JPS5828591B2 - Drive circuit for thin film EL element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thin film electroluminescent device (hereinafter simply E).
(referred to as L element).

As an example of a conventional thin film EL pulse drive circuit, there is an LC series resonant drive circuit as a drive circuit that can reduce the power supply voltage compared to the applied voltage (peak value) of the EL element.

Figure 1 shows the capacitance of the EL upper segment. An example of an LC series resonant drive circuit using the quantity C will be shown and its operating principle will be explained.

Generally, the resonant frequency f in a series resonant circuit is given by the following equation.

Here, L is the inductance of the coil and Co is the total capacity.

Further, the sharpness Q value of this circuit is expressed by the following equation.

Here, R is the resistance valve of the resonant circuit.

The series resonant circuit has a frequency equal to the resonant frequency, and excitation pulses 180° out of phase with each other are applied to the synchronized transistor Tr.
13. When an electromotive force ±E is applied to the series resonant circuit in addition to the input terminals A and B of the Tr24, the terminal voltage Ec of the capacitor is given by the following equation.

Therefore, at the time of resonance, by substituting equation (2) into equation (3), the following equation is obtained.

Ec is That is, IEcI is Q times the applied voltage E.

Therefore, the power supply voltage may be 1/Q times the voltage required for display.

In FIG. 1, 1 is an EL element, 2 is a coil 5 and 6 is a protection diode.

Further, FIG. 2 shows a voltage waveform at each point in FIG. 1 and a time chart of the light emission waveform of the EL element.

FIGS. 2a and 2b show the input waveforms to the input terminals A and B of Trl and Tr2, respectively, FIG. 2C shows the voltage waveform of the EL hydrogen electrode C, and d shows the EL hydrogen light emission waveform.

By the way, when selectively connecting segments on EL using one coil, for example, 7 segments/number display (
In the case of Japanese-character segments), the number of segments to be connected naturally depends on the display content.

For this reason, the Lc resonance frequency changes as is clear from equation (1), and when externally excited at a certain frequency, it deviates from the resonance condition depending on the number of connected segments, causing problems as the voltage value required for display cannot be obtained. .

Two further conventional drive circuit embodiments are shown in FIGS. 3 and 4 as ways to overcome the above drawbacks.

Two of the seven segments are illustrated.

In the case of Fig. 3, a capacitor 11 with a sufficiently large capacity compared to the capacitance Cξ per segment of EL is connected to EL.
Segment) 12, 13, etc. are connected in parallel, and EL hydrogen atoms are selectively connected by a switch constituted by transistors 14, 15, etc. and diodes 16, 17, etc.
This method allows the resonance frequency to be kept almost constant even when the number of connections differs.

In addition, in FIG. 4, capacitors 23, 24, etc. having a capacitance almost equal to the EL hydrogen element capacitance are connected in parallel facing the EL element Cξ, and the EL hydrogen element is connected by transistors 29, 30, etc. or 31, 32, etc. This is a method of keeping the total capacity of the resonant circuit constant and keeping the resonant frequency constant by cutting off the capacitor when connecting, and connecting a capacitor when cutting off the EL hydrogen.

In FIGS. 3 and 4, 18 and 33 are excitation pulse drive circuits, and 10 and 20 are coils 16° 17.25, 2.
6, 27, and 28 are diodes for one-way connection.

As mentioned above, these two drive circuits add a compensation capacitor to keep the resonant frequency constant, and the purpose of keeping the resonant frequency constant and the voltage applied to the EL hydrogen element at the time of resonance constant is achieved, but the capacitance is In order to increase the load, increase the load on the excitation pulse drive unit, or (
2) It had the disadvantage of lowering the Q value shown by the formula.

Therefore, the present invention provides a drive circuit that can maintain a constant resonant frequency even when the number of connected (lighted) segments differs without adding a compensation capacitor.

The present invention provides a display device that uses a thin-film EL element to display a pattern in which the area of the Japanese-shaped segments is approximately the same, and when series LC resonance driving is performed using the electric capacitance C of the segment. One electrode of each segment is connected to the coil L, and each of the other electrodes is connected to a switch controlled by a lighting segment selection signal and a lighting segment sequential signal, and the selected segments are lit by time-division sequential driving. , is a drive circuit for a thin film EL element that can keep the LC resonance frequency constant regardless of the number of lighting segments and can drive without deviation from the external excitation frequency.

An embodiment of the drive circuit of the present invention will be explained by showing the same embodiment in FIG. 5, taking the case of a 7-segment square display as an example.

This diagram shows three of the seven segments.

Each segment A of the Japanese-shaped segment 65.

B, C1, that is, one electrode of the EL elements 59, 60, 61 is connected to the coil 58, and the other electrode is connected to the switch circuit 62, 6 consisting of a transistor and a diode, respectively.
3 and 64 to form an LC series resonant circuit.

In order to add an excitation pulse having a frequency equal to the resonance frequency to this series resonant circuit, the pulse output from the oscillator 40 is ANDed through a monostable multi 41 and a bistable multi 42.
) 43 and 44 to obtain outputs that are 180° out of phase with each other, and input terminals IN1.
Add to IN2.

Next, the switch circuit 62°63.64 which is a feature of the present invention
0 opening/closing will be explained.

Input terminals INA, IN of switch circuits 62, 63, 64
The inputs to B and ING are the lighting segment selection signals a - g, which are the outputs of the 7-segment decoder 56 that receives the signal from the display selection switch 55, and the lighting segment sequential signal 8.
When the lighting segment selection signal and the lighting segment sequential signal are both logic υH, the switch 62
゜63.64 turns on.

Therefore, even when displaying an arbitrary number, a plurality of segments will not light up at the same time, and the resonance wave number is determined to be f- by the capacitance C per segment and the inductance L of the coil.

Furthermore, if the lighting 2πV signal is repeated sequentially at a frequency above a certain level (approximately 40 to 50H), it appears to the human eye as continuous lighting, and an arbitrary number can be displayed.

Note that the clock signal of the lighting segment sequential signal is obtained by frequency-dividing the clock signal of the excitation pulse by appropriate steps via the divider 47.

In addition, in Fig. 5, 45, 46, 52° 53.54
is the inverter buffer.

In one embodiment of the drive circuit according to the present invention described above, "2
6 shows a voltage waveform time chart at each point when "" is displayed.

In FIG. 6, ■ is the output of the shift register 48, ■
is the output of the 7-segment decoder 56, ■ is the output of the AND gates 49, 50, 51, and ■ is the excitation Hals drive unit 57
0 manual power and V indicate the voltage waveforms applied to the EL upper segment, respectively.

By the way, in the case of the drive circuit according to the present invention (when lighting is performed sequentially in time division), when the instantaneous luminance of the display element is B and the number of time divisions is N, the average luminance of the display element is reduced to B/N, and the display It may reduce the ease of use.

However, in the case of a thin film EL element, high brightness can be obtained depending on the driving voltage value and driving frequency, as shown in FIG. 7, which shows an example of measuring brightness versus frequency characteristics.

For example, if the driving frequency (LC resonant frequency) is 2 KF (and the applied voltage value is 200 V), a luminance of about 700 FL (7-T Lambert) is obtained.

Therefore, when the seven segments shown in one embodiment of the present invention are driven in a time-division manner, a luminance of approximately 100 F.L can be obtained, which is sufficiently light as a display element and does not impair visibility.

In FIG. 7, the applied voltage waveform is a sine wave, and Vp indicates the peak voltage value.

As described above, when the drive circuit according to the present invention uses a thin film EL to display a pattern in which the area of the sun-shaped segments is approximately the same, the lighted segments are Regardless of the number, the LC resonance frequency can be kept constant, and the LC resonance frequency can be driven without deviation from the external excitation frequency.

Furthermore, since the segments are sequentially driven by time division, the capacitance load is small, so the load on the driver can be reduced, and the Q value of the resonant circuit can be increased, which has the additional advantage of being possible.

It should be noted that the present invention is not limited to the 7-segment diagonal shape, and it is clear that the gist of the invention will not be impaired even if the number of segments and display patterns are different.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of an LC resonant circuit using the capacitance C of a conventional EL element segment, Fig. 2 is a voltage waveform diagram of each part of the resonant circuit and a time chart of the light emission waveform of the EL element, Figs. 3 and 4. 5 is a configuration diagram of a conventional LC resonant circuit, FIG. 5 is a configuration diagram of a drive circuit according to an embodiment of the present invention, FIG. 6 is a voltage waveform time chart at each point in the same embodiment, and FIG. 7 is a diagram of a thin film EL.
FIG. 2 is a diagram of brightness versus frequency characteristics of an element. 57...Excitation pulse drive section, 58...
Coils 59, 60 and 61... EL element segment, 62, 63 and 64... switch circuit.

Claims (1)

[Claims] 1. In a display device that uses a thin film EL element to display a pattern in which each segment has an approximately equal area, one electrode of the segment, which has a capacitance C, is connected to a coil L to perform series LC resonance drive. The other electrode of the segment is lit segment selection signal and lit segment respectively)/
A thin film EL element connected to a switch circuit controlled by an IIN order signal, lights up and displays selected segments by time-division sequential driving, and drives a display while keeping the frequency of the series LC resonance constant. drive circuit.