Miniature LED pixel circuit driven in PWM mode

Abstract

A micro LED pixel circuit driven in PWM according to the present invention comprises: a scan signal generation unit for outputting a scan signal having a slope adjusted according to gradation data; the constant current control unit is used for providing constant current and controlling whether to stop providing the constant current or not according to the scanning signal output by the scanning signal generation unit; and the micro LED is used for driving according to the constant current.

Description

Technical Field

[0001] The invention relates to a micro LED pixel circuit driven in a PWM mode. Background Art

[0002] As we all know, compared with OLED displays, micro light-emitting diode (hereinafter referred to as Micro LED) displays can achieve higher resolution, lower power consumption, and higher temperature stability. In particular, Micro LED has the advantage of no burn-in problem because its light-emitting layer uses inorganic materials. However, the wavelength of light emitted by Micro LED varies according to the current density, which may cause color distortion. Therefore, Micro LED displays must adopt pulse width modulation (PWM) drive that provides constant current and expresses grayscale by adjusting the light emission time.

[0003] When implementing PWM drive based on pixel circuits, a ramp signal called SWEEP is usually used to control the switching of the driving thin film transistor (DRT) of the PWM part. The DRT switch of the PWM part directly affects the DRT operation of the constant current generation (CCG) part, causing the constant current generation circuit to stop providing current to the micro-LED. However, this method based on the scan signal essentially involves a falling time of hundreds of microseconds before the current is interrupted. This is because the DRT of the PWM part cannot act as an ideal switch, and the scan signal also has a certain slope.

[0004] These fall times of hundreds of microseconds can be a significant problem for grayscale expression, especially for low grayscales where the light emission duration is shorter than the fall time. Typically, the peak current starts to fall around 60 to 90 grayscales, and the fall time shortens as the current waveform deteriorates. These results indicate that operation is not proceeding as expected. Therefore, shortening the fall time of constant current may be the most important issue for stable grayscale expression of micro-LED displays.

[0005] Recently, in order to reduce the fall time, an inverter is used in the PWM circuit, and a method is being studied. When the scanning signal passes through the inverter, its slope becomes steeper, so the fall time can be shortened. However, even if the inverter is applied to the pixel circuit, it is confirmed that there is still a limitation caused by the fall time for grayscale expression below 37G.

[0006] Therefore, the present invention proposes a new driving method of adjusting the slope of the scanning signal itself as a method of accurately expressing extremely low grayscales.

[0007] [Prior art literature]

[0008] [Patent Literature]

[0009] (Patent Document 1) Korean Patent Publication No. 2023-0013608 (Invention Title: Display Device) Summary of the invention

[0010] Technical issues

[0011] The present invention aims to solve the above-mentioned problems of the prior art, and its purpose is to provide a micro LED pixel circuit capable of adjusting the slope of the scanning signal itself according to grayscale data.

[0012] However, the technical problems to be achieved by this embodiment are not limited to the above technical problems, and there may be other technical problems.

[0013] Technical Solution

[0014] As a technical means to solve the above-mentioned technical problems, a micro LED pixel circuit driven in a PWM manner according to the first aspect of the present invention includes: a scanning signal generating unit, which is used to output a scanning signal with a slope adjusted according to grayscale data; a constant current control unit, which is used to provide a constant current and control whether to stop providing the constant current according to the scanning signal output by the scanning signal generating unit; and a micro LED, which is used to be driven according to the constant current.

[0015] In addition, a micro LED display according to the second aspect of the present invention includes a plurality of micro LED pixel circuits.

[0016] Effects of the Invention

[0017] According to the configuration of the present invention, the slope of the scanning signal itself is adjusted according to the grayscale data. In particular, the slope of the scanning signal is set to the maximum for low grayscale data. Therefore, even for low grayscale data, the falling time of the constant current can be significantly shortened compared with the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a view used to illustrate the problems of existing micro LED pixel circuits.

[0019] Figure 2 A micro LED pixel circuit according to an embodiment of the present invention is shown.

[0020] Figure 3 FIG. 4 shows a scanning signal output by a micro LED pixel circuit according to an embodiment of the present invention.

[0021] Figure 4 A micro LED pixel circuit according to another embodiment of the present invention is shown.

[0022] Figures 5 to 9is a view showing the configuration and operation of a micro LED pixel circuit according to another embodiment of the present invention.

[0023] Fig.10 and Fig.11 is a diagram showing experimental results of a micro LED pixel circuit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0024] The embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings so that those skilled in the art can easily implement the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. In order to clearly describe the present invention in the accompanying drawings, parts that are not related to the description are omitted, and similar parts are given similar reference numerals throughout the specification.

[0025] Throughout the specification, when it is mentioned that a part is "connected" to another part, this includes not only the case where they are "directly connected" but also the case where they are "electrically connected" with another element interposed therebetween. In addition, when a part "includes" a certain element, this means that it may also include other elements rather than excluding other elements, unless explicitly stated otherwise.

[0026] Throughout the specification of the present application, when it is mentioned that a member is located “on” another member, this includes not only a case where the member is in contact with another member but also a case where another member exists between them.

[0027] Below, the preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings and the following content. However, the present invention is not limited to the embodiments described herein and can be implemented in other forms. Throughout the specification, the same reference numerals refer to the same elements.

[0028] Figure 1 is a view used to illustrate the problems of existing micro LED pixel circuits.

[0029] like Figure 1 As shown in (a), the circuit for generating a PWM signal receives the PWM data voltage DATA through the first switch element T1. PWM and stores it in the first capacitor C PWM Then, in the first capacitor C PWM The bottom of the pixel receives the scanning signal of the form shown in (b). At this time, regardless of the grayscale of the data, the scanning signal has the same voltage waveform slope, and the scanning signal is provided from outside the pixel circuit. PWM , the first capacitor C PWM The voltage stored in the first drive element DRT is different. PWMThe on / off timing of the light source changes, so that the light emission time can be adjusted.

[0030] In addition, the constant current control circuit receives the constant current source data voltage DATA through the second switch element T2. CCG and stores it in the second capacitor C CCG The second driving element DRT CCG Based on the second capacitor C CCG The voltage stored in the first drive element DRT is used to provide a constant current to the micro LED. PWM When turned on, the second capacitor C CCG The voltage stored in the second drive element DRT is discharged. CCG The current is no longer supplied and the micro LED stops emitting light.

[0031] As shown in (b), since the scanning signal has the same slope regardless of the gray scale, the falling time of the constant current becomes the same, which leads to a problem that it is difficult to accurately express low gray scales.

[0032] Figure 2 FIG. 2 shows a micro LED pixel circuit according to an embodiment of the present invention. Figure 3 FIG. 4 shows a scanning signal output by a micro LED pixel circuit according to an embodiment of the present invention.

[0033] The micro LED pixel circuit 100 is driven in a PWM manner, and includes a scan signal generating unit 110 , a constant current control unit 120 , and a micro LED 130 .

[0034] The scan signal generating unit 110 outputs a scan signal having a slope adjusted according to grayscale data.

[0035] The constant current control unit 120 controls whether to stop providing the constant current according to the scan signal output by the scan signal generation unit 110 .

[0036] The micro LED 130 is driven according to the constant current provided by the constant current control unit 120 .

[0037] The grayscale data includes a plurality of data representing low grayscale and high grayscale. The scanning signal generating unit 110 generates a scanning signal so that the slope of the voltage waveform of the scanning signal increases as the grayscale represented by the grayscale data decreases. Therefore, the constant current control unit 120 can control the falling time of the constant current so that it is shortened as the slope of the scanning signal increases.

[0038] Looking at the more detailed configuration, the scanning signal generating unit 110 includes a scanning signal generating unit 111 according to the grayscale data DATA SG The first capacitor C is charged with voltage SG Based on the first capacitor C SGThe voltage charged in the second capacitor C SWEEP The driving element DRT that transmits the current SG , and using the driving element DRT SG Based on the first capacitor C SG The voltage charged in the capacitor generates a current to charge the second capacitor C. SWEEP The second capacitor C SWEEP The voltage charged in the grayscale data DATA is output as a scan signal. In addition, the scan signal generating unit 110 may also include a SG Transmitted to the first capacitor C SG The switching element T1.

[0039] The scanning signal generating unit 110 operates so that the driving element DRT SG Based on the first capacitor C SG The voltage charged in the second capacitor C SWEEP In this way, the first capacitor C SG The voltage charged in the SG The second capacitor C SWEEP The magnitude of the charging current also changes accordingly, so the voltage waveform slope of the scanning signal generated thereby changes. SG Indicates that the grayscale decreases, the first capacitor C SG and the second capacitor C SWEEP The voltage charged in the MOSFET increases more rapidly, which subsequently shortens the falling time of the constant current. In this way, the emission time and the falling time of each gray scale can be adjusted based on the slope difference of the voltage waveform.

[0040] like Figure 3 As shown, in the present invention, the scanning signal is generated in a form in which the slope of the voltage waveform changes more sharply as the grayscale decreases. Figure 1 There is a clear technical difference in that the scanning signal in has the same slope regardless of the grayscale.

[0041] The constant current control unit 120 includes a first capacitor C charged with a constant current source data voltage. CCG According to the first capacitor C CCG The voltage charged in the micro LED 130 transmits the constant current to the driving element DRT CCG , and when the scan signal output by the scan signal generating unit 110 reaches a preset voltage or above, the first capacitor C CCG The voltage charged in the DRT is discharged to turn off the driving element CCG The switching element SWT. In this way, when the driving element DRT CCGWhen the scanning signal is turned off, the constant current supply to the micro LED 130 is stopped. At this time, the steeper the slope of the voltage waveform of the scanning signal, the faster the switching element SWT is turned on, and the corresponding driving element DRT is CCG The faster the shutdown is, the faster the light-emitting time of the micro LED 130 and the falling time of the constant current can be shortened.

[0042] Figure 4 A micro LED pixel circuit according to another embodiment of the present invention is shown.

[0043] Figure 4 is characterized by Figure 2 Compared with the embodiment of the present invention, an inverter 140 is added.

[0044] The inverter 140 is connected between the scan signal generating unit 110 and the constant current control unit 120 so that when the scan signal reaches a preset voltage or above, the constant current control unit 120 outputs a signal for stopping the constant current supply.

[0045] The inverter 140 may include a first driving element DRT of a first polarity connected in series between a first power voltage ELVDD1 and a ground voltage ELVSS. INV1 and a second driving element DRT of a second polarity INV2 At this time, the first driving element DRT INV1 and the second driving element DRT INV2 The gates of the inverter 140 are connected to the output terminal of the scan signal generating unit 110, and the output terminal of the inverter 140 is connected to the gate of the switch element SWT of the constant current control unit 120. Therefore, when the scan signal reaches a preset voltage, the inverter 140 can quickly turn on the switch element SWT to quickly turn off the drive element DRT. CCG Thus, due to the addition of the inverter 140, the switching can be performed more quickly and the falling time of the constant current can be further shortened.

[0046] Figures 5 to 9 is a view showing the configuration and operation of a micro LED pixel circuit according to another embodiment of the present invention.

[0047] Basically, although the common Figure 4 In order to apply this PWM drive method, it is necessary to understand the electrical characteristics (V TH ) deviations are compensated, and this configuration is also used in existing micro-LED circuits.

[0048] The illustrated micro LED pixel circuit 200 includes a scan signal generating unit 210 , a constant current control unit 220 , a micro LED 230 , and an inverter 240 .

[0049] The scan signal generating unit 210 includes a first capacitor C1 charged with a voltage according to grayscale data DATA_SG, a driving element T_SG generating a current for charging a second capacitor C2 based on the voltage charged in the first capacitor C1, and a second capacitor C2 charged by a current generated by the driving element T_SG based on the voltage charged in the first capacitor C1. The voltage charged in the second capacitor C2 is output as a scan signal.

[0050] In addition, the scan signal generating unit 110 may further include a switch element T1 turned on according to the first scan signal Scan1b[n-1] and connected in parallel to both ends of the first capacitor C1, a switch element T2 turned on according to the first scan signal Scan1b[n-1] and connected in parallel to both ends of the second capacitor C2, a switch element T3 turned on according to the second scan signal Scan1b[n] and connected between a node A which is one side terminal of the first capacitor C1 and the other side terminal of the driving element T_SG, and a switch element T4 turned on according to the second scan signal Scan1b[n] and connected between a node A which is one side terminal of the first capacitor C1 and the other side terminal of the driving element T_SG, and a switch element T5 which transmits the grayscale data DATA_SG to the first capacitor C1 according to the third scan signal Scan1[n]. SG A switching element T4 is connected to the first power supply voltage ELVDD1, a switching element T5 is connected to the first power supply voltage ELVDD1, and a switching element T6 is connected to the first power supply voltage ELVDD1, and a switching element T7 is connected to the first power supply voltage ELVDD1, and a switching element T8 is connected to the first power supply voltage ELVDD1, and a switching element T9 is connected to the first power supply voltage ELVDD1, and a switching element T10 is connected to the first power supply voltage ELVDD1, and a switching element T2 is connected to the first power supply voltage ELVDD1, and a switching element T11 is connected to the first power supply voltage ELVDD1, and a switching element T12 is connected to the first power supply voltage ELVDD1, and a switching element T13 is connected to the first power supply voltage ELVDD1, and a switching element T14 is connected to the first power supply voltage ELVDD1, and a switching element T15 is connected to the first power supply voltage ELVDD1, and a switching element T16 is connected to the first power supply voltage ELVDD1, and a switching element T17 is connected to the first power supply voltage ELVDD1, and a switching element T18 is connected to the first power supply voltage ELVDD1, and a switching element T19 is connected to the first power supply voltage ELVDD1, and a switching element T11 is connected to the first power supply voltage ELVDD1, and a switching element T12 is connected to the first power supply voltage ELVDD1, and a switching element T13 is connected to the first power supply voltage ELVDD1, and a switching element T14 is connected to the first power supply voltage ELVDD1, and a switching element T15 is connected to the first power supply voltage ELVDD1, and a switching element T16 is connected to the first power supply voltage ELVDD1, and a switching element T18 is connected to the first power supply voltage ELVDD1, and a switching element T19 is connected to the first power supply voltage ELVDD1, and a switching element T11 is connected to the first power supply voltage ELVDD1,

[0051] Next, the constant current control unit 220 includes a first capacitor C4 charged by a constant current source data voltage, a driving element T_CCG that transmits a constant current to the micro LED 130 according to the voltage charged in the first capacitor C4, and a switch element T8 that discharges the voltage charged in the first capacitor C4 to turn off the driving element T_CCG when the scan signal output by the scan signal generating unit 210 reaches a preset voltage or above. In this way, when the driving element T_CCG is turned off, the constant current supply to the micro LED 230 is stopped. At this time, the steeper the slope of the voltage waveform of the scan signal, the faster the switch element T8 is turned on, and accordingly, the faster the driving element T_CCG is turned off, so that the light emission time of the micro LED 230 and the falling time of the constant current can be quickly shortened.

[0052] In addition, the constant current control unit 220 may also include a switching element T9 that is turned on according to the first scan signal Scan1b[n-1] and connected in parallel to both ends of the first capacitor C4, a switching element T10 that is turned on according to the second scan signal Scan1b[n] and connected between the node E which is one side terminal of the first capacitor C4 and the other side terminal of the driving element T_CCG, a switching element T11 that transmits the constant current source data voltage DATA_CCG to the first capacitor C4 according to the third scan signal Scan1[n], a switching element T12 that is turned on according to the fourth scan signal Scan2[n] and has one side terminal connected to the other side terminal of the driving element T_CCG and the other side terminal connected to the second power supply voltage ELVDD2, and a switching element T13 that is turned on according to the fourth scan signal Scan2[n] and has one side terminal connected to one side terminal of the driving element T_CCG and one side terminal of the switching element T11 and the other side terminal connected to the micro LED.

[0053] In addition, the inverter 240 may include a first switching element T_INV1 of a first polarity and a second switching element T_INV2 of a second polarity connected in series between the first power supply voltage ELVDD1 and the ground voltage ELVSS. In addition, the inverter 240 may further include a switching element T7 that is turned on according to the first scan signal Scan1b[n-1] and connected between the input node C and the output node D of the inverter 240.

[0054] The operation of the pixel circuit can be divided into Figure 6 The initialization and compensation operation of the inverter 240 shown, Figure 7 The compensation operation of the scanning signal generating unit 210 and the constant current control unit 220 shown in FIG. Figure 8 and Fig. 9 The light emitting operation and other steps are shown.

[0055] (1) Initialization and compensation operation of inverter 240

[0056] First, see Figure 6, the first scan signal Scan1b[n-1] is set to a high level, and the switch elements T1, T2, T7 and T9 are turned on respectively. Therefore, the voltages of nodes A and E are reset to the ground voltage ELVSS, and the voltage of node B is reset to the second power supply voltage ELVDD2, which is the high level of the reference signal REF[n]. In addition, since node C, which is the input node of the inverter 240, and node D, which is the output node, are connected through the switch element T7, the inverter switch threshold Vm defined as VC=VD is sensed at the two nodes. At this time, the third scan signal Scan1[n] and the fourth scan signal Scan2[n] are set to a high level, so the PMOS switch elements T4, T5, T6, T11, T12, and T13 to which the corresponding signals are applied are all turned off. In addition, since the second scan signal Scan1b[n] is set to a low level, the NMOS switch elements T3 and T10 to which the corresponding signals are applied are also turned off.

[0057] (2) Compensation Operation of the Scan Signal Generating Unit 210 and the Constant Current Control Unit 220

[0058] like Figure 7 As shown, the first scan signal Scan1b[n-1] and the third scan signal Scan1[n] are set to a low level, and the second scan signal Scan1b[n] and the fourth scan signal Scan2[n] are set to a high level. Thus, the switch elements T1, T2, T7, and T9 are turned off, and the switch elements T3, T4, T10, and T11 are turned on.

[0059] As current flows through the first capacitor C1 of the scan signal generating unit 210 and the first capacitor C4 of the constant current controlling unit 220, the voltages of the nodes A and E increase to V DATA_SG -V TH_SG and V DATA_CCG -V TH_CCG Here, V TH_SG and V TH_CCG They respectively represent the threshold voltages of the driving element T_SG and the driving element T_CCG.

[0060] At the same time, the reference signal REF[n] is set to the low-level ground voltage ELVSS, so the voltages of the nodes B and C are reduced to ELVSS and Vm+ELVSS-ELVDD2, respectively. Then, the first driving element T_INV1 of the inverter 240 is turned on, and the voltage at the node D becomes the first power supply voltage ELVDD1. As a result, the switching element T8 remains off throughout the step.

[0061] (3) Lighting operation

[0062] like Figure 8As shown, the first scan signal Scan1b[n-1], the second scan signal Scan1b[n] and the fourth scan signal Scan2[n] are set to a low level, and the third scan signal Scan1[n] is set to a high level. As a result, the switch elements T3, T4, T10, and T11 are turned off, and the switch elements T5, T6, T12, and T13 are turned on.

[0063] As the second capacitor C2 is charged by the current flowing through the driving element T_SG, the voltage of the node B increases linearly with time and acts as a scan signal. At the same time, due to charge conservation, the voltage of the node C also increases together with the voltage of the node B.

[0064] In addition, if Fig. 9 As shown, when the voltage of node B reaches the second power supply voltage ELVDD2 and the voltage of node C exceeds Vm, the first drive element T_INV1 of inverter 240 is turned off and the second drive element T_INV2 is turned on. Then, the voltage at node D becomes the ground voltage ELVSS and the second switch element T8 is turned on. After that, the voltage of node E becomes the second power supply voltage ELVDD2.

[0065] Therefore, the source-gate voltage (VSG) of the driving element T_CCG becomes smaller than the threshold voltage and is turned off, so that the driving element T_CCG cannot provide current to the micro LED 230 .

[0066] Fig.10 and Fig.11 is a diagram showing experimental results of a micro LED pixel circuit according to an embodiment of the present invention.

[0067] exist Fig.10 In the figure, (a) shows the voltage level of the scanning signal for high grayscale data, and (b) shows the voltage level of the scanning signal for low grayscale data. As proposed by the present invention, it can be confirmed that the slope of the scanning signal changes according to the grayscale data, especially for low grayscale data, the slope of the scanning signal becomes larger.

[0068] In addition, (c) shows a constant current for high grayscale data, and (d) shows a constant current for low grayscale data. As the slope of the scan signal becomes larger for low grayscale data, the falling time can be further shortened.

[0069] This is in Fig.11 This can also be confirmed in the graph of (a) showing the grayscale data V DATA_SG grayscale levels, and in (b) it can be confirmed that the fall time of each grayscale is different.

[0070] By including a plurality of micro LED pixel circuits configured as above and arranging them in an array form, a micro LED display can be realized.

[0071] The above description of the present invention is intended to be illustrative, and those skilled in the art will appreciate that the present invention can be easily modified into other specific forms without changing the technical ideas or essential features of the present invention. Therefore, the above embodiments should be understood as exemplary and non-restrictive in all aspects. For example, each element described as a single form can be implemented in a distributed manner, and similarly, the elements described as distributed can also be implemented in a combined form.

[0072] The scope of the present invention is presented by the appended claims rather than the detailed description above, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention.

[0073] [Description of Reference Numerals]

[0074] 100: Micro LED pixel circuit

[0075] 110: Scanning signal generator

[0076] 120: Constant current control unit

[0077] 130: Micro LED

[0078] 140: Inverter

[0079] 200: Micro LED pixel circuit

[0080] 210: Scanning signal generator

[0081] 220: Constant current control unit

[0082] 230: Micro LED

[0083] 240: Inverter.

Claims

1. A micro LED pixel circuit, which is driven in a PWM manner, the micro LED pixel circuit comprising: a scanning signal generating unit, configured to output a scanning signal having a slope adjusted according to grayscale data; a constant current control unit, which is used to provide a constant current and control whether to stop providing the constant current according to the scan signal output by the scan signal generating unit; and A micro LED is used to be driven according to the constant current.

2. The micro-LED pixel circuit according to claim 1, wherein the grayscale data includes multiple data representing low grayscale and high grayscale, the scanning signal generating unit generates the scanning signal so that the slope of the voltage waveform of the scanning signal increases as the grayscale represented by the grayscale data decreases, and the constant current control unit controls the falling time of the constant current so that it shortens as the slope of the scanning signal increases.

3. The micro LED pixel circuit according to claim 1 further includes an inverter connected between the scan signal generating unit and the constant current control unit, wherein the inverter causes the constant current control unit to output a signal for stopping the constant current supply when the scan signal reaches a preset voltage or above.

4. The micro LED pixel circuit according to claim 1, wherein the scanning signal generating unit comprises: A first capacitor, which performs voltage charging according to the grayscale data; a driving element that generates a current for charging a following second capacitor based on the voltage charged in the first capacitor; as well as a second capacitor that is charged by a current generated by the driving element based on the voltage charged in the first capacitor, The voltage charged in the second capacitor is output as the scanning signal, and the voltages charged in the first capacitor and the second capacitor increase as the grayscale represented by the grayscale data decreases.

5. The micro LED pixel circuit according to claim 4, wherein the constant current control unit comprises: A first capacitor, which is charged by a constant current source data voltage; a driving element that transmits a constant current to the micro LED according to the voltage charged in the first capacitor; as well as a switch element, which discharges the voltage charged in the first capacitor to turn off the driving element when the scanning signal reaches a preset voltage or above, The constant current stops being provided when the driving element is turned off.

6. The micro LED pixel circuit according to claim 5 further includes an inverter connected between the scan signal generating unit and the constant current control unit, wherein the inverter turns on the switching element of the constant current control unit to turn off the driving element of the constant current control unit when the scan signal reaches a preset voltage or above.

7. A micro LED display, comprising a plurality of micro LED pixel circuits according to any one of claims 1 to 6.

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