JPH11142809A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display device capable of preventing the occurrence of malfunction at the reset operation or the like of a power supply in a personal computer by connecting a discharging means to a power supply line. SOLUTION: In a liquid crystal display device 10 provided with a DC-DC converter for generating voltage of a voltage level different from power supply voltage supplied from the exterior through a power supply line 16, a discharging means 17 is connected to the power supply line 16 to which power supply voltage is supplied from the exterion. The means 17 is a discharging resistor connected between the line 16 and a reference power supply line to which reference potential is supplied, a switching element to be conducted when the voltage of the line 16 is dropped to prescribed voltage or less or a switching element to be conducted when the power supply voltage supplied from the exterion is interrupted.

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, and more particularly, to a liquid crystal display device in which a power supply is momentarily cut off and turned on again, that is, a liquid crystal display device in which a malfunction such as a so-called power supply reset operation is prevented.

[0002]

2. Description of the Related Art A simple matrix type liquid crystal display device or an active matrix type liquid crystal display device having an active element (for example, a thin film transistor) for each pixel and switchingly driving the active element is used as a display device of a notebook personal computer or the like. Widely used. As one of these liquid crystal display devices, an STN type or a TFT type liquid crystal display module (LCM) is known.

This liquid crystal display module is provided with display data, display control signals (clock signal, horizontal synchronizing signal, vertical synchronizing signal, display timing signal) and display data from an external unit, for example, a personal computer (hereinafter referred to as a personal computer). The power supply voltage is supplied. The liquid crystal display module has a DC-DC converter. The DC-DC converter boosts a power supply voltage supplied from outside, and uses the boosted voltage to apply a liquid crystal to each pixel of the liquid crystal display panel. For example, in the case of a TFT type liquid crystal display module, a driving voltage is generated, such as a video signal voltage supplied to a drain signal line or a common voltage supplied to a common electrode (counter electrode).

[0004] Such a technique is described, for example, in Japanese Patent Application No. 7-169648.

[0005]

In a personal computer, there is a case where a so-called power-supply resetting operation, that is, a momentary power-off and power-on, is required. However, the conventional liquid crystal display module sometimes malfunctions at the time of resetting the power supply. That is, during the reset operation of the power supply, the power supply voltage supplied from the personal computer to the liquid crystal display module is temporarily cut off, and after a predetermined time, the power supply voltage is supplied again from the personal computer to the liquid crystal display module. However, even when the power supply voltage is not supplied from the personal computer to the liquid crystal display module, the voltage of the power supply line (hereinafter referred to as a power supply line) in the liquid crystal display module does not immediately decrease, but decreases at a predetermined time constant. I do. Therefore, before the voltage of the power supply line in the liquid crystal display module does not drop to a predetermined voltage,
In some cases, the power supply voltage was supplied again from the personal computer body side, which caused the liquid crystal display module (LCM) to malfunction.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a liquid crystal display device with a discharge means provided on a power supply line to reset a power supply of a personal computer. It is an object of the present invention to provide a technique capable of preventing a malfunction at the time.

[0007] The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0008]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows.

In a liquid crystal display device provided with a DC-DC converter for generating a voltage having a voltage level different from the power supply voltage based on a power supply voltage supplied from the outside via a power supply line, an external power supply voltage is supplied. The power supply line is provided with discharge means.

That is, according to the present invention, there is provided a switching resistor which is turned on when a voltage of a power supply line becomes lower than a predetermined voltage between a power supply line and a reference power supply line to which a reference potential is supplied. Or a switching element that conducts when a power supply voltage supplied from the outside is cut off.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a TFT type liquid crystal display module will be described below in detail with reference to the drawings.

In all the drawings for describing the embodiments, those having the same functions are denoted by the same reference numerals, and their repeated description will be omitted.

FIG. 1 is a block diagram showing a schematic configuration of a TFT type liquid crystal display module according to an embodiment of the present invention. Liquid crystal display module (LCM) 10 of the present embodiment
A horizontal scanning driver (drain driver) 13 is disposed above a liquid crystal display panel (TFT-LCD) 15, and a vertical scanning driver (gate driver) 14 and a power supply circuit 11 are disposed on side surfaces of the liquid crystal display panel 15. , A timing controller 12. In FIG. 1,
Horizontal scanning driver 13 and vertical scanning driver 14
Is shown by one block, but in practice, the horizontal scanning driver 13 and the vertical scanning driver 14
Is composed of a plurality of LSIs (semiconductor integrated circuits).
In FIG. 1, reference numeral 20 denotes a personal computer main body, and reference numeral 21 denotes a power control switch (SW).

FIG. 2 is a diagram showing an equivalent circuit of one example of the liquid crystal display panel 15 shown in FIG.

As shown in FIG. 2, the liquid crystal display panel 1
5 has a plurality of pixels formed in a matrix.
Each pixel includes two adjacent signal lines (a drain signal line (D) or a gate signal line (G)) and two adjacent signal lines (a gate signal line (G) or a drain signal line (D)). And is arranged in the intersection area with. Each pixel has a thin film transistor (TFT1, TFT2), and the source electrode of the thin film transistor (TFT1, TFT2) of each pixel is connected to the pixel electrode (ITO1). In addition, the pixel electrode (ITO1) and the counter electrode (or common electrode) (IT
O2), a liquid crystal layer is provided, so that the pixel electrode (I
A liquid crystal capacitor (CLC) is equivalently connected between the common electrode (TO1) and the common electrode (ITO2). Further, an additional capacitance (CADD) is connected between the source electrodes of the thin film transistors (TFT1 and TFT2) and the preceding gate signal line (G). In FIG. 2, AR is a display area.

In the liquid crystal display panel 15 shown in FIG.
The thin film transistor (TF) of each pixel arranged in the column direction
The drain electrode of T) is a drain signal line (D)
, And each drain signal line (D) is connected to a horizontal scanning driver 13 for applying a gradation voltage to the liquid crystal of each pixel in the column direction.
Connected to. The gate electrodes of the thin film transistors (TFTs) in the pixels arranged in the row direction are connected to gate signal lines (G), respectively. Each gate signal line (G) is connected to each pixel in the row direction for one horizontal scanning time. Is connected to a vertical scanning driver 14 that supplies a scanning drive voltage (positive bias voltage or negative bias voltage) to the gate electrode of the thin film transistor (TFT).

The timing controller 12 shown in FIG.
Is composed of one semiconductor integrated circuit (LSI), and the display data (R) transmitted from the computer body side.
G.B) and a display control signal (clock signal, display timing signal, horizontal synchronization signal, vertical synchronization signal) to control and drive the horizontal scanning driver 13 and the vertical scanning driver 14.

The horizontal scanning driver 13 latches display data for one horizontal scan sent from the timing controller 12 based on the display data latch clock signal sent from the timing controller 12, and sends out the data from the timing controller 12. A grayscale voltage corresponding to the latched display data is output to each drain signal line (D) based on the output timing control clock signal. The vertical scanning driver 14 includes a plurality of thin film transistors connected to each gate signal line (G) of the liquid crystal display panel 15 based on a frame start instruction signal sent from the timing controller 12 and a shift clock signal having one horizontal scanning time period. (TFT) are sequentially turned on for one horizontal scanning time. With the above operation, the liquid crystal display panel 1
An image is displayed in 5.

FIG. 3 is a block diagram showing a schematic configuration of the power supply circuit 11 shown in FIG. As shown in the figure, the power supply circuit 11 includes a DC-DC converter 31, a gradation reference voltage generation circuit 32, a common electrode voltage generation circuit 33, and a gate electrode voltage generation circuit 34. The grayscale reference voltage generation circuit 32 generates a grayscale reference voltage used when generating a grayscale voltage corresponding to display data in the horizontal scanning driver 13 based on the output voltage from the DC-DC converter 31. Then, the output is output to the horizontal scanning driver 13. Generally, the gray scale reference voltage generation circuit 32 includes a positive voltage generation circuit that generates a gray scale reference voltage of a positive polarity, and a negative voltage generation circuit that generates a gray scale reference voltage of a negative polarity. The common electrode voltage generation circuit 33 includes the DC-DC converter 31
A driving voltage to be applied to the counter electrode (ITO2) is generated based on the output voltage from
Generates a drive voltage (positive bias voltage and negative bias voltage) to be applied to the gate electrode of the thin film transistor (TFT) based on the output voltage from the DC-DC converter 31.

FIG. 4 is a block diagram showing a schematic configuration of an example of the DC-DC converter 31 shown in FIG. A DC-DC converter 31 shown in FIG. 4 includes a switching control circuit 41 and a switching element (npn transistor) 4.
2 is turned on and off, thereby generating a high voltage induced voltage in the coil 43, and rectifying and smoothing the high voltage induced voltage by the diode 44 and the capacitor 45 and outputting the rectified and smoothed voltage. .

FIG. 5 is a diagram for explaining the operation when the switching control circuit 41 shown in FIG. 4 is started and when the operation is stopped. When the power supply voltage (VDD) is applied as shown by the solid line at the time of activation of the switching control circuit 41, the switching control circuit 41 sets the power supply voltage (VDD) to the first threshold voltage (VTH). Fires when crossing). When the operation of the switching control circuit 41 is stopped, the power supply voltage (VDD) as shown by a dotted line in FIG.
Is applied, the switching control circuit 41 stops operating when the power supply voltage (VDD) falls below the second threshold voltage (VTL). Generally, the switching control circuit 41 is provided as a protection measure against overvoltage.
An overvoltage protection circuit that stops operation when an overvoltage is applied is incorporated.

In FIG. 1, the power control switch (SW) 21 is controlled by the personal computer body 20 to reset the power, that is, the power control switch (SW) 21 is controlled from ON to OFF to ON. Consider the case. In this case, the voltage (VC) of the power supply line 16 in the liquid crystal display module 10 changes as shown in FIG. In the power-on state, the voltage (VC) of the power supply line 16 is VC = VD
D, electric charges are accumulated in the capacitance component 18 connected to the power supply line 16. Thereafter, the power control switch (S
W) 21 is turned off, and when the power supply voltage (VDD) is not supplied from the personal computer body 20 side, the charge of the capacitance component 18 is discharged via the resistance component 19 in the liquid crystal display module 10 shown in FIG. .

FIG. 7 shows the liquid crystal display module 1 shown in FIG.
It is a schematic diagram showing the capacitance component and the resistance component in 0. In the figure, reference numeral 17 denotes a discharge resistor constituting discharge means;
Denotes a capacitance component in the liquid crystal display module 10, and 19 denotes a resistance component in the liquid crystal display module 10. Here, the capacitance component 18 represents a capacitance component obtained by combining all capacitors connected to the power supply line 16 or a floating capacitor connected to the power supply line 16, and Ra is a resistance value of the discharge resistor 17. , Cin are the capacitance value of the capacitance component 18, R
in represents the resistance value of the resistance component 19. In this case, the resistance value (Rin) of the resistance component 19 depends on the liquid crystal display module 1.
Although it varies depending on the size of 0 (in other words, the size of the liquid crystal display panel 15), the value is approximately several hundreds KΩ to several MΩ.

When the electric charge of the capacitance component 18 is discharged through the resistance component 19 in the liquid crystal display module 10,
As shown by the dotted line in FIG. 6, the voltage (VC) of the power supply line 16 decreases exponentially according to the exponential function shown in the following equation (1), and approaches the ground potential (GND).

[0025]

## EQU00001 ## VCC = VDD.exp (-t / (Cin.Rin)) (1) Here, when the resistance value (Rin) of the resistance component 19 is large, the discharge time becomes long. Power control switch (S
If the period (Ta) in which the W) 21 is off is short, as shown by the dotted line in FIG. 6, the power control switch (SW) 21
During the period (Ta) when the power supply line 16 is off, the power supply line 16
May not be reduced to a voltage equal to or lower than the second threshold voltage (VTL) of the DC-DC converter 31 in some cases. In addition, during a period (Ta) in which the resistance value (Rin) of the resistance component 19 is not so large and the power supply control switch (SW) 21 is off, the voltage (VC) of the power supply line 16 is changed to a DC-DC converter. Even if the voltage can be reduced to a voltage equal to or lower than the second threshold voltage (VTL) of the PC 31, for example, as shown in FIG. In this case, the voltage (VC) of the power supply line 16 is changed during the period (Ta) in which the power control switch (SW) 21 is off.
The second threshold voltage of the DC-DC converter 31 (VT
L) In some cases, the voltage did not decrease to the following.

When the voltage (VC) of the power supply line 16 is DC-
If the voltage has not dropped below the second threshold voltage (VTL) of the DC converter 31, the DC-DC converter 31 is still operating. However, the power supply voltage (VD
D) is equal to or less than the first threshold voltage (VTH), and DC-D
The C converter 31 regards it as an abnormal state, and after a predetermined time,
When the overvoltage protection circuit of the DC-DC converter 31 is activated, D
The operation of the C-DC converter 31 stops. In this state, when the power supply control switch (SW) 21 is turned on and the power supply voltage (VDD) is supplied from the personal computer body 20 side, when viewed from the DC-DC converter 31, the second threshold voltage ( (VTL) After that, since the power is not turned on, the DC-DC converter 31 keeps its operation stopped. When the DC-DC converter 31 stops operating, no output voltage is output from the DC-DC converter 31, so that both ends of the liquid crystal layer of the liquid crystal display panel 15 are provided.
No voltage is applied.

Therefore, if the liquid crystal display module 10 malfunctions and the liquid crystal display panel 15 is a normally white type liquid crystal display panel, the display surface of the liquid crystal display panel 15 is white or the liquid crystal display panel 15 is normally black. Liquid crystal display panel, a liquid crystal display panel 1
In some cases, the display surface of No. 5 was black. Then, in order to return from the malfunction state to the normal state, the power supply control switch (SW) 21 is turned off, the power supply control switch (SW) is reduced to the second threshold voltage (VTL) or less of the DC-DC converter converter 31, and the power supply control switch (SW) is turned off. SW) 21 had to be turned on.
As described above, in the conventional liquid crystal display module 10, the DC-DC
When the converter 31 stops, the liquid crystal display module may malfunction.

However, in this embodiment, since the discharge resistor 17 is connected between the power supply line 16 and the ground line to which the ground potential (GND) is supplied, the power supply control switch (SW) 21 is Turns off and PC body 2
When the power supply voltage (VDD) is no longer supplied from the 0 side, the charge of the capacitance component 18 becomes as shown by the solid line in FIG.
It decreases exponentially according to the exponential function shown in the following equation (2), and approaches the ground potential (GND).

[0029]

[Mathematical formula-see original document] VCC = VDD * exp (-t / (Cin * (RP)) where RP = Rin * Ra / (Rin + Ra) (2) where the discharge resistor 17 Is the resistance value (Rin) of the resistance component 19 in the liquid crystal display module 10.
If the value is sufficiently smaller than (Ra 充分 Rin), the electric charge of the capacitance component 18 is discharged through the discharging resistor 17, so that the discharging time is substantially equal to the resistance value (Ra of the discharging resistor 17). ), The discharge time can be shortened.

Therefore, in the present embodiment, as shown by the solid line in FIG. 6, within the period (Ta) in which the power supply control switch (SW) 21 is off, the period is shorter than the period (Ta). , The voltage of the power supply line 16 (VCC)
Second threshold voltage (VTL) of C-DC converter 31
It is possible to reduce the voltage to the following. Therefore, after the period (Ta), the power control switch (SW) 21
Is turned on, and the power supply voltage (VD
Even if D) is supplied, the overvoltage protection circuit of the DC-DC converter 31 operates and the DC-DC converter 31 does not stop operating, so that the liquid crystal display module 10 does not malfunction. Thus, in the present embodiment, it is possible to prevent a malfunction at the time of resetting the power supply,
Power control switch (SW) 2 during power reset operation
1 can be shortened in the off period (Ta).

It should be noted that the resistance value (R
a) is not uniquely determined, and is a preset off period (T) of the power control switch (SW) 21.
a) Based on the capacitance value (Cin) of the capacitance component 18 and the resistance value (Rin) of the resistance component 19, the power supply line 1
6 is determined so that the voltage (VCC) of the DC-DC converter 6 can be reduced to a voltage equal to or lower than the second threshold voltage (VTL) of the DC-DC converter 31.
The optimum resistance value (Ra) may be determined in consideration of the power consumption consumed by the discharging resistor 17 during normal operation (when the power control switch (SW) 21 is on). An example of the resistance value (Ra) of the discharge resistor 17 is the resistance value (Rin) of the resistance component 19 in the liquid crystal display module 10.
However, if it is several 100 KΩ, it is preferably about several KΩ.

As described above, in the liquid crystal display module 10 of the present embodiment, it is possible to prevent an erroneous operation when the power supply is momentarily shut off and turned on, for example, at the time of resetting the power supply of the personal computer. Further, in the liquid crystal display module 10 of the present embodiment, the period during which the power is shut off (Ta in FIG. 6), such as when resetting the power of the personal computer, can be shortened as compared with the conventional example. .

As the discharging means, in addition to the discharging resistor 17 shown in FIG. 1, for example, a switching element as shown in FIGS. 9 and 10 can be used. In the example shown in FIG. 9, the power supply line 16 is connected to a ground potential (GND).
When a switching element (npn transistor) 51 is connected via a resistor between the power supply control switch (SW) 21 and the power supply control switch (SW) 21 on the personal computer main body 20 side, the power supply control switch (SW) 21 The switching signal (npn transistor) 51
Are turned on, and the voltage (VC) of the power supply line 16 is
This is to quickly reduce the voltage to a voltage lower than the second threshold voltage (VTL) of the DC-DC converter 31.

The example shown in FIG.
A switching element 51 is connected through a resistor between the power supply line 16 and a ground line to which the ground potential (GND) is supplied. The voltage monitoring / control circuit 52 constantly monitors the voltage (VC) of the power supply line 16. , Power supply line 16 voltage (VC)
When the power supply voltage (VDD) is 3.3 V or less, for example, when the voltage (VC) of the power supply line 16 becomes 2.5 V or less, the switching element 51 is turned on. State, the voltage (VC) of the power supply line 16 is quickly changed to the second voltage of the DC-DC converter 31.
The threshold voltage (VTL) is lowered to a voltage lower than the threshold voltage (VTL). The above-mentioned resistance is the switching element 51.
It is needless to say that the switching element 51 is not limited to an npn transistor.

Here, the voltage monitoring / control circuit 52 has a circuit configuration operating at a voltage lower than the power supply voltage (VDD). For example, if the power supply voltage (VDD) is 3.3 V, the voltage monitoring and control circuit
It must be configured with a circuit specification that operates on. As the voltage monitoring / control circuit 52, for example, a known circuit such as a watchdog circuit can be used.
Since a current always flows through the discharge resistor 17 shown in FIG. 1, the power consumption of the liquid crystal display module 10 increases.
Since the switching element 51 shown in FIG. 9 or FIG.
By using the switching element 51 shown in (1), an increase in power consumption of the liquid crystal display module 10 can be suppressed.

In the above-described embodiment, the present invention is applied to T
Although the embodiment applied to the FT type liquid crystal display module has been described, the present invention is not limited to this, and it is needless to say that the present invention is also applicable to the STN type liquid crystal display module.

As described above, the invention made by the present inventor is:
Although a specific description has been given based on the above-described embodiment, the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist of the invention.

[0038]

The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows.

(1) According to the present invention, the discharging means is provided on the power supply line in the liquid crystal display device. Therefore, when the supply of the power supply voltage to the liquid crystal display device is interrupted, the power supply line is connected to the power supply line. The electric charge accumulated in the capacitance component can be quickly discharged, and the time until the power is turned on again can be shortened.

(2) According to the present invention, it is possible to prevent a malfunction of the liquid crystal display device during a reset operation of the power supply of the personal computer or the like.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of a TFT-type liquid crystal display module according to an embodiment of the present invention.

FIG. 2 is a diagram showing an equivalent circuit of an example of the liquid crystal display panel shown in FIG.

FIG. 3 is a block diagram illustrating a schematic configuration of a power supply circuit illustrated in FIG. 1;

FIG. 4 is a block diagram illustrating a schematic configuration of an example of the DC-DC converter illustrated in FIG. 3;

FIG. 5 is a diagram for explaining an operation when the switching control circuit shown in FIG. 4 is started and when the operation is stopped.

FIG. 6 is a diagram for explaining a voltage change of a power supply line during a power supply reset operation.

FIG. 7 is a schematic diagram showing a capacitance component and a resistance component in the liquid crystal display module shown in FIG.

FIG. 8 is a diagram for explaining a malfunction of the liquid crystal display module during a reset operation of the power supply of the personal computer.

FIG. 9 is a block diagram illustrating a schematic configuration of an example of a discharging unit using a switching element in the first embodiment.

FIG. 10 is a block diagram showing a schematic configuration of another example of a discharging unit using a switching element in the first embodiment.

[Explanation of symbols]

Reference Signs List 10: liquid crystal display module (LCM), 11: power supply circuit, 12: timing controller, 13: horizontal scanning driver (drain driver), 14: vertical scanning driver (gate driver), 15: liquid crystal display panel (TF)
T-LCD), 16 power supply line, 17 resistance for discharge, 18 capacitance component, 19 resistance component, 20 personal computer body, 21 power control switch (SW), 22, 45 ...
Capacitor, 31 ... DC-DC converter, 32 ... Grayscale reference voltage generation circuit, 33 ... Counter electrode voltage generation circuit, 34
... Gate electrode voltage generation circuit, 41 switching control circuit, 42, 51 switching element (npn transistor), 43 coil, 44 diode, 52 voltage monitoring / control circuit, D drain signal line, G gate signal Line, TFT: thin film transistor, ITO1: pixel electrode,
ITO2: counter electrode (common electrode), CLC: liquid crystal capacitance,
CADD: Additional capacity.

Claims (4)

[Claims] 1. A liquid crystal display device comprising a DC-DC converter for generating a voltage of a voltage level different from the power supply voltage based on a power supply voltage supplied from the outside via a power supply line, comprising: A liquid crystal display device characterized in that discharge means is provided on a power supply line to which is supplied. 2. The liquid crystal display device according to claim 1, wherein said discharging means is a resistor connected between a power supply line and a reference power supply line to which a reference potential is supplied. 3. The discharging means constantly monitors a switching element provided between a power supply line and a reference power supply line to which a reference potential is supplied, and a voltage of the power supply line. 2. The liquid crystal display device according to claim 1, further comprising power supply monitoring / control means for turning on the switching element when the voltage becomes lower than the voltage. 4. The discharging means is provided between a power supply line and a reference power supply line to which a reference potential is supplied, and cuts off a power supply voltage supplied from the outside based on a control signal supplied from the outside. 2. The liquid crystal display device according to claim 1, wherein the switching device is a switching element that conducts when the liquid crystal panel is turned on.