# Patent

## Title
Device with OLED matrix of active pixels with cathode voltage regulation, and corresponding method

## Abstract

A device includes a matrix of active pixels, with each active pixel having an OLED diode having a cathode to receive a cathode voltage, and a control circuit coupled to an anode of the OLED diode. The device also includes at least one dummy pixel having a dummy OLED diode having a cathode to receive the cathode voltage, and an anode, and a dummy control circuit coupled to the anode of the OLED diode and having a power supply terminal. The dummy OLED diode and the dummy control circuit are substantially similar to the OLED diode and the control circuit. First regulation circuitry is configured to deliver a reference current to the power supply terminal to thereby generate a voltage, and second regulation circuitry is configured to regulate the cathode voltage so as to maintain the voltage at the power supply terminal at a given level.

## Background
RELATED APPLICATION
This application claims the benefit and priority of French Application No. 1456579, filed Jul. 8, 2014, the contents of which are hereby incorporated by reference in their entirety to the maximum extent allowable under the law.
TECHNICAL FIELD
Implementations and embodiments according to this disclosure relate to devices equipped with a matrix or matrices of active OLED pixels, and more specifically the cathode voltage of the OLED diodes of these matrices.
BACKGROUND
Devices are known from the prior art that have matrices of active pixels, in which each pixel comprises an OLED diode and a control circuit constructed from transistors. In these matrices, the cathodes of the OLED diodes are coupled together to receive a cathode voltage, for example a negative voltage.
The OLED diodes exhibit a voltage threshold which can vary with temperature. To obtain a light image which is unaffected by temperature, it has been proposed to use a temperature sensor, and to use the measured temperature value to generate a cathode voltage which makes it possible to maintain the light emission despite the temperature variation.
This approach presents a variety of drawbacks. For example, a calibration step after the fabrication of each device is used to associate the temperature values with the corresponding cathode voltage levels and with the luminosity levels. This step can be time consuming, and the temperature sensors used may not be sufficiently accurate with an inaccuracy on the order of a few degrees. The circuitry used to generate a cathode voltage may not be sufficiently accurate, and the use of a single temperature sensor provides information relating to the temperature at but a single point.
Therefore, further developments in devices with matrices of active OLED pixels capable of compensating for temperature are desired.
SUMMARY
According to one implementation and embodiment, a device includes a pixel configured to receive a cathode voltage, and a dummy pixel coupled between a power supply terminal and the cathode voltage, the dummy pixel being substantially similar to the pixel. A reference current generator is coupled to the power supply terminal to apply a reference current thereto, causing generation of a reference voltage. A voltage regulator is configured to generate the cathode voltage so as to maintain the reference voltage at a threshold level.
According to one implementation and embodiment, it is proposed to generate a cathode voltage that is better matched to the temperature of the device, that is to say that makes it possible to have a desired luminosity, and to generate that cathode voltage without the use of a temperature sensor.
According to one aspect, a device is proposed that includes a matrix of active pixels, each active pixel being an OLED diode, the cathode of which is intended to receive a cathode voltage. A control circuit coupled to the anode of the OLED diode.
According to a general feature of this aspect, the device includes at least one dummy pixel which is an OLED diode, the cathode of which is intended to receive the cathode voltage, and a dummy control circuit coupled to the anode of the OLED diode and having a power supply terminal. First circuitry is configured to deliver a reference current to the power supply terminal, and second regulation circuitry is configured to regulate the cathode voltage so as to maintain a reference voltage at the power supply terminal.
Thus, no temperature sensor is used, and the cathode voltage is regulated with an aim toward maintaining the pair formed by the reference current and the reference voltage. Since the reference current is supplied constantly, it is the voltage at the power supply terminal which is observed and maintained by acting on the cathode voltage.
Upon a temperature rise, the threshold voltage of the OLED diode of the dummy pixel decreases. The voltage measured at the lower supply terminal also decreases. The cathode voltage of the OLED diode, which is biased with a negative potential, decreases as an absolute value. To have a constant current flow in the OLED diode, which results in a constant luminosity, the cathode voltage is increased in absolute value.
Also, by using a dummy pixel which includes a control circuit and an OLED diode, it is possible to observe on this circuit the same impact that the temperature has on a pixel of the matrix of active OLED pixels. “Dummy pixel” should be understood to mean a pixel which is not used for the display of an image by the matrix of active pixels, but which includes an OLED diode and a control circuit.
The at least one dummy pixel can be arranged at the periphery of the matrix of active pixels. Advantageously, the device can have a plurality of dummy pixels with their power supply terminals coupled together.
The reference current value is then chosen to correspond to the product of the number of dummy pixels and of the current required by one pixel. It is easier to provide a higher current value, and the accuracy of the device is therefore enhanced with a plurality of dummy pixels.
Furthermore, by using a plurality of dummy pixels, the effect of the temperature is taken into account over a greater number of dummy pixels which are not all positioned in the same place but which can be arranged around the matrix of active pixels. The temperature variations at a higher number of points are thus taken into account and the device is therefore more accurate.
The device also has second circuitry including an analog-to-digital converter with an input configured to receive the voltage at the power supply terminal(s), third circuitry configured to compare the reference voltage with the output of the analog-to-digital converter, and fourth circuitry configured to generate the cathode voltage from the output of the third circuitry.
The reference current value and the reference voltage value can be chosen for the OLED diode of the at least one dummy pixel to supply a reference luminosity. It will be possible, for example, to choose a reference luminosity which can be easily maintained. Such a reference luminosity is different from black. It can be noted that it is easier to maintain a luminosity which corresponds to a current that is high enough to be controlled.
The device can be used to determine the cathode voltage at certain moments so as not to disrupt the light emission. It is possible to determine the cathode voltage at regular intervals.
According to another aspect, there is proposed a method for regulating a cathode voltage of an OLED diode of an OLED matrix of active pixels, each active pixel having an OLED diode, the cathode of which is intended to receive the cathode voltage and a control circuit coupled to the anode of the OLED diode.
According to one feature of this aspect, the method includes a delivery of a reference current to a power supply terminal of at least one dummy pixel having an OLED diode, the cathode of which is intended to receive the cathode voltage and a dummy control circuit coupled to the anode of the OLED diode and having the power supply terminal. The method also includes a regulation of the cathode voltage so as to maintain a reference voltage at the power supply terminal.
The reference current can be delivered to a plurality of power supply terminals of a plurality of dummy pixels having their power supply terminals coupled together.
The regulation of the voltage applied to the negative power supply line can include an analog-to-digital conversion of the voltage at the power supply terminal(s), a comparison of the reference voltage with the value obtained by the analog-to-digital conversion, and a generation of the cathode voltage from the result of the comparison.
The reference current value and the reference voltage value can be chosen for the OLED diode of the at least one dummy pixel to supply a reference luminosity.



BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages and features will become apparent on examining the detailed description of nonlimiting implementations and embodiments, and the attached drawings in which:
FIG. 1 is a diagram of a device according to one embodiment, and
FIG. 2 is a diagram of a device according to another embodiment.



DETAILED DESCRIPTION
One or more embodiments will be described below. These described embodiments are only examples of implementation techniques, as defined solely by the attached claims. Additionally, in an effort to provide a focused description, irrelevant features of an actual implementation may not be described in the specification.
In FIG. 1, a device DIS is represented comprising a matrix of active pixels MPA. The matrix of active pixels comprises active pixels PIX provided with an OLED diode DI0 and a control circuit CC0. The OLED diode DI0 has its cathode coupled to a cathode power supply line LAC which is intended to receive a cathode voltage. The anode of the OLED diode DI0 is coupled to the control circuit CC0, and more specifically to the source of transistor TR0 which has its drain coupled to a power supply line VDD. Upon the refreshing of a pixel PIX, a voltage is applied to the gate of this transistor TR0 by controlling a switch IT0, and this voltage is maintained by virtue of the capacitor formed by the gate and the substrate of the transistor TR0. The switch IT0 is controlled by a selection line LS to switch this voltage from a sampling line LE to the gate of the transistor TR0.
For the purposes of simplification, a single active pixel PIX of the matrix of pixels MPA has been represented, but it should be understood that the OLED matrix of active pixels MPA includes a plurality of pixels PIX organized in rows and in columns.
To generate the cathode voltage to be applied to the cathode power supply line of the pixels PIX, the device comprises a dummy pixel PIXF, first circuitry GI configured to deliver a reference current, and regulation circuitry MREG configured to regulate the cathode voltage so as to maintain a reference voltage at the power supply terminal.
As can be seen in FIG. 1, the dummy pixel PIXF is arranged outside of the matrix of active pixels MPA, at the periphery of this matrix.
The dummy pixel PIXF comprises a dummy control circuit CCF and an OLED diode DIF, the cathode of which is coupled to the cathode power supply line LAC and is therefore intended to receive the cathode voltage. The control circuit CCF of the dummy pixel PIXF comprises a transistor TRF, the source of which is coupled to the anode of the OLED diode DIF. The drain and the gate of the transistor TRF are coupled together at a power supply terminal BA, which can receive, via a switch ITF, a current delivered by the first circuitry GI configured to deliver a reference current. The first circuitry GI can be a current generator.
The appearance of a current at the power supply terminal of a transistor causes a voltage to appear which controls the gate of the transistor TRF to make the transistor TRF conduct, and the current flows through the transistor to the diode DIF.
When the switch ITF is closed and a reference current is delivered to the power supply terminal BA, to generate the cathode voltage, the voltage can be measured at the power supply terminal. To this end, the regulation circuitry MREG for regulating the cathode voltage comprises second circuitry including an analog-to-digital converter CAN. Third circuitry MC is configured to compare the reference voltage with the output of the analog-to-digital converter CAN, and fourth circuitry MG is configured to generate the cathode voltage from the output of the third circuitry.
The analog-to-digital converter CAN generates a digital value from the voltage measured at the power supply terminal BA. This digital value is compared to a reference value within the circuitry MC which can comprise a logic circuit and memories.
At the output of the circuitry MC, a digital value is obtained which corresponds to the new cathode voltage value to be applied. This value is determined in such a way that the voltage measured at the power supply terminal BA is maintained around a reference value which corresponds with the reference current value delivered, to a chosen luminosity value. A voltage variation is thus taken into account whereas, in the prior art, it is a temperature variation measured by a sensor which is taken into account.
The fourth circuitry MG configured to generate the cathode voltage from the output of the third circuitry comprises a voltage generator coupled to the cathode power supply line LAC.
FIG. 2 shows a diagram of another device DIS which has several dummy pixels PIXF. In this FIG. 2 four dummy pixels PIXF have been represented around the matrix of active pixels MPA. The pixels PIXF are distributed evenly around the matrix of pixels PIXF and their number can, for example, be on the order of several hundred. The increase in the number of dummy pixels PIXF makes it possible to improve the accuracy of the device, since the temperature variations at a plurality of points are taken into account. The accuracy of the device is also improved because the reference current corresponds to the product of the reference current of one dummy pixel by the number of dummy pixels PIXF. This makes it possible to correctly take into account the temperature variations and therefore obtain a well-controlled luminosity. It can be noted that the power supply terminals BA are intercoupled, and such is also the case with the power supply lines LAC.
According to one aspect, the devices described herein make it possible to take into account the temperature in a more accurate manner to control the luminosity of an OLED pixel matrix.
The technology described herein also makes it possible to simplify the calibration of the device for which there is no need to measure the temperature, the cathode voltage, and the luminosity during calibration phases. Here, the selection of a reference current/reference voltage pair with an associated luminosity makes it possible to calibrate the device. A faster calibration phase is thus obtained.
While the disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be envisioned that do not depart from the scope of the disclosure as disclosed herein. Accordingly, the scope of the disclosure shall be limited only by the attached claims.

## Claims

1. A device, comprising:
an anode side terminal;
a current source configured to generate a reference current;
a switch coupled between the current source and the anode side terminal and configured to selectively deliver the reference current to the anode side terminal to thereby generate a voltage;
a matrix of active pixels, each active pixel comprising:
an OLED diode having a cathode configured to receive a cathode voltage and an anode, and
a control circuit comprising a transistor having a source coupled to the anode of the OLED diode, a drain coupled to a supply voltage, and a gate coupled to a refresh switch;

at least one dummy pixel comprising:
a dummy OLED diode having a cathode configured to receive the cathode voltage and an anode, and
a dummy control circuit comprising a transistor having a source coupled to the anode of the dummy OLED diode, a drain coupled to the anode side terminal, and a gate coupled to the anode side terminal, the transistor operating to turn on when the switch is closed and turn-off when the switch is open,
wherein the dummy OLED diode and the dummy control circuit are substantially similar to the OLED diode and the control circuit;

and
a regulation circuitry configured to sense a voltage at the anode side terminal and generate the cathode voltage so as to regulate the voltage at the anode side terminal at a given level.

2. The device of claim 1, wherein the at least one dummy pixel is arranged at a periphery of the matrix of active pixels.

3. The device of claim 1, wherein the at least one dummy pixel comprises a plurality of dummy pixels; and wherein the anode side terminals of each dummy pixel are coupled together.

4. The device of claim 1, wherein the regulation circuitry comprises an analog-to-digital converter having an input to receive the voltage at the anode side terminal; and further comprising a comparator configured to compare a reference voltage with an output of the analog-to-digital converter, and circuitry configured to generate the cathode voltage from the output of the comparator.

5. The device of claim 4, wherein the reference current and the reference voltage are chosen for the OLED diode of the at least one dummy pixel to supply a reference luminosity.

6. A method for generating a variable cathode voltage of a matrix of active pixels, each active pixel comprising an OLED diode having a cathode coupled to the cathode voltage and an anode coupled to a control circuit, the method comprising:
selectively delivering a reference current to a switch coupled to an anode side terminal associated with at least one dummy pixel comprising an OLED diode having a cathode terminal configured to receive the cathode voltage and an anode coupled to a dummy control circuit, the at least one dummy pixel being substantially similar to the active pixel; and
varying the cathode voltage so as to maintain a voltage at the anode side terminal;
wherein regulating the cathode voltage comprises performing an analog-to-digital conversion of a voltage at the anode side terminal, comparing a reference voltage with the anode side terminal voltage, and generating the cathode voltage based upon the comparison to regulate the voltage at the anode side terminal to a given voltage level.

7. The method of claim 6, wherein delivering the reference current comprises delivering the reference current to a node coupling anode side terminals of a plurality of dummy pixels together.

8. The method of claim 6, wherein the reference current and the reference voltage are such that the OLED diode of the at least one dummy pixel supplies a reference luminosity.

9. A device, comprising:
a pixel comprising an OLED diode having a cathode configured to receive a cathode voltage;
a dummy pixel comprising a dummy OLED diode having a cathode coupled to the cathode voltage and an anode coupled to a dummy control circuit, and a transistor having a drain coupled to an anode side terminal, a source coupled to the anode of the dummy OLED diode, and a gate coupled to the anode side terminal, the dummy pixel being substantially similar to the pixel;
a reference current generator selectively coupled to the anode side terminal via a switch to apply a reference current thereto and cause generation of a voltage at the anode side terminal; and
a voltage regulator configured to generate the cathode voltage so as to maintain the voltage at the anode side terminal at a threshold level;
wherein the gate and drain of the transistor of the dummy OLED diode being coupled to the anode side terminal results in turn-on of the transistor of the dummy OLED diode when the switch is closed and turn-off of the transistor of the dummy OLED diode when the switch is open.

10. The device of claim 9, wherein the pixel comprises an OLED diode having a cathode coupled to the cathode voltage and an anode coupled to a control circuit.

11. The device of claim 10, wherein the control circuit comprises a transistor having a drain coupled to the anode side terminal, a source coupled to the anode of the OLED diode, and a gate coupled to a refresh switch.

12. The device of claim 9, wherein the voltage regulator comprises an analog-to-digital converter configured to convert the voltage at the anode side terminal to a digital voltage, comparison circuitry configured to compare the digital voltage to the threshold level and to generate a control signal, and regulation circuitry configured to adjust the cathode voltage such that digital voltage is at the threshold level in response to the control signal.

13. The device of claim 9, wherein the pixel comprises an array of pixels; wherein the dummy pixel comprises a plurality of dummy pixels, each dummy pixel located at a different edge of the array of pixels; wherein the anode side terminal of each dummy pixel is coupled together.