JP2007171543A - Drive circuit for organic EL display - Google Patents

Abstract

To reduce power consumption.
A drive circuit for supplying a drive current to an organic EL element (104) provided in a passive matrix organic EL display. Switch means (2) interposed between the power supply (4) and the current output terminal (5), and when the switch means (2) is turned on, it is output from the output terminal (5) to the organic EL element (104). The current monitoring means for monitoring the current (3), the integration means (10) for integrating the monitored output current, and the switch means (2) based on the scanning line switching signal, and the integral value of the output current is Switch control means (6) for turning off the switch means (2) when a predetermined value corresponding to the value of the analog image signal is reached.
[Selection] Figure 1

Description

  The present invention relates to a drive circuit applied to a passive matrix organic EL display, and more particularly to a constant voltage drive type drive circuit that drives an organic EL element with a constant voltage.

  Organic EL displays are attracting attention as high-quality and thin image display devices. FIG. 3 conceptually shows a typical overall structure of the display panel 100 of the organic EL display. The display panel 100 is sandwiched between a glass substrate 101, an anode 102 made of ITO (Indium Tin Oxide) transparent electrode provided on the glass substrate 101, a cathode 103 as a back electrode, and the anode 102 and the cathode 103. The organic EL element 104 is provided.

  In the display panel 100, when a current is passed through the organic EL element 104, that is, when holes are injected from the anode 102 and electrons are injected from the cathode 103 into the organic EL element 104, these are generated in the organic EL element 104. Recombination in the light emitting layer (described later) excites organic molecules. Then, the organic EL element 104 emits light by energy released when returning from the excited state to the ground state. The emitted light is emitted downward through the glass substrate 101 as shown by the arrows.

FIG. 4 illustrates the structure of the organic EL element 104. There are two types of organic EL elements: a single-layer type consisting only of a light-emitting layer and a multilayer type having layers other than the light-emitting layer. FIG. 4 illustrates the organic EL element belonging to the latter (five-layer type).
This organic EL element 104 has a hole injection layer 104a for improving the bonding property with the anode 102 and improving the hole injection efficiency, and a hole transport for improving the transportability of holes injected from the hole injection layer 104a. An electron injection layer 104e for improving the bonding efficiency between the layer 104b and the cathode 103 to increase the electron injection efficiency, and an electron transport layer 104d for improving the transportability of electrons injected from the electron injection layer 104e. And a light emitting layer 104c which is an organic phosphor sandwiched between the hole transport layer 104b and the electron transport layer 104d, and emits light by recombination of holes and electrons in the light emitting layer 113 as described above. To do.

The characteristics of the organic EL element will be described below.
・ Light emission brightness is almost proportional to current.
-It has rectification like a diode.
-Voltage-current characteristics as shown in FIG. When a voltage higher than the threshold voltage (Vth) is applied in the forward direction, a current flows.
-It has a large parasitic capacitance.

FIG. 6 shows the principle configuration of a passive matrix organic EL display. In the display panel 100 of the organic EL display, a plurality of data lines 102a, 102b,... Constituting a column and a plurality of scanning lines 103a, 103b constituting a row are arranged. Are arranged in a grid pattern, and an organic EL element (denoted by a diode symbol) 104 is arranged at the intersection of each data line 102a, 102b,... And each scanning line 103a, 103b,. .
The data lines 102a, 102b,... And the scanning lines 103a, 103b,... Correspond to the anodes 102 and the cathodes 103 shown in FIG. Reference numeral 105 indicates the parasitic capacitance of the organic EL element 104.

  A column driver 200 connected to each of the data lines 102a, 102b,... Is provided at the column side end of the display panel 100, and scanning lines 103a, 103b are connected to the row side end of the display panel 100. 103b,... Are provided. The column driver 200 includes a changeover switch 202i that connects each data line 102i (i = a, b, c...) To either the drive current source 201i or a ground point, and the row driver 300 includes A changeover switch 302j for connecting the scanning line 103j (j = a, b, c...) To either the positive power supply 301 or the ground point is provided.

  In this passive matrix organic EL display, an image is displayed by so-called “line sequential scanning”. That is, only one scanning line 103m is grounded by the changeover switch 302m provided in the row driver 300, and the other scanning lines 103k are connected to the positive power source 301 by the changeover switch 302k. On the other hand, the column driver 200 supplies a current corresponding to the image data to the EL element 104 on the scanning line 103m by connecting the changeover switch 202n to the drive current source 201n. At this time, no current flows through the EL element 104 on the other scanning line 103k because a reverse bias voltage is applied. Then, by executing the above operation while sequentially changing the scanning lines 103j, an image of a full screen is displayed (for example, see Patent Document 1).

  FIG. 7 shows a conventional example of a drive circuit provided in the column driver 200. The drive circuit 400 includes the drive current source 201 i and the changeover switch 202 i, and is formed in the IC chip of the column driver 200 by the number corresponding to the number of output terminals 401 necessary for driving the display panel 100. .

The drive circuit 400 employs a so-called “constant current pulse width modulation method”. That is, when the output is turned on, a constant current is supplied from the output terminal 401 to the display panel 100 via the pMOS transistor 402, and when the output is turned off, the output terminal 401 is dropped to the ground potential via the nMOS transistor 403 to supply current to the display panel 100. Supply is stopped (see FIG. 8).
At this time, the output ON time is defined based on the image data (pulse width modulated image data). When the pixel (organic EL element 104) is brightly illuminated, the output on time is set to be long by image data having a large pulse width. Conversely, when the pixel is to be illuminated darkly, the output on time is short due to image data having a small pulse width. Is set.

The pMOS transistor 403 forms a current mirror circuit together with the pMOS transistor 402. The nMOS transistor 404 is turned on / off by an image data signal, and supplies a reference current Iref defined by the constant current source 405 to the pMOS transistor 403 when turned on. Since the output current from the output terminal 401 is equal to the current flowing through the pMOS transistor 402,
Output current (Iout) = Miller ratio (k) x reference current (Iref)
The relationship is established. Note that the transistor 406 functions as the changeover switch 202 i illustrated in FIG. 6, and is turned on and off by the image data inverted by the inverter 407. Further, the transistors 402 to 404 and the constant current source 405 constitute a drive current source 201i shown in FIG.

JP 2005-258259 A

The output stage pMOS transistor 402 used in the drive circuit operates in a so-called analog region. When a transistor is used in an analog region, a voltage drop (drain-source voltage) in the transistor increases, which causes a problem that power consumption inside the transistor increases.
An object of the present invention is to provide a drive circuit for an organic EL display with low power consumption in view of such a conventional problem.

  In order to achieve the above object, the present invention has the following configuration. That is, the present invention is a drive circuit for supplying a drive current to an organic EL element provided in a passive matrix type organic EL display, the switch means interposed between a power supply and a current output terminal, and the switch Current monitoring means for monitoring a current output from the output terminal to the organic EL element when the means is on, an integration means for integrating the monitored output current, and the switch based on a scanning line switching signal Switch control means for turning on the switch means and turning off the switch means when the integral value of the output current reaches a predetermined value corresponding to the value of the analog image signal.

The switch means is composed of, for example, a push-pull circuit composed of a pair of transistors.
The current monitoring means may be configured by a resistor that converts the output current into a corresponding voltage, and the integrating means may include a capacitor that allows the monitored output current to flow.
The switch control means includes comparison means for comparing a voltage based on the electric charge accumulated in the capacitor and a value of the analog image signal, and is configured to turn off the switch means based on a comparison result of the comparison means. can do.

  According to the present invention, a constant voltage driving system is employed in which the output current is turned on and off by the switch means. Therefore, when a semiconductor switch element such as a transistor is used as the switch means, the power consumption of the transistor is only due to the on-resistance, so that the power consumption can be reduced. Moreover, it is possible to control the total amount of the current value supplied to the organic EL element while being driven at a constant voltage, that is, to control the light emission amount of the organic EL element.

FIG. 1 shows an embodiment of a drive circuit for an organic EL display according to the present invention. The drive circuit 1 corresponds to the drive circuit 400 shown in FIG. 7, and is provided in the column driver 200 shown in FIG.
In FIG. 1, a pMOS transistor 2 has a source connected to a constant voltage source 4 via a current detection resistor 3, a drain connected to an output terminal 5, and a gate connected to an output of a comparator 6. The nMOS transistor 7 has a drain connected to the output terminal 5, a gate connected to the output of the comparator 6 via the inverter 8, and a source grounded. The pMOS transistor 9 has a source and a gate connected to one end and the other end of the current detection resistor 3, respectively. One end and the other end of the capacitor 10 are connected to the source and drain of the pMOS transistor 10, respectively. The nMOS transistor 11 has a drain connected to the other end of the capacitor 10 (the drain of the pMOS transistor 10), a gate connected to the input terminal 12, and a source grounded. The comparator 6 has one input connected to the line 14 connecting the capacitor 10 and the nMOS transistor 11, and the other input connected to the input terminal 13.

  Next, the operation of the drive circuit according to this embodiment will be described. When a scanning line switching signal shown in FIG. 2B is input to the input terminal 12 from a control unit (not shown), the analog image signal shown in FIG. Input voltage signal). On the other hand, the nMOS transistor 11 is turned on based on the input of the scanning line switching signal. When the nMOS transistor 11 is turned on, the capacitor 10 is rapidly charged by the current from the constant voltage source 4, and as a result, the potential on the line 14 is lowered as shown in FIG. The nMOS transistor 11 is turned on only during the period when the scanning line switching signal is input.

  The comparator 9 compares the voltage of the analog image signal with the potential of the line 14, and when the condition of (image signal voltage)> (potential of the line 14) is satisfied as shown in FIG. The output becomes “H” level (indicated as ON in the figure). When the output of the comparator 9 becomes “H” level, the pMOS transistor 2 in the output stage is turned on, so that the organic EL element 104 to be driven emits light by the current output from the output terminal 5.

  Here, assuming that the current flowing through the pMOS transistor 2 is I and the value of the current detection resistor 3 is R, a voltage of R × I = V is generated at both ends of the current detection resistor 3. This voltage V is applied to the gate of the pMOS transistor 9. Therefore, a current corresponding to the current I flows through the transistor 9, and this current means a discharge current of the capacitor 10. That is, the current detection resistor 3 has a function as a monitoring unit that converts the current output from the output terminal 5 into a corresponding voltage and monitors it. Since the capacitor 10 discharges while the pMOS transistor 2 is on, the capacitor 10 has a function as an integrating means for integrating the output current.

When the discharge of the capacitor 10 progresses and the relationship of (image signal value) ≦ (potential of the line 14) is established, the output of the comparator 9 becomes the “L” level (indicated as OFF in the figure). The transistor 2 is turned off, and at the same time, the nMOS transistor 7 is turned on (see FIG. 2F). As a result, the organic EL element 104 to be driven is deactivated and its light emission stops.
2 (g) shows the waveform of the current output from the output terminal 5, and FIG. 2 (h) shows the waveform of the discharge current of the capacitor 10.

  As described above, in the drive circuit of the organic EL display according to the present embodiment, the output terminal 5 is connected to the positive potential of the constant voltage source and the ground using the switch means configured by the push-pull circuit including the pair of transistors 2 and 7. Since the constant voltage drive method of switching to the potential is adopted, the power consumption of the transistor 2 in the output stage is only due to the on-resistance. Therefore, it is possible to reduce power consumption and suppress heat generation of the column driver 200 shown in FIG.

  Further, according to the drive circuit of the present embodiment, since the output current is integrated and the total amount of the current is controlled, the organic EL element 104 to be driven can emit light with appropriate luminance. That is, the larger the voltage of the image signal, the longer the on-time of the transistor 2 through which the output current flows, so that the organic EL element 104 can emit light with appropriate luminance corresponding to the voltage level of the image signal.

  The present invention is not limited to the embodiment described above, and includes various modifications. That is, in the above-described embodiment, the on-time of the pMOS transistor 2 in the output stage is defined based on the comparison result of the comparator 6, but this on-time is defined at the rise time or the fall time of the scanning line switching signal. It is also possible.

It is a circuit diagram which shows embodiment of the drive circuit of the organic EL display which concerns on this invention. 2 is a timing chart showing the operation of the drive circuit of FIG. 1. It is a conceptual diagram which shows the typical whole structure of the display panel of an organic electroluminescent display. It is a perspective view which shows notionally the structure of an organic EL element. It is the graph which illustrated the voltage-current characteristic of the organic EL element. It is a circuit diagram which shows the fundamental structure of a passive matrix type organic electroluminescent display. It is a circuit diagram which shows an example of the drive circuit of the conventional organic EL display. 8 is a timing chart illustrating an output current waveform of the drive circuit of FIG.

Explanation of symbols

2 pMOS transistor 3 current detection resistor 4 constant voltage source 5 current output terminal 6 comparator 7 nMOS transistor 8 inverter 9
9 pMOS transistor 10 capacitor 11 nMOS transistor 12, 13 input terminal 104 organic EL element

Claims (5)

A drive circuit for supplying a drive current to an organic EL element provided in a passive matrix type organic EL display,
Switch means interposed between the power source and the current output terminal;
Current monitoring means for monitoring a current output from the output terminal to the organic EL element when the switch means is on;
Integrating means for integrating the monitored output current;
Switch control means for turning on the switch means on the basis of a scanning line switching signal, and turning off the switch means when the integral value of the output current reaches a predetermined value corresponding to the value of the analog image signal;
A drive circuit for an organic EL display, comprising:   2. The drive circuit for an organic EL display according to claim 1, wherein the switch means comprises a push-pull circuit comprising a pair of transistors.   2. The drive circuit for an organic EL display according to claim 1, wherein the current monitor means is a resistor that converts the output current into a corresponding voltage.   2. The organic EL display drive circuit according to claim 1, wherein the integration unit includes a capacitor for supplying a current corresponding to the monitored output current. 3.   The switch control means includes comparison means for comparing a voltage based on the electric charge accumulated in the capacitor and a value of the analog image signal, and is configured to turn off the switch means based on a comparison result of the comparison means. The organic EL display driving circuit according to claim 4, wherein the driving circuit is an organic EL display driving circuit.

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