Abstract:
According to one aspect of the present invention, there is provided a driver of a display unit including a latch circuit holding gradation information, a D/A converter outputting analog signal based on the gradation information held by the latch circuit, a test circuit provided between the latch circuit and the D/A converter, the test circuit inputting or outputting test signal regarding the latch circuit, a switch connecting voltage output of the D/A converter and a driver output terminal in normal operation, and a test switch connecting the test circuit and the driver output terminal in test operation and disconnecting the test circuit and the driver output terminal in normal operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a driver of a display unit such as a liquid crystal display, an organic light emitting display, a plasma display or the like. More particularly, the present invention relates to a driver such as a column driver, a source driver, or a horizontal driver or the like. 
     2. Description of Related Art 
     A display unit has recently become larger and larger in size due to development of manufacturing technique. The display unit having large size requires ability of driving large capacitance load of output of the driver. It means that output impedance of the driver needs to be decreased. If the output impedance is not substantially small, there is caused a problem such as lack of driving ability, increase of power consumption, or heat generation. 
     Further, recent display unit performs multi-gradation display, and there is developed a multi-bit driver of the display unit. Moreover, the driver of typical display unit has hundreds of driving outputs and includes latch circuits, level shifters, D/A converters, and buffer amplifiers. 
       FIG. 13  shows an example of a driving output circuit in the driver of the display unit according to a related art. 
     The driver shown in  FIG. 13  is an output circuit having two outputs. A driver  10  of the display unit includes a latch circuit  11 , a level shifter  12 , a D/A converter  13 , an output amplifier  15 , an output switch  16 , and an output pin  17 . In this example, it is assumed that the display unit is a liquid crystal display and includes a polarity switching circuit  14  and the output switch  16 . In this example, the polarity switching circuit  14  is provided between the D/A converter  13  and the output amplifier  15 . However, the polarity switching circuit may be provided between the output amplifier  15  and the output pin  17 . In this case, the polarity switching circuit may also function as the output switch. 
     Hereinafter, a behavior of the driver of the display unit shown in  FIG. 13  will be described in brief. The latch circuit  11  holds digital gradation information for each driving output and outputs the digital gradation information to the level shifter  12  as the output signal. The level shifter  12  performs voltage level conversion between the latch circuit  11  which is a low voltage circuit and the D/A converter  13  which is a high voltage circuit. The digital gradation information output from the level shifter  12  is converted into gradation information signal having analog value by the D/A converter  13  according to its digital value. The gradation information signal having analog value that is output from the D/A converter  13  is alternately switched by the polarity switching circuit  14  in a predetermined cycle and input to the output amplifier  15 . The output amplifier  15  amplifies the analog gradation information signal and outputs the amplified signal to the output pin  17  when the output switch  16  is in ON state. 
     In the multi-bit driver as stated above, the test may take longer time and accuracy is not high. In order to overcome these problems, Japanese Unexamined Patent Application Publication No. 2006-227168 discloses a technique to provide a driver of a display unit in which inspection time is reduced and inspection accuracy is improved. 
     In the prior art disclosed in Japanese Unexamined Patent Application Publication No. 2006-227168, the driver includes a selector selecting output of the latch circuit to output latch data from a predetermined bit, and an output selector switching a level shifter output corresponding to the predetermined bit and gradation voltage output. In normal operations, the selector is switched so as to output the gradation voltage to the driving output pin. In test operations, the selector is switched so as to output voltage (test output voltage) according to the level shifter output corresponding to the predetermined bit. 
     As stated above, the display unit having large size requires ability of driving large capacitance load of output of the driver. If the output impedance of the driver is not substantially small, there is caused a problem such as lack of driving ability, increase of power consumption, or heat generation. 
     In the prior art as in Japanese Unexamined Patent Application Publication No. 2006-227168, there is provided an output selector where gradation voltage is output to the driving output pin of the driver in the normal operation and test output voltage is output in the test operation. This output selector needs to be composed of the transistor since the output selector is implemented in the integrated circuit. The switch made of transistor has impedance in accordance with its size. Therefore, if the transistor having lower impedance is employed in order to maintain large driving ability, the size of the integrated circuit composing the selector increases. On the other hand, if the selector is composed of small transistor in order to avoid increase in size, the output impedance increases and the ability of driving the output load is lacked. Further, if the driving ability of the amplifier is enhanced in order to compensate lack of driving ability, there are caused other problems such as increase in power consumption and heat generation. 
     Therefore, there is a need to connect the driving output pin and the test signal without directly adding the selector which is one of factors for increasing impedance to the driving output pin which requires driving ability. 
     There is Japanese Unexamined Patent Application Publication No. 2006-053480 as a prior art. 
     SUMMARY 
     According to one aspect of the present invention, there is provided a driver of a display unit including a latch circuit holding gradation information, a D/A converter outputting analog signal based on the gradation information held by the latch circuit, a test circuit provided between the latch circuit and the D/A converter, the test circuit inputting or outputting test signal regarding the latch circuit, a switch connecting voltage output of the D/A converter and a driver output terminal in normal operation, and a test switch connecting the test circuit and the driver output terminal in test operation and disconnecting the test circuit and the driver output terminal in normal operation. 
     According to the driver of the display unit of the present invention, it is possible to output test result of an internal circuit from the output terminal of the driver and to input test signal to the output terminal of the driver with little or no change of output performance of the driver of the display unit. 
     According to the driver of the display unit of the present invention, it is possible to perform test without substantially degrading performance of the driver. Therefore, the test can be carried out in easier manner, and both of test time and test cost can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows an example of a configuration of a driver of a display unit according to a first embodiment of the present invention; 
         FIG. 2  shows an example of a specific configuration of a test circuit according to the first embodiment of the present invention; 
         FIG. 3  shows an example of a specific configuration of a switch of the test circuit according to the first embodiment of the present invention; 
         FIG. 4  is a table showing a relationship between test data and test signal according to the first embodiment of the present invention; 
         FIG. 5  shows an example of a specific configuration of an output amplifier according to the first embodiment of the present invention; 
         FIG. 6  is a timing chart of a behavior of the test circuit according to the first embodiment of the present invention; 
         FIG. 7  shows another example of a specific configuration of the test circuit according to the first embodiment of the present invention; 
         FIG. 8  shows an example of a configuration of a driver of a display unit according to a second embodiment of the present invention; 
         FIG. 9  shows an example of a configuration of an output amplifier of the display unit according to the second embodiment of the present invention; 
         FIG. 10  shows an example of a configuration of a driver of a display unit according to a third embodiment of the present invention; 
         FIG. 11  shows an example of a configuration of an output amplifier of the display unit according to the third embodiment of the present invention; 
         FIG. 12  shows an example of a configuration of a driver of a display unit according to a fourth embodiment of the present invention; and 
         FIG. 13  shows an example of a configuration of a driver of a display unit according to a related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
     First Embodiment 
     Hereinafter, the first specific embodiment to which the present invention is applied will be described in detail with reference to the drawings. In the first embodiment, the present invention is applied to a driver of a display unit. 
       FIG. 1  shows an example of a configuration of the driver of the display unit according to the first embodiment of the present invention. Note that the driver shown in  FIG. 1  is applied to a liquid crystal display as a display unit.  FIG. 1  shows an example of an output circuit having only two outputs for the sake of simplicity. 
     The driver  100  includes a latch circuit  101 , a test circuit  102 , a D/A converter  103 , a polarity switching circuit  104 , an output amplifier  105 , an output switch  106 , an output pin  107 , a test switch  108 , a test controller  109 , and a test signal line  110 . As stated above, the driver  100  of the first embodiment has two outputs. Therefore, symbol of a or b is given to each signal of the configuration as necessary to make a distinction. 
     The latch circuit  101  holds digital gradation information for each driving output and outputs the digital gradation information to the test circuit  102  as output signals. The output signals of the digital gradation information are input to the test circuit  102  through data buses DB0 to DB7. 
     The test circuit  102  tests the output signals of the latch circuit  101  and is connected to the test signal line  110 . The test circuit  102  performs normal operation when the test switching signal is in low level and directly outputs the signals from the latch circuit  101  to the D/A converter  103 . The test circuit  102  performs test operation when the test switching signal is in high level and outputs the test signal which is the test information of the output signals of the latch circuit  101  to the test signal line  110 . The test signal of the test circuit  102  is determined by the configuration of the test circuit  102  and the test signal may be either input signal or output signal. The test data controls a behavior of the test circuit  102 . This test data is typically input to the driver  100  from a test device (not shown). 
     The latch circuit  101  and the D/A converter  103  are connected by the data buses DB0 to DB7. The symbol “DB0 to DB7” indicates both of the name of the data bus and the signal (having a value of 0 or 1) output to the data bus for the sake of convenience. 
       FIG. 2  shows a specific configuration example 1 of the test circuit  102 .  FIG. 2  only shows a configuration of a part of the test circuit  102  where the test operation is performed for the sake of simplicity. Therefore, although not specifically shown, the test circuit  102  directly outputs the signals from the latch circuit  101  to the D/A converter  103  in normal operation as stated above. 
     As shown in  FIG. 2 , the test circuit  102  includes switches SW 151  to SW 157 . Each of the switches SW 151  to SW 157  has two input terminals a and b and one output terminal c. Eight data buses DB0 to DB7 are connected to first-stage switches SW 151  to SW 154 . For example, the DB0 is connected to the input terminal a of the SW 151 , and the DB1 is connected to the input terminal b of the SW 151 . The output terminals of the first-stage switches SW 151  to SW 154  are further connected to the input terminals of the second-stage switches SW 155  and SW 156 . The output terminals of the second-stage switches SW 155  and SW 156  are further connected to the third-stage switch SW 157 . These switches SW 151  to SW 157  are controlled by the test data TB0 to TB2 so as to connect the output terminal c and the input terminal a or b. For example, when the test data TB0 is 0, which means the test data is in low level, the input terminals a and the output terminals c of the switches SW 151  to SW 154  are connected. On the other hand, when the test data TB0 is 1, which means the test data is in high level, the input terminals b and the output terminals c are connected. This can also be applied to the test data TB1 and the switches SW 155  and SW 156 , or the test data TB2 and the switch SW 157 . 
     The switches SW 151  to SW 157  include CMOS transfer gates TG 151  and TG 152  and an inverter Inv 151  as shown in  FIG. 3 . The transfer gates TG 151  and TG 152  are connected in parallel, and the input terminal a and the transfer gate TG 151  are connected and the input terminal b and the transfer gate TG 152  are connected. Both of the outputs of the transfer gates TG 151  and  152  are connected to the output terminal c. Further, the test data input terminal d and the input of the inverter Inv 151  are connected to each other. One of the transfer gates is exclusively selected by the input signal of the test data input terminal d and the output signal of the inverter Inv 151 . The input signal to the inverter Inv 151  is the test data. 
     The first-stage switches SW 151  to SW 154 , the second-stage switches SW 155  and SW 156 , and the third switch SW 157  are controlled by the test data TB0, TB1, and TB2. Note that each of the test data TB0, TB1, and TB2 is binary signal. As shown in  FIG. 4 , the test circuit  102  outputs one of the eight signals output from the latch circuit  101  as one of the test signals DB0 to DB7 by eight combinations made of test data TB0 to TB2. 
     The D/A converter  103  converts the digital signal output from the test circuit  102  into the analog signal to output the analog signal. The analog output signal output from the D/A converter  103   a  or  103   b  is positive voltage output signal or negative voltage output signal. For example, if the D/A converter  103   a  outputs positive voltage output signal, the D/A converter  103   b  outputs negative voltage output signal. 
     The polarity switching circuit  104  is the switch for inverting polarity of voltage applied between a liquid crystal pixel electrode and a counter electrode in a certain cycle to prevent degradation that is occurred due to the characteristics of the liquid crystal material. Therefore, the positive voltage output of the D/A converter  103   a  and the negative voltage output of the D/A converter  103   b  are switched in a certain cycle by the polarity switching circuit  104  to be output to the output amplifier which is in the later stage. 
     The output amplifier  105  amplifies the signal from the polarity switching circuit  104  to output the amplified signal to the output switch  106 . Note that the output amplifier  105   a  or  105   b  may be for positive voltage or negative voltage. 
       FIG. 5  shows a specific configuration of the output amplifier  105 . As shown in  FIG. 5 , the output amplifier  105  includes an input stage  161  and an output stage  162 . The input stage  161  includes PMOS transistors M 161  and M 162 , NMOS transistors M 163  to M 165 , and a capacitance element C 161 . The output stage  162  includes a PMOS transistor M 166  and an NMOS transistor M 167 . The input stage  161  forms differential amplifier, and the output of the D/A converter  103   a  or  103   b  is applied to an input IN+ in  FIG. 5  through the polarity switching circuit  104 . The output of the output stage  162  is applied to an input IN−. Although the output amplifier  105  shown in  FIG. 5  has differential input configuration, the output amplifier  105  may be replaced with the amplifier having single-phase input. 
     The test switch  108  connects the test signal line  110  to the output pin  107  in test operation. The test switch  108  can use CMOS transfer gate, for example. 
     The output switch  106  is the switch disconnecting the output amplifier  105  and the output pin  107  of the driver. The output switch  106  is connected when the mode is not in test mode (when the test switching signal is in low level) and the output control signal is in high level, and is disconnected when the output control signal is in low level. The output control signal is in high level while the output is driven. Note that connection between panel terminals are shorted out in order to collect charges of panel pixel immediately before the polarity of the data line is inversed. At this time, the output control signal is set to low level and the output switch  106  is turned off. Hence, the output switch  106  also has a function of effectively collecting charges of the panel during this period. 
     The test controller  109  forces to disconnect the output switch  106  in test operation when there is provided the output switch  106 . In the first embodiment, the output switch  106  also needs to be disconnected when the test switching signal is in high level. Therefore, the test controller  109  is formed by an inverter INV  111  inverting the test switching signal and an AND circuit AND  111  to which the output control signal and signal from the inverter INV  111  are input. 
     In the present invention, the level shifter described in the related art is omitted for simplicity. This is because some test circuits need to have the level shifter between the latch circuit and the test circuit, and other circuits need to have the level shifter between the test circuit and the D/A converter. Such combination is not related to the essential part of the present invention, and therefore the level shifter is not shown in this invention. 
     Now, the behavior of the driver of the display unit according to the first embodiment will be described. The description of the behaviors of the latch circuit, the D/A converter, the polarity switching circuit, and the output amplifier is omitted since they have already been explained in the related art. 
     Now, the description will be made on a case where the test switching signal is in low level (normal state). In normal state, the test switching signal is in low level, and therefore the test circuit  102  outputs the signals from the latch circuit  101  directly to the D/A converter  103 . At this time, the test switch  108  is in disconnection state. In the normal state, there are output driving period and panel charge collecting period. In the output driving period, the output control signal is in high level and the output switch  106  is in conduction state. Therefore, the output amplifier  105  and the output pin  107  are connected. The rest of the operation is the driver operation which is the same as the operation described in the related art. 
     In the first embodiment, it is assumed that the test signal is the output signal from the test circuit  102 . Because the test switch  108  is disconnected, the test signal output from the test circuit  102  may be either in output state or in high-impedance state. On the other hand, when the test signal is in input state, the test signal may be fixed to high level or low level since the high-impedance state occurred by disconnecting the test switch is not preferable. The test switch may be in connection state if the test circuit is not influenced by the test signal and the test signal does not influence the output pin  107  when the test switching signal is in low level (normal operation). If the test signal of the test circuit  102  has withstand voltage that can withstand gradation voltage output from the output amplifier, the connection state that is stated above may be conduction state. The connection state mentioned here means the state where the signal can be transmitted. The level may be changed in transmission. The conduction state mentioned here means the state connection is made in a relatively low impedance. 
     Now, the description will be made on a case where the test switching signal is in high level (test state). In the test state, the test switching signal is in high level, and the output of the latch circuit  101  is input to the test circuit  102  and the test circuit  102  performs the test and outputs the test signal to the test signal line  110 . At the same time, the output switch  106  is forced to be disconnected by the test controller  109  regardless of the state of the output control signal. The test switch  108  is in connection state at this time. Therefore, the output pin  107  does not output the output gradation voltage from the output amplifier  105  but outputs the test signal. Otherwise, it is possible to input the test control signal from an external device through the output pin  107 . 
       FIG. 6  shows an operation of the specific configuration example 1 of the test circuit  102  shown in  FIG. 2 . The test data TB0, TB1, and TB2 each controls connection between the input terminal a or b and the output terminal c of the first-stage switches SW 151  to SW 154 , the second-stage switches SW 155  and SW 156 , and the third-stage switch SW 157  forming the test circuit  102 . We assume here that the input terminal a and the output terminal c are connected when the test data is 0, which means the test data is in low level, and the input terminal b and the output terminal c are connected when the test data is 1, which means the test data is in high level in the switches SW 151  to  157 . 
     The test data TB0 repeats binary data of 0 and 1 in predetermined clock cycle. The test data TB1 repeats binary data of 0 and 1 in clock cycle that is twice as long as that in the test data TB0. The test data TB2 repeats binary data of 0 and 1 in clock cycle that is three times as long as that in the test data TB0. The values of the data buses DB0 to DB7 (output data of the latch circuit  101  which is the test target) are sequentially output to the test signal line  110  from the test circuit  102  by periodically changing the test data TB0, TB1, and TB2. 
     Instead of periodically changing the test data TB0 to TB2, it is also possible to output the values of the data buses DB0 to DB7 to the test signal line  110  by specific bit combination. In this case, the test circuit  102  may specify one of the output data of the latch circuit  101  selected by three bits of test data TB0 to TB2 to output the data as the test signal  110 . For example, when all of the test data TB0 to TB2 are 0, the data bus DB0 is output as the test signal. 
       FIG. 7  shows a specific configuration example 2 of the test circuit  102  shown in  FIG. 1 . The configuration example of the test circuit  102  detects match or mismatch between two sets of 8-bit data. The test circuit  102  of this example includes XOR circuits XOR  161  to  168  and an NOR circuit NOR  161 . As shown in  FIG. 7 , the XOR circuits XOR  161  to  168  have one terminals to which 8-bit data of the latch circuit  101  output to the data buses DB0 to DB7 are input and the other terminals to which the 8-bit test data TB0 to TB7 input to the driver  100  from the test device (not shown) are input. The outputs of the XOR circuits XOR  161  to  168  are input to the NOR circuit NOR  161  and output to the test signal line  110  from the test circuit  102  as the test signal. When the 8-bit data output from the latch circuit  101  (measurement value) and the 8-bit data of the test data (expectation value) completely match, the test circuit  102  outputs the value of “True” and otherwise outputs the value of “False”. In the second example, it is possible to reduce test time since 8-bit data is compared in parallel. 
     The connection between the data buses DB0 to DB7 and the test circuit  102  is controlled by the test switching signal in both test circuits  102  shown in  FIGS. 2 and 7 . Although the controller is not especially shown in  FIGS. 2 and 7 , the connection can be realized by providing another switch between the data buses DB0 to DB7 and the input part of the test circuit  102 . The switch is closed when the test switching signal is in high level and the switch is opened when the test switching signal is in low level. 
     In the driver  100  according to the first embodiment, the switch between the output amplifier and the output pin does not influence the driving ability of the driver even when the test circuit is added to the driver. Therefore, there is not caused a problem that the driving ability is lacked due to increase of output impedance. Further, since it is not needed to improve the driving ability of the output amplifier to compensate the lack of the driving ability, there is not caused a problem of increased power consumption or the heat generation. 
     Second Embodiment 
     Hereinafter, the driver of the display unit according to the second embodiment of the present invention will be described with reference to  FIG. 8 .  FIG. 8  shows an example of a configuration of the driver of the display unit according to the second embodiment. The configurations to which the same symbols as in  FIG. 1  are given are the same or similar to the configurations in  FIG. 1 . The difference between the first embodiment and the second embodiment is that the output amplifier has an output enable function in the second embodiment, and there is difference in configurations of the output amplifier  120  and the test controller  124 . 
     The test controller  124  makes the output of the output amplifier  120  high-impedance state in test operation when the output amplifier  120  has the output enable function. In other words, the output stage of the amplifier  120  in test operation is made deactivation state. The test controller  124  is formed by an inverter INV  121  inverting the test switching signal in order to make the output of the amplifier  120  high-impedance state when the test switching signal is in high level (test operation). 
     Now, the amplifier  120  will be described.  FIG. 9  shows an example of the amplifier having output enable function. As shown in  FIG. 9 , the amplifier  120  includes an input stage  121 , a test switching circuit  122 , and an output stage  123 . 
     The signal from the D/A converter  103  is input to the input stage  121 . Note that the specific configuration of the input stage  121  is the same as the configuration of the amplifier input stage  161  shown in  FIG. 5 . 
     The test switching circuit  122  includes switches SW 121  and SW 122 . The SW 121  switches the signal output from the input stage  121  and VDD voltage, and the SW 122  switches the signal output from the input stage  121  and ground voltage according to the test switching signal. The switch SW 121  is connected to the output side of the input stage  121  when the test switching signal is in low level and is connected to VDD side when the test switching signal is in high level. Similarly, the switch SW 122  is connected to the output side of the input stage  121  when the test switching signal is in low level and is connected to ground when the test switching signal is in high level. 
     The output stage  123  includes a PMOS transistor M 121  and an NMOS transistor M 122  in series between VDD and ground. The output from the switch SW 121  is input to a gate of the PMOS transistor M 121 . Similarly, the output from the switch SW 122  is input to the gate of the NMOS transistor M 122 . There is provided an output terminal of the amplifier  120  between the PMOS transistor M 121  and the NMOS transistor M 122 . 
     Hereinafter, the behavior of the driver of the display unit according to the second embodiment will be described. The description of the configurations other than the amplifier  120  and the test controller  124  are omitted since these configurations are the same as those in the first embodiment. The specific configuration and the description of the behavior of the test circuit  102  are omitted as well. 
     The test switching signal is in high level and the signal output from the test controller  124  is in low level in test operation. Therefore, the switch SW 121  of the test switching circuit  122  is connected to VDD side and the switch SW 122  is connected to ground side. Therefore, the high level signal is input to the gate of the PMOS transistor M 121  of the output stage  123 , and the PMOS transistor M 121  is turned off. On the other hand, the low level signal is input to the gate of the NMOS transistor M 122  and the NMOS transistor M 122  is turned off as well. Therefore, both of the transistors of the output stage  123  are in disconnection state and the amplifier output terminal is in high-impedance state. In other words, the output stage  123  is in deactivation state in test operation. At this time, the test switch  108  is in connection state and the test signal is connected to the output pin  107 . Therefore, the output pin  107  can be used as the pin for test signal. 
     On the other hand, the test switching signal is in low level and the signal output from the test controller  124  is in high level in normal operation. Therefore, the switches SW 121  and SW 122  of the test switching circuit  122  are connected to the output side of the input stage  121 . Therefore, the output signal of the input stage  121  is input to the output stage  123  and the output stage  123  functions as inverter amplifier. The signal from the D/A converter  103  input to the output amplifier  120  is output to the amplifier output terminal with predetermined driving ability. The rest of the operation is the same as in the normal operation of the first embodiment. 
     In the driver  100  according to the second embodiment, the switch between the output amplifier and the output pin does not influence the driving ability of the driver even when the test circuit is added to the driver as well as in the first embodiment. Therefore, there is not caused a problem that the driving ability is lacked due to increase of output impedance. Further, since it is not needed to improve the driving ability of the output amplifier to compensate the lack of the driving ability, there is not caused a problem of increased power consumption or the heat generation. 
     Third Embodiment 
     The driver of the display unit according to the third embodiment of the present invention will be described with reference to  FIG. 10 .  FIG. 10  shows an example of a configuration of the driver of the display unit according to the third embodiment. The configurations to which the same symbols are given as in  FIGS. 1 and 8  indicate same or similar configurations as those in  FIGS. 1 and 8 . The specific configuration and the description of the behavior of the test circuit  102  are the same as well. The difference between the second embodiment and the third embodiment is that the circuit of the output stage of the output amplifier is configured as the output buffer of the test signal in the third embodiment. There is a difference in the configurations of the output amplifier  130  and the test controller  134 . However, the third embodiment is effective only when the test signal of the test circuit  102  is output signal. 
     The test controller  134  connects the test signal line  110  to the output stage of the output amplifier  130  when the test switching signal is in high level (test operation). Therefore, the test controller  134  is formed by an inverter INV  131  inverting the test switching signal. 
       FIG. 11  shows an example of the amplifier  130  according to the third embodiment. In  FIG. 11 , the amplifier  130  includes an input stage  131 , a test switching circuit  132 , and an output stage  133 . The input stage  131  and the output stage  133  have the same configurations as those of the input stage  121  and the output stage  123  shown in the second embodiment and therefore the description thereof is omitted. 
     The test switching circuit  132  includes switches SW 131  and SW 132 . The switches SW 131  and SW 132  switch the test signal and the signal output from the input stage  131  according to the signal obtained by inverting the test switching signal by the inverter INV  131 . The switch SW 131  is connected to the output side of the input stage  131  when the test switching signal is in low level (normal operation). The switch SW 131  is connected to the test signal line  110  side when the test switching signal is in high level (test operation). Similarly, the switch SW 132  is connected to the output side of the input stage  131  when the test switching signal is in low level (normal operation). The switch SW 132  is connected to the test signal line  110  side when the test switching signal is in high level (test operation). 
     Next, the behavior of the driver of the display unit according to the third embodiment will be described. However, configurations other than the test switching circuit  132  forming the amplifier  130  are the same as those in the second embodiment. Therefore, the overlapping description is omitted. The behavior in the normal operation is the same as that in the second embodiment as well, and therefore the overlapping description is omitted. 
     In test operation, the test switching signal is in high level and the signal output from the test controller  134  is in low level. Therefore, the switch SW 131  of the test switching circuit  132  is connected to the test signal line  110  side. Similarly, the switch SW 132  is connected to the test signal line  110  side as well. Therefore, the output stage  133  functions as the logic output buffer outputting the test signal, and the signal is output to the amplifier output terminal with predetermined driving ability. 
     Therefore, in the driver according to the third embodiment of the present invention, it is possible to connect the test signal and the output pin  107  in the test operation. In the normal operation, the relationship between the output amplifier  130  and the output pin  107  is equivalent to the configuration without the test circuit  102 . Therefore, there is no problem that the output impedance is increased. Further, the test signal is output through strong logic output buffer configured by the output stage  133  of the output amplifier  130 . Therefore, since there is no test switch having impedance as the driver in the first and second embodiments, it is possible to output high-speed test signal in the test operation. Hence, the test time can be reduced. 
     Fourth Embodiment 
     Now, the driver of the display unit according to the fourth embodiment of the present invention will be described with reference to  FIG. 12 .  FIG. 12  shows an example of a configuration of the driver of the display unit according to the fourth embodiment. The configurations to which the same symbols are given as in  FIG. 1  indicate the same or similar configurations as those in  FIG. 1 . The specific configuration and the description of the behavior of the test circuit  102  are the same as well. The difference between the first embodiment and the fourth embodiment is that the switch is forced to be disconnected in test operation when there is provided a switch circuit (polarity switching circuit  104  in this example) between the D/A converter  103  and the output amplifier  105 . Therefore, the configurations of the test controller  141  and the test switch  142  are different from those in the first embodiment. However, the fourth embodiment is effective only when the test signal of the test circuit is output signal. 
     The test controller  141  turns off the polarity switching circuit (the control signal of the polarity switching circuit is in low level) when the test switching signal is in high level (test operation). Therefore, the test controller  141  includes an inverter INV  141 , an inverter INV  142 , an AND circuit AND  141 , and an AND circuit AND  142 . The inverter INV  141  inverts the test switching signal, and the inverter INV  142  inverts the polarity switching signal. The AND circuit AND  141  outputs the output signal of the inverter INV  142  and the polarity switching signal to the polarity switching circuit as input signals, and the AND circuit AND  142  outputs the output signal of the inverter INV  141  and the output signal of the inverter INV  142  to the polarity switching circuit as the input signals. 
     The test switch  142  connects the test signal line  110  to the input of the output amplifier  105  when the test switching signal is in high level (test operation). 
     Now, the behavior of the driver of the display unit according to the fourth embodiment will be described. When the test switching signal is in low level (normal operation), the high level signal inverted by the inverter INV  141  is input to the AND circuits AND  141  and AND  142 . Therefore, the polarity switching signal and the signal obtained by inverting the polarity switching signal are directly output from the test controller  141  and the behavior is the same as that in the related art. Similarly, the test switch  142  is turned off and the test signal line  110  and the input of the output amplifier  105  are disconnected with each other. 
     On the other hand, in the test controller  141 , the low level signal inverted by the inverter INV  141  is input to the AND circuits AND  141  and AND  142  when the test switching signal is in high level (test operation). Therefore, the AND circuits AND  141  and AND  142  both output the low level signals, and all the polarity inverting switch  104  are in disconnection state. At the same time, the test switch  142  is in ON state and therefore the test signal line  110  and the input of the output amplifier  105  are connected. Therefore, the test signal is output to the output pin  107  with predetermined driving ability by the output amplifier  105 . 
     Therefore, since the relationship between the output amplifier  105  and the output pin  107  is equivalent to the configuration without the test circuit in normal operation, there is not caused a problem that the output impedance is increased. Further, the test signal is also output through the output amplifier. Therefore, there is no test switch having impedance between the test signal and the output pin as the driver in the first embodiment and the second embodiment. Therefore, it is possible to output the high-speed test signal in the test operation, which makes it possible to reduce test time. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. For example, the driver may be applied to an organic light emitting display, a plasma display, an SED or the like.