Patent Document

RELATED APPLICATION 
     This application claims the benefit and priority of French Application No. 1458632, filed Sep. 15, 2014, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     Various embodiments described herein relate to devices equipped with a matrix or matrices of active OLED pixels, and more precisely to the determination of the refresh frequency of these matrices of active OLED pixels. 
     BACKGROUND 
     Devices are known in the prior art in which organic light-emitting diodes, more simply denoted by the term “OLED diodes”, are controlled at a refresh frequency by control circuits comprised of transistors. Such control circuits allow a voltage to be applied across the terminals of the OLED diodes after a refresh operation, and for this voltage to be maintained until the next refresh operation. This is made possible by using the capacitance associated with NMOS transistors which are respectively coupled to anodes of the OLED diodes. 
     The control circuits include other MOS transistors which act as switches. These transistors are conducting during the refresh operation (which has a short duration), then are in a non-conducting state when the voltage is maintained between refresh operations. Current leakages may nevertheless appear within these transistors acting as a switch. 
     The effect of these leakage currents causes a drop in the voltage across the terminals of the OLED diodes, which progressively decreases starting from the moment when the transistors acting as switches go into the non-conducting state. 
     This drop in the voltage across the terminals of the OLED diodes is associated with a reduction in the light intensity emitted by the OLED diodes. If this reduction in light intensity is too great between two refresh operations, it may be noticed by the user looking at the matrix of active OLED pixels. This phenomenon is commonly denoted by those skilled in the art by the term “flicker”. 
     The period between two refresh operations is associated with a refresh frequency. In order to limit the appearance of the flicker phenomenon, those skilled in the art know that it is preferable to use a high refresh frequency, for example 50 or 60 Hz. These refresh frequencies have the drawback of being associated with an increase in electrical power consumption. 
     SUMMARY 
     According to one embodiment, a determination is made of a refresh frequency, allowing the appearance of the flicker phenomenon to be limited. 
     The inventors have observed that, by determining the period of time starting from which a decrease in brightness becomes noticeable, a refresh frequency may be deduced at which flicker is rendered imperceptible. This refresh frequency may be lower than the frequencies generally used, but still without the flickering being noticeable. 
     According to one aspect, a method is thus provided for determining a refresh frequency for a matrix of OLED active pixels, having, for each pixel, a control circuit controllable at the refresh frequency. The method includes a simultaneous control, at a first moment in time, of first and second dummy control circuits analogous to the control circuit. The control of the first dummy control circuit includes the application of a first voltage to the input of the first dummy control circuit in such a manner so as to obtain a first output voltage. The control of the second dummy control circuit includes the application of the first voltage to the input of the second dummy control circuit, then the placing of the second dummy control circuit into a leaky state, in such a manner as to obtain a second output voltage. The method also includes a determination of the period of time separating the first moment in time from a second moment in time at which the difference between the first and second output voltages reaches a threshold, so as to facilitate deduction of the refresh frequency. 
     By maintaining the first voltage on the input of the first dummy control circuit, the first output voltage remains constant and equal to the voltage across the terminals of an OLED diode of the matrix of active pixels while it is being refreshed. 
     Putting the second dummy control circuit into a leaky state allows the second dummy control circuit to be placed in a state similar to that of a control circuit having just been refreshed. 
     The threshold may be chosen such that it corresponds to a reduction in the voltage across the terminals of an OLED diode which produces a noticeable reduction in light intensity. 
     As soon as this threshold is reached, the period corresponding to the time between the first and the second moments in time is inverted and this period of time corresponds to the refresh frequency sought: the lowest frequency that allows the flicker phenomenon to be rendered imperceptible. 
     The time between the first and the second moment in time can be directly determined in order to deduce the refresh frequency. A mapping could be used that has as input the period of time determined and as output a refresh frequency. Frequency determination algorithms might also be used. 
     The control of the second circuit can include the charging of a capacitance associated with an NMOS transistor and the placement into a leaky state includes the discharging of this capacitance. 
     “A capacitance associated with an NMOS transistor” shall be understood to mean the gate capacitance formed between the gate and the substrate of the transistor as separated by a gate oxide. 
     According to another aspect, a device is provided and includes a matrix of OLED active pixels. For each pixel, a control circuit is associated therewith and designed to be controlled at a refresh frequency. The device also includes first and second dummy control circuits analogous to the control circuit, and a controller configured for simultaneously controlling, at a first moment in time, the first dummy pixel circuit and the second dummy pixel circuit. The controller is configured for controlling the first dummy pixel circuit by applying a first voltage to the input thereof in such a manner as to obtain a first output voltage, and for controlling the second dummy pixel circuit by applying the first voltage to the input thereof and then placing the second dummy pixel circuit into a leaky state in such a manner as to obtain a second output voltage. The device also includes a determination circuit configured for determining the period of time separating the first moment in time from a second moment in time at which the difference between the first and second output voltages reaches a threshold, and configured for deducing from this the refresh frequency for the matrix of active OLED pixels. 
     The controller can be configured for controlling the second dummy pixel circuit by charging a gate capacitance associated with an NMOS transistor of the second circuit, and the placement into a leaky state includes the discharging of this gate capacitance. 
     The first dummy pixel control circuit and the second dummy pixel control circuit can each include a first NMOS transistor having its source forming the input of the dummy pixel control circuit and its drain coupled to the gate of a second NMOS transistor having its source forming the output of the dummy pixel control circuit. 
     Such a circuit corresponds to a pixel control circuit of a conventional OLED matrix, which allows leakage currents corresponding to those of the matrix of pixels to be observed. 
     The controller can include a circuit configured for placing the second control circuit into a leaky state. The circuit includes a first NMOS transistor having its source designed to receive the first voltage, its drain coupled to the input of the second control circuit, and a second NMOS transistor coupled between the input of the second control circuit and ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of this disclosure will become apparent upon examining the detailed description of non-limiting embodiments and appended drawings, in which: 
         FIG. 1  is a circuit diagram of a device according to one embodiment, and 
         FIG. 2  is a diagram illustrating the time variation of the output voltages from the dummy control circuits of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a device DIS having a matrix of active OLED pixels MPA. The matrix of active OLED pixels MPA has active pixels PIX, which each are comprised of an OLED diode DI and a control circuit CC 0 . The control circuit for each pixel of the matrix of active OLED pixels MPA is controlled at a refresh frequency F, which is supplied to a logic circuit GPI for management of the pixels. This management circuit GPI controls lines for selecting rows of the matrix of pixels and lines which are designed to supply a voltage to the input of the control circuits of the pixels in order to refresh them. 
     For the purposes of simplification, one active pixel PIX of the matrix of pixels MPA has been shown. 
     The control circuit CC 0  includes a first NMOS transistor TR 01  and a second NMOS transistor TR 02 . The OLED diode DI is designed to receive at its cathode a cathode voltage VCA, which is the same for other cathodes of diodes of the matrix of active OLED pixels MPA. The anode of the OLED diode DI is coupled to the control circuit CC 0 , and more precisely to the source of the second NMOS transistor TR 02 . The second transistor TR 02  has its drain coupled to a power supply line VDD and its gate coupled to the drain of the first NMOS transistor TR 01 , which has its source forming the input E 0  of the control circuit CC 0 . The input E 0  is controlled by the logic circuit for management of the pixels GPI, and this is also the case for the gate  10  of the first NMOS transistor TR 01 . 
     The device DIS includes a first dummy control circuit CCF 1  and a second dummy control circuit CCF 2 . These dummy control circuits CCF 1  and CCF 2  are analogous to the control circuits CC 0  of the active pixels of the matrix of active OLED pixels MPA. Thus, they include the same or similar components and are configured in the same or similar way. The dummy control circuits CCF 1  and CCF 2  are not, however, coupled to OLED diodes. 
     The first dummy control circuit CCF 1  includes a first NMOS transistor TR 11  having its source forming the input E 1  of the first control circuit CCF 1  and its drain coupled to the gate of a second NMOS transistor TR 12 , the source of which forms the output S 1  of the first dummy control circuit. The drain of the second NMOS transistor TR 12  is coupled to the power supply line VDD. 
     The second dummy control circuit CCF 2  comprises a first NMOS transistor TR 21  having its source forming the input E 2  of the second control circuit CCF 2  and its drain coupled to the gate of a second NMOS transistor TR 22 , the source of which forms the output S 2  of the second dummy control circuit. The drain of the second NMOS transistor TR 22  is coupled to the power supply line VDD. 
     In the dummy control circuits CCF 1  and CCF 2 , the first NMOS transistors allow a voltage level to be passed from the input of the dummy control circuit to charge a gate capacitance associated with each of the second NMOS transistors, in other words the capacitor formed between the gate and the substrate of the second NMOS transistor. 
     When a voltage is held by the gate capacitance associated with a second NMOS transistor, an output voltage appears at the sources of these transistors, in other words at the output S 1  of the first dummy control circuit CCF 1  and at the output S 2  of the second dummy control circuit CCF 2 . 
     When a first voltage is applied to the input E of the device, this voltage can be transmitted to the gate of the second NMOS transistor TR 12  of the first dummy control circuit which will maintain this voltage by virtue of its gate capacitance. The first NMOS transistor TR 11  is then controlled in a conducting state for at least a particularly short lapse of time for stabilization to allow transmission of the first voltage to the gate of the second NMOS transistor TR 12  and the charging of its gate capacitance. 
     By maintaining the first voltage on the input E of the device, and on the input E 1  of the first dummy control circuit CCF 1 , the flow of a leakage current IF 1  through the transistor TR 11  is prevented. The voltage maintained by virtue of the gate capacitance associated with the transistor TR 11  is therefore conserved. This is also the case for the first output voltage obtained at the point S 1 . 
     The device further includes a circuit CEF configured for placing the second dummy control circuit CCF 2  into a leaky state. The circuit CEF comprises an NMOS transistor TRF 1  having its source coupled to the input E of the device and its drain coupled to the drain of an NMOS transistor TRF 2 , the source of which is coupled to ground. The common point of the transistors TRF 1  and TRF 2  is coupled to the input E 2  of the second dummy control circuit CCF 2 . 
     In order to control the ON and OFF states of the transistors TR 11 , TR 21 , TRF 1  and TRF 2 , their gates, respectively referenced  1 ,  2 ,  3  and  4  in the figure, are controlled by suitably configured logic control circuitry MC. This control circuit can also apply a first voltage to the input E of the device. 
     The logic circuitry MC is configured for simultaneously controlling, at a first moment in time, the first dummy pixel circuit CCF 1  and the second dummy pixel circuit CCF 2 . 
     More precisely, the logic circuitry MC is configured for controlling the first dummy control circuit CCF 1  by applying the first voltage to the input E 1  of the circuit in order to obtain the first output voltage. For this purpose, the gate  1  of the first NMOS transistor TR 11  is controlled such that this transistor is conducting. 
     The logic circuitry MC is configured for controlling the second dummy control circuit CCF 2  by applying the first voltage to the input E 2  of the second circuit for a very short period of time for stabilization, then by placing the second circuit into a leaky state, in such a manner as to obtain a second output voltage. For this purpose and during this very short period of time for stabilization, the gate  2  of the first NMOS transistor TR 21  is controlled such that this transistor is conducting, the gate  4  of the NMOS transistor TRF 2  is controlled such that this transistor is turned off, and the gate  3  of the transistor TRF 1  is controlled such that this transistor is conducting. At the expiration of the very short period of time for stabilization, the gate  3  of the transistor TRF 1  is controlled such that this transistor is turned off, the gate  4  of the transistor TRF 2  is controlled such that this transistor is conducting, and the gate  2  of the first NMOS transistor TR 21  is controlled such that this transistor is turned off, as is the case for such a pixel transistor which has just been refreshed. A leakage current IF 2  can then flow through the first NMOS transistor TR 21  to ground. For this reason, the voltage maintained on the gate of the transistor TR 22  can drop, and this is also the case for the second output voltage obtained at the output S 2  of the second dummy control circuit CCF 2 . 
     At the expiration of the very short period of time for stabilization, and in order to achieve an operation similar to that of a pixel of the matrix, the first NMOS transistor TR 11  is also controlled by its gate  1  in a non-conducting state, so the control of this gate is the same as that of the gate of the first NMOS transistor TR 12 . 
     The logic circuitry MC can include one or more logic circuits, or any other circuit suitable for applying gate voltages to control transistors to be in ON or OFF states. 
     It may be noted that the time variation of the second output voltage corresponds to the time variation of the voltage across the terminals of the OLED diode DI of the matrix of active OLED pixels MPA after a command has been applied to its control circuit. 
     The first and second output voltages are compared by a comparator CMP. A current generator GI 1  is coupled between the output S 1  of the first dummy control circuit CCF 1  and the corresponding input ECMP 1  of the comparator CMP. In the same way, a current generator GI 2  is coupled between the output S 2  of the second dummy control circuit CCF 2  and the corresponding input ECMP 2  of the comparator CMP. The current generators GI 1  and GI 2  allow the second transistors NMOS TR 12  and TR 21  to be biased so that they operate in a source follower mode. 
     The device DIS includes determining circuitry M configured for determining the period of time separating a first moment in time starting from which the two dummy control circuits have been simultaneously controlled, and a second moment in time where the difference obtained at the output of the comparator CMP reaches a threshold. This may notably be carried out by determining the period of time separating the first moment in time from the second moment in time. 
     This determining circuitry M can include a logic circuit or any other circuit suitable for determining this period, for example a comparator or a counter. This threshold can correspond to a voltage drop that is sufficiently high for flicker to be noticeable by the human eye observing the matrix of OLED active pixels. 
     The determining circuitry M is furthermore configured for deducing from this period the refresh frequency F for the matrix of OLED pixels MPA. This may be carried out by mapping or by using algorithms for frequency determination. 
     More precisely, taking into account a desired voltage drop across the terminals of the diode, this period of time provides an approximation to the first order of the slope of the time variation of the diode voltage as a function of the refresh frequency. Taking into account the characteristics of the OLED diode, the reduction in brightness may be deduced from the voltage drop, and consequently a first approximation of the refresh frequency may be obtained from the ratio between the reduction in brightness and the slope. This first approximation is subsequently corrected with a correction factor taking into account the sensitivity of the human eye at various frequencies. Such correction factors are for example available in the standard VESA IDMS (Informative Display Measurement Standard) version 1.03, which is hereby incorporated by reference in its entirety. 
     In other words, the combination of the slope, the characteristics of the OLED diode, and the sensitivity of the human eye as a function of frequency, supplies a mapping allowing the desired refresh frequency to be determined for a measured period of time. 
     This refresh frequency is subsequently supplied to the management circuit for the pixels GPI which manages the matrix of pixels MPA by applying commands to the circuits for controlling the pixels at the refresh frequency F. 
       FIG. 2  is a diagram illustrating the time variation of the output voltages of the dummy control circuits CCF 1  and CCF 2 . In this figure, the mixed dashed line corresponds to the output voltage VS 1  at the point S 1 , in other words at the output of the first dummy control circuit CCF 1 . This value VS 1  is constant since no leak can appear within this control circuit. 
     The solid line curve corresponds to the output voltage at the point S 2 , in other words at the output of the second dummy control circuit CCF 2 . At a first moment in time I 1 , a command is applied to this second control circuit and the input voltage of the device is applied to the gate of the second NMOS transistor TR 22  which delivers at the output a voltage equal to that obtained at the output of the first dummy control circuit CCF 1 . 
     This application of a voltage is implemented for a very short stabilization time, and subsequently the circuit is no longer controlled, and the voltage is maintained owing to the MOS capacitance associated with the second NMOS transistor TR 22 . 
     It is also after the application of the voltage for a very short stabilization time that a command is applied to the circuit CEF configured for placing the second control circuit CCF 2  in a leaky state. These leaks cause a drop in the output voltage of the second dummy control circuit. 
     At the time I 2 , the difference between the output voltages exceeds the threshold TH, and the time period T 1  separates the two moments in time. The determination of the refresh frequency may be implemented at any time during the operation of the device. 
     According to one aspect, a refresh frequency is obtained that is the lowest possible for an observer not to notice the flicker, which allows the consumption of electrical power to be limited. 
     The various embodiments described herein are well adapted to the displaying of fixed images by matrices of OLED active pixels.

Technology Category: 3