JPH06314076A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain an always optimum display contrast with the liquid crystal display device having an automatic contrast adjusting function by assuring the driving voltage necessary for an LCD (liquid crystal element) even in a low-temp. region. CONSTITUTION:This liquid crystal display device is provided with a first control circuit A which has a thermistor (temp. detecting element) TH for detecting the temp. of the LCD and sets the driving voltage of the LCD in accordance with the output value of this thermistor TH and a second control circuit B which compares the output value of this thermistor TH and the voltage values preset by resistors R4 to R7 by an OP amplifier Q3 and sets the driving voltage in the low temp. region of the LCD by the output meeting the result of the comparison. The output sides of the respective control circuits A, B are connected by diodes D1, D2 so that the output Vo is switched to either Voa or Vob at a certain temp. in the low temp. region.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device having an automatic contrast adjusting function, and more particularly to detecting the temperature of a liquid crystal element by a temperature detecting element such as a thermistor and automatically setting a driving voltage of the liquid crystal element. The present invention relates to a liquid crystal display device.

[0002]

2. Description of the Related Art FIG. 11 is a diagram showing a main part of a general auto contrast (contrast automatic adjustment) circuit of a conventional liquid crystal display device. This circuit divides a negative voltage of -10V with resistors R1 to R3, and a resistor R2 at the center of the series circuit is used.
And a thermistor TH, which is a temperature detection element of an LCD (liquid crystal element), are connected in parallel. Then, the output of this thermistor TH is passed through an OP amplifier (operational amplifier) Q1 which constitutes a voltage follower, and a driver IC of the LCD.
Is supplied to the LCD, and the LCD is driven by the drive voltage based on the output voltage Vo. Further, a series circuit of a diode D3 connected to a + 5V power source and resistors R10 to R14 is connected to the output side, and the voltage output at each connection point is also supplied to the driver IC. .

FIG. 12 is a VT characteristic diagram showing the relationship between the negative voltage (V) for driving the LCD and the temperature (° C.) of the LCD. The solid line shows the calculated value (Typical) by the circuit, and the dotted line shows the measurement. Indicates the value. As you can see from this figure, 0
Stable characteristics can be obtained at temperatures above ℃, and sufficient display contrast can be obtained for consumer use.

Next, V-when using STN-LCD
The T characteristic is shown in FIG. As shown in the figure, it can be seen that the driving voltage (negative voltage) required for optimum contrast rapidly increases in the low temperature range of 0 ° C. or lower. The calculated value indicated by the solid line in this figure is a value calculated by the above-described auto contrast circuit, and obviously does not follow in the low temperature range, and the voltage is completely insufficient.

[0005]

In the conventional liquid crystal display device, since the auto-contrast circuit is constructed as described above, a good display contrast can be obtained in the normal temperature range, but it is 0 ° C. or less. In the low temperature range of, the calculated value does not follow, and the drive voltage may be insufficient,
There is a problem that the optimum display contrast may not be obtained.

The present invention has been made in view of the above problems, and the calculated value and the measured value of the driving voltage match even in a low temperature range, and the necessary driving voltage is constantly supplied to obtain the optimum display contrast. It is an object of the present invention to provide a liquid crystal display device that can obtain the above.

[0007]

A liquid crystal display device of the present invention has a temperature detecting element for detecting the temperature of a liquid crystal element, and sets a drive voltage of the liquid crystal element according to an output value of the temperature detecting element. A first control circuit, and a second control circuit that sets a drive voltage in a low temperature range of the liquid crystal element based on an output value of the temperature detection element and a preset value.
The setting voltage of the control circuit and the setting voltage of the second control circuit can be switched at a predetermined temperature in a low temperature range.

[0008]

In the liquid crystal display device of the present invention, the control circuit for setting the driving voltage of the liquid crystal element in the low temperature range is provided, and when the temperature of the liquid crystal element detected by the temperature detecting element falls below a certain temperature, This control circuit is selected from the circuits of, and the liquid crystal element is driven by the set voltage by the control circuit.

[0009]

1 is a circuit diagram showing a first embodiment of the present invention, showing the configuration of an auto contrast circuit of a liquid crystal display device, and the same reference numerals as those in FIG. 11 represent the same components. In the figure, A is a first control circuit that sets the drive voltage of the LCD according to the output value of a thermistor (temperature detection element) TH that detects the temperature of the LCD (liquid crystal element), and B is the output value of the thermistor. A second control circuit that sets a drive voltage in a low temperature range of the LCD based on a value (voltage value) set in advance by the resistors R4 to R7.
The diodes D1 and D2 are connected to the respective output sides so that the set voltage and the set voltage of the second control circuit B can be switched at a predetermined temperature in a low temperature range.

The first control circuit A includes a series circuit of resistors R1 to R3 for dividing a negative voltage of -20V, a thermistor TH connected in parallel to a resistor R2 in the center of the series circuit, and an OP amplifier forming a voltage follower. It is composed of Q1, and the diode D1 is connected to the output side thereof.

The second control circuit B forms a voltage follower with a resistor R7 connected in series with a resistor R4 to R6 for voltage division similar to the above and a resistor R5 in the center.
P amplifier Q2, comparison OP amplifier Q3 and resistor R
8 and R9, and the diode D2 is connected to the output side thereof.

The anode sides of the diodes D1 and D2 are connected to each other, and a series circuit of a diode D3 and resistors R10 to R14 similar to that shown in FIG. 11 is connected to this, and the output at each connection point is the LCD. It is supplied to the driver IC.

Next, the operation will be described. The first control circuit A is an auto-contrast circuit for the normal temperature range and the high temperature range, which is the same as the conventional one, and the second control circuit B is a circuit newly added for low-temperature countermeasures. Then, the second control circuit B makes an improvement in the low temperature range, and a voltage sufficient for driving the LCD is secured even in the low temperature range.

That is, the output Voa of the first control circuit A
, The voltage Vra determined by the resistance value of the thermistor TH and the resistors R1 to R3 is output as it is. The value of the voltage Vra from the thermistor TH changes with the temperature change of the thermistor TH, and is a value corresponding to the temperature of the thermistor TH. At the same time, the output Vob of the second control circuit B outputs a voltage obtained by multiplying the potential difference between the voltage Vra (= Voa) and the output voltage Vrb of the OP amplifier Q2 by R9 / R8. Then, the final output Vo has a diode D1,
The lower voltage of Voa and Vob is selected by D2, and the LCD is selected according to the selected voltage.
Drive voltage is determined.

The point at which the selected voltage Voa and Vob are switched is determined by the setting of the voltage Vrb of the second control circuit B. When Vra ≧ Vrb, Voa ≦ Vo
The voltage Voa is selected as b, and when Vra <Vrb, Voa> Vob and the voltage Vob is selected.

At this time, resistors R4 to R6 are set as circuit conditions.
And resistors R1 to R3 are equal, and R1 = R4 and R2
= R5, R3 = R6, when the value of the resistor R7 and the resistance value of the thermistor TH are equal, the voltages Vra and Vrb are
Have the same value. Therefore, by setting the resistance value of the thermistor TH at a certain temperature to the value of the resistor R7, it is possible to switch between the voltages Voa and Vob at that temperature.

That is, it is possible to set Vo = Voa above a certain set temperature in the low temperature range and Vo = Vob below the set temperature. Therefore, as shown in FIG. 2, the calculated value of the LCD drive voltage and the actual measured value can be substantially matched from the low temperature range of −20 ° C. to the high temperature range of + 80 ° C. Can be supplied to obtain the optimum display contrast.

Here, in practice, there are variations in the thermistor TH, the resistors R1 to R9, and the negative power source, so that a circuit error occurs. In particular, the deviation of the switching point due to the dispersion of the resistance and the error of the output voltage caused by the slight change of the negative power source are very large. An example thereof is shown in FIGS. 3 and 4.

FIG. 3 shows the shift of the switching point due to the variation of the resistance value. Even though only the resistors R4 to R6 vary only ± 1%, the switching point greatly shifts. . The solid line and the dotted line in the figure show the above-mentioned calculated value and the measured value, the broken line shows the lower limit calculated value (Minimum), and the chain line shows the upper limit calculated value (Maximum). Further, FIG. 4 shows an error of the output voltage when the tolerance of the negative power source of −20V is ± 0.5V.

Therefore, in order to absorb the shift of the switching point, the resistor R3 in FIG. 1 is replaced with a fixed resistor connected to a variable resistor, and Vr at the switching point is changed.
It is effective to perform the initial adjustment so that the voltages a and Vrb become equal. As a result, the shift of the switching points can be greatly improved as shown in FIG.

However, set value ± 0.1 for negative power supply
There is no method other than the initial adjustment so that it will be within V.
Therefore, in the end, the error is always ±
If you try to put it within 0.3V, the tolerance of each resistor is ±
1%, the tolerance of the negative power supply becomes ± 0.1V. The total error is shown in FIG.

FIG. 7 shows an error due to the variation of the thermistor TH. 1 at 25 ° C for the thermistor TH
If a resistance value of 00 KΩ is used, the resistance value R at that temperature is not required to be more accurate than R = 100 KΩ ± 3%, and even if it is ± 3% at 25 ° C., it is actually −30 ° C. to + 8%.
There is an error of ± 5% or more in the range of 0 ° C. But,
Since the thermistor TH is connected in parallel with the resistor R2,
The output error due to the variation in the thermistor TH is so small that it can be ignored as shown in FIG.

Even if the auto-contrast circuit operates correctly, it does not make any sense if the temperature of the LCD and the temperature detected by the thermistor TH do not match in any case when actually assembled in the module. For example, it is considered that there is almost no temperature difference between the LCD and the thermistor when the switch is off, but when the temperature of the LCD rises due to the bulb illumination after the switch turns on, the temperature of the thermistor TH also does not rise. Is good just after the switch is turned on, but the proper drive voltage is
It will not cover D. Therefore, it is necessary to mount the thermistor TH in an appropriate position and method so that this may not occur.

FIG. 8 is a circuit diagram showing a second embodiment of the present invention, and the same reference numerals as those in FIG. 1 indicate the same components.
In the above-mentioned embodiment, the calculated value and the measured value deviate from each other in a low temperature range of -20 ° C or lower. Therefore, in this embodiment, the resistor R9 of FIG. 1 is divided into resistors R15 and R16, and a series circuit of the resistor R17 and the diode D5 is connected between the dividing point and the output side of the OP amplifier Q2.

In this way, the resistor R9 is appropriately divided, and the circuit of the resistor R17 and the diode D5 is bypass-connected between the dividing point and the resistor R8 to form a circuit having three characteristics, as shown in FIG. Thus, the calculated value and the measured value match even at -20 ° C or lower.

FIG. 10 is a circuit diagram showing a third embodiment of the present invention, and the same reference numerals as those in FIG. 1 indicate the same components. In this embodiment, the third control circuit C is provided, and the voltage in the low temperature region where the calculated value and the measured position do not match in FIG. 2 is set by the resistors R18 to R21, and the output voltage is changed from Vob to Voc at the switching point. It is intended to switch to.

That is, the above resistors R18 to R21
R1 = R4 = R18, R2 = R5 = R19, R3 =
It is set so that R6 = R20, and has a similar configuration in which a resistor R21 is connected in parallel with the central resistor R19. Then, the output voltage Vob of the second control circuit B
And an output voltage Vrc of the resistance circuit are compared with each other, and an OP amplifier Q4 is provided.
The output voltage Voc multiplied by R23 / R22 is selected by the diode D4.

The second control circuit B and the third control circuit C
Is similar to the relationship between the first control circuit A and the second control circuit B, and operates in exactly the same way according to the relationship between the voltages Vob and Vrc. Therefore, by setting an appropriate switching point, the output voltage Vo can be reduced to Vob at a temperature lower than a certain temperature.
Can be changed from Voc to Voc, and the optimum display contrast can be obtained even in a low temperature range.

[0029]

As described above, according to the present invention, the LCD based on the output value of the temperature detecting element and the preset value.
A control circuit that sets the drive voltage in the low temperature range of
Since the drive voltage of D is switched to the set voltage of this control circuit at a certain temperature in the low temperature range or less, the calculated value of the drive voltage of the auto-contrast circuit matches the measured value even in the low temperature range, and the always required drive voltage is set to LCD. To obtain an optimum display contrast.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the relationship between calculated values and measured values of the circuit of FIG.

FIG. 3 is an explanatory diagram showing an error due to a variation in resistance value of the circuit of FIG.

FIG. 4 is an explanatory diagram showing an error due to a fluctuation of a negative power supply in the circuit of FIG.

5 is an explanatory diagram showing an error of the circuit of FIG. 1 after initial adjustment.

6 is an explanatory diagram showing a total error of the circuit of FIG.

7 is an explanatory diagram showing an error due to variations in the thermistor of the circuit of FIG.

FIG. 8 is a circuit diagram showing a second embodiment of the present invention.

9 is an explanatory diagram showing the relationship between calculated values and measured values of the circuit of FIG.

FIG. 10 is a circuit diagram showing a third embodiment of the present invention.

FIG. 11 is a circuit diagram showing a conventional example.

12 is an explanatory diagram showing voltage-temperature characteristics of the circuit of FIG.

FIG. 13 is an explanatory diagram showing voltage-temperature characteristics of STN-LCD.

[Explanation of symbols]

 A first control circuit B second control circuit C third control circuit TH Thermistor

[Procedure amendment]

[Submission date] March 1, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

That is, Vo = Voa + V _F above a certain set temperature in the low temperature region, and Vo = Vob + V _F below that.
(V _F : diode forward voltage) .
Therefore, as shown in FIG. 2, from the low temperature range of −20 ° C.,
The calculated value of the LCD drive voltage and the actual measured value can be made to almost match up to a high temperature range of 80 ° C, and the LCD always needs
It is possible to obtain the optimum display contrast by supplying the drive voltage of.

[Procedure Amendment 2]

[Document name to be corrected] Drawing

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[Figure 3]

[Procedure 3]

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[Figure 4]

[Procedure amendment 4]

[Document name to be corrected] Drawing

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[Figure 5]

[Procedure Amendment 5]

[Document name to be corrected] Drawing

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[Figure 6]

[Procedure correction 6]

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[Figure 7]

[Procedure Amendment 7]

[Document name to be corrected] Drawing

[Name of item to be corrected] Fig. 10

[Correction method] Change

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[Figure 10]

Claims (1)

[Claims] 1. A first control circuit having a temperature detection element for detecting the temperature of a liquid crystal element, and setting a drive voltage of the liquid crystal element according to an output value of the temperature detection element; and an output of the temperature detection element. A second control circuit for setting a drive voltage of the liquid crystal element in a low temperature range based on a value and a preset value, the set voltage of the first control circuit and the set voltage of the second control circuit A liquid crystal display device characterized in that it is configured such that and can be switched at a predetermined temperature in a low temperature range.