e.g. using light-emitting diodes [LED]
• CPC Class Codes: G09G 3/3614 Control of polarity reversal in general

Abstract

A circuit and method of driving a display panel requiring gray scale control wherein the voltage applied to a row of pixels is equal to the sum of voltages of opposite sign with respect to ground applied respectively to the row electrode and column electrodes whose intersection with the row defines the pixels. Gray scale is realized through modulation of the voltage applied to the column electrodes. Typically for video application, 256 individual gray levels are required corresponding to luminance levels ranging from zero (no emissive luminance) to full luminance. The required luminance for each gray level is not a linear function of the gray level number but rather corresponds to an approximate quadratic function of this number. The present invention facilitates generation of luminance values for each gray level that approximates this functional dependence (i.e. Gamma corrected) with a non-linear voltage ramp terminated by a digital clock having 256 (8 bit) resolution. The voltage at the ramp termination is held at a constant value and fed to the output buffer of the gray scale drivers for the display columns.

Claims

1. A gray scale column driver circuit for an alternating current dielectric electroluminescent display comprising rows, columns that intersect the rows and pixels at the intersections of said rows and columns, said column driver circuit comprising:

a counter receiving video signal gray level data and in response counting for a time interval proportional to said gray level data;

a non linear analogue voltage ramp generator connected to said counter, said non linear analogue voltage ramp generator outputting a first ramping voltage during said time interval when a positive voltage is applied to a row of said electroluminescent display and outputting a second ramping voltage when a negative voltage is applied to a row of said electroluminescent display, wherein each said ramping voltage is defined by a function representing a curve having an initial convex portion and signifying a positive second derivative over time followed by a concave portion and signifying a negative second derivative over time; and

a column driver receiving the ramping voltage and in response applying driving pulses to the columns of said dielectric electroluminescent display.

2. The gray scale column driver circuit of claim 1, wherein said counter is an 8-bit counter for delineating said time interval to define 256 gray levels.

3. The gray scale column driver circuit of claim 1, wherein said non linear analogue voltage ramp generator further comprises an integrator circuit and at least two current sources generating and applying currents to said integrator circuit such that when a first one of said current sources is connected to said integrator circuit the convex portion of said ramping voltage being output is generated, when said at least two current sources are connected in parallel to said integrator circuit a transition portion of said ramping voltage being output that is between said convex portion and said concave portion is generated, and when a second one of said current sources is connected to said integrator circuit the concave portion of said ramping voltage being output is generated.

4. The gray scale column driver circuit of claim 3, wherein said first one of said current sources generates a current that decreases during said time interval, and said second one of said current sources generates a current that increases during said time interval.

5. The gray scale column driver circuit of claim 3, wherein said at least two current sources are time-dependent voltage feedback controlled current sources.

6. The gray scale column driver circuit of claim 3, wherein said at least two current sources are constant current sources.

7. The gray scale column driver circuit of claim 3, wherein said non linear analogue voltage ramp generator further comprises a threshold control circuit for controlled switching of said at least two current sources.

8. The gray scale column driver circuit of claim 3, wherein said non linear analogue voltage ramp generator further comprises a frame polarity control circuit selecting between said first ramping voltage for said positive row voltage and said second ramping voltage for said negative row voltage.

9. The gray scale column driver circuit of claim 7, wherein said threshold control circuit further includes a control input setting a voltage of the transition portion between the convex and concave portions of said ramping voltage being output.

10. The gray scale column driver circuit of claim 9, wherein said at least two current sources are time-dependent voltage feedback controlled current sources.

11. The gray scale column driver circuit of claim 9, wherein said at least two current sources are constant current sources.