Driving circuit of display

Details Driving circuit of display Driving circuit of display

2002-09-18

2004-12-28

Shalwala, Bipin (Department: 2673)

Computer graphics processing and selective visual display system

Plural physical display element control system

Display elements arranged in matrix

C345S082000, C345S204000

Reexamination Certificate

active

06836264

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application serial no. 91114785, filed Jul. 4, 2002.
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to the driving circuit of a display. More particularly, the present invention relates to a driving circuit capable of maintaining a constant driving current in a display.
2. Description of Related Art
People are always interested in watching recorded images and movies. Since the invention of the cathode ray tube (CRT), televisions have become commercialized and owned by almost every family. Accompanying the rapid progress in technology, the CRT has been used in many applications including the desktop monitor of a personal computer. However, the CRT poses a radiation hazard and due to the bulkiness of the electron gun, the CRT display is hard to lighten up and flatten.
Because of intrinsic bulkiness, researchers are now developing more slim-line displays. The so-called ‘flat panel displays’ now include liquid crystal displays (LCDs), field emission displays (FEDs), organic light-emitting diode (OLED) displays and plasma display panel (PDP) displays.
The organic light-emitting diode (OLED) is also known as an organic electroluminescence display (OELD) due to its self-illuminating character. OLED is driven by a low DC voltage and has properties including high brightness level, high energy efficiency, high contrast values as well as being slim and lightweight. Moreover, the display is able to emit light of a range of colors from the three primary colors red (R), green (G) and blue (B) to white light. Hence, OLED is considered to be the display panel of the next generation. Aside from having high resolution and light just like the LCD and having self-illuminating capacity, a quick response and a low energy consumption just like the LED, OLED also has other advantages including a wide viewing angle, good color contrast and a low production cost. Thus, OLED is often used in LCD or as a background light source for indicator panels, mobile phone, digital cameras and personal digital assistant (PDA).
According to the type of driver selected to drive the OLED, OLED can be divided into passive matrix driven or active matrix driven type. Passive matrix OLED has the advantage of structural simplicity. It does not have to be driven by a thin film transistor (TFT) and hence has a lower production cost. However, the passive matrix OLED has a relative low resolution rendering it unsuitable for producing high-quality images. Moreover, the passive matrix OLED consumes a lot of power, has a shorter working life and sub-optimal displaying capacity. On the other hand, although the active matrix OLED is slightly more expensive to produce, it can be assembled to form a huge screen aside from having a large viewing angle, the capacity for producing high brightness level and a quick response.
According to the driving method, a flat display panel is also divided into voltage-driven type or a current-driven type. In general, the voltage-driven type is employed in TFT-LCD. By inputting different voltages to the data lines, different shades of gray are produced to generate a full color palette. Voltage-driven TFT-LCD is technically mature, stable and cost-effective to produce. The current-driven type is mainly employed in OLED display. To operate the current-driven flat display panel, different currents are fed into data lines to produce different shades of gray for generating a full color palette. Since new types of circuits and ICs must be developed to drive the current-driven pixels, development cost for this type of panel is huge. Thus, if TFT-LCD voltage-driven circuit can somehow be tapped to drive the OLED, production cost will be greatly reduced. However, if the TFT-LCD voltage-driven circuit is deployed to drive the OLED, threshold voltage of the driving TFT may shift after a long period of operation leading to a rise in the threshold voltage. The drain current of TFT in the saturation region is given by the formula:
I
ds
=(1/2)×&mgr;
n
×C
ox
×(
W/L
)×(
V
gs
−V
th
)
2
Here, electron mobility &mgr;
n
and gate capacitor on unit area C
ox
are constants, V
th
is the threshold voltage of the TFT, W is the channel width of the TFT and L is the channel length of the TFT. According to the aforementioned formula, a rise in the threshold voltage leads to a lowering of the driving current flowing between the drain terminal and source terminal of the driving TFT. Since the driving current is used to drive the OLED and produce light, a lowering of the driving current results in a dimming of the OLED emission.
To provide a better explanation refer to the circuit in FIG.
1
.
FIG. 1
is a diagram showing the circuit of a pixel
10
in a conventional display. As shown in
FIG. 1
, the pixel
10
circuit includes a conventional driving circuit
102
and an OLED (
104
). The aforementioned driving circuit
102
further includes a TFT
1
(
106
), a capacitor C (
108
) and a TFT
2
(
110
). TFT
2
(
110
) is a driving thin film transistor that generates a driving current for driving the OLED (
104
) and producing light. The drain terminal of TFT
1
(
106
) is coupled to a data voltage (V
data
). The gate terminal of TFT
1
(
106
) is coupled to a scanning voltage (V
scan
). The source terminal of TFT
1
(
106
) is coupled to a first terminal of the capacitor C (
108
) and the gate terminal of TFT
2
(
110
). The drain terminal of TFT
2
(
110
) is coupled to a positive voltage (V
dd
) terminal. The source terminal of TFT
2
(
110
) is coupled to the positive terminal of the OLED (
104
). The second terminal of the capacitor C (
108
) is coupled to a voltage (V
ss1
) terminal. V
ss1
is a negative voltage or a ground voltage. The negative terminal of the OLED (
104
) is coupled to a voltage (V
ss
) terminal. The voltage V
ss
is a negative voltage or a ground voltage.
FIG. 2
is a timing diagram showing the variation of V
dd
, V
scan
, V
data
, V
g2
of the gate terminal of TFT
2
(
110
) in the driving circuit
102
in FIG.
1
. As shown in
FIG. 2
, when V
scan
is set to a high potential, TFT
1
(
104
) will conduct. When V
scan
is set to a low potential, TFT
1
(
104
) will shut down. In addition, the interval between the appearance of a high potential and a low potential is called a frame period (indicated by T in FIG.
2
). In general, a frame period is {fraction (1/60)} second. In other words, the driving circuit
102
operates at a 60 Hz frequency and one frame constitutes a pixel of image. Since V
data
is at a high potential when V
scan
is at a high potential, V
g2
maintains a positive voltage and rises gradually. The gradual rising of V
g2
leads to the accumulation of more trap charges in the oxide layer at the gate terminal of TFT
2
(
110
). Consequently, there is a shift in the threshold voltage of TFT
2
(
110
) towards a higher voltage. As a result, there is a lowering of driving current from the drain terminal to the source terminal of TFT
2
(
110
) and a corresponding reduction in the brightness level of the OLED (
104
).
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a driving circuit for a display. Two thin film transistors are added to the original driving circuit of display so that the driving circuit now includes four thin film transistors such that the gate terminal of the thin film transistor's voltage level increases as the threshold voltage of the driving thin film transistor increases. Therefore, the driving current of the thin film transistor is able to maintain a constant value and hence the initial luminance of the display remains unchanged.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a driving circuit for a display. The driving circuit is used for driving a light-emitting device. The light-emitting device has a positive terminal and a negative terminal. The drivin

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