JP4219997B2 - Organic EL drive circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a drive circuit for a light-emitting element used in a display, and more particularly to a drive circuit for a current-controlled light-emitting element in which light emission such as organic EL (Electro Luminescence) is controlled by a constant current.
[0002]
[Prior art]
Conventionally, an inorganic EL display using an inorganic material for a light emitting layer has been put to practical use as a display element used in a display. However, the drive voltage is as high as 100 to 200 V, and the formation of the drive circuit therefor is expensive, so the price is high and the use is limited only to specific applications such as for medical devices.
[0003]
On the other hand, research and development of an EL display (organic EL) using an organic compound having some characteristics, which is not available in the above-mentioned inorganic EL and is used in the light emitting layer, has been conducted in recent years. It has been difficult to always drive the EL element with a constant current value, and this has been a factor that causes a decrease in light emission efficiency and destruction of the element due to a slight current fluctuation or the like. This is because non-uniformity is likely to occur in the molecular structure of the organic EL element itself, and electric field concentration occurs due to this slight non-uniformity. There is a disadvantage that the light emission life is also shortened.
[0004]
In order to solve the above problem, it is known that the EL emission lifetime can be extended by applying a driving waveform applied to the organic EL. This is a method of driving with a constant current or applying a reverse bias.
[0005]
An ideal waveform for the organic EL driving is generally as shown in FIG. In order to realize such a drive waveform, it is necessary to periodically apply a waveform such as a forward bias to the positive electrode side, a reverse bias to the negative electrode side, and a zero bias. For this reason, the simplest method is to use a constant current source having both positive and negative polarities on the power supply side and periodically switch them. When driven by an alternating current having similar characteristics and having both positive and negative polarities, almost no current flows on the negative electrode side. Therefore, when the positive and negative electrodes are driven by a constant current source, a large amount of current flows on the negative electrode side, resulting in saturation. For this reason, as shown in FIG. 12, the forward bias applied to the positive electrode side is the constant current source 1, while the reverse bias applied to the negative electrode side is the constant voltage source 2, as shown in FIG. The generated signal source is input to the switch circuit 3, and the organic EL 5 is driven by controlling the switching signal from the timing generating circuit 4 so as to approach the ideal waveform shown in FIG.
[0006]
[Problems to be solved by the invention]
However, in the method shown in FIG. 12, the forward bias constant current source 1 and the reverse bias constant voltage source 2 must be switched and driven by a signal from the timing generation circuit 4. Large fluctuation on the power supply side. In particular, in the case of driving with a constant current source, the fluctuation of the current on the power source side is larger due to the connected load side capacitance than in the case of using a constant voltage source, and the waveform for driving the organic EL is distorted. Generally, when the current feedback gain of the constant current source is high, an overshoot waveform is seen as shown in FIG. 13A, and conversely when the current feedback gain is low, the waveform is as shown in FIG. 13B. It becomes distorted and breaks down from the square wave. Such an overshoot gives an excessive load to the organic EL, and thus becomes a factor for shortening the light emission lifetime, and the rounding of the waveform causes a decrease in the light emission efficiency. In addition, since it is difficult to follow when the current is changed from the current-off state, a deviation from the fundamental period occurs sequentially and the ideal square wave is lost, and a stable current cannot be supplied to the organic EL.
[0007]
Therefore, the present invention uses a constant voltage source with little power supply fluctuation for both forward bias and reverse bias, and switches forward bias and reverse bias through a control circuit in which operating conditions are set, and is optimal for the characteristics of the organic EL to be used. An organic EL driving circuit for supplying a driving current and timing conditions and at the same time extending the light emission lifetime of the organic EL is provided.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, an organic EL driving circuit according to claim 1 of the present invention is a driving circuit for an organic EL using a constant voltage source, and a forward bias voltage and a reverse bias voltage generated by the constant voltage source. A switching circuit that regularly switches the operation condition of the organic EL, an operating condition setting circuit that initially sets the operating condition of the organic EL, and an initial setting value of the operating condition setting circuit, and a control signal for the switching circuit And a current detection circuit provided between the output stage of the constant voltage source and the organic EL on the load side, the control circuit measuring the organic EL side measured by the current detection circuit An error between the current value supplied to the current and the above-mentioned initial drive current value is detected, and a voltage value obtained by calculating a forward bias voltage of the next period is set as a forward bias constant voltage source based on the error. The And butterflies.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an organic EL drive circuit according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows a basic configuration of an organic EL driving circuit according to the present invention.
[0011]
As shown in FIG. 1, the organic EL drive circuit according to the present invention includes a power supply unit composed of two power supply circuits, a forward bias constant voltage source 6 and a reverse bias constant voltage source 7, and a forward bias and a reverse bias. A switch circuit 8 for switching the current, a current detection circuit 9 for measuring a current value supplied to the organic EL 5, and an optimum current value, voltage value, square wave period, etc. according to the characteristics of the organic EL 5 to be driven An operation condition setting circuit 10; and a control circuit 11 for controlling the forward bias constant voltage source 6, the reverse bias constant voltage source 7, and the switch circuit 8 in response to signals from the current detection circuit 9 and the operation condition setting circuit 10. Consists of
[0012]
Voltage signals from the forward bias constant voltage source 6 and the reverse bias constant voltage source 7 are respectively input to the switch circuit 8 and supplied to the organic EL 5 through the current detection circuit 9. At this time, the current value supplied to the organic EL 5 side is measured by the current detection circuit 9, input to the control circuit 11, compared with the operation condition value set by the operation condition setting circuit 10, and a new voltage value and A switching cycle is set, and a control signal is sent to the forward bias constant voltage source 6, the reverse bias constant voltage source 7, and the switch circuit 8. In this way, the organic EL 5 can always be driven with an appropriate current value and waveform by comparing the immediately preceding current value for each cycle.
[0013]
3 and 4 show the contents of each block of the basic configuration shown in FIG. 1 in detail block diagrams and circuit element levels, and are of the first embodiment adopting a microcomputer control system. . Hereinafter, a description will be given based on FIGS. 3 and 4.
[0014]
A constant voltage source for supplying power to the organic EL 5 is composed of DA converters 12 and 14 and amplifiers 13 and 15, and has two systems for forward bias and reverse bias. The DA converter 12 converts the digital value of the forward bias setting voltage sent from the control circuit 11 into an analog positive voltage value and supplies it to the amplifier 13. On the other hand, the DA converter 14 converts the digital value of the reverse bias setting voltage sent from the control circuit 11 into an analog negative voltage value and supplies it to the amplifier 15.
[0015]
The switch circuit 8 is a circuit for electrically switching three states of a forward bias, a reverse bias, and a zero bias with respect to a switching signal from the control circuit 11 based on a set constant period. The configuration includes four switches (SW1, SW2, SW3, SW4), four terminals on the input side, two terminals on the output side, and an input terminal for control signals. Among the switches, the input sides of SW2 and SW3 are both grounded and set to a reference voltage of 0 V, the forward bias voltage from the amplifier 13 is applied to the input side of SW1, and the reverse bias from the amplifier 15 is applied to the input side of SW4. A voltage is input. Then, with the control signal from the control circuit 11, the states S0 to S3 shown in FIG. 5 are alternately switched and output.
[0016]
The current detection circuit 9 is a circuit for sequentially measuring the current value actually flowing through the organic EL 5. The circuit mainly includes a current detection resistor 16, a comparator 17, and an AD converter 18. One end of the current detection resistor 16 is on the forward bias supply side, and the other end is on the organic EL 5 side to be driven. Connected. Then, the two signal lines output from both ends of the current detection resistor 16 are input to the comparator 17, and after the difference between the current values of both signal lines is detected, it is controlled as a digital quantity via the AD converter 18. It is stored in the RAM 19 in the circuit 11.
[0017]
The operation condition setting circuit 10 is an initial setting of frequency, current, voltage, and the like, which are operation conditions necessary for organic EL driving. In particular, for the forward bias constant voltage source 6, a sufficiently small initial voltage is set so that the elements of the organic EL 5 are not broken. The configuration includes an input / output port and a memory unit, and the initial setting value is input via the input / output port and stored in the memory unit. The stored contents are read into the RAM 19 of the control circuit 11 at the same time as the power is turned on.
[0018]
The control circuit 11 includes a CPU 20, a RAM 19, a ROM 21, a timer / counter 22, and a parallel I / O port 23, and a detected current value from the current detection circuit 9 and various initial setting values from the operating condition setting circuit 10. In response, the operating conditions corrected up to the next driving cycle are the voltage value for the constant voltage source 6, the forward bias (S1), the reverse bias (S2) for the switch circuit 8, A switching signal for alternately switching the three states of zero bias (S3) and data of the repetition period of the three states S1, S2, and S3 are sent. The forward bias (S1), reverse bias (S2), zero bias (S3) duty ratio and one cycle timing are set by setting information initially set in the operating condition setting circuit 10 in the timer / counter 22. . In addition, the voltage value, current value, duty ratio of the square wave pulse, and the like can be set in the ROM 21 in advance.
[0019]
Next, the overall control method will be described with reference to FIGS. First, as an initial setting, an initial voltage is set in a forward bias constant voltage source, and a voltage equivalent to the initial voltage and opposite in polarity is set in a reverse bias constant voltage source (P1). The initial voltage is set to a sufficiently small value so that the organic EL element to be driven is not destroyed. However, when the initial voltage / current characteristic of the organic EL to be driven is known, a voltage value corresponding to the initial voltage is given. Generally, the voltage value is about 5 to 20V. The state up to this point corresponds to S0 shown in FIG.
[0020]
Next, after the initial setting performed during the state S0, the control circuit 11 sends a control signal for switching the output to the state S1 in which the output is forward biased to the switch circuit 8 (P2). The current detection circuit waits for a time (ts) until the signal selected by the control signal from the control circuit 11 and output from the switch circuit 8 rises to reach a predetermined voltage value and becomes stable (P3). The output current value is measured by 9 and current information is read (P4). This current information is temporarily stored in the control circuit. Next, the timer / counter 22 in the control circuit 11 counts the time corresponding to the set pulse width of t1 (P5). After the count for t1 is completed, the control circuit 11 then switches to the switch circuit. A control signal for switching the output to a state S2 in which the output is reverse biased is sent to P8 (P6). Here, the timer / counter 22 counts the time t2 corresponding to the set reverse bias pulse width (P7). After the count for t2 hours is completed, the output is switched from the control circuit 11 to the switch circuit 8 to the zero bias of the state 3 (P8). During this zero bias state S3, the forward bias voltage value of the next period is calculated from the difference between the current value measured in (P4) and the initial set drive current value, and the voltage is converted (P9). The converted voltage value is set as a forward bias constant voltage source (P10). When the zero bias state of state S3 is completed (P11), the switch of the switch circuit 8 is switched to the forward bias constant voltage source side 6 and the state returns to state S2. By repeating such operations from process (P2) to (P11), a constant continuous drive waveform is generated.
[0021]
Next, a circuit example for controlling the organic EL drive circuit according to the present invention by a method other than the microcomputer control will be described.
[0022]
FIG. 9 is a circuit example when the control method based on the microcomputer configuration described in FIGS. 3 and 4 is configured by an analog circuit. In this embodiment, for simplicity, the reverse bias constant voltage source 24 is fixed to the output voltage value, and the forward bias constant voltage source 25 is mainly controlled. The current value on the forward bias constant voltage source 25 side measured by the current detection circuit 9 is temporarily held by the hold amplifier 26, and then the current value on the organic EL side that is the load side is temporarily held by the hold amplifier 27. The Then, the current value corresponding to the error of both the hold amplifiers 26 and 27 is set to a necessary voltage value in the forward bias constant voltage source 25 via the filter circuit 28. The timing generation circuit 4 is controlled by sending corrected timing information.
[0023]
FIG. 10 is a circuit example when the control circuit unit is configured by only random logic without using a CPU. The current value on the forward bias constant voltage source 25 side measured by the current detection circuit 9 is first converted from an analog value to a digital value by the AD converter 18 and is held by the first latch circuit 29. When detected, the data of the latch circuit 29 held in the previous cycle is shifted to the latch circuit 30, and the newly detected current value is input to the latch circuit 29. At this time, the error amount between the data in the latch circuit 29 and the data in the latch circuit 30 is decoded by the decode circuit 31. Subsequently, the data of the decode circuit 31 is set in the forward bias constant voltage source 25 through the DA converter 32 as an appropriate voltage value. At the same time, the data of the latch circuit 29 and the latch circuit 30 is sent to the timing generation circuit 4, and the timing generation circuit corrects the data to an appropriate timing and sends it to the switch circuit 8. In this way, the current current value and the previous current value are sequentially held in the two latch circuits and compared, whereby driving can be performed while correcting the supply voltage value and the cycle.
[0024]
The circuit examples shown in FIGS. 9 and 10 have the advantage that the circuit configuration is simplified, although the accuracy is not as high as that of the microcomputer control system shown in FIGS.
[0025]
Next, FIG. 11 shows an example in which the organic EL drive circuit according to the present invention is applied to the display panel drive of a watch. This application example includes an oscillation circuit 33, a counter circuit 34, a decoder circuit 35, an EL drive circuit 36, and an organic EL panel 37. First, a 1 Hz clock signal is generated by the oscillation circuit 33 and applied to the counter circuit 34. The counter circuit 34 is composed of a “second counter” and a “minute counter” which are 60-digit counters, and a “hour counter” which is a 12-digit counter. The EL segment 38 to be lit is selected and sent to the EL drive circuit 36 according to the present invention. From the EL drive circuit 36, a predetermined drive waveform is sent to the EL segment 38 for which a lighting command has been issued to emit light. In this manner, the timepiece is configured by causing the EL segment 38 necessary for the time display pattern to be synchronized with the clock signal from the oscillation circuit 33. Since such a digital timepiece display panel is required to always emit EL with low power consumption, the organic EL driving circuit according to the present invention that can be driven at a low voltage and supports long-life EL emission is optimal. It becomes one of the application examples.
[0026]
【The invention's effect】
As described above, according to the organic EL drive circuit according to the present invention, constant current drive having pseudo AC characteristics can be realized by using a constant voltage source for both forward bias and reverse bias applied as an organic EL drive source. By doing so, it is possible to obtain an effect that current fluctuation is small, a stable driving waveform is obtained, and the light emission life of the organic EL is extended.
[0027]
In addition, there are advantages that the circuit configuration is simpler and the size and the IC can be easily formed as compared with the case where the equivalent function is configured by a constant current source.
[0028]
In addition, since it is driven at a constant voltage, adjustment and control are relatively easy, and digital control or the like by a microcomputer becomes possible.
[0029]
In addition, by sharing a control circuit and adding an output stage signal line to be supplied to the organic EL side, the organic EL driving circuit of the present invention enables matrix driving of an organic EL display composed of a plurality of segments. Can be easily expanded.
[Brief description of the drawings]
FIG. 1 is a basic configuration diagram of an organic EL drive circuit according to the present invention.
FIG. 2 is an ideal drive waveform by an organic EL drive circuit according to the present invention.
FIG. 3 is a block diagram of a first embodiment of the present invention.
FIG. 4 is a circuit diagram of a first embodiment of the present invention.
FIG. 5 is a switching state table of the first embodiment of the present invention.
FIG. 6 is a control flow of the first embodiment of the present invention.
FIG. 7 is a basic waveform of the first embodiment of the present invention.
FIG. 8 is a combination table of each state and selected voltage value in the first embodiment of the present invention.
FIG. 9 is a block diagram of a second embodiment of the present invention.
FIG. 10 is a block diagram of a third embodiment of the present invention.
FIG. 11 is a block diagram showing an application example of the present invention.
FIG. 12 is a block diagram of a conventional organic EL driving method.
FIG. 13 shows a driving waveform by a conventional driving circuit.
[Explanation of symbols]
5 Organic EL
6 Forward bias constant voltage source 7 Reverse bias constant voltage source 8 Switch circuit 9 Current detection circuit 10 Operating condition setting circuit 11 Control circuit

Claims (1)

In an organic EL drive circuit using a constant voltage source,
A switch circuit that regularly switches a forward bias voltage and a reverse bias voltage generated by the constant voltage source based on a certain period ;
An operating condition setting circuit for initially setting the operating conditions of the organic EL;
A control circuit that receives an initial setting value of the operating condition setting circuit and sends a control signal to the switch circuit;
A current detection circuit provided between the output stage of the constant voltage source and the organic EL on the load side, and the control circuit has a current value supplied to the organic EL side measured by the current detection circuit And a voltage value obtained by calculating a forward bias voltage of the next period based on the error amount is set as a forward bias constant voltage source. Driving circuit.

Images