JP4701836B2 - Backlight control device - Google Patents

Description

  The present invention relates to a backlight control device that controls a light emission amount of a backlight of a liquid crystal display device.

  Conventionally, a liquid crystal display device is known as one of flat panel displays, and is widely used. This liquid crystal display device performs display by controlling the light transmission characteristics of the liquid crystal, and the liquid crystal element itself does not emit light. Therefore, there are a transmissive type in which a backlight is provided and light from the backlight is transmitted through the liquid crystal for display, and a reflective type in which external light transmitted through the liquid crystal is reflected for display.

  Further, transflective liquid crystal display devices are also used as liquid crystal display devices for devices used under various conditions such as indoors, daytime outdoors, and nighttime, such as portable devices. This transflective liquid crystal display device has a transmissive display portion and a reflective display portion for each pixel. In a room or at night, light is transmitted from the backlight to the transmissive portion for display. In the outdoors, the reflection part is used for display.

  Here, the light quantity of the backlight may be considerably small when the illuminance of outside light is very small. On the other hand, when the illuminance of outside light is quite bright, the backlight does not contribute to display. For this reason, there is a demand for controlling the amount of light emitted from the backlight according to the brightness of external light. In particular, portable devices are usually battery-powered, and in order to increase the continuous operation time of portable devices, there is a great demand for increasing the efficiency of use of the backlight and reducing power consumption.

  Thus, it has been proposed to detect the amount of external light with an optical sensor and control the amount of backlight light based on the detection result (see Patent Document 1). With such a configuration, the light amount of the backlight can be PWM-controlled based on the illuminance of the external light obtained by the optical sensor, and the backlight can be efficiently controlled.

JP 9-43569 A

  However, the configuration of Patent Document 1 requires various circuits such as an optical sensor and a unique circuit for PWM control, and the control circuit becomes large. In particular, mounting such a large-scale circuit in a portable device such as a cellular phone hinders downsizing and cost reduction of the apparatus, which is not practical.

  The present invention is a backlight control device that controls the amount of light emitted from a backlight of a liquid crystal display device, and is formed at a site irradiated with external light and generates a detection current in accordance with the irradiated light. An element, a first current detection means for detecting a detection current of the first light detection element, and an element that generates a detection current in response to the irradiated light, and is formed in a portion that is not irradiated with external light, and is in a dark state A second photodetecting element to be placed, a second current detecting means for detecting a detection current of the second photodetecting element, a magnitude of the detected current detected by the first current detecting means, a first threshold value, The external light illuminance is determined from the comparison result between the first threshold illuminance determining means for determining the external light illuminance from the comparison result of the first threshold, the ratio of the two detected currents detected by the first and second current detecting means, and the second threshold value. Second illuminance determination means for performing the first Beauty based on both the determination result of the second illuminance determination unit, and controls the light emission amount of the backlight.

  Further, the liquid crystal display device is a transflective type having a transmissive display portion and a reflective display portion for each pixel. When the illuminance of external light is low, the amount of light emitted from the backlight is set to a low level. When the illuminance of light is at an intermediate level, the light emission amount of the backlight is set to a high level, and when the illuminance of external light is at a high level, the backlight is turned off, and the determination result of the second illuminance determination means In the case where it is determined that the ambient light illuminance is lower than the second threshold value, the external light illuminance is determined to be low, and the determination result of the second illuminance determination means exceeds the second threshold value, and the first illuminance determination If it is determined in the determination result of the means that it is equal to or less than the first threshold value, the ambient light illuminance is determined to be an intermediate level, and the determination result of the first illuminance determination means is determined to exceed the first threshold value. If the ambient light illuminance is high, It is preferable to constant.

  Further, the first illuminance determination means compares the magnitude of a current value obtained by subtracting the detection current detected by the second current detection means from the detection current detected by the first current detection means, and a first threshold value. It is preferable to determine the external light illuminance from the result.

  The first and second photodetecting elements are transistors that generate a detection current in response to the irradiated light, and are used for processing for display on the liquid crystal display device on the panel of the liquid crystal display device. It is preferable to form by the same process.

  As described above, according to the present invention, the amount of external light is detected using the leakage current of a current leakage transistor that generates a leakage current according to external light, such as a low-temperature polysilicon transistor. Therefore, a special optical sensor or the like is not necessary. Further, since the amount of light is controlled step by step by comparison with the threshold value, the processing circuit can be simplified. For example, the processing circuit can be mounted on the liquid crystal panel. Then, the ratio of the leak current of the current leak transistor that receives external light and the leak current of the current leak transistor that does not receive external light and the leak current of the current leak transistor that receives external light are determined, and the light amount of the backlight is controlled. The leakage current of the current leakage transistor is greatly affected by temperature, but by using two types of determination, the amount of external light can be detected with a simple circuit with relatively little influence of temperature.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram illustrating an overall configuration of a backlight control apparatus according to an embodiment. The current leak transistor 10 is provided in a portion of the liquid crystal panel that is exposed to external light. For example, in the case of a liquid crystal display for a mobile phone, the liquid crystal display is provided in a portion that is exposed to external light, such as a portion that can be seen from the outside or a portion that has a small hole in the cover member even at the periphery of the panel.

  Here, the current leak transistor 10 is manufactured in a manufacturing process of a liquid crystal panel. That is, the liquid crystal panel uses, for example, a low-temperature polysilicon TFT (thin film transistor), and is an active matrix type in which a switching TFT is formed in each pixel of the display portion and display is controlled for each pixel. When the liquid crystal panel is formed, TFTs for each pixel and peripheral circuit TFTs are manufactured, and the current leak transistor 10 is manufactured by the same process as other TFTs on these panels.

  The backlight control device is formed on the liquid crystal panel and outputs a backlight control signal to the outside.

  The current leak transistor 10 is an n-channel TFT, a source is connected to the ground, and a reverse bias power source 12 is provided between the gate and the source. The drain is connected to the power supply via the leak current detection circuit 14.

  The leak current detection circuit 14 detects the leak current Ileak (photo) of the current leak transistor 10. Here, since the leakage current Ileak (photo) is relatively small, the integrated value of the leakage current in a predetermined time may be detected intermittently. For example, it is possible to charge the capacitor to the power supply voltage, and then disconnect from the power supply and supply the current leakage transistor 10 from the capacitor to detect the voltage value of the capacitor after a predetermined time.

  In addition, a current leak transistor 20 formed in the same manner as the current leak transistor 10 is also provided, and the current leak transistor 20 is formed in a light-shielded portion so as not to be exposed to external light. Further, the reverse bias power supplies 12 and 22 are about −5 V with respect to the ground, whereby the n-channel current leak transistors 10 and 20 are always set to an off state.

  A reverse bias power source 22 is also arranged between the gate and source of the current leak transistor 20, the source is connected to the ground, and the drain is connected to the power source via the leak current detection circuit 24. The leak current detection circuit 24 detects the leak current Ileak (dark) of the dark current leak transistor 20 that is not exposed to external light. Note that since the leakage current in the TFT is increased by light irradiation, the leakage current Ileak (dark) is basically smaller than the leakage current Ileak (photo).

  The detection result of the leak current detection circuit 14 is input to the positive input terminal of the comparator 30. The comparator 30 is supplied with the reference voltage Vref <b> 1 from the reference voltage source 32, and the comparator 30 supplies a signal of the comparison result to the light quantity control unit 50.

  The detection result of the leakage current detection circuit 14 and the detection result of the leakage current detection circuit 24 are supplied to the ratio calculation circuit 40. Here, the ratio (Ileak (photo) / Ileak (dark)) of the leak current Ileak (photo) of the current leak transistor 10 that receives external light and the leak current Ileak (dark) of the current leak transistor 20 that does not receive external light is Calculated.

  The calculation result of the ratio calculation circuit 40 is supplied to the positive input terminal of the comparator 42. The comparator 42 is supplied with the reference voltage Vref 2 from the reference voltage source 44, and the comparator 42 supplies a signal of the comparison result to the light quantity control unit 50.

  The light amount control unit 50 determines whether or not the light amount (illuminance) of external light is about 10 to 500 lx based on the output from the comparator 42, and the light amount (illuminance) of external light is 5000 lx or less based on the output from the comparator 30. It is determined whether or not.

  Here, Vref2 is set to a voltage corresponding to the ratio of the leakage current amount of the current leakage transistors 10 and 20 when the amount of external light is 10 to 500 lx, and the comparator 42 has a light amount of 10 to 500 lx. The L level is output in the following cases, and the H level is output when exceeding 10 to 500 lx. Vref1 is set to a voltage corresponding to the amount of leakage current of the current leakage transistor 10 when the amount of external light is 5000 lx, and the comparator 42 exceeds the L level and 5000 lx when the amount of external light is 5000 lx or less. In this case, H level is output.

  As shown in FIG. 2, the light amount control unit 50 controls the light amount of the backlight to about 2 nit when the amount of external light is 10 to 500 lx or less (the outputs of the comparators 42 and 30 are both L level). However, when the amount of external light is more than 10 to 500 lx and less than or equal to 5000 lx (the output of the comparator 42 is H level and the output of the comparator 30 is L level), the light amount of the backlight is controlled to about 100 to 150 nit, When the amount of light exceeds 5000 lx (the outputs of the comparators 42 and 30 are both H level), the backlight is turned off.

  Actually, the power supply to the backlight is performed by another circuit, and the light quantity control unit 50 outputs a control signal for controlling the luminance of the backlight to the above three types of luminance.

  In addition, various things, such as a cold cathode tube and LED, are employable as a backlight. The brightness control may be any control such as PWM control.

  Here, FIG. 3A shows the leakage current of the current leakage transistor 10 irradiated with light input to the comparator 30 at room temperature (20 ° C.), and FIG. The ratio of the leakage current of the current leakage transistor 10 that is irradiated with light and the leakage current of the current leakage transistor 20 that is not irradiated with light is shown.

  4A shows a leakage current of the current leakage transistor 10 irradiated with light input to the comparator 30 at a high temperature (80 ° C.), and FIG. 4B shows a high temperature (80 ° C.). The ratio of the leakage current of the current leakage transistor 10 irradiated with light and the leakage current of the current leakage transistor 20 not irradiated with light, which is input to the comparator 42 in FIG. Further, FIG. 5 shows an enlarged view of the portion of the low external light quantity in FIGS. 3 (b) and 4 (b).

  3 (a), 3 (b), 4 (a), 4 (b), 5 (a), and 5 (b) are n-channel TFTs (channel length L / channel width W = 6/600). Gate-source voltage Vgs = −5 V, drain-source voltage Vds = 2.6 V (◯), 5.1 V (black Δ). Since the drain-source voltage Vds = 2.6 V is easier to use based on the actual detection result, the drain-source voltage Vds = 2.6 V is adopted in the following description.

"Decision of 10-500 lx"
From this, when trying to use the leakage current amount of the current leakage transistor 10 (input of the comparator 30) for the determination of 10 to 500 lx assuming nighttime, the leakage current amount at room temperature is about 0.04 nA. This is a very weak current, and it is difficult to set a threshold value in consideration of variations in TFT characteristics.

  On the other hand, in the ratio of the two leakage currents (input to the comparator 42), a value of about 5 times is obtained at the ambient light illuminance of 500 lx at room temperature. If the current leak transistors 10 and 20 are formed in the same process and relatively close to each other, variations in their characteristics can be compensated to some extent. Therefore, by setting the voltage corresponding to about three times the output of the ratio calculation circuit 40 as the voltage Vref2 of the reference voltage source 44, the threshold value at the ambient light illuminance of about 10 to 500 lx in the range of 50 to 60 ° C. from room temperature. Is determined to be possible.

"Judgment of 5000 lx"
Next, in the determination of 5000 lx, which is the brightness at which the backlight is turned off, when trying to use the ratio of the two leak currents (input to the comparator 42), the sensitivity is about 30 times at room temperature with respect to 5000 lx, It is enough. However, at a high temperature (80 ° C.), the ratio of the two leakage currents is three to four times. Thus, the influence of temperature is very large and it is difficult to set an appropriate threshold value.

  On the other hand, the leakage current of the current leakage transistor 10 at 5000 lx (input of the comparator 30) is about 0.22 nA at room temperature and about 0.31 nA at high temperature (80 ° C.). Accordingly, by setting the threshold value to about 0.25 nA, it is possible to determine 6000 lx at room temperature and about 4000 lx at high temperature. Considering the NI (nematic) transition temperature of the liquid crystal, it is not necessary to consider the use of 60 ° C. or higher. Therefore, it is possible to make a temporary determination by setting the threshold value of the comparator 30 to about 0.25 nA (the reference voltage Vref1 is set to a voltage corresponding to the leakage current).

  Furthermore, at high temperatures, the leakage current in the dark state increases. Therefore, it is preferable to subtract the leakage current Ileak (dark) of the current leakage transistor 20 from the leakage current Ileak (photo) of the current leakage transistor 10.

  The leak current Ileak (dark) of the current leak transistor 20 is about 0.08 nA. Therefore, when the leakage current Ileak (dark) of the current leakage transistor 20 is subtracted from the leakage current Ileak (photo) of the current leakage transistor 10, the value at a high temperature corresponding to 5000 lx is 0.23, which is 0.22 nA at room temperature. Is almost equal to Therefore, with this configuration, an external light amount of 5000 lx can be detected appropriately.

  Here, a configuration in which the leakage current Ileak (dark) of the current leakage transistor 20 is subtracted from the leakage current Ileak (photo) of the current leakage transistor 10 is shown in FIG. As described above, the subtractor 60 that subtracts the output from the leak current detection circuit 24 from the output of the leak current detection circuit 14 is provided, and the output of the subtractor 60 is input to the comparator 30. Then, by setting the reference voltage Vref1 input to the comparator 30 to a voltage corresponding to 0.22 nA, it is possible to determine whether the external light amount is 5000 lx or less from room temperature to high temperature.

Note that Lx is a unit representing illuminance (brightness of an object surface when light emitted from the light source illuminates a certain object surface), and Lx (lux) = lm (lumen) / m ² , where lm is a light source This is a unit of the amount of light (light flux) emitted from. Nit is a unit representing luminance (luminance per unit area of a light source that emits light) and is nit (unit) = cd (candela) / m ² , and the candela illuminates a certain object surface with light emitted from the light source. Is a unit representing the brightness (illuminance) of the object surface.

  In the present embodiment, a transistor using low-temperature polysilicon as an active layer is used as the light detection element, but a pin structure or the like can also be used. In addition, a transistor having an amorphous active layer can be used as the light detection element.

It is a figure which shows the structure of embodiment. It is a figure which shows the example of control of the backlight according to external light quantity. It is a figure which shows the leakage current in room temperature, and leakage current ratio. It is a figure which shows the leakage current and leakage current ratio in high temperature. It is a figure which shows the leakage current ratio in the low external light quantity. It is a figure which shows the structure of other embodiment.

Explanation of symbols

  10, 20 Current leakage transistor, 12, 22 Reverse bias power supply, 14, 24 Leak current detection circuit, 30, 42 Comparator, 32, 44 Reference voltage source, 40 Ratio calculation circuit, 50 Light quantity control unit, 60 Subtractor.

Claims (6)

An external light illuminance determination device that determines the illuminance of external light applied to a liquid crystal display device ,
A first photodetecting element that is formed at a site irradiated with external light and generates a detection current according to the irradiated light;
First current detection means for detecting a detection current of the first photodetecting element;
A second photodetecting element that generates a detection current in response to the irradiated light, is formed in a portion that is not irradiated with external light, and is placed in a dark state;
Second current detection means for detecting a detection current of the second photodetecting element;
First illuminance determination means for determining the illuminance of outside light from the comparison result between the magnitude of the detected current detected by the first current detection means and the first threshold value;
2. A liquid crystal display, comprising: a second illuminance determining unit that determines an illuminance of external light from a comparison result between a ratio of two detected currents detected by the first and second current detecting units and a second threshold value. Device for determining ambient light illuminance. A liquid crystal display device comprising the external light illuminance determination device according to claim 1 on a panel. The liquid crystal display device according to claim 2,
It has a backlight control device that controls the amount of light emitted from the backlight,
The liquid crystal display device, wherein the backlight control device controls the light emission amount of the backlight based on the determination results of both the first and second illuminance determining means of the external light illuminance determining device. The liquid crystal display device according to claim 3 .
The liquid crystal display device is a transflective type having a transmissive display portion and a reflective display portion for each pixel,
The backlight control device includes:
When the illuminance of outside light is low, the amount of light emitted from the backlight is set low,
When the illuminance of outside light is at an intermediate level, the backlight emission level is set to a high level,
If the illuminance of outside light is high, turn off the backlight,
In the determination result of the second illuminance determining means, when it is determined that the second illuminance is less than or equal to the second threshold value, the external light illuminance is determined to be low level,
In the determination result of the second illuminance determination means, when the second threshold value is exceeded and the determination result of the first illuminance determination means is determined to be equal to or less than the first threshold value, the external light illuminance is an intermediate level. Judgment,
The liquid crystal display device according to claim 1, wherein in the determination result of the first illuminance determination means, the external light illuminance is determined to be high when it is determined that the first threshold value is exceeded. The liquid crystal display device according to claim 3 or 4 ,
The first illuminance determining means is based on a comparison result between the magnitude of the current value obtained by subtracting the detected current detected by the second current detecting means from the detected current detected by the first current detecting means and the first threshold value. A liquid crystal display device characterized by determining the illuminance of outside light. A liquid crystal display device according to any one of claims 2 to 5,
The first and second photodetecting elements are transistors that generate a detection current in response to irradiated light, and are formed on the panel by the same process as a transistor used for processing for display of the liquid crystal display device. A liquid crystal display device.

Images