JP2002289374A - Light source, display equipment, and electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make dimming of light easy in addition to make a light source have high efficiency. SOLUTION: A capacitor is periodically connected by turns with a power supply and a light-emitting element. Here, dimming is performed by changing a cycle and by changing capacity of the capacitor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

2. Description of the Related Art In recent years, liquid crystal display devices having characteristics of small size, light weight and low power consumption have been widely used as display devices. However, unlike a self-luminous display device such as a cathode ray tube (CRT), the liquid crystal element itself does not emit light, so that it is necessary to provide a light source for the liquid crystal display device to function even in a dark place.

[0002] Incandescent lamps and cathode tubes are also used as light sources.
Called. ) And electroluminescence (hereinafter referred to as EL) are being widely used.

[0003] Here, if the LED or EL is driven to emit light by constant voltage driving, the current flowing due to a slight variation in the characteristics of these elements or the ambient temperature changes greatly, and therefore the brightness greatly changes, which is not preferable. Therefore, in general, constant current driving is performed to emit light of a predetermined brightness. FIG.
FIG. 2 is a diagram showing an example of a circuit configuration for performing the constant current driving. In the figure, 101 is a voltage source, 801 is a resistor, 103 is LE
Light emitting elements such as D and EL. The resistor 801 and the light emitting element 103 are connected in series, and the voltage of the voltage source 101 is applied. Here, if the light-emitting element 103 tries to flow a current larger than a predetermined value due to variations in characteristics and the like,
A large current also flows through the resistor 801 that is in series, which causes a voltage drop, and the voltage applied to the light emitting element 103 decreases, thereby suppressing an increase in the current. Note that the brightness can be adjusted by using the resistor 801 as a variable resistor.

By the way, a PDA (Personal Digital Assista)
In general, portable electronic devices such as nt) and mobile phones are driven by batteries, and this light source is also required to have low power consumption. Further, it is preferable that the light source of the display device incorporated in the portable electronic device can adjust the brightness of the light source according to the intensity of external light, in other words, the surrounding brightness. This is because when the surroundings are bright, the visibility can be increased by brightening the light source, and when dark, the brightness of the light source can be reduced to increase the antiglare property.
This can be achieved by making the resistor 802 in FIG. 8 a variable resistor.

[0005]

SUMMARY OF THE INVENTION However, the conventional
In the constant current driving of the LED or the EL, there is a problem that a current flows through a resistor for making the current constant, and wasteful power not involved in light emission is consumed in this portion.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a light source and an electronic device capable of improving the efficiency of a light source and performing dimming easily. It is in.

[0007]

In order to achieve the above object, a light source according to a first aspect of the present invention comprises a capacitor, a power supply, a light emitting element, and the capacitor alternately connected to either the power supply or the light emitting element. A first switch element to be connected is provided.

According to this configuration, assuming that the power supply voltage is V, the capacitance of the capacitor is C, and the switching period of the first switch element is T, the capacitor is first connected to the power supply and charges the electric charge CV. Next, the capacitor is connected to the light emitting element and supplies the charge CV to the light emitting element. Since this is repeated 1 / T times per second, a constant current of CV / T flows through the light emitting element. At this time, there is no useless power consumption because there is no prior art resistor.

A light source according to a second aspect of the present invention includes a plurality of capacitors, a power supply, a light emitting element, a state in which the plurality of capacitors are connected in parallel with the power supply, and a method in which the plurality of capacitors are connected in series. A second switch element for alternately switching to one of a state of connection with the element.

According to this configuration, for example, two capacitors are first connected in parallel with the power supply, respectively, and the voltage V (charge C)
V). Thereafter, the capacitors are connected in series, and the voltage across both ends becomes 2V. The light emitting element is driven by this voltage. Thus, the light emitting element can be driven with a power supply lower than the minimum voltage required for driving the light emitting element. Here, the description has been made with two capacitors, but if the switching is performed in parallel with an arbitrary number n, the voltage becomes nV.

According to another aspect of the present invention, in the first or second light source, the first or second switch element varies a switching cycle for alternately connecting the capacitor to either the power supply or the light emitting element. It is characterized in that it comprises a period changing means for performing the operation.

According to this configuration, the current flowing through the light emitting element
By changing the switching period T of (CV / T), the brightness of the light emitting element can be adjusted. As a method of changing the switching period, there is a method of manually changing the switching period, and the user can manually change the switching period according to his / her preference, and can achieve a desired brightness. In addition, when used for a backlight of a transmissive liquid crystal display device, the external light becomes more intense by increasing the brightness of the light source as the external light becomes stronger. Therefore,
By automatically shortening the switching period when the external light becomes strong, it is possible to obtain the optimum brightness. Of course,
By adjusting the switching period both manually and by the intensity of the external light, it is possible to obtain the optimum brightness.

Further, another light source according to the present invention is characterized in that, in the first and second light sources, a capacitance changing means for changing a capacitance of the capacitor or the plurality of capacitors is provided.

According to this configuration, the current flowing through the light emitting element
By changing (CV / T) the capacitance C of the capacitor, the brightness of the light emitting element can be adjusted. Note that there is a method of manually changing the capacitance of the capacitor, and the user can manually change the capacitance according to his / her preference, and can achieve a desired brightness. In addition, when external light becomes strong, the capacity of the capacitor is automatically increased, so that optimum brightness can be obtained.
Of course, it is possible to obtain the optimum brightness by adjusting the capacitance of the capacitor both manually and by the intensity of the external light.

The brightness can be adjusted by changing both the switching period and the capacitance of the capacitor, and the variable method can be adjusted manually or in accordance with external light, or both, to obtain the optimum brightness adjustment range. Can be expanded or fined.

Many types of light emitting elements used for the light source of the present invention, such as incandescent lamps, cathode tubes, ELs, and LEDs, are suitable. Is preferred.

Since the electronic apparatus of the present invention includes the above-described light source, power consumption is reduced and the configuration is simplified. Note that the light source is used as a light source of the display device, and this display device can be incorporated in a higher-order electronic device, so that the power consumption of the display device and, consequently, the higher-order electronic device can be reduced and the configuration can be simplified.

As such an electronic device, a personal computer
Computers, mobile phones, digital still cameras, and the like.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

First Embodiment <Structure> First, the electrical structure of a light source according to a first embodiment of the present invention will be described. FIG. 1 is a diagram showing this electric circuit configuration. In the figure, 101 is a power supply, 102 is a switch circuit, 103 is a light emitting element, 104 is a capacitor,
105 is a logic oscillation circuit. Power supply 101 has voltage V
Is output. The switch circuit 102 has terminals a, b, c,
The terminal a is connected to one end of the power supply 101, the terminal b is connected to one end of the light emitting element, and the terminal c is connected to one end of the capacitor 104. The other ends of the power supply 101, the capacitor 104, and the light emitting element 103 are grounded to a common potential. (In the figure, ▽
Symbol) The terminal s of the switch circuit 102 is an input terminal to which an output signal of the logic oscillation circuit 105 is input. Note that the logic oscillation circuit 105 outputs a binary clock signal.
Here, when one of the two values is L and the other value is H, the switch circuit 102 conducts the terminals a and c when the signal is L, and conducts the terminals c and b when the signal is H. As the light emitting element 103, many types of light emitting elements such as an incandescent light bulb, a cathode ray tube, an EL, and an LED can be applied. However, from the viewpoints of low voltage, small size, light weight, durability, and high efficiency, EL and LED are particularly preferable.
Is preferred. Here, an LED is used. Here, a specific configuration example of the switch circuit 102 is shown in FIG. In the figure,
The terminals a, b, c, and s correspond to the respective terminals of the switch circuit 102 in FIG. In FIG. 2, reference numeral 201 denotes a Pch field effect transistor (hereinafter, referred to as an FET), and reference numeral 202 denotes an Nch FET. Terminal a is the source of FET 201, terminal b
Is the source of the FET 202, and the terminal c is the FETs 201 and 20.
2 are connected to both drains. And the terminal s
Is connected to both gates of the FETs 201 and 202. Therefore, when the potential of the terminal s becomes lower than the potential of the terminal a and becomes lower than the potential of the terminal b or less, the FET 201 conducts, and F
The ET 202 becomes non-conductive. Conversely, when the potential of the terminal s becomes higher than the potential of the terminal b and becomes equal to or higher than the potential of the terminal a, FE
T202 becomes conductive and FET 201 becomes nonconductive.

Next, FIG. 3 shows a specific configuration example of the oscillation circuit 105 of FIG. In the figure, 301 is a Schmitt trigger type logic inversion circuit, 302 is a capacitor having a capacitance Cosc, and 303 is a resistor having a resistance value Rosc. Here, one end of the capacitor is grounded to a certain potential, and the other end is connected to the input a of the logic inversion circuit 301. One end of the resistor 303 is connected to the input a of the logic inversion circuit 301, and the other end is connected to the output Y of the logic inversion circuit 301. With such a configuration, a clock signal having a cycle substantially proportional to the product of Rosc and Cosc is output. The oscillation frequency f “Hz” at this time is f
= 1 / T.

The light source of the first embodiment has the above configuration.

<Operation> Next, the operation will be described. First, FIG.
When the output of the oscillation circuit 105 is in the L state, the switch circuit 102 conducts the terminals a and c, so that the capacitor 104 is charged with the voltage V of the power supply 101. That is, the charge CV is stored in the capacitor 104. Note that the period during which the terminals a and c are conducted needs to be a period during which charges are sufficiently stored in the capacitor 104. Conversely, the capacitance value of the capacitor 104 needs to be a value that can sufficiently charge the electric charge during the period when the terminals a and c are conducting.

Next, when the output of the oscillation circuit 104 becomes H level, the switch circuit 102 conducts the terminals b and c, so that the capacitor 104 discharges the electric charge CV to the light emitting element 103. That is, current flows through the light emitting element 103. here,
When the light emitting element is an incandescent light bulb or the like, the current continues to flow until the applied voltage becomes 0. However, when the light emitting element is an LED, the current stops flowing when the applied voltage becomes lower than a certain applied voltage (voltage VF). Therefore, the amount of charge released by the capacitor 104 is C
(V-VF). The period for conducting the terminals b and c is
It is necessary that the period be such that the charges stored in the capacitor 104 are sufficiently released. Conversely, the capacitance value of the capacitor 104 and the resistance value of the light emitting element 103 are determined by the terminal b
It is necessary for the capacitor to have a value that can sufficiently release the electric charge from the capacitor during the period when and c are conducting.

Since this is repeated f times in one second, the average current becomes C (V-VF) f. Here, the consumed power is only the light emitting element 103, and unnecessary power consumption is eliminated. Note that the oscillation frequency f of the logic oscillation circuit 104
Is too low, flicker occurs, so at least 30Hz
Preferably, the frequency is set to 50 Hz or more. Note that the specific configuration examples of the switch circuit 102 and the oscillation circuit 105 described herein facilitate embodying the present invention, and the present invention is not limited to this configuration. Any configuration is acceptable as long as it is suitable.

[Second Embodiment] <Configuration> As described in the first embodiment, the average current flowing through the light emitting element is defined by C (V-VF) f, and is proportional to the frequency f. Therefore, by changing the switching frequency f of the switch circuit 102, the average current flowing through the light emitting element can be adjusted, and light can be adjusted. This will now be described with reference to the drawings. FIG. 4 is a diagram showing an electrical configuration of a light source according to the second embodiment of the present invention. In the figure, components other than 405 have the same configuration and operation as those in FIG. Reference numeral 405 denotes an oscillation circuit that can change the frequency of the clock signal. FIG. 5 shows a logic oscillation circuit 405.
FIG. 3 is a diagram showing a specific example of the configuration of FIG. In FIG. 5, components other than 501 have the same configuration and operation as the logic oscillation circuit 105 of FIG. Reference numeral 501 denotes a variable resistor, and a resistor 3 is provided between the input and output of the inverting circuit 301.
03 to be connected in series.

<Operation> With the above configuration, the oscillation frequency of the oscillation circuit 405 in FIG. 4 is changed by, for example, manually changing the resistance value of the variable resistor 501 in FIG. The average current flowing through 103 changes, and light can be adjusted to an optimal brightness.

Here, if necessary, a fixed resistor may be added in parallel between the input and output of the variable resistor 501 or the inverting circuit 301. Thereby, the light control range and the ease of light control can be made appropriate.

As described above, just by changing the oscillation frequency,
In other words, dimming can be easily performed only by adding a variable resistor, and high efficiency can be achieved as in the first embodiment.

[Third Embodiment] In the light source according to the second embodiment, instead of the variable resistor 501 shown in FIG. 5, an optical sensor whose resistance value changes according to ambient brightness, that is, the intensity of external light. An element can be used. Such devices include photoconductive devices such as cadmium sulfide (CdS) cells. This element has such a property that the resistance value decreases when light hitting the element increases. Therefore, when the intensity of external light increases, the resistance value decreases.
The oscillation frequency f of 01 becomes higher as the external light becomes stronger, and the light emitting element becomes brighter. When the external light is weak, the light emitting element becomes darker, and is automatically adjusted to the optimal brightness.

[Fourth Embodiment] In the light source of the second embodiment, an optical sensor element can be connected in series or in parallel to the variable resistor 501 shown in FIG. As a result, manual light control and automatic light control using external light can be performed, and light control can be performed to more optimal brightness.

Also in this embodiment, the fixed resistor 303 and the variable resistor 50 interposed between the input and output of the inverting circuit 301 are used.
1. A fixed resistor may be added in parallel between any of the optical sensor elements. This makes it possible to properly adjust the light control range including the light control range and the automatic adjustment. [Fifth Embodiment] As described in the first embodiment, the average current flowing through the light emitting element 103 is C (V-VF) f. Therefore, even if the capacitance C of the capacitor 104 in FIG. 1 is changed, the average current flowing through the light emitting element 103 changes and the brightness changes. Therefore, dimming is possible.

<Structure> FIG. 6 is a diagram showing an electric circuit structure of the light source according to the fifth embodiment. In the figure, 6041 and 604
Components and operations other than 2, 6061 and 6062 are the same as those in FIG. 6041 and 604
2 is a capacitor having a capacity of c1 and c2, respectively.
Reference numerals 061 and 6062 denote switch circuits for connecting or disconnecting the capacitors 6041 and 6042 with a certain potential, respectively.

<Operation> With the above configuration, when the switch circuit 6061 is turned on and the switch circuit 6062 is turned off (this state is referred to as state 1), the average current flowing through the light emitting element is c1 (V-VF). ) f. Next, switch circuit 6
When 061 is turned off and 6062 is turned on, the average current flowing through the light emitting element is c2 (V-VF) f. When the switch circuits 6061 and 6062 are both turned on, the average current flowing through the light emitting element is (c1 + c2).
(V-VF) f. Therefore, by setting c1 and c2 to have different capacities, light control in three stages can be performed. In particular, when the capacitance of the capacitor is set so that c2 = 2c1, the average current can be set to be equal to 1, 2, or 3 times as long as the average current in state 1 is 1. .

In this embodiment, the capacitor 604
1 and 6042, switch circuits 6061 and 6062
Are inserted, but one may be always grounded.

Further, in this embodiment, the number of capacitors and the number of capacitors corresponding thereto are two, but the number is not limited to this, and three or more capacitors may be used.
The capacity of each of these capacitors may be arbitrarily set, but it is possible to set the average current at equal intervals by selecting a capacitor which is twice, four times, eight times and a power of 2 and the grounding is appropriately performed. It becomes possible.

[Sixth Embodiment] In the fifth embodiment, the on / off control of the switch circuits 6061 and 6062 may be controlled by external light. That is, the intensity of the external light may be sensed by an arbitrary optical sensor, converted into a digital amount by an analog-to-digital converter, and the ON / OFF control of the switch circuits 6061 and 6062 may be performed according to the state of each bit. Thereby, the same effect as in the third embodiment can be obtained.

[Seventh Embodiment] In the fifth embodiment, the on / off control of the switch circuit 6061 is controlled by external light as in the sixth embodiment.
2 may be manually controlled. This allows
The same effects as in the fourth embodiment can be obtained. Of course, the number and capacity of the capacitors may be arbitrary in the present embodiment.

[Eighth Embodiment] In the second to fourth embodiments, the method of dimming by changing the switching period of the switch circuit 102 will be described. In the fifth to seventh embodiments, the capacitance of the capacitor will be changed. The dimming method was described by doing so. Here, a configuration in which the switching cycle of the switch circuit 102 is varied and the capacitance of the capacitor is varied may be used. This configuration will not be described in detail because it is a simple combination of the above embodiments and can be easily embodied. For example, rough dimming is performed by using a method of varying the capacitance of a capacitor. Light can be finely adjusted over a wide range by using a method of changing the switching cycle.
If necessary, a part of the light may be manually adjusted to adjust the light in accordance with the external light, and this can be easily realized.

[Ninth Embodiment] In the first embodiment, an example is shown in which an LED is used as a light emitting element. However, a low voltage V battery or the like is used as the power supply 101, and V <V with respect to the LED VF.
In the case of F, no current flows through the LED and the LED does not function as a light source. In this embodiment, an electrical configuration of a light source that functions even with a power supply voltage equal to or lower than the minimum voltage necessary for the light emitting element is described.

<Structure> FIG. 7 is a diagram showing an electric structure of the light source of the present embodiment. In the figure, a power supply 101, a light emitting element 103
And the oscillating circuit 105 operate in the same manner as those of FIG. 7021, 7022, 7023 in FIG.
Is a switch circuit, and 7041 and 7042 are capacitors having a capacitance C. Switch circuits 7021, 7022, 702
3 has terminals a, b, c, and s, respectively. All terminals s of the switch circuits 7021, 7022, and 7023 are connected to the output of the oscillation circuit 105, and when the output signal of the oscillation circuit 105 is an L signal, the switch circuits 7021, 7022
The terminals 22 and 7023 conduct the terminals a and c, and conduct the terminals c and b when the terminals are at H level. Here, the terminal a of the switch circuit 7021 is connected to one end of the power supply 101 and the terminal b is connected to the switch circuit 7.
The terminal b and the terminal c of the switch circuit 022 are connected to one end of the capacitor 7041, and the terminal c of the switch circuit 7022 is connected to one end of the capacitor 7042. Further, the switch circuit 70
23, a terminal c is connected to the other end of the capacitor 7042, a terminal a is connected to one end of the power supply 101, and a terminal b is
03 is connected to one end. The other end of the power supply 101, the terminal a of the switch circuit 7022, and the other end of the light emitting element are grounded to a certain common potential (symbol ▽). The configuration is as described above.

<Operation> Next, the operation will be described. First, when the output of the oscillation circuit 105 is L, the switch circuits 7021, 7
022 and 7023 conduct the terminals a and c, so that one end of the capacitor 7041 is grounded at the other end of the power supply 101,
Similarly, one end of the capacitor 7042 is connected to the power supply 101 and the other end is grounded. Therefore, the capacitors 7041 and 7042 are connected in parallel to the power
41 and 7042 are charged. Next, when the output of the oscillation circuit 105 becomes H level, the switch circuits 7021 and 702
2, 7023 conducts the respective terminals b and c of
The capacitors 7041 and 7042 are connected in series, and the voltage of the two capacitors 7041 and 7042 becomes 2V,
This voltage is applied to the light emitting element 103. Here, 2V>
If VF (> V), the amount of charge released by the capacitors 7041 and 7042 is C (2V-VF). Since this is repeated f times per second, the average current is C (2V-V
F) f.

As described above, it is possible to function as a light source even with a power supply voltage equal to or lower than the minimum voltage necessary for the light emitting element, and high efficiency is achieved because there is no useless power consumption as in the first embodiment. In this embodiment, two capacitors are switched in parallel to obtain a voltage of 2 V. However, it is needless to say that an arbitrary number n of capacitors can be switched in parallel to obtain a voltage of nV.

Even when V> VF, depending on the light emitting element, application of a high voltage may have higher luminous efficiency, and this embodiment may be used.

Further, similarly to the first embodiment, the light control means of the second to eighth embodiments can be applied, and the same effect can be obtained.

Tenth Embodiment <Display Device> By using the above-mentioned light source for a light source such as a backlight of a liquid crystal display device, the power consumption of the liquid crystal display device is reduced. In addition, since the dimming is easy, a display with good visibility can be obtained.

Eleventh Embodiment <Electronic Apparatus> By incorporating the above-described display device into an electronic apparatus, the power consumption of the electronic apparatus can be reduced. In addition, an easy-to-read display is provided, and the product quality of the electronic device can be improved.

As such electronic equipment, personal computers
Examples include a computer, a mobile phone, a digital still camera, a liquid crystal television, and a monitor of a video recording device.

[0049]

As described above, according to the present invention, a highly efficient light source can be obtained and light can be easily adjusted.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an electrical configuration of a light source according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a specific configuration example of a switch circuit 102 according to the first embodiment of the present invention.

FIG. 3 shows an oscillation circuit 10 according to the first embodiment of the present invention.
FIG. 5 is a diagram showing a specific configuration example of FIG.

FIG. 4 is a diagram showing an electric circuit configuration of a light source according to a second embodiment of the present invention.

FIG. 5 shows an oscillation circuit 40 according to a second embodiment of the present invention.
FIG. 5 is a diagram showing a specific configuration example of FIG.

FIG. 6 is a diagram showing an electric circuit configuration of a light source according to a fifth embodiment of the present invention.

FIG. 7 is a diagram showing an electric circuit configuration of a light source according to a ninth embodiment of the present invention.

FIG. 8 is a diagram showing an electric circuit configuration of a light source according to the related art.

[Brief description of reference numerals]

 DESCRIPTION OF SYMBOLS 101 ... Power supply 102 ... Switch circuit 103 ... Light emitting element 104 ... Capacitor 105 ... Oscillation circuit 201 ... Pch field effect transistor 202 ... Nch field effect transistor 301 ... Logical inversion circuit of Schmitt trigger input 302 ... Capacitor 303 ... Resistor

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H091 FA44Z FA45Z FD07 FD22 GA11 3K007 AB03 EB00 EC00 GA04 3K073 AA16 AA54 BA28 CE17 CG35 CG42 CG45 CG46 CJ17

Claims (16)

[Claims] 1. A light source comprising: a capacitor; a power supply; a light-emitting element; and a first switch element that connects the capacitor alternately with the power supply or the light-emitting element. 2. A plurality of capacitors, a power supply, a light emitting element, and a state in which the plurality of capacitors are connected in parallel with the power supply or a state in which the plurality of capacitors are connected in series and connected to the light emitting element. A light source comprising a second switch element that alternately switches the light source. 3. The apparatus according to claim 1, wherein said first or second switch element includes a cycle changing means for changing a switching cycle for alternately connecting said capacitor to either said power supply or said light emitting element. 3. The light source according to any one of claims 1 to 2. 4. The light source according to claim 3, wherein said period changing means manually changes said switching period. 5. The light source according to claim 3, wherein said variable means changes said switching period according to the intensity of external light. 6. The light source according to claim 3, wherein said variable means changes said switching period manually or according to the intensity of external light. 7. The light source according to claim 1, further comprising a capacitance changing unit that changes a capacitance of the capacitor or the plurality of capacitors. 8. The light source according to claim 7, wherein said variable capacitance means manually changes the capacitance of said capacitor or said plurality of capacitors. 9. The light source according to claim 7, wherein said capacitance varying means changes the capacitance of said capacitor or said plurality of capacitors according to the intensity of external light. 10. The light source according to claim 7, wherein said capacitance varying means changes the capacitance of said condenser or said plurality of condensers manually and according to the intensity of external light. 11. The light source according to claim 1, further comprising the variable period unit and the variable capacitance unit. 12. The light source according to claim 11, wherein at least one of the period variable unit and the capacitance variable unit is manually or in accordance with the intensity of external light, the switching period or the capacitor or the plurality of capacitors. A light source characterized by changing the capacity of a capacitor. 13. The light source according to claim 1, wherein the light emitting element is a light emitting diode or electroluminescence. 14. An electronic apparatus comprising the light source according to claim 1. 15. A display device comprising the light source according to claim 1. Description: 16. An electronic apparatus comprising the display device according to claim 15. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source used for, for example, a backlight of a liquid crystal display device.