JP2009042485A - Display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a display device capable of suppressing variance in the operation of TFTs even when characteristics of TFTs vary by each panel manufactured or characteristics of TFTs on the same panel simultaneously vary depending on an environment. <P>SOLUTION: An active matrix type display device having TFTs as selection elements of pixels is equipped with: a detecting means for detecting an ON-current (Ion) in a preliminarily determined gate voltage (Vgh1) of a TFT; and a gate voltage control means for controlling a gate voltage (Vgh) to be applied to the TFT upon turning TFT into an on-state so as to render the ON-current of the TFT into a preliminarily determined level (I1) based on the detection result (I2) of the ON-current (Ion) by the detecting means. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to variations in TFT characteristics.

  FIG. 7 shows a structure of a general picture element built in an active matrix type liquid crystal display device.

  The picture element PIX is provided corresponding to each intersection of the gate bus line GL and the source bus line SL. The picture element PIX includes a TFT 11, a liquid crystal capacitor CL, and an auxiliary capacitor Cs. The gate 11g of the TFT 11 is connected to the gate bus line GL, the source 11s of the TFT 11 is connected to the source bus line SL, and the drain 11d of the TFT 11 is connected to the pixel electrode 12. A liquid crystal layer is disposed between the picture element electrode 12 and the counter electrode COM, and a liquid crystal capacitor CL is formed between the picture element electrode 12 and the counter electrode COM. The auxiliary capacitor Cs is formed between the pixel electrode 12 and the auxiliary capacitor bus line CSL. The liquid crystal capacitor CL and the auxiliary capacitor Cs constitute a picture element capacitor Cp.

  The TFT 11 is a selection element of the picture element PIX, and is turned on when a high gate voltage Vgh is output to the gate bus line GL during the selection period of the picture element PIX. While the TFT 11 is on, a data signal is written to the picture element PIX via the source bus line SL, and the picture element electrode 12 becomes a voltage corresponding to the data signal, and the liquid crystal capacitor CL and the auxiliary capacitor Cs are charged. . During the non-selection period of the picture element PIX, the low gate voltage Vgl is output to the gate bus line GL, whereby the TFT 11 is turned off, and the picture element PIX maintains the state where the data signal is written.

Next, FIG. 8 shows current-voltage characteristics of the TFT 11 used in the picture element PIX. Such a current-voltage characteristic is described in Patent Document 1, for example. Here, the polarity of the TFT 11 is assumed to be n-type. The vertical axis represents the on-current Ion, which means the magnitude of the drain current Id of the TFT 11, and is shown on a log scale. The horizontal axis represents the gate voltage Vg. As indicated by the curve a (solid line), the gate voltage Vg is applied to the gate 11g so that the drain current Id is negligibly small. This state turns off the TFT 11g. When the gate voltage Vg is increased in the positive direction from the off state, the TFT 11 is turned on and the on current Ion starts to flow, and the on current Ion increases rapidly as the gate voltage Vg increases. When the gate voltage Vg increases to some extent, the increase in the on-current Ion becomes moderate and gradually becomes saturated. On the other hand, even if the gate voltage Vg is increased in the negative direction from the OFF state, the ON current Ion starts to flow from a certain gate voltage Vg and increases. Thus, in order to turn off the TFT 11g, it is necessary to apply to the gate a gate voltage Vg within a limited range Ra in which the on-current Ion is substantially zero. Therefore, the low gate voltage Vgl is set within this range Ra.
JP 2001-51292 A (published February 23, 2001) JP 2006-106187 A (published April 20, 2006) Japanese Patent Laid-Open No. 2001-13930 (published on January 19, 2001) Japanese Patent Laid-Open No. 10-333178 (published on December 18, 1998)

  However, since the characteristics of the TFT 11 vary from panel to panel or the characteristics of the TFT 11 change all at once in the same panel depending on the environment, the current-voltage of the TFT 11 as shown by a curve b (broken line) in FIG. The characteristic may be different from the curve a. In this example, the curve b is shifted to the left from the curve a.

  Since the high gate voltage Vgh is set corresponding to the curve a, when the characteristic is represented by the curve b, the on-current Ion increases from Ia to Ib at the gate voltage Vgh. As a result, depending on the current-voltage characteristics of the TFT 11 at that time, the amount of charge to the liquid crystal capacitor CL and the auxiliary capacitor Cs during the selection period of the picture element PIX differs, which may cause variations in luminance and gamma characteristics. .

  The range where the on-current Ion is zero is also shifted to the left side Rb. Therefore, when the low gate voltage Vgl is set corresponding to the curve a, when the characteristic is represented by the curve b, the on-current Ion represented by Ic flows in the gate voltage Vgl. As a result, there are cases where sufficient off characteristics of the TFT 11 cannot be obtained.

  As described above, the conventional liquid crystal display device has a problem in that the TFT operation is likely to vary due to variations in TFT characteristics for each panel to be manufactured, or TFT characteristics in the same panel that change simultaneously depending on the environment. was there.

  The present invention has been made in view of the above-described problems, and the purpose of the present invention is that even if the characteristics of TFTs vary among panels to be manufactured, or the characteristics of TFTs on the same panel change all at once due to the environment. An object of the present invention is to realize a display device capable of making the operation of TFTs less likely to vary.

  In order to solve the above-described problem, the display device of the present invention is an active matrix display device including a TFT as a pixel selection element. In the active matrix display device, a detection unit that detects an on-current at a predetermined gate voltage of the TFT; Based on the detection result of the on-current by the detecting means, the gate voltage applied to the TFT when the TFT is turned on is controlled so that the on-current of the TFT becomes a predetermined magnitude. And a control means.

  According to the above invention, the detecting means detects the on-current of the TFT used for the selection element of the picture element at a predetermined gate voltage. Then, the gate voltage control means controls the gate voltage applied to the TFT when the TFT is turned on based on the detection result of the on current so that the on current of the TFT becomes a predetermined magnitude. Therefore, even when the current-voltage characteristics of the TFT vary or change, the gate voltage applied to the TFT when the TFT is turned on is set so that the on-current is always the same.

  As a result, it is possible to realize a display device that can make TFT operations difficult to vary even if the TFT characteristics vary from panel to panel or even if the TFT characteristics change all at once in the same panel due to the environment. There is an effect that can be done.

  In order to solve the above problems, the display device according to the present invention is configured so that a gate voltage applied to the TFT when the TFT is turned off is a predetermined voltage from the gate voltage controlled by the gate voltage control means. It is generated as a subtracted voltage.

  Variations and changes in the current-voltage characteristics of TFTs have a relationship in which the characteristic curves move in parallel with each other. Therefore, only the gate voltage for turning on the TFT is controlled by the gate voltage control means, and based on the gate voltage. If the gate voltage when the TFT is turned off is generated, the gate voltage when the TFT is turned off becomes a voltage suitable for the TFT.

  According to the above invention, the gate voltage when the TFT is turned off is generated as a voltage obtained by subtracting a predetermined voltage from the gate voltage controlled by the gate voltage control means. Since the circuit for subtracting the predetermined voltage can be easily configured, the gate voltage when the TFT is turned off can be easily generated.

  In the display device of the present invention, in order to solve the above problem, the detection unit measures an on-current of a dummy TFT provided separately from the TFT, and measures the on-current of the TFT. It is characterized by detecting by associating with an on-current.

  According to the above invention, the dummy TFT is provided separately from the TFT as the pixel selection element, and the detecting means measures the on-current of the dummy TFT, and the on-current of the measured TFT is turned on. Detection is performed by associating with current. Therefore, there is an effect that means for detecting the on-current can be configured without interfering with the TFT actually used for display.

  In order to solve the above problems, the display device of the present invention is characterized in that the dummy TFT has the same structure as the TFT.

  According to the above invention, since the dummy TFT has the same structure as the TFT as the pixel selection element, the detection result of the on current of the dummy TFT can be directly used as the detection result of the TFT as the selection element. There is an effect that can be done.

  In order to solve the above problem, the display device of the present invention is characterized in that the dummy TFT is provided in a light shielding region provided outside a region where a picture element used for display is formed.

  According to the above invention, since the dummy TFT is provided in the light-shielding area provided outside the area where the picture element used for display is formed, the area of the dummy TFT obstructs the area used for display. There is an effect that there is nothing. In addition, there is an effect that the on-current can be detected without causing a variation in characteristics of the dummy TFT due to light irradiation.

  In order to solve the above problems, the display device of the present invention is characterized in that a constant voltage for turning on the dummy TFT is applied to the gate of the dummy TFT.

  According to the above invention, since the constant voltage for turning on the dummy TFT is applied to the gate of the dummy TFT, the detection means can always output the detection result of the on-current, and the gate voltage control means The gate voltage can be controlled to an appropriate value one by one.

  In the display device of the present invention, in order to solve the above-described problem, the detection unit temporarily converts the detected on-current of the TFT into a voltage, and then generates a detection result of the on-current to control the gate voltage. Output to the means.

  According to the above invention, the detection means converts the detected on-current of the TFT into a voltage, then generates a detection result of the on-current and outputs it to the gate voltage control means. Can be easily transmitted to the gate voltage control means.

  In order to solve the above problems, the display device of the present invention includes a charge pump circuit that generates and outputs a gate voltage to be applied to the TFT when the TFT is turned on. And controlling the gate voltage applied to the TFT when the TFT is turned on by controlling the output voltage of the charge pump circuit based on the detection result of the on-current by the detecting means. It is a feature.

  According to the above invention, the gate voltage applied to the TFT when the TFT is turned on is controlled by controlling the voltage itself generated by the charge pump circuit based on the detection result of the on-current by the detecting means. As a result, the gate voltage control means can be configured using a power supply circuit that generates a gate voltage.

  In the display device of the present invention, in order to solve the above problem, the gate voltage control unit adds the detection result of the on-current by the detection unit to the feedback to the input side of the output voltage of the charge pump circuit. It is characterized by receiving.

  According to the above invention, the detection result of the on-current by the detection means is added to the feedback to the input side of the output voltage of the charge pump circuit, so the output voltage of the charge pump circuit is adjusted so that the feedback voltage becomes constant. Only by controlling, the gate voltage applied to the TFT can be controlled when the TFT is turned on.

  In order to solve the above problems, the display device of the present invention is characterized in that the charge pump circuit is a circuit that generates an AC rectangular wave and boosts the AC rectangular wave to smooth it to a direct current. .

  According to the above invention, the charge pump circuit is a circuit that generates an AC rectangular wave, boosts the AC rectangular wave, and smoothes it to a direct current. Therefore, the output voltage is controlled using the detection result of the on-current. Thus, there is an effect that it can be controlled by controlling the waveform of the AC rectangular wave.

  In the display device of the present invention, in order to solve the above-described problem, the gate voltage control unit changes the detection result of the on-current by the detection unit, and changes the waveform of the AC rectangular wave of the output voltage of the charge pump circuit. It is characterized in that it is received so as to be added to the feedback for making it happen.

  According to the above invention, the detection result of the on-current by the detecting means is added to the feedback for changing the waveform of the AC rectangular wave of the output voltage of the charge pump circuit, so that the feedback voltage becomes constant. Only by controlling the waveform of the AC rectangular wave, the gate voltage applied to the TFT can be controlled when the TFT is turned on.

  In order to solve the above problem, the display device of the present invention stores the detection result of the on-current by the detection unit, and the gate voltage control unit stores the TFT based on the stored detection result of the on-current. The gate voltage applied to the gate is controlled.

  According to the above invention, the gate voltage control means controls the gate voltage applied to the gate of the TFT based on the stored detection result of the on-current, so that it is not necessary to constantly detect the on-current of the TFT. There is an effect.

  In order to solve the above problems, the display device of the present invention is characterized in that a constant drain-source voltage is applied to the dummy TFT.

  According to the above invention, since a constant gate voltage and a constant drain-source voltage are applied to the dummy TFT, one point on a specific current-voltage curve can be specified, and the accuracy is high. The detection result can be obtained, and the gate voltage applied to the TFT can be accurately set.

  In order to solve the above problems, a display device of the present invention is a liquid crystal display device.

  According to the above invention, in the liquid crystal display device, the gate voltage applied to the TFT can be optimized.

  As described above, the display device of the present invention is configured to detect the on-current at a predetermined gate voltage of the TFT, and to turn the TFT on based on the detection result of the on-current by the detection unit. And a gate voltage control means for controlling the gate voltage applied to the TFT so that the on-current of the TFT has a predetermined magnitude.

  As described above, it is possible to realize a display device that can make TFT operations less likely to vary even if the TFT characteristics vary from panel to panel or even if the TFT characteristics change all at once in the same panel depending on the environment. There is an effect that can be done.

  An embodiment of the present invention will be described below with reference to FIGS.

  FIG. 2 shows a configuration of the liquid crystal display device 1 according to the present embodiment.

  The liquid crystal display device 1 is an active matrix display device, and includes a gate driver 3 as a scanning signal line drive circuit, a source driver 4 as a data signal line drive circuit, a liquid crystal display panel 21, a gate driver 3 and a source driver. 4 includes a display control circuit 5 for controlling 4 and a power supply circuit 6.

  The liquid crystal display panel 21 further includes a polarizing plate or the like outside the configuration in which a liquid crystal layer is disposed between a TFT substrate and a CF substrate, and includes a display unit 2 used as a display region. It is assumed that the TFT substrate occupies the area 21a and the CF substrate has an area occupying the area 21b. The region 21b includes the region of the display unit 2 on the inner side and is included in the region 21a, and the region outside the display unit 2 is a light shielding region BM of a solid region indicated by hatching. The light shielding area BM is connected to the black matrix in the display unit 2.

  The display unit 2 includes gate bus lines GL1 to GLm as a plurality (m) of scanning signal lines and a plurality (n) of data signal lines that intersect with each of the gate bus lines GL1 to GLm. Source bus lines SL1 to SLn, and a plurality (m × n) of picture elements PIX... Provided corresponding to the intersections of the gate bus lines GL1 to GLm and the source bus lines SL1 to SLn, respectively. . The configuration of the picture element PIX is the same as that shown in FIG. Although not shown here, the display unit 2 includes the auxiliary capacitance bus lines CSL... Shown in FIG. 7 in parallel with the gate bus lines GL1 to GLm, and includes n picture elements arranged in the direction. One auxiliary capacity bus line CSL is assigned to each picture element row.

  The plurality of picture elements PIX are arranged in a matrix to form a picture element array, and each picture element PIX includes a TFT 11, a liquid crystal capacitor CL, and an auxiliary capacitor Cs. The TFT 11, the liquid crystal capacitor CL, and the auxiliary capacitor Cs have the same configuration as that in FIG. A voltage Vcom is applied from the power supply circuit 6 to the counter electrode COM. The liquid crystal capacitor CL and the auxiliary capacitor Cs constitute a pixel capacitor Cp, but there is also a parasitic capacitor formed between the pixel electrode 12 and the peripheral wiring as another capacitor constituting the pixel capacitor Cp. To do.

  The display control circuit 5 supplies the gate driver 3 with the gate start pulse GSP and the gate clock GCK, and supplies the source driver 4 with the source start pulse SSP, the source clock SCK, and the display data DA.

  In addition, a dummy TFT D11 is provided in a region of the TFT substrate that is shielded by the light shielding region BM, such as a portion indicated by A in the drawing. FIG. 3 shows the configuration of the dummy TFT D11. The dummy TFT D11 has the same structure as the TFT 11 of the picture element PIX, and is manufactured simultaneously by the same process as the TFT 11. The gate D11g of the dummy TFT D11 is connected to the dummy gate bus line DGL, and the source D11s of the dummy TFT D11 is connected to the dummy source bus line DSL. The dummy gate bus line DGL is simultaneously manufactured in the same process as the gate bus line GL connected to the picture element PIX of the display unit 2, and the dummy source bus line DSL is a source bus connected to the picture element PIX of the display unit 2. It is manufactured simultaneously with the same process as the line SL. A set voltage is applied from the gate driver 3 to the dummy gate bus line DGL, and a set voltage is applied from the source driver 4 to the dummy source bus line DSL. The drain D11d of the dummy TFT D11 is connected to the input of the drain current detection circuit 31 shown in FIG.

  As shown in FIG. 4, the drain current detection circuit (detection means) 31 includes a current-voltage conversion circuit 32, a resistance dividing circuit 33, and a resistor R4. The current-voltage conversion circuit 32 includes an operational amplifier OP and a resistor R1. The non-inverting input terminal of the operational amplifier OP is connected to GND, and the inverting input terminal of the operational amplifier OP is connected to the drain D11d of the dummy TFT D11. The resistor R1 is connected between the output terminal and the inverting input terminal of the operational amplifier OP. The resistance dividing circuit 33 includes a resistor R2 and a resistor R3. One end of the resistor R2 is connected to the output terminal of the operational amplifier OP, and the other end of the resistor R2 is connected to one end of the resistor R3. The other end of the resistor R3 is connected to GND. One end of the resistor R4 is connected to a connection point between the resistor R2 and the resistor R3. The other end of the resistor R4 is connected to the gate voltage generation circuit 41 as will be described later.

  The gate voltage generation circuit (gate voltage control means) 41 includes a charge pump circuit including an AC rectangular wave generation circuit 42, a booster circuit 43, and an output detection circuit 44. The AC rectangular wave generation circuit 42 uses a weak alternating current signal generated inside, and from a direct current voltage, an AC rectangular wave Vout (AC) having a waveform corresponding to the waveform of the alternating current signal as shown in FIG. ) Is generated and output. The booster circuit 43 boosts the AC rectangular wave Vout (AC) output from the AC rectangular wave generating circuit 42 as shown in FIG. 6A, and further smoothes it into direct current as shown in FIG. 6B. ,Output. The output detection circuit 43 detects the voltage output from the booster circuit 43 and feeds it back to the AC rectangular wave generation circuit 42. The AC rectangular wave generating circuit 42 changes the waveform of the AC signal generated internally so that the duty ratio of the pulse after boosting in the boosting circuit 43 changes, and thereby the output voltage of the gate voltage generating circuit 41 is set to a predetermined value. Stabilize.

  FIG. 5B shows a configuration example of the AC rectangular wave generation circuit 42. The AC rectangular wave generating circuit 42 includes capacitors C1 and C2, diodes D1 and D2, and MOS transistors 51 and 52. One terminal of the capacitors C1 and C2 is an input terminal for the AC signal AC, the other terminal of the capacitor C1 is connected to the gate of the MOS transistor 51, and the other terminal of the capacitor C2 is the MOS transistor. 52 is connected to the gate. The MOS transistor 51 is a p-channel type, its source is connected to a power supply that outputs a positive DC voltage V +, and its drain is connected to the output terminal of the AC rectangular wave generation circuit 42. The MOS transistor 52 is an n-channel type, its source is connected to a power supply that outputs a negative DC voltage V−, and its drain is connected to the output terminal of the AC rectangular wave generation circuit 42. The anode of the diode D1 is connected to the gate of the MOS transistor 51, and the cathode of the diode D1 is connected to the source of the MOS transistor 51. The anode of the diode D2 is connected to the source of the MOS transistor 52, and the cathode of the diode D2 is connected to the gate of the MOS transistor 52.

  As described above, the AC rectangular wave generation circuit 42 in FIG. 5B is configured by the inverter circuit. When the MOS transistors 51 and 52 are turned on and off by the input of the AC signal AC, the positive peak voltage is V + and the negative peak voltage is V− from the output terminal as shown in FIG. AC rectangular wave Vout (AC) is output. The total amplitude of the AC rectangular wave Vout (AC) is (V +) − (V−).

  FIG. 6C shows a configuration example of the booster circuit 43. The booster circuit 43 includes a capacitor C3, diodes D3 and D4, and a capacitor C4. One terminal of the capacitor C3 is an input terminal for an AC rectangular wave Vout (AC), and the other terminal of the capacitor C3 is connected to the anode of the diode D3 and the cathode of the diode D4. The cathode of the diode D3 is connected to one terminal of the capacitor C4, and the anode of the diode D4 is connected to GND. The other terminal of the capacitor C4 is connected to GND. The one terminal of the capacitor C3 is an output terminal of the booster circuit 43. The output terminal of the booster circuit 43 is also an output terminal of the gate voltage generation circuit 41.

  With such a configuration of the booster circuit 43, the input AC rectangular wave Vout (AC) is boosted to a waveform having a peak voltage of (V +) − (V−) with reference to GND as shown in FIG. Then, as shown in FIG. 6B, it is output as a voltage Vout (boot) smoothed to a direct current. FIG. 6B also shows a ripple component included in the voltage Vout (boot). This voltage Vout (boot) is used as the high gate voltage Vgh. If the diodes in the booster circuit 43 are arranged in two stages, a booster pulse having twice the amplitude (= 2 × (V +) − (V−)) is obtained in FIG. 6A.

  The output detection circuit 44 includes a resistor R5 and a resistor R6. One end of the resistor R5 is connected to the output terminal of the booster circuit 43, and the other end of the resistor R5 is connected to one end of the resistor R6. The other end of the resistor R6 is connected to GND. A connection point P between the resistors R5 and R6 is connected to a feedback terminal of the AC rectangular wave generating circuit 42. The connection point P is also connected to the other end of the resistor R4 included in the drain current detection circuit 31. Therefore, the detection result of the on-current Ion by the drain current detection circuit 31 is added to the feedback of the output voltage of the gate voltage generation circuit 41 to the AC rectangular wave generation circuit 42.

  Such a gate voltage generation circuit 41 is provided, for example, in the display control circuit 5 or the power supply circuit 6, but can be mounted in an arbitrary form such as an IC at an arbitrary position of the liquid crystal display panel 21. The arrangement position of the drain current detection circuit 31 provided between the dummy TFT D11 and the gate voltage generation circuit 41 is the same as that of the gate voltage generation circuit.

  In the liquid crystal display device 1 having the above configuration, a constant high gate voltage Vgh is continuously applied from the gate driver 3 to the dummy gate bus line DGL. In addition, a constant source voltage is continuously applied from the source driver 4 to the dummy source bus line DSL. At this time, the dummy TFT D11 is turned on, and the drain-source voltage Vds of the dummy TFT D11 is constant. By fixing the drain-source voltage Vds in addition to the gate voltage Vg, the current-voltage characteristic curve of the TFT shown in FIG. 1 described later and FIG. 8 described above is specified as one, and one point on the curve Therefore, the accuracy of the detection result is increased. The drain current can be detected even by fixing only the gate voltage Vg without fixing the drain-source voltage Vds. In this case, the drain-source voltage in a region where the change in the drain current Id is small is possible. Vds is preferably applied. A drain current Id determined by this voltage state, that is, an on-current Ion flows through the dummy TFT D11. The drain current Id flows through the resistor R1 of the current-voltage conversion circuit 32 included in the drain current detection circuit 31, and is once converted into the voltage Vd and appears at the output terminal of the operational amplifier OP. The voltage Vd is divided by the resistance dividing circuit 33 to become a voltage Vd ′.

  If the drain current Id is larger than the reference value, the voltage Vd ′ increases to increase the feedback voltage Vfb at the connection point P of the output detection circuit 44 of the gate voltage generation circuit 41. Uses the result of comparing the feedback voltage Vfb with the reference voltage corresponding to the reference value of the drain current Id inside the AC rectangular wave generating circuit 42 to control the gate voltage Vgh to be lowered. On the contrary, when the drain current Id is smaller than the reference value, the voltage Vd ′ decreases and the feedback voltage Vfb at the connection point P of the output detection circuit 44 of the gate voltage generation circuit 41 is decreased. 41 performs control to increase the gate voltage Vgh using the result of comparing the feedback voltage Vfb with the reference voltage inside the AC rectangular wave generation circuit 42. Thus, the gate voltage Vgh is adjusted so that the feedback voltage Vfb at the connection point P is kept constant. The adjusted gate voltage Vgh is used for application to the gate bus line GL, and the initially set gate voltage Vgh is continuously applied to the dummy gate bus line DGL.

  Therefore, if the voltage applied to the dummy gate bus line DGL and the voltage or voltage range applied to the dummy source bus line DSL are set, the drain current Id of the dummy TFT D11 at that time is compared with the reference drain current Id. As a result, a deviation of the current-voltage characteristic of the TFT 11 from the reference characteristic can be detected, and the gate voltage Vgh can be set in accordance with the deviation. If the low gate voltage Vgl is generated as a voltage lower than the adjusted gate voltage Vgh by a predetermined voltage, the low gate voltage Vgl is a voltage suitable for shifted characteristics. In order to generate a voltage lower by a predetermined voltage, a conventional circuit such as a forward voltage drop of a diode or a subtraction circuit including an operational amplifier can be used.

  The above example will be specifically described with reference to FIG. Assume that Vgh1 is set as the high gate voltage Vgh applied from the gate driver 3 to the dummy gate bus line DGL. This gate voltage Vgh1 is the gate voltage Vgh when I1 flows as the on-current Ion through the TFT 11 having the reference current-voltage characteristic represented by the curve a (solid line). In order to fix the drain-source voltage Vds and fix the on-current Ion to one value to the dummy source bus line DSL, a source voltage that is appropriately selected and determined is applied from the source driver 4. The AC rectangular wave generation circuit 42 of the gate voltage generation circuit 41 holds a voltage having a magnitude corresponding to the on-current I1 as a reference voltage for comparison with the feedback voltage Vfb.

  At this time, the current-voltage characteristic of the TFT 11 on the actual panel is represented by a curve b (broken line), and the on-current Ion flowing through the dummy TFT D11 formed in the same manner is I2 larger than I1. Suppose. Since the AC rectangular wave generation circuit 42 determines that the feedback voltage Vfb is larger than the reference voltage, the output voltage of the gate voltage generation circuit 41, that is, the gate voltage Vgh is lowered until the feedback voltage Vfb becomes equal to the reference voltage. The gate voltage Vgh at this time is the gate voltage Vgh2 at which the on-current Ion becomes I1 in the curve b. Therefore, the gate voltage Vgh2 is applied to the TFT 11 as the high gate voltage Vgh.

  Further, by setting the high gate voltage Vgh to the gate voltage Vgh2, the low gate voltage Vgl is set to change from Vgl1 corresponding to the curve a to Vgl2 corresponding to the curve b. Vgl1 is initially set together with Vgh1, and curve b is obtained by shifting curve a in parallel. Therefore, a voltage reduced from Vgh2 by the difference between Vgh1 and Vgl1 is defined as Vgl2. As a result, the low gate voltage Vgl1 set in the region R1 where the on-current Ion is zero in the curve a is changed to the gate voltage Vgl2 in the region R2 where the on-current Ion is zero in the curve b. It becomes.

  Thereby, even if the current-voltage characteristics of the TFT 11 vary or change, the high gate voltage Vgh of the TFT 11 is set so that the on-current is always the same. As a result, it is possible to realize a display device that can make the operation of the TFTs 11 less likely to vary even if the characteristics of the TFTs 11 vary from panel to panel or the characteristics of the TFTs 11 change all at once in the same panel depending on the environment. it can. Therefore, variations in luminance and gamma characteristics can be suppressed. Further, since the low gate voltage Vgl is also set to an appropriate value based on the high gate voltage Vgh, there is an effect that a sufficient off characteristic of the TFT 11 can be obtained. In order to set the high and low gate voltages Vgh, the on-current Ion at the high gate voltage Vgh is detected, so that detection can be performed where the current is large, and detection accuracy is increased. Can do.

  The set value of the low gate voltage Vgl has a certain allowable range of the region R1 and the region R2, and therefore there is a range in which the on-current Ion can be kept zero even if the current-voltage characteristic curve of the TFT 11 is shifted. In this case, by setting the range, it is not always necessary to change the same as the high gate voltage Vgh.

  In the above example, the on-current Ion of the dummy TFT D11 is measured, and the measurement result is obtained as the detection result of the on-current Ion of the TFT11. Further, in the present embodiment, the dummy TFT D11 has the same structure as the TFT 11, but the present invention is not limited to this, and the dummy TFT D11 having a different gate length and gate width is created from the TFT 11 and associated with the on-current Ion. The gate voltage Vgh of the TFT 11 may be set.

  In the above example, the configuration in which the constant gate voltage Vgh and the source voltage are always applied to the dummy TFT D11 has been described. However, the present invention is not limited to this, and the detection result of the on-current Ion is stored and the stored result is used. The gate voltage Vgh may be controlled. In order to store the detection result of the on-current Ion, a switch is inserted before and after the current-voltage conversion circuit 32. When the on-current Ion is measured, the switch is turned on, and the voltage is further connected to the output side switch. Examples include a configuration in which a holding capacitor is provided, the measurement result converted into the voltage Vd is stored therein, and then the switch is turned off.

  The present embodiment has been described above. In the above example, a liquid crystal display device is used as the display device. However, the present invention is not limited to this, and any display device using TFTs such as an EL display device is a subject of the present invention.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  The present invention can be suitably used for a display device using TFT, particularly a liquid crystal display device.

4 is a graph showing the current-voltage characteristics of a TFT for illustrating the control of the gate voltage to an appropriate value according to the embodiment of the present invention. 1, showing an embodiment of the present invention, is a block diagram illustrating a configuration of a liquid crystal display device. FIG. It is a top view which shows the structure of a dummy TFT. FIG. 3 is a circuit block diagram showing a configuration of a circuit for detecting an on-current of a TFT and a circuit for controlling a gate voltage of the TFT provided in the liquid crystal display device of FIG. 2. It is a figure explaining the production | generation of AC rectangular wave, (a) is a waveform diagram of AC rectangular wave, (b) is a circuit diagram which shows the structure of an AC rectangular wave generation circuit. It is a figure explaining the pressure | voltage rise of AC rectangular wave, (a) is a wave form diagram of the pulse after pressure | voltage rise of AC rectangular wave, (b) is a figure which shows the waveform which smoothed the pulse after pressure | voltage rise of (a), (c). FIG. 3 is a circuit diagram showing a configuration of a booster circuit. It is a circuit diagram which shows the structure of a pixel with an equivalent circuit. It is a graph explaining the dispersion | variation and change of the current-voltage characteristic of TFT.

Explanation of symbols

1 Liquid crystal display device (display device)
11 TFT
31 Drain current detection circuit (detection means)
41 Gate voltage generation circuit (gate voltage control means, charge pump circuit)
D11 Dummy TFT
BM Shading area Ion On-current Vout (AC) AC rectangular wave

Claims (14)

In an active matrix display device including a TFT as a picture element selection element,
Detecting means for detecting an on-current at a predetermined gate voltage of the TFT;
Based on the detection result of the on-current by the detecting means, the gate voltage applied to the TFT when the TFT is turned on is controlled so that the on-current of the TFT becomes a predetermined magnitude. And a control means.   The gate voltage applied to the TFT when the TFT is turned off is generated as a voltage obtained by subtracting a predetermined voltage from the gate voltage controlled by the gate voltage control means. Display device.   The detection means measures an on-current of a dummy TFT provided separately from the TFT, and detects the on-current of the TFT by associating it with the measured on-current of the dummy TFT. 3. The display device according to 1 or 2.   The display device according to claim 3, wherein the dummy TFT has the same structure as the TFT.   5. The display device according to claim 3, wherein the dummy TFT is provided in a light-shielding region provided outside a region where a picture element used for display is formed.   The display device according to claim 3, wherein a constant voltage that turns on the dummy TFT is applied to a gate of the dummy TFT.   7. The detection unit according to claim 1, wherein the detection unit converts the detected on-current of the TFT into a voltage, generates a detection result of the on-current, and outputs the detection result to the gate voltage control unit. The display device according to any one of the above.   The gate voltage control means includes a charge pump circuit that generates and outputs a gate voltage applied to the TFT when the TFT is turned on, and based on the detection result of the on-current by the detection means. 8. The gate voltage applied to the TFT when the TFT is turned on is controlled by controlling an output voltage of the charge pump circuit, according to any one of claims 1 to 7. The display device described.   The said gate voltage control means receives the detection result of the said on-current by the said detection means so that it may be added to the feedback to the input side of the output voltage of the said charge pump circuit. Display device.   10. The display device according to claim 8, wherein the charge pump circuit is a circuit that generates an AC rectangular wave and boosts the AC rectangular wave to smooth the DC rectangular wave.   The gate voltage control means receives the detection result of the on-current by the detection means so as to be added to feedback for changing the waveform of the AC rectangular wave of the output voltage of the charge pump circuit. The display device according to claim 10. Storing the detection result of the on-current by the detection means;
The display device according to claim 1, wherein the gate voltage control unit controls a gate voltage applied to the gate of the TFT based on the stored detection result of the on-current.   The display device according to claim 6, wherein a constant drain-source voltage is applied to the dummy TFT.   The display device according to claim 1, wherein the display device is a liquid crystal display device.

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