JPS5942311B2 - Signal current separation method during write state of light emitting element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a light emitting device having a memory function such as an AC plasma display or a ZnS thin film light emitting device, and in particular to a method for driving a light emitting device having a memory function such as an AC plasma display or a ZnS thin film light emitting device. It has the following characteristics.

In light-emitting devices such as AC plasma displays and ZnS thin-film light-emitting devices, which have a hysteresis effect in their emission brightness characteristics with respect to AC applied voltage, once a high voltage is applied and writing is performed, the device emits light, and then the device emits light depending on the application of the next sustain pulse. The light emission continues.

In this case, since the display element has strong capacitance, the first
As shown in the figure, a fairly large displacement current iD and a signal current ip superimposed thereon (discharge current in an AC plasma display, polarization current in a ZnS thin film light emitting element) flow. This signal current ip is not detected when the element is not written and does not emit light. Therefore, written information can be electrically detected by knowing the presence or absence of this signal current IP. It can be read out. 1A shows the waveform of a sustaining pulse (readout pulse) P applied to the element for reading, and the opening of the figure shows the waveform of the electron current I due to the sustaining pulse P. However, in the reading method based on the application of the pulse P as shown in FIG. 1, the displacement current i
The element current is detected in the form of signal current ip superimposed on D, and if the displacement current ID is considered as noise, the S/N is very poor and it is difficult to accurately detect the signal current IP. The present invention has been made in view of this point, and is a signal current I indicating the write state which is superimposed on the displacement current iD due to voltage application.
The present invention provides a method of separating P from the displacement current iD, thereby making it possible to accurately detect the signal current 1p and reliably read out the information written in the display element.

That is, in the separation method of the present invention, the above-mentioned AC plasma display and ZnS thin film EL element have a fairly high emission threshold (threshold voltage Vth), and hardly emit light at an applied voltage below this value. , the signal current 1p of the displacement current 1D is controlled by controlling the rising part of the application sustaining pulse by utilizing the property that the signal current 1p does not flow.
The feature is that the signal current 1p is separated with a good S/N ratio by removing the influence on the signal.

The method of the present invention will be described in detail below with reference to Examples.

FIGS. 2 and 3 are drawings for explaining first and second embodiments of the present invention, respectively.

Now, the embodiment shown in FIG. 2 will be explained with reference to the drawings. In this embodiment, the pulse P2 applied for reading has a stepped portion of the sustain pulse P, as shown in FIG. A voltage Vs lower than the threshold value Th of the light emission of the element is controlled to
This is a waveform having a prefix portion A whose amplitude is τs and whose application time is τs. The waveform of the element current due to the pulse P2 is shown. That is, since the amplitude s of the front portion A is lower than the threshold for light emission of the element, only the displacement current 1D1 due to this rise flows through the element, and the signal current 1p associated with the light emission of the element does not flow. However, at the next rising portion B, the amplitude f exceeds the threshold value Vth for light emission, and therefore, if the element is being written and is in the light emitting state, a signal current 1p flows. The displacement current 1D2 at this time is (Vf-s)
Since the value of is small, it can be almost ignored. In other words, almost the displacement current 1D1 is passed during the application period of the prefix A,
At the rising edge B of the sustain pulse P', the signal current 1p can be detected with high accuracy without being affected by the displacement current 1D1. The embodiment shown in FIG.
The gradient of is selected to be sufficiently larger than the time constant of the element so that the influence of the displacement current 1D1 can be ignored compared to the signal current 1p at the rising edge B, so that the signal current Ip can be separated.

In FIG. 3, the figure A shows the voltage waveform of the applied pulse, and the figure opening shows the element current waveform. FIG. 4 is a diagram showing the relationship between the luminance brightness B and the applied voltage of this device. Using this diagram, the relationship between the amplitude f1 of the sustain pulse P1 and the amplitude s of the prefix portion A of the pulse P will be described.

The present device exhibits a hysteresis phenomenon in its light emitting mechanism as shown in the figure. Therefore, when this element is driven once by a pulse (AC pulse) whose amplitude (or peak value) is the threshold value f of light emission when the voltage increases, writing is performed (a high voltage Vw is momentarily applied). ), the element will remain stable and emit light at a high brightness Bw in the area shown in the figure, but
When no writing is being performed, the device remains stable at point A in the figure, maintaining a state in which almost no light is emitted. Therefore, this pulse is called a sustain pulse. In the present invention, the electron current is detected using this sustaining pulse P.
The amplitude Vs needs to be at a voltage such that the element in the written state does not emit light, that is, below the threshold value Vth for light emission when the voltage decreases as shown in the figure. As described above, in the above embodiment, s represents a voltage below the threshold value Th for light emission when the voltage decreases, and Vf represents a voltage near the threshold value for light emission when the voltage increases. Furthermore, in the embodiments shown in FIGS. 2 and 3 above, the amplitude s of the prefix A is made as close to Th as possible within the range of s<Vth, thereby further improving the S/N during signal current detection. be done. As described above, according to the present invention, the signal current that flows superimposed on the displacement current during the write state can be separated from the displacement current by a simple method, and the S/N can be used as the signal current for knowing the write state to the light emitting element. It is possible to obtain a signal current that can be detected with high accuracy and has improved (displacement current is viewed as noise).

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining a conventional example of this invention, FIG.
FIG. 3 is a diagram for explaining an embodiment of the invention, and FIG. 4 is a diagram for explaining the invention.

Claims (1)

[Claims] 1 After writing into a light-emitting element having a memory function, the above-mentioned written state is maintained by applying a sustaining voltage pulse whose amplitude is a voltage near the threshold of light emission, and in the written state, the voltage is In a light emitting element that flows an applied displacement current and a signal current indicating a write state superimposed on the displacement current, when reading the write state,
τ_ for applying a bias voltage lower than the light emission threshold to the light emitting element at the rising edge of the sustain voltage pulse.
A prefix of s period is provided, and during the τ_s period of the prefix,
A displacement current due to the application of a voltage lower than the threshold of the light emission is almost caused to flow, and the main component of the current flowing through the light emitting element when a voltage higher than the threshold after the prefix is applied is the signal current component indicating the write state. A signal current separation method in a write state of a light emitting element, characterized in that the displacement current and the signal current are separated by: