JPS6212902B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching element built-in liquid crystal display device in which a switching element is provided on a cell substrate.
More specifically, the present invention relates to a liquid crystal display device that has stable display quality even under environmental changes.

Liquid crystal display devices have features that other devices do not have, such as low power and low voltage, and are widely used mainly in portable devices such as calculators and watches. However, since the display characteristics do not have sharp threshold characteristics with respect to voltage, they are not suitable for high-division multiplex driving. Therefore, in order to enable high-division multiplex driving, a method was proposed in which a switching element is arranged for each display element (switching element built-in method) (see
BJLechner et al.Proc.IEEE vol 59,
Nov.1971, p.1566-1579). However, with conventional switching element built-in systems, there are few measures taken to prevent deterioration of display quality due to changes in the external environment such as temperature and light, making it difficult to use for applications such as mobile devices that must operate reliably even under harsh usage conditions. It has not been put into practical use. The present invention relates to a technique for eliminating such drawbacks, and provides means for extremely efficiently and reliably compensating for the effects of changes in the external environment.

Prior to explaining the present invention, a switching element built-in system will be described. FIG. 1 is an explanatory diagram of one example, where Sij is a switching element, Zij is an impedance element made of a liquid crystal layer narrowed by electrodes, and Cij is an additional capacitance that is appropriately provided and added as necessary.
However, the subscripts i and j are arbitrary natural numbers. The liquid crystal layer impedance element Zij is approximately composed of a wiring resistance R, a liquid crystal layer capacitance C, and a liquid crystal layer resistance R as shown in FIG.
Assuming that the gate voltage V _G vs. drain current I _D characteristic of the switching element Sij takes the characteristic shown by the solid line 15 in Figure 7, if an appropriate gate voltage is applied to the Yi electrode, the liquid crystal layer impedance Zji and additional capacitance will be changed from the Xj electrode to the liquid crystal layer impedance Zji. The drain current to be injected can be controlled arbitrarily. For example, the time-divided control signals y ₁ , y ₂ , y ₃ , . . . shown in FIG. 3 are applied to the electrodes Y ₁ , Y ₂ , Y ₃ , . The period during which v ₂ is applied is selected period t, v ₁
The period during which is applied is called a non-selection period T-t. During the selection period t, the switching element becomes conductive and a drain current flows, and if the wiring resistance R is sufficiently small and the liquid crystal resistance is sufficiently large, the potential of the liquid crystal capacitance and the additional capacitance (for example, V ₁₁ in Fig. 3) will be the same at that time.
It is charged and discharged with a time constant τ ₁ so that the potential of the Xj electrode becomes, for example, V _ON in FIG. During the non-selection period, the switching element is non-conductive, and the potential determined during the previous selection period is maintained with a time constant τ ₂ . As described above, in the switching element built-in system, the charging/discharging characteristics during the selection period and the holding characteristics during the non-selection period are important, and high-quality, high-division multiplex driving is only possible when both characteristics are sufficient. The charging/discharging time _τ1 during the selection period depends on the on-resistance R _ON of the switching element, the liquid crystal capacitance C, and the additional capacitance Cij, and is R _ON
It is proportional to (C + Cij), and the retention time τ in the non-selection period
₂ is the off-resistance R _OFF of the switching element and the above C
Proportional to the product of +Cij. Therefore, in order to obtain stable operation, a switching element that can realize small R _ON and large R _OFF is required.

An example of a practical configuration of a switching element stacked type liquid crystal display panel is shown in FIGS. 4, 5, and 6. FIG. 4 is a perspective view. An electrode layer 4 is formed on the substrate 1, a layer 5 including switching elements is formed on the substrate 2, and a liquid crystal layer 3 is sandwiched between the two substrates. Fifth
The figure is a partial sectional view. The switching element on the substrate 2 includes an electrode Y serving as a gate, an insulating layer 10, semiconductor layers 11, 12, and 13, an electrode X serving as a source electrode, and a liquid crystal electrode 7 serving as a drain electrode. 8 and 9 are surface treatment layers that also serve as protective films.
FIG. 6 shows a portion of the substrate 2 corresponding to about one display element.
FIG. Y is the gate electrode, X
is a source electrode, 6 is a semiconductor layer, and 7 is a drain electrode and a liquid crystal electrode. The cross section along the dashed line 14 corresponds to FIG. Although an additional capacitor is not provided in FIGS. 5 and 6, it may be formed between the drain electrode and the gate electrode if necessary.

FIG. 8 shows the entire block diaphragm.
Reference numeral 21 denotes a display panel section having a configuration as shown in FIGS. 4 to 6, and 22 a gate driver that supplies gate signals such as y ₁ , y ₂ , y ₃ , etc. shown in FIG. 23 is a source driver that supplies a source signal to the display signal generation circuit 2.
It is composed of a temporary storage circuit, etc., in which display signals serially input from 6 are sequentially stored in accordance with a signal from a high-speed sweep shift register 24 synchronized by a clock circuit 25. The display signal generation circuit 26 operates by receiving external information from 27 as necessary. The power line is omitted.

The liquid crystal display device with a built-in switching element as described above is, in principle, capable of displaying as much information as on a TV, and without sacrificing its advantages of low voltage and low power drive, it can be used as a small portable display device. can be said to be a very superior method compared to other methods.

However, although the method is excellent in principle, the conventional method has not been put into practical use. One of the causes is variation and deterioration in display quality due to external factors such as temperature and light. This effect is particularly significant when a semiconductor that is extremely sensitive to temperature, light, etc. is used for the switching element. For example, when using a field effect transistor type switching element as shown in FIG.
The solid line 15 in the figure changes to the dashed line 17 due to temperature rise, and changes to the broken line 16 due to light irradiation. In this way, the drain current I _D changes and the effective on-resistance R _ON
When the off-resistance R _OFF changes, the signal waveform applied to the liquid crystal layer, for example, the time constants τ ₁ and τ ₂ of V ₁₁ in FIG. . In order to compensate for this quality deterioration in advance during manufacturing, a large operating margin must be provided for the switching element characteristics, which imposes a heavy burden on the manufacturing method and yield. The present invention provides technology that improves quality, expands the degree of freedom in manufacturing, and reduces manufacturing costs by improving the effects of characteristic deterioration caused by external factors such as those described above by devising a driving method. do. A detailed explanation will be given below based on the drawings.

FIG. 9 is a block diagram of an embodiment of the present invention. 28 is a detection circuit for detecting a change in the characteristics of the switching element, and a variable clock circuit 29
A signal 61 corresponding to the conductance of the switching element is supplied to the variable clock circuit 29 by a control signal 66 from a measurement control circuit included in the circuit. The variable clock circuit 29 sets a basic clock signal f capable of compensating for fluctuations in the conductance of the switching element, and outputs clock signals 35, 3 based on f.
6, 38, and 37 are supplied to the source high-speed sweep shift register 24, the gate drive low-speed sweep shift register 22, the source driver 23, and the display signal generation circuit 26, respectively. 25 is a fixed reference clock circuit which receives reference clock signals 30, 34.
Display signal generation circuit 26 and variable clock circuit 2
Supply to 9.

In the present invention, variations in the stitching characteristics are compensated for by changing the driving frequency based on the basic clock signal. That is, the charging/discharging time constant τ ₁ during the selection period is proportional to the on-resistance R _ON of the switching element, and the holding time constant τ ₂ during the non-selection period is proportional to the off-resistance R _OF
Since it is proportional to _F , if the selection time t and non-selection time (Tt) are changed by the amount of variation in R _ON and R _OFF , the variation in the effective voltage applied to the liquid crystal layer becomes zero.

Both T and t may be changed _{, but as can be seen from the example in Figure 7, the fluctuations in R ON and R OFF are often} of the same degree, and t→ Even if the frequency is changed from (1+α)tT to (1+α)T to compensate, the effect is sufficiently large. At this time, it is sufficient to apply an appropriate gate voltage, measure the rate of change α, and multiply the basic clock signal by 1+α.

FIG. 10 shows a specific embodiment of the present invention.
Three blocks 56, 50, and 51 correspond to the detection circuit 28 of FIG. 9, and blocks 52, 53, 54, and 55 correspond to the variable clock circuit 29 of FIG. Reference numeral 56 denotes an oscillator control circuit, which operates and stops the oscillation of the oscillator 50 for measuring switching element characteristics by turning on and off the power supply line 68 using a switch 85 in response to a control signal 66 from a measurement control circuit included in 53; A gate voltage v is supplied to the switching element 80 for detecting characteristics on the liquid crystal pulse. Characteristic detection oscillator 50
consists of amplifiers 82, 83, 84, an additional capacitance 81, a stray capacitance 88, and a switching element 80 acting as a resistance element, and oscillates at a frequency that is a function of the conductance of the switching element. Oscillation output 6
1 is converted into a pulse train proportional to the oscillation frequency by the detection signal control gate 51 and sent to the counter section 52 at 62.
Input from The counter section 52 counts the oscillation frequency and inputs the signal 63 to the processing circuit 53. The processing circuit 53 has a circuit for generating a signal 64 for controlling the clock generating circuit 54 from the count signal 63, and the aforementioned measurement control circuit. In this embodiment, the clock generation circuit 54 is constructed mainly of a preset counter 89, and the processing circuit 53 performs arithmetic processing on the counter output and supplies a signal 64 to the preset counter 89. Arithmetic processing may be performed using a microprocessor, an arithmetic circuit combining random logic gates, or a logic circuit using a programmable logic array.
A basic variable clock signal 65 generated by the clock generation circuit 54 is appropriately frequency-divided by a frequency divider 55 and supplied to each circuit.

As explained above, a detection circuit is provided to detect the characteristics of the switching elements on the liquid crystal panel, and the detection output of the detection circuit controls the clock circuit to determine the fundamental period T and the reference signal supplied to the liquid crystal panel. The present invention, which optimizes the selection period t, only requires the addition of a few circuits, so the manufacturing burden is extremely small. It is an extremely effective technology that can maintain constant quality and reduce manufacturing costs since it requires only a small manufacturing margin.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the built-in switching element system.
Figure 2 is an equivalent circuit diagram of liquid crystal layer impedance, Figure 3 is a diagram of gate control signals and signal waveforms applied to the liquid crystal layer in a switching element built-in type, and Figure 4 is a perspective view of a liquid crystal display panel with a built-in switching element. 5 is a partial sectional view of FIG. 4, FIG. 6 is a partial plan view of FIG. 4, FIG. 7 is a switching element characteristic diagram,
8 and 9 are block diagrams of the conventional drive circuit and the present invention, and FIG. 10 is a specific circuit diagram of the present invention. 21... Liquid crystal display panel with built-in switching element, 28... Detection circuit, 29... Variable clock circuit.

Claims (1)

[Claims] 1. A liquid crystal display panel having a liquid crystal layer sandwiched between a substrate provided with a switching element and a substrate having at least one electrode, and a drive circuit section that applies a drive signal to the liquid crystal layer through the electrode. A liquid crystal display device, characterized in that the liquid crystal display device is provided with a detection means for detecting a change in the characteristics of the switching element, and further includes means for changing the basic period of a drive signal based on the detected information.