JPS5828592B2 - Drive circuit for three-layer thin film EL element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a drive circuit for lighting a plurality of segments of a three-layer thin film EL element by LC series resonance driving.

In particular, the present invention relates to a circuit that can keep the LC resonant frequency constant regardless of the number of lit segments.

A three-layer thin film EL element has a transparent electrode Y2 on a transparent substrate such as glass.
Insulating thin film such as O3, Si 3N4, Mn 0.5
% doped ZnS EL thin film, same as above, Y2
It is constructed by laminating in this order an insulating thin film such as O3, Si3N4, and a back electrode such as AI.

The above two insulating thin films are about 0.2μ771. EL thin film is 0
．． It is a 5μ door and is made by vapor deposition, sputtering, etc.

Since a three-layer thin film EL element has an EL thin film sandwiched between an extremely thin and high dielectric constant insulating layer between a transparent electrode and a back electrode, it is an element that equivalently has a fairly large capacitive component. it is conceivable that.

By actively utilizing this capacitance component, an inductance component (coil) is connected in series to the external circuit of the three-layer thin film EL element, and the power supply voltage is applied to the three-layer thin film EL element by an LC series resonant circuit made up of this capacitance component and inductance component. It can be driven at a lower voltage than the light emitting voltage of the device.

FIG. 1 shows a diagram of an LC series resonant circuit using a capacitance Ce of a three-layer thin film EL element, and its operation will be explained.

Generally, the resonant frequency 1 in a series resonant circuit is given by the following equation.

Here, L is the inductance of the coil, and co is the total capacitance.

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

Here, R is the resistance of the resonant circuit.

Transistor Q
, Q2's input terminals A and B, and when an electromotive force ±E is applied to the series resonant circuit, 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.

c. teeth That is, IEcl 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 a thin film EL element having a capacitance Ce, 2 is a coil, and 5 and 6 are protection diodes.

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.

Figures 2a and 2b are transistors Ql, respectively. Input waveforms to input terminals A and B of Q2, C shows the voltage waveform of electrode C of the EL element, and d shows the light emission waveform of the EL element.

By the way, when selectively connecting segments on the EL using one coil, for example when displaying 7 segments (Japanese character shape), the number of segments to be connected naturally depends on the number to be displayed.

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

The inventors have filed two patent applications for circuits that eliminate the above drawbacks.

One is Japanese Patent Application No. 108032/1983 ``Method for Driving Three-Layer Thin Film EL Element'' and is shown in Figure 3; the others are Japanese Patent Application No. 1983-1
No. 12256 ``Driving device for thin film EL element'' and is shown in FIG.

In the case of Fig. 3, a capacitor 7 having a sufficiently large capacity compared to the capacitance Ce per segment of the EL is connected in parallel to the upper EL segments -1, . . . , 8-n, and the transistor 10-1. .......

10-n and diode 11-1. ......, 11
This is a drive circuit that selectively connects EL elements by n, reduces the rate of change in capacitance even when the number of connections differs, and keeps the resonant frequency almost constant.

In addition, in FIG. 4, capacitors 18-1, .
8n are connected in parallel opposite to the EL elements 16-1, . . . , 16-n, and the transistors 21-1, .
・, 21-n or 22-1,...

22-n, when turning on the transistor on the EL element side, turn off the transistor on the capacitor side, and when turning off the transistor on the EL element side, turning on the transistor on the capacitor side, thereby creating a resonant circuit. This method keeps the total capacitance constant and the resonant frequency constant.

In addition, in FIGS. 3 and 4, 14 and 28 are excitation pulse drive circuits, 2 and 15 are coils 11-1°...
, 11-n, 24-1. ......, 24-n, 22
5-1, . . . , 25-n are diodes for one-way connection.

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

The present invention solves the above-mentioned problems, and the present invention uses a plurality of segments as one unit, and when the total number of segments is N and the number of segments in one set is n,
The selected segments are sequentially driven at N cycles.

FIG. 5 shows a block diagram of a drive circuit for driving the seven segments of the present invention, including a ten-key keyboard 29, an encoder 30,
, launch circuit 31. BcDt7 segment decoder 32
, excitation pulse generator 33, n-stage frequency dividing circuit 34.4-phase gate pulse 35, monostable 36, flip-flop 37, gate 38, gate 39, switches S1 to S8
It consists of a circuit 40 and a resonance drive circuit 41.

FIG. 6 is a detailed circuit diagram of portion 42 of FIG.

The latch circuit 31 is a circuit that holds the numerical signals of the numeric keyboard 29 in binary code, and outputs the signal a - by the BCDtc 7 segment decoder 32 and the gate circuit.
get h.

FIG. 7 is a detailed circuit diagram of portion 43 of FIG.

The frequency dividing circuit 34 divides the pulse signal with a frequency of 21 from the excitation pulse generator 33 and inputs it to the n 4-bit parallel output shift register 35.
Create 4-phase gate pulses ■～■.

The reset switch R8 serves to return the output of the shift register 35 to the initial Xll] state.

FIG. 8 is a detailed circuit diagram of portion 44 of FIG.

The excitation pulse 33 is input to a variable pulse width circuit 36 and a flip-flop circuit 31.

The Q and Q outputs of the circuits 36 and 37 are input to the gate circuit 38, and a signal Sig whose phase is shifted by 1800,
1 and Sig, 2.

FIG. 9 shows details of the gate circuit 45 of FIG. 5.

This circuit receives the decoder output signal a from the section shown in FIG.
-h and the timing of the four-phase gate signals (1) to (4) from the section shown in FIG.

81 to S8 indicate connections to each switch.

FIG. 10 shows a somewhat schematic circuit diagram of the portion 46 in FIG.
49, and is represented by the * symbol of a capacitor, and the connection relation diagram of those capacitors and a switch is shown.

FIG. 11 shows its detailed circuit diagram.

42 is a coil; 43 to 49 are capacitances of each segment a to g of the thin film EL element; and 50 is a capacitor approximately equal to the capacitance Ce of one segment of the thin film EL connected to the outside.

(Not required if the number of display segments is even).

51 to 58 indicate changeover switches 81 to S8.

59 to 62 are switching transistors of the resonance drive circuit, 63 and 64 are backflow prevention diodes, and 65 to 72
is a unidirectional application diode.

00 in one digit (excluding decimal point) when lighting two segments of the present invention as one set of load.
Table 1 shows the relationship between the timings of the R-cycle four-phase gate pulse signals and the switch signals 81 to S8 in numbers 9 to 9.

h is a capacitor, regardless of whether it is lit or not, so the number ■
It is always in the ON state at 4 timings except for 6".

FIG. 12a shows a time chart of the display logic driving circuit.

As an example, a numerical display example of 'X3'' is shown. A is an excitation pulse signal with a frequency of 2f, B and C are Q of the circuit 37 in FIG.
The Q signals, D and E are Sig, 1.51g2 in FIG.

F is the frequency divided output of the circuit 34, here the frequency L, G-J is the parallel output signal of the circuits 4 and 35 in FIG. 1, and K-R is the decoder 32 and gate output a in FIG. -h signal is shown.

S-z is a timing output signal of each gate in FIG. 9, and corresponds to 81 to S8, respectively.

In FIG. 12, a to h are applied voltage waveforms applied to each segment (a to h) in FIG. 10, and the frequency is f.

In the case of number ゝゝ3'', segment a at the first timing
.

d lights up, and the number of segments is displayed at the next second timing.

C lights up, and at the third timing, no segment lights up.

Segment g lights up at the fourth timing.

Note that the light emission period of the thin film EL is 2f. The lighting is then repeated in sequence.

Other numbers can also be lit using the same process.

As shown in Table 1 and Figure 12, the load capacity of the series LC resonant circuit is kept almost constant regardless of the number displayed by the 0N-OFF connection of the capacitor 50 inserted from the outside. .

Since the loss during light emission of the three-layer thin film EL element is sufficiently smaller than that of L, the resonance condition is almost satisfied, and the display can be lit with almost uniform brightness at a constant resonance frequency.

Further, in the present invention, since a plurality of segments are driven as a unit, the driving cycle is longer than that in a case where each segment is driven one after another, so that the lighting time becomes longer and the average luminance improves.

Alternatively, if the average luminance is the same, the voltage applied to the thin film EL element can be lowered.

In the present invention, since the resonant circuit is always connected to only two segments of electrical capacitance, the capacitive load of the resonant circuit is also thick (not large), and therefore the circuit can be driven without significantly deteriorating the Q value. There is no need to increase the voltage.

Furthermore, since the present invention does not use a dummy capacitor as a load, unnecessary power consumption is reduced.

It goes without saying that the present invention is not limited to the 7-segment diagonal shape, but can also be applied to cases where the number of segments and display patterns are different.

[Brief explanation of drawings]

Figure 1 is an example of the LC resonant circuit of a three-layer thin film EL element,
The figure shows a time chart of the voltage waveform at each point of the resonant circuit in Fig. 1 and the emission waveform of the EL element, Fig. 3 shows an example of the resonant frequency compensation circuit of the earlier invention, and Fig. 4 shows an example of the other earlier invention. An example of a resonant frequency compensation circuit, FIG. 5 is a block diagram of a drive circuit according to an embodiment of the present invention, FIGS. 6, 7, 8,
9, 10, and 11 are detailed circuit diagrams of each part of FIG. 5, and FIGS. 12a and 12b are time charts of the voltage waveforms of FIG. 5. 32 is a BCDto 7 segment decoder, 34 is a frequency dividing circuit, 35 is a 4-phase gate pulse, 39 is a gate circuit,
42 is a coil, 43 to 49 are one segment of a thin film EL element, 50 is a capacitor, and 51 to 58 are switches.

Claims (1)

[Claims] 1. In a circuit in which an inductance component is connected in series to a three-layer thin film EL element and the three-layer thin film EL element is driven by an LC series resonant circuit, a plurality of segments among the segments for character display are regarded as one unit, and each of the segments is A driving circuit for a three-layer thin film EL element, characterized in that the units are sequentially driven and the resonant frequency of the LC series resonant circuit is kept constant.