Patent Grant
10762836
The present disclosure relates generally to techniques for driving a display and, more particularly, to techniques for driving of the electronic display.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Electronic display uniformity is a valuable factor to ensure images are displayed on a display properly. Uniformity may be decreased by fluctuations based on temperature, threshold voltage variations, voltage drop due to electrical resistance in the display (IR drop), or supply variation. Specifically, IR drop in the panel can impact the overdrive voltage of the current source inside and cause brightness errors and display artifacts. Severity of the artifacts is display pattern dependent, and the problem may worsen as more pixels serially share a current or voltage supply. In other words, more pixels sharing a current or voltage supply may increase the IR drop to cause non-uniformity of the display and/or artifacts, which degrade display quality.
A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below.
Row drivers and column drivers may be used to provide driving signals for micropixels to microdrivers that then distribute the driving signals to the micropixels connected to the microdrivers. Micropixels may include any display pixels that are driven by a microdriver. For example, a pixel may be a unit of a display that includes a single color (e.g., red, green, white, or blue) or a pixel may be a unit of sub-pixels of single individual colors with the pixel capable of displaying any color that the display is capable of achieving due to combinations of the individual colors.
The row and column drivers, in combination with the microdrivers, enable the display to accurately pinpoint individual micropixels and/or sub-pixels or groups of pixels and/or sub-pixels that are to be driven. However, as the communications extend further from the drivers, voltage may drop due to electrical resistance in the display. In this disclosure, this drop in voltage is referred to as IR drop. IR drop may be compensated for by shipping current to the micropixels by generating a current in the microdrivers, in the row drivers, in the column drivers, a timing controller, or other suitable circuitry prior to shipment to the micropixels.
Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which:
One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
As discussed above, IR drop is a voltage drop due to an internal resistance of an electronic display that may cause display artifacts on the electronic display. The IR drop may refer to an analog IR drop or a digital IR drop. Analog IR drop is at a low frequency due to the current through the passing through the micro light emitting diodes. Digital IR drop refers to an IR drop caused by digital switching (e.g., emission scanning).
Suitable electronic devices that may include a micro-LED (μ-LED) display and corresponding circuitry of this disclosure are discussed below with reference to
The CPU/GPU 12 of the electronic device 10 may perform various data processing operations, including generating and/or processing image data for display on the display 18, in combination with the storage device(s) 14. For example, instructions that can be executed by the CPU/GPU 12 may be stored on the storage device(s) 14. The storage device(s) 14 thus may represent any suitable tangible, computer-readable media. The storage device(s) 14 may be volatile and/or non-volatile. By way of example, the storage device(s) 14 may include random-access memory, read-only memory, flash memory, a hard drive, and so forth.
The electronic device 10 may use the communication interface(s) 16 to communicate with various other electronic devices or components. The communication interface(s) 16 may include input/output (I/O) interfaces and/or network interfaces. Such network interfaces may include those for a personal area network (PAN) such as Bluetooth, a local area network (LAN) or wireless local area network (WLAN) such as Wi-Fi, and/or for a wide area network (WAN) such as a long-term evolution (LTE) cellular network.
Using pixels containing an arrangement of μ-LEDs, the display 18 may display images generated by the CPU/GPU 12. The display 18 may include touchscreen functionality to allow users to interact with a user interface appearing on the display 18. Input structures 20 may also allow a user to interact with the electronic device 10. For instance, the input structures 20 may represent hardware buttons. The energy supply 22 may include any suitable source of energy for the electronic device. This may include a battery within the electronic device 10 and/or a power conversion device to accept alternating current (AC) power from a power outlet.
As may be appreciated, the electronic device 10 may take a number of different forms. As shown in
The electronic device 10 may also take the form of a slate 40. Depending on the size of the slate 40, the slate 40 may serve as a handheld device such as a mobile phone. The slate 40 includes an enclosure 42 through which several input structures 20 may protrude. The enclosure 42 also holds the display 18. The input structures 20 may allow a user to interact with a GUI of the slate 40. For example, the input structures 20 may enable a user to make a telephone call. A speaker 44 may output a received audio signal and a microphone 46 may capture the voice of the user. The slate 40 may also include a communication interface 16 to allow the slate 40 to connect via a wired connection to another electronic device.
A notebook computer 50 represents another form that the electronic device 10 may take. It should be appreciated that the electronic device 10 may also take the form of any other computer, including a desktop computer. The notebook computer 50 shown in
A block diagram of the architecture of the μ-LED display 18 appears in
As noted above, the video TCON 66 may generate the data clock signal (DATA_CLK). An emission timing controller (TCON) 72 may generate an emission clock signal (EM_CLK). Collectively, these may be referred to as Row Scan Control signals, as illustrated in
In particular, the display panel 60 includes column drivers (CDs) 74, row drivers (RDs) 76, and micro-drivers (μDs) 78. Each μD 78 drives a number of pixels 80 having μ-LEDs as subpixels 82. Each pixel 80 includes at least one red μ-LED, at least one green μ-LED, and at least one blue μ-LED to represent the image data 64 in RGB format. Although the μDs 78 of
A power supply 84 may provide a reference voltage (Vref) 86 to drive the μ-LEDs, a digital power signal 88, and an analog power signal 90. In some cases, the power supply 84 may provide more than one reference voltage (Vref) 86 signal. Namely, subpixels 82 of different colors may be driven using different reference voltages. As such, the power supply 84 may provide more than one reference voltage (Vref) 86. Additionally or alternatively, other circuitry on the display panel 60 may step the reference voltage (Vref) 86 up or down to obtain different reference voltages to drive different colors of μ-LED.
To allow the μDs 78 to drive the μ-LED subpixels 82 of the pixels 80, the column drivers (CDs) 74 and the row drivers (RDs) 76 may operate in concert. Each column driver (CD) 74 may drive the respective image data 70 signal for that column in a digital form. Meanwhile, each RD 76 may provide the data clock signal (DATA_CLK) and the emission clock signal (EM_CLK) at an appropriate to activate the row of μDs 78 driven by the RD 76. A row of μDs 78 may be activated when the RD 76 that controls that row sends the data clock signal (DATA_CLK). This may cause the now-activated μDs 78 of that row to receive and store the digital image data 70 signal that is driven by the column drivers (CDs) 74. The μDs 78 of that row then may drive the pixels 80 based on the stored digital image data 70 signal based on the emission clock signal (EM_CLK).
A block diagram shown in
When the pixel data buffer(s) 100 has received and stored the image data 70, the RD 76 may provide the emission clock signal (EM_CLK). A counter 102 may receive the emission clock signal (EM_CLK) as an input. The pixel data buffer(s) 100 may output enough of the stored image data 70 to output a digital data signal 104 represent a desired gray level for a particular subpixel 82 that is to be driven by the μD 78. The counter 102 may also output a digital counter signal 106 indicative of the number of edges (only rising, only falling, or both rising and falling edges) of the emission clock signal (EM_CLK) 98. The signals 104 and 106 may enter a comparator 108 that outputs an emission control signal 110 in an “on” state when the signal 106 does not exceed the signal 104, and an “off” state otherwise. The emission control signal 110 may be routed to driving circuitry (not shown) for the subpixel 82 being driven, which may cause light emission 112 from the selected subpixel 82 to be on or off. The longer the selected subpixel 82 is driven “on” by the emission control signal 110, the greater the amount of light that will be perceived by the human eye as originating from the subpixel 82.
A timing diagram 120, shown in
It should be noted that the steps between gray levels are shown by the steps between emission clock signal (EM_CLK) edges. That is, based on the way humans perceive light, to notice the difference between lower gray levels, the difference between the amount of light emitted between two lower gray levels may be relatively small. To notice the difference between higher gray levels, however, the difference between the amounts of light emitted between two higher gray levels may be comparatively much greater. The emission clock signal (EM_CLK) therefore may use relatively short time intervals between clock edges at first. To account for the increase in the difference between light emitted as gray levels increase, the differences between edges (e.g., periods) of the emission clock signal (EM_CLK) may gradually lengthen. The particular pattern of the emission clock signal (EM_CLK), as generated by the emission TCON 72, may have increasingly longer differences between edges (e.g., periods) so as to provide a gamma encoding of the gray level of the subpixel 82 being driven.
Displays may use PMOS or NMOS LED drivers that do not use huge level shifters. In some embodiments, these drivers are driven and/or drive using specific voltage levels (e.g., voltage driven). However, some LED drivers (e.g., PMOS drivers) are sensitive to threshold voltage variation of one or more transistors in the driver varying a voltage used to drive the transistor to a different state. Temperature and oxide thickness each have an effect on the threshold voltage some transistor types (e.g., CMOS device).
Specifically, with temperature, the surface potential has a direct relationship with the temperature. While the threshold voltage may not have a direct relationship to temperature and some other effects, the threshold voltage is not independent of these effects. For example, a change of 30° C. results in significant variation from the 500 mV design parameter (e.g., VTH) commonly used for a 90 nm technology node.
Impurity concentrations may also effect different threshold voltages across different portions of a display. For example, random dopant fluctuation (RDF) is a form of process variation resulting from variation in the implanted impurity concentration. In MOSFET transistors, RDF in the channel region can alter the transistor's properties, especially threshold voltage. As the number of dopants decreases, such as in modem dopings, the effects of RDF can be greater.
Pixels may also vary based on voltage fluctuations of supplied power (e.g., Vdd). These voltages may vary due to IR drop as well as other voltage fluctuation effects. For example,
The micropixels (e.g., sub-pixels) may be driven using a voltage mode or a current mode. For example, a voltage mode may include row drivers providing a reference voltage to microdrivers for each pixel with the microdrivers forwarding the reference voltage to the micropixels. Additionally or alternatively, the current mode may include row drivers providing a reference current (e.g., at a constant voltage) to the microdrivers to the micropixels, a timing controller providing a reference current to the row drivers, row drivers receiving the reference voltage from the timing controller and locally generating the reference current, column drivers providing the reference current to the microdrivers to the micropixels, and/or other suitable pathways for sending a reference current to the micropixels.
Voltage Mode
Various components of the electronic device 10 may be used to control the current signal supplied to drive LED devices 208 of the pt-LED display 18. The LED devices 208 may include micropixels/subpixels/pixels of the display 18. For example, as will be further appreciated, the μDs 78 may include a p-type metal-oxide-semiconductor (PMOS) device, an n-type metal-oxide-semiconductor (NMOS) device, or some combination of PMOS and NMOS devices.
In certain embodiments, the number of LED devices 208A may each be coupled to a high voltage potential rail (e.g., “VDD”) and a low voltage potential rail or ground (e.g., “VSS” or “GND”). For example, the high voltage potential rail (e.g., “VDD”) may be set to a voltage of 1.2V, 1.5V, 1.8V, 2.5V, 3.3V, 5V, or other similar voltage that may be used to supply power to the subpixels 82 for operation. Similarly, the low voltage potential rail or ground (e.g., “VSS” or “GND”) 212A may be generally set to a ground voltage (e.g., 0V or approximately 0V).
In some embodiments, the μDs 78 may each include a PMOS driver used to drive the Subpixels 82. For example, PMOS drivers may be used as part of the μDs 78 in order to conserve physical area of the μ-LED display 18 by avoiding level shifters that may be otherwise involved. However, in some embodiments, utilizing PMOS drivers as part of the μDs 78 may lead to image artifacts (e.g., flicker) becoming present on the μ-LED display 18, as the PMOS drivers may be sensitive to variations of the high voltage potential rail (e.g., “VDD”) 210A. The variations of the high voltage potential rail (e.g., “VDD”) 210A may be caused by IR drop (e.g., voltage drops across the resistance R of the power supply 198A between supply pins and one or more components drawing a current I). As noted above,
Indeed, the VDD variations may vary depending on the incoming image data and the image pattern, as the luminance of the μ-LED display 18 and the characteristics of the subpixels 82 may also be variable. Furthermore, variations in the threshold voltage (e.g., “VTH”) of the subpixels 82 may also adversely impact the drive currents (e.g., “ILED”) of the subpixels 82. As may be further appreciated, the VDD and VTH variations may be exacerbated for larger area μ-LED displays 18. Thus, as will be further appreciated with respect to
Turning now to
For example, in an initial phase (e.g., “Phase 1”), the voltage VB may be low (e.g., approximately “GND” or 0V). Thus, a PMOS transistor 216A (e.g., “M5”) coupled (e.g., in series) between a PMOS transistor 218A (e.g., “M5A”) and the high voltage potential rail (e.g., “VDD”) 210A coupled directly to the high voltage potential rail (e.g., “VDD”) 210A may be “ON” (e.g., activated). The PMOS transistor 218A may also be “ON,” as the voltage EM may also be low (e.g., approximately “GND” or 0V) in the initial phase (e.g., “Phase 1”). Accordingly, a drive current may be allowed to flow from the high voltage potential rail (e.g., “VDD”) 210A to the LED device 208A. In some embodiments, the PMOS transistor 216A may be susceptible to VDD voltage variations, while the PMOS transistor 218A may be susceptible to VTH voltage variations.
In certain embodiments, in a reset phase 229 (e.g., “Phase 2”), the voltage EM may be low (e.g., approximately “GND” or 0V), while the voltages VA and VB may be expressed as:
VA=VRef (Equation 1);
VB=VDD_CL−VTH (Equation 2).
Specifically, VRef may be the reference supply voltage for the LED device 208A that may be controlled by the PMOS 228A. VDD_CL may be an additional high voltage potential rail (e.g., “VDD_CL”) 217A (e.g., independent of the high voltage potential rail (“VDD”) 210A). Thus, in the reset phase (e.g., “Phase 2”), when VA=VRef and VB=VDD_CL VTH, the following condition may exist:
VB=VDD_CL−VTH, for VB<VTH_LED (Equation 3).
In this case, the LED device 208A may not turn “ON.” Furthermore, in the reset phase (e.g., “Phase 2”), the voltage VC (e.g., voltage across a compensation capacitance 230A) may be expressed as:
VC=VRef−VDD_CL−VTH (Equation 4).
As may be appreciated from the foregoing equation, the voltage VC may be a voltage across a compensation capacitance 230A that may, in some embodiments, be the difference between the reference voltage VRef and the voltage VB.
In certain embodiments, in another reset phase 231 (e.g., “Phase 3”), the voltages VA and VB may be then expressed as:
VA=VDD (Equation 5);
VB=VA−VC (Equation 6).
Expanding equations (5) and (6) based on equations (1), (2), and (4), the voltage VB may be then expressed as:
VB=VDD−VRef+VDD_CL−VTH (Equation 7).
Thus, when VB<VDD−VTH and VTH<VDD_CL<VRef, the PMOS transistor 216A (e.g., “M1”), the PMOS transistor 216A (e.g., “M5”), and the PMOS transistor 228A (e.g., “M6”) may each be “ON” (e.g., conductive or in the saturation mode). Indeed, further, when VRef<VTH<VTH Diode, the LED device 208A drive current ILED may be expressed as:
ILED=K(VGS−VTH)2=K(VDD−VB−VTH)2 (Equation 8).
Expanding equation 8 based on equation 7, the LED device 208A drive current ILED may be then expressed as:
ILED=K(VDD−(VDD−VRef+VDD_CL−VTH)−VTH)2 (Equation 9).
Lastly, simplifying equation 9, the LED device 208A drive current ILED may be expressed as:
ILED=K(VRef+VDD_CL)2 (Equation 10).
Accordingly, equation 10 illustrates that LED device 208A drive current ILED may be independent of the high voltage potential rail (e.g., VDD) and the threshold voltage (e.g., VTH), and may thus compensate for VDD and VTH variations that may otherwise adversely affect drive current ILED (e.g., due to IR drop). Indeed, instead of being a function of VDD and VTH (e.g., as expressed by equation (8)) and, by extension, being susceptible to VDD and VTH variations (e.g., due to IR drop), the LED device 208A drive current ILED may be function of the μDs 78 reference voltage VRef and the compensation voltage potential rail VDD_CL. In this way, any possible occurrence of image artifacts becoming apparent on the μ-LED display 18 may be reduced or substantially eliminated.
As a further example of the presently disclosed embodiments,
Turning now to
Current Mode
In the current mode, pixel data is displayed relative to a reference current, IREF. The reference current causes the pixel performance to be independent of VDD and ground variations thereby reducing the IR drop effect on pixel performance. In some embodiments, a timing controller 72 passes a reference current to row drivers that is then passed further down. Alternatively, the IREF may be generated by a respective row driver 76 and passed to the μD 78. In some embodiments, Vref may be passed all the way to the microdriver and locally converted to a current to be passed to the micropixels.
Once the current has been generated, the row driver ships the current to a microdriver (block 247). The microdrivers then drive micropixels using a selective current mirror or other suitable circuitry (block 248). Moreover, although the foregoing discussion relates to row driver current generation, some embodiments may include column driver current generation and shipping the current to the microdrivers using the column drivers.
For example,
By shipping the current horizontally, the TCON 322, 324 can be used with adding pins for each row. Furthermore, each row provides a current for a section of a column of microdrivers 330. For example, the number of rows driven in a section may be determined by number of columns in the display. For example, the illustrated embodiment includes 9 columns in a segment and thus 9 rows in a section. However, these numbers may vary by the number of microdrivers of the display. For example, the display may have 10, 20, 30, 39, 50, or more columns and rows of microdrivers in a segment. Within a segment, each row driver provides current for the portion of the column in the segment. For example, in the illustrated embodiment, the first column of microdrivers 350 in a segment is driven by the first row driver 308 and so on. Alternatively, the pattern for each row driver shipping current to each column in the segment.
By segmenting the columns into segments, the panel is segmented with reduced parasitic capacitance for each line. Thus, for each segment, the current is time-multiplexed for the number of micropixels in a column/line. Thus, if the segment has 39 microdrivers per column/line in a segment, each line is loaded with 1/20th of the line driving time. However, each segment is independent from other segments, but thus, even with time-multiplexing, timing requirements may be relaxed from a single segment display. Also, these currents may be provided through column lines 352 that are used to drive data using column drivers 354.
Although the embodiment shown in
Since the foregoing discussion contemplates row drivers 308 that receive a reference voltage and generate a reference current, the row drivers 308 include a current generator.
The current generator 400 has three phases: a reset phase, a sample VTH phase, and a compensation phase.
Although the foregoing discussion discusses that two row drivers may exist per line, some embodiments include row drivers that may drive a whole row while dividing the row horizontally into 1, 2, 3, or more segments.
As noted above, the current generator 400 may be omitted from the row drivers 308 if the TCON 322 were to provide current sources to the row drivers 308. The trade off for this scheme in simplicity in circuitry of the row drivers 308 is that the current sources would have to be shared with segments in a time-multiplexed fashion. In other words, the segments are no longer independent and requires more stringent timing requirements than the local current generation in the row drivers 308.
Microdrivers
As discussed above, the microdrivers receive or generate a reference current for transport to the micropixels that the microdrivers are responsible for driving.
The voltage drive scheme 520 may also include VTH compensation circuitry 530 that compensations for possible fluctuations of a VTH of a control transistor 531 for the microdriver causing the VTH compensation circuitry 530 to supply Vref plus the VTH for the control transistor 531 to the gate of the control transistor M4. The VTH compensation circuitry 530 may be similar to the foregoing discussed VTH compensation circuits.
The voltage drive scheme 520 also includes a connection to a Vbottom 532 that mitigates for IR drop by reducing current further down the display.
Current-Driven Microdrivers
To drive the micropixels, the bias for the micropixels is changed once every data upload (e.g., every 16 microseconds) with alternation between red and blue-green (or one or more other) bias nodes between consecutive uploads or time-multiplexed within a single data upload. Moreover, a microdriver may drive micropixels in rows above and/or below a location of the microdriver or in columns left and/or right of the microdriver. In other words, the microdriver may drive more than a single row and column of row drivers and the selectable current mirror may be used for all of the connected micropixels. Thus, circuitry may be reused for multiple micropixels increasing area efficiency over dedicated microdrivers restricted to a single row, column, or pixel.
IR Drop Techniques
The following discussion refers to some additional techniques that may be employed to reduce IR drop and the mura artifacts that result from the IR drop. Some of these additional techniques may be adopted along with some of the foregoing techniques into a single device in any combination.
Current is determined by the display pattern/switching scheme. Once the current (I) is decided, resistance (R) can still be reduced to reduce IR drop. To reduce R, an electronic display may use wider power buses and more vias wherever possible. Given same routing area, different power distribution network can be used. For example, a power stripes formation may be replaced with a power grid as illustrated in
Resistance reduction may also be limited by the routing area available and/or a complexity of the power grid that is feasible for use in the display. When choosing the power distribution network, equalizing the resistance between each pixel to the power supply input point decreases IR drop by ensuring that the resistance between pixels are substantially the same as illustrated in
Additionally or alternatively, given that VDD is reduced by the IR drop, the Vref may be manipulated to compensate for the IR drop. For example, as illustrated in
To avoid the LUT, a diode connected device 706 located at the panel bottom 702 to find out the Vref_bot to be used. Moreover, in some embodiments, M1 may be located inside a row driver. The diode connected device 706 may share its pin with other functions, and be enabled by a configuration bit.
The bias current M1 may be adjusted with different brightness, to have an adaptive control that works for different brightnesses and display patterns.
The device 706 resolves first order IR drop error. To compensation for more pattern-dependent parts of the IR drop, more buffers using the same configuration may be used. A similar device may be in the X dimension as well (in column driver) to compensate for X gradients.
Microdriver Local Sampling
In addition to or alternative to the foregoing implementations for reducing IR drop and threshold voltage and B compensation. Local sampling may be used at each current driver in each microdriver to reduce or eliminate IR drop, eliminate threshold voltage and B mismatch from PMOS output drivers, and eliminate temperature dependence of threshold voltage and B while enabling usage of low-voltage transistor devices as current sources and MOS capacitors. The local sampling may also eliminate cross talk between sub-pixels.
The first slice 800 includes multiple current drivers 804 that each drives a micropixel 82 in the first row. The number of current drivers 804 corresponds to the number of micropixels 82 in the first row. The second slice 802 includes multiple current drivers 806 that each drives a micropixel 82 in the second row using to a specific current. In some embodiments, the number of current drivers 806 and micropixels 82 in a slice may be 3, 6, 9, 12, 15, or more. In some embodiments, each slice includes a multiple of a number (e.g., 3) of colors of micropixels 82 included in a pixel 80. In some embodiments, some colors may be omitted from some pixels 80 but included in other pixels 80 causing a slice to include any number of micropixels 82 and/or current drivers 806. As is discussed below, each current driver 804 drives a respective LED of a micropixel 82 to a predetermined level in a manner that is robustly resistant to temperature variation effects on transistor characteristics, such as threshold voltage and/or B. Specifically, as discussed below, current calibration circuitry 810 generates a calibration current that is provided to a calibration portion of each current driver 804, 806 for use in ensuring that a predetermined current is used to power an LED regardless of temperature variations and resultant transistor characteristics of a transistor that controls access to the LED.
where Ical is the calibration current 826 and R is the resistance of the resistor 836.
The calibration current 826 is used to sequentially charge current driver 804. The calibration mode for each current driver 804 may be set using a calibration signal. For example, the current driver 804A that corresponds to an LED 838 is driven to a calibration mode by a calibration signal 840A, and the current driver 804B that corresponds to an LED 839. The calibration signals 840 cause respective transistors 842 and 844 to enable current to travel through the respective transistors 842 and 844 to charge a respective calibration capacitor 828. Once the calibration capacitor 828 is charged for a current driver 804, the current driver 804 may be taken out of calibration mode by deasserting the respective calibration signal 840. During an emission mode, the calibration capacitor 828 provides a voltage the causes a specific current to pass through a transistor 846 during emission. Furthermore, by using the capacitor to supply the gate voltage to the transistor 846, the voltage may be supplied when the capacitor is disconnected from the calibration current 826 when the transistors 842 and 844 shut off connection due to deassertion of the calibration signal for the current driver 804. An emission transistor 848 controls whether a respective current driver is emitting in an emission mode based on a pulse signal 849. The pulse signal 849 may be a pulse width modulated (PWM) signal that controls a level of luminance of the respective LED (e.g., LED 838). Each current driver 804 may also include an emission transistor 850 that controls whether the current driver 804 is in an emission mode. Essentially, the transistor 850 may have a first mode (e.g., transmissive) during an emission mode and a and a second mode (e.g., non-transmissive) during the calibration mode. In some embodiments, the PWM driving scheme to modulate luminance of the respective LED in addition to or in place of the PWM driving scheme applied to the transistor 848.
In some embodiments, the supplied voltages (e.g., Vbottom 822, Vref 824, a VDD, etc.) may be supplied using operational amplifiers.
Using the reference voltages, the calibration circuitry 810 generates a calibration current 826 (block 1004). The calibration current is generated across the resistor 836. In some embodiments, the reference voltages are used to generate the calibration current 826 with at least one of the reference voltages captured in a capacitor (e.g., capacitor 830). Once the calibration current 826 is generated, the calibration circuitry 810 provides the calibration current 826 appropriate current drivers 804 in the μD 78 (block 1006). Specifically, the calibration current 826 is connected to calibration capacitors 828 using calibration transistors 842 and 844 sequentially. A PWM transistor 848 is also connected. Using these connections, a capacitor 828 is charged such that an output voltage is placed at the gate of a current driver transistor 846 to produce an output current to an LED that is substantially independent of transistor parameter changes of the transistor 846 based at least in part on temperature. In other words, the gate voltage, stored in the capacitor 828, accounts for variations in the current driver transistor and/or variations in the power supply (e.g., a VDD). Using the gate voltage, each current driver is used to operate the display using a gate voltage that is substantially independent of variations to the transistor and/or the power supply (block 1008).
It should be noted that more than a single calibration current may be used. For example, the calibration current may be specific to a particular color. In other words, in a RGB display, a calibration current for red current drivers may differ from a calibration current for blue or green current drivers. In some embodiments, red current drivers may have their own calibration current while blue and green current drivers share a calibration current. Alternatively, red, green, and blue current drivers may have their own calibration current specific to a respective color.
The calibration scheme may performed multiple times per frame. For example, a first calibration process for a first portion (e.g., first group of μDs and/or first group of rows) and a second calibration process for a second portion of the display. Furthermore, since voltage in the capacitor 828 may gradually decrease over time due to leakage, increasing frequency of calibrations may improve maintenance of a constant calibration current via a constant voltage stored in the capacitor.
The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure. Moreover, although the foregoing discusses row drivers that send data to μD s and column drivers that send data to microdrivers and row drivers that control which μD in a row receives the data, it should be appreciated that the foregoing discussion about row drivers may be applied to column drivers and vice versa merely by rotating orientation of the display. Thus, recitations of columns and rows may be interchangeable in meaning herein.
This application claims priority from and the benefit of U.S. Provisional Application Ser. No. 62/297,108, filed Feb. 18, 2016 entitled “Electronic Display”, and is incorporated by reference herein in its entirety.
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