Pressure sensing display device

A pressure sensing architecture for use with liquid crystal (LC), organic light emitting diode (OLED), electrophoretic, or other similarly fabricated displays. The described architecture includes a bottom TFT structure and a top structure with color filter material, and with liquid crystal, electrophoretic or OLED material provided in between. A piezoelectric or equivalent material is provided within the display assembly. Transmitting and receiving electrodes can be used to electrically bias the piezoelectric or equivalent material, which provides an analog electrical signal in response to incident touch pressure.

FIELD

The described embodiments relate to touch sensing digital displays and, in particular, to force or pressure sensing in touch sensing displays.

BACKGROUND

Touch panel displays are widely used in consumer electronics, such as smartphones and computing tablets, among other devices. Broadly speaking, there are two types of touch panel technologies currently used in consumer electronics: projected capacitance and resistive. Both types of touch panels typically can only sense the location and time of a touch event on the touch panel (e.g., from a finger or stylus). The location of a touch event is typically recorded only in two dimensions (e.g., x-y coordinates). Conventional touch panels are unable to sense in a third dimension to determine the magnitude of a touch force (e.g., a z-coordinate). Prior attempts at three-dimensional sensing have typically focused on the inclusion of a sensitive analog element. Conventionally, the inclusion of an analog element in what is otherwise a digital system has been costly, bulky and non-trivial.

In contrast, in concert with trends in the smartphone industry, and the computing industry more generally, touch panel displays continue to become thinner and less costly. One approach to reduce costs while making displays thinner is to integrate touch panel elements with display elements in a so-called “in-cell” fashion, as opposed to “on-cell” approaches.

On-cell approaches typically provide a touch panel display by stacking transparent touch location sensing elements (e.g., traces for capacitive touch sensing) on top of display elements. In-cell approaches typically provide a touch panel display by interspersing touch location sensing elements between layers of the display elements. For example, in one approach, touch location sensing traces may be provided between the liquid crystal layer and a color filter layer in an LCD device.

SUMMARY

in a first broad aspect, there is provided a pressure sensing display device, comprising: a cover layer; a base layer; a plurality of display pixels provided above the base layer and below the cover layer; a plurality of pressure sensing elements provided above the base layer, wherein each of the pressure sensing elements is addressable to provide an output based on a force applied to the pressure sensing element; and at feast one amplifier configured to detect the output of each addressed pressure sensing element and provide an output pressure value.

An integrated circuit layer may be provided above the base layer, and the integrated circuit layer comprises a thin film transistor circuit.

An output of the pressure sensing element may be coupled to an input of the at least one amplifier.

The at least one amplifier may be a charge amplifier, and charge at an output of the pressure sensing element may be coupled to an input of the at least one amplifier by an addressing transistor.

Each of the pressure sensing elements may further comprise an integrator, and an amplifier output of the integrator may be coupled to an input of the at least one amplifier by an addressing transistor.

An output of a pressure sensing portion of the pressure sensing element may be resettable by a reset transistor.

An output of the at least one amplifier may be coupled to an input of a correlated double sampler.

An insulating layer may be provided between the pressure sensing elements and the base layer.

An insulating layer may be provided between multiple layers of pressure sensing elements and the base layer.

The insulating layer may be formed of a dielectric material. The dielectric material may be aluminum oxide, silicon dioxide or silicon nitride, for example. The dielectric material may be optically transparent.

The pressure sensing elements may be provided in a layer above the display pixels. The pressure sensing elements may be provided in a layer below the display pixels. The pressure sensing elements may be substantially co-planar with the display pixels.

The display pixels may be arranged in a grid, and the pressure sensing elements may replace display pixels in the grid at selected intervals.

The display pixels may be arranged in a first grid, and the pressure sensing elements may be arranged in a second grid. The second grid may correspond to the first grid.

A controller may also be provided. The controller may detect a blanking interval of the display pixels, and each pressure sensing element may be addressed during the blanking interval. The blanking interval may be a horizontal blanking interval or a vertical blanking interval. The blanking interval may be selectively determined by the controller between a horizontal and a vertical blanking interval.

In another broad aspect, there is provided a method of pressure sensing in a display device comprising: providing a plurality of display pixels provided above a base layer and below a cover layer of the display device; providing a plurality of pressure sensing elements above the base layer; addressing each of the pressure sensing elements is addressable to provide an output based on a force applied to the pressure sensing element; and detecting the output of each addressed pressure sensing element at at least one amplifier to provide an output pressure value.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Existing approaches to pressure sensing in a touch panel display have focused on the use of a separate sensor film layer with multiple cells or with separate pressure sensing traces. In a cell-based approach, an applied force deforms the sensor film layer, causing a change in the distance between cells, and a corresponding change in a voltage or current. Such an approach provides only a linear measurement of the applied pressure, and thus may be limited by the small deformation distance typical in most displays. In a trace-based approach, a non-anisotropic film is used, which limits the ability of the device to determine the x-y location of the touch, particularly in multi-touch systems.

Another known approach measures the force of a touch event by detecting deflections of a screen or housing and measuring the resulting changes in capacitance values due to changes in a gap distance between the screen and a sensing pad. Such an approach is limited by the arrangement of the sensing pads (e.g., beneath the keys of a virtual keyboard), as it is only able to detect force at the pads.

The described embodiments generally provide an integrated display assembly that has both touch location-sensing and pressure-sensing elements and circuits for driving and controlling the display assembly. The location-sensing elements can be conventional resistive or capacitive touch panel elements. The pressure-sensing elements can be analog elements integrated with and distributed among digital display elements (e.g., one pressure sensing element per display pixel, one pressure sensing element for every n pixels). The pressure-sensing elements may be formed from a material that provides a non-linear output response to an applied force, and which can be current biased to provide high gain and tenability. Moreover, the described embodiments provide methods for achieving low noise operation.

Generally, the assembly includes a substrate upon which the display is fabricated and biasing and operating elements for touch-detection and pressure-detection. The biasing and operating elements may be provided on the same layer as the display elements, or may be provided separately on another layer of the assembly.

Integration of the described pressure-sensing elements into digital display elements can be done during display backplane fabrication, thereby reducing or eliminating manufacturing steps, and the handling of multiple substrates, to minimize fabrication cost.

The described embodiments also provide for low power operation to improve device battery life.

Referring now toFIG. 1, there is illustrated a simplified plan view of a display assembly. Display assembly100includes a pixel array110, a gate driver120and a source driver130.

Pixel array110may include a backplane with an active matrix comprising individually addressable pixels (e.g., LCD or LED elements), and a frontplane for optical modulation (e.g., color filters, polarizers, etc.). The backplane may include a plurality of layers, formed of various materials such as glass, polyester and paper.

Each addressable pixel may comprise one or more transistors and, in particular, a thin-film transistor (TFT), for controlling the operation of the pixel. In some embodiments, each pixel may consist of separate sub-pixels, each individually controllable, that are provided with different color filters.

Gate driver120and source driver130are generally integrated circuits that drive the operation of pixel array110. Both gate driver120and source driver130may be integrated into pixel array110, or provided as separate circuits in a display module using, for example, a flexible printed circuit, chip on glass or chip on flex approach.

In operation, display assembly100forms an image by scanning lines of pixels in pixel array110. Gate driver120provides a signal to open or activate selected pixels (or sub-pixels) in each line of pixel array110. Source driver130then charges each pixel in the line to a preconfigured voltage.

Referring now toFIG. 2A, there is illustrated a simplified cross-sectional view of a portion of a prior art display assembly. Display assembly200A generally includes an LCD device with a separate touch panel.

In particular, display assembly200A has a diffuser250that serves as a bottom layer or substrate. Diffuser250may be a light guide plate (LGP), a brightness enhancing film (BEF) or other suitable diffusing element that serves to diffuse light from, for example, an LED backlight that produces broad spectrum (e.g., white) light.

A polarizer240is stacked atop diffuser250to polarize light from diffuser250and direct it through a TFT layer235. TFT layer235includes integrated circuits for controlling each pixel or sub-pixel element in the display assembly200A. In some cases, TFT layer235may also serve as a base layer, providing a substrate upon which further layers may be stacked.

A color filter and liquid crystal layer230is stacked atop TFT layer230. Layer230includes liquid crystal elements that respond to control outputs from TFT layer230to become selectively opaque or partially opaque. Color filter elements are used to admit only selected wavelengths to cause the pixels or sub-pixels to appear to provide only light of the desired color (e.g., red, green, blue).

A color filter substrate layer225is stacked atop layer230. Color filter substrate layer225may be a glass substrate, for example, upon which the color filter portion of layer230is adhered or affixed. Generally, the liquid crystal portion of layer230is below the color filter portion. A further polarizer220is provided to ensure that stray light does not escape.

Layers250,240,235,230,225and220generally comprise a conventional LCD display assembly, in a conventional approach, a touch panel layer215may be stacked atop the conventional LCD display assembly, and a cover layer (e.g., lens) may be affixed to the assembly.

Each of the layers of display assembly200A may be fixed to the other, for example by lamination using a resin or other optically clear adhesive (OCA), portions of the assembly may also be sealed together during fabrication.

Touch panel layer215generally consists of an array of transparent conducting traces, as described herein with reference toFIG. 3.

Referring now toFIG. 2B, there is illustrated a simplified cross-sectional view of a portion of another prior art display assembly. Display assembly200B generally includes an OLED device with a separate touch panel.

Display assembly200B is fabricated using a similar approach to display assembly200A, but differs in that a back layer265may be used in place of diffuser250of assembly200A.

Back layer265is generally a black tape or moisture barrier layer that is optically absorbent. TFT layer235is stacked atop back layer265.

In contrast to display assembly200A, an OLED material layer260is provided atop TFT layer235. OLED elements respond to control outputs from TFT layer230to produce light at varying intensities, or colors, or both.

An encapsulation layer245is provided to seal the OLED elements, and a polarizer220is also provided. As with assembly200A, a touch panel layer215and a cover210are provided to complete display assembly200B.

It will be appreciated that various modifications can be made to the basic stacks of display assembly200A or200B.

Referring now toFIG. 2C, there is illustrated a simplified cross-sectional view of a portion of another prior art display assembly. Display assembly200C is generally analogous to display assembly200A, but differs in that touch sensing elements217may be provided on an underside of cover layer212. For example, transparent conductive traces (e.g., indium tin oxide (ITO), indium zinc oxide (IZO), etc.) may be coated or deposited on the cover layer212. Accordingly, a separate touch panel layer215may be omitted to provide a thinner assembly.

Referring now toFIG. 2D, there is illustrated a simplified cross-sectional view of a portion of another prior art display assembly. Display assembly200D is generally analogous to display assembly200A, but differs in that touch sensing elements223may be deposited or coated atop color filter substrate layer225. For example, transparent conductive traces (e.g., indium tin oxide (ITO), indium zinc oxide (IZO), etc.) may be coated or deposited on the color filter layer225.

Referring now toFIG. 2E, there is illustrated a simplified cross-sectional view of a portion of another prior art display assembly. Display assembly200E is generally analogous to display assembly200A, but differs in that touch sensing elements224may be deposited or coated beneath color filter substrate225or atop color filter layer230. For example, transparent conductive traces (e.g., indium tin oxide (ITO), indium zinc oxide (IZO), etc.) may be coated or deposited on the underside of color filter substrate225.

Referring now toFIG. 2F, there is illustrated a simplified cross-sectional view of a portion of another prior art display assembly. Display assembly200F is generally analogous to display assembly200B, but differs in that touch sensing elements228may be deposited or coated atop encapsulation layer245. For example, transparent conductive traces (e.g., indium tin oxide (ITO), indium zinc oxide (IZO), etc.) may be coated or deposited on the encapsulation layer245.

Referring now toFIG. 2G, there is illustrated a simplified cross-sectional view of a portion of another prior art display assembly. Display assembly200G is an electrophoretic display assembly, also known as an “e-Ink” display.

Display assembly200G may have a diffuser250and a TFT layer235. A liquid polymer layer290containing e-Ink capsules is provided above TFT layer235. A biasing layer292with transparent electrodes is provided atop the liquid polymer layer. Transistors in TFT layer235work in conjunction with electrodes in biasing layer292to bias the e-Ink capsules in liquid polymer layer290to a selected orientation (i.e., either admitting or blocking light from diffuser250).

Referring now toFIG. 3, there is illustrated a plan view of an example display assembly with touch panel traces shown. Display assembly300is a projected capacitive touch screen, and includes a plurality of transmitter traces310(shown as vertical traces) and a plurality of receiver traces320(shown as horizontal traces) forming a touch sensing grid. Traces310and320are electrically conductive and substantially optically transparent in order to be overlaid on a display pixel array350. For example, the traces may be formed by depositing ITO or IZO on a glass substrate. Moreover, transmitter traces310are electrically insulated from receiver traces320, for example with an intermediate insulating barrier formed of glass or some other dielectric.

In operation, a driver circuit312applies a voltage to the transmitter traces310to create an electrostatic field. In the absence of an external stimulus (e.g., finger or stylus), the electrostatic field is uniform across the grid. Each intersection of a transmitter and receiver trace forms a capacitor, which has a corresponding capacitance that can be measured by a receiver circuit322.

When a conductive object contacts the panel, the uniform electrostatic field becomes locally distorted. This distortion causes a change in capacitance (e.g., reduction in mutual capacitance) at an intersection of the transmitter and receiver traces. This change in capacitance can be determined by measuring a voltage on each of the receiver traces320, to identify the location of a touch event on the grid.

Typically, there are between 10 to 16 transmitter traces310, and 10 to 16 receiver traces320, resulting in between 100 to 258 distinct touch locations. Traces are typically between 4 to 8 mm in width, to capture a typical finger touch.

Display pixel array350has a portion410consisting of a subset of the pixel array350.

Referring now toFIG. 4A, there is illustrated an example pixel array portion in accordance with some embodiments.

Pixel array portion410A may be a portion of a pixel array, such as pixel array350. Pixel array portion410A generally includes a regularly arranged array of display pixels430A, which may comprise sub-pixels. However, at least one of the display pixels430A in portion410A may be replaced with a pressure sensing element420A. Pressure sensing element420A may be a pressure sensing element as described herein.

Referring now toFIG. 4B, there is illustrated another example pixel array portion in accordance with some embodiments.

Pixel array portion410B may be a portion of a pixel array, such as pixel array350. Pixel array portion410B generally includes a regularly arranged array of display pixels430B, which may comprise sub-pixels. Each of display pixels430B may have a corresponding pressure sensing element420B, which may be co-located adjacent to, or even integrated with, the display pixel430B. In some cases, only a subset of display pixels430B has a corresponding pressure sensing element420B, although the grid pattern may be spaced to accommodate the omitted pressure sensing elements. Pressure sensing element420B may be a pressure sensing element as described herein.

Referring now toFIG. 4C, there is illustrated another example pixel array portion in accordance with some embodiments.

Pixel array portion410C may be a portion of a pixel array, such as pixel array350. Pixel array portion410C generally includes a regularly arranged array of sub-pixels431,432and433, each of which may correspond to a different color. However, at least one of the sub-pixels may be replaced with a pressure sensing element420C. Pressure sensing element420C may be a pressure sensing element as described herein.

The arrangements of pixel array portion410A,410B or410C may be mixed, repeated or altered throughout a large display assembly, causing the display assembly, to have a regular or semi-regular arrangement of the respective pressure sensing elements. In general, the number and spacing (pitch) of pressure sensing elements may determine the degree of sensitivity to pressure. For example, relatively few pressure sensing elements may be used for a high sensitivity device. However, to provide more fine-grained sensitivity detection, a larger number of pressure sensing elements may be used.

Referring now toFIG. 5A, there is illustrated a simplified cross-sectional view of a portion of an example display assembly in accordance with some embodiments. Portions of the example display assembly, such as a diffuser, polarizer layers, and adhesive layers, are omitted to aid in understanding. Display assembly portion500A is generally an in-plane switching (IPS) display device with pressure sensing ability.

Display assembly portion500A integrated circuit layer535, which, includes in integrated circuit substrate529and an interlayer dielectric536. Integrated circuit layer535may be a TFT layer and substrate529maybe a TFT substrate, which may act as the base layer. A plurality of display pixel electrodes571are provided atop interlayer dielectric536, in addition, a pressure sensing electrode572is also provided atop interlayer dielectric layer536. Both types of electrode may be provided by, for example, deposition in an integrated circuit fabrication process, or post-fabrication by mechanical application.

Pressure sensing electrode572is electrically coupled to a detection circuit537via a conductor539. Detection circuit537may comprise one or more thin film transistors, tier example. Examples of detection circuits are described herein with reference, for example, toFIGS. 6A and 7A. One or more common voltage electrodes573may be embedded within interlayer dielectric536to provide for the proper operation of detection circuit537, and to reduce noise from, for example, operation of the display pixel elements.

Circuitry associated with the operation of display pixels is omitted fromFIG. 5A, so as not to obscure description of the pressure sensing elements.

In some embodiments, each display pixel electrode571may also serve as a pressure sensing electrode572, or vice versa, and there may be a combined display circuit and a pressure sensing circuit537for each electrode. In other embodiments, display pixel electrodes571may have separate circuits.

A pressure sensitive layer570is provided atop display pixel electrodes571and pressure sensing electrodes572. The pressure sensitive layer is generally formed of pressure sensitive material, such as a piezoelectric material, a quantum-tunneling composite material, or equivalent material, that converts a change in an applied pressure into a corresponding voltage or electric output signal. The pressure sensitive material is preferably substantially optically transparent.

The pressure sensitive layer may be screen printed, deposited, or laminated into a substrate, where the substrate may be glass, plastic, polyester, thin metal, or the like in a semiconductor display fabrication process.

Each pressure sensing electrode572and the surrounding region of pressure sensitive material579may form a distinct pressure sensing element. Thus, when a force is applied incident to the region of pressure sensitive material579, causing a compression or deformation of the region of material, this change in pressure within the pressure sensitive material may cause a corresponding change in voltage at pressure sensing electrode572. This voltage change can be detected by pressure sensing circuit537via conductor539.

Although pressure sensitive layer570may be monolithic, the piezoelectric effect in response to applied pressure is generally localized. Thus, an applied pressure in one location of the pressure sensitive layer570will produce a larger voltage change at a nearby electrode than at a more distant electrode.

A liquid crystal layer531may be provided atop the pressure sensitive layer570. In some embodiments, the liquid crystal layer531may be an in-plane switched (IPS) LCD layer. A color filter layer526and a cover layer510can also be provided, as with other layers.

As shown inFIG. 5A, the pressure sensing layer may be separate from a liquid crystal layer. In other embodiments, the pressure sensing layer may be integrated with, or provided upon a cover layer. In some cases, the pressure sensing layer may be provided above a layer containing display pixels, or below the layer containing display pixels. In some cases, pressure sensing elements may be co-planar with display pixels. In still other embodiments, the pressure sensing layer may be integrated with an integrated circuit layer. In still other embodiments, pressure sensing material may be interspersed with a liquid crystal (or other display) material in a set spatial pattern, either full pixel or sub-pixel.

Referring now toFIG. 5B, there is illustrated a simplified cross-sectional view of a portion of another example display assembly in accordance with some embodiments. In contrast to assembly portion500A, which illustrates an example IPS assembly, assembly portion500B depicts an example Twisted Nematic (TN) or Vertical Alignment (VA) assembly with pressure sensing ability.

As with assembly500A, assembly500B includes an integrated circuit layer535and a pressure sensing electrode572in contact with a pressure sensitive layer570.

A liquid crystal layer581is provided atop the pressure sensitive layer570. The liquid crystal layer581may be a TN medium or a VA medium, for example.

A common voltage electrode583is provided atop the liquid crystal layer581.

As with assembly500A, a color filter layer528and cover layer510are provided. Other portions of the example display assembly, such as a diffuser, polarizer layers, and adhesive layers, are omitted to aid in understanding.

Referring now toFIG. 5C, there is illustrated a simplified cross-sectional view of a portion of another example display assembly in accordance with some embodiments. In contrast to assembly portions500A and500B, which illustrate LCD assemblies, assembly portion5000illustrates an example OLED device with pressure sensing ability.

Display assembly portion500C includes an integrated circuit layer535, which may comprise a TFT substrate529, which may act as base layer, and interlayer dielectric536.

Interlayer dielectric536may have embedded therein a display pixel element596and a pressure sensing element597.

Display pixel element596may be a conventional OLED display pixel, including a driving circuit538, a conductor589connecting to a display electrode571which contacts OLED material583.

Pressure sensing element597includes a detection circuit537connected to a pressure sensing electrode572via a conductor539. Pressure sensing electrode contacts pressure sensitive material580, which is deposited generally in the same layer as OLED material583(e.g., not atop OLED material583).

An encapsulation layer511is provided to seal the display assembly portion.

Other portions of the example display assembly, such as polarizer and adhesive layers, are omitted to aid in understanding.

Referring now toFIG. 5D, there is illustrated a simplified cross-sectional view of a portion of another example display assembly in accordance with some embodiments. Display assembly portion500D illustrates an example electrophoretic display device with pressure sensing ability.

Assembly portion500D includes an integrated circuit layer535with a TFT substrate529, a plurality of driving and detection circuits561, which combine the driving capabilities of a driving circuit538with the detection capabilities of a detection circuit537.

A combined display and pressure sensing electrode562is connected to circuit561, and a pressure sensitive layer570and liquid polymer layer590, respectively, are provided atop the integrated circuit layer535.

In some embodiments, pressure detection can be suspended while the display is not being actively refreshed. In such a mode, power consumption may be close to zero. Pressure sensitive buttons (not shown) can be provided at a periphery of the display assembly, which can be used to trigger further pressure detection in the display when in a power saving mode.

Referring now toFIG. 5E, there is illustrated a simplified cross-sectional view of a portion of another example display assembly in accordance with some embodiments. Display assembly portion500E illustrates an example IPS display assembly with improved noise performance.

Assembly portion500E may be generally analogous to assembly portion500A. However, in contrast to assembly portion500A, assembly portion500E has an insulating layer599provided between pressure sensitive layer570and other portions of the display assembly.

In assembly portion500E, pressure sensitive layer570is provided above other layers, including the liquid crystal layer597and TFT layer529. A separate electrode layer596may also be provided.

The insulating layer599may be formed of a dielectric, such as aluminum oxide, silicon dioxide or silicon nitride (each of which can be optically transparent). The insulating layer599generally serves to prevent capacitive charge leakage from pressure sensitive layer570through electrodes572to other layers and, in particular, a ground plane of substrate598, which can make it difficult to measure capacitance modulation and thus the applied force.

By providing insulating layer599, capacitive charge leakage is mitigated, and charge is transferred primarily via conductive traces (not shown) provided directly to electrodes572, for example, in the pressure sensing layer. In particular, electrodes572may be coupled to detection circuits separated from the display assembly (e.g., located off-panel).

Although the described approach is shown in the context of an IPS display, a similar approach of providing an insulating layer between the pressure sensitive layer and other layers may also be used with other assembly types, such as OLED, VA-LCD, TN-LCD, and the like.

Referring now toFIG. 6A, there is illustrated a schematic diagram of an example detection circuit in accordance with some embodiments.

Detection circuit600uses a passive approach, and may be used, for example, to provide a detection circuit such as detection circuit537or561of display assembly portions500A to500E, for example.

Detection circuit600has an in pixel portion620, which includes a pressure sensing element605, such as pressure sensing element579of assembly portion500A, and an addressing transistor610, which may be a PMOS or NMOS transistor, for example. An output of pressure sensing element605is connected to addressing transistor610. A gate of addressing transistor610is addressable, such that when addressing transistor610is switched on, the output of pressure sensing element605is coupled to a detection line660via transistor610.

Thus, when pressure sensing element805detects a higher voltage corresponding to a touch, and when addressing transistor610is switched on, a higher current Itouchis transferred to detection line660.

Detection line660is input to a column charge amplifier630, which may for example include an op-amp635and capacitor640connected in parallel. The output of column charge amplifier630may be converted to a digital value by analog-to-digital converter (ADC)650.

Generally, there may be one column charge amplifier630for a plurality of pixels (e.g., line, column, row of pixels).

FIG. 6Billustrates signals at various nodes of detection circuit600when a touch is or is not registered. It can be observed that a voltage measured at the output of a column charge amplifier (Vout) is generated whether or not a touch is detected, although the output voltage is lower when no touch is detected. In particular, when no touch is registered, Itouch is zero, and Vout is low. When a touch is registered, Itouch increases, and Vout is relatively higher.

Detection circuit600may be implemented in whole or in part in an integrated circuit layer of a display assembly using, for example, TFTs. In some embodiments, only the in-pixel portion620is implemented in the integrated circuit layer, while the amplifier630and ADC650may be provided externally.

Referring now toFIG. 7A, there is illustrated a schematic diagram of another example detection circuit in accordance with some embodiments.

Detection circuit700uses an active approach whereby integration of the pressure sensing element output occurs in-pixel, and may be used, for example, to provide a detection circuit such as detection circuit537or561of display assembly portions500A to500E, for example.

Detection circuit700has an in-pixel portion720, which includes a pressure sensing element705, such as pressure sensing element579of assembly portion500A, a reset transistor712, an integrating transistor715and an addressing transistor710. Each transistor712,715and710may be a PMOS or NMOS transistor, for example, depending on the specific circuit configuration.

In operation, an output of pressure sensing element705is connected to a gate of transistor715, which serves as an integrator. In the example shown, a source of transistor715is connected to a bulk supply voltage, causing a drain terminal of transistor715to integrate the input to the gate of transistor715. This integrated output can be coupled to a detection fine760when an addressing transistor710is switched on.

The integrated output can be provided to a column amplifier730, correlated double sampler735, and digitized using ADC750.

Correlated double sampler735may be used to improve signal accuracy and signal-to-noise ratio. Generally, correlated double sampling is a technique used when measuring sensor outputs, which allows an undesired offset to be removed from a measured value (e.g., voltage, current). To perform correlated double sampling, the output of a sensor may be measured twice: once in a known condition and again in an unknown condition. The value measured during the known condition can be subtracted from the value measured during the unknown condition.

Correlated double sampling is used, for example, in switched capacitor op-amps to improve the gain of a charge-sharing amplifier, while adding an extra phase.

In the described embodiments, correlated double sampling may be performed by measuring the output of a pixel or group of pixels after a reset is performed (e.g., the known condition) and subtracting this output from the output at the end of an integration period (e.g., the unknown condition). The reset may be performed, for example, by triggering reset transistor712.

In some embodiments, the correlated double sampler735may be omitted.

In contrast to detection circuit600, detection circuit700does not require that a pressure sensing element provide an output voltage concurrently with the detection and amplification. Accordingly, a users touch may occur separately from the detection event. As illustrated inFIG. 7B, this allows for greater detection sensitivity, since detection can occur during less noisy times.

Reset transistor712can be activated to reset the output node of pressure sensing element705to a default voltage (e.g., Vreset). This reset pulse may also act to erase any material memory effect that may exist in the pressure sensing element, which could affect the measurement calibration.

Detection circuit700may be implemented in whole or in part in an integrated circuit layer of a display assembly using, for example, TFTs. In some embodiments, only the in-pixel portion720is implemented in the integrated circuit layer, while the amplifier730, CDS735and ADC750may be provided externally.

Referring now toFIG. 8, there is illustrated an example timing diagram for a display assembly with pressure sensing.

Generally, data line switching during display programming can be a major source of noise for capacitive touch sensing and pressure sensing. In particular, capacitive coupling through data lines can interfere with touch detection, reducing signal-to-noise ratio.

In some embodiments, touch detection and reporting, including sensing of the touch events, can be synchronized with display programming quiet periods. In particular, touch detection and reporting can be synchronized with vertical or horizontal blanking intervals, or both. The vertical blanking interval may be particularly suited to touch detection and reporting, since there is generally little or no display programming signaling in this period.

Referring now toFIG. 9, there is illustrated an example current-force characteristic for a pressure sensitive material, such as a pressure sensitive material for use in pressure sensing layer570.

As described herein, the pressure sensitive material may exhibit a piezoelectric effect or quantum tunneling effect. In general, the pressure sensitive material may have an analog response characteristic as shown inFIG. 9, with a non-linear response to pressure sensing. Various other response characteristics may also be suitable.

The described embodiments generally provide a pressure sensing architecture suitable for use with liquid crystal (LC), organic light emitting diode (OLED), electrophoretic, or other similarly fabricated displays. The described architecture generally includes, but is not limited to, a bottom TFT structure and a top structure with color filter material, and with liquid crystal, electrophoretic or OLED material provided in between. A piezoelectric or equivalent material is inserted or integrated into the display assembly. The piezoelectric or equivalent material is capable of providing an analog electrical signal in response to incident touch pressure. Transmitting and receiving electrodes can be provided and used to electrically bias the piezoelectric or equivalent material to allow detection of applied force.

The described embodiments can be fully integrated within a pixel array, semi-integrated within a pixel array (e.g., where parts of the pixel elements are re-used for pressure sensing), or separately disposed on an individual substrate and later integrated with a pixel array (e.g., laminated or otherwise affixed atop or below the pixel array).

Moreover, the describe pressure sensing architecture can be fabricated using existing display fabrication techniques and sequences, reducing complexity and cost.

Although the described embodiments have been described primarily with reference to “in-cell” display technologies, the described techniques are applicable to many other display assembly structures, including “in-cell”, “on-cell”, “one-glass solution” and laminated panel approaches, in general, the described embodiments provide analog pressure sensing combined with conventional digital touch methods, such as projected capacitive or resistive touch sensing. The various methods of fabrication combined with the accompanied methods of operation allow the tuning of analog sensitivity to provide high gain and low noise operation. Moreover, tight integration can be realized in the backplane fabrication sequence, eliminating manufacturing steps and logistical handling of multiple substrates to minimize cost. The described embodiments also allow for high signal to noise ratio, enabling accurate, fast and sensitive touch and force sensing, as well as ultra-low power modes that significantly improve battery life and device operation times compared to known touch panel architectures and technologies.

It will be appreciated that numerous specific details are set forth in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. The scope of the claims should not be limited by the preferred embodiments and examples, but should be given the broadest interpretation consistent with the description as a whole.