Patent Publication Number: US-2022238064-A1

Title: Display device

Description:
The invention relates to a display device having a plurality of pixels and a control unit for driving the plurality of pixels. 
     This patent application claims priority to German patent application 102019105001.4, the disclosure content of which is hereby incorporated by reference. 
     Conventional controls for pixels of a display device work in a cross-matrix arrangement and with current dimming to influence the brightness by changing the intensity of the emitted light of the pixels. This is also referred to as analog dimming. It is used for OLEDs and LCDs, for example. Such a control is disadvantageous for LED displays because of the unfavorable influence on color location or color palette gamut. 
     The object is to provide a display device with an alternative control. 
     A display device according to claim  1  is provided for this purpose. It comprises a plurality of pixels, the pixels being arranged in an array having rows and columns. It further comprises a plurality of column lines respectively connected to the pixels of one of the columns, and a plurality of row lines respectively connected to the pixels of one of the rows. A control unit is connected to the plurality of column lines and adapted to generate a column pulse for a selected one of the plurality of column lines. The control unit is also connected to the plurality of row lines and adapted to generate a data signal for a selected row line from the plurality of row lines. The data signal includes a set pulse that, when the pixel is set to a radiating state, is applied at least in part to the pixel connected to the selected column line and row line when the column pulse is applied to the pixel, and drives the pixel such that a light emission of the pixel depends on the time offset between the column pulse and the set pulse. 
     This control enables the offset-dependent control of the display device with high dynamics. The light is emitted only when the set pulse is applied, provided that the column pulse is applied at the same time. If this occurs at an early point in time while the column pulse is applied, i.e. the offset is small, the radiation starts earlier than if the set pulse is only applied towards the end of the column pulse, i.e. the offset is larger. The pixel can be switched off by a reset pulse or automatically after a preset time. A pulse width concept that can be implemented with this, in which radiating and non-radiating periods alternate, allows the bit depth and gray gradation to be set and a dimming effect to be achieved. This pulse width modulation for active matrix pixels enables optimum image quality and allows pixel-fine control. 
     In one embodiment, the control unit is configured to adjust the time offset between the start of the column pulse and the start of the settling pulse to influence the emission duration. 
     Another degree of freedom results from the fact that the light emission depends on the amplitude of the set pulse and the control unit is suitable for adjusting the amplitude of the set pulse. This allows the intensity of the radiation to be changed. 
     In one embodiment, the control unit is configured to reset the pixel before setting it again so that it is in a non-radiating state and no light is emitted. The mandatory reset before each setting puts the pixel in a well-defined non-radiating rest state during which it does not emit, so that no interference with the previously set information occurs in the pixel electronics during subsequent setting. A pixel has a light-emitting semiconductor device and a pixel controller. If the pixel control has a capacitor as charge storage, and information storage, the reset results in its complete discharge before it is again charged by the set pulse. 
     In one embodiment, the control unit is configured to reset the pixel when a reset pulse is applied to the pixel if a column pulse is also applied to the pixel. 
     In an alternative embodiment, the pixel is configured to reset with a time delay to setting. In other words, the pixel is automatically reset to the non-radiating state after a predetermined time has elapsed without a reset pulse. 
     In one embodiment, the control unit is configured to generate as a data signal a bipolar pulse that is a sequence of the reset pulse followed by the set pulse, wherein when the column pulse is applied to the pixel, at least a portion of the sequence that is at least the reset pulse is also applied to the pixel. The bipolar pulse comprises a reset pulse with, for example, a negative sign, followed by a set pulse with an opposite, for example positive, sign, so that a reset occurs before each new set. Advantageously, the control unit is configured to adjust the distance between the reset pulse and the set pulse in order to influence the radiation characteristic. 
     In one embodiment, the row line of a row has two galvanically separated sections, each connected to a group of pixels in the row. The control unit is configured to generate a data signal for each of the two sections. Illustratively, this means that the array is separated into two areas, one of which comprises the first row sections of the rows and the other of which comprises the second row sections of the rows. The areas can be controlled largely independently of each other. It is thus possible to drive the two areas in parallel, which, compared to an embodiment with continuous row lines, can be done at half the speed. It should also be noted that the row line can also be divided into more than two sections. 
     In one embodiment, the control unit is configured to generate two column pulses for selecting two column lines whose offset from a set pulse traveling along the selected row line is so selected that if at least one of the column pulses is applied to one of the pixels connected to one of the selected column line and the row line, at least a portion of the set pulse is also applied to the same pixel. This allows two pixels to be driven by the same set pulse. Again, selecting two column pulses allows the display device to operate at half the speed. 
     Advantageously, in the above embodiment, the control unit comprises a first pulse generator that applies the set pulse to one side of the row line and a second pulse generator that applies the reset pulse to the other side of the row line. 
     The two unipolar pulse generators have a simpler design than bipolar pulse generators. In addition, the unipolar pulses can be wider, which reduces circuit complexity. 
     In one embodiment, the control unit is configured to select the column lines cyclically successively, so that if a pixel is set to the radiating state in a first cycle and in a subsequent second cycle, it is first reset and set in both cycles when the column pulse is applied. The selection allows the columns to be selected successively one after the other and their pixels to be controlled in the process, so that line-specific control is possible. 
     In one embodiment, the control unit is configured to successively select the column lines cyclically so that when a pixel is set to the active state in a first cycle and remains therein in the subsequent second state, in the first cycle the data signal comprises only a set pulse and in the second cycle the data signal comprises no reset or set pulse or only a recovery pulse. With this control, the pixel is not reset and newly set each time its column is selected, but remains in its state until reset or only a parasitic charge loss is compensated by recovering in the pixel control. The recovery pulse is smaller than the set pulse with the same polarity. 
     The reset is performed by a reset pulse. The control unit is configured to successively select the column lines cyclically so that when a pixel is reset from the radiating state to the non-radiating state in one cycle, the data signal comprises only one reset pulse. The described control allows the selective pulse width modulation control, so that periods in which the set pixel radiates and periods in which the reset pixel does not radiate alternate, which corresponds to a pulse width modulation. 
     In one embodiment, supply lines of two of the rows are galvanically isolated at the array level so that voltage drops in pixel control are negligible and impose lower circuit requirements on the pixel controls. 
     In one embodiment, the pixel control comprises a charge storage, for example a capacitor, for the energy of the set pulse, the charge amount of the charge storage being limited by a protection diode or circuit arrangement in parallel to the charge storage. The pixel control may be based on a 2T1C structure comprising two transistors and a capacitor. 
     In one embodiment, the pixel control includes a toggle circuit that returns to an idle state after triggering with the reset pulse after a predetermined time such that the pixel is reset. This toggle circuit allows automatic resetting, for example, after a time period that depends on the circuit dimensions has elapsed, without requiring the application of a reset pulse. Such a toggle circuit can be a mono-flop, for example. 
     The above described display device and its embodiments allow the control with and the generation of fast pulses. Thereby the duration of the pulses intended for the individual pixels is short. 
    
    
     
       In the following, the display device is explained in more detail with reference to embodiments and the associated figures. 
         FIG. 1  schematically shows an embodiment of a display device. 
         FIG. 2  shows an embodiment of a pixel. 
         FIG. 3  shows a schematic embodiment of a display device. 
         FIG. 4  schematically shows another embodiment of a display device. 
         FIG. 5  schematically shows another embodiment of a display device. 
         FIG. 6  schematically shows another embodiment of a display device. 
         FIG. 7  schematically shows another embodiment of a display device. 
         FIG. 8  shows an embodiment of a pixel controller and a light-emitting semiconductor device. 
         FIG. 9  shows another embodiment of a pixel controller and a light-emitting semiconductor device. 
         FIG. 10  shows another embodiment of a pixel controller and a light-emitting semiconductor device. 
         FIG. 11  shows another embodiment of a part of a pixel controller and light emitting semiconductor devices. 
     
    
    
       FIG. 1  schematically shows the structure of an embodiment of a display device having a plurality of pixels P 0101 , P 0102 , P 0103 , P 01 Y, P 0201 , P 0301 , PX 01  arranged in an array having a plurality of rows, namely X, and a plurality of columns, namely Y. For clarity, not all pixels are shown. The array has X*Y pixels P 0101 , P 0102 , P 0103 , P 01 Y, P 0201 , P 0301 , PX 01 . 
     The display device comprises a plurality of row lines L 1  . . . LX, not all of which are shown for the sake of clarity. The number of row lines L 1  . . . LX is X. Each row line L 1  . . . LX is connected to the pixels of one of the rows. For example, the first row line L 1  is connected to the pixels P 0101 , P 0102 , P 0103  . . . P 01 Y in the first row. 
     The display device comprises a plurality of column lines C 1  . . . CY, not all of which are shown for clarity. The number of column lines C 1  . . . CY is Y. Each column line C 1  . . . CY is connected to the pixels of one of the columns. Thus, the first column line C 1  is connected to the pixels P 0101 , P 0201 , P 0301  . . . PX 01  in the first column. 
     A control unit  2  is provided for driving the pixels. It is connected to the plurality of column lines C 1  . . . CY and adapted to generate column signals CS 1  . . . CSY for the column lines C 1  . . . CY, and it is connected to the plurality of row lines L 1  . . . LX and adapted to generate data signals LP 1  . . . LPX for the row lines L 1  . . . LX. 
     Each of the pixels P 0101 , P 0102 , P 0103 , P 01 Y, P 0201 , P 0301 , PX 01  is connected to one of the row lines L 1 , LX and one of the column lines C 1 , CY and can be controlled by selecting the column line C 1 , CY connected to it by means of a column signal CS 1 , CSY and simultaneously applying a data signal LP 1 , LPX to the row line L 1 , LX connected to the selected pixel, so that the pixels emit light in the desired manner, i.e. in a manner dependent on the data signal LP 1 , LPX. The column signals CS 1 , CSY are generated by means of a column signal generator  4  for the plurality of column lines C 1  . . . CY in such a way that a column pulse is applied to a column line in order to select it. The data signals LP 1  . . . LPX are generated by means of a row signal generator  6  connected to the plurality of row lines L 1  . . . LX for driving the pixels in the selected column. 
     The control of the pixels P 0101 , P 0102 , P 0103 , P 01 Y, P 0201 , P 0301 , PX 01  is carried out column by column by selecting the columns cyclically successively and applying a data signal to the selected pixels of the column if necessary. First the pixels of the first column are controlled, then those of the second column and so on. After the pixels of the Yth column have been controlled, the pixels of the first column are controlled again. The sequence of all controlled columns is called a cycle. The frequency with which the cycles are repeated is called the frame rate. 
       FIG. 2  shows an embodiment of a pixel P from the array having a light emitting semiconductor device  200 , for example, an LED or a μLED, and a pixel controller by means of which a current for driving the light emitting semiconductor device  200  is adjusted. The light emission depends on the current that is set. If no current flows, no light emission occurs. 
     A capacitor  210 , a switching transistor  220  and a driver transistor  230  are assigned to the semiconductor component  200  for control. In addition, a row line L, via which data LP is applied, a column line C for switching by means of a column signal CS and two supply lines for a supply potential VDD and a reference potential GND are assigned to the pixel P for control. The switching transistor  220  is arranged to apply a voltage to the capacitor  210  and thus to charge or discharge it. The capacitor  210  is arranged to provide a voltage controlling the driver transistor  230 , which in turn can be used to adjust the current through the driver transistor  230  and the semiconductor device  200 . The capacitor  210  is used as an analog storage element. In this embodiment, a common anode and an n-FET are used in the pixel control. Other transistor structures and thus polarizations are conceivable. 
     The pixel drive described above is also called a 2T1C structure because it has two transistors and a capacitor. 
       FIG. 3  shows an embodiment of a display device with a plurality of pixels arranged in rows and columns, as already described in connection with  FIG. 1 . The control unit  2  has been omitted for the sake of clarity. 
     In this embodiment  1000  columns are provided, which are cyclically selected successively by applying a rectangular column pulse as column signal CS 1 , CS 2 , CS 3  to one column line C 1 , C 2 , C 3  each. With a frame rate of 60 Hz and 1000 columns (Y=1000), this results in a column pulse width T=1/60/1000 of 16.6 μs. In this time window, the pixels of the selected column can be controlled by applying data signals to the row lines.  FIG. 3  shows an example of a data signal LP 1  applied to the first row line L 1 . 
     The data signal LP 1  comprises a sequence of a reset pulse RP with negative amplitude and a set pulse SP with positive amplitude. The duration of the sequence is such that both the reset pulse RP and the set pulse SP are present at the pixel as long as the column pulse CS 1  is present. In other words, the sequence of reset pulse RP and set pulse SP is shorter than or equal to the column pulse width T. 
     The reset pulse RP causes the pixel to be set to a non-light emitting state, so it can also be referred to as an off pulse. It causes the capacitor  210  in the pixel control to be discharged to put it into a well-defined state. The set pulse SP causes the pixel to be set to a light emitting state, so it can also be referred to as a turn-on pulse. It charges the capacitor  210  so that, depending on the amplitude level, and thus the amount of charge charged on the capacitor  210 , the current through the light emitting semiconductor device  200  and thus its radiation is adjusted. 
     During the time period from the start of the reset pulse RP to the start of the set pulse SP, the semiconductor device  200  does not emit light. This time period is also referred to as the turn-on time TOT. It can be set by the control unit  2 . A short turn-on time TOT with a small interval between the reset pulse and the set pulse causes the semiconductor device  200  to emit light for a longer period during the pulse duration T than is the case with a long turn-on time TOT with a larger interval between the reset pulse and the set pulse. After the end of the set pulse SP, the pixel remains in the light emitting state because the charge applied to the capacitor  200  during the set still controls the current flow through the semiconductor device  200  in an unchanged manner. The next pixel in the row can be similarly controlled by applying the next set pulse SP. If a pixel is no longer to be lit, only the reset pulse RP can be applied as the data signal. 
     Due to the amplitude level and the setting of the turn-on time TOT, two degrees of freedom are available for setting the pixel brightness and radiation duration. Resetting and setting is performed while a column pulse is present. 
     During the column pulse, which is 16.6 μs in this embodiment, the pixel control electronics are discharged and, if necessary, recharged. During at least a portion of this time period, whether analog or digitally discretized, current may flow through the light emitting semiconductor device  200  and the light emitting semiconductor device  200  emits light. The ratio of the on-time, in which light is emitted, to the sum of the on-time and off-time, in which no light is emitted, then results in the duty cycle, which is, for example, a value for the set gray level. 
     In one embodiment, the supply lines for supply and reference potential VDD, GND of two of the lines to which the same potential is applied are galvanically isolated at array level. In other words, the supply lines for the supply potential VDD and the reference potential GND for each row are galvanically isolated from those of another row. Alternatively, it can be provided that the supply lines for the supply potential VDD and the reference potential GND for one group of rows are galvanically isolated from those of another group of rows. With such a setup, voltage drops on the common supply potential or reference potential line are negligible. Only one pixel per line is selected at a time. In such a case, the pixel control can be configured as a 2T1C structure as described in  FIG. 2 , since voltage drops on the common supply potential or reference potential lines are negligible. The electronics of the pixel driver in such an embodiment can be TFT-based (e.g. IGZO, LTPS) or silicon-based (e.g. crystalline, ASIC). 
     It should also be noted that the circuit advantages result in an increased space requirement for the separate supply lines in each row. 
       FIG. 4  shows another example of a display device. To avoid repetition, the description focuses on differences from the previous embodiment in  FIG. 3 . 
     The row line L 1  . . . LX of each row have two sections L 1   a  . . . LXa and L 1   b  . . . LXb separated from each other. The sections L 1   a  . . . LXa and L 1   b  . . . LXb are each connected to a group of pixels in the row. With 1000 columns, in this embodiment, 500 pixels are provided in the first section L 1   a  . . . LXa of each row in the first to the five hundredth column. In the second section of each row L 1   b  . . . LXb, 500 pixels are also provided in the five hundredth to the thousandth column. 
     The data signal generator  6  (not shown in  FIG. 4 ) is configured to generate a data signal for each of the two sections so that a column in the first group and a column in the second group can be selected simultaneously and their pixels reset and set. Exemplarily, the first and five hundredth columns can be selected simultaneously, then the second and five hundredth columns, and so on. Other sequences are conceivable, for example, the first and thousandth columns may be selected simultaneously, then the second and nine hundred and ninety-ninth columns, and so on. 
     As indicated in  FIG. 4 , the data signals can be applied on both sides (which corresponds to left and right in  FIG. 4 ) of the row lines L 1  . . . LXa and L 1   b  . . . LXb separated into sections L 1   a  . . . LX. 
     Since two pixels can be set simultaneously, this results in a larger column pulse width T compared to the previous embodiment for the same number of columns Y and frame rate. It is twice as large for two sections per row line. With a frame rate of 60 Hz and 1000 columns (Y=1000), this results in a pulse width T of 33 μs. 
     The provision of separate sections of line conductors is usually accompanied by the provision of separate supply conductors for the pixels of the different sections. 
     Although the data signal generator  6  must generate two pulses simultaneously, which involves increased circuitry, there is more time for reset and set pulse generation and width, which means a reduction in effort for these aspects. 
     It should be noted that although only two sections L 1   a  . . . LXa and L 1   b  . . . LXb have been described in the embodiment, embodiments with more than two sections per row line L 1  . . . LY are also conceivable. 
       FIG. 5  shows another example of a display device. In order to avoid repetition, the description concentrates on differences to the embodiment in  FIG. 3 . 
     The data signal generator  6  comprises a first pulse generator  61  which applies the reset pulse RP to one side of the row lines L 1  . . . LX, and a second pulse generator  62  which applies the set pulse SP to the other side of the row lines L 1  . . . LX. Such an arrangement would also be suitable for generating the sequence of reset and set pulses described in connection with  FIG. 3  within the column pulse duration required in  FIG. 3 . 
     However, this embodiment in  FIG. 5  has twice the dynamics of the embodiment described in  FIG. 3 , where the column pulse duration T is twice as long, i.e. 33 μm. Two column pulses are generated to select two columns, the time spacing of which corresponds to the transit time of the set and reset pulses between the selected columns. The pulse generators  61 ,  62  can be arranged on the two sides of the row lines L 1  . . . LX. 
     The column pulse at one of the pixels, which is connected to one of the selected column and row lines, is selected in such a way that within the time in which the same column pulse is applied to the pixel, the set pulse PS is also applied to the same pixel in addition to the reset pulse PR, if the pixel is to be set. If both pixels are to be set, the above is the case for both pixels. 
     During the time when the pulse generators  61 ,  62  are not generating pulses, the so-called off-time, they are at high impedance so as not to influence the generation of the other pulse. In this way, two pixels can be programmed by the same reset and set pulse RP, SP. The offset of the column pulses relative to the set pulse is selectable, as is the spacing of the reset and set pulses RP, SP and the amplitudes of the set pulse SP, so that the drive level and the brightness of the pixels can also be set. Usually, the same reset and set pulses RP, SP are used to drive pixels that are spaced from one to twenty pixels apart. In  FIG. 5 , an exemplary distance of ten was selected. 
     Although two pulse generators  61 ,  62  are provided, there is more time for reset and set pulse generation and width, which means a reduction in effort for these aspects. 
       FIG. 6  shows another example of a display device. In order to avoid repetition, the description concentrates on differences to the embodiment in  FIG. 3 . 
     The embodiment in  FIG. 6  has a higher dynamic, where the columns are scrolled with a much higher frequency than described in  FIG. 3 . For example, the example in  FIG. 6  has an x-fold frame rate, so that at x=256 and a frame rate of 256*60 
     In this embodiment, however, while the column signal pulse is applied to the pixel, a sequence of reset and set pulses is not applied, but while the column pulse is applied to the pixel, either a set pulse SP or a reset pulse RP is applied to the pixel. The pixel, if it is to remain in the radiating state, is not reset and newly set in the next cycle, but can remain in the radiating state for several cycles without newly setting. Here, too, the degree of actuation can be adjusted by the offset of the column pulse and the set pulse and can thus be changed. The later the set pulse SP occurs within the column pulse duration, the longer the switch-on time TOT. 
     The reset does not occur until one of the following column pulses is applied to the pixel. If no column pulse is applied, the driver transistor  230  of the pixel control remains conductive and the charge in the capacitor  210  is stored almost indefinitely until it is discharged during one of the next column selection operations. The column line is selected again after 65 μs (that is 1000*65 ns). 
     The on-time in a time window of 16.6 ms (corresponding to 60 Hz) is n*65 μs+(x−n)*m with m&lt;65 ns, n&lt;256, x=256. 
     Advantageously, small leakage currents in the pixel control are compensated by a recovery by means of a recovery pulse SPN. The recovery pulse SPN has the same polarity as the set pulse SP, but has a lower amplitude, which only compensates for charge losses at capacitor  210  due to leakage currents. 
     The pixel is set to the non-radiating state by a reset pulse RP while the column pulse is applied to the pixel. The data signals SP, RP, SPN mentioned are sketched as an example in  FIG. 6 . If no pulse is generated, the data signal generator  6  has a high impedance. 
     In one embodiment, a bipolar pulse with reset and set pulses is generated and the control is such that either the reset or set pulse is applied within the column pulse. 
     The shorter column signal pulses and the targeted setting and resetting allow even finer adjustability of dynamics and brightness. Fine granular tuning of duration and grayscale is possible. 
       FIG. 7  shows another example of a display device. In order to avoid repetition, the description concentrates on differences to the embodiment in  FIG. 6 . 
     In this embodiment, resetting of the pixels to the non-radiating state occurs after a predetermined time after setting. In other words, a reset pulse is not required. The set pulse SP may have a width that is less than, equal to, or greater than the column pulse. In the latter case, the same set pulse SP can be used to drive several adjacent pixels in a row. 
     The preset time for resetting can be in the range of the frame rate, so for a column pulse width T=65 ns and 1000 columns 65 μs=1000*65 ns. The time for resetting can be greater in another embodiment. 
     For example, the on-time in a time window of 16.6 ms (corresponding to 60 Hz) is n*65 μs+(x−n)*m with m&lt;65 ns, n&lt;256, x=256. 
     This embodiment also allows finer adjustability of dynamics and brightness. Fine granular adjustment of duration and grayscale is possible. 
       FIG. 8  shows a circuit diagram for an embodiment of a pixel controller and a light-emitting semiconductor device as exemplary pixel electronics. 
     The embodiment differs from the embodiment shown in  FIG. 2  in that a protective diode (English “clamping diode”) is connected in parallel with the capacitor  210 , which is configured as a Z-diode  240  and which limits the voltage applied to the capacitor  210 . In this simple form of pixel electronics, voltage limiting and current pinning occurs even when the gate terminal of the driver transistor  230  is fully driven with the set pulse SP as the data signal LP. The current is limited by the LED  200 , consequently set to a fixed value and largely independent of the amplitude, provided that the set pulse SP has sufficient energy. 
     In addition, this design allows an automatic, albeit slow, reset, since the predetermined amount of charge also determines the time until the capacitor is discharged due to the leakage currents and thus the pixel is reset to the non-radiating state. 
       FIG. 9  shows a circuit diagram for an embodiment of a pixel drive and a light-emitting semiconductor device  200 , namely an LED. The pixel drive enters a non-radiating state after a predetermined time, in which no current flows through the semiconductor device  200 . The pixel drive comprises a mono-flop with two transistors  310 ,  320 , which is triggered by the set pulse SP as a data signal LP and returns to a non-radiating state by itself after a time determined by the circuit dimensioning. Bias current limiting, i.e. current spinning, is also present in this circuit arrangement. 
     A switching transistor  220  and a driver transistor  230  are assigned to the semiconductor component  200  for control. Between the driver transistor  230  and the LED  200 , the transistors  310 ,  320  connected as a mono-flop are provided, which are triggered by the switching transistor  220 . In this embodiment, the transistors  220 ,  230 ,  310 ,  320  are N-channel enhancement MOSFETs. 
       FIG. 10  shows a circuit diagram for an embodiment of a pixel driver and a light emitting semiconductor device  200 , namely an LED. The pixel drive also has a toggle circuit that goes into the idle state after a predetermined time, which interrupts the flow of current through the LED  200 . A switching transistor  220  and a capacitor  210  are associated with the semiconductor device  200  for driving. 
     Four transistors  330 ,  340 ,  350 ,  360  are connected in such a way that they are triggered by set pulse SP to allow current flow through LED  200  and the toggle circuit returns to the rest position by itself after a time determined by its dimensioning. 
     A first and a second transistor  330 ,  340  are connected in series with the LED  200 , with the gate terminal of the first transistor  330  connected to the capacitor  210  so that it acts as a driver capacitor. In parallel with the series connection of the LED  200  and the first and second transistors  330 ,  340 , a third and fourth transistor  350 ,  360  are connected in series, with the drain and gate terminals of the third transistor  350  connected together. The gate terminal of the fourth transistor  360  is connected between the LED  200  and the first transistor  330 . The gate terminal of the second transistor  340  is connected between the third and fourth transistors  350 ,  360 . In this embodiment, the first, third, and fourth transistors  330 ,  350 ,  360  are N-channel enhancement MOSFETs, as is the switching transistor  220 . The second transistor  340  is formed as a P-channel enhancement MOSFET. 
       FIG. 11  shows an embodiment of a circuit that also returns to the idle state by itself. In addition to a series of LEDs  200  and the driver transistor  230 , a first resistor  410  is connected in series. A transistor  370  is connected in parallel with the gate terminal of the driver transistor  230  and the first resistor  410 , and a Z-diode  510  is also connected in parallel. The gate terminal of the transistor  370  is connected between the driver transistor  230  and the first resistor  410 . A second resistor  420  is connected in parallel with the LEDs  200  and the gate terminal of the driver transistor  230 . 
     The previously described circuit arrangements are more complex designs with a toggle circuit and can, for example, enable current pinning despite a short set pulse SP. The current is limited by the LED  200  and thus set to a fixed value that is independent of the set pulse SP, provided it has sufficient energy. 
     The features of the embodiments can be combined with each other. The invention is not limited by the description based on the embodiments to these. Rather, the invention encompasses any new feature as well as any combination of features, which in particular includes any combination of features in the patent claims, even if this feature or combination itself is not explicitly stated in the patent claims or embodiments. 
     LIST OF REFERENCE SIGNS 
       2  Control unit 
       4  Column signal generator 
       6  Data signal generator 
       200  Semiconductor device 
       210  Capacitor 
       220 ,  230 ,  310 ,  320 ,  330 ,  340 ,  350 ,  360 ,  370  Transistor 
       410 ,  420  Resistor 
       240 ,  510  Z-diode 
     C, C 1  . . . CY Column line 
     CS, CS 1  . . . CSY Column signal 
     L, L 1  . . . LX Row line 
     LP, LP 1  . . . LPX Data signal 
     P, P 0101 , P 0102 , P 0103 , P 01 Y, P 0201 , P 0301 , PX 01  Pixel 
     VDD, GND Potential 
     TOT Switch-on time 
     SP Set pulse 
     RP Reset pulse 
     NSP Recovery pulse 
     T Pulse width