Abstract:
An optoelectronic circuit receiving a variable voltage containing alternating ascending and descending phases, which circuit comprises sets of light-emitting diodes mounted in series, a module for each set for comparing the voltage at one of the terminals of the set with at least a first threshold and a control module which is connected to the comparison modules and is suitable, during each ascending phase, for interrupting the flow of a current in each set when said voltage of said set goes above the second threshold or when said voltage of the set which is adjacent to said set and through which current passes goes above the first threshold and is suitable, during each descending phase, for controlling the flow of a current in each set when said voltage of the set which is adjacent to said set and through which current passes goes below the first threshold.

Description:
[0001]    The present patent application claims the priority benefit of French patent application FR14/56180 which will be incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present description relates to an optoelectronic circuit, particularly to an optoelectronic circuit comprising light-emitting diodes. 
       DISCUSSION OF THE RELATED ART 
       [0003]    It is desirable to be able to power an optoelectronic circuit comprising light-emitting diodes with an AC voltage, particularly a sinusoidal voltage, for example, the mains voltage. 
         [0004]      FIG. 1  shows an example of an optoelectronic circuit  10  comprising input terminals IN 1  and IN 2  having an AC voltage V IN  applied therebetween. Optoelectronic circuit  10  further comprises a rectifying circuit  12  comprising a diode bridge  14 , receiving voltage V IN  and supplying a rectified voltage V ALIM  which powers light-emitting diodes  16 , for example, series-assembled with a resistor  15 . Call I ALIM  the current flowing through light-emitting diodes  16 . 
         [0005]      FIG. 2  is a timing diagram of power supply voltage V ALIM  and of power supply current I ALIM  for an example where AC voltage V IN  corresponds to a sinusoidal voltage. When voltage V ALIM  is greater than the sum of the threshold voltages of light-emitting diodes  16 , light-emitting diodes  16  become conductive. Power supply current I ALIM  then follows power supply voltage V ALIM . There thus is an alternation of phases OFF with no light emission and of light-emission phases ON. 
         [0006]    A disadvantage is that as long as voltage V ALIM  is smaller than the sum of the threshold voltages of light-emitting diodes  16 , no light is emitted by optoelectronic circuit  10 . An observer may perceive this lack of light emission when the duration of each phase OFF with no light emission between two light-emission phases ON is too long. A possibility, to increase the duration of each phase ON, is to decrease the number of light-emitting diodes  16 . A disadvantage then is that the electric power lost in the resistor is significant. 
         [0007]    Publication US 2012/0056559 describes an optoelectronic circuit where the number of light-emitting diodes receiving power supply voltage V ALIM  progressively increases during a rising phase of the power supply voltage and progressively decreases during a falling phase of the power supply voltage. This is achieved by a switching circuit capable of short-circuiting a variable number of light-emitting diodes according to the variation of voltage V ALIM . This enables to decrease the duration of each phase with no light emission. 
         [0008]    A disadvantage of the optoelectronic circuit described in publication US 2012/0056559 is that the light-emitting diode power supply current does not continuously vary, that is, there are abrupt interruptions of the current flow during the voltage variation. This causes time variations of the light intensity supplied by the light-emitting diodes, which may be perceived by an observer. This further causes a degradation of the harmonic factor of the current powering the light-emitting diodes of the optoelectronic circuit. 
         [0009]    A current-limiting circuit may be interposed between the rectifying circuit and the light-emitting diodes to keep the power supply current at a substantially constant level. The structure of the optoelectronic circuit may then be relatively complex and the bulk of the optoelectronic circuit may be significant. Further, it may be difficult to at least partly form the rectifying circuit and the current-limiting circuit in integrated fashion with the light-emitting diodes in order to still further decrease the bulk of the optoelectronic circuit. 
       SUMMARY 
       [0010]    An object of an embodiment is to overcome all or part of the disadvantages of the previously-described optoelectronic circuits. 
         [0011]    Another object of an embodiment is to decrease the duration of phases with no light emission by the optoelectronic circuit. 
         [0012]    Another object of an embodiment is for the current powering the light-emitting diodes to substantially continuously vary. 
         [0013]    Another object of an embodiment is to decrease the bulk of the optoelectronic circuit. 
         [0014]    Thus, an embodiment provides an optoelectronic circuit intended to receive a variable voltage containing an alternation of increasing and falling phases, the optoelectronic circuit comprising: 
         [0015]    a plurality of assemblies of light-emitting diodes, said assemblies being series-assembled; 
         [0016]    for each assembly, a comparison unit capable of comparing the voltage at one of the terminals of the assembly, and/or a voltage depending on said voltage at one of the terminals of the assembly, with at least a first threshold and possibly with a second threshold; and 
         [0017]    a control unit connected to the comparison units and capable, during each rising phase, of interrupting the flowing of a current in each assembly from among certain assemblies of the plurality of assemblies when said voltage of said assembly rises above the second threshold or when said voltage of the assembly, adjacent to said assembly and conducting the current, rises above the first threshold and, during each falling phase, of controlling the flowing of a current in each assembly from among certain assemblies of the plurality of assemblies when said voltage of the assembly, adjacent to said assembly and conducting the current, decreases below the first threshold. 
         [0018]    According to an embodiment, the optoelectronic circuit comprises: 
         [0019]    a current source; 
         [0020]    for each assembly, a switch connecting the current source to said terminal of said assembly, 
         [0021]    and the control unit is capable, for each assembly from among certain assemblies of the plurality of assemblies, of ordering the turning-on of the switch associated with said assembly when said voltage of the assembly, adjacent to said assembly and conducting the current, decreases below the first threshold in each falling phase. 
         [0022]    According to an embodiment, the control unit is capable, for each assembly from among certain assemblies of the plurality of assemblies, of ordering the turning-on of the switch associated with said assembly when said voltage of the assembly, adjacent to said assembly and conducting the current, rises above the second threshold in each rising phase. 
         [0023]    According to an embodiment, the control unit is capable, after the turning-on of the switch associated with said assembly, of ordering the turning-off of the switch associated with said adjacent assembly. 
         [0024]    According to an embodiment, the control unit is capable, for each assembly from among certain assemblies of the plurality of assemblies, of ordering the turning-off of the switch associated with said assembly when said voltage of the assembly, adjacent to said assembly, rises above the first threshold in each rising phase. 
         [0025]    According to an embodiment, the optoelectronic circuit comprises, for each assembly, a current source, the control unit being capable, for each assembly, of ordering the activation of the current source associated with said assembly when said voltage of the assembly, adjacent to said assembly and conducting the current, rises above the second threshold in each rising phase and decreases below the first threshold in each falling phase. 
         [0026]    According to an embodiment, the control unit is further capable, after the activation of the current source associated with said assembly, of ordering the deactivation of the current source associated with said adjacent assembly. 
         [0027]    According to an embodiment, the optoelectronic circuit further comprises a fullwave rectifying circuit capable of supplying said voltage. 
         [0028]    According to an embodiment, at least one of the light-emitting diodes is a planar light-emitting diode comprising a stack of layers resting on a planar surface, having at least one active layer capable of emitting light. 
         [0029]    According to an embodiment, the light-emitting diodes of at least one of the assemblies of light-emitting diodes comprise three-dimensional semiconductor elements in the form of microwires, of nanowires, or of pyramids, each semiconductor element being covered with an active layer capable of emitting light. 
         [0030]    According to an embodiment, the optoelectronic circuit comprises a first integrated circuit comprising the control unit and at least one second integrated circuit, distinct from the first integrated circuit and attached to the first integrated circuit, and comprising at least one of the assemblies of light-emitting diodes. 
         [0031]    According to an embodiment, the second integrated circuit comprises all the assemblies of light-emitting diodes. 
         [0032]    According to an embodiment, the optoelectronic circuit further comprises a third integrated circuit, distinct from the first integrated circuit and from the second integrated circuit and attached to the first integrated circuit, and comprising at least one of the assemblies of light-emitting diodes. 
         [0033]    An embodiment also aims at a method of controlling a plurality of assemblies of light-emitting diodes, said assemblies being series-assembled and powered with a variable voltage, containing an alternation of rising and falling phases, the method comprising: 
         [0034]    for each assembly, comparing the voltage at one of the terminals of the assembly, and/or a voltage depending on said voltage at one of the terminals of the assembly, with at least a first threshold and possibly with a second threshold; and 
         [0035]    during each rising phase, interrupting the current flow in each assembly from among certain assemblies of the plurality of assemblies when said voltage of said assembly rises above the second threshold or when said voltage of the assembly, adjacent to said assembly and conducting the current, rises above the first threshold and, during each falling phase, controlling the flowing of a current in each assembly from among certain assemblies of the plurality of assemblies when said voltage of the assembly, adjacent to said assembly and conducting the current, decreases below the first threshold. 
         [0036]    According to an embodiment, a current source is connected, for each assembly, to said terminal of said assembly via a switch, the method further comprising, for each assembly from among certain assemblies of the plurality of assemblies, turning on the switch associated with said assembly when said voltage of the assembly, adjacent to said assembly and conducting the current, decreases below the first threshold in each falling phase. 
         [0037]    According to an embodiment, the method comprises, for each assembly from among certain assemblies of the plurality of assemblies, turning on the switch associated with said assembly when said voltage of the assembly, adjacent to said assembly and conducting the current, rises above the second threshold in each rising phase. 
         [0038]    According to an embodiment, the method further comprises, after the turning on of the switch associated with said assembly, turning off the switch associated with said adjacent assembly. 
         [0039]    According to an embodiment, the method comprises, for each assembly from among certain assemblies of the plurality of assemblies, turning off the switch associated with said assembly when said voltage of the assembly, adjacent to said assembly, rises above the first threshold in each rising phase. 
         [0040]    According to an embodiment, for each assembly, a current source is connected to said assembly, the method comprising, for each assembly, activating the current source associated with said assembly when said voltage of the assembly, adjacent to said assembly and conducting the current, rises above the second threshold in each rising phase and decreases below the first threshold in each falling phase. 
         [0041]    According to an embodiment, the method further comprises, after the activation of the current source associated with said assembly, deactivating the current source associated with said adjacent assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0043]      FIG. 1 , previously described, is an electric diagram of an example of an optoelectronic circuit comprising light-emitting diodes; 
           [0044]      FIG. 2 , previously described, is a timing diagram of the power supply voltage and current of the light-emitting diodes of the optoelectronic circuit of  FIG. 1 ; 
           [0045]      FIG. 3  shows an electric diagram of an embodiment of an optoelectronic circuit comprising light-emitting diodes; 
           [0046]      FIGS. 4 and 5  illustrate two layouts of the light-emitting diodes of the optoelectronic circuit of  FIG. 3 ; 
           [0047]      FIGS. 6 and 7  are more detailed electric diagrams of embodiments of portions of the optoelectronic circuit of  FIG. 3 ; 
           [0048]      FIG. 8  is a timing diagram of voltages of the optoelectronic circuit of  FIG. 3 ; 
           [0049]      FIG. 9  shows an electric diagram of another embodiment of an optoelectronic circuit comprising light-emitting diodes; 
           [0050]      FIGS. 10 and 11  are drawings respectively similar to  FIGS. 6 and 7  and show electric diagrams of more detailed embodiments of portions of the optoelectronic circuit of  FIG. 9 ; 
           [0051]      FIG. 12  shows an electric diagram of another embodiment of an optoelectronic circuit comprising light-emitting diodes; and 
           [0052]      FIGS. 13 and 14  are partial simplified cross-section views of two embodiments of an optoelectronic circuit comprising light-emitting diodes. 
       
    
    
     DETAILED DESCRIPTION 
       [0053]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. In the following description, unless otherwise indicated, terms “substantially”, “approximately”, and “in the order of” mean “to within 10%”. 
         [0054]      FIG. 3  shows an electric diagram of an embodiment of an optoelectronic circuit  20  comprising two input terminals IN 1  and IN 2  receiving input voltage V IN . As an example, input voltage V IN  may be a sinusoidal voltage having a frequency, for example, in the range from 10 Hz to 1 MHz. Voltage V IN  for example corresponds to the mains voltage. 
         [0055]    Circuit  20  may comprise a fullwave rectifying circuit  22 , for example comprising a diode bridge formed, for example, of four diodes  14 . Rectifying circuit  22  receives power supply voltage V IN  between terminals IN 1  and IN 2  and supplies a rectified voltage V ALIM  between nodes A 1  and A 2 . As a variation, circuit  20  may directly receive a rectified voltage, and it is then possible for the rectifying circuit not to be present. 
         [0056]    Optoelectronic circuit  20  comprises N series-connected assemblies of elementary light-emitting diodes, called general light-emitting diodes D i  in the following description, where i is an integer in the range from  1  to N and where N is an integer in the range from  2  to  200 . Each general light-emitting diode D 1  to D N  comprises at least one elementary light-emitting diode and is preferably formed of the series and/or parallel assembly of at least two elementary light-emitting diodes. In the present embodiment, the N general light-emitting diodes D i  are series-connected, the cathode of general light-emitting diode D i  being connected to the anode of general light-emitting diode D i+1 , for i varying from 1 to N−1. The anode of general light-emitting diode D 1  is connected to node A 1 . General light-emitting diodes D i , with i varying from 1 to N, may comprise the same number of elementary light-emitting diodes or different numbers of elementary light-emitting diodes. 
         [0057]      FIG. 4  shows an embodiment of general light-emitting diode D 1  where general light-emitting diode D 1  comprises R branches  26  assembled in parallel, each branch comprising S elementary light-emitting diodes  27  series-assembled in the same conduction direction, R and S being integers greater than or equal to 1. 
         [0058]      FIG. 5  shows another embodiment of general light-emitting diode D 1  where general light-emitting diode D 1  comprises P series-assembled blocks  28 , each block comprising Q elementary light-emitting diodes  27  assembled in parallel, P and Q being integers greater than or equal to 1 and Q being likely to vary from one block to the other. 
         [0059]    The other general light-emitting diodes D 2  to D N  may have a structure similar to that of general light-emitting diode D 1  shown in  FIG. 4 or 5 . 
         [0060]    Elementary light-emitting diodes  27  are, for example, planar light-emitting diodes, each comprising a stack of layers resting on a planar surface, having at least one active layer capable of emitting light. Elementary light-emitting diodes  27  are, for example, planar light emitting diodes, light-emitting diodes formed from three-dimensional semiconductor elements, particularly microwires, nanowires, or pyramids, for example comprising a semiconductor material based on a compound mainly comprising at least one group-III element and one group-V element (for example, gallium nitride GaN), called III-V general hereafter, or mainly comprising at least one group-II element and one group-VI element (for example, zinc oxide ZnO), called II-VI general hereafter, each three-dimensional semiconductor element is covered with an active layer capable of emitting light. 
         [0061]    Referring to  FIG. 3 , optoelectronic circuit  20  comprises a current source  30  having a terminal connected to node A 2  and having its other terminal connected to a node A 3 . Call V CS  the voltage across current source  30  and I CS  the current supplied by current source  30 . Optoelectronic circuit  20  may comprise a circuit, not shown, for supplying a reference voltage to power the current source, possibly obtained from voltage V ALIM . 
         [0062]    Circuit  20  comprises N controllable switches SW 1  to SW N . Each switch SW i , with i varying from 1 to N, is assembled between node A 3  and the cathode of general light-emitting diode D i . Each switch SW i , with i varying from 1 to N, is controlled by a signal S i . As an example, signal S i  is a binary signal and switch SW i  is off when signal S i  is in a first state, for example, the low state, and switch SW i  is on when signal S i  is in a second state, for example, the high state. Call V Ci  the voltage between the cathode of general light-emitting diode D i  and node A 2 . In the following description, unless otherwise mentioned, the voltages are referenced to node A 2 . Switch SW i  is, for example, a switch based on at least one transistor, particularly a field-effect metal-oxide gate transistor or enrichment (normally on) or depletion (normally off) MOS transistor. 
         [0063]    Optoelectronic circuit  20  further comprises N comparison units COMP i , with i varying from 1 to N, capable of each receiving voltage V Ci  and of supplying a signal H i  and a signal L i . Optoelectronic circuit  20  further comprises a control unit  32  receiving signals L 1  to L N  and H 1  to H N  and supplying signals S 1  to S N  for controlling switches SW 1  to SW N . Control unit  32  preferably corresponds to a dedicated circuit. 
         [0064]    Control unit  32  is capable of ordering the turning-on or off of switches SW i , with i varying from 1 to N, according to the value of voltage V Ci  at the cathode of each general light-emitting diode D i . To achieve this, each comparison unit COMP i , with i varying from 1 to N, is capable of comparing voltage V Ci  at the cathode of general light-emitting diode D i  with at least two thresholds Vhigh i  and Vlow i . As an example, signal L i  is a binary signal which is in a first state when voltage V Ci  is smaller than threshold Vlow i  and which is in a second state when voltage V Ci  is greater than threshold Vlow i . As an example, signal H i  is a binary signal which is in a first state when voltage V Ci  is smaller than threshold Vhigh i  and which is in a second state when voltage V Ci  is greater than threshold Vhigh i . The first states of binary signals H i  and L i  may be equal or different and the second states of binary signals H i  and L i  may be equal or different. 
         [0065]      FIG. 6  shows an electric diagram of a more detailed embodiment of a portion of optoelectronic circuit  20 . According to the present embodiment, each comparator COMP i  comprises a first operational amplifier  40 , operating as a comparator, having its inverting input (−) connected to the cathode of general light-emitting diode D i , and having its non-inverting input (+) receiving voltage threshold Vhigh i  which is supplied by a unit  42 . Comparator  40  supplies signal H i . Each comparator COMP i  further comprises a second operational amplifier  44 , operating as a comparator, having its inverting input (−) connected to the cathode of general light-emitting diode D i , and having its non-inverting input (+) receiving voltage threshold Vlow i  which is supplied by a unit  46 . Comparator  44  supplies signal L i . 
         [0066]      FIG. 7  shows an electric diagram of a more detailed embodiment of current source  30  and of switch SW i . In the present embodiment, current source  30  comprises an ideal current source  50  having a terminal connected to a first source of a reference voltage VREF. The other terminal of current source  50  is connected to the drain of a diode-assembled N-channel MOS transistor  52 . The source of MOS transistor  52  is connected to node A 2 . The gate of MOS transistor  52  is connected to the drain of MOS transistor  52 . Reference potential VREF may be supplied from voltage V ALIM . It may be constant or vary according to voltage V ALIM . The intensity of the current supplied by current source  30  may be constant or be variable, for example, vary according to voltage V ALIM . 
         [0067]    For each general light-emitting diode D i , current source  30  comprises an N-channel MOS transistor  54  having its gate connected to the gate of transistor  52  and having its source connected to node A 2 . MOS transistors  52  and  54  form a current mirror, current I CS  supplied by current source  50  being copied, possibly with a multiplication factor. 
         [0068]    According to the present embodiment, switch SW i  comprises an N-channel MOS transistor  56  having its drain connected to the cathode of general light-emitting diode D i  and having its source connected to the drain of transistor  54 . The voltage applied to the gate of transistor  56  corresponds to previously-described signal S i . 
         [0069]      FIG. 8  shows timing diagrams of power supply voltage V ALIM  and of the voltages V Ci  measured by each comparator COMP i , with i varying from 1 to N, illustrating the operation of optoelectronic circuit  20  according to the embodiment shown in  FIG. 3  in the case where N is equal to 4 and in the case where each general light-emitting diode D i  comprises the same number of elementary light-emitting diodes arranged in the same configuration, and thus has the same threshold voltage Vled. Call t 0  to t 20  successive times. 
         [0070]    As an example, voltage V ALIM  supplied by rectifying bridge  100  is a rectified sinusoidal voltage comprising a succession of cycles having voltage V ALIM  increasing from the zero value, crossing a maximum value, and decreasing to the zero value, in each of them. As an example, two successive cycles of voltage V ALIM  are shown in  FIG. 8 . 
         [0071]    At time t 0 , at the beginning of a cycle, switch SW 1  is turned on and all switches SW i , with i varying from 2 to N, are turned off. Voltage V ALIM  rises from the zero value and distributes between general light-emitting diode D 1 , switch SW 1 , and current source  30 . Voltage V ALIM  being smaller than threshold voltage Vled of general light-emitting diode D 1 , there is no light emission (phase P 0 ) and voltage V C1  remains substantially equal to zero. 
         [0072]    At time t 1 , when the voltage across general light-emitting diode D 1  exceeds threshold voltage Vled, general light-emitting diode D 1  becomes conductive (phase P 1 ). The voltage across general light-emitting diode D 1  then remains substantially constant and voltage V C1  keeps on increasing along with voltage V ALIM . As soon as power supply voltage V C1  is sufficiently high to allow the activation of current source  30 , current I CS  flows through the general light-emitting diode D 1 , which emits light. As an example, voltage V CS , when current source  30  is in operation, is preferably substantially constant. 
         [0073]    At time t 2 , when voltage V C1  exceeds threshold Vhigh 1 , unit  32  successively orders the turning-on of switch SW 2  and then the turning-off of switch SW i . Voltage V ALIM  then distributes between general light-emitting diodes D 1  and D 2 , switch SW 2 , and current source  30 . Preferably, threshold Vhigh 1  is substantially equal to the sum of the threshold voltage of general light-emitting diode D 2  and of operating voltage V CS  of current source  30  so that, at the turning-on of switch SW 2 , general light-emitting diode D 2  conducts current I CS  and emits light. The fact for switch SW 2  to be turned on before the turning-off of switch SW i  ensures that there will be no interruption of the current flow in general light-emitting diode D 1 . Phase P 2  corresponds to a phase of light emission by general light-emitting diodes D 1  and D 2 . 
         [0074]    Generally, during a rising phase of power supply voltage V ALIM , for i varying from 1 to N−1, while switch SW i+1  is on and the other switches are off, unit  32  successively orders the turning-on of switch SW i+1  and the turning-off of switch SW i  when voltage V Ci  exceeds threshold Vhigh i . Voltage V ALIM  then distributes between general light-emitting diodes D 1  to D i+1 , switch SW i+1 , and current source  30 . Preferably, threshold Vhigh i  is substantially equal to the sum of the threshold voltage of general light-emitting diode D i+1  and of operating voltage V CS  of current source  30  so that, at the turning-on of switch SW i+1 , general light-emitting diode D i+1  conducts current I CS  and emits light. Phase P i+1  corresponds to the emission of light by general light-emitting diodes D 1  to D i+1 . The fact for switch SW i+1  to be turned on before the turning-off of switch SW i  ensures that there will be no interruption of the current flow in general light-emitting diodes D 1  to D i . 
         [0075]    Thus, at time t 3 , unit  32  orders the turning-on of switch SW 3  and the turning-off of switch SW 2 . Phase P 3  corresponds to the emission of light by general light-emitting diodes D 1 , D 2 , and D 3 . At time t 4 , unit  32  orders the turning-on of switch SW 4  and the turning-off of switch SW 3 . Phase P 4  corresponds to the emission of light by general light-emitting diodes D 1 , D 2 , D 3 , and D 4 . 
         [0076]    Power supply voltage V ALIM  reaches its maximum value at time t 5  during phase P 4  in  FIG. 8  and starts a falling phase. 
         [0077]    At time t 6 , when voltage V C4  decreases below threshold Vlow 4 , unit  32  successively orders the turning-on of switch SW 3  and the turning-off of switch SW 4 . Voltage V ALIM  then distributes between general light-emitting diodes D 1 , D 2 , and D 3 , switch SW 3 , and current source  30 . Preferably, threshold Vlow 4  is selected to be substantially equal to the sum of operating voltage V CS  of current source  30  and of the minimum operating voltage of switch SW 4  so that, at the turning-on of switch SW 3 , there is no interruption of the current flow. 
         [0078]    Generally, during a falling phase of power supply voltage V ALIM , for i varying from 2 to N, when voltage V Ci  decreases below threshold Vlow i , unit  32  successively orders the turning-on of switch SW i−1  and the turning-off of switch SW i . Voltage V ALIM  then distributes between general light-emitting diodes D 1  to D i−1 , switch SW i−1 , and current source  30 . Preferably, threshold Vlow i  is selected to be substantially equal to the sum of operating voltage V CS  of current source  30  and of the minimum operating voltage of switch SW i  so that, at the turning-on of switch SW i−1 , there is no interruption of the current flow. 
         [0079]    Thus, at time t 7 , unit  32  orders the turning-on of switch SW 2  and the turning-off of switch SW 3 . At time t 8 , unit  32  orders the turning-on of switch SW 2  and the turning-off of switch SW 1 . At time t 9 , voltage V C1  becomes zero so that general light-emitting diode D 1  is no longer conductive and current source  30  is off. At time t 10 , voltage V ALIM  becomes zero and a new cycle starts. Times t 11  to t 20  are respectively similar to times t 1  to t 10 . In the present embodiment, comparator COMP 1  may have a simpler structure than comparators COMP i , with i varying from 2 to N, since threshold Vlow 1  is not used. 
         [0080]    According to another embodiment of optoelectronic circuit  20 , each comparator COMP i  of optoelectronic circuit  20  only supplies signal L i . An advantage of this embodiment is that the structure of comparator COMP i  can be simplified. Indeed, it is possible for comparator COMP i  not to comprise operational amplifier  40 . 
         [0081]    The operation of the optoelectronic circuit according to this other embodiment is then identical to what has been previously described, with the difference that switches SW i , with i varying from 1 to N−1, are initially on and that, in a rising phase of power supply voltage V ALIM , switch SW i−1  is off when voltage V Ci  is greater than threshold Vlow i . Indeed, this means that current starts flowing through switch SW i . 
         [0082]    More specifically, in a rising phase of power supply voltage V ALIM , for i varying from 1 to N−1 , while light-emitting diodes D 1  to D i−1  are conductive and light-emitting diodes D i  to D N  are blocked, when voltage V Ci  falls below threshold Vlow i , unit  32  orders the turning-off of SW i−1 . Indeed, a rise in voltage V Ci  means that the voltage across light-emitting diode D i  becomes greater than the threshold voltage of light-emitting diode D i  and that the latter becomes conductive. 
         [0083]    The operation of the optoelectronic circuit according to this other embodiment in a falling phase of power supply voltage V ALIM  may be identical to that which has been previously described for optoelectronic circuit  20 . 
         [0084]      FIG. 9  shows an electric diagram of another embodiment of an optoelectronic circuit  60 . All the elements common with optoelectronic circuit  20  are designated with the same reference numerals. Unlike optoelectronic circuit  20 , optoelectronic circuit  60  does not comprise switch SW N . Further, unlike optoelectronic circuit  20 , for i varying from 1 to N−1, optoelectronic circuit  60  comprises a resistor  62   i  provided between node A 3  and switch SW i , and optoelectronic circuit  60  comprises a resistor  62   N  provided between node A 3  and the cathode of general light-emitting diode D N . Call B i  a node between resistor  62   i  and switch SW i , for i varying from 1 to N−1, and B N  a node between resistor  62   N  and the cathode of general light-emitting diode D N . Further, each comparator COMP i , with i varying from 1 to N, further receives the voltage at node B i . Signal H i  then is a binary signal which is in a first state when the voltage at node B i  is smaller than a threshold MIN i  and which is in a second state when the voltage at node B i  is greater than threshold MIN i . 
         [0085]      FIG. 10  shows an electric diagram of a more detailed embodiment of a portion of optoelectronic circuit  60 . In the present embodiment, comparator COMP i  comprises all the elements of comparator COMP i  shown in  FIG. 6  with the difference that operational amplifier  40  is replaced with a hysteresis comparator  64  receiving the voltage across resistor  62   i  and supplying signal H i . 
         [0086]      FIG. 11  shows an electric diagram of a more detailed embodiment of current source  30  and of switch SW i  for optoelectronic circuit  60 . Current source  30  comprises all the elements of the current source shown in  FIG. 7 . Resistor  62   i  is interposed between MOS transistor  54  and node B i , a terminal of resistor  62   i  being connected to the drain of transistor  54  and the other terminal of resistor  62   i  being connected to node B i . 
         [0087]    The operation of optoelectronic circuit  60  may be identical to the operation of previously-described optoelectronic circuit  20  with the difference that, in a rising phase of power supply voltage V ALIM , switch SW i  is turned off when current starts flowing through resistor  62   i+1 . 
         [0088]    More specifically, switches SW i , with i varying from 1 to N−1, are initially on. In a rising phase of power supply voltage V ALIM , for i varying from 1 to N−1, while light-emitting diodes D 1  to D i−1  are conductive and light-emitting diodes D i  to D N  are blocked, when the voltage across light-emitting diode D i  becomes greater than the threshold voltage of light-emitting diode D i , the latter becomes conductive and a current starts flowing through resistor  62   i . This results in a rise in the voltage at node B i . As soon as the voltage at node B i  rises above threshold MIN i , unit  32  orders the turning-on of switch SW i−1 . 
         [0089]    The operation of optoelectronic circuit  60  in a falling phase of power supply voltage V ALIM  may be identical to that which has been previously described for optoelectronic circuit  20 . 
         [0090]    Optoelectronic circuit  60  has the advantage that thresholds MIN i  and Vlow i  can be independent from the characteristics of light-emitting diodes D i . In particular, they do not depend on the threshold voltage of each light-emitting diode D i . 
         [0091]      FIG. 12  shows an electric diagram of another embodiment of an optoelectronic circuit  70 . All the elements common with optoelectronic circuit  20  are designated with the same reference numerals. Optoelectronic circuit  70  comprises, for each general light-emitting diode D i , a current source  72   i , with i varying from 1 to N, associated with general light-emitting diode D i . A terminal of current source  72   i , with i varying from 1 to N, is connected to node A 2  and the other terminal is connected to the cathode of general light-emitting diode D i . 
         [0092]    Each current source  72   i , with i varying from 1 to N, is controlled by a signal S′ i  supplied by control unit  32 . As an example, signal S′ i  is a binary signal and current source  72   i  is activated when signal S′ i  is in a first state and current source ‘ 72   i  is deactivated when signal S′ i  is in a second state. 
         [0093]    The operation of optoelectronic circuit  70  may be identical to the operation of previously-described optoelectronic circuit  20 , with the difference that the steps of turning-off and turning-on of switches SW i  of optoelectronic circuit  20  are respectively replaced with steps of activation and of deactivation of current sources  72   i . 
         [0094]    More specifically, in a rising phase of power supply voltage V ALIM , for i varying from 1 to N−1, while current source  72   i  is activated and the other current sources are deactivated, unit  32  successively orders the activation of current source  72   i+1  and the deactivation of current source  72   i  when voltage V Ci  exceeds threshold Vhigh i . Voltage V ALIM  then distributes between general light-emitting diodes D 1  to D i+1  and current source  72   i+1 . Preferably, threshold Vhigh i  is selected to be substantially equal to the threshold voltage of general light-emitting diode D i+1  so that on activation of current source  72   i+1 , general light-emitting diode D i+1  conducts current I CS  and emits light. The fact for current source  72   i+1  to be activated before current source  72   i  is deactivated ensures that there is no interruption in the current flow in general light-emitting diodes D 1  to D i . 
         [0095]    Generally, in a falling phase of power supply voltage V ALIM , for i varying from 2 to N, when voltage V Ci  decreases below threshold Vlow i , unit  32  successively orders the activation of current source  72   i−1  and the deactivation of current source  72   i . Voltage V ALIM  then distributes between general light-emitting diodes D 1  to D i−1  and current source  72   i+1 . The fact for current source  72   i−1  to be activated before current source  72   i  is deactivated ensures that there is no interruption in the current flow in general light-emitting diodes D 1  to D i−1 . 
         [0096]      FIG. 13  is a partial simplified cross-section view of another embodiment of an optoelectronic circuit  80  having an equivalent electric diagram which may correspond to one of the diagrams shown in  FIG. 3, 9 , or  12 . In this embodiment, each general light-emitting diode D 1  to D N  is formed on a different monolithic circuit  82 . The other components of optoelectronic circuit  80  are formed in another integrated circuit  84 . Each monolithic circuit  82  is connected to integrated circuit  84 , for example, by a flip-chip-type connection. Each general light-emitting diode D 1  to D N  may correspond to a planar light-emitting diode or to a light-emitting diode formed from three-dimensional elements, particularly semiconductor microwires or nanowires. 
         [0097]    According to a variation, at least one of monolithic circuits  82  may comprise more than one general light-emitting diode. 
         [0098]      FIG. 14  is a partial simplified cross-section view of another embodiment of an optoelectronic circuit  90  having an equivalent electric diagram which may correspond to one of the diagrams shown in  FIG. 3, 9 , or  12 . In this embodiment, general light-emitting diodes D 1  to D N  are formed in integrated fashion on a different circuit  92 . The other components of optoelectronic circuit  90  are formed in another integrated circuit  94 . Integrated circuit  92  is connected to integrated circuit  94 , for example, by a flip-chip-type connection. Each general light-emitting diode D 1  to D N  may correspond to a planar light-emitting diode or to a light-emitting diode formed from three-dimensional elements, particularly semiconductor microwires or nanowires. 
         [0099]    According to another embodiment, all the components of the optoelectronic circuit according to one of the equivalent electric diagrams shown in  FIG. 3, 9 , or  12  are formed on a same integrated circuit. Each general light-emitting diode D 1  to D N  may correspond to a planar light-emitting diode or to a light-emitting diode formed from three-dimensional elements, particularly semiconductor microwires or nanowires. 
         [0100]    According to another embodiment, each general light-emitting diode D 1  to D N  may correspond to a discrete component, particularly comprising a light-emitting diode protection package. Each component is for example attached to a support, particularly a printed circuit, having the other components of the optoelectronic circuit attached thereto. 
         [0101]    Various embodiments with different variations have been described hereabove. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step.