Light emitting element driving circuit

Abstract

The present invention relates to a light emitting element driving circuit for individually driving and controlling a plurality of light emitting elements having cathodes of equal potential. The light emitting element driving circuit comprises a plurality of driving circuit units arranged one-to-one corresponding to the plurality of light emitting elements. Each driving circuit unit comprises a switching element, a first power storage element, a current control element and a current cutoff element. The current control element is connected in parallel with the semiconductor light emitting element. In the first power storage element, the first electrode is connected to the first node, and the second electrode is connected to the first constant potential line. In the switching element, the first current terminal is connected to the second electrode, and the second current terminal is connected to the second node. The second nodes of the plurality of driving circuit units are connected to the second constant potential line through the current cutoff element without short-circuiting each other. The current cutoff element cuts off or suppresses the current during the period when the switching element is in the on state.

Description

Technical Field

[0001] The invention relates to a light emitting element driving circuit. Background Art

[0002] Patent document 1 discloses a technology for a current generating circuit for array elements. The circuit has a reference voltage input unit and transistors that form a constant current generating circuit according to different channels. The reference voltage input unit is connected to the base of each transistor via a passive circuit. The emitter of each transistor is connected to a common ground line via a resistor. The constant current generating circuit of each channel is centrally arranged in a specific area on an integrated circuit chip. The collector output current of each transistor is individually supplied to the array functional circuit block dispersed in the integrated circuit chip.

[0003] Patent document 2 discloses a technology related to a light source device of an optical head. The device comprises: a common package, a single-mode oscillating semiconductor laser and a multi-mode oscillating semiconductor laser built into the package, and a high-frequency oscillation circuit. In order to convert the single-mode oscillating semiconductor laser into a multi-mode oscillation, the high-frequency oscillation circuit generates a high-frequency current overlapping with the driving current of the semiconductor laser. The device has a means for preventing unnecessary radiation caused by the high-frequency current. The means includes a high-frequency component absorption circuit provided in the power supply line of the multi-mode oscillating semiconductor laser. The high-frequency component absorption circuit is a bandpass filter, a choke coil or a ferrite bead.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 9-270554

[0007] Patent Document 2: Japanese Patent Application Publication No. 2002-288864 Summary of the invention

[0008] Technical problem to be solved by the invention

[0009] The inventors have studied the above-mentioned prior art, and as a result, the following technical problems have been discovered. That is, when a plurality of semiconductor light-emitting elements are driven individually, normally, an independent driving circuit unit is used for each semiconductor light-emitting element. However, for example, in a plurality of semiconductor light-emitting elements monolithically integrated on a common substrate, there is a case where the cathode is set to the same potential (a structure in which the cathode of each semiconductor light-emitting element is connected to a common wiring). In such a case, a plurality of driving circuit units are short-circuited to each other via the common wiring, and when each semiconductor light-emitting element is driven individually, it is necessary to design so that the driving current does not flow into other semiconductor light-emitting elements through the common wiring.

[0010] The present invention is made to solve the above-mentioned technical problems, and its object is to provide a light emitting element driving circuit having a structure for individually pulse-driving a plurality of semiconductor light emitting elements, wherein the plurality of semiconductor light emitting elements respectively have cathodes electrically connected via a common wiring.

[0011] Technical means for solving technical problems

[0012] The light emitting element driving circuit of the present embodiment is configured as a circuit for driving and controlling a plurality of semiconductor light emitting elements individually, and has a plurality of driving circuit units corresponding to the plurality of semiconductor light emitting elements one to one. The cathodes of the plurality of semiconductor light emitting elements may be cathode electrodes that are commonized through a monolithic structure, and may be electrically connected via common wiring and / or common terminals. Thus, the cathodes of the plurality of semiconductor light emitting elements are set to be equipotential. The plurality of driving circuit units individually pulse drive the corresponding semiconductor light emitting elements. In addition, the plurality of driving circuit units respectively have a current control element, a first power storage element, and a switching element. The current control element has: a first terminal electrically connected to the cathode of the corresponding semiconductor light emitting element via a first node; and a second terminal electrically connected to the anode of the corresponding semiconductor light emitting element via a second node. In addition, the current control element flows current from the first node to the second node during the period when the corresponding semiconductor light emitting element is in a non-light emitting state, and limits the current during the period when the corresponding semiconductor light emitting element is in a light emitting state. The first power storage element has a first electrode electrically connected to the first node, and a second electrode. The switch element includes: a control terminal to which a drive control signal of a corresponding semiconductor light emitting element is input; a first current terminal electrically connected to the second electrode of the first power storage element; and a second current terminal electrically connected to the second node.

[0013] Further, in the light emitting element driving circuit, the second electrode of the first storage element in the plurality of driving circuit units is electrically connected to the first constant potential line, respectively. The second nodes in the plurality of driving circuit units are electrically connected to the second constant potential line, respectively, via the first current cutoff element, or directly. In addition, the first current cutoff element is arranged on the wiring between two second nodes adjacent to each other, and a short circuit between the two adjacent second nodes adjacent to each other is avoided. The second constant potential line is set to a lower potential than the first constant potential line. The control object of the first current cutoff element is at least a current of a specific frequency or a current of a specific period. The first current cutoff element cuts off or suppresses the control object. The first current cutoff element cuts off or suppresses the current at least during the period when the switching element is in the on state.

[0014] Effects of the Invention

[0015] According to the present invention, it is possible to provide a light emitting element driving circuit capable of individually driving a plurality of semiconductor light emitting elements each having a cathode set to the same potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 1 is a circuit diagram showing a configuration of a light emitting element driving circuit 1 according to an embodiment of the present invention.

[0017] Figure 2 It is a plan view schematically showing the semiconductor light emitting element array 14 .

[0018] Figure 3 The diagram shows the current Ia flowing during a period when the transistor 3 of each of the drive circuit units 2A to 2D is in the off state (that is, when the light emitting element 4 is not emitting light).

[0019] Figure 4 1 is a diagram showing the driving current Ib that flows during a period in which the transistor 3 of the driving circuit unit 2A is in the on state.

[0020] Figure 5 : is a circuit diagram showing the structure of the light emitting element driving circuit unit 20A according to the first comparative example.

[0021] Figure 6 : is a circuit diagram showing the structure of a light emitting element driving circuit unit 20B according to a second comparative example.

[0022] Figure 7 : is a circuit diagram showing the structure of a light emitting element driving circuit unit 20C according to a third comparative example.

[0023] Figure 8 : is a circuit diagram showing the structure of the light emitting element driving circuit unit 30 according to the fourth comparative example.

[0024] Fig. 9 It means in Figure 8 In the circuit shown, a diagram is a diagram of the current flowing during a period when a certain transistor 3 is in the on state.

[0025] Fig.10 It is a circuit diagram conceptually showing a power supply system included in the light emitting element driving circuit 1 according to one embodiment.

[0026] Fig.11 This is a diagram showing the current path from the capacitor 7 during a period when the transistor 3 is in the on state (that is, when the light emitting element 4 emits light).

[0027] Fig.12 4 is a graph showing the results of simulating the operation of a light emitting element driving circuit that independently drives two light emitting elements 4 (that is, includes only the driving circuit units 2A and 2B).

[0028] Fig.13 This is an image of light emission captured in a light emitting element driving circuit actually produced as the light emitting element driving circuit 1 of one embodiment that drives four light emitting elements 4 individually.

[0029] Fig.14 1 is a circuit diagram showing a configuration of a light emitting element driving circuit 1A according to a modified example. DETAILED DESCRIPTION

[0030] [Description of Embodiments of the Invention of the Present Application]

[0031] First, the contents of the embodiments of the present invention will be listed and described separately.

[0032] (1) One mode of the light emitting element driving circuit of the present embodiment is a circuit configured to drive and control a plurality of semiconductor light emitting elements individually, and comprising a plurality of driving circuit units corresponding one to one to the plurality of semiconductor light emitting elements. The cathodes of the plurality of semiconductor light emitting elements may be cathode electrodes common to each other through a monolithic structure, and may be electrically connected via a common wiring and / or a common terminal. Thus, the cathodes of the plurality of semiconductor light emitting elements are set to be at the same potential. The plurality of driving circuit units individually pulse drive the corresponding semiconductor light emitting elements. In addition, the plurality of driving circuit units respectively include a current control element, a first power storage element, and a switching element. The current control element includes: a first terminal electrically connected to the cathode of the corresponding semiconductor light emitting element via a first node; and a second terminal electrically connected to the anode of the corresponding semiconductor light emitting element via a second node. In addition, the current control element flows current from the first node to the second node during a period when the corresponding semiconductor light emitting element is in a non-light emitting state, and limits the current during a period when the corresponding semiconductor light emitting element is in a light emitting state. The first power storage element includes a first electrode electrically connected to the first node, and a second electrode. The switch element includes: a control terminal to which a drive control signal of a corresponding semiconductor light emitting element is input; a first current terminal electrically connected to the second electrode of the first power storage element; and a second current terminal electrically connected to the second node.

[0033] Further, in the light emitting element driving circuit, the second electrode of the first storage element in the plurality of driving circuit units is electrically connected to the first constant potential line, respectively. The second nodes in the plurality of driving circuit units are electrically connected to the second constant potential line, respectively, via the first current cutoff element, or directly. In addition, the first current cutoff element is arranged on the wiring between two second nodes adjacent to each other, and a short circuit between the two adjacent second nodes adjacent to each other is avoided. The second constant potential line is set to a lower potential than the first constant potential line. The control object of the first current cutoff element is at least a current of a specific frequency or a current of a specific period. The first current cutoff element cuts off or suppresses the control object. The first current cutoff element cuts off or suppresses the current at least during the period when the switching element is in the on state.

[0034] As described above, in the first storage element, the first electrode is electrically connected to the first node, and the second electrode is electrically connected to the first constant potential line (e.g., a positive power supply wiring). Furthermore, a current control element is connected to the first node, and the current control element flows current from the first node to the second node when the semiconductor light-emitting element is not emitting light. The second node is electrically connected to a second constant potential line (e.g., a ground wiring) having a lower potential than the first constant potential line. Therefore, during the period when the switch element of each drive circuit unit is in the off state (i.e., during the non-emitting period of the semiconductor light-emitting element), current is supplied from the first constant potential line to the first storage element of each drive circuit unit, and charge is accumulated in the first storage element.

[0035] Furthermore, during the period when the switch element of a certain driving circuit unit is in the on state, the charge accumulated in the first storage element of the driving circuit unit returns from the second electrode of the first storage element through the second node, the semiconductor light emitting element and the first node to the first electrode of the first storage element. As a result, the semiconductor light emitting element corresponding to the driving circuit unit one-to-one emits light (is driven). At this time, because the current control element limits the current, the charge can flow to the semiconductor light emitting element without a detour. In addition, assuming that the first current interruption element is not provided, a part of the charge flows from the second node to the second node of other driving circuit units, and returns to the first storage element of the driving circuit unit through the semiconductor light emitting element of the other driving circuit unit and the cathode of the semiconductor element. In this case, the semiconductor light emitting element of the other driving circuit unit will also emit light, and the single drive of the semiconductor light emitting element cannot be realized. In contrast, in the light emitting element driving circuit having the above-mentioned structure, the first current interruption element exists between two second nodes adjacent to each other among the second nodes of the plurality of driving circuit units, and the two second nodes adjacent to each other are not short-circuited. Therefore, it is possible to block or suppress the flow of a part of the charge from the second node to the second node of the other driving circuit unit. As a result, it is possible to prevent the semiconductor light emitting element of the other driving circuit unit from emitting light, thereby achieving independent driving.

[0036] (2) As one embodiment of the present invention, each of the plurality of drive circuit units further includes a second current cutoff element disposed on a wiring line between the first constant potential line and the second electrode of the first storage element. The control object of the second current cutoff element is at least a current of a specific frequency or a current of a specific period. The second current cutoff element is an element that cuts off or suppresses the control object. The second current cutoff element cuts off or suppresses the current at least during the period when the switch element is in the on state. In this case, it is possible to prevent the current from flowing from the first constant potential line through the switch element to the second node during the period when the switch element is in the on state (the potential rise of the second node can be suppressed). Thus, the potential difference between the control terminal of the switch element and the second current terminal (in one example, the gate-source voltage of the FET) can be maintained (a stable on / off action of the switch element can be achieved).

[0037] (3) As one embodiment of the present invention, each of the plurality of drive circuit units may further include a second storage element, the second storage element including: a first electrode electrically connected to a third node between a second electrode of the first storage element and a second current interruption element; and a first electrode electrically connected to the second node. In this case, during a period when the switch element of each drive circuit unit is in an off state (i.e., when the semiconductor light emitting element is not emitting light), current is supplied from the first constant potential line to the second storage element of each drive circuit unit, and charge is accumulated in the second storage element. Then, after the semiconductor light emitting element emits light and the charge of the first storage element is discharged, the first storage element can be instantly recharged with the charge accumulated in the second storage element, regardless of whether the second current interruption element interrupts or suppresses the current.

[0038] (4) As one embodiment of the present invention, the capacity of the second power storage element is preferably greater than the capacity of the first power storage element. In this case, for example, when the semiconductor light emitting element emits light multiple times at short time intervals, the first power storage element can be recharged each time light is emitted, regardless of whether the second current interruption element interrupts or suppresses the current.

[0039] (5) As one embodiment of the present invention, the first current interruption element may include an inductor. In this case, the inductor functions as a low-pass filter and can interrupt or suppress a high-frequency current under pulse drive. Therefore, the current that is to pass when the switch element is turned on can be interrupted or suppressed.

[0040] (6) As one embodiment of the present invention, the current control element may include a resistor element. In this case, the impedance of the current control element is higher than the forward impedance of the semiconductor light emitting element, so that the current can be limited when the semiconductor light emitting element is emitting light. In addition, because the impedance of the current control element is lower than the reverse impedance of the semiconductor light emitting element, the current can flow when the semiconductor light emitting element is not emitting light.

[0041] (7) As one embodiment of the present invention, a plurality of driving circuit units may individually drive a plurality of semiconductor light-emitting elements integrated on a common semiconductor substrate. As an example, a semiconductor layer including an active layer is provided on a semiconductor substrate, and a cathode electrode serving as a common cathode of a plurality of semiconductor light-emitting elements is provided on the back side of the semiconductor layer. On the other hand, as anodes of a plurality of semiconductor light-emitting elements, a plurality of anode electrodes corresponding to the anodes of the plurality of semiconductor light-emitting elements are provided on the upper surface of the semiconductor layer. In addition, a structure may be provided in which the cathodes (cathode electrodes) of a plurality of semiconductor light-emitting elements are electrically connected via common wiring and / or common terminals. Any of the above structures can reduce the size of the semiconductor substrate, thereby contributing to miniaturization of the light-emitting device.

[0042] Each of the aspects listed in the column [Description of Embodiments of the Invention of the Present Application] above can be applied to each of all the remaining aspects, or to all combinations of the remaining aspects.

[0043] [Details of the embodiments of the present invention]

[0044] Below, the specific structure of the light-emitting element driving circuit of the present embodiment is described in detail with reference to the accompanying drawings. In addition, the present invention is not limited to these examples, but is intended to be shown in the claims, including all changes within the meaning and scope equivalent to the claims. In addition, in the description of the accompanying drawings, the same symbol is given to the same element, and repeated description is omitted. In addition, in the following description, electrical connection means, unless otherwise specified, also including: the case of connection by a conductive wire with substantially zero resistance, and the case of connection via electronic components such as resistors. In addition, short circuit means connection to each other through a resistance that is substantially zero.

[0045] Figure 1 FIG. 1 is a circuit diagram showing the structure of a light emitting element driving circuit 1 according to an embodiment of the present invention. Figure 1 As shown in FIG. 1 , the light emitting element driving circuit 1 includes a plurality of driving circuit units 2A to 2D. Figure 1 In the example of FIG. 4 , four drive circuit units are shown. The drive circuit units 2A to 2D are circuits for individually pulse-driving a plurality of semiconductor light-emitting elements 4 (hereinafter referred to as light-emitting elements 4) having cathodes set to the same potential. Figure 1 In the example, four light emitting elements are shown. The light emitting element 4 is, for example, a laser diode. The output wavelength of the light emitting element 4 is, for example, included in the near-infrared region. In one embodiment, the peak oscillation wavelength of each light emitting element 4 is 905 nm, and the peak output of each light emitting element 4 is, for example, 100 W. In this case, for example, a semiconductor light emitting element array suitable for LiDAR (Light Detection and Ranging) can be provided.

[0046] Figure 2 It is a top view schematically showing the semiconductor light emitting element array 14. The semiconductor light emitting element array 14 has a monolithic structure in which a plurality of light emitting elements 4 are integrated on a common semiconductor substrate 17. When the output wavelength is in the near-infrared region, the material of the semiconductor substrate 17 is, for example, GaAs or InP, and the material of the semiconductor layer provided on the semiconductor substrate 17 and including the active layer is, for example, AlGaAs, InGaAs or InGaAsP. On the back side of the semiconductor layer, a cathode electrode (common cathode) in which the cathodes of the plurality of light emitting elements 4 are common is provided, and on the upper surface of the semiconductor layer, a plurality of anode electrodes corresponding to the anodes of the plurality of light emitting elements 4 and separated from each other are provided. The semiconductor light emitting element array 14 has: a plurality of anode terminals 15 respectively connected to the plurality of anode electrodes; and a cathode terminal 16 connected to the common cathode electrode.

[0047] Refer again Figure 1 The driving circuit units 2A to 2D have the same circuit structure as each other. The driving circuit units 2A to 2D respectively have a transistor 3 (switching element), a capacitor 5 (first storage element), a current control element 6, a capacitor 7 (second storage element), a current interruption element 8 (first current interruption element), a current interruption element 9 (second current interruption element) and a resistor 13.

[0048] The transistor 3 is an example of a switching element in the present embodiment, and is a selection means for driving each light emitting element 4 individually. The transistor 3 is, for example, a field effect transistor (FET). In the following description, the case where the transistor 3 is an enhancement type n-channel MOSFET is illustrated, but the transistor 3 is not limited to this. The transistor 3 is not limited to a FET, and for example, it can also be a bipolar transistor. The transistor 3 has a control terminal and a pair of current terminals. When the transistor 3 is a FET, the control terminal is a gate, and the pair of current terminals are a source and a drain. When the transistor 3 is a bipolar transistor, the control terminal is a base, and the pair of current terminals are a collector and an emitter. Figure 1 FIG. 2 clearly shows the parasitic diode 12 included in the transistor 3 .

[0049] The current control element 6 is connected in parallel with the light emitting element 4 driven by the driving circuit to which the current control element 6 belongs. Specifically, the current control element 6 is connected in parallel with the light emitting element 4 between the node N1 (the first node) on the cathode side and the node N2 (the second node) on the anode side of the light emitting element 4. The current control element 6 allows current to flow from the node N1 to the node N2 during the non-light emitting period of the light emitting element 4, and limits the current from the node N2 to the node N1 during the light emitting period of the light emitting element 4. The current control element 6 can include, for example, various electronic components such as resistors and diodes, either individually or in combination. Figure 1 In the example, a resistor element is illustrated as the current control element 6. In this case, with respect to the current direction from the node N1 to the node N2, since it becomes the reverse direction of the light-emitting element 4, the current flows through the resistor element, and with respect to the current direction from the node N2 to the node N1, since it becomes the forward direction of the light-emitting element 4, the current flows through the light-emitting element 4. In the case where the current control element 6 is a resistor element, its resistance value is, for example, not less than 5Ω and not more than 1kΩ. In addition, in the case where the current control element 6 is a diode, the connection direction of the diode is opposite to that of the light-emitting element 4 (node ​​N1 is the anode side, and node N2 is the cathode side). In addition, the smaller the resistance value of the current control element 6, the faster the storage speed of the capacitor 5 described later can be made.

[0050] Capacitor 5 is, for example, a chip capacitor mounted on a circuit substrate. Capacitor 5 has a pair of electrodes, and charges are stored between the pair of electrodes. The capacitance value of capacitor 5 is, for example, greater than 500 pF and less than 1500 pF. In addition, the capacitance value of capacitor 5 affects the peak output and pulse width of the pulse light output from light emitting element 4. The larger the capacitance value of capacitor 5, the larger the peak output and pulse width of the pulse light. One electrode of capacitor 5 is electrically connected to node N1. The other electrode of capacitor 5 is electrically connected to a current terminal (for example, a drain) of transistor 3 in a state where the resistance value is substantially zero. The other current terminal (for example, a source) of transistor 3 is electrically connected to node N2 in a state where the resistance value is substantially zero. In addition, the potential of node N2 serves as a reference potential for the voltage applied to the control terminal of transistor 3 (the terminal to which the driving pulse signal of light emitting element 4 is input).

[0051] The other electrode of the capacitor 5 is electrically connected to a power supply line 10 (first constant potential line) via a resistor 13 and a current interruption element 9 connected in series. The power supply line 10 is a positive potential power supply line provided as a common wiring for driving the circuit units 2A to 2D. The power supply line 10 supplies a predetermined power supply voltage between a reference potential line 11 (second constant potential line) having a lower potential than the power supply line 10. The reference potential line 11 is also provided as a common wiring for the driving circuit units 2A to 2D.

[0052] The current cutoff element 8 is electrically connected between the node N2 and the reference potential line 11. Specifically, one end of the current cutoff element 8 is electrically connected to the node N2, and the other end is electrically connected to the reference potential line 11. That is, the adjacent nodes among the nodes N2 of the drive circuit units 2A to 2D are electrically connected via the current cutoff element 8, but are not short-circuited. In addition, the node N2 of the drive circuit units 2A to 2D must be electrically connected to the reference potential line 11 via the current cutoff element 8. The resistance value between the node N2 and the current cutoff element 8, and the resistance value between the current cutoff element 8 and the reference potential line 11 are substantially zero.

[0053] The current cutoff element 8 is an element that cuts off or suppresses at least a current of a specific frequency or a current of a specific period. In this specification, a specific frequency refers to the frequency of the rise and fall of the pulse drive current, and a specific period refers to the period from the rise to the fall of the pulse drive current. That is, the current cutoff element 8 cuts off or suppresses the current at least during the period when the transistor 3 is in the on state. As an element that cuts off or suppresses the current of a specific frequency, for example, inductors such as ferrite beads, resistor elements, etc. can be listed. As an element that cuts off or suppresses the current of a specific period, for example, a transistor can be listed. The current cutoff element 8 can include these electronic components individually or in combination. Figure 1 A circuit consisting of only an inductor is exemplified as the current cutoff element 8. In this case, the current cutoff element 8 functions as a low-pass filter.

[0054] The current cutoff element 9 is connected between the power supply line 10 and the other electrode of the capacitor 5 (i.e., the electrode on the power supply line 10 side). In other words, the other electrode of the capacitor 5 is electrically connected to the power supply line 10 via the current cutoff element 9. The current cutoff element 9, like the current cutoff element 8, is an element that cuts off or suppresses at least a current of a specific frequency or a specific period, and cuts off or suppresses the current at least when the transistor 3 is in the on state. As an element that cuts off or suppresses a current of a specific frequency, for example, an inductor, a resistor element, etc. can be listed. As an element that cuts off or suppresses a current of a specific period, for example, a transistor can be listed. The current cutoff element 9 can include these electronic components individually or in combination. Figure 1 A circuit in which the current interruption element 9 is composed only of an inductor is exemplified. In this case, the current interruption element 9 functions as a low-pass filter. In addition, when the current interruption elements 8 and 9 are low-pass filters, their cut-off frequencies may be equal to or different from each other. In addition, when the current interruption elements 8 and 9 are both inductors, their inductances may be equal to or different from each other.

[0055] Capacitor 7 is, for example, a chip capacitor mounted on a circuit substrate. Capacitor 7 has a pair of electrodes, and charges are stored between the pair of electrodes. The capacitance value of capacitor 7 is greater than the capacitance value of capacitor 5, for example, 0.1 μF or more. In other words, the capacitance value of capacitor 7 is, for example, 100 times or more of the capacitance value of capacitor 5. One electrode of capacitor 7 is electrically connected to node N3 (third node). Node N3 is a node between the electrode on the power line 10 side of capacitor 5 and the current interruption element 9 (in the present embodiment, between the resistor 13 and the current interruption element 9). The other electrode of capacitor 7 is electrically connected to node N2.

[0056] In the light emitting element driving circuit 1 having the above-mentioned structure, one electrode of the capacitor 5 is electrically connected to the node N1, and the other electrode is electrically connected to the power supply line 10. Furthermore, the node N1 is connected to the current control element 6, and the current control element 6 flows the current from the node N1 to the node N2 when the light emitting element 4 is not emitting light. The node N2 is electrically connected to the reference potential line 11 having a lower potential than the power supply line 10. Therefore, during the period when the transistor 3 of each of the driving circuit units 2A to 2D is in the off state (that is, when the light emitting element 4 is not emitting light), as shown in FIG. Figure 3 As shown, a current Ia is supplied from the power supply line 10 to the capacitor 5 of each of the drive circuit units 2A to 2D, and electric charges are accumulated in the capacitor 5 .

[0057] Furthermore, during the period when the transistor 3 of a certain driving circuit unit (hereinafter referred to as the driving circuit unit 2A) is in the on state, the charge accumulated in the capacitor 5 of the driving circuit unit 2A is as follows: Figure 4 As shown, the driving current Ib flows instantaneously from the electrode on the power line 10 side of the capacitor 5 through the transistor 3, the node N2, the light emitting element 4 and the node N1, and returns to the electrode on the opposite side of the capacitor 5. As a result, the light emitting element 4 of the driving circuit unit 2A is pulse driven to emit light. At this time, because the current control element 6 limits the current, the driving current Ib can flow to the light emitting element 4 without detour. In addition, the resistor 13 prevents the charge accumulated in the capacitor 7 from flowing to the transistor 3. In addition, the same operation is possible in any of the driving circuit units 2B, 2C, and 2D.

[0058] Here, the effects obtained by the light emitting element driving circuit 1 of the present embodiment will be described together with a comparative example. Figure 52 is a circuit diagram showing the structure of a driving circuit 20A of the first comparative example. The driving circuit 20A is a circuit for driving a single light emitting element 4A, and includes: a transistor 21 connected in series between the cathode of the light emitting element 4A and the reference potential line 11; and a resistor 23 and a capacitor 22 connected in series between the power supply line 10 and the reference potential line 11, and connected in series with each other. The anode of the light emitting element 4 is connected to the node between the resistor 23 and the capacitor 22.

[0059] In the drive circuit 20A, while the transistor 21 is in the off state, the current from the power line 10 is supplied to the capacitor 22, and the charge is accumulated in the capacitor 22. Then, when the transistor 21 is turned on, the charge accumulated in the capacitor 22 flows instantaneously to the reference potential line 11 through the light emitting element 4A and the transistor 21. As a result, the light emitting element 4A is pulse driven to emit light.

[0060] Figure 6 1 is a circuit diagram showing the structure of a driving circuit 20B of a second comparative example for driving four light-emitting elements 4 each having a cathode electrically connected via a common wiring. The driving circuit 20B includes a resistor 23 and a capacitor 22 provided for each light-emitting element 4, and a transistor 21 provided in common with the four light-emitting elements 4. The connection between each light-emitting element 4 and the resistor 23 and the capacitor 22 is similar to the connection between the two light-emitting elements 4 and the resistor 23 and the capacitor 22. Figure 5 The transistor 21 is connected in series between the common cathode of the four light emitting elements 4 and the reference potential line 11 . The power supply line 10 is common to the light emitting elements 4 .

[0061] In the drive circuit 20B, during the period when the transistor 21 is in the off state, the current from the power supply line 10 is supplied to each capacitor 22, and the charge is accumulated in each capacitor 22. Then, when the transistor 21 is turned on, the charge accumulated in each capacitor 22 is instantly circulated to the reference potential line 11 through each light emitting element 4 and the transistor 21. Thus, the four light emitting elements 4 are pulse-driven at the same time to emit light. Therefore, in the drive circuit 20B, the four light emitting elements 4 cannot be driven individually.

[0062] Therefore, an improved method for individually driving four light emitting elements 4 each having a cathode electrically connected via a common wiring line 2 is studied. Figure 7 20C is a circuit diagram showing an improved drive circuit 20C of the third comparative example. Figure 6 In addition to the structure of the driving circuit 20B, it also includes four transistors 24, four transistors 25, and a resistor element 26.

[0063] Transistors 24 and 25 constitute a switch circuit provided for each light emitting element 4. Transistor 24 is connected in series between a common power supply line 10, capacitor 22, and light emitting element 4 to control the current supplied to capacitor 22. Transistor 25 and resistor 27 constitute a series circuit, one end of the resistor 27 side of the series circuit is connected to power supply line 10, and the other end is connected to reference potential line 11. The node between transistor 25 and resistor 27 is connected to the control terminal of transistor 24, and the potential of the node (driving control signal) is applied to the control terminal of transistor 24. That is, the on / off of transistor 24 can be switched corresponding to the on / off of transistor 25. Transistor 24 is, for example, a p-channel MOSFET, and transistor 25 is, for example, an n-channel MOSFET. According to such a structure, a high withstand voltage switch circuit can be formed. Resistor 26 is connected in series between four capacitors 22 and a common reference potential line 11.

[0064] In the drive circuit 20C, first, in order to accumulate charge in the capacitor 22 corresponding to the light emitting element 4 to be made to emit light, the transistor 25 corresponding to the light emitting element 4 is turned on. As a result, the transistor 24 corresponding to the light emitting element 4 is turned on, and the charge is accumulated in the capacitor 22. Thereafter, when the transistor 21 is turned on, current is instantaneously supplied from the capacitor 22 to the light emitting element 4, and the light emitting element 4 is pulse driven to emit light.

[0065] However, the drive circuit 20C has the following problem. That is, when charge is accumulated in the capacitor 22 corresponding to the light-emitting element 4 to be made to emit light, the voltage across the capacitor 22 rises. During the period when the transistor 21 is in the off state, the voltage across the capacitor 22 is applied to the cathodes of the other three light-emitting elements 4 via the light-emitting element 4 as a reverse bias, and a reverse current proportional to the reverse bias flows through the light-emitting element 4 and is supplied to the other three capacitors 22. As a result, charge is also accumulated in the other three capacitors 22. Therefore, when the transistor 21 is turned on, current is instantly supplied from each capacitor 22 to each light-emitting element 4, and the four light-emitting elements 4 are pulse-driven at the same time to emit light. That is, it is difficult for the drive circuit 20C to drive the four light-emitting elements 4 individually.

[0066] Therefore, the inventors of the present invention further studied a circuit for individually driving four light-emitting elements 4 each having a cathode electrically connected via a common wiring. Figure 8 3 is a circuit diagram showing a drive circuit 30 of the fourth comparative example. The drive circuit 30 includes a transistor 3, a capacitor 5, and a current control element 6 provided for each light emitting element 4. The interconnection relationship between the transistor 3, the capacitor 5, and the current control element 6 is similar to Figure 1However, in the drive circuit 30, the electrode of the capacitor 5 on the power line 10 side is connected to the common power line 10 only via the resistor 13, and the node N2 is short-circuited to the reference potential line 11. In addition, the capacitor 7 (see Figure 1 ).

[0067] In the driving circuit 30, during the period when each transistor 3 is in the off state (i.e., during the non-light-emitting period of the light-emitting element 4), current is supplied from the power line 10 to each capacitor 5, and charge is accumulated in the capacitor 5. Then, when a certain transistor 3 (for example, the transistor 3 at the left end) becomes in the on state, the charge accumulated in the capacitor 5 connected to the transistor 3 is increased. Fig. 9 As shown, the driving current Ib flows instantaneously through the transistor 3, the node N2, the light emitting element 4 and the node N1, and returns to the capacitor 5. As a result, the light emitting element 4 is pulse driven to emit light. At this time, because the current control element 6 limits the current, the driving current Ib can flow to the light emitting element 4 without making a detour.

[0068] However, if Fig. 9 As shown, a part Iba of the driving current Ib flows from the node N2 to the node N2 of the other light emitting element 4, passes through the other light emitting element 4 and the common cathode and returns to the original capacitor 5. In this case, the other light emitting element 4 also emits light, and the light emitting element 4 cannot be driven alone.

[0069] In contrast, Figure 1 In the light emitting element driving circuit 1 of the present embodiment shown, there is a current cutoff element 8 between the nodes N2 of the driving circuit units 2A to 2D, and the nodes N2 of the driving circuit units 2A to 2D are not short-circuited. The current cutoff element 8 is an element that cuts off or suppresses the current at least at a specific frequency or a specific period, and cuts off or suppresses the current at least during the period when the transistor 3 is in the on state. Therefore, it is possible to cut off or suppress a part of the driving current Ib from flowing from the node N2 to the node N2 of other driving circuits, and it is possible to prevent the light emitting element 4 of other driving circuits from emitting light and realize independent driving.

[0070] In addition, as in the present embodiment, the driving circuit units 2A to 2D may each include a current cutoff element 9 connected between the common power supply line 10 and the electrode on the power supply line 10 side of the capacitor 5. The current cutoff element 9 is an element that cuts off or suppresses at least a current of a specific frequency or a specific period, similar to the current cutoff element 8, and cuts off or suppresses the current at least during the period when the transistor 3 is in the on state. In this case, it is possible to prevent the current from flowing from the power supply line 10 through the transistor 3 to the node N2 when the transistor 3 is in the on state. Therefore, it is possible to suppress the potential rise of the node N2 caused by the current passing through the current cutoff element 8. As a result, it is possible to maintain the potential difference (gate-source voltage) between the control terminal (e.g., gate) of the transistor 3 and other current terminals (e.g., source) and stably turn on / off the transistor 3.

[0071] As in the present embodiment, the drive circuit units 2A to 2D may each include a capacitor 7 as a bypass capacitor, one electrode of which is connected to the node N3 between the electrode on the power line 10 side of the capacitor 5 and the current interruption element 9, and the other electrode of which is connected to the node N2. In this case, when the transistor 3 is in the off state (i.e., when the light emitting element 4 is not emitting light), current is supplied from the power line 10 to the capacitor 7, and charge is accumulated in the capacitor 7. Furthermore, after the light emitting element 4 emits light and the charge of the capacitor 5 is discharged, the capacitor 5 can be instantly recharged using the charge accumulated in the capacitor 7, regardless of whether the current interruption element 9 interrupts or suppresses the current. Therefore, the light emitting element 4 can be made to emit light again in a short time, and each light emitting element 4 can be driven at a high speed.

[0072] here, Fig.10 1 is a circuit diagram conceptually showing a power supply system of the light emitting element driving circuit 1 of the present embodiment. Fig.10 The figure shows a capacitor 7, an inductor 8 as a current interruption element, and an inductor 9 as a current interruption element, which are respectively included in two of the drive circuit units 2A to 2D. Fig.10 , a common power supply line 10 and a common reference potential line 11 are also shown. In the light emitting element driving circuit 1, a plurality of inductors constituting the current interruption elements 8 and 9 constitute a filter circuit, and the power supply systems of the driving circuit units 2A and 2B are virtually separated from each other by the filter circuit. Fig.10 A virtual power supply line 10 a and a reference potential line 11 a for supplying a power supply voltage to the drive circuit unit 2A, and a virtual power supply line 10 b and a reference potential line 11 b for supplying a power supply voltage to the drive circuit unit 2B are shown.

[0073] In this filter circuit, each inductor acts as a low-pass filter that cuts off the high-frequency region including the rising and falling frequencies of the pulse drive current. In addition, the capacitor 7 acts as a bypass capacitor for reducing the power supply impedance (the impedance of the power supply side when viewed from the part connected to the load) at this frequency.

[0074] Fig.11 3 is a diagram showing the current path of the current from the capacitor 7 during the period when the transistor 3 is in the on state (that is, during the light emitting period of the light emitting element 4). Fig.11 As shown, the current Ic output from the electrode on the power supply line 10 side of the capacitor 7 returns to the electrode on the reference potential line 11 side of the capacitor 7 via the node N2 after passing through the transistor 3. In this way, when the light-emitting element 4 emits light, the current Ic output from the capacitor 7 is all returned to the capacitor 7 and does not flow to the current cutoff element 8, so it does not become a cause of the potential increase of the node N2.

[0075] Fig.12 The diagram shows the results of simulating the operation of a light emitting element driving circuit that independently drives two light emitting elements 4 (i.e., only has driving circuit units 2A and 2B). In this simulation, one light emitting element 4 is made to emit light, while the other light emitting element 4 is kept off. Fig.12 The horizontal axis represents time (unit: nanoseconds), and the vertical axis represents the magnitude of the current flowing through the light emitting element 4. Fig.12 As shown, it can be seen that the current of one light emitting element 4 (graph G1) changes in a pulsed manner, and the pulse drive current Ib is supplied to the one light emitting element 4. In addition, it can be seen that the current of the other light emitting element 4 (graph G2) does not change at all, and the pulse drive current Ib is hardly supplied to the other light emitting element 4. That is, according to the light emitting element driving circuit 1 of this embodiment, the ratio (Pb / Pa) of the light output Pa of the light emitting element 4 that is made to emit light and the light output Pb of the light emitting element 4 that is not made to emit light can be reduced to a very small value.

[0076] Fig.13 This is an image of light emission captured in a light emitting element driving circuit actually manufactured as the light emitting element driving circuit 1 of this embodiment that drives four light emitting elements 4 individually. Fig.13 , an area A2 corresponding to one light-emitting element 4 that emits light, and areas A1 and A3 corresponding to two light-emitting elements 4 that are adjacent to it and do not emit light are shown. Fig.13 As shown, it can be seen that the degree of darkness in the non-luminous regions A1 and A3 is extremely high compared to the luminous bright region A2.

[0077] As in the present embodiment, the capacitance of capacitor 7 may be larger than that of capacitor 5. In this case, for example, when light emitting element 4 emits light multiple times at short time intervals, capacitor 5 can be recharged each time light is emitted, regardless of whether current interruption element 9 interrupts or suppresses the current.

[0078] As in the present embodiment, the current cutoff element 8 may include an inductor. In this case, the inductor functions as a low-pass filter and can cut off or suppress the high-frequency current of the pulse drive. Therefore, the current that is to pass can be cut off or suppressed while the transistor 3 is in the on state.

[0079] As in the present embodiment, the current control element 6 may include a resistor. In this case, since the impedance of the current control element 6 is greater than the forward impedance of the light-emitting element 4, the current can be limited during the light-emitting period of the light-emitting element 4. In addition, since the impedance of the current control element 6 is less than the reverse impedance of the light-emitting element 4, the current can flow during the non-light-emitting period of the light-emitting element 4. Therefore, during the non-light-emitting period of the light-emitting element 4, the current flows from the node N1 to the node N2, and the current control element 6 that limits the current during the light-emitting period of the light-emitting element 4 can be easily realized. In addition, by appropriately selecting the resistance value of the resistor (more preferably, selecting the resistance value of the resistor 13 together), the speed of accumulating charge in the capacitor 5 can be controlled.

[0080] As in the present embodiment, a plurality of light emitting elements 4 may be integrated on a common semiconductor substrate 17. In this case, the cathodes of the plurality of light emitting elements 4 are made common (one cathode electrode is used as the cathode of the plurality of light emitting elements 4), and the semiconductor substrate 17 can be reduced in size, which contributes to miniaturization of the light emitting device.

[0081] (Variation Example)

[0082] Fig.14 1 is a circuit diagram of a light emitting element driving circuit 1A showing a modification of the above-mentioned embodiment. In this modification, the node N2 of one of the driving circuit units 2A to 2D (for example, the driving circuit unit 2D) is electrically connected directly (with a substantially zero resistance) to the reference potential line 11 without passing through the current interruption element 8. In addition, the nodes N2 of the driving circuit units 2A to 2D are connected to each other via the current interruption element 8 without short-circuiting. Therefore, the node N2 of the driving circuit unit 2C is electrically connected to the reference potential line 11 via one current interruption element 8, the node N2 of the driving circuit unit 2B is electrically connected to the reference potential line 11 via two current interruption elements 8, and the node N2 of the driving circuit unit 2A is electrically connected to the reference potential line 11 via three current interruption elements 8.

[0083] The connection method between the node N2 of the driving circuit units 2A to 2D and the reference potential line 11 is not limited to Figure 1 In this variation, the nodes N2 of the drive circuit units 2A to 2D are connected to each other via the current interruption element 8 without short-circuiting each other, and are electrically connected to the reference potential line 11 via the current interruption element 8 or directly, so that the same effect as the above-mentioned embodiment can be obtained.

[0084] The light emitting element driving circuit of the present invention is not limited to the above-mentioned embodiments and modifications, and other various modifications are possible. For example, Figure 1 In the example of the filter constituting the current cutoff elements 8 and 9, an inductor is shown. However, as the filter constituting the current cutoff elements 8 and 9, if it is a low-pass filter or a band-pass filter that can cut off or suppress the high-frequency (e.g., GHz band) AC current of pulse drive and pass the low-frequency (e.g., DC to kHz band) current, then various other elements such as chip ferrite beads and RC filter circuits can be applied. In particular, the chip ferrite beads can be a single chip for each drive circuit, and the light-emitting element drive circuit 1 can be easily constructed.

[0085] In addition, at least one of the current interruption elements 8 and 9 does not necessarily have to be a filter. Figure 1 The current cutoff elements 8 and 9 shown, as elements for at least cutting off or suppressing current of a specific frequency, may also include a resistance element instead of the inductor (or in addition to the inductor). In particular, when the driving cycle of the light-emitting element 4 is relatively long, the current cutoff elements 8 and 9 may also be composed of only resistance elements. In this case, one end of the resistance element as the current cutoff element 8 is electrically connected to the node N2, and the other end is electrically connected to the reference potential line 11. In addition, the other electrode of the capacitor 5 is electrically connected to the power supply line 10 via the resistance element as the current cutoff element 9. However, in this case, since the current is suppressed in the entire frequency band area, the power loss becomes large, and the recharging of the capacitor 5 takes time. In the case where the current cutoff elements 8 and 9 are composed of filters, the power loss can be reduced, and the recharging time of the capacitor 5 can be shortened.

[0086] In addition, as mentioned above, Figure 1At least one of the current cutoff elements 8 and 9, as an element for at least cutting off or suppressing the current during a specific period, may also include a switching element such as a transistor instead of the inductor (or in addition to the inductor). In this case, the switching element is turned off during a certain period including the period when the transistor 3 is in the on state (that is, the light-emitting period of the light-emitting element 4), and the switching element is turned on during other periods. Even in such a case, the same effect as the above-mentioned embodiment can be obtained. However, it is further necessary to use a number of switching elements and a control circuit for driving these switching elements as many as the number of driving circuits. In the case where the current cutoff elements 8 and 9 are composed of filters, a control circuit is not required, and they can be constructed more cheaply than switching elements, so it is advantageous in reducing manufacturing costs and miniaturizing the device.

[0087] In addition, Figure 1 , the example in which each of the driving circuit units 2A to 2D drives one light emitting element 4 is shown, but it is also possible that each of the driving circuit units 2A to 2D drives two or more light emitting elements 4. In other words, the number of light emitting elements 4 may be greater than the number of the driving circuit units 2A to 2D. In this case, Figure 1 Each of the light emitting elements 4 shown is replaced with two or more light emitting elements 4 connected in parallel or in series.

[0088] Explanation of symbols

[0089] 1, 1A... light-emitting element driving circuit; 2A to 2D... driving circuit unit; 3... transistor; 4, 4A... light-emitting element (semiconductor light-emitting element); 5... capacitor (first power storage element); 6... current control element; 7... capacitor (second power storage element); 8... current cutoff element (first current cutoff element); 9... current cutoff element (second current cutoff element); 10... power supply line (first constant potential line); 10a, 10b... virtual power supply line; 11... reference potential line (second constant potential line); 11a , 11b...virtual reference potential line; 12...parasitic diode; 13...resistance element; 14...semiconductor light emitting element array; 15...anode terminal; 16...cathode terminal; 17...semiconductor substrate; 20A, 20B, 20C...driving circuit; 21...transistor; 22...capacitor; 23...resistance element; 24, 25...transistor; 26, 27...resistance element; 30...driving circuit; N1...node (1st node); N2...node (2nd node); N3...node (3rd node).

Claims

1. A light emitting element driving circuit, wherein: The invention relates to a light emitting element driving circuit which is configured to individually drive and control a plurality of semiconductor light emitting elements each having a cathode set to the same potential. The light emitting element driving circuit comprises: a plurality of driving circuit units which are provided in one-to-one correspondence with the plurality of semiconductor light emitting elements and drive the semiconductor light emitting elements corresponding to the respective ones individually; The plurality of driving circuit units respectively include: a current control element having a first terminal electrically connected to the cathode of the corresponding semiconductor light emitting element via a first node, and a second terminal electrically connected to the anode of the corresponding semiconductor light emitting element via a second node, wherein the current control element flows a current from the first node to the second node during a period when the corresponding semiconductor light emitting element is in a non-light emitting state, and limits the current during a period when the corresponding semiconductor light emitting element is in a light emitting state; a first power storage element having a first electrode electrically connected to the first node and a second electrode; and a switch element having a control terminal to which a drive control signal of the corresponding semiconductor light emitting element is input, a first current terminal electrically connected to the second electrode of the first power storage element, and a second current terminal electrically connected to the second node, The second electrodes of the first power storage elements in the plurality of drive circuit units are electrically connected to first constant potential lines, respectively. The second nodes in the plurality of driving circuit units are respectively arranged on wirings between two second nodes adjacent to each other, and are electrically connected via a first current interruption element for avoiding a short circuit between the two second nodes adjacent to each other, or directly to a second constant potential line set to a lower potential than the first constant potential line, The first current cutoff element is an element that cuts off or suppresses at least a current of a specific frequency or a current of a specific period, and the first current cutoff element cuts off or suppresses the current at least during a period in which the switching element is in an on state.

2. The light emitting element driving circuit according to claim 1, wherein: Each of the plurality of drive circuit units further includes a second current interruption element disposed on a wiring line between the first constant potential line and the second electrode of the first power storage element. The second current cutoff element is an element that cuts off or suppresses at least a current of a specific frequency or a current of a specific period, and the second current cutoff element cuts off or suppresses the current at least during a period in which the switching element is in an on state.

3. The light emitting element driving circuit according to claim 2, wherein: Each of the plurality of drive circuit units further includes a second power storage element including a first electrode electrically connected to a third node on a wiring line between the second electrode of the first power storage element and the second current interruption element, and a second electrode electrically connected to the second node.

4. The light emitting element driving circuit according to claim 3, wherein: The capacity of the second power storage element is larger than the capacity of the first power storage element.

5. The light emitting element driving circuit according to any one of claims 1 to 4, wherein: The first current interruption element includes an inductor.

6. The light emitting element driving circuit according to any one of claims 1 to 4, wherein: The current control element includes a resistance element.

7. The light emitting element driving circuit according to claim 5, wherein: The current control element includes a resistance element.

8. The light emitting element driving circuit according to any one of claims 1 to 4, wherein: The plurality of driving circuit units individually drive the plurality of semiconductor light emitting elements integrated on a common semiconductor substrate.

9. The light emitting element driving circuit according to claim 5, wherein: The plurality of driving circuit units individually drive the plurality of semiconductor light emitting elements integrated on a common semiconductor substrate.

10. The light emitting element driving circuit according to claim 6, wherein: The plurality of driving circuit units individually drive the plurality of semiconductor light emitting elements integrated on a common semiconductor substrate.

11. The light emitting element driving circuit according to claim 7, wherein: The plurality of driving circuit units individually drive the plurality of semiconductor light emitting elements integrated on a common semiconductor substrate.

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