Abstract:
The present invention relates to a device for individually driving OLED/LED elements of an OLED/LED string, comprising for each OLED/LED element of the string: a controllable shunting switch ( 22, 42 ) coupled with the respective OLED/LED element ( 14, 15 ), switch controller means ( 30, 44 ) for controlling said shunting switch ( 22, 42 ) and having a control output port coupled to said switch ( 22, 42 ), a data input port and a clock input port, level shifting means ( 32 ) assigned to said switch controller means ( 30, 44 ) and adapted to bring the control input data to a level sufficient to be accepted by the switch controller means ( 30, 44 ) during a programming mode and to allow the control of said shunting switch ( 22, 42 ). Said switch controller means ( 30, 44 ) of said OLED/LED elements ( 14, 15 ) are provided to form a serial-to-parallel converter means ( 31 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for individually driving OLED/LED elements of an OLED/LED string. The invention also relates to a method of driving OLED/LED elements of an OLED/LED string individually. 
     BACKGROUND OF THE INVENTION 
     Lighting devices using LEDs or OLEDs (Organic Light Emitting Diode) gain more and more interest for general lighting applications. LEDs and OLEDs can be used to produce large amounts of light and have the benefit that they allow fast switching. On the one hand they can be used as general light sources, on the other hand they can be used as displays or design elements. In order to control the LEDs/OLEDs in a lighting device, so-called driver devices are used. In the art, there are a couple of solutions with respect to how to design such driver devices. For example, US 2006/0038803 A1 or WO 2006/107199 A2 disclose approaches regarding how to control LEDs coupled in series to form an LED string. 
     Generally, using a scanning matrix is the most obvious way to control a plurality of LEDs (or OLEDs) individually. However, the drawback is the low utilization of the individual LEDs. Due to multiplexing, only a fraction of the time cycle is used as activation time for each LED. Thus, the optical output of the LEDs will be lower than their nominal value. It is not possible to compensate for the dark time in larger setups, because in high power LEDs, the peak current is limited to a certain value. 
     In cases where only one string (i.e. a couple of LEDs arranged in one dimension, for example as a column or a row) is controlled, two possibilities are proposed in the art: 
     First, all LEDs can be connected to a common potential at one terminal, and the other terminal is switched. In this case, some current limiting means are necessary for each individual LED. 
     Second, it is possible to connect the LEDs in series. Here, only one current limiting block is required, but it is complicated to switch the individual LEDs due to their floating reference potential within the string. 
     SUMMARY OF THE INVENTION 
     With respect to the second possibility, it is an object of the present invention to provide a device which overcomes the above-mentioned problem and which is simple, cost-effective and scalable. 
     This and other objects are solved by the device for individually driving OLED/LED elements of an OLED/LED string, which comprises for each OLED/LED element of the string: 
     a controllable shunting switch coupled with the respective OLED/LED element, 
     switch controller means for controlling said shunting switch and having a control output coupled to said switch, a data input port and a clock input port, 
     level shifting means assigned to said switch controller means and adapted to bring the control input data to a level sufficient to be accepted by the switch controller during a programming mode and to allow the control of said shunting switch, said switch controller means of said OLED/LED elements being provided to form a serial-to-parallel converter means. 
     In other words, the inventive device is built up modularly and each OLED/LED element of the string is assigned one modular unit. Each modular unit comprises a switch controller means which is adapted to form one stage of a serial-to-parallel converter if the modular units are appropriately coupled in series with each other. The switch controller means of the modular units store the binary control value for controlling the respective shunting switch. Each switch controller means receives its control value by serially supplying the control values to the first modular unit of the device and clocking the serial data stream through the stages of the serial-to-parallel converter. 
     Since the individual OLED/LED elements of the string have floating reference potentials, each modular unit is arranged such that it includes the level shifting means which is adapted to align the potential of the switch controller means output signal and the reference point of the shunting switch which is coupled parallel to the OLED/LED element. 
     The inventive device allows to control OLED/LED elements of a string separately, e.g. switching the elements on and off. This feature enables the use of pixelated LED lamps without drawbacks with respect to efficiency. As mentioned, the inventive device includes three essential features, namely a serial-to-parallel converter to connect a string to a simple serial data source, the level shifting features which are required to drive a series connection of OLEDs/LEDs, and the shunting switch. 
     In case that the individual activation of the OLED/LED elements is not used, there are no additional losses due to the control electronics. In summary, the inventive device has the advantage that a very good efficiency may be achieved when it is used for a normal string of OLEDs/LEDs without individual control, and a very high utilization of the OLEDs/LEDs is achieved, when use is made of the individual addressing features. 
     It is to be noted that in the context of the present application the expression “LED” or “LED element” means on the one hand LED as well as OLED elements, and on the other hand not only one LED or OLED but also a series or parallel connection or a mixed series and parallel connection of two or more LEDs or OLEDs. Further “LED” also means laser diode or any other similar or related light element. 
     In a preferred embodiment, said shunting switches are switched on when said switch controller means are programmed by a serial data stream. In a preferred embodiment, the LED elements are switched off by setting the supply current to zero or to a small negative value during the programming mode, avoiding the activation of the LED elements when the control values are shifted through the stages of the serial-to-parallel converter. 
     In a preferred embodiment, said shunting switch is a transistor, preferably a field effect transistor. More preferably, said switch controller means is a D-latch circuit triggered via said clock input port. It is further preferred that said level shifting means comprises a capacitor and a first diode coupled in series and provided between a potential reference point of said latch means and its clock input port, said clock input port being supplied with a Clock_and_Supply signal via a second diode during programming of said serial-to-parallel converter means formed by said latch means. It is further preferred that a resistor is coupled between said clock input port of said latch means and said reference point. 
     In a preferred embodiment, said data input port of a latch means is coupled with said output port of the predecessor latch in order to form said serial-to-parallel converter. 
     In a preferred embodiment, a current limiting resistor is provided between the data input port of said latch means and the data output port of said predecessor latch means. 
     The aforementioned features have been proven advantageous in practice. However, it is to be noted that it is a preferred approach to designing the inventive device, but not the only design. Of course, the inventive concept may also be implemented differently. 
     The object of the present invention is also solved by a method of driving OLED/LED elements of an OLED/LED string individually, which method comprises the steps of: 
     providing a serial-to-parallel converter, 
     providing a shunting switch for each OLED/LED element in the string, each shunting switch being assigned to a respective stage of said serial-to-parallel converter, 
     programming said converter by supplying a serial datastream, said shunting switches being switched on during programming, and 
     operating said string by supplying power to said string and controlling said shunting switches by the parallel output signals of the serial-to-parallel converter. 
     The inventive method achieves the same advantages as described with respect to the inventive device, so that for its description reference is made to the respective description above. 
     Further features and advantages can be taken from the following description and the enclosed drawings. 
     It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are shown in the drawings and will be explained in more detail in the description below with reference to same. In the drawings: 
         FIG. 1  schematically shows a device for individually driving LED elements of an LED string according to a preferred embodiment; 
         FIG. 2  shows a further implementation of the inventive device; 
         FIG. 3  is a signal diagram showing the programming sequence of the device of  FIG. 2 ; and 
         FIG. 4  is a further implementation of the inventive device. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In  FIG. 1 , a driver device is schematically shown and indicated by means of reference numeral  10 . The driver device  10  may be used for general illumination lamps with an enhanced control possibility or for pixelated lamps or to support spatial dimming or local highlighting in backlight and signage applications. 
     In particular, the driver device  10  is used in the shown embodiment to control light elements  14  which are coupled in series to form a string  16 . The light elements  14  are provided as light emitting diodes  15  or organic light emitting diodes (OLED). Further, it is to be noted that each light element  14  may comprise one or more LEDs or OLEDs arranged in series, in parallel or a combination thereof. In the context of the following description, the expression LED  15  means generally a light element  14  of the afore-mentioned kind. 
     The LEDs  15  of the string  16  are powered by a power supply  18 , which is for example a current source  19 . The current source may be controlled via control signals supplied to a control input  17 . 
     Each LED  15  in the string  16  is assigned a modular circuit  12 . 1 - 12 . n  which serve to control the respective LED  15 . The number of provided modular circuits  12  corresponds to the number of light elements  14  forming the string  16 . 
     Since the modular circuits  12 . 1 - 12 . n  are similarly constructed, the following description only refers to one modular circuit, namely the modular circuit  12 . 2 . 
     The modular circuit  12 . 2  comprises a shunting switch  22  which is coupled parallel to the LED  15 . The shunting switch  22  serves to bypass the LED  15  if the LED should be switched off. If it is desired that the LED radiates light, the respective shunting switch  22  is opened, i.e. switched off, so that no bypass exists. The supplied power may therefore reach the LED 15  to cause the radiation of light. 
     The shunting switch  22  is controlled by a control unit  24  which supplies a control signal via a control signal line  26  to the shunting switch  22 . Further, the control unit  24  is electrically coupled with one side of the shunting switch  22  and with the cathode side of the LED  15  to have a common reference potential  28 . 
     The control unit  24  comprises a register  30  for storing a control value applied via the control signal line to the shunting switch  22 , and a level shifting element  32 . 
     The register  30  is adapted to form one stage of a serial-to-parallel converter  31  with the other registers  30  of the modular circuits  12 . 1 - 12 . n.    
     For realizing a serial-to-parallel converter, each register  30  has a Data_in input and a clock input, the data_out output being the control signal on the control signal line  26 . As is known by a skilled person, the serial datastream supplied to a serial-to-parallel converter is shifted from stage to stage with each clock signal. 
     With respect to the embodiment shown in  FIG. 1 , the serial datastream is supplied to the first modular circuit  12 . 1  and is then transferred to the following modular circuits  12 . 2 - 12 . n . Hence, the input of n data values takes n clock signals. 
     In order to pass the serial datastream from one modular circuit to the next, the modular circuits have Data_in ports  33  and Clock_and_Supply ports  35 . Further, each modular circuit  12  has a Data_out port  43  and a Clock_and_Supply output port  45 . 
     As is apparent from  FIG. 1 , the Data_in port  33  of a modular circuit is electrically coupled with the Data_out port  43  of the predecessor modular circuit. Further, the Clock_and_Supply input port  35  is electrically coupled with the Clock_and_Supply output port  45  of the predecessor modular circuit  12 . 
     It is further shown in  FIG. 1 , that the respective ports of the modular circuit are connected via lines  34 ,  36 ,  38  and  40 , respectively. 
     The control unit  24  comprises the level shifting element  32 , as mentioned before, which is necessary, since the potential of the reference point  28  differs dependent on the position of the respective modular circuit within the string  16  and the status (on/off) of the LEDs. The level shifting element of each modular circuit  12  guarantees that the shunting switches  22  may be switched on and off although the reference potential of the respective LED is floating. The level shifting element  32  ensures that the potential of the reference point  28  and the control output of said register  30  is raised during normal operation (not during a programming mode). 
     As is also apparent from  FIG. 1 , each modular circuit comprises two LED ports  20 ,  21  between which the light element  14  is connected and which are coupled with succeeding modular circuits and the power supply  18 , respectively, in order to achieve the illustrated series connection of n light elements  14  forming the string  16 . 
     With respect to  FIG. 2 , a preferred embodiment of the device  10  is shown and will be described in detail below. Since the modular circuits  12 . 1 - 12 . n  are similarly designed, the structure of the modular circuit  12 . 2  will now be described in detail. 
     The shunting switch  22  is provided as a field effect transistor, preferably a MOSFET,  42 , the drain of which is coupled with the anode of the LED  15  and the source is coupled with the cathode. 
     The register  30  is provided in form of a so-called D-latch  44  which is generally known in the art. The D-latch  44  has a data output Q which is coupled to the gate of the MOSFET  42 . The data input port D of the D-latch  44  is connected with the Data_in port  33  to receive the data output of the predecessor circuit (here modular circuit  12 . 1 ). 
     Between the Clock_and_Supply port  35  and the reference point  28  of the modular circuit  12 . 2 , a series connection of two diodes D 1 _ 2  and D 2 _ 2  and a capacitor C 1 _ 2  is provided. The bridging point between both diodes D 1 _ 2  and D 2 _ 2  is connected to an inverter A 2 _ 2 , the output of which is connected to the clock input CLK of the D-latch  44 . Further, a resistor R 1 _ 2  is coupled between the bridging point and the reference point  28 . Finally, the CLR input of the D-latch  44  is also connected to the reference point  28 . 
     As an alternative, the PRE-Input or the CLR-Input might be connected to a pulse-forming network deriving a pulse upon the appearance of a positive voltage at the bridging point between D 1 _n and D 2 _ 2 . This would result in closing or opening the switch automatically and hence propagating this status throughout the complete serial connection automatically. This could be used to have a defined starting status for each data transmission and a defined charging of all capacitors C 1 _n 
     The signal to be applied to the Clock_and_Supply output port  45  is taken from the bridging point between the diodes D 1 _ 2  and D 2 _ 2 . 
     The Clock_and_Supply input port of the modular circuit  12 . 2  is coupled via the Clock_and_Supply line  36  with the anode of the diode D 1 _ 2 . 
     In order to build up a serial-to-parallel converter, the output signal of the D-latch  44 , namely the Q signal, is supplied via a resistor R 2 _ 2  to the Data_out port  43  which itself is connected to the Data_in port  33  of the successor modular circuit  12 . 3 . Finally, between the Data_out port  43  and the reference point  28 , a capacitor C 2 _ 2  is provided. This capacitor C 2 _ 2  serves for a dedicated delay in the signal propagation. Based on the speed of the logic devices used, it might be omitted. 
     Generally, the output of the D-latch  44  drives the MOSFET  42 . Dependent on the signal (low or high) of the D-latch  44 , the MOSFET is switched on or off. The capacitor C 1 _ 2  is provided to stabilize the supply voltage of the respective modular circuit  12 . The supply voltage is referenced to the cathode side (reference point  28 ) of the LED  15 . The clock signal portion of the Clock_and_Supply signal which is applied to the port  35 , is derived, due to decoupling, with the diodes D 1 _ 2  and D 2 _ 2  and the pull-down resistor R 1 _ 2 . 
     The driver device  10  shown in  FIG. 2  operates as follows: 
     During a programming mode used to program the respective D-latches  44 , a negative current I 1 &lt;0 is forced through the circuit which is caused by an appropriate control signal at the control input  17  of the current source. Thus, depending on the previous switch status either the MOSFET is conducting in reverse direction or the body diodes of the MOSFETs  42  of the modular circuits  12  are conducting. In either case, all LEDs  15  are switched off. Using the aforementioned pulse-forming network to clear the latches (using the CLR-input), the situation described in the following can be achieved. A positive voltage V cc  is applied to the Clock_and_Supply port  35  of the first modular circuit  12 . 1 . Via the diodes D 1 _ 1  . . . D 2 _n, the supply voltage capacitors of the modules (C 1 _n) are recharged to
 
 V   supply   =V   cc   −V   f  
 
     where V f  is the forward voltage of the diode type used for D 2 _x. This supply voltage is nearly equal for each modular circuit. With an appropriate selection of the diode type used for D 1 _x and the negative current I 1 , the voltage drop across the body diodes of the MOSFETs  42  and the voltage drop across the diode D 1 _x is the same and, thus, they cancel each other out. 
     At each clock input CLK (across R 1 _x), there is a high level. 
     Then a datum (control value) is applied to the Data_in port  33  of the first modular circuit  12 . 1 . Next, the Clock_and_Supply signal is set to 0 V. Thus, the data at the input of each modular circuit (Data_in) is copied to the output of each D-latch  44 . Due to the delay related to the speed of the logic (represented or intentionally created by the RC network R 2 _x, C 2 _x), each modular circuit copies the data present at the falling edge of the Clock_and_Supply port to its output Q. 
     Of course, there is a potential difference between two neighboring modular circuits which are sending and receiving the shifted data. But this shift cannot exceed the voltage drop of one body diode (˜0.5 V). A high level outputted from modular circuit  12 . x  can easily be read as high from block x+1 (e.g. V cc =5 V, CMOS_high=4.95 V, will result in 5.45 V input signal for the upper modular circuit). A low level (CMOS_low=0.05 V) will be 0.45 V for the upper modular circuit. Usually, all logic devices have clamping diodes from the signal terminals to both supply and reference potential. Using a simple current limiting resistor (e.g. the R 2 _x shown in  FIG. 2 ) will allow safe and stable operation. 
     Then, the Clock_and_Supply signal at port  35  of modular circuit  12 . 1  is set to V cc  again. A new datum is applied to the Data_in port  33  of the modular circuit  12 . 1 . The cycle mentioned above repeats, and the serial data stream at the Data_in port  33  of modular circuit  12 . 1  is parallelized over the other modular circuits  12 . 2 - 12 . n . At each falling edge of Clock_and_Supply input signal, the data is shifted up one modular circuit, meaning from  12 . x  to  12 . x+ 1. 
     When all the desired information is clocked into the latches  44  (typically after n clock cycles, when all latches are updated), the Clock_and_Supply input is kept at 0 V. 
     Then, the programming mode is left by applying a respective control signal to the current source  18 , and setting the current source to a positive current I 1 &gt;0, which is the desired forward current of the LEDs  15 . Within each block, the current I 1  will flow either through LED  15  if a “0” is stored in the latch  44  and the MOSFET  42  is open, or through MOSFET  42 , if the latch  44  was programmed to “1”. 
     Due to the forward current in the LEDs  15  or the MOSFETs  42 , there will be positive voltage drops across each modular circuit. The data signal is not monitored during this operation and anyway is in the allowed input voltage range or protected due to the current limiting resistor R 2 _x described above. All reference potentials  28  will be positive with respect to GND, so the Clock_and_Supply signal is low (or negative) for each modular circuit. The negative voltage is blocked by the diodes D 1 _x. There is no transition on the clock input of the driver device, and the latched information in the latches  44  is kept stable. 
     For a change in the states of the LEDs, the complete cycle as mentioned above is repeated, starting with the setting of a negative current and followed by the clocking of new data into the structure. 
     The programming sequence mentioned above is shown in  FIG. 3 , for an example of seven modular circuits  12 . 1 - 12 . 7 . When the current is set to I 1 &gt;0, LED_ 1  . . . LED_n will be lit according to the inverted data D 6  . . . D 0 . The length of the clocking interval scales with the number of LEDs. The data source could control several LED strings  16  when generating one Clock_and_Supply signal and several data_in signals at the same time. In that way, an easy control of display light elements is possible. 
     A slightly modified embodiment of the driver device  10  shown in  FIG. 2  is illustrated in  FIG. 4 . The main difference is that the Clock_and_Supply signal is set in parallel with all modular circuits  12 . 1 - 12 . n . Hence, the modular circuits  12  do not have the Clock_and_Supply output port  45  any more. In other words, the Clock_and_Supply signal enters each modular circuit but does not leave it after D 1 _x. This results in a slightly different distribution of the electrical potential during the programming mode. In this case, it would be required to place a bypassing resistor  52  across the LED  15  and set the current source to zero (and not to a negative current) during programming. 
     As already mentioned, the inventive driver device is applicable as an enhanced control possibility for generating illumination lamps or as a core functionality for pixelated lamps, or to support spatial dimming or local highlighting in backlight and signage applications.