JPS5828596B2 - EL element reading device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Overview The present invention relates to a memory readout device for an EL element having memory characteristics.

The present invention provides an improved device for reliably reading out the memory contents of a thin film EL display element by detecting the polarization current of a thin film EL display element having a hysteresis between applied voltage and luminance. be.

Prior art The structure of an EL display element will be briefly explained.

First, a transparent electrode is placed on a glass substrate.

On top of that, a dielectric material such as Y2O3, Zn doped with Mn
A fluorescent layer made of S or the like is formed, and a dielectric material and a back electrode are formed in this order on this layer.

These layers are formed by sequentially depositing them by vapor deposition, sputtering, or the like.

Such an EL element, in which a dielectric material is deposited on both sides of a fluorescent layer, has a memory effect based on the history characteristic between the applied voltage and the luminance, so it can be written with a writing pulse of a large voltage level, or The state of an EL element having a memory characteristic erased by a low-level erase pulse can be maintained by applying a sustain drive wave of an appropriate level.

Thin-film EL elements are considered to be a type of capacitive element, so a displacement current flows when a voltage is applied, but when the element emits light, a current corresponding to the luminance of the emitted light is superimposed on this displacement current and flows. .

This current is called a polarization current.

In reality, even in the erased state, there is a slight rise in the background, so a small amount of polarization current flows accordingly, but the displacement current that is added to this small amount of polarization current is simply referred to as the displacement current. I'll decide.

For example, when a voltage waveform as shown in FIG. 1a is applied to an EL element, the current flowing through the element is
The waveform of the displacement current is shown as a solid line 11 in the figure.
In the light emitting state, as shown by a broken line 12, a waveform obtained by superimposing a polarization current thereon is shown.

Characters, symbols, or patterns can be displayed by configuring the transparent electrodes and back electrodes that make up the EL element in a matrix or by configuring them with segment electrodes.
Electrically reading out the displayed stored information is an extremely important function when used as an information input/output terminal.

For reading the memory of an EL element, a method has already been proposed in which a read waveform with a constant rising slope is added to separate the displacement current and polarization current of the EL element, and the presence or absence of the polarization current is detected.

However, with this method, for example in matrix-type EL panels, there are variations in displacement current due to variations in characteristics due to non-uniformity of film thickness between elements corresponding to each pixel, so it is difficult to compare If this is not corrected for each element, there is a risk of erroneous reading.

Further, there is an influence of other picture elements capacitively coupled to the read picture element, resulting in inaccurate reading.

The present invention has been made to overcome the above-mentioned drawbacks.

<Summary of the Invention> According to the present invention, when a capacitor having a capacitance sufficiently larger than the capacitance of the EL element is connected in series with the EL element, different terminal voltages are obtained at the capacitor when the EL element emits light and when it erases the light. It will be done.

A 1:1 relationship between this terminal voltage and the luminance of the EL element can be obtained from measurements.

Based on this fact, the present invention is based on the fact that E
This is for reading out the L element.

(Description of Examples) A capacitor 2 having a capacitance C6 sufficiently larger than the capacitance of the element is connected in series to the EL element 1 having the above configuration.

A driving power source 10 for applying write pulses, erase pulses, and sustain pulses is connected to this series circuit.

In order to detect the voltage across the capacitor 20, the gate circuit 2
The capacitor voltage is taken out during the period when no driving pulse is applied through 0.

The driving power source 10 applies a pulse train to the EL element 1, which consists of a pulse C1, a pulse C1, and a write pulse a, as shown in FIG. 3A.

The voltage across the capacitor 20 when the driving pulse is applied is 5 to 5000 when the EL element is not emitting light.
A voltage (Vd) d corresponding to the amount of displacement current flowing through the EL element having the same phase as the sustaining pulse C having a pulse width of μS is obtained as shown at the beginning of FIG.

On the other hand, if the EL element emits light by the sustain pulse 8 after the write pulse a is applied, as shown in Figure 3, the voltage corresponding to the displacement current component of the same phase and the integral of the polarization current component A voltage phase (Vp)e corresponding to the value is obtained.

During the time f when the sustaining pulse C is not applied, the EL element stops emitting light as shown in FIG.
Due to the polarized charges stored near the interface between the layer and the dielectric layer,
A polarization voltage g corresponding to the amount of polarization charge is obtained between both ends of the capacitor 20.

The voltage characteristics of the polarization charge amount Qp and the luminance B of the EL element were measured as shown in FIG. 4.

That is, the amount of polarized charge Qp and the luminance B have a one-to-one correspondence, and the luminance of the EL element is basically expressed as the product of the drive pulse frequency f and the amount of polarized charge Qp.

Furthermore, the amount of polarized charge has the following relationship.

However, for C6, the capacitance value Vp of the capacitor 2 is the polarization voltage value of the capacitor 2 at the time of light emission, and therefore the polarization voltage ■.Once the value of the polarization voltage is recognized, an appropriate coefficient (a value determined by the drive pulse frequency and the capacitance value of the capacitor 2) is determined. By multiplying by , the luminance of the EL element can be detected as an analog quantity.

After the write, erase, and sustain pulses are applied from the terminal 30, the gate circuit 20 applies the gate pulse h shown in FIG. With the power on, the polarization voltage of capacitor 2 is V.

Take out.

The output 40 is an output proportional to the polarization voltage vp (Fig. 3 E)
is obtained.

This output is multiplied by an appropriate coefficient to obtain the polarization voltage V of the EL element.
Read out.

<Effects of the Invention> As described above, the light emitting state and erasing state of the EL element can be adjusted to E.
Detection can be performed without changing the state of the L element.

That is, since the voltage across the extrapolation capacitor 20 is taken out, the polarized charges inside the EL element are not erased.

The voltage across the capacitor 20 is all based on the light emitting state, and there is no need to remove the voltage component due to the displacement current, so it is not only possible to distinguish between the light emitting state and the erased state, but also to recognize them as positive and negative analog quantities of polarization voltage. It is also possible to read out the magnitude of the luminance of the emitted light, and it can be used as an analog memory element.

The polarization voltage of the non-light-emitting point is extremely small, and a signal can be obtained from the S/H without being adversely affected by the background floating effect caused by the non-uniformity of the film thickness of each picture element of the EL element.

[Brief explanation of the drawing]

Fig. 1 is a time relationship diagram between applied voltage and current flowing through an EL element, Fig. 2 is a constituent country of an EL element readout device according to an embodiment of the present invention, and Fig. 3 is a time chart of the circuit shown in Fig. 2. ,
FIG. 4 shows a diagram of the relationship between the luminance of light emission and the polarization voltage with respect to the applied voltage of the EL element. 1 is an EL element, 2 is a capacitor, 10 is a drive circuit, 20
is a gate circuit, and 30 is a gate signal input terminal.

Claims (1)

[Claims] 1. A readout device for an EL element that has a history characteristic between applied voltage and luminance, comprising: a drive power source that applies write pulses, erase pulses, and sustain pulses; and a capacitor connected in series to the EL element. A reading device for an EL element, comprising: a circuit that detects the terminal voltage of the capacitor after the write pulse, erase pulse, and sustain pulse are applied to read out the luminance of the EL element.