Display device

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.

CLAIM OF PRIORITY

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

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

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.

FIG. 16is 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. 17is an explanatory drawing of detection operation points of the transitional organic EL display panel shown inFIG. 16. InFIG. 17, the horizontal axis indicates anode voltages (V) of organic EL element, and the vertical axis indicates a current density (mA/cm2) flowing through an organic EL element. InFIG. 16, the display device includes a display part and a detection unit. In a display area15of the display part100, plural pixels10are matrix-arrayed. Each pixel10is formed at an intersection of a signal line11and a select switch line (scanning line)12. Moreover, each pixel10is provided with an illumination switch line13provided in common for pixels connected to the select switch12, and a power line14connected in common for pixels connected to a common signal line11.

The signal line11is connected to a signal line driving circuit16, and supplies a display signal to a pixel selected by the select switch line12and the illumination switch line13connected to a display scanning circuit17. The power line14supplies an illumination current to the selected pixel10from the power circuit18and illuminates the pixel with brightness corresponding to the display signal. A display signal and a timing signal29are inputted to the signal line driving circuit16and the display scanning circuit17from a signal source (not shown) such as a host computer.

The power circuit18is provided with a detection unit200that includes a detection unit200that includes current source41, a monitor element20to detect environmental temperatures, a buffer amplifier21, an analog/digital converter22(AD converter: ADC), and a power control unit28. The power control unit28controls the power circuit18, according to the output of the ADC22, based on an environmental temperature detected by the monitor element20. Here, an organic EL element is used for the monitor element20.

In the organic EL display panel constructed shown inFIG. 16, a current I1is fed to the monitor element20from the current source41. At this time, as shown inFIG. 17, the voltage of the anode of the organic EL device being the monitor element20is set to a voltage V1as a high temperature region when an environmental temperature is a defined temperature abnormality, and set to a voltage V1′ in the case of low temperatures lower than it. The voltages V1and V1′ are inputted to the AD converter22through the buffer amplifier21for conversion into a digital value. The power control unit28, 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 element20, the same element as that of the pixel10provided in the display area, brightness deterioration and variations due to secular changes can be corrected.

FIG. 18is 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. 19is an explanatory drawing of detection operation of the transitional organic EL display panel shown inFIG. 18. InFIG. 18, only portions different fromFIG. 16are described, and descriptions of common portions are omitted because they overlap. Detection control lines33are disposed in parallel with the select switch lines12and the illumination switch lines13. The detection control lines33detect current values of pixels connected in common to the select switch lines12, and output them to the detection scanning circuit32.

For the detection scanning circuit32to 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 source31, buffer amplifier21, AD converter22, and signal correction control unit34is provided. Changeover switches43that include switches SWA (1 to n) turning on and off between the signal driving circuit16and the signal lines11, and switches SWB (1 to n) turning on and off between the signal lines11and the current source31are provided. The changeover switches43operate so that when one switch is on, the other is off, and vice versa.

In a normal display mode, switches SWA (1 to n) of the changeover switches43are on, and switches SWB (1 to n) are off. In this state, a signal is supplied from the signal driving circuit16to a pixel connected to a select switch line12selected by the display scanning circuit17through the signal line11, and the pixel illuminates with brightness corresponding to the value of the display signal by an illumination signal of the illumination switch lines13to display a required two-dimensional image.

On the other hand, in a detection mode, switches SWB (1 to n) of the changeover switches43are 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.

In the detection mode, a current I3is fed from the current source31to organic EL elements of pixels through the signal lines11of the pixel side to monitor properties. At this time, a voltage of the anode of the organic EL elements is V3before deterioration and V3′ after deterioration, as shown inFIG. 19. The voltages V3and V3′ are inputted to the AD converter (ADC)22through 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 unit34affords a control signal to the signal driving circuit16to correct the display signal.

For individual pixels, their current values are individually detected by scanning of the detection scanning circuit32and the signal timing of the signal driving circuit16, and determined in the signal correction control unit34. 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.

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

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 V1′ and V1as shown inFIG. 17is large. When the voltage difference is large, since a voltage range necessary for the buffer amplifier and the AD converter ofFIG. 16become 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.

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.

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 V3′ as shown inFIG. 19, a voltage difference is large with respect to voltage V3before 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 inFIG. 18becomes 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.

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.

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.

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,

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.

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.

By the construction for achieving the second object, by correcting a display signal supplied to the pixels according to a voltage value detected in the detection unit, variations in luminous intensity due to secular changes can be reduced.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described in detail below.

First Embodiment

FIG. 1is 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. 2is an explanatory drawing of detection operation of the organic EL display panel shown inFIG. 1.FIG. 2is an explanatory drawing of detection operation of the organic EL display panel shown inFIG. 1. InFIG. 1, plural pixels10are matrix-arrayed in a display area15of a display part100of the organic EL display panel. Each pixel10is formed in an intersection of a signal line11and a select switch (scanning line)12. Each pixel10is also provided with a luminance switch line13provided in common for pixels connected to the select switch line12, and a power line14connected in common to pixels connected to the common signal lines11.

The signal lines11, which are connected to a signal line driving circuit16, supply a display signal to a pixel selected by the select switch lines12connected to the display scanning circuit17and the luminance switch lines. The power lines14supply a luminance current to the selected pixel10from a power circuit18and light the pixel10with brightness corresponding to the display signal. A display signal and a timing signal29(not shown in the drawing) are inputted to the signal line driving circuit16and the display scanning circuit17from a signal source (not shown in the drawing) such as a host computer.

The power circuit18is provided with a signal from a detection unit200that includes a first current source25, a second current source26, changeover switch44, a monitor element20to detect environment temperatures, a buffer amplifier21, an analog/digital converter (AD converter: ADC)22, a power control unit28, a decoder control unit26, and a decoder27. According to the output of the AD converter22based on an environmental temperature detected by the monitor element20, the power control unit28controls the power circuit18, and the output of the AD converter22is supplied to the decoder27from the decoder control unit26to switch the changeover switch44. Organic EL elements are used for the monitor element20.

The changeover switch44includes a first switch (hereinafter referred to as a high temperature side switch) SW1and a second switch (hereinafter referred to as a low temperature side switch) SW2. The changeover switch44enables the first current source25and the second current source26to be switched on and off, or switched off and on.

The changeover switch44is on in the high temperature side switch SW1, and off in the low temperature side switch SW2. In this state, a current I1flows through the organic EL element20being a monitor element from the first current source25. At this time, a voltage of the anode of the organic EL device20is V1as shown inFIG. 2. The voltage V1rises as temperatures become lower, and digital values converted by the AD converter22also increase.

A threshold value is provided for the digital values, and when the decoder control unit26is equal to or greater than a digital value corresponding to a voltage V2, the decoder control unit turns off the high temperature side switch SW1and turns on the low temperature side switch SW2. When the low temperature side switch SW2has been switched on, the second current source26is supplied to the organic EL element20. A detection voltage at this time is in a range from V1to V2.

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 V1and V2can be reduced, enabling the display device to operate with low power consumption.

Second Embodiment

FIG. 3is 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 comparator30is used. That is, an analog output of the buffer amplifier21is inputted directly to the comparator30for 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 switch44of a detection side. Other constructions are the same as those in the first embodiment. The comparator30is an analog circuit. Use of such an analog circuit also enables changeover control of current sources.

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 V1and V2can be reduced, enabling the display device to operate with low power consumption.

Third Embodiment

FIG. 4is 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 unit200in the first embodiment. The constant current source31of band gap type includes a parallel circuit of a first external resistor R1and a second external resistor R2that have different resistance values, and a detection unit changeover switch44that selectively connects a first external resistor R1and a second external resistor R2to the constant current source31. Other constructions are the same as those in the first embodiment.

Since current amounts supplied by the constant current source31of 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.

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 V1and V2can be reduced, enabling the display device to operate with low power consumption.

Fourth Embodiment

FIG. 5is 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 part200to detect detects environmental temperatures. On the other hand, in the fourth embodiment, the organic EL element to constitute the pixels of the display part100is used as a detection element of environmental temperatures. Therefore, a display part changeover switch43is inserted between the signal lines11and the signal driving circuit of the display part100, detection control lines33to detect a current value of the pixel10are provided in parallel with the select switch lines12, and a detection scanning circuit32to apply a scanning signal to the detection control lines33is provided.

InFIG. 5, when a signal for displaying images is supplied to the pixel10, SWA1, SWA2, . . . , SWAn of the display part changeover switch43are selectively turned on, and when an organic EL element of a pixel is monitored, any of SWB1, SWB2, . . . , 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 circuit32and horizontally by turning on any of switches SWB1, SWB2, . . . , SWBn. The organic EL element to be selected is optional.

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 V1and V2described previously can be reduced, enabling the display device to operate with low power consumption.

Fifth Embodiment

FIG. 6is 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. 7is an explanatory drawing of detection operation of the organic EL display panel shown inFIG. 6. In the fourth embodiment ofFIG. 5, one output of the AD converter22is afforded to the power control unit28to change over a voltage of the power circuit18. In contrast to this, in the fifth embodiment, a signal correction circuit34is provided that inputs one output of the AD converter22to correct a display signal supplied from the signal driving circuit16to the signal lines11. The same power control unit28as that inFIG. 5may be provided inFIG. 6.

InFIG. 6, as is conventionally done, the switch SW3of the detection unit changeover switch44is selected, and the switches SWA3to SWAn of the display part changeover switch43are selected, whereby a current I3is fed from the first power source (high-voltage side power source)25to the organic EL element of the pixel10. At this time, a voltage of the anode of the organic EL device is V3as shown inFIG. 7. The voltage V3rise as the element deteriorates, and digital values converted by the AD converter22also increase. Here, a threshold value is provided in advance for the digital values, and the decoder27is provided that, when a digital value corresponding to a voltage V4or greater is reached, turns off the switch SW3of the detection unit changeover switch44, and turns on the switch SW4. A detection voltage at this time is in a range from V3to V4. Voltage ranges of V3and V4are small.

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 V3and V4described previously are small, enabling the display device to operate with low power consumption.

Sixth Embodiment

FIG. 8is 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 comparator30is provided in place of the decoder control unit26and the decoder27of the fifth embodiment described inFIG. 6. That is, analog output of the buffer amplifier21is inputted directly to the comparator30for 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 switch44. Other constructions are the same as those in the fifth embodiment. The comparator30is an analog circuit. Even use of such an analog circuit allow changeover control of current sources.

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 V3and V4described previously are small, enabling the display device to operate with low power consumption.

Seventh Embodiment

FIG. 9is 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 source31of band gap type is used in place of the first and second current sources25and26in the sixth embodiment. The constant current source31of band gap type includes a parallel circuit of a first external resistor R1and a second external resistor R2that have different resistance values, and a detection unit changeover switch44consisting of switches SW1and SW2that selectively connects a first external resistor R1and a second external resistor R2to the constant current source31. Other constructions are the same as those in the first embodiment.

Since current amounts supplied by the constant current source31of 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.

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 V3and V4described previously are small, enabling the display device to operate with low power consumption.

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. 10is a circuit diagram for describing a first construction example suitable for a pixel circuit in the embodiments ofFIGS. 1,3, and4. InFIG. 10, a portion enclosed by the dotted line indicates one pixel. One pixel includes a select switch36connected to a signal line11and a select switch12, a holding capacitor37to hold a display signal, an OLED driving switch38that drives an organic EL element (OLED element)35according to the magnitude of the display signal held in the holding capacitor37, and an illumination switch39that supplies an illumination current from a power line14to the OLED element35through the OLED driving switch38in illumination timing of the OLED element35.

FIG. 11is a circuit diagram for describing a second construction example suitable for a pixel circuit in the embodiments ofFIGS. 1,3, and4. InFIG. 11, a portion enclosed by the dotted line indicates one pixel. The pixel circuit ofFIG. 11is constructionally almost the same as that ofFIG. 10, except that the disposition of the select switch36and the holding capacitor37is different from that ofFIG. 10.

FIG. 12is a circuit diagram for describing a third construction example suitable for a pixel circuit in the embodiments ofFIGS. 5,6,8, and9. InFIG. 12, a portion enclosed by the dotted line indicates one pixel. The pixel circuit ofFIG. 11is an addition of a detection line33and a detection switch40connected to the detection line33to the circuit ofFIG. 10.

FIG. 13is a circuit diagram for describing a fourth construction example suitable for a pixel circuit in the embodiments ofFIGS. 5,6,8, and9. InFIG. 13, a portion enclosed by the dotted line indicates one pixel. The pixel circuit ofFIG. 13is an addition of the detection line33and the detection switch40connected to the detection line33to the circuit ofFIG. 11.

FIGS. 14 and 15are drawings showing an example of electronic equipment equipped with the display device of the present invention.FIG. 14Ashows a mobile electronic equipment50, a so-called cellular phone, and its display part51is equipped with the display device of the present invention.FIG. 14Bshows a television receiver60, and its display part61is equipped with the display device of the present invention.

FIG. 15Ashows a digital portable terminal70, a so-called PDA, and its display part71is equipped with the display device of the present invention. A touch panel is mounted in the display part71. A reference numeral72indicates a stick for screen input.FIG. 15Bshows a video camera80, and its monitor part81and finder part82each 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.