Patent Application
20070091030
The present invention relates to an electronic control cell for organic light-emitting diode of active matrix display as well as operating methods. It finds applications in the domain of the display units, notably flat screens, whereof elementary display units, pixels or segments, with organic light-emitting diodes are controlled individually by control cells arranged in the form of one or several matrices.
The development of electronic equipment and/or industrial data processing equipment or mass public equipment requires the use of interfaces of interaction with the users and notably of visual interfaces with display units or segment or pixel monitors, these four terms being considered two by two in an equivalent fashion below. In order to provide enhanced display features, it is preferred currently to act individually on the display elementary units (segments or pixels) and it is thus that the display units with active matrix have been developed.
On top of a possible cost reduction, miniaturising and searching for increased stand-alone capacity have led to implementing technologies enabling to reduce the space requirements of the display units and to lower the power consumption as with liquid crystals. However, the latter technology exhibits a few limitations and shortcomings whereof a relative complexity due to the fact that the display is indirect inasmuch as polarization conditions of an external lighting should be acted upon. Other technologies based on a direct display, i.e. wherein the elementary units produce light, have been therefore developed and in particular that relative to the light-emitting diodes whereof a specific domain is considered more particularly here, that of the organic light-emitting diodes or OLED which enable to provide display units on various substrates such as glass or plastic materials and under interesting manufacturing conditions.
In the known OLED display units with active matrix, the control of each diode or of a group of light-emitting diodes of a pixel or segment is conducted in current which enables to obtain a linear control law between the log of the intensity Id running through the diode and the log of the luminosity Lum, i.e. log(Lum)=A*log(Id). However, the control circuit associated with a pixel is generally complex and requires control transistors which may sustain relatively high currents. The purpose of this control circuit is to maintain the control and the extinction of the OLED(s) of the pixel by, at an appropriate instant, an additional control signal, of the same type as that used for switching on or selecting the pixel and, generally, by a short ignition control pulse in one case and an extinction pulse in the other.
The major defect of such a control in current, results from the fact that it is generally realised by a complex assembly of at least four transistors, so-called in “current mirror”. This involves passing a high current through all the transistors of the pixel as well as in the control circuits situated upstream, and this, throughout a control cycle. Besides the fact that two control lines are necessary for operating the current mirror, these high currents should circulate via control lines provided on the display unit with relatively significant ohmic losses. This creates naturally constraints in terms of size and regarding the electronic mobility of these transistors, which leads, on top of the difficulties of realisation, to high energy consumption of the monitor.
In the matrix display units, the control of each of the pixels is multiplexed on a line×column basis and the display of a frame is carried out on a line×line basis (or a column×column basis according to the embodiment selected). Moreover, since the pixel remains turned on with substantially constant luminous level throughout the duration of a frame causes the transition of a light level from one frame to the other may be sudden. Such transitions may for example take place because an object displayed in a scene moves in the scene with the course of time, Still, such sudden transitions are perceived by the eye and disturb the visual perception of the scene animated on the screen. This causes a blurring effect which may be rather unpleasant.
The invention offers to solve these difficulties while providing a pixel control in voltage which enables additionally to simplify the control circuit associated with each pixel or segment. It uses the memory effect of an additional or intrinsic capacitor being discharged in an additional or intrinsic resistor of an electronic current switch of the OLED(s) of the pixel. The implementation of a voltage-based control enables additionally to limit the constraints on the size of the transistors and the electronic mobility (load carriers). It is thus possible to realise such display units with thin-film transistors, so-called TFT, with small mobility or not and, for example in amorphous or micro-crystalline or poly-crystalline silicon, possibly even organic transistors.
The invention relates to therefore an electronic control cell for at least one organic light-emitting diode (OLED) of a pixel or segment of an active matrix display, the cell including at least:
According to the invention, the storage is temporary by discharging the capacitor through a resistor Rf parallel to the capacitor.
In diverse embodiments of the invention, the following means which may be combined according to all the technical possibilities, are employed:
The invention also relates to an operating method of an electronic control cell for at least one organic light-emitting diode (OLED) of a pixel or segment of an active matrix display, the cell having at least:
According to the method, there is implemented a cell which is according to the one or several previous features and wherein the discharge of the capacitor is caused through a resistor Rf arranged parallel to the capacitor in order to provide a temporary storage of a turned-on state, and wherein, under average operating conditions the storage duration of a turned-on state is smaller than the duration of a frame, and preferably smaller than or equal to half the duration of a frame.
In a variation of the method, for turning the OLED(s) on a selection pulse Vsel is applied to the selection line of such a duration that at the end of the selection pulse the voltage at the terminals of the capacitor is a fraction of Vcom. In other variations which may be combined with the latter:
The invention relates finally to a display unit with organic light-emitting diodes (OLED) of pixels and/or segments implementing a set of electronic control cells of said diodes organised into a matrix, each pixel or segment being controllable individually by line×column multiplexing of the matrix, wherein the cells are according to one or several cell features indicated previously.
In a modality for manufacturing the display unit, the selection signals Vsel correspond to the lines of the matrix and the control voltages Vcom correspond to the columns of the matrix.
The invention enables the realisation of a simplified display unit and if the simplification of the electronic control cells of the pixels of the display unit may be accompanied by an increase in complexity of the driving circuits upstream of the display unit and of its cells, this enhanced complexity concerns circuits implementing well-known technologies, such as the integrated circuits built from silicon slices, and whereof the global impact in cost and/or consumption in a complete electronic or data processing piece of equipment is minimum with respect to the gain provided by the invention at the level of the display unit. It may be implemented for the realisation of flexible flat screens.
Among the advantages of the invention in the case of using a control transistor, one may quote the suppression of the blurring effect which is conversely observed on the display units of the state of the art. This is due to the fact that the voltage at the terminals of the capacitor decreases gradually with time, which reduces the light intensity of the OLED down to the threshold of the control transistor where, from that moment on, the control transistor is not conductive any longer and does not supply the OLED any longer. There is not any sudden transition any longer from a constant level to another constant luminosity level, from one frame to the next. One may also modify the display luminosity relative to the load sent to the capacitor during the selection of the cell of the pixel, a load which depends on the voltage Vcom (and/or Vsel). The current circulating through the OLED(s) and the light-up duration depend on Vcom (and/or Vsel). Moreover, the capacitor being discharged at the time when the cell of the pixel is accessed for displaying the following frame, there is no significant memory effect at the level of luminosity from one frame to the next.
The invention enables to obtain additionally structural simplification of the display unit, enhanced display features in terms of reduced consumption and, possibly as explained herein, reduced visual perception.
Indeed, among the other advantages of the invention, one may also quote the fact that refreshing the display of each diode OLED may enable to modulate, notably in all or in part, the light energy produced with times at high frequencies (pulsed rate) not enabling conscientious perception of the modulation by the human user, but which provides however enhanced perception with respect to a display which would be continuous. Besides, such a modulation enables to use in each diode OLED discontinuous (pulsed) currents which may be vastly greater than the currents that each diode may accept continuously, hence a possibility of increasing still further the perception by the user.
The present invention will now be exemplified by the following description, without being limited thereto, and in relation with:
According to the invention in its entirety, the electronic control cell for organic light-emitting diode(s) (OLED) of a pixel/segment of an active matrix display, includes a matrix set of such cells. Such a display unit operates sequentially by time units corresponding each to the duration of display of a frame. Throughout a frame duration, the columns or lines of the matrix are scanned to enable the configuration of display (level/intensity of turning on or off) of each of the pixels/segments. The OLED(s) of the pixel/segment are supplied by means of a control circuit which operates as an electronic switch relative to a control signal arriving via a control line and enabling to circulate or not in the OLED a current of variable intensity obtained between a ground and a positive power supply terminal Vdd.
The leadthrough impedance (resistor) of the control circuit in the conductive state is relatively small so as to turn on the OLEDs and to avoid ohmic dissipation (Joule effect) and excessive losses. In locked, non conductive, state the control circuit exhibits high leadthrough impedance (resistor), such that the leakage current is negligible and does not turn on the OLEDs.
In a preferred embodiment, the control circuit exhibits high control input impedance and hardly stresses the control line which includes a capacitor C and a resistor Rf which returns to the ground or Vdd according to the case. The capacitor C and the resistor Rf may be added-on and/or intrinsic elements of other elements of the cell. In the latter case, C may be the <<spurious>> input capacitor of the control circuit and/or Rf the input impedance (resistor) of the control circuit (the control circuit has not high impedance/input resistor any longer). One contemplates the case when Rf is the own leakage resistor of the capacitor (or conversely C is the spurious capacitor of the resistor Rf) which involves the manufacture of a particular capacitor (or conversely of a resistor) since the components available conventionally are generally practically pure components, i.e. resistors which are practically pure resistors and capacitors which are practically pure capacitors.
This section of the cell with the control circuit and the control line with its capacitor C and resistor Rf, forms a switching element with temporary memory: when the voltage on the control line exceeds the conduction threshold Vsl of the control circuit, the latter becomes conductive and, conversely, when the voltage on the control line falls below the conduction threshold Vsl of the control circuit, the latter becomes locked, non-conductive. The control circuit may operate on an all-or-nothing basis (substantially constant conductive/non-conductive) or linearly as will be seen with transistors in the case of
Once loaded, the capacitor C will be discharged gradually and if the initial load of C is such that the voltage on the control line is greater than the threshold Vsl, the OLED(s) will remain turned on as long as the decreasing voltage on the control line will be greater than the conduction threshold Vsl, of the control circuit.
To be able to load the capacitor C, a selection circuit which operates also as a switch controlled by a selection signal Vsel, may apply (conductive state) or not (locked, insulating state) to the control line a voltage Vcom. The voltage Vcom may be comprised between a voltage smaller than the threshold Vsl, preferably minimum 0V (at the ground) and a voltage greater than the threshold Vsl, preferably maximum Vdd. This voltage Vcom is one of the means for adjusting the display luminosity in the case of a transistor control circuit as represented on
It may be noted that because of the use of a transistor which exhibits at least one substantially linear operating zone 62 or 61, for the control circuit and because the voltage on the control line, 5 or 5′, varies with time, the current circulating in the OLED(s) will also vary with time and therefore the light intensity produced also up to the conduction threshold, a moment from which no more current run through the transistor and therefore through the OLED(s).
In the case of several organic light-emitting diodes operated by the control transistor, said diodes may be in series and/or in parallel. Besides, the invention can be implemented in a display unit including redundant components, notably cells and/or transistors and/or light-emitting diodes, which may be substituted for faulty components in order to reduce the production wastage of the display units which may include millions of components.
It has been seen therefore that in its easiest embodiment, the invention consists, basically, in controlling in voltage a pixel by charging a capacitor by a selection transistor M2 with a control voltage Vcom (which is preferably held substantially constant during the charging but it can be varied from one frame to another in order to modify the luminosity of the successive pixels of a column) throughout the pulse duration of the selection signal Vsel corresponding to the pixel. This voltage-operate control circuit behaves like a sample and hold which enables to charge a capacitor throughout the sampling period and to keep the charge (here decreasing) throughout the blockage period. This capacitor is directly connected to the gate of a switching transistor M1 which enables to feed the OLED(s) of the pixel. This gate exhibits high input impedance and the discharge of the capacitor through the gate (and the possible resistor parallel to the capacitor) is relatively slow, preferably such that the OLED(s) are supplied throughout half the duration of a frame.
This capacitor may be an added-on capacitor or the input capacitor, possibly increased by construction, of the control gate of the switching transistor M1. An added-on resistor or a leakage current of the capacitor or of the gate of the switching transistor, then causes gradual discharge of the capacitor and therefore automatic turning-off of the OLED(s) as soon as the voltage of the gate of the control transistor M1 falls below the threshold voltage Vsl, of the switching transistor. This extinction takes place at the end of a duration which depends on the threshold Vsl of M1, on the control voltage Vcom, on the value of the capacitor, the value of the impedances limiting the charge and the value of the discharge impedances. According to these values and the duration of the selection (selection pulse) tsel, the value of the maximum voltage applied to the gate varies, hence the time control effect of the OLED(s). One may therefore modify the duration of the lighting of the OLED(s) simultaneously by construction, once for all (for example with a capacitor value C determined by construction), or dynamically, in operation (for example in modifying the duration of the selection pulse tsel and/or the value of the voltage Vcom, possibly of the voltage Vsel).
The operating principle of a cell as that represented on
One may correlate the evolution of the voltage of the control line 5 of
One may therefore obtain a variation in luminosity of the pixels while modulating the control signal in duration and/or in level of voltage (initial, at the completion of the selection pulse) from one frame to another. This modulation may be obtained in several ways, according to whether the control voltage Vcom is modulated in level of voltage and/or the selection signal Vsel is modulated in duration, let alone the selection pulse Vsel is modulated in level of voltage.
To have an idea of the durations of the different signals implemented, one may consider the case of a display unit including 768 lines and 1024 pixels per line and for which the frame frequency is 75 Hz, i.e. 13.3 ms. The duration of a line is then 17.6 μs, which corresponds to the width of the selection pulse Vsel.
It may be noted that with a selection pulse of not too long a duration, the capacitor is only charged partially (discharged) during the selection pulse of the line, the maximum voltage at the terminals of the capacitor does not reach the voltage applied Vcom. This means that the voltage at the terminals of this capacitor (i.e. the gate voltage of the control transistor M1) is not brought to the value Vcom upon completion of this impulsion, but at a potential which is a fraction of Vcom. It may also be contemplated that the capacitor is charged up to substantially Vcom throughout the duration of the selection pulse Vsel.
It is useful to limit the charge current of the capacitor through the selection transistor in order to limit the size of the selection transistor and to prevent it from being charged completely to the control voltage Vcom with the duration tsel of the selection pulses used since a circuit, which would ensure complete charging of the capacitor, would not exhibit many advantages with respect to a conventional control in current. This limitation of the charging current may be obtained in several ways, possibly combined, whereof five examples are given below. Firstly while increasing the internal resistance of the source Vcom with the shortcoming of having variations in the maximum charging voltage relative to the number of cells selected in case when several cells are selected simultaneously. Secondly, using a selection transistor which exhibits a relatively high leadthrough impedance in conductive state, hence the possibility of using transistors with small mobility. Thirdly by addition of a resistor in series with the selection transistor. Fourthly, by addition of a non-linear component limiting the current peak and arranged in series with the selection transistor. Fifthly, by addition of a constant current generator in series or combined with the selection transistor.
The assemblies suggested wherein the capacitor and the control transistor have both a direct common point (Vdd for
It should be noted that with the invention and in the case of using transistors as represented on
The preferred operating method is the one wherein the OLEDs are turned on only throughout a section of the frame duration, i.e. there is a non-productive time during which each OLED is not turned on throughout a frame duration (it can be understood that an OLED of a pixel which should not be visible, will be turned off throughout the duration of the frame and that an OLED of a pixel which should be visible will be turned on only for a section of the duration of the frame). The non-productive time enables to place the OLED in idle mode and may have a beneficial effect on the life duration of the OLEDs. Besides, on top of the fact that a greater ridge current may be sent into an OLED which has an idle time, there might be favourable psychovisual effects with cyclic ignition of the OLEDs.
Thanks to the device and method of the invention, a voltage control enables modulation of duration of the current sent through the OLED. Indeed, for simplification purposes, the control circuit 61, 62 operates substantially in all or nothing mode, letting current through and turning on the OLED when the voltage on its control line 5, 5′ is greater than a threshold and locked beneath. Still the selection circuit 41, 42 which receives a substantially binary selection signal Vsel is made conductive or not, relative to said signal Vsel for substantially constant duration (pulse duration of Vsel) and the charge received by the capacitor C (hence the voltage at its terminals) depends therefore substantially on the level of the control voltage Vcom. One acts therefore on the lighting duration of the OLED while varying the voltage Vcom supplied to the capacitor C. The variation in the voltage Vcom therefore enables modulation encoding of the lighting pulse width of the OLED.
Preferably, the voltage Vcom remains substantially constant for the duration of the pulse Vsel (while neglecting the impact of the internal resistor of the source Vcom) and will be modified outside the pulses Vsel. The generator Vcom may be a digital/analogue converter with voltage output.
The choice of the values of Rf and C (own components or intrinsic to others as for example leakage current) will therefore be made notably relative to the frame duration and to the possible values of Vcom provided as well as that of the threshold of the control circuit so that there is a non-productive time (non lighting) within a frame for a OLED for which the maximum of Vcom has been sent to the capacitor during the pulse Vsel. One may also take into account the source resistor of the generator of Vcom and/or the leadthrough resistor of the selection circuit and/or of a possible additional circuit limiting the rise/fall time.
The time constant may be computed as follows:
The first step is the adjustment of the time constants of the assembly to the type of screen contemplated, in that instance a 1024×768 pixel display at a 75 Hz frequency gives a duration of the frame equal to 13.3 ms, and a selection time smaller than or equal to 17 μs.
The main characteristic time of the assembly is the constant RC, where C designates the storage capacitor of the control, and R is the leakage resistor at the terminals thereof. At the time scales considered, the transient phenomena in the transistors, of gate length fixed to 10 microns, are not perceptible. A solution with RC of the order of the microsecond is therefore requested.
More precisely, the point is to keep the OLED turned on for a duration close to half the frame duration. Indeed, in a screen-type application, liable to produce high dynamics display, it is essential not to maintain the control of display of a pixel throughout the frame duration, since this would cause, because of the visual remanence, blurred perception of any movement on the screen. At the frequency contemplated, the frame duration is roughly double that of the temporal perception of human vision system, whereof the generally accepted value is approx. 5 ms. To avoid superimposition of two frames, without modifying the refreshing frequency, therefore to limit the lighting of a pixel to approx. half the duration of the frame, and this for an OLED screen as well as for an LCD display (for which, the response time of the pixel itself should also be taken into account).
In the case of a purely voltage-control circuit, the discharge of the capacitor should turn off naturally the OLED before the end of the frame. An improvement in the dynamic visual qualities may even be expected thanks to the more regular variation of the lighting than in the case of the step-type control realised by an intensity/time driver. The point is to avoid the generation of too short an ignition cycle. Too quick discharge of the capacitor would have negative consequences on the display, and further involve higher peak intensity, in order to maintain the same average luminosity. An additional constraint is associated with the “staircase” effect: if the discharge is conversely too slow, the voltage at the terminals of the capacitor increases from one frame to another. Such behaviour corresponds to the storage phenomenon, which is specific to the voltage control par partial charging of the capacitor, and never occurs in the case of an intensity control, wherein the voltage at the terminals of the capacitor is forced independently for each frame relative to the current applied. It is therefore necessary, to maximise the discharge duration under the constraint of the stability of the assembly over a large number of frames for which the circuit is subjected systematically to a maximum lighting control, since the memory of the simulation computer does not enable in practice to exceed 500 cycles. A last constraint is of more concrete nature: given the size of a pixel, the capacitor is limited to a few pF maximum, and the more so because the selection duration does not enable to charge a greater capacitor.
Finally, the solution adopted is a constant RC equal to 6 ms, with R=kΩ, C=2 pF.
These values correspond to the best feasible time constant while preserving stability, and generate a significant current in the OLED for a duration close to half the frame. The current in the OLED is not cancelled out completely before the end of the frame, but the plotting of the voltage curve at the terminals of the OLED shows that said voltage falls again below the threshold voltage of the diode, estimated at approx. 4.9 V, after 6 ms at most. The current passing through the diode below this threshold may be considered very small in terms of lighting power with respect to the peak, and the OLED is in practice turned off before the end of the frame. This remanent current is not accompanied by a staircase-type behaviour that ought to be avoided, it appears however as soon as values slightly greater than the time constant are observed.
It should be understood that the examples of embodiment which have been given are purely illustrative and that other variations are considered within the framework of the invention. Notably, relative to the reversing type or not of the control circuit, in particular the control transistor M1, and to the type of selection circuit, in particular the transistor M2, the ignition of the OLED(s) may be obtained with a voltage greater than the threshold at the terminals of the capacitor or, conversely equal to zero and the charging/discharging of the capacitor may be obtained with a positive voltage Vsel or, conversely, with a zero voltage. Finally, the expression ‘positive voltage’ is relative and according to the reference used and/or the components used, positive and negative voltages, possible only negative, with respect to the ground may be implemented. It is however preferable to use cells in an apparatus fitted with a display unit which relies on a single voltage, and, in particular that of its own power supply source which may be formed of throw-away batteries or of rechargeable batteries.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0351026 | Dec 2003 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR04/50685 | 12/13/2004 | WO | 6/12/2006 |
