Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2002-06-21
2004-08-10
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C315S169300
Reexamination Certificate
active
06774877
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a current driver circuit for driving a current-driven element such as an organic EL (electroluminescent) element, and to an image display device that both incorporates this type of current driver circuit and uses a current-driven element as a luminous element.
2. Description of the Related Art
In recent years, devices using current-driven luminous elements such as organic EL elements have been receiving increasing attention for use as image display devices used in portable telephones or the output devices of computers. Organic EL elements are also called “organic light-emitting diodes” and have the advantage of allowing drive by direct current (dc). When organic EL elements are used in a display device, organic EL elements for each picture element (pixel) are typically arranged in matrix form on a substrate to constitute a display panel. As the construction of a display device, an active matrix arrangement is under investigation in which TFTs (thin-film transistors) are formed on this substrate and the organic EL elements of respective picture elements are driven by way of the TFTS.
Since an organic EL element is a current-driven element, however, driving an organic EL element by a TFT precludes the use of a circuit configuration that is the same as an active matrix liquid crystal display device that uses liquid crystal cells, which are voltage-driven elements. Conventionally, active matrix drive circuits have been proposed in which organic EL elements and TFTs, which are MOS (metal-oxide semiconductor) transistors, are connected in a series and inserted between a power supply line and a ground line so as to allow the application of a control voltage to the gates of the TFTS, and further, in which holding capacitors that retain this control voltage are connected to the gates of the TFTs with switching elements provided between the TFTs and signal lines for applying the control voltage to respective picture elements. In such a circuit, the control voltage is outputted in a time-division manner to each picture element on the signal lines, and each switching element is controlled to enter a conductive state (ON state) only at the timings at which the control voltage is outputted to the corresponding picture elements. Thus, when a switching element enters the conductive state, the control voltage at that time is applied to the gates of the TFTS, whereby a current that accords with the control voltage flows through the organic EL element and the holding capacitor is charged by this control voltage. If the switching element transits to the cut-off state (OFF state) in this state, the already applied control voltage continues to be applied to the gates of the TFTs under the effect of the holding capacitor, and a current that accords with this control voltage therefore continues to flow to the organic EL element. This type of the circuit is disclosed in, for example, W099/65011.
In this circuit of the prior art, however, the occurrence of variations in the characteristics of the TFT brings about variations in the current that flows to the organic EL element of each picture element despite the application of the same control voltage, and these variations therefore prevent the realization of a suitable display, particularly when performing a gray-scale display. In addition, the occurrence of voltage drops on the fine signal lines also results in variations in the current that flows to organic EL elements.
In the interest of solving the above-described problems when constituting an active matrix display device, the assignee of this invention has previously proposed in Japanese Patent Laid-open Application No. 11-282419 (JP, 11282419, A), which corresponds to U.S. Pat. No. 6,091,203 of Kawashima et al., a current driver circuit that is directed toward driving current-driven active elements such as the organic EL elements that constitute the picture elements of this type of display device.
FIG. 1
is a circuit diagram showing the basic circuit configuration of the current driver circuit proposed in JP, 11282419, A. This figure shows the circuit of one picture element.
The circuit shown in
FIG. 1
is arranged such that signal current on signal line
53
is converted, by means of a current mirror circuit composed of n-channel transistors
56
and
58
, to a driving current that flows to organic EL element
61
, and such that organic EL element
61
is driven at a constant current by the driving current that accords with the signal current. Power-supply line
51
and ground line
52
are provided, the power supply voltage being positive, the anode of organic EL element
61
, which is provided as the load of transistor
58
, is connected to power-supply line
51
, and the cathode of organic EL element
61
is connected to the drain of transistor
58
. The sources of transistors
56
and
58
are each connected to ground line
52
. The gate and drain of transistor
56
are connected to each other and further connected to the gate of transistor
58
by way of switch element
62
. Holding capacitance
60
is provided between the gate of transistor
58
and ground line
52
. The drain of transistor
56
is connected to signal line
53
by way of switch element
63
. Switch elements
62
and
63
are constituted by, for example, MOS switches, and the control terminals of each are connected to selection line
54
. If MOS transistors are used for switch elements
62
and
63
, the control terminals are the gate terminals the MOS transistors.
When selection line
54
become active and switch elements
62
and
63
become conductive, the signal current supplied from signal line
53
flows to transistor
56
that is diode-connected by way of switch element
63
, and holding capacitor
60
is charged until the voltage across both ends of holding capacitor
60
reaches the gate-to-source voltage of transistor
56
. Since transistors
56
and
58
constitute a current mirror circuit, a current that has the same magnitude as the signal current from signal line
53
flows to transistor
58
if the channel length and channel width of transistors
56
and
58
are the same, and this current flows to organic EL element
61
, which is the load of transistor
58
.
When selection line
54
becomes inactive and switch elements
62
and
63
enter the cutoff state, the signal current is not supplied from signal line
53
because switch element
63
is in the cutoff state, but the voltage level in holding capacitor
60
that is connected to the gate of transistor
58
remains at the same value as when switch elements
62
and
63
were in the conductive state because switch element
62
is in the cutoff state, and transistor
58
therefore continues to direct to organic EL element
61
a current of the same value as when switch elements
62
and
63
were conductive.
In this circuit, causing a signal current to flow instead of applying a control voltage to the signal line can curtail the effect of voltage drops in the signal line, and using a current mirror circuit allows a driving current to be obtained that accords with the signal current and that is unaffected by differences in transistor characteristics between the picture elements.
Nevertheless, in contrast with transistors formed on a single-crystal silicon semiconductor substrate, when the transistors that make up the above-described current driver circuit are constituted by amorphous silicon TFTS (thin-film transistors) or polycrystalline silicon TFTS, variations in threshold voltage V
th
on the order of several tens of millivolts may occur even when these TFTS are arranged contiguous to each other. Thus, despite the contiguous arrangement of transistors
56
and
58
that make up the current mirror circuit in the circuit shown in
FIG. 1
, variations in threshold are difficult to suppress and matching of the two transistors
56
and
58
is therefore difficult to achieve. In addition to variations in threshold value, variations in the carrier mobility or gate oxide fil
Iguchi Koichi
Nishitoba Shigeo
Mengistu Amare
NEC Corporation
Young & Thompson
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