Abstract:
To implement brightness change of pixels due to variations in environmental temperatures with low electric power, the display device includes a display part having a display area arrayed with plural pixels, a display scanning circuit and a signal driving circuit for driving the plural pixels, and a power circuit that supplies a current for illuminating each of the plural pixels with brightness corresponding to a display signal from the signal driving circuit; and a detection unit that includes: a monitor element for driving a constant current that detects environmental temperatures; and plural constant current sources, detects a voltage value relating to the luminous intensity of the pixels by the monitor element to generate a signal to control an output voltage of the power circuit, and changes over a constant current source of the monitor element according to a voltage value detected in the detection unit.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application JP 2007-191296 filed on Jul. 23, 2007, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a display device, and more particularly to a display device that curbs a driving voltage range for light emitting elements corresponding to a change in ambient temperatures to achieve lower power consumption. 
       BACKGROUND OF THE INVENTION 
       [0003]    A spontaneous light emitting display device that configures pixels with light emitting elements such as organic EL elements (OLED: Organic Light Emitting Diode, also referred to as OLED elements) is in a practical stage. An image display device using spontaneous light emitting display elements is characterized by high visibility, not requiring an auxiliary lighting device such as the backlight of a liquid crystal display device, and quick response speed. An organic EL display panel that uses organic EL elements being a paradigm of spontaneous light emitting display elements for current driving changes in light emission luminance, depending on environmental temperatures. The light emission luminance of individual organic EL elements changes also due to secular changes, causing variations in surface brightness of a display area. 
         [0004]      FIG. 16  is a circuit diagram showing a first construction example of an organic EL display panel that constitutes a display device equipped with a traditional temperature correction system.  FIG. 17  is an explanatory drawing of detection operation points of the transitional organic EL display panel shown in  FIG. 16 . In  FIG. 17 , the horizontal axis indicates anode voltages (V) of organic EL element, and the vertical axis indicates a current density (mA/cm 2 ) flowing through an organic EL element. In  FIG. 16 , the display device includes a display part and a detection unit. In a display area  15  of the display part  100 , plural pixels  10  are matrix-arrayed. Each pixel  10  is formed at an intersection of a signal line  11  and a select switch line (scanning line)  12 . Moreover, each pixel  10  is provided with an illumination switch line  13  provided in common for pixels connected to the select switch  12 , and a power line  14  connected in common for pixels connected to a common signal line  11 . 
         [0005]    The signal line  11  is connected to a signal line driving circuit  16 , and supplies a display signal to a pixel selected by the select switch line  12  and the illumination switch line  13  connected to a display scanning circuit  17 . The power line  14  supplies an illumination current to the selected pixel  10  from the power circuit  18  and illuminates the pixel with brightness corresponding to the display signal. A display signal and a timing signal  29  are inputted to the signal line driving circuit  16  and the display scanning circuit  17  from a signal source (not shown) such as a host computer. 
         [0006]    The power circuit  18  is provided with a detection unit  200  that includes a detection unit  200  that includes current source  41 , a monitor element  20  to detect environmental temperatures, a buffer amplifier  21 , an analog/digital converter  22  (AD converter: ADC), and a power control unit  28 . The power control unit  28  controls the power circuit  18 , according to the output of the ADC  22 , based on an environmental temperature detected by the monitor element  20 . Here, an organic EL element is used for the monitor element  20 . 
         [0007]    In the organic EL display panel constructed shown in  FIG. 16 , a current I 1  is fed to the monitor element  20  from the current source  41 . At this time, as shown in  FIG. 17 , the voltage of the anode of the organic EL device being the monitor element  20  is set to a voltage V 1  as a high temperature region when an environmental temperature is a defined temperature abnormality, and set to a voltage V 1 ′ in the case of low temperatures lower than it. The voltages V 1  and V 1 ′ are inputted to the AD converter  22  through the buffer amplifier  21  for conversion into a digital value. The power control unit  28 , when the digital value is small, determines that the system is in the high temperature region, and lowers a power supply voltage of the power circuit. When the digital value is large, it determines that the system is in a low temperature region, and raises a power supply voltage. By using, as the monitor element  20 , the same element as that of the pixel  10  provided in the display area, brightness deterioration and variations due to secular changes can be corrected. 
         [0008]      FIG. 18  is a circuit diagram showing a second construction example of an organic EL display panel that constitutes a display device equipped with a traditional temperature correction system.  FIG. 19  is an explanatory drawing of detection operation of the transitional organic EL display panel shown in  FIG. 18 . In  FIG. 18 , only portions different from  FIG. 16  are described, and descriptions of common portions are omitted because they overlap. Detection control lines  33  are disposed in parallel with the select switch lines  12  and the illumination switch lines  13 . The detection control lines  33  detect current values of pixels connected in common to the select switch lines  12 , and output them to the detection scanning circuit  32 . 
         [0009]    For the detection scanning circuit  32  to detect the respective current values of organic EL elements constituting individual pixels to detect variations in brightness within the display area, and correct them, a detection unit that includes current source  31 , buffer amplifier  21 , AD converter  22 , and signal correction control unit  34  is provided. Changeover switches  43  that include switches SWA (1 to n) turning on and off between the signal driving circuit  16  and the signal lines  11 , and switches SWB (1 to n) turning on and off between the signal lines  11  and the current source  31  are provided. The changeover switches  43  operate so that when one switch is on, the other is off, and vice versa. 
         [0010]    In a normal display mode, switches SWA (1 to n) of the changeover switches  43  are on, and switches SWB (1 to n) are off. In this state, a signal is supplied from the signal driving circuit  16  to a pixel connected to a select switch line  12  selected by the display scanning circuit  17  through the signal line  11 , and the pixel illuminates with brightness corresponding to the value of the display signal by an illumination signal of the illumination switch lines  13  to display a required two-dimensional image. 
         [0011]    On the other hand, in a detection mode, switches SWB (1 to n) of the changeover switches  43  are on, and switches SWA (1 to n) are off. Changeover to the detection mode may be made when main power to the image display device is turned on or off, during flyback period, or by a manual operation. 
         [0012]    In the detection mode, a current I 3  is fed from the current source  31  to organic EL elements of pixels through the signal lines  11  of the pixel side to monitor properties. At this time, a voltage of the anode of the organic EL elements is V 3  before deterioration and V 3 ′ after deterioration, as shown in  FIG. 19 . The voltages V 3  and V 3 ′ are inputted to the AD converter (ADC)  22  through the buffer amplifier for change to digital values. When the digital values are below a specific value, the system determines that the organic EL elements do not deteriorate, and does not perform special brightness adjustment. However, when the digital values are greater than the specific value, the system determines that the organic EL elements deteriorate, and the signal correction control unit  34  affords a control signal to the signal driving circuit  16  to correct the display signal. 
         [0013]    For individual pixels, their current values are individually detected by scanning of the detection scanning circuit  32  and the signal timing of the signal driving circuit  16 , and determined in the signal correction control unit  34 . Thereby, even when the organic EL elements deteriorate due to secular changes, high-quality image display free of variations is achieved while maintaining a given brightness. 
         [0014]    This system configuration achieves stable brightness control regardless of large variations in environmental temperatures. Such a related art is disclosed in JP-A-2006-048011. 
       SUMMARY OF THE INVENTION 
       [0015]    Organic EL elements depend on current values for their luminous intensity. In the conventional temperature correction control system as described above, the buffer amplifiers and the AD converter require large power consumption. That is, since the temperature coefficient of the organic EL elements is as large as several tens mV/degree, voltages for securing currents for obtaining brightness corresponding to temperature changes change greatly, a voltage difference V 1 ′ and V 1  as shown in  FIG. 17  is large. When the voltage difference is large, since a voltage range necessary for the buffer amplifier and the AD converter of  FIG. 16  become large, the display device does not operate with a low power supply voltage, and electric power consumed in the buffer amplifier and the AD converter becomes large. 
         [0016]    In JP-A 2006-48011, a monitor element for driving a constant current is provided, a voltage applied to the monitor element is detected, and the voltage is applied to a light emitting element, whereby brightness variations due to changes in environmental temperatures and secular changes are curbed. However, since the organic EL element change greatly in its properties, depending on environmental temperatures and secular changes, the range of detected voltages are wide. Therefore, since the range of voltages necessary for the buffer amplifier and the like to buffer a detected voltage becomes wide, high power supply voltages are required to constitute circuits such as the buffer amplifier, resulting in large power consumption. 
         [0017]    A buffer amplifier and an AD converter provided for transitional secular change correction systems have large power consumption. When the deterioration of organic EL elements halves brightness, since the systems operate at voltage V 3 ′ as shown in  FIG. 19 , a voltage difference is large with respect to voltage V 3  before the deterioration of the organic EL elements. When a system is built with the deterioration of organic EL elements in mind, since a voltage range necessary for the buffer amplifier and the AD converter in  FIG. 18  becomes large, the system does not operate at a low power supply voltage, and electric power consumed in the buffer amplifier and the AD converter becomes large. 
         [0018]    A first object of the present invention is to provide a display device that realizes brightness change of pixels due to variations in environmental temperatures with low electric power. A second object of the present invention is to provide a display device that realizes brightness variations among pixels due to deterioration as a result of secular changes with low electric power. 
         [0019]    To achieve the first object, a display device of the present invention includes: a display part including a display area arrayed with plural pixels, a display scanning circuit and a signal driving circuit for driving the plural pixels, and a power circuit that supplies a current for illuminating each of the plural pixels with brightness corresponding to a display signal from the signal driving circuit; and a detection unit that includes: a monitor element for driving a constant current that detects environmental temperatures; and plural constant current sources, detects a voltage value relating to the luminous intensity of the pixels by the monitor element to generate a signal to control an output voltage of the power circuit, and changes over a constant current source of the monitor element according to a voltage value detected in the detection unit. 
         [0020]    To achieve the second object, a display device of the present invention includes: a display part including a display area arrayed with plural pixels, a display scanning circuit and a signal driving circuit for driving the plural pixels, a power circuit that supplies a current for illuminating each of the plural pixels with brightness corresponding to a display signal from the signal driving circuit, a detection control line to detect current values of the pixels, 
         [0000]    a detection scanning circuit that applies a scanning signal to the detection control line, and a display part changeover means that alternatively selects the signal driving circuit and the detection unit changeover means for the signal line; and a detection unit that includes a current source to output plural constant current values, a detection unit changeover means to select one of the current sources, and a signal correction control unit that is connected to the signal driving circuit and corrects a display signal supplied to the signal line. 
         [0021]    By the construction for achieving the first object, by changing over a constant current value of the monitor element according to a voltage value detected in the detection unit, a variation range of voltages for feeding a current value corresponding to an environmental temperature to the monitor element can be reduced. 
         [0022]    By the construction for achieving the second object, a display signal supplied to the pixels according to a voltage value detected in the detection unit to reduce variations in luminous intensity due to secular changes. 
         [0023]    Display elements used for pixels and monitor elements are not limited to organic EL elements, and the present invention can also apply to a display device using spontaneous light emitting display elements that is reduced in luminous intensity due to variations in environmental temperatures and deterioration due to secular changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a first embodiment of a display device of the present invention; 
           [0025]      FIG. 2  is an explanatory drawing of detection operation of the organic EL display panel shown in  FIG. 1 ; 
           [0026]      FIG. 3  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a second embodiment of a display device of the present invention; 
           [0027]      FIG. 4  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a third embodiment of a display device of the present invention; 
           [0028]      FIG. 5  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a fourth embodiment of a display device of the present invention; 
           [0029]      FIG. 6  is a block diagram of an organic EL display panel that corrects reduction in light emission luminance caused by deterioration due to secular change, to describe a fifth embodiment of a display device of the present invention; 
           [0030]      FIG. 7  is an explanatory drawing of detection operation of the organic EL display panel shown in  FIG. 6 ; 
           [0031]      FIG. 8  is a block diagram of an organic EL display panel that corrects reduction in light emission luminance caused by deterioration due to secular change, to describe a sixth embodiment of a display device of the present invention; 
           [0032]      FIG. 9  is a block diagram of an organic EL display panel that corrects reduction in light emission luminance caused by deterioration due to secular change, to describe a seventh embodiment of a display device of the present invention; 
           [0033]      FIG. 10  is a circuit diagram for describing a first construction example suitable for a pixel circuit in the embodiments of  FIGS. 1 ,  3 , and  4 ; 
           [0034]      FIG. 11  is a circuit diagram for describing a second construction example suitable for a pixel circuit in the embodiments of  FIGS. 1 ,  3 , and  4 ; 
           [0035]      FIG. 12  is a circuit diagram for describing a third construction example suitable for a pixel circuit in the embodiments of  FIGS. 5 ,  6 ,  8 , and  9 ; 
           [0036]      FIG. 13  is a circuit diagram for describing a fourth construction example suitable for a pixel circuit in the embodiments of  FIGS. 5 ,  6 ,  8 , and  9 ; 
           [0037]      FIG. 14A  and  FIG. 14B  are drawings showing an example of electronic equipment equipped with a display device of the present invention; 
           [0038]      FIG. 15A  and  FIG. 15B  are drawings showing an example of electronic equipment equipped with a display device of the present invention; 
           [0039]      FIG. 16  is a circuit diagram showing a first construction example of an organic EL display panel that constitutes a display device equipped with a traditional temperature correction system; 
           [0040]      FIG. 17  is an explanatory drawing of detection operation points of a transitional organic EL display panel shown in  FIG. 16 ; 
           [0041]      FIG. 18  is a circuit diagram showing a second construction example of an organic EL display panel that constitutes a display device equipped with a traditional temperature correction system; and 
           [0042]      FIG. 19  is an explanatory drawing of detection operation of the transitional organic EL display panel shown in  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    Preferred embodiments of the present invention will be described in detail below. 
       First Embodiment 
       [0044]      FIG. 1  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a first embodiment of a display device of the present invention.  FIG. 2  is an explanatory drawing of detection operation of the organic EL display panel shown in  FIG. 1 .  FIG. 2  is an explanatory drawing of detection operation of the organic EL display panel shown in  FIG. 1 . In  FIG. 1 , plural pixels  10  are matrix-arrayed in a display area  15  of a display part  100  of the organic EL display panel. Each pixel  10  is formed in an intersection of a signal line  11  and a select switch (scanning line)  12 . Each pixel  10  is also provided with a luminance switch line  13  provided in common for pixels connected to the select switch line  12 , and a power line  14  connected in common to pixels connected to the common signal lines  11 . 
         [0045]    The signal lines  11 , which are connected to a signal line driving circuit  16 , supply a display signal to a pixel selected by the select switch lines  12  connected to the display scanning circuit  17  and the luminance switch lines. The power lines  14  supply a luminance current to the selected pixel  10  from a power circuit  18  and light the pixel  10  with brightness corresponding to the display signal. A display signal and a timing signal  29  (not shown in the drawing) are inputted to the signal line driving circuit  16  and the display scanning circuit  17  from a signal source (not shown in the drawing) such as a host computer. 
         [0046]    The power circuit  18  is provided with a detection unit  200  that includes a first current source  25 , a second current source  26 , changeover switch  44 , a monitor element  20  to detect environment temperatures, a buffer amplifier  21 , an analog/digital converter (AD converter: ADC)  22 , a power control unit  28 , a decoder control unit  26 , and a decoder  27 . According to the output of the AD converter  22  based on an environmental temperature detected by the monitor element  20 , the power control unit  28  controls the power circuit  18 , and the output of the AD converter  22  is supplied to the decoder  27  from the decoder control unit  26  to switch the changeover switch  44 . Organic EL elements are used for the monitor element  20 . 
         [0047]    The changeover switch  44  includes a first switch (hereinafter referred to as a low temperature side switch) SW 1  and a second switch (hereinafter referred to as a high temperature side switch) SW 2 . The changeover switch  44  enables the first current source  25  and the second current source  26  to be switched on and off, or switched off and on. 
         [0048]    The changeover switch  44  is on in the high temperature side switch SW 1 , and off in the low temperature side switch SW 2 . In this state, a current I 1  flows through the organic EL element  20  being a monitor element from the first current source  25 . At this time, a voltage of the anode of the organic EL device  20  is V 1  as shown in  FIG. 2 . The voltage V 1  rises as temperatures become lower, and digital values converted by the AD converter  22  also increase. 
         [0049]    A threshold value is provided for the digital values, and when the decoder control unit  26  is equal to or greater than a digital value corresponding to a voltage V 2 , the decoder control unit turns off the high temperature side switch SW 1  and turns on the low temperature side switch SW 2 . When the low temperature side switch SW 2  has been switched on, the second current source  26  is supplied to the organic EL element  20 . A detection voltage at this time is in a range from V 1  to V 2 . 
         [0050]    By the first embodiment, a variation range of voltages for feeding current values corresponding to variations in environmental temperatures to the monitor element can be reduced. Therefore, voltage ranges of V 1  and V 2  can be reduced, enabling the display device to operate with low power consumption. 
       Second Embodiment 
       [0051]      FIG. 3  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a second embodiment of a display device of the present invention. In a second embodiment, a decoder is not used for switching control of current sources as it is in the first embodiment, but a comparator  30  is used. That is, an analog output of the buffer amplifier  21  is inputted directly to the comparator  30  for comparison with a specific value set in advance by a resistance dividing circuit or the like. A result of the comparison is used as a changeover signal of the changeover switch  44  of a detection side. Other constructions are the same as those in the first embodiment. The comparator  30  is an analog circuit. Use of such an analog circuit also enables changeover control of current sources. 
         [0052]    Also by the second embodiment, a variation range of voltages for feeding current values corresponding to variations in environmental temperatures to the monitor element can be reduced. As a result, voltage ranges of V 1  and V 2  can be reduced, enabling the display device to operate with low power consumption. 
       Third Embodiment 
       [0053]      FIG. 4  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a third embodiment of the display device of the present invention. The third embodiment is characterized in that a constant current source of band gap type is used as a current source of the detection unit  200  in the first embodiment. The constant current source  31  of band gap type includes a parallel circuit of a first external resistor R 1  and a second external resistor R 2  that have different resistance values, and a detection unit changeover switch  44  that selectively connects a first external resistor R 1  and a second external resistor R 2  to the constant current source  31 . Other constructions are the same as those in the first embodiment. 
         [0054]    Since current amounts supplied by the constant current source  31  of band gap type equipped with the external resistors are inversely proportional to resistance values of the external resistors, current amounts can be adjusted simply by changing over the external resistors. Therefore, one external current source has only to be provided, with the result that there are fewer external parts. 
         [0055]    By the third embodiment, a variation range of voltages for feeding current values corresponding to variations in environmental temperatures to the monitor element can be reduced. As a result, voltage ranges of V 1  and V 2  can be reduced, enabling the display device to operate with low power consumption. 
       Fourth Embodiment 
       [0056]      FIG. 5  is a block diagram of an organic EL display panel equipped with a temperature correction system to describe a fourth embodiment of the display device of the present invention. In the first to fourth embodiments described previously, the same organic EL element as the display element to constitute the pixels of the display part is used for the monitor element of the detection part  200  to detect detects environmental temperatures. On the other hand, in the fourth embodiment, the organic EL element to constitute the pixels of the display part  100  is used as a detection element of environmental temperatures. Therefore, a display part changeover switch  43  is inserted between the signal lines  11  and the signal driving circuit of the display part  100 , detection control lines  33  to detect a current value of the pixel  10  are provided in parallel with the select switch lines  12 , and a detection scanning circuit  32  to apply a scanning signal to the detection control lines  33  is provided. 
         [0057]    In  FIG. 5 , when a signal for displaying images is supplied to the pixel  10 , SWA 1 , SWA 2 , . . . , SWAn of the display part changeover switch  43  are selectively turned on, and when an organic EL element of a pixel is monitored, any of SWB 1 , SWB 2 , . . . , SWBn is selected. The organic EL element to be monitored of a pixel of a specific signal line is selected vertically by the detection scanning circuit  32  and horizontally by turning on any of switches SWB 1 , SWB 2 , . . . , SWBn. The organic EL element to be selected is optional. 
         [0058]    According to the fourth embodiment, without needing elements for monitor, a variation range of voltages for feeding current values corresponding to variations in environmental temperatures to the monitor element can be reduced. Therefore, voltage ranges of V 1  and V 2  described previously can be reduced, enabling the display device to operate with low power consumption. 
       Fifth Embodiment 
       [0059]      FIG. 6  is a block diagram of an organic EL display panel that corrects reduction in light emission luminance caused by deterioration due to secular change, to describe a fifth embodiment of a display device of the present invention.  FIG. 7  is an explanatory drawing of detection operation of the organic EL display panel shown in  FIG. 6 . In the fourth embodiment of  FIG. 5 , one output of the AD converter  22  is afforded to the power control unit  28  to change over a voltage of the power circuit  18 . In contrast to this, in the fifth embodiment, a signal correction circuit  34  is provided that inputs one output of the AD converter  22  to correct a display signal supplied from the signal driving circuit  16  to the signal lines  11 . The same power control unit  28  as that in  FIG. 5  may be provided in  FIG. 6 . 
         [0060]    In  FIG. 6 , as is conventionally done, the switch SW 3  of the detection unit changeover switch  44  is selected, and the switches SWA 3  to SWAn of the display part changeover switch  43  are selected, whereby a current I 3  is fed from the first power source (high-voltage side power source)  25  to the organic EL element of the pixel  10 . At this time, a voltage of the anode of the organic EL device is V 3  as shown in  FIG. 7 . The voltage V 3  rise as the element deteriorates, and digital values converted by the AD converter  22  also increase. Here, a threshold value is provided in advance for the digital values, and the decoder  27  is provided that, when a digital value corresponding to a voltage V 4  or greater is reached, turns off the switch SW 3  of the detection unit changeover switch  44 , and turns on the switch SW 4 . A detection voltage at this time is in a range from V 3  to V 4 . Voltage ranges of V 3  and V 4  are small. 
         [0061]    According to the fifth embodiment, a variation range of voltages for feeding current values to correct variations in light emission luminance caused by deterioration due to secular change of organic EL elements can be reduced. Therefore, voltage ranges of the V 3  and V 4  described previously are small, enabling the display device to operate with low power consumption. 
       Sixth Embodiment 
       [0062]      FIG. 8  is a block diagram of an organic EL display panel that corrects reduction in light emission luminance caused by deterioration due to secular change, to describe a sixth embodiment of the display device of the present invention. In the sixth embodiment, the comparator  30  is provided in place of the decoder control unit  26  and the decoder  27  of the fifth embodiment described in  FIG. 6 . That is, analog output of the buffer amplifier  21  is inputted directly to the comparator  30  for comparison with a specific value set previously by a resistance dividing circuit or the like. A result of the comparison is used as a changeover signal of the detection side changeover switch  44 . Other constructions are the same as those in the fifth embodiment. The comparator  30  is an analog circuit. Even use of such an analog circuit allow changeover control of current sources. 
         [0063]    Also by the sixth embodiment, a variation range of voltages for feeding current values to correct variations in light emission luminance caused by deterioration due to secular change of organic EL elements can be reduced. Therefore, voltage ranges of the V 3  and V 4  described previously are small, enabling the display device to operate with low power consumption. 
       Seventh Embodiment 
       [0064]      FIG. 9  is a block diagram of an organic EL display panel that corrects reduction in light emission luminance caused by deterioration due to secular change, to describe a seventh embodiment of the display device of the present invention. The seventh embodiment is characterized in that the constant current source  31  of band gap type is used in place of the first and second current sources  25  and  26  in the sixth embodiment. The constant current source  31  of band gap type includes a parallel circuit of a first external resistor R 1  and a second external resistor R 2  that have different resistance values, and a detection unit changeover switch  44  consisting of switches SW 1  and SW 2  that selectively connects a first external resistor R 1  and a second external resistor R 2  to the constant current source  31 . Other constructions are the same as those in the first embodiment. 
         [0065]    Since current amounts supplied by the constant current source  31  of band gap type equipped with the external resistors are inversely proportional to resistance values of the external resistors, current amounts can be adjusted simply by changing over the external resistors. Therefore, one external current source has only to be provided, with the result that there are fewer external parts. 
         [0066]    Also by the seventh embodiment, a variation range of voltages for feeding current values to correct variations in light emission luminance caused by deterioration due to secular change of organic EL elements can be reduced. Therefore, voltage ranges of the V 3  and V 4  described previously are small, enabling the display device to operate with low power consumption. 
         [0067]    The following describes a pixel configuration provided in a display area of the display device of the present invention. The same reference numerals as those in the previous embodiments in each drawing correspond to same functional portions.  FIG. 10  is a circuit diagram for describing a first construction example suitable for a pixel circuit in the embodiments of  FIGS. 1 ,  3 , and  4 . In  FIG. 10 , a portion enclosed by the dotted line indicates one pixel. One pixel includes a select switch  36  connected to a signal line  11  and a select switch  12 , a holding capacitor  37  to hold a display signal, an OLED driving switch  38  that drives an organic EL element (OLED element)  35  according to the magnitude of the display signal held in the holding capacitor  37 , and an illumination switch  39  that supplies an illumination current from a power line  14  to the OLED element  35  through the OLED driving switch  38  in illumination timing of the OLED element  35 . 
         [0068]      FIG. 11  is a circuit diagram for describing a second construction example suitable for a pixel circuit in the embodiments of  FIGS. 1 ,  3 , and  4 . In  FIG. 11 , a portion enclosed by the dotted line indicates one pixel. The pixel circuit of  FIG. 11  is constructionally almost the same as that of  FIG. 10 , except that the disposition of the select switch  36  and the holding capacitor  37  is different from that of  FIG. 10 . 
         [0069]      FIG. 12  is a circuit diagram for describing a third construction example suitable for a pixel circuit in the embodiments of  FIGS. 5 ,  6 ,  8 , and  9 . In  FIG. 12 , a portion enclosed by the dotted line indicates one pixel. The pixel circuit of  FIG. 11  is an addition of a detection line  33  and a detection switch  40  connected to the detection line  33  to the circuit of  FIG. 10 . 
         [0070]      FIG. 13  is a circuit diagram for describing a fourth construction example suitable for a pixel circuit in the embodiments of  FIGS. 5 ,  6 ,  8 , and  9 . In  FIG. 13 , a portion enclosed by the dotted line indicates one pixel. The pixel circuit of  FIG. 13  is an addition of the detection line  33  and the detection switch  40  connected to the detection line  33  to the circuit of  FIG. 11 . 
         [0071]      FIGS. 14 and 15  are drawings showing an example of electronic equipment equipped with the display device of the present invention.  FIG. 14A  shows a mobile electronic equipment  50 , a so-called cellular phone, and its display part  51  is equipped with the display device of the present invention.  FIG. 14B  shows a television receiver  60 , and its display part  61  is equipped with the display device of the present invention. 
         [0072]      FIG. 15A  shows a digital portable terminal  70 , a so-called PDA, and its display part  71  is equipped with the display device of the present invention. A touch panel is mounted in the display part  71 . A reference numeral  72  indicates a stick for screen input.  FIG. 15B  shows a video camera  80 , and its monitor part  81  and finder part  82  each are equipped with the display device of the present invention. It goes without saying that the display device of the present invention can find various applications as described above.