Patent Abstract:
A device for driving a dot matrix display panel has a number of first terminals that can be connected to different signal lines in the dot matrix display panel, to carry current to or from picture elements in the dot matrix display panel. The invented device also has a second terminal and a set of switches that can selectively connect the first terminals to the second terminal. These switches enable a measurement device connected to the second terminal to measure electrical parameters at the first terminals individually, so that the electrical parameters of the device can be tested efficiently and accurately, without having to contact each of the first terminals with a probe.

Full Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a display driving circuit for a current-driven display panel such as an electroluminescent (EL) panel, and more particularly to the testability of the display driving circuit.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 1 shows a conventional display driving circuit, disclosed in Japanese Unexamined Patent Application Publication No. 11-95723, and illustrates how it is tested. The display driving circuit  10  drives an organic electroluminescent panel  1  to create a dot matrix display. The organic electroluminescent panel  1  comprises an intersecting grid of m data lines SGi (i=1 to m) and n scanning lines CMj (j=1 to n), where m and n are integers greater than one. Organic electroluminescent picture elements or pixels PE i,j  are disposed at the intersections of the grid; each pixel PE i,j  has an anode coupled to data line SGi and a cathode coupled to scanning line CMj.  
           [0005]    The display driving circuit  10  comprises m constant-current sources  11   i  (i=1 to m), a switching unit  12 , a switching unit  13 , and a driving control unit  14 .  
           [0006]    The i-th constant-current source  11   i  drives data line SGi. On their input side, the constant-current sources  11   i  are connected to a shared power terminal  15  from which they receive a supply voltage VS; on their output side they are connected to electrodes (a) of switches  12   i  in the switching unit  12 . The switches  12   i  also have respective electrodes (b) connected to a common ground terminal  16 , to which a ground potential (GND) is supplied, and further electrodes (c) connected to respective current output terminals  17   i . Data line SGi in the organic electroluminescent panel  1  is connected to current output terminal  17   i .  
           [0007]    The switching unit  13  comprises n switches  13   j  having respective electrodes (a) connected to the ground terminal  16 , electrodes (b) connected to the power terminal  15 , and electrodes (c) connected through respective terminals  18   j  to the corresponding scanning lines CMj in the organic electroluminescent panel  1 .  
           [0008]    The driving control unit  14  controls the switching units  12 ,  13  according to display data DT received from a data input terminal  19 .  
           [0009]    In this type of display driving circuit  10 , the switches  13   j  in switching unit  13  are selected cyclically, one at a time, by the driving control unit  14 , and switch over to their a-electrodes when selected. The scanning line CMj in the organic electroluminescent panel  1  corresponding to the selected switch  13   j  is thereby connected to ground, while the other (non-selected) scanning lines are connected to the power-supply voltage VS.  
           [0010]    The switches  12   i  in switching unit  12  operate under control of the driving control unit  14  according to the data to be displayed on the selected scanning line. Pixel PE i,j  in the organic electroluminescent panel  1  emits light if switches  12   i  and  13   j  are both set to the a-side, so that current supplied by constant-current source  11   i  flows through pixel PE i,j  to ground. As switching unit  13  selects the scanning lines CMj in sequence, the emitted light produces a dot matrix display.  
           [0011]    The organic electroluminescent panel  1  and its display driving circuit  10  are manufactured separately and tested as independent units. The display driving circuit  10  is fabricated on a semiconductor wafer and undergoes various electrical tests in the semiconductor wafer state. If it passes these tests, then after the wafer has been diced into chips, the display driving circuit  10  is packaged and connected to the organic electroluminescent panel  1 . Accurate testing of the constant-current sources  11   i  is particularly necessary, because the uniformity of the current output therefrom has a major effect on the quality of the display. If the constant-current sources  11   i  do not output uniform amounts of current, the pixel elements cannot put out uniform amounts of light.  
           [0012]    The display driving circuit  10  is tested in the wafer state by a testing apparatus  30  of the type shown in FIG. 1. The testing apparatus  30  has a switch setting unit  31  that supplies data DT to the driving control unit  14  to set the switches in the switching units  12 ,  13 , a constant voltage source  32  that supplies voltage VP to the constant voltage source  32 , a constant voltage source  33  that outputs a lower voltage, and an ammeter  34  connected in series with the constant voltage source  33 . The testing apparatus  30  also has a constant-current source  35  that supplies current to switching unit  13  and a voltmeter  36  that measures the resulting voltage drop. The testing apparatus  30  is connected to the display driving circuit  10  by a cable equipped with probes.  
           [0013]    In this configuration, the ammeter  34  is connected to the current output terminals  17   i  of the display driving circuit  10  one after another, and measures the current supplied by the corresponding constant-current sources  11   i . The constant-current source  35  and voltmeter  36  are connected to the scanning terminals  18   j  of the display driving circuit  10  one after another, and measure the voltage drops on the different paths leading through the switches  13   j  in switching unit  13 . A decision is then made as to whether the measured currents and voltages are within specified tolerance limits.  
           [0014]    One problem with this type of test is that it takes too much time, since the probes have to be moved repeatedly from one terminal to another, and each time a probe is moved to a new terminal, a certain time must be allowed before the flow of current stabilizes and accurate values can be measured.  
           [0015]    Another problem is that the test results tend to vary according to random variations in the force with which the probes make contact with the terminals, the area of contact, and other such factors. To ensure the quality of the display, tight tolerances are set on the test results, so random variations can easily cause a device that actually meets its specifications to be rejected as defective.  
         SUMMARY OF THE INVENTION  
         [0016]    An object of the present invention is to provide an efficiently testable device for driving a dot matrix display panel.  
           [0017]    Another object is to provide an accurately testable device for driving a dot matrix display panel.  
           [0018]    The invented device for driving a dot matrix display panel has a plurality of first terminals connected to different signal lines in the dot matrix display panel, for carrying current to or from the picture elements in the dot matrix display panel. The invented device also has a second terminal and a plurality of switches for selectively connecting the first terminals to the second terminal. These switches enable test apparatus connected to the second terminal to measure electrical parameters at the first terminals individually.  
           [0019]    The invented device can be tested efficiently because it is not necessary to contact each of the first terminals individually with a probe.  
           [0020]    The invented device can be tested accurately because the test results are not affected by contact force, contact area, and other factors that may vary from one terminal to another.  
           [0021]    The first terminals may be current output terminals connected to different data signal lines in the dot matrix display panel, and the measured electrical parameters may be current values.  
           [0022]    Alternatively, the first terminals may be scanning terminals connected to different scanning signal lines in the dot matrix display panel, and the measured electrical parameters may be voltage drops.  
           [0023]    The invention also provides a device with switches for connecting current output terminals to one test terminal and switches for connecting scanning terminals to another test terminal, so that electrical parameters can be measured at both types of terminals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    In the attached drawings:  
         [0025]    [0025]FIG. 1 is a circuit diagram illustrating a conventional display driving circuit and test circuit; and  
         [0026]    [0026]FIG. 2 is a circuit diagram illustrating a conventional display driving circuit and test circuit embodying the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    An embodiment of the invention will now be described with reference to FIG. 2, in which elements similar to the corresponding elements in FIG. 1 are indicated by the same reference characters.  
         [0028]    The display driving circuit  10 A in FIG. 2 drives an organic electroluminescent panel  1  to generate a dot matrix display. The organic electroluminescent panel  1  is of the conventional type comprising an intersecting grid of data lines SGi and scanning lines CMj with organic electroluminescent pixels PE i,j  disposed at the grid intersections, the pixels being connected by their anodes to the data lines and by their cathodes to the scanning lines. The data lines SGi have respective terminals TS i  (i=1 to m) at which they are connected to the display driving circuit  10 A; the scanning lines CMj have respective terminals TC j  (j=1 to n) at which they are connected to the display driving circuit  10 A (m and n are integers greater than one).  
         [0029]    The display driving circuit  10 A includes the conventional constant-current sources  11   i , switching units  12 ,  13 , driving control unit  14 , common power terminal  15 , common ground terminal  16 , current output terminals  17   i , scanning terminals  18   j , and data input terminal  19 , and a novel test control unit  21 , test switching unit  22 , and test switching unit  23 .  
         [0030]    The i-th constant-current source  11   i  supplies a constant current to drive the pixel elements PE i,j  connected to data line SGi. The constant current value is typically in the range from several tens of microamperes to several hundred microamperes. Each constant-current source  11   i  receives a power-supply potential or supply voltage VS from the common power terminal  15 , and supplies current to the a-electrode of switch  12   i  in switching unit  12 . The b-electrode of switch  12   i  is coupled to the common ground terminal  16 , and the c-electrode of switch  12   i  is coupled to the current output terminal  17   i  that connects with terminal TS i  in the organic electroluminescent panel  1 . After the display driving circuit  10 A has been connected to the organic electroluminescent panel  1 , the c-electrode of switch  12   i  is thereby connected to data line SGi.  
         [0031]    The switches  13   j  in switching unit  13  have their a-electrodes connected to the common ground terminal  16 , their b-electrodes connected to the common power terminal  15 , and their c-electrodes connected to scanning terminals  18   j  that connect to the corresponding terminals TC j  in the organic electroluminescent panel  1 , thus to the scanning lines CMj.  
         [0032]    The switches  12   i ,  13   j  operate under control of the driving control unit  14 , connecting their c-electrodes selectively to their a-electrodes and b-electrodes. The driving control unit  14  operates according to the data DT received at the data input terminal  19 .  
         [0033]    A test signal TST received at a test control terminal  20  controls the test control unit  21 , which in turn controls the test switching units  22  and  23 . Test switching unit  22  comprises on-off switches  22   i  through which the corresponding current output terminals  17   i  can be selectively coupled to a test output current terminal  24 . Test switching unit  23  comprises on-off switches  23   j  through which the corresponding scanning terminals  18   j  can be selectively coupled to a test input current terminal  25 .  
         [0034]    Next the electrical testing of the display driving circuit  10 A in the wafer state will be described.  
         [0035]    As shown in FIG. 2, the test apparatus  30 A comprises the conventional switch setting unit  31 , constant voltage sources  32 ,  33 , constant-current source  35 , and voltmeter  36 , and a novel test setting unit  37 , voltmeter  38 , and resistor  39 .  
         [0036]    The switch setting unit  31  supplies data DT to the data input terminal  19  of the display driving circuit  10 A. The constant voltage source  32  supplies a supply voltage VP (for example, 7 V) to the power terminal  15  of the display driving circuit  10 A. The constant voltage source  33  outputs a voltage (for example, 4 V) corresponding to the voltage drop that occurs in an organic electroluminescent pixel PE i,j  when the pixel is driven. The constant-current source  35  supplies the test input current terminal  25  of the display driving circuit  10 A with a current (of several tens of milliamperes, for example) equivalent to the maximum current that may be carried on a scanning line CMj in the organic electroluminescent panel  1 . The voltmeter  36  measures the voltage at the test input current terminal  25 .  
         [0037]    The test setting unit  37  is connected to the test control terminal  20  of the display driving circuit  10 A, and outputs a test signal TST that controls the test control unit  21 , thereby controlling the switches in the test switching units  22 ,  23 .  
         [0038]    The voltmeter  38  and resistor  39  are connected in series between the constant voltage source  33  and the test output current terminal  24  of the display driving circuit  10 A. By measuring the voltage at a point between constant voltage source  33  and resistor  39 , the voltmeter  38  obtains a value proportional, by Ohm&#39;s law, to the current flowing through resistor  39 .  
         [0039]    In the test procedure, the switch setting unit  31  and test setting unit  37  drive the data signal DT and test control signal TST in a predetermined pattern. The driving control unit  14  sets the switches in the switching units  12 ,  13  according to the data signal DT. The test control unit  21  sets the switches in the test switching unit  22 ,  23  according to the test control signal TST.  
         [0040]    The current output by the constant-current sources  11   i  is measured by having the switches  12   i  in switching unit  12  connect the constant-current sources  11   i  to the current output terminals  17   i , and having the switches  22   i  in test switching unit  22  connect the test output current terminal  24  to terminals  17   1  to  17   m  in turn. Since the current is measured by measuring the voltage drop in resistor  39 , and since it is not necessary to connect and disconnect a probe for each individual measurement, the outputs of all of the constant-current sources  11   i  can be measured quickly, and little current is consumed in the measurement process.  
         [0041]    The voltage drop on different electrical paths through the switching unit  13  is measured in a similar way. The switches  23   j  in the test switching unit  23  connect the scanning terminals  18   j  to the test input current terminal  25  in turn. The switches  13   j  in switching unit  13  are set to connect the a-terminal to the c-terminal, to measure the voltage drop on the current path leading to ground. Measurement of the voltage drops on all current paths in switching unit  13  can be completed quickly because it is not necessary to connect and disconnect probes, and the measurement process consumes little current.  
         [0042]    The invented display driving circuit  10 A can be tested efficiently in that the test procedure can be completed quickly without using very much current. The display driving circuit  10 A can also be tested accurately, because the test results are not affected by variations in electrical contact quality at different terminals.  
         [0043]    If the measured voltages and currents are within specified tolerances, then after the display driving circuit  10 A has been diced from the wafer, it is packaged and connected to the organic electroluminescent panel  1 , with the switches  22   i ,  23   j  in the test switching units  22 ,  23  all placed permanently in the off state. During subsequent operation, the display driving circuit  10 A operates in the same way as the conventional display driving circuit  10 .  
         [0044]    The present invention is not limited to the embodiment described above. For example, the display driving circuit  10 A is not limited to driving an organic electroluminescent panel  1 ; any type of current-driven matrix display panel may be driven. The potentials supplied to the common terminals  15 ,  16  can be any two different potentials, not necessarily a power-supply potential and ground potential.  
         [0045]    The test apparatus  30 A is not limited to the configuration shown. Other test equipment configurations and measurement methods are possible.  
         [0046]    The display driving circuit  10 A need not have test switching units  22 ,  23  for both the data lines SGi and scanning lines CMj. The invention can be practiced with just one of these two test switching units.  
         [0047]    Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined by the appended claims.

Technology Classification (CPC): 8