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Output circuit for gray scale control, testing apparatus thereof, and method for testing output circuit for gray scale controlOutput circuit for gray scale control, testing apparatus thereof, and method for testing output circuit for gray scale control description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060176252, Output circuit for gray scale control, testing apparatus thereof, and method for testing output circuit for gray scale control. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] The present invention relates to an output circuit for gray scale control which is used for a display apparatus and an output apparatus, and more particularly to a driver IC for performing gray scale control in accordance with an electrical current or voltage, a testing apparatus thereof, and a method for testing the driver IC. [0002] In general, an active matrix type image display apparatus includes a large number of pixels arranged in a matrix, and controls light intensity for each pixel in accordance with given luminance information to display images. Therefore, a display panel having, for example, a rectangular shape has TFTs (Thin-Film-Transistors) which are arranged in a matrix and control a state of a liquid crystal or optical substance, a data line driving circuit provided along upper and lower sides of the panel, and a gate line driving circuit provided at the side end of the panel. [0003] Conventionally, image display apparatuses such as display panels, using liquid crystal as an optical substance, have been mainstream. In these image display apparatuses, a liquid crystal driving circuit (liquid crystal driver) supplies display information in the form of a voltage to each pixel, and changes transmittances of the pixels in accordance with the display information. [0004] In comparison, in recent years, proposals have been frequently made for image display apparatuses using an organic EL (Electro Luminescence) material as light emitting devices. Dissimilar to liquid crystal, since the organic EL material itself emits light, a display panel using it offers advantages in that visibility is improved and no backlight is necessary. The organic EL material used in the display panel has a function of serving as a diode and emits in reaction to electric current. Two driving schemes are employed for the organic EL panels. [0005] FIG. 24 is a diagram for describing the driving schemes for the organic EL panel. [0006] As shown in FIG. 24, a first one of the driving schemes for the organic EL panel is a voltage write scheme. In this scheme, display data is supplied in the form of voltage V.sub.0 from a voltage driver to a TFT (low temperature polysilicon pixel Tr). Charges accumulated in a load such as a capacitor are charged or discharged corresponding to the voltage V.sub.0, whereby a current I.sub.0 is flown to an organic EL diode. However, there occurs a problem in that while the driving scheme is advantageous in that an existing liquid crystal driver IC technique can be used, voltage supply is unstable, thereby making it difficult to compensate characteristics variations of a low temperature polysilicon TFT. [0007] The second one of the driving schemes for the organic EL panel is a current write scheme. In this scheme, gray scale display is controlled by changing the amount of current from the panel. The TFTs made of the low temperature polysilicon on the panel constitute current mirrors, to which a current equal to a current I.sub.0 taken out from the panel to a signal line is flown. According to this scheme, it is possible to compensate TFT characteristics variations, and realize an organic EL panel with high image quality. [0008] In an organic EL panel capable of performing a color display, pixels of three colors R (red), green (G) and B (blue) are arranged. In the case of the current write scheme, the pixel luminance is varied in accordance with current supplied from a current driver, thereby enabling pixel luminance gray scale display. [0009] FIGS. 25A and 25B are, respectively, a circuit diagram showing the configuration of a conventional voltage driver used for performing voltage driving of a display apparatus for implementing the above-described gray scale display, and a graph showing the relationship between a power-supply potential of a power supply voltage supply line and the distance from a power-supply voltage supply unit. [0010] As shown in FIG. 25A, the conventional voltage driver (output circuit for gray scale control) includes: a power-supply voltage supply unit 1112; gray scale control units 1101a, 1101b, . . . , and 1101.sub.N (N: a natural number) each of which are connected to the power-supply voltage supply unit 1112 and has an output unit 1116; a current supply unit 1110 connected to the ground; a first MISFET 1111 which is a P-channel MISFET provided between the power-supply voltage supply unit 1112 and the current supply unit 1110 and having a drain and a gate electrode connected to each other; a first node 1118 provided between the first MISFET 1111 and the power-supply voltage supply unit 1112; a gate bias supply line 1115 connected to the gate electrode of the first MISFET 1111; a power-supply voltage supply wire 1121 connected to the first node 1118 and used for supplying the power supply voltage to each of the gray scale control units; a power-supply voltage supply node 1117 provided on the power-supply voltage supply wire 1121 and connected to each of the gray scale control units 1101a, 1101b, . . . , and 1101.sub.N; and resistor 1113 individually provided between the power-supply voltage supply nodes 1117 and between the power-supply voltage supply node 1117 and the first node 1118. Herein, there is shown an example that the N gray scale control units are provided. In many cases, one output circuit for gray scale control includes about 400 to 500 gray scale control units. [0011] In the conventional output circuit for gray scale control, current mirror circuits are utilized for the gray scale control units 1101a, 1101b, . . . , and 1101.sub.N [0012] In specific, as shown FIG. 25A, the gray scale control unit 1101a has: a P-channel second MISFET 1102a and a P-channel third MISFET 1103a of which the sources are connected to each other and which are connected to the power-supply voltage supply node 1117; a voltage selection switch 1120a; an operational amplifier 1106a in which the voltage selection switch 1120a is connected to a (+) side of an input unit, and the output unit 1116 is connected to a (-) side thereof; an output-side transistor 1105a which is an N-channel MISFET having a source connected to the ground, a drain connected to the third MISFET 1103a, and a gate electrode connected to the output unit of the operational amplifier 1106a; a first node 1114a provided between the output-side transistor 1105a and the third MISFET 1103a and connected to the output unit 1116; and an oscillation-preventing capacitor 1119a provided between wires connecting between the output unit of the operational amplifier 1106a and the gate electrode of the output-side transistor and connecting between the output-side transistor 1105a and a second node. In addition, the second MISFET 1102a and the operational amplifier 1106a together constitute a differential circuit 1107a, and the third MISFET 1103a, the first node 1114a, the oscillation-preventing capacitor 1119a and the output-side transistor 1105a together constitute an output buffer unit 1108a. Herein, in the conventional gray scale control unit 1101a, electrical characteristics of the second MISFET 1102a and the third MISFET 1103a are homogenized mutually, and the gate electrodes thereof are both connected to the gate bias supply line 1115, thereby together constituting the current mirror circuit. In addition, the configuration is designed such that a current I.sub.2 flowing through the third MISFET 1103a is higher than a current I.sub.1 flowing through the second MISFET 1102a in order to driving a load. [0013] In the conventional output circuit for gray scale control, each of the N gray scale control units 1101a, 1101b, . . . , and 1101.sub.N has the same circuit configuration as the above-described gray scale control unit 1101a. Each of the gate electrodes of the second MISFETs 1102a, 1102b, . . . , and 1102.sub.N and the third MISFETs 1103a, 1103b, . . . , and 1103.sub.N are connected to the gate bias supply line 1115. As shown in FIG. 25B, equal voltages are applied from the gate bias supply line 1115 to the gate electrodes of these MISFETs so that these MISFETs are turned on. [0014] In addition, for the voltage selection switch, the conventional output circuit for gray scale control uses a multiplexer capable of selecting a plurality of reference voltages corresponding to digital data. Voltages selected herein are current-amplified by the operational amplifier and are outputted to the panel using, for example, a liquid crystal or organic EL material. [0015] The conventional output circuit for gray scale control which is used for current-driving and for the current-write-scheme employed organic EL panel has a configuration including current adding-type D/A converters instead of the gray scale control units 1101a, 1101b, . . . , and 1101.sub.N of the output circuit for gray scale control shown in FIG. 25A. From the D/A converters, currents having magnitudes corresponding gray scale data are supplied to the TFTs and the pixels, thereby enabling a gray scale display with the organic EL panel. [0016] The above-described output circuit for gray scale control which is used for current-driving can be utilized not only as the driver for the organic EL panel but also as a head of an output apparatus such as a printer. Further, the circuit can also be used as a display-apparatus driver or printer head using an inorganic EL or LED (Light Emitting Diode) in addition to the organic EL. [0017] Hereinafter, description will be given of a method for testing the conventional output circuit for gray scale control which is used for current-driving. [0018] FIGS. 26A and 26B are, respectively, a cross-sectional view showing a conventional probe card for testing the conventional output circuit for gray scale control which is used for current-driving, and a block circuit diagram showing a cross section of the conventional probe card. [0019] As shown in FIG. 26A, a test of the conventional output circuit for gray scale control which is used for current-driving is performed in such a manner that a probe card 1156 which is connected, on the upper surface side, to a head 1153 of a semiconductor tester 1152 and has, on its lower surface, probes 1155 made of a conductor is mounted on a wafer to be tested 1151 in which a large number of current drivers are provided. [0020] In specific, as shown in FIG. 26B, testing currents are supplied to flow from the head 1153 of the semiconductor tester 1152 in a state where a testing pad 1154 (or bump) provided on the wafer is brought into contact with the probe 1156, and currents outputted from the testing bump are then detected. [0021] Many organic EL diodes exhibit a peak luminance at a supplied current of 1 .mu.A or less. As such, in an organic EL panel having 6-bit gray scales (64 gray scales), the current per gray scale is about 10 to 20 nA. Therefore, the semiconductor tester 1152 can detect currents of about 10 to 20 nA. Devices used herein such as the semiconductor tester, the probe card, jigs for connection with the semiconductor tester and the probe card are similar to those used for general wafer testing. [0022] Hereinafter, description will be given of drawbacks occurring with the conventional technique. [0023] As can be seen from FIG. 25B, in the conventional voltage driver, the identical gray scale control units are connected to the single power-supply voltage supply wire 1121. Therefore, the supplied voltage drops due to the presence of the resistors 1113 and the like in the power-supply voltage supply node 1117 placed at a position away from the power-supply voltage supply unit 1112. On the other hand, since the potential of the gate bias supply line 1115 is constant regardless of the position, a voltage V.sub.GS applied between the gate and the source of each of the second MISFET 1102 and the third MISFET 1103 is varied depending on the distance from the power-supply voltage supply unit 1112. Continue reading about Output circuit for gray scale control, testing apparatus thereof, and method for testing output circuit for gray scale control... 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