This application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/JP2007/070338 filed on Oct. 18, 2007, which claims priority to Japanese Application No. 2006-285323 filed on Oct. 19, 2006, incorporated herein by reference.
1. Field of the Invention
The present invention relates to an LED (Light Emitting Diode) driving device, an illuminating device using an LED as its light source, and a projection-type display device.
2. Description of the Related Art
A field-sequential display device is an example of projection-type display devices, and forms a color image by time-divisionally displaying R (red), G (green), B (blue). The formation of a color image in an exemplar DLP (Digital Light Processing) projector is performed by: employing a high-pressure mercury lamp or the like as a light source; separating the white light from the light source into colors by means of a color wheel; modulating the color-separated light by means of a reflective device such as a DMD (Digital Micromirror Device); and then projecting the modulated light on a screen through a projection optical system.
The quantization noise causes a problem in low-intensity display of the brightness expression implemented by a display device with such a reflective device. To address this problem, a conventional display device employing a lamp such as a high-pressure mercury lamp is equipped with an ND (Neutral Density) filter attached to the segments of the color wheel. The ND filter is designed to lower the intensity down to approximately 10% so that the apparent bits in low-intensity display can be increased and thus the quantization noise can be reduced.
Another example of field-sequential display devices employs LEDs of RGB colors as light sources in place of a white-lamp light source with a color wheel. The RGB LEDs emit light in a time-dividing manner, and the light thus emitted enters a reflective device to be modulated. The resultant light is then projected on a screen through a projection optical system to form a color image. Note that, in this case, the light emission for each LED is turned on and off by pulsing.
Meanwhile, liquid-crystal displays are examples of direct-view display devices. The light source of the liquid-crystal display has come to employ solid-state illumination (i.e., LEDs) in place of fluorescent tubes. An improvement in the performance of the liquid-crystal display has been achieved by a technique (known as an area-active technique). In the technique, the intensities of the multiple LEDs that the liquid-crystal display device is equipped with are changed for such groups of LEDs as determined in accordance with the video image to be displayed by the liquid-crystal display device. The visual dynamic range is thus changed resulting in the above-mentioned improvement in the performance.
The display device equipped with LED light source also has the problem of quantization noise at low-intensity display. Being provided with no color wheel, the kind of countermeasures such as the ND filter employed in the conventional display device with the lamp light source cannot be taken in the case of the display device with the LED light source.
The same effect as that obtainable by use of the ND filter can be obtained by reducing the amount of the light emitted from each LED. The reduction in the light-emitting amounts of the LEDs can be achieved, for example, by pulse-based light modulation. The light emission of each LED, however, is controlled by pulsing in a field-sequential display device, so that the pulse-based light modulation is not operable.
Reducing the current that flows through the LEDs is another way of achieving the reduction in the amount of light emitted from each LED. In the light modulation by changing the amount of current, lowering the power is a problem. The detected current is converted to a voltage and the resultant voltage is used for feedback in this type of modulation. A small current, however, results in a small feedback voltage, which makes the light modulation control difficult.
Patent Document 1 discloses a light-emitting diode driving device that employs a technique based on a switching circuit. The disclosed device employs a current-detection method based on the control using a single resistor, and cannot deal with a minute current. In addition, the configuration of the circuit may have a problem caused by the offset in the comparator.
Patent Document 2 discloses a light-emitting element driving circuit that is configured to improve the efficiency by means of peak-value detection. The disclosed circuit, however, is not suitable for constant current regulation from a large current to a minute current.
Patent Document 3 also discloses a light-emitting diode driving device that employs a technique based on a switching circuit. The disclosed device employs a current-detection method based on the control using a single resistor, and cannot deal with a minute current. In addition, the configuration of the circuit may have a problem caused by the offset in the comparator.
Patent Document 4 discloses a light-emitting diode driving device that employs a light modulation method based on switching, and thus cannot be used in the field-sequential display device. In addition, the switching makes the influence of the noise more likely to be produced.
In view of what has been described thus far, the present invention provides an LED driving device with the following features. The LED driving device, if employed in a projection-type display device, is capable of stably reducing the brightness of the LEDs down to such a level that the same effect as in a case of using the ND filter can be obtained. The LED driving device, if employed in a direct-view type display device, is capable of driving the display device by the area-active technique based not on the pulse-based light modulation control but on the current-based light modulation control. The present invention also provides an illuminating device and a display device each of which employs the LED light source with the above-mentioned features and which thereby reduces the generation of the noise.
The present invention provides an LED driving device including: a driving voltage switching means for switching between a first driving voltage and a second driving voltage in accordance with a timing signal; and a feedback circuit to which any one of the first and second driving voltages is applied and which thereby determines a current flowing through an LED. The feedback circuit includes a current controlling means for controlling, in accordance with the timing signal, a current flowing through the LED.
The current controlling means may be a resistor switching means for switching, in accordance with the timing signal, between opposings that determine the current flowing through the LED.
The present invention provides an illuminating device including: an LED driving device such as one described above; and an LED driven by the LED driving device.
The present invention provides a display device including: an LED driving device such as one described above; a green LED driven by the LED driving device; a red LED; a blue LED; a controlling means for switching between the green LED, the red LED, and the blue LED and making the selected one of the LEDs emit light; a reflective device which is controlled by the controlling means in synchronization with the light emission of the green LED, the red LED, and the blue LED, and which modulates the light emitted by the green LED, the red LED, and the blue RGB; and a projection optical system which projects light reflected by the reflective device.
In addition, the present invention provides a direct-view type display device including an LED driving device such as one described above; and a backlighting system that can achieve an area-active control and a wide dynamic range. The area-active control and the wide dynamic range are made possible not by means of an ON/OFF pulse modulation of light of the green LED, the red LED, and the blue LED all of which are driven by the LED driving device but by means of a current modulation of light of the LEDs, that is, by changing the driving currents for the LEDs.
What is obtained according to the present invention is an LED driving device is capable of stably reducing the brightness of the LEDs down to such a level that the same effect as in a case of using an ND filter can be obtained.
The DLP system equipped with the LED light source and shown in
Reference numerals R1001 to R1015 denote resistors, and reference numerals Q1001 to Q1010 denote transistors. LED_VCC shown in the upper right-hand portion of
The portion enclosed by the dotted lines in
The driving voltage for the LEDs denoted by the LED ON passes through an and-circuit including transistors Q1009 and Q1010 into which the LED-ON and the ND supplied by the DAC, and then is switched by the transistor Q1008. The transistor Q1003 is provided for the regulation of the driving voltage thus switched.
In the LED driving device shown in
The control of a minute current, however, is difficult by use of the above-described system which controls the current in a feedback route in which a current-voltage conversion is performed. Even when the base potential of the transistor Q1005 is set to zero, the occurrence of a dark current (leakage current) prevents the transistor Q1001 from having a zero base voltage. In this case, it is difficult to reduce the light amount down to approximately 10%, which can be easily done by use of the ND filter.
The portion enclosed by the dotted lines in
The driving voltage for the LEDs denoted by the LED ON passes through an and-circuit including transistors Q1009 and Q1010 into which the LED-ON and the ND supplied by the DAC, and then is switched by the transistor Q1008. The transistor Q1003 is provided for the regulation of the driving voltage thus switched.
In the LED driving device shown in
The control of a minute current, however, is difficult by use of the above-described system which controls the current in a feedback route in which a current-voltage conversion is performed. Even when the base potential of the transistor Q1005 is set to zero, the occurrence of a dark current (leakage current) prevents the transistor Q1001 from having a zero base voltage.
As will be described below, in the LED driving device of the present invention shown in
The gate of the transistor Q2001 is controlled by the LED ON. When the LEDs emit the normal light, that is, when the LED ON is high, the transistor Q2001 is in operation and, in the circuit, the resistor 82001 is made to be equivalent to the ground. Accordingly, in this case, the current flowing through the LEDs is determined by the value of the combined resistor including the two resistors R1001 and R1002.
When the LEDs emit the ND light, that is, when the LED ON is low, the transistor Q2001 is not in operation, so that the circuit as a whole becomes equivalent to a circuit without the Q2001. Accordingly, in this case, the current flowing through the LEDs is determined by the value of the combined resistor including the three resistors R1001, R1002, and R2001.
As has been described above, in the LED driving device of the present invention, the current is controlled by switching the voltage supplied from the DAC, and the value of the current is switched by the feedback route. Accordingly, a minute current can be controlled. The use of the LED driving device of the present invention in a display device can result in the effect obtainable by use of the ND filter in a conventional system equipped with a color wheel. As a consequence, a video image can be formed with a reduced quantization noise.
In the above-described embodiment, a case where green LEDs are driven to emit normal light and ND light has been described using an example of a field-sequential DLP system equipped with a light source of LEDs. The present invention, however, is not limited to the above-described embodiment, but is applicable to other uses. For example, the present invention can be carried out as an illuminating device equipped with a light source of LED and a device enabling the adjustment of the amount of light.
The LED driving device of the present invention is applicable to an area-active circuit of a backlighting system (driven by LEDs) for a liquid-crystal display. The backlighting system for a liquid-crystal display of today employs either CCFLs (Cold-Cathode fluorescent lamps) or LEDs as its light source. As to the LEDs, some of the backlighting systems for liquid-crystal displays, which now has a wider gamut of colors, employ LEDs of RGB colors. Occurrence of shallow black expression is one of the drawbacks of liquid-crystal displays, and it is pointed out that liquid-crystal displays have a weakness in the expressions of the black gradation. A method known as the area-active control is one of the means for addressing the above-mentioned problem. In the area-active control, the backlighting system is divided into several blocks, and the amount of light emitted from the light source for each of the blocks thus divided is controlled in synchronization with the video signals. An area-active circuit employed in the LED driving device of the present invention is capable of linearly changing the amount of emitted light, so that a wider dynamic range of the amount of emitted light can be obtained.
When the conventional LED driving device shown in
Reference numerals R1001 to R1015 denote resistors, and reference numerals Q1001 to Q1010 denote transistors. LED_VCC shown in the upper right-hand portion of
The portion enclosed by the dotted lines in
The driving voltage for the LEDs denoted by the LED ON switches the transistor Q1008 by means of the transistor Q1009 (the transistor Q1010 is not mounted on the circuit). The transistor Q1003 is provided for the regulation of the driving voltage thus switched.
In the LED driving device shown in
The control of a minute current, however, is difficult by use of the above-described system which controls the current in a feedback route in which a current-voltage conversion is performed. Even when the base potential of the transistor Q1005 is set to zero, the occurrence of a dark current (leakage current) prevents the transistor Q1001 from having a zero base voltage. In this case, it is difficult to reduce the light amount.
LED ON denotes a signal that is high when the backlight is lit. GND denotes a reference ground of the circuit. DAC IN denotes a variable value ranging basically from the GND level to the VCC level. This signal allows the current flowing through the LED to be changed.
The portion enclosed by the dotted lines in
The driving voltage for the LEDs denoted by the LED ON switches the transistor Q1008 by means of the transistor Q1009 (the transistor Q1010 is not mounted on the circuit). The transistor Q1003 is provided for the regulation of the driving voltage thus switched.
In the LED driving device shown in
The control of a minute current, however, is difficult by use of the above-described system which controls the current in a feedback route in which a current-voltage conversion is performed. Even when the base potential of the transistor Q1005 is set to zero, the occurrence of a dark current (leakage current) prevents the transistor Q1001 from having a zero base voltage.
As will be described below, in the LED driving device of the present invention shown in
The gate of the transistor Q2001 is controlled by the inversion signal of the ND. When the LEDs do not emit the ND light, the transistor Q2001 is in operation and, in the circuit, the resistor R2001 is made to be equivalent to the ground. Accordingly, in this case, the current flowing through the LEDs is determined by the value of the combined resistor including the two resistors R1001 and R1002.
When the LEDs emit the ND light, that is, when the gate voltage of the transistor Q2001 is low, the transistor Q2001 is not in operation, so that the circuit as a whole becomes equivalent to a circuit without the Q2001. Accordingly, in this case, the current flowing through the LEDs is determined by the value of the combined resistor including the three resistors R1001, R1002, and R2001.
As has been described above, in the LED driving device of the present invention, the current is controlled by the video image applied to the DACIN, and the value of the current is switched by the feedback route. Accordingly, a minute current can be controlled. The use of the LED driving device of the present invention in a direct-view type display device can result in the effect obtainable by the conventional light modulation method with the pulse light emission. As a consequence, a reduction in the switching noise is possible.
In the above-described embodiment, a second case has been described using an example of a liquid-crystal display system equipped with a backlighting system including a LED light source. The present invention, however, is not limited to the above-described embodiment, but is applicable to other uses. For example, the present invention can be carried out as an illuminating device equipped with a light source of LED and as a device enabling the adjustment of the amount of light.
The LED driving device of the present invention can also be used as a driving device for LEDs used in a display device. Conventionally, LEDs have been used in the displays of electric signboard and the like for expressing simple characters and the like. Some of these electric signboards used in pachinko parlors and the like express animation and the like, but the quality of the video image has not reached a level equivalent to liquid-crystal displays.
With the LED driving device of the present invention, the driving current for LEDs to be driven can be changed dynamically. Accordingly, the use of the LED driving device of the present invention allows not only the expression of colors achieved conventionally by the simple combination of the ON/OFF of the RGB colors but also the expression of a wider variety of colors.
As described above with reference to
The present invention is applicable to an LED driving device.
Number | Date | Country | Kind |
---|---|---|---|
2006-285323 | Oct 2006 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/JP2007/070338 | 10/18/2007 | WO | 00 | 4/17/2009 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2008/047862 | 4/24/2008 | WO | A |
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