Patent Publication Number: US-7215307-B2

Title: Drive unit of self-luminous device with degradation detection function

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a drive unit of a self-luminous device such as an organic electroluminescent device and the like. 
   2. Description of the Related Art 
   An image display device used in a portable terminal such as a hand-held mobile phone and the like requires a thin-profile display panel. As the conventional thin-profile display panel, a liquid crystal display panel is generally used. However, a display panel which is constituted of a matrix of a plurality of organic electroluminescent devices, hereinafter called organic EL devices, is more preferable as the image display device for portable terminal, because the display panel with the organic EL devices is not only thin but also lightweight. 
   Two methods are generally used to drive the organic EL device, those are, a current driving method and a voltage driving method. The organic EL device emits light, luminance of which is corresponding to a supplied current level, so that the drive unit adopting the current driving method keeps a current supplied to the organic EL device at a constant current level, and the drive unit adopting the voltage driving method keeps voltage applied to the organic EL device at a constant voltage level. 
   However, since the organic EL device is a self-luminous device, a current-luminance characteristic is varied depending on cumulative driving period and the operating environment. When the organic EL device is driven with a constant current, the luminance decreases as the driving time increases. On the other hand, the luminance increases as the ambient temperature increases, and it decreases as the ambient temperature decreases. When the organic EL device is driven with a constant voltage, the rate of variation in the luminance is larger than that in the case where the organic EL device is driven with the constant current. This is because the amount of the current flowing through the organic EL device changes as a consequence of the variation in impedance of the organic EL device depending on the driving time and the operating environment. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a drive unit which can prevent a problem such as the lowering of luminance intensity of a self-luminous device such as an organic electroluminescent device and the like due to a change of a characteristic of the self-luminous device. 
   A drive unit according to the present invention drives a self-luminous device to make it emit light. The drive unit includes a semiconductor device having an electric characteristic substantially equal to an electric characteristic of the self-luminous device, a driver for driving the semiconductor device in accordance with the frequency of light emission from the self-luminous device, a characteristic change detector for generating a characteristic change detection signal indicating a degree of change in an electric characteristic of the semiconductor device, and a drive signal supply device for supplying the self-luminous device with a drive signal having a current level or a voltage level based on the characteristic change detection signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the configuration of a drive unit driven by a current driving method according to the present invention; 
       FIG. 2  is a graph showing variations in impedance and luminance of an organic EL device with a lapse of time; 
       FIG. 3  is a block diagram showing a drive unit adopting a voltage driving method according to the present invention; 
       FIG. 4  is a block diagram showing a part of a drive unit according to another embodiment of the present invention; 
       FIG. 5  is a block diagram showing a part of a drive unit according to still another embodiment of the present invention; 
       FIG. 6  is a block diagram showing the configuration of a drive unit adopting the current driving method according to still another embodiment of the present invention; and 
       FIG. 7  a block diagram showing a further embodiment of the drive unit according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be hereinafter described in detail with reference to the accompanying drawings. 
     FIG. 1  shows an embodiment of a drive unit of a display panel adopting a current driving method according to the present invention. This drive unit has a display panel  1 , a display control circuit  2 , an anode line driving circuit  3 , and a cathode line scanning circuit  4 . The display panel  1  is a matrix display panel on which an organic EL device (an organic electroluminescent device) is disposed at each intersection of a plurality of anode lines A 1  to Am (m is a positive integer larger than or equal to 2) and a plurality of cathode lines B 1  to Bn (n is a positive integer larger than or equal to 2). 
   The display control circuit  2 , consisting of a CPU, controls the anode line driving circuit  3  and the cathode line scanning circuit  4 , so that an image based on input image data is displayed on the display panel  1  in accordance with a line sequential scanning method. The display control circuit  2  issues a scanning command to the cathode line scanning circuit  4  in synchronization with predetermined scanning timing, and simultaneously issues a driving command to an after-mentioned switch circuit  15  in the anode line driving circuit  3 . 
   The anode line driving circuit  3  is connected to each of the anode lines A 1  to Am of the display panel  1 , and selectively supplies the anode lines A 1  to Am with a driving current in response to the driving command from the display control circuit  2 . The cathode line scanning circuit  4  is connected to each of the cathode lines B 1  to Bn. The cathode line scanning circuit  4  chooses any one of the cathode lines B 1  to Bn in predetermined order in response to the scanning command from the display control circuit  2 , and applies a predetermined scanning voltage (ground voltage, for example). The organic EL device emits light, when the predetermined voltage is applied to the connected cathode line and the organic EL device itself is supplied with the driving current via the anode line. 
   The anode lines driving circuit  3  is provided with a degradation detection circuit  11 , a sample hold circuit  12 , a current supply circuit  13 , a current mirror circuit  14 , and the switch circuit  15 . 
   The degradation detection circuit  11 , as an example of the characteristic change detection circuit, which has a constant current generator  21 , a switch  22 , and an organic EL device  23 , outputs a voltage Ve 1  indicating degree of degradation of the organic EL device  23  as a degradation detection signal which typically constitutes the characteristic change detection signal. The degradation detection circuit  11  may be driven by a voltage generator via an appropriate resistor instead of the constant current generator  21 . The EL device  23  has the same electrical characteristics as the EL devices of the display panel  1 . The EL device  23  is disposed inside the display panel  1  in order to be placed in the same operating environment as the display panel  1 , or disposed in the vicinity of the display panel  1 . It is preferable that the EL device  23  is disposed in a position where it is exposed to outside light as with the display panel  1 . 
   A power supply voltage VB is applied to one end of the constant current generator  21 , and the other end is connected to an anode of the EL device  23  via the switch  22 . A cathode of the EL device  23  is connected to ground. An anode voltage of the EL device  23  is output as a degradation level voltage. The switch  22  is turned on and off in accordance with usage of the display panel  1 , namely a lighting rate of each EL device of the display panel  1 . The EL device  23 , for example, is turned on while the display panel  1  is driven, and is turned off at all other times. Switching of the switch  22  is controlled by the display control circuit  2 . 
   The sample hold circuit  12  holds the degradation level voltage (the degradation detection signal) output from the degradation detection circuit  11  with predetermined timing, and outputs it to the current supply circuit  13 . When the switch  22  is ON, for example, the sample hold circuit  12  outputs the degradation level voltage just as it is, and when the switch  22  is OFF the sample hold circuit  12  holds and keeps on outputting the degradation level voltage at just a moment before the switching. The current supply circuit  13 , which includes a differential amplifier  33 , an NPN transistor  34 , and a resistor  35 , constitutes a voltage follower circuit. In other words, a positive input terminal of the differential amplifier  33  is supplied with an output voltage of the sample hold circuit  12 , and an output terminal thereof is connected to a base of the transistor  34 . An emitter of the transistor  34  is connected to ground via the resistor  35 . A connection line between the emitter and the resistor  35  is connected to a negative input terminal of the differential amplifier  33 . The differential amplifier  33  makes a voltage across the resistor  35  equal to a hold voltage supplied from the sample hold circuit  12  due to its circuitry configuration, so that a collector current of the transistor  34  is controlled corresponding to the hold voltage of the sample hold circuit  12 . The collector current is supplied to the current mirror circuit  14  as a reference current Iref. 
   The current mirror circuit  14  includes m+1 paired resistors R 0  to Rm and PNP transistors Tr 0  to Trm. The power supply voltage VB is applied to an end of each resistor R 0  to Rm. The other end of the resistor R 0  is connected to an emitter of the PNP transistor Tr 0 , and both a base and a collector of the transistor Tr 0  are connected to a collector of the transistor  34  of the current supply circuit  13 . A common connection line between the base of the transistor Tr 0  and the collector thereof is connected to a base of each transistor Tr 1  to Trm. Emitters of the transistors Tr 1  to Trm are connected to the other ends of the corresponding resistors R 1  to Rm, respectively, and collectors thereof are connected to the switch circuit  15 . In the current mirror circuit  14  with the above configuration, it is possible to feed a current I through each of the resistors R 1  to Rm and emitter-to-collector of the transistors Tr 1  to Trm. The amount of the current I is proportional to the reference current Iref flowing through the resistor R 0  and emitter-to-collector of the transistor Tr 0 . 
   The switch circuit  15  has m units of switches SW 1  to SWm, and the switches SW 1  to SWm are disposed between the current mirror circuit  14  and the anode lines A 1  to Am of the display panel  1 , respectively. Each of the switches SW 1  to SWm is turned on and off in response to the driving command described above. 
   In the drive unit with this configuration, since the switch  22  of the EL device  23  is turned on in accordance with emission time of each EL device of the display panel  1 , degradation in characteristics of the EL device  23  is almost equal to average degradation of each EL device of the display panel  1 . A terminal voltage Ve 1  of the EL device  23  which is corresponding to the impedance thereof is held in the sample hold circuit  12 . 
   While the switch  22  is ON, the sample hold circuit  12  updates and holds the terminal voltage Ve 1  of the EL device  23  with predetermined timing, and then outputs it. The voltage held by the sample hold circuit  12  is applied to the current supply circuit  13 , and a voltage equal to the terminal voltage Ve 1  is applied to the resistor  35 . When resistance of the resistor  35  is R 35 , the current Iref, which can be expressed as Ve 1 /R 35 , runs through the resistor R 0 , emitter-to-collector of the transistor Tr 0 , collector-to-emitter of the transistor  34 , and the resistor  35 . Suppose that a switch SWi (i is any number from 1 to m) out of the switches SW 1  to SWm of the switch circuit  15  is turned on in response to the driving command from the display control circuit  2 , and a cathode line Bj (j is any number from 1 to n) is selected in response to the scanning command. The current I an amount of which is proportionate to the reference current Iref passes through a resistor R 1  and emitter-to-collector of a transistor Tri, and flows into ground through the switch SWi, an anode line Ai, an EL device ELi,j, and a cathode line Bj. Thus, the EL device ELi,j emits light. 
   The terminal voltage Ve 1  of the EL device  23  is varied with degradation in each EL device of the display panel  1 , because when each EL device of the display panel  1  is degraded, the EL device  23  is also degraded in like manner. In other words, the more degraded an organic EL device, the higher internal impedance of the organic EL device becomes, and the lower luminance becomes. Thus, the terminal voltage Ve 1  increases in accordance with the degradation in each EL device of the display panel  1 . The terminal voltage Ve 1  is the degradation detection signal indicating degree of degradation in the EL device  23 . When the terminal voltage Ve 1  increases, the current Iref increases in accordance with variation of the terminal voltage ΔVe 1 . The current I increased in proportion to increase in the current Iref passes through the EL device ELi,j. Therefore, increase in the current I compensates lower luminance of the EL device ELi,j due to the degradation thereof, so that luminance of the EL device ELi,j is prevented from being lowered. 
   The same is true in a case where a plurality of switches out of the switches SW 1  to SWm are turned on (including a case where all switches are selected) and a plurality of EL devices connected to the cathode line Bj simultaneously emit light. In other words, when the plurality of switches out of the switches SW 1  to SWm are turned on, the current I flows into the EL devices through each anode line corresponding to the plurality of switches which has been turned on. The amount of the current I includes compensation for lower luminance due to the degradation of the EL device, so that luminance is prevented from being lowered in each EL device through which the current I passes. 
     FIG. 2  shows variations in impedance and in luminance of an organic EL device with respect to a lapse of driving time. In  FIG. 2 , solid lines are in a case of the drive unit according to the present invention, and broken lines are in a case of a conventional drive unit. It can be seen from characteristic curves in  FIG. 2  that the luminance of the present drive unit is prevented from being lowered as compared with that of the conventional one, even if the variation in impedance of the present drive unit is larger than that of the conventional one. 
     FIG. 3  shows another embodiment of a drive unit of the display panel adopting a voltage driving method according to the present invention. The drive unit is provided with the display panel  1 , the display control circuit  2 , an anode line driving circuit  3 , and the cathode line scanning circuit  4 , as in the case of the drive unit shown in  FIG. 1 . The anode line driving circuit  3  has a different configuration from that of  FIG. 1 . Referring to  FIG. 3 , the anode line driving circuit  3  includes a degradation detection circuit  41 , a sample hold circuit  42 , a voltage generator circuit  43 , a monitor circuit  44 , and a switch circuit  45 . The degradation detection circuit  41  includes an organic EL device  51 , a constant current generator  52 , and a switch  53 . The organic EL device  51 , the constant current generator  52 , and the switch  53  are connected in series in order. The power supply voltage VB is applied to an end of the series circuit, that is, an anode of the organic EL device  51 , and the other end of the series circuit in the switch  53  side is connected to ground. As in the case of the organic EL device  23  and the constant current generator  21  in the driving device of  FIG. 1 , it is preferable that the EL device  51  has the same characteristics as each EL device of the display panel  1 , and the constant current generator  52  may be a resistor. The switch  53 , as in the case of the switch  22 , is turned on and off in response to the usage of display panel  1 , namely the lighting rate of each EL device of the display panel  1 . A degradation level voltage Ve 1  (a degradation detection signal) which is applied to a cathode of the organic EL device  51  connected to the constant current generator  52  is supplied to the sample hold circuit  42 . 
   The sample hold circuit  42  holds the degradation level voltage Ve 1  output from the degradation detection circuit  41  with predetermined timing, and outputs it to the voltage generator circuit  43 . The voltage generator circuit  43 , which includes a differential amplifier  63 , an NPN transistor  64 , and resistors  65  and  66 , constitutes a voltage follower circuit. In other words, a positive input terminal of the differential amplifier  63  is supplied with an output voltage from the sample hold circuit  42 , and an output terminal thereof is connected to a base of the transistor  64 . An emitter of the transistor  64  is connected to a line of a power supply voltage VB via the resistor  65 . A connection line between the emitter and the resistor  65  is connected to a negative input terminal of the differential amplifier  63 . A collector of the transistor  64  is connected to ground via the resistor  66 . According to the above-mentioned configuration of circuitry, the differential amplifier  63  makes a voltage across the resistor  65  equal to a hold voltage supplied from the sample hold circuit  42 , so that a collector current of the transistor  64  is controlled corresponding to the hold voltage of the sample hold circuit  42 . Since the collector current flows into ground through the resistor  66  as the reference current Iref, a voltage across the resistor  66  is generated corresponding to the current Iref. The voltage is applied to the monitor circuit  44 . 
   The monitor circuit  44  includes a differential amplifier  71 , a resistor  72 , and an organic EL device  73 . An output voltage from the voltage generator circuit  43  is supplied to a positive input terminal of the differential amplifier  71 , and a negative input terminal is connected to ground through the resistor  72 . The organic EL device  73 , which is connected between an output terminal of the differential amplifier  71  and the negative input terminal, constitutes a feedback circuit of the differential amplifier  71 . The organic EL device  73  is provided as an emission monitor device. The differential amplifier  71  amplifies the output voltage from the voltage generator circuit  43  with a gain, which is based on a ratio between the forward resistance of the organic EL device  73  and the resistance of the resistor  72 , in order to output a driving voltage V. Since the forward resistance of the organic EL device  73  becomes large with a lapse of driving time, the gain of the differential amplifier  71  also increases. The driving voltage V output from the monitor circuit  44  is applied to the switch circuit  45 . 
   The switch circuit  45 , as with the above-mentioned switch circuit  15 , has m units of switches SW 1  to SWm which are disposed between the monitor circuit  44  and the anode lines A 1  to Am of the display panel  1 . 
   In the drive unit with this configuration, the sample hold circuit  42  updates and holds the terminal voltage Ve 1  of the EL device  51  as the degradation level voltage with predetermined timing and outputs it, while the switch  53  is ON. The voltage held by the sample hold circuit  12  is supplied to the voltage generator circuit  43 , and a current Iref which is proportionate to the terminal voltage Ve 1  flows into ground through emitter-to-collector of the transistor  64  and the resistor  66 . When resistance of the resistor  65  is R 65 , the current Iref can be expressed as Ve 1 /R 65 . A collector voltage of the transistor  64  is generated corresponding to the current Iref as the driving voltage V through the monitor circuit  44 . The driving voltage V is applied to the EL device  73  for monitoring and makes the EL device  73  emit light. The driving voltage V is applied to any EL device of the display panel  1  through any of switches SW 1  to SWm, which is turned on, in the switch circuit  45 . 
   Suppose that a switch SWi (i is any number from 1 to m) out of the switches SW 1  to SWm in the switch circuit  45  is turned on in response to the driving command from the display control circuit  2 , and a cathode line Bj (j is any number from 1 to n) is selected in response to the scanning command. In this case, the driving voltage V is applied to an EL device ELi,j via the switch SWi, so that a current flows into ground through the switch SWi, an anode line Ai, the EL device ELi,j, and the cathode line Bj. Thus, the EL device ELi,j emits light. 
   When each EL device of the display panel  1  is degraded, the EL device  51  is also degraded in like manner, so that the terminal voltage Ve 1  of the EL device  51  is varied in accordance with degradation in each EL device of the display panel  1 . In other words, the more degraded an organic EL device, the higher internal impedance the organic EL device has, and the lower luminance becomes. Thus, the terminal voltage Ve 1  increases in accordance with the degradation in each EL device of the display panel  1 . When the terminal voltage Ve 1  increases, the current Iref also increases in accordance with variation in the terminal voltage ΔVe 1 . The driving voltage V increased in proportion to an increase in the current Iref is applied to the EL device ELi,j. Therefore, an increase in the driving voltage compensates for a decrease in the luminance of the EL device ELi,j due to the degradation thereof, so that the luminance of the EL device ELi,j is prevented from being lowered. 
   The same is true in a case where a plurality of switches out of the switches SW 1  to SWm are turned on (including a case where all switches are selected) and a plurality of EL devices connected to the cathode line Bj simultaneously emit light. When the plurality of switches out of the switches SW 1  to SWm are turned on, the driving voltage V is applied to the plurality of EL devices through each anode line corresponding to the plurality of switches which has been turned on. 
   In order to cope with the situation that variation in impedance of the EL device is not linearly proportionate to variation in the luminance with respect to a lapse of driving time, as shown in  FIG. 4 , the drive unit may be provided with an analog-to-digital converter  81  for analog-to-digital conversion of an output voltage from the sample hold circuit  12 , an arithmetic circuit  82  for nonlinearly converting an output digital value from the analog-to-digital converter  81  using a predetermined table, and a digital-to-analog converter  83  for digital-to-analog conversion of an output value from the arithmetic circuit  82 . In the configuration shown in  FIG. 4 , an output voltage from the digital-to-analog converter  83  is applied to the current supply circuit  13 . Furthermore, as shown in  FIG. 5 , a constant current circuit  84  with digital input may be provided instead of the digital-to-analog converter  83  and the current supply circuit  13  shown in  FIG. 4 . 
   In the drive unit of the display panel according to the present invention, it is also possible to supply a suitable driving voltage for the impedance of the EL device by means of using an output voltage from a booster circuit  17  as the power supply voltage. Thus, it is possible to keep power consumption of a current driving circuit to a minimum. In a conventional drive unit adopting the current driving method, a power supply voltage for a display panel has a margin of approximately 5 volts in consideration of variation in impedance of EL devices. The voltage margin becomes heat loss in a driving circuit, and the heat loss brings about increase in power consumption. In the drive unit according to the present invention, however, the increase in power consumption is prevented due to the booster circuit  17 . 
   A further embodiment of the drive unit according to the present invention will be described with reference to  FIG. 7 . 
   In  FIG. 7 , circuit elements or parts that corresponds to those depicted in the preceding drawings are denoted by like reference numerals and the explanation thereof will not be repeated. 
   In this embodiment, the output signal of the sample/hold circuit  12  is supplied to a booster circuit  101  whose output current is in turn supplied to a cathode drive circuit  103 . The booster circuit  101  is analogous to the booster circuit  17  used in the first embodiment shown in  FIG. 1 , and generates a voltage higher than a potential applied to the cathode of the organic electroluminescent device driven to emit light, as explained later. 
   The plurality of anode lines of the display panel  1  are connected to an anode driver  102  that selectively supplies a drive current in response to the driving command from the display controller  2 . The plurality of cathode lines of the display panel  1  are connected to a cathode driver  103  that selects one of the plurality of cathode lines in response to a scanning command from the display controller  2  and applies a scanning electric potential to the selected one of the scanning lines. As illustrated in  FIG. 7 , the anode drive circuit  102  has a plurality of switches, each of which connects the anode line to the drive current source or a ground potential. A second electric potential is set to be higher than the scanning potential, so that the second electric potential higher than the scanning electric potential is applied to cathode lines other than the cathode line of scanning row. 
   As a result, among the organic electroluminescent devices connected to the anode lines to which the drive current is supplied (in the illustrated example, the first and third anode lines from the left end), the devices other than the devices driven to emit light are prevented from being supplied with the drive current. In  FIG. 7 , the organic electroluminescent devices marked with the double circle are devices driven to emit light, and the devices marked with the single circle are devices that are reverse-biased by the application of the second electric potential of the scanning drive. In this way, the driving current is surely prevented from flowing through these devices marked with the single circle. 
   Thus, by the application of the present invention in driving structures using the so-called cathode reset method in which a second electric potential other than the drive potential is applied to the cathode of each of organic electroluminescent devices of non-lit rows, a sufficient current control function can be maintained even if the impedance of the organic electroluminescent device changes. Consequently, the advantageous effects of the so called cathode reset method, e.g., the reduction of electric power consumption of the display panel and the prevention of the crosstalk of the drive current between organic electroluminescent devices, can be surely maintained. 
   Furthermore, it is possible to adopt an arrangement in which the voltage of current control of the anode driver based on the output signal of the sample hold circuit  12  in each of the preceding embodiment is used in combination with the voltage control of the cathode driver based on the output voltage of the sample hold circuit  12  which has been explained referring to  FIG. 7 . 
   The present invention is applicable to both the drive unit of the display panel adopting the current driving method and that adopting the voltage driving method. The present invention is furthermore applicable not just to a passive drive unit, but also to an active drive unit. The present invention is applicable not just to a dot presentation panel described above, but also to a segment presentation panel. 
   The organic EL devices  23  and  51  in the respective embodiments described above are emission devices. However, the present invention is also applicable to a nonluminous organic semiconductor device which has equal electrical characteristics to the organic EL devices. 
   In each embodiment described above, an organic EL device is used as a self-luminous device. However, the self-luminous device is not limited to the organic EL device, but may be another luminous device luminance of which is proportionate to supplied current level. 
   As described above, the present invention can prevent lowered luminance of a self-luminous device due to degradation thereof. 
   This application is based on Japanese Patent Application No. 2002-111464 which is herein incorporated by reference.