Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-08-13
2001-02-13
Hjerpe, Richard A. (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S077000, C345S078000, C345S079000, C345S080000, C345S081000
Reexamination Certificate
active
06188375
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to active matrix electroluminescent (AMEL) displays, and more particularly relates to an AMEL pixel driving circuit and quasi-analog method of operating same for improved gray scale and color operation.
2. Description of the Related Art
Thin film electroluminescent (EL) displays are well known in the art and are typically used for flat screen displays in a variety of applications. A typical display includes a plurality of pixels arranged in an array of rows and columns. Each pixel comprises an electroluminescent phosphorus active layer interposed between a pair of insulators and a pair of electrodes. A typical embodiment is a 640×480 pixel array placed in a 0.75 inch square area for use in heads-up displays and the like.
The pixels in a conventional AMEL display are either active (illuminated) or inactive (dark) at any given time. To generate shades of gray, typical AMEL displays are operated in a time multiplexed mode. Referring to
FIG. 1
, prior art AMEL displays generally include a driver circuit at each pixel comprising a first transistor
100
having a gate connected to a select line
110
, a source connected to a data line
108
and a drain connected to a gate of a second transistor
102
and through a first capacitor
104
to an AC ground potential. A source of the second transistor
102
is connected to a ground terminal, a drain is connected to one electrode of an EL cell
106
. A second electrode EL cell
106
is connected to a high voltage alternating current source (AC Source)
112
for excitation of the phosphor in the EL cell
106
.
A conventional AMEL pixel is operated in consecutive time frames. During a first portion of a time frame (load cycle) a digital signal is consecutively applied to all data lines
108
in the display. For those pixels which will be active, the corresponding select lines
110
are strobed consecutively by row. This turns on transistor
100
thereby allowing charge from data line
108
to accumulate on capacitor
104
and on the gate of transistor
102
, thereby turning transistor
102
on. The charge is stored in capacitor
104
when the select line
110
returns to an inactive state. At the completion of the load cycle, the second transistor
102
associated with each activated pixel is on. During a second portion of the frame (illumination cycle), the AC source
112
is applied to all pixels in the display. Current flows from the AC source
112
through the EL cell
106
and the transistor
102
to ground in each activated pixel. This produces an electroluminescent light output from each activated EL cell
106
. For each load/illuminate frame, each pixel is either on or off. Therefore, gray scale operation is only achieved by time multiplexing the pixel over a sequence of multiple load/illumination frames and allowing the eye to integrate the light pulses into a gray image.
Efforts have been made in the prior art to improve the gray scale operation of AMEL display devices. For example, U.S. Pat. No. 5,302,966 to Stewart discloses an AMEL display that employs an analog driving technique for each pixel.
FIG. 2
illustrates one embodiment disclosed in the Stewart patent. This circuit includes a first transistor
200
having a gate terminal coupled to a select line
210
, a drain terminal connected to a data line
208
and source terminal connected to the gate terminal of a second transistor
202
. The source terminal of the first transistor
200
is also connected to capacitor
204
which has a second terminal coupled to circuit ground. The second transistor
202
has a source terminal connected to the data line
208
and a drain terminal connected to an EL cell
206
. The EL cell
206
is further connected to an AC source
212
.
As with the driver circuit of
FIG. 1
, the circuit of
FIG. 2
operates in consecutive frames having load/illuminate cycles. However, during the load cycle, an analog voltage is applied to the data line
208
and this analog voltage is stored in capacitor
204
when an active pixel receives a row select strobe. During the subsequent illumination cycle, the data line receives an analog ramp signal. So long as the ramp signal has a voltage potential less than the voltage stored in capacitor
204
(minus a threshold), transistor
202
conducts, thereby illuminating EL cell
206
. When the ramp voltage on the data line exceeds the stored voltage in capacitor
204
minus the threshold, transistor
202
turns off and current no longer flows through EL cell
206
. Therefore, El cell
206
is only operating during a portion of each illumination cycle, thus providing a variable illumination period during each frame which provides gray scale operation.
To achieve 64 levels of gray scale using the method of the Stewart patent, all of the transistors in the AMEL driver circuit of
FIG. 2
must consistently respond to a small change in voltage equal to (V
DD
−V
DMOS
)/64, where V
DMOS
is a threshold voltage where the transistors are active. For a threshold voltage of 2 volts and V
DD
of 5 volts, the resolution of each voltage step is about 47 millivolts. However, current manufacturing technology and the errors related to high voltage coupling components do not provide sufficient uniformity across the AMEL display substrate to support this requirement. Therefore, the step size must be increased either by using non-conventional power supply voltages or by reducing the number of gray scale steps of the display.
The operation of the AMEL display in accordance with the Stewart patent also supplies a highly non-uniform current versus time profile. When only the least significant bit is active, the EL cell
206
will be active for only 1 out of 32 pulses. When the most significant bit (MSB) is active, the EL cell will be active for all 32 pulses. This broad range of duty cycle (32:1) makes power recovery techniques impractical, thereby reducing the overall operating efficiency of the display.
Therefore, there remains a need for an improved AMEL display. Such a display shall feature a more uniform activation profile and also feature increased step size for a given resolution of gray scale operation. Such circuits and methods will also be applicable to color displays as well as monochrome displays.
SUMMARY OF THE INVENTION
In accordance with a first embodiment of the present invention, a pixel driver circuit for an AMEL display has a first transistor with a gate terminal coupled to a row select line, a source terminal coupled to a data column line and a drain terminal coupled to a gate terminal of a second transistor. A capacitor is coupled from the gate terminal of the second transistor to a profile voltage input terminal. The second transistor has a source terminal connected to a circuit ground terminal and a drain terminal connected to an electroluminescent cell. The electroluminescent cell is also coupled to a voltage high AC source input terminal.
In accordance with another embodiment of the present invention, a pixel driver circuit for an AMEL display includes a first transistor with a gate terminal coupled to a row select line, a source terminal coupled to a data column line and a drain terminal coupled to a gate terminal of a second transistor. A capacitor is coupled from the gate terminal of the second transistor to a circuit ground terminal. The second transistor has a source terminal connected to the profile voltage input terminal and a drain terminal connected to an EL cell. The EL cell is also coupled to a high voltage AC source input terminal.
A method of operating an electroluminescent cell in accordance with the present invention includes the steps of dividing an illumination frame into a plurality of passes; determining the desired illumination intensity for a given pixel during said illumination frame; and activating the pixel for a substantially equal duration in each pass to provide the desired gray scale level for each frame.
In accordance with a further method of the present invention, each frame is di
Allied Signal Inc.
Fredrick Kris T.
Hjerpe Richard A.
Lesperance Jean E.
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