Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-01-18
2001-11-27
Nguyen, Matthew (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06323631
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for driving the organic light emitting diode (OLED) display panel and, more particularly, to a constant current driver with auto-clamped pre-charge function.
2. Description of Related Art
The organic light emitting diode (OLED) is known as an organic thin film semiconductor based light emitting device. Thus, a display panel can be provided by a two-dimensional array of OLEDs.
In general, an OLED panel may be driven by a constant voltage, which is deemed to be less energy consumed. However, because the cut-in voltages of the OLEDs on the display panel are not uniform, each OLED may de conducted in different voltage level, which results in that the emitted light is not even.
Furthermore, it is known that the light intensity of the OLED is proportional to the current generated by combining the electrons and holes at the junction area. This current is an exponential function of the junction voltage, so that it is very sensitive to the variance of the junction voltage. Hence, in order to achieve a uniform light intensity of the whole OLED array, it is preferable to drive the OLED panel by constant current.
FIG. 8
is a system architecture showing the conventional constant current driven OLED display panel and the driver. As shown, the driver includes a column driving circuit
81
and a row driving circuit
82
. The column driving circuit
81
includes a reference bias generator
811
and a plurality of constant current column driver cells
812
.
FIG. 9
is a detailed circuit diagram of the column driving circuit
81
. The reference bias generator
811
is coupled to each constant current column driver cell
812
to form a current mirror, so as to turn on the switch transistor MPS based on an input from a column data shift register
83
via an input terminal COLI, thereby an output transistor MPO providing a constant current output on the output terminal COLO. Furthermore, a discharge transistor MND, controlled by a discharge control terminal DIS, is provided in each constant current column driver cell
812
for eliminating the possible residual image caused by the junction capacitance and the wiring stray capacitance of OLEDs. The discharge transistor MND is turned on for a short period of time before the driving current is applied, so as to leak out the charge stored in the junction capacitors and the wiring stray capacitors of OLEDs.
With reference to
FIG. 8
again, the row driving circuit
82
includes a plurality of inverters
821
connected to a row scanning shift register
84
. Hence, under the control of the synchronous signals (HSYNC and VSYNC) and clock signal (HCLK), current from the output terminal COLO of a selected constant current column driver cell
812
is outputted to the OLEDs of a corresponding column. Furthermore, a selected inverter
821
drains the conducting current of a row of OLEDs, so as to turn on the desired OLEDs to emit light.
In a typical application, only dozens of micro amperes (e.g., 25 &mgr;A) of driving current is sufficient for driving a pixel having a size of 0.1 mm
2
to emit a required light intensity under a {fraction (1/64)} duty cycle operating condition. However, taking a 64×64 OLED display panel as an example, a parasitic capacitance of several hundreds pico farads (e.g., 600 pF) may be generated from the stray capacitor on the thin film electrode layout and the junction capacitance of the diode array in driving each pixel. Therefore, if the constant current driving circuit as shown in
FIG. 8
is employed for driving, the parasitic capacitor is charged by the driving current at first. As shown in
FIG. 10
, in a driving duration of about 200 micro seconds (&mgr;s), it takes about 150 &mgr;s to charge the OLED to have an enough voltage (e.g., about 7V) for conducting a current of about 25 &mgr;A at the junction. Therefore, the actual duration for emitting light is greatly reduced, and the intensity of emitting light is not satisfactory.
To eliminate such a problem, a pre-charge capability is provided in the constant current driving circuit. A known driver with pre-charge circuit is shown in
FIG. 11
, wherein the gate of a PMOS transistor MPPRE, which is used as a pre-charge device, is temporarily grounded at the front edge of a driving period by a switch, so as to generate a large current in a short period of time rapidly charging a stray capacitor to a high voltage. However, such a design suffers from several disadvantages. With reference to
FIG. 12
, the first disadvantage is that the voltage of stray capacitor may be over-charged, resulting in a much larger junction current generated in OLED as compared to the predetermined driving current at this time period. The second disadvantage is that the over-charged voltage of the stray capacitor may be slowly discharged through OLED after the pre-charge process, resulting in a junction current being difficult to control. Particularly, the pre-charge process may produce a product of large current and time, i.e., a considerable amount of constant charge. As a result, it is difficult to adjust the driving current for obtaining a desired intensity of display panel. The third disadvantage is that an independent pre-charge control pulse signal with a very small width is required for alleviating the problem of uneven light emission caused by the first disadvantage. In view of above, the conventional constant current OLED drivers are not satisfactory, and thus there is a need to have an improved constant current driver to mitigate and/or obviate the aforementioned problems.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a constant current driver with auto-clamped pre-charge function, which allows the OLED display panel to emit light uniformly without the need of an additional pre-charge signal, thus eliminating the drawbacks of the conventional OLED driver.
Another object of the present invention is to provide a constant current driver with auto-clamped pre-charge function, which can be switched into a voltage driven mode by a multiplexer, so as to be used in an application requiring a low energy consumption, instead of requiring uniform light illumination.
According to one aspect, the present invention which achieves these objects relates to a constant current driver with auto-clamped pre-charge function, which comprises: a reference bias generator having a bias output terminal for providing a reference bias; and a plurality of constant current driver cells, each being connected to the reference bias generator to form a respective current mirror. The constant current driver cell comprises: a switch transistor controlled by an input terminal for being turned on or off; a current output transistor connected to the switch transistor and the bias output terminal of the reference bias generator for outputting a constant current when the switch transistor is on; and a pre-charge transistor having a gate connected to the gate of the current output transistor and further connected to the bias output terminal of the reference bias generator, a drain and a source connected to the drain and source of the current output transistor, respectively, whereby, when a constant current is outputted from the current output transistor for driving an organic light emitting diode, the pre-charge transistor is turned on due to the gate to source voltage thereof being larger than its threshold voltage, so as to provide a drain to source current as an additional large current for rapidly pre-charging the organic light emitting diode until the gate to source voltage of the pre-charge transistor is smaller than the threshold voltage.
According to another aspect, the present invention which achieves these objects relates to a constant current driver with auto-clamped pre-charge function, which comprises: a reference bias generator having a bias output terminal for providing a reference bias; and a plurality of constant current driver cells, each being connected to the reference bias generator to
Bacon & Thomas PLLC
Nguyen Matthew
Sunplus Technology Co. Ltd.
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