Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-11-23
2001-02-20
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S076000, C345S182000
Reexamination Certificate
active
06191535
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a passive matrix electroluminescence (EL) display apparatus in which organic EL devices are driven using pulse width modulation signals.
2. Description of the Related Art
Organic EL devices are ideal for thin configurations as they emit light and do not require the backlight that is required in liquid crystal displays, and they do not have restrictions in viewing angle. Thus, the application of organic EL devices is highly expected in the next generation of display devices.
As shown in an example in
FIG. 1
, an organic EL device
1
is formed from a hole-transport layer
5
, which is formed from MTDATA (4,4′-bis(3-methylphenylphenylamino)biphenyl), an emissive layer
6
, which is formed from TPD (4,4′,4″-tris(3-methylphenylphenylamino)triphenylanine) and Rubrene, and an electron-transport layer
7
, which is formed from Alq3, between an anode (first electrode)
3
, which is formed from a transparent electrode, such as ITO, on a transparent glass substrate
2
, and a cathode (second electrode)
4
, which is formed from an MgIn alloy. Holes injected from the anode
3
and electrons injected from the cathode
4
are recombined within the emissive layer
6
to emit light, which is radiated outward from the transparent anode side in the direction of the arrow shown in the figure.
Display apparatuses for driving this sort of organic EL device can be divided into two types: a passive matrix type, and an active matrix type using TFTs. A schematic circuit diagram of the passive matrix type is shown in FIG.
2
.
Namely, of the pair of electrodes of the EL devices described above, the anodes
3
are designated for columns, the cathode
4
are designated for rows, and they are arranged in a matrix configuration so as to cross each other and sandwich an organic layer. To the cathodes
4
are supplied scan signals ROW
1
, ROW
2
, ROW
3
, and so forth, from a row driver
8
, the scan signal of only the selected row of a plurality of rows becomes a low level for one horizontal period while the scan signals for the other rows become a high level. Meanwhile, a column driver
9
inputs gray-scale data mDATA for expressing the display gray scale of each pixel, and pulse signals having pulse widths proportional to this gray-scale data are output as column driving signals COL
1
, COL
2
, COL
3
, . . . , COLm. The column driving signals COL
1
, COL
2
, COL
3
, . . . , COLm are at a high level during the pulse width period, thus, the EL device of the row that inputs the low level scan signal emits light.
The configuration of the column driver
9
will be described in detail with reference to FIG.
3
.
The column driver
9
comprises a shift register
10
for inputting n-bit gray-scale data mDATA for each column according to a shift clock CL, a latch circuit
11
for latching the data input by the shift register
10
according to a latch pulse, an n-bit counter
12
for expressing the gray-scale level, and m pulse width modulation circuits
13
for comparing the n-bit gray-scale data from the latch circuit
11
provided for every column and the n-bit counter value, and respectively outputting the column driving signals COL
1
, COL
2
, COL
3
, . . . , COLm of pulse widths proportional to the gray-scale data. In the passive matrix EL display apparatus, the column driving signals COL
1
, COL
2
, COL
3
, . . . , COLm are output from the respective pulse width modulation circuits
13
as shown in FIG.
4
.
The counter value of the n-bit counter
12
, as shown in
FIG. 4
when n=3, for example, changes in a sequence of “0”, “1”, . . . , “7” during one horizontal scan period (1H), and the column driving signals COL
1
, COL
2
, COL
3
, . . . , COLm all simultaneously start their output at a timing when the counter value reaches “1”. The high level during the pulse width period is maintained in proportion to the gray-scale data of the respective pixel. Therefore, pixels PX
1
, PX
2
, PX
3
, . . . , PXm of the same row shown in
FIG. 2
emit light during the pulse width periods shown in
FIG. 4
, and the gray scales are expressed by these light emitting periods.
In the above-mentioned EL display apparatus, gray scales are expressed by the pulse widths of the pulse width modulation signals that are output as the column driving signals COL
1
, COL
2
, COL
3
, . . . , COLm as described above, and the output start timing is the same for all signals. Therefore, at the initial timing when the counter value becomes “1”, the current concentrates in its flow from the anode
3
to the cathode
4
to result in an extremely high current consumption at this time. However, the gray scale of the pixel is dependent on the high level period during one horizontal scan period and is not dependent on the generated position of the pulse width modulation signal.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a passive matrix EL display apparatus that avoids the concentration of current consumption and achieves reliable gray-scale displays.
To achieve the above-mentioned object, the electroluminescence display apparatus employing electroluminescence devices as light emitting pixels in the present invention comprises: the electroluminescence devices, each having an emissive layer between a first electrode and a second electrode, in which the first electrodes and the second electrodes are disposed in a matrix configuration so as to mutually cross; a first driver circuit for supplying scan signals to the second electrodes; and a second driver circuit for supplying pulse width modulation signals, having a pulse width proportional to a gray scale, as driving signals to the first electrodes, and for outputting the driving signals at different timings for every column of the matrix.
In another aspect of the present invention, the second driver circuit shifts the output timing of the driving signals by a predetermined timing between adjacent first electrodes.
By shifting the output timing of the driving signal for every column in this manner, concentration of the supply current can be prevented and the power consumption of the apparatus can be reduced.
In a further aspect of the present invention, the second driver circuit comprises: a counter for counting a counter pulse that is generated at every period, which is one horizontal scan period divided by the number of display gray scales; and a pulse width modulation circuit for determining the pulse width and amount of delay of the output start timing of the driving signal according to the gray-scale level to be displayed, and the number of the column to which output is to be performed, on the basis of the count value at the counter, and for outputting the driving signal to the relevant column.
In another aspect of the present invention, the pulse width modulation circuit: for a case where the less significant n bits of the column number are expressed as m, compares a less significant n bit data nP of addition data of n-bit gray-scale data mDATA and m−1 data for column m, with a count value nT of the counter, and generates a signal A at a high level during the period when the count value nT satisfies nT≦nP; for a case where the most significant bit Q of the addition data is 0, inhibits the output of the signal A at a high level by a signal B until the count value nT of the counter is nT>m−1, and outputs signal A, which is enabled by the signal B, as the driving signal, and for a case where the most significant bit Q of the addition data is 1, generates a signal C at a high level only when the count value nT of the counter is nT≧m−1, and outputs signal A and signal C as driving signals.
As mentioned above, the gray-scale data for the corresponding column, the count value of the counter proportional to the number of gray-scale levels set in the apparatus, the pulse width of the driving signal, and the output timing of the driving signal according to the number m of the column to which the driving signal is to be supp
Cantor & Colburn LLP
Sanyo Electric Co,. Ltd.
Tran Thuy Vinh
Wong Don
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