Lightning circuit

Abstract

To provide a lightning circuit capable of suppressing heat generation.SOLUTION: A lightning circuit adopted to a vehicle lighting tool including a light emission element, comprises: a driving circuit that outputs a driving current for driving the light emission element when a power supply voltage is applied to a power supply line; a first resistor that is serially connected to the light emission element; a first switch that is parallely connected to the first resistance; and a switch control circuit that turns on the first switch in the case where the power supply voltage is lower than a predetermined level, and turns off the first switch in the case where the power supply voltage is higher than a predetermined level.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a lighting circuit. [Background technology]

[0002] A known lighting circuit applied to vehicle lamps is one in which a constant current circuit (drive circuit) is provided for a light source using a light-emitting element, and the light-emitting element is driven by a constant current (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-80999 A Summary of the Invention [Problem to be solved by the invention]

[0004] Meanwhile, a vehicle may be provided with a detection device that detects that a light source (light-emitting element) is disconnected when the current (hereinafter, input current) supplied from a power source on the vehicle side to a vehicle lamp is smaller than a predetermined value. In such vehicles, a current adjustment resistor is provided to increase the input current so that the light source is not mistakenly detected as disconnected when the input current is small. However, increasing the input current may increase heat generation in the drive circuit, etc.

[0005] An object of the present invention is to provide a lighting circuit capable of suppressing heat generation. [Means for solving the problem]

[0006] The main invention for achieving the above-mentioned object is a lighting circuit applied to a vehicle lamp including a light-emitting element, the lighting circuit comprising: a drive circuit that outputs a drive current for driving the light-emitting element when a power supply voltage is applied to a power supply line; a first resistor connected in series to the light-emitting element; a first switch connected in parallel to the first resistor; and a switch control circuit that turns on the first switch when the power supply voltage is lower than a predetermined level and turns off the first switch when the power supply voltage is higher than the predetermined level. Effect of the Invention

[0007] According to the present invention, it is possible to provide a lighting circuit capable of suppressing heat generation. [Brief description of the drawings]

[0008] [Figure 1] 1 is a diagram showing a configuration of a vehicle lamp 1 including a lighting circuit 10 according to a first embodiment. [Diagram 2] 1 is a diagram for explaining a change in input current Iin depending on the presence or absence of resistor Rd. FIG. [Diagram 3] FIG. 2 is a diagram showing an example of the configuration of a switch control circuit 15. [Figure 4] 4 is an explanatory diagram of the operation of the switch control circuit 15. FIG. [Diagram 5] FIG. 13 is a diagram showing a simulation result when the turn voltage Vt is high. [Figure 6] FIG. 13 is a diagram showing a simulation result when the turn voltage Vt is low. [Figure 7] FIG. 11 is a diagram showing a modified example of the first embodiment. [Figure 8] 1 is a diagram showing a configuration of a vehicle lamp 1A including a lighting circuit 10A according to a second embodiment. [Figure 9] FIG. 2 is a diagram illustrating an example of the configuration of a VDD circuit 16. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] At least the following points will become apparent from the description of this specification and the accompanying drawings.

[0010] In the following, the same or equivalent components, members, etc. shown in each drawing will be denoted by the same reference numerals, and duplicated explanations may be omitted as appropriate.

[0011] In this embodiment, "connection" refers to a state in which two components are electrically connected unless otherwise specified. Therefore, "connection" includes cases in which two components are connected not only through wiring but also through, for example, a resistor.

[0012] =====First embodiment===== <<Configuration of vehicle lamp 1>> 1 is a diagram showing the configuration of a vehicle lamp 1 including a lighting circuit 10 according to a first embodiment. The vehicle lamp 1 of this embodiment is a turn signal lamp (vehicle direction indicator lamp).

[0013] As shown in FIG. 1, a vehicle lamp 1 includes a switch 4, a light source 6, and a lighting circuit 10.

[0014] The switch 4 is an element for applying a power supply voltage Vbat (hereinafter also simply referred to as voltage Vbat) of the vehicle battery 2 to the power supply line L1 of the vehicle lamp 1. For example, a mechanical contact relay or a contactless relay using a semiconductor element is used as the switch 4.

[0015] The voltage Vbat of the battery 2 is applied to one end of the switch 4, and the other end is connected to the power supply line L1.

[0016] For example, when a driver of a vehicle operates a turn signal to turn on a vehicle lamp 1 (turn signal lamp), a control circuit (not shown) such as a microcomputer controls the on / off of switch 4 (periodically turns switch 4 on and off).

[0017] When the switch 4 is turned on, the voltage Vbat of the battery 2 is applied to the power supply line L1 via the switch 4. As a result, the voltage of the power supply line L1 becomes the voltage Vbat. Note that here, the voltage drop due to elements (e.g., diodes) (not shown) arranged on the power supply line L1 is ignored.

[0018] When the switch 4 is turned off, the voltage Vbat of the battery 2 is no longer applied to the power supply line L1. As a result, the voltage of the power supply line L1 drops to zero due to the influence of, for example, the resistance and circuit between the power supply line L1 and the ground.

[0019] Therefore, by turning the switch 4 on and off, the voltage of the power supply line L1 becomes a pulsed voltage in which a high level (hereinafter, H level) and a low level (hereinafter, L level) are alternately repeated. This pulsed voltage is hereinafter referred to as a turn voltage Vt. In this manner, the voltage Vbat of the battery 2 is periodically applied to the power supply line L1. Note that the voltage Vbat of the battery 2 is not always constant, but fluctuates between, for example, 8V and 16V.

[0020] The power supply line L1 is a wiring that supplies a turn voltage Vt to the circuit inside the lighting circuit 10. In this embodiment, all the lines (inside and outside the lighting circuit 10) connected to the switch 4 are defined as the power supply line L1. The vehicle lamp 1 is also provided with a ground line (not shown). The parts of each circuit in the lighting circuit 10 that are grounded are connected to the ground line.

[0021] The light source 6 includes a plurality of light-emitting elements (two light-emitting elements D1 and D2 in this embodiment) connected in series, and is lit by being supplied with a drive current Ik from a drive circuit 12 of a lighting circuit 10 described later. The light-emitting elements D1 and D2 in this embodiment are light-emitting diodes (LEDs). However, the light-emitting elements D1 and D2 are not limited to LEDs, and may be other semiconductor light-emitting elements such as laser diodes (LDs) and organic EL elements, or incandescent lamps. The light source 6 in this embodiment is composed of two light-emitting elements D1 and D2, but is not limited to this, and may be composed of one or three or more light-emitting elements. For example, a plurality of light-emitting elements may be connected in parallel.

[0022] The lighting circuit 10 is a circuit that lights up the light source 6 based on the application of a voltage Vbat to the power supply line L1. The lighting circuit 10 will be described in detail later, but the lighting circuit 10 is a module in which a plurality of circuits for lighting the light source 6 (light-emitting elements D1, D2) are attached to a board.

[0023] <<Configuration of lighting circuit 10>> As shown in FIG. 1, the lighting circuit 10 includes a drive circuit 12, a control IC 13, a switch control circuit 15, a resistor Rd, and an NMOS transistor Q1.

[0024] When the switch 4 is turned on and a voltage Vbat is applied to the power supply line L1, the drive circuit 12 generates a voltage according to the load based on the power supplied from the power supply line L1, and generates a predetermined drive current Ik (constant current) for driving the light emitting elements D1 and D2 of the light source 6. The method for generating the constant current is not particularly limited, and may be, for example, a switching method or a linear method. The generated drive current Ik is output from an output node 12a of the drive circuit 12 and supplied to the light emitting elements D1 and D2 of the light source 6 (via a resistor Rd or an NMOS transistor Q1).

[0025] Furthermore, when the switch 4 is turned off and the voltage Vbat is no longer applied to the power supply line L1, the drive circuit 12 stops supplying the drive current Ik.

[0026] The control IC 13 operates when a voltage Vbat is applied to the power supply line L1 (when the turn voltage Vt becomes H level), and while monitoring the output of the drive circuit 12, controls the drive circuit 12 so that a drive current Ik is output from the drive circuit 12.

[0027] The resistor Rd is provided between the output node 12a of the drive circuit 12 and the light source 6 (light-emitting elements D1, D2), and is connected in series to the light-emitting elements D1, D2. As will be described later, this resistor Rd is a resistor for adjusting the input current Iin, and corresponds to a "first resistor." In this embodiment, the resistor Rd is one, but is not limited to this, and for example, a plurality of resistors may be connected in parallel.

[0028] The NMOS transistor Q1 is connected in parallel to the resistor Rd and has a switch function. The NMOS transistor Q1 corresponds to a "first switch." The connection point between the NMOS transistor Q1 and the light source 6 (specifically, the anode of the light-emitting element D1) is defined as a node N1. The node N1 corresponds to a "node." The voltage Vs of the node N1 is higher than the ground potential by the sum of the forward voltages of the light-emitting elements D1 and D2.

[0029] If a PMOS transistor is used as the switch, the configuration of the switch control circuit 15 (described later) can be simplified. However, PMOS transistors are expensive and have a large on-resistance. Therefore, in this embodiment, an NMOS transistor Q1 is used. This makes it possible to reduce the on-resistance when the NMOS transistor Q1 is turned on, and to suppress heat generation. Note that the switch is not limited to a MOS transistor (NMOS transistor Q1), and a bipolar transistor (NPN transistor), for example, may be used.

[0030] The switch control circuit 15 controls the on / off of the NMOS transistor Q1 according to the voltage level of the turn voltage Vt (power supply voltage Vbat). The switch control circuit 15 will be described in detail later.

[0031] <Tolerance of input current Iin> Meanwhile, a vehicle incorporating the vehicular lamp 1 of this embodiment is provided with a detection device (not shown) that detects whether or not there is a disconnection in the light-emitting elements D1, D2 of the vehicular lamp 1 based on the input current Iin from the battery 2. For example, when the vehicular lamp 1 is operating, this detection device detects that there is a disconnection in either of the light-emitting elements D1, D2 of the vehicular lamp 1 if the current value of the input current Iin is smaller than a predetermined threshold value.

[0032] Therefore, when the vehicle lamp 1 is operating in a normal state, the current value of the input current Iin needs to be greater than the threshold value so as not to erroneously detect that the light-emitting elements D1, D2 are disconnected. Here, the "normal state" refers to, for example, a state in which no disconnection occurs in the light-emitting elements D1, D2 of the light source 6 of the vehicle lamp 1. Also, the "disconnection of the light-emitting element" refers to, for example, a state in which the resistance value between the cathode and anode of the light-emitting element is sufficiently greater than the normal resistance value.

[0033] <About resistance Rd> The resistor Rd is a current adjusting resistor (dummy resistor) provided to increase the input current Iin so as to prevent erroneous detection by the detection device on the vehicle side when the input current Iin is small as described above.

[0034] 2 is a diagram for explaining the change in input current Iin depending on the presence or absence of resistor Rd. The horizontal axis of the diagram is the power supply voltage Vbat, and the vertical axis is the input current Iin. In addition, the dashed line in the diagram indicates the case where resistor Rd is not present, and the solid line indicates the case where resistor Rd is present. Note that NMOS transistor Q1 and switch control circuit 15 are not taken into consideration here.

[0035] As shown in the figure, the input current Iin decreases as the power supply voltage Vbat increases, and without resistor Rd (dashed line), the input current Iin is lower than the threshold value at voltages equal to or higher than V1 in the figure. In this case, when the power supply voltage Vbat exceeds V1, the vehicle-side detector may erroneously detect that the light source 6 (light-emitting elements D1 and D2) is disconnected.

[0036] Therefore, in this embodiment, a resistor Rd for adjusting the current is provided between the output node 12a of the drive circuit 12 and the light source 6 (light-emitting elements D1, D2). When the resistor Rd is provided in this manner, the load increases and the power consumption increases, so that the input current Iin becomes higher overall, as shown in Fig. 2. This makes it possible to make the input current Iin higher than the threshold value regardless of the voltage level of the power supply voltage Vbat, and prevents erroneous detection by the detection device on the vehicle side.

[0037] Note that, on the input side (switch 4 side) of the drive circuit 12, for example, a resistor for adjusting current may be provided between the power supply line L1 and ground. In this case, however, the current flowing through the resistor increases as the power supply voltage Vbat increases. This may result in a large input current Iin on the side where the power supply voltage Vbat is high. In this case, even when the light-emitting elements D1 and D2 are disconnected, a current continues to flow through the resistor for adjusting current, so it is necessary to provide a current interruption circuit or the like to interrupt the current.

[0038] In contrast, in this embodiment, if a resistor Rd is provided on the output side (the side where the output node 12a is located) of the drive circuit 12, when the light-emitting elements D1 and D2 are disconnected, no current will flow through the resistor Rd either, making the above-mentioned cut-off circuit unnecessary.

[0039] However, when resistor Rd is provided on the output side of drive circuit 12, as shown in Fig. 2, the input current Iin increases when the power supply voltage Vbat is low, and the amount of heat generated in drive circuit 12 increases. Furthermore, since resistor Rd also generates heat, there is a risk that the heat generation will further increase due to thermal interference with the heat generated in drive circuit 12. Furthermore, when an upper limit threshold is set for the input current Iin so that it does not exceed the rated current, for example, there is a risk that the upper limit threshold will be exceeded.

[0040] Therefore, in this embodiment, by providing the NMOS transistor Q1 and the switch control circuit 15, when the power supply voltage Vbat becomes equal to or lower than V2 (<V1) in FIG. 2, the input current Iin is made to transition from "with Rd" to "without Rd" (details will be described later). That is, in this embodiment, the input current Iin when the power supply voltage Vbat is higher than V2 is "with Rd", and the input current Iin when the power supply voltage Vbat is lower than V2 is "without Rd". By doing so, when the power supply voltage Vbat is low, an increase in the input current Iin (that is, heat generation) is suppressed.

[0041] <<Configuration of Switch Control Circuit 15>> FIG. 3 is a diagram showing an example of the configuration of the switch control circuit 15.

[0042] The switch control circuit 15 includes a capacitor C1, resistors R1 to R3, a Zener diode D10, an NMOS transistor Q2, a charging circuit 20, and an on-off circuit 30. Note that the capacitor C1 is described in the charging circuit 20 for convenience.

[0043] The capacitor C1 is a bootstrap capacitor provided to surely turn on the NMOS transistor Q1 (to ensure the potential difference between the gate and source of the NMOS transistor Q1). One end of the capacitor C1 is connected to the power supply line L1 (via a resistor 22 and a diode 21, which will be described later), and the other end is connected to the node N1. Note that the capacitor C1 corresponds to the "first capacitor".

[0044] <Charging Circuit 20> The charging circuit 20 is a circuit that charges the capacitor C1 for a predetermined period when the turn-on voltage Vt becomes the H level (in other words, when the voltage Vbat is applied to the power supply line L1). The charging circuit 20 includes a diode 21, resistors 22, 24, 25, a capacitor 23, a Zener diode 26, and an NMOS transistor 27.

[0045] The diode 21 is a diode for preventing reverse current. The anode of the diode 21 is connected to the power supply line L1, and the cathode is connected to one end of the capacitor C1 via a current limiting resistor 22. The connection point between the resistor 22 and the capacitor C1 is referred to as a node N2.

[0046] The NMOS transistor 27 is a transistor that controls the charging of the capacitor C1. The drain of the NMOS transistor 27 is connected to the other end of the capacitor C1 and the node N1, and the source is grounded.

[0047] The capacitor 23 and the resistors 24 and 25 are connected in series, the capacitor 23 is connected to the power supply line L1 (to which a turn voltage Vt is applied), and the resistor 25 is grounded. In addition, the resistors 24 and 25 are connected to the gate of an NMOS transistor 27.

[0048] The capacitor 23 and resistors 24 and 25 form a differentiation circuit (high-pass filter), which applies a voltage equal to or higher than the threshold to the gate electrode of the NMOS transistor 27 only for a predetermined period during the rising edge of the turn voltage Vt, thereby turning on the NMOS transistor 27. When the NMOS transistor 27 is turned on, the capacitor C1 is charged.

[0049] The Zener diode 26 is an element (clamp element) for protecting the gate of the NMOS transistor 27, and is connected in parallel to the resistor 25. Specifically, the anode of the Zener diode 26 is grounded, and the cathode is connected to the gate of the NMOS transistor 27.

[0050] <On / Off circuit 30> The on / off circuit 30 is a circuit that operates based on the charging voltage of the capacitor C1, and turns on the NMOS transistor Q1 when the turn voltage Vt (voltage Vbat) is lower than a predetermined level (V2 in FIG. 2, 10 V in this embodiment), and turns off the NMOS transistor Q1 when the turn voltage Vt is higher than the predetermined level (10 V). However, the on / off circuit 30 is configured to turn on the NMOS transistor Q1 after the period for charging the capacitor C1 of the charging circuit 20 (the above-mentioned predetermined period) has elapsed, even if the turn voltage Vt is lower than 10 V.

[0051] The on / off circuit 30 includes a comparator COM1, a capacitor C2, Zener diodes 41 and 46, and resistors 42-45.

[0052] The comparator COM1 has a positive power supply terminal, a negative power supply terminal, a non-inverting input terminal (hereinafter, + terminal), an inverting input terminal (hereinafter, - terminal), and an output terminal. The output terminal of the comparator COM1 is connected to the gate electrode (control electrode) of the NMOS transistor Q1 via a resistor R1. The comparator COM1 outputs a signal according to a comparison between the + terminal and the - terminal, and controls the ON / OFF of the NMOS transistor Q1. The output terminal of the comparator COM1 is connected to a node N2 via a resistor R2, and is connected to the drain of the NMOS transistor Q2 via a resistor R3.

[0053] The positive power supply terminal of the comparator COM1 is connected to the node N2, and a voltage based on the charging voltage of the capacitor C1 is applied to it. The positive power supply terminal of the comparator COM1 is also connected to the cathode of a Zener diode D10. The voltage Vs of the node N1 is applied to the anode of the Zener diode D10. This Zener diode D10 is provided to protect (clamp) the voltage of the positive power supply terminal of the comparator COM1 from rising too high.

[0054] The voltage Vs of the node N1 is applied to the negative power supply terminal of the comparator COM1. Therefore, when the voltage of the + terminal is lower than the voltage of the - terminal, the comparator COM1 outputs the voltage Vs. Note that the comparator COM1 in this embodiment is an open-drain type, and when the voltage of the + terminal is higher than the voltage of the - terminal, the output is open (high impedance).

[0055] The negative terminal of the comparator COM1 is connected to a connection point (hereinafter, node N3) between the anode of the Zener diode 41 and the resistor 42 of the Zener diode 41 and resistors 42 and 43 that are connected in series between the power supply line L1 and the ground. The connection point between the resistors 42 and 43 is node N4, and the voltage of the node N4 is Vp.

[0056] The positive terminal of the comparator COM1 is connected to a connection point (hereinafter, node N5) between resistors 44 and 45 that are connected in series between the power supply line L1 and the ground. The resistor 45 not only divides the turn voltage Vt together with the resistor 44, but also has the function of discharging the charge stored in a capacitor C2 (described later) to the ground while the turn voltage Vt is at the L level.

[0057] The capacitor C2 is connected in parallel to the resistor 45. Specifically, one end of the capacitor C2 is connected to the positive terminal of the comparator COM1, and the other end is grounded. Therefore, when the turn voltage Vt becomes H level, the capacitor C2 is charged, but the time constant is set so that the charging time of the capacitor C2 is longer than the charging time of the capacitor C1 by the charging circuit 20. Then, for a predetermined period after the turn voltage Vt becomes H level, the output of the comparator COM1 is set to be L level (voltage Vs) regardless of the level of the turn voltage Vt. The capacitor C2 corresponds to the "second capacitor."

[0058] The Zener diode 46 is connected in parallel to the resistor 45. Specifically, the cathode of the Zener diode 46 is connected to the node N5, and the anode is grounded. The Zener diode 46 clamps the voltage of the node N5 (the charging voltage of the capacitor C2 and the + terminal of the comparator COM1) to a breakdown voltage (for example, 2.4V). That is, the capacitor C2 is charged only up to the breakdown voltage of the Zener diode 46, regardless of the magnitude of the turn voltage Vt. This ensures that the charge stored in the capacitor C2 can be discharged through the resistor 45 during the period when the turn voltage Vt is at the L level. In addition, even if the power supply voltage Vbat fluctuates, the voltage (reference voltage) of the + terminal of the comparator COM1 can be fixed to a constant voltage.

[0059] In this embodiment, the Zener diode 41, the resistors 42 and 43, the Zener diode 46, and the resistors 44 and 45 correspond to a "voltage output circuit." That is, when the turn voltage Vt (voltage Vbat) is lower than 10V, the voltage level of the node N5 (positive terminal of the comparator COM1) is higher than the voltage level of the node N3 (negative terminal of the comparator COM1), and when the turn voltage Vt (voltage Vbat) is higher than 10V, the voltage level of the node N5 is lower than the voltage level of the node N3, so that their respective resistance values ​​and breakdown voltages are set. Note that the voltage (voltage output to the positive terminal of the comparator COM1) generated at the node N5 by the Zener diode 46 and the resistors 44 and 45 corresponds to a "first voltage," and the voltage (voltage output to the negative terminal of the comparator COM1) generated at the node N3 by the Zener diode 41 and the resistors 42 and 43 corresponds to a "second voltage."

[0060] <NMOSトランジスタQ2> The NMOS transistor Q2 is a transistor for discharging the capacitance of the capacitor C1 and the gate electrode of the NMOS transistor Q1 when the turn-on voltage Vt (voltage Vbat) is higher than 10 V (V2 in FIG. 2). The transistor Q2 corresponds to a "second switch."

[0061] The drain of the NMOS transistor Q2 is connected to the output terminal of the comparator COM1 via a current limiting resistor R3, and the source is grounded. The voltage Vp of the node N4 is applied to the gate of the NMOS transistor Q2. The breakdown voltage of the Zener diode 41 and the resistance values ​​of the resistors 42 and 43 are set so that the voltage Vp of the node N4 is greater than the threshold of the NMOS transistor Q2 when the turn voltage Vt is greater than 10V. Therefore, when the turn voltage Vt is greater than 10V, the NMOS transistor Q2 is turned on.

[0062] <<Operation of switch control circuit 15>> FIG. 4 is a diagram illustrating the operation of the switch control circuit 15. As shown in FIG.

[0063] First, a description will be given of a state in which the turn voltage Vt is at the L level and the capacitors C1 and C2 are not charged. In this case, the NMOS transistor Q1 and the NMOS transistor Q2 are off, and the comparator COM1 is not operating.

[0064] <When turn voltage Vt rises> When the power supply voltage Vbat is applied to the power supply line L1, the turn voltage Vt goes from L level to H level. During a certain period of time during the rise of the turn voltage Vt, a voltage higher than the threshold is applied to the gate of the NMOS transistor 27 of the charging circuit 20, and the NMOS transistor 27 turns on. When the NMOS transistor 27 turns on, a current flows from the power supply line L1 through a path (path kj in FIG. 4) of the diode 21 → resistor 22 → capacitor C1 → NMOS transistor 27 → ground, and the capacitor C1 is charged. As a result, the voltage of the capacitor C1 becomes higher than the voltage Vs of the node N1. Specifically, if the forward voltage of the diode 21 is Vf, the charging voltage of the capacitor C1 is Vt(Vbat)+Vs-Vf.

[0065] Furthermore, when the turn voltage Vt becomes H level, the capacitor C2 is charged via the resistor 44. The time constant and the like are set so that the charging period of the capacitor C2 is longer than the charging period of the capacitor C1, and regardless of the magnitude of the turn voltage Vt, the voltage of the node N5 becomes lower than the voltage of the node N3 during the charging period of the capacitor C2 (i.e., the output of the comparator COM1 is L level). As a result, the NMOS transistor Q1 is turned off.

[0066] <When turn voltage Vt is higher than 10V> When the turn-on voltage Vt is higher than 10V, the voltage Vp of the node N4 becomes higher than the threshold of the NMOS transistor Q2, and the NMOS transistor Q2 turns on. Therefore, the capacitance of the capacitor C1 and the gate electrode of the NMOS transistor Q1 is discharged. By discharging in this manner, it is possible to prevent malfunctions such as the NMOS transistor Q1 turning on based on the charging voltage of the capacitor C1. In addition, in this case, the voltage of the negative terminal (node ​​N3) of the comparator COM1 becomes higher than the voltage of the positive terminal (node ​​N5), and the output of the comparator COM1 becomes the L level (voltage Vs). Therefore, the NMOS transistor Q1 turns off, and the drive current Ik output from the drive circuit 12 flows to the light emitting elements D1 and D2 of the light source 6 via the resistor Rd (by the path ka shown in FIG. 4), and the light source 6 turns on. In this case, the power consumption can be increased by flowing to the light emitting elements D1 and D2 via the resistor Rd, and the input current Iin can be increased (see FIG. 2). Therefore, it is possible to prevent the detection device on the vehicle from erroneously detecting that a wire is broken.

[0067] <When turn voltage Vt is lower than 10V> When the turn-on voltage Vt is lower than 10V, the voltage Vp of the node N4 becomes lower than the threshold voltage of the NMOS transistor Q2, and the NMOS transistor Q2 turns off. As a result, the charging voltage of the capacitor C1 is supplied to the positive power supply terminal of the comparator COM1 via the node N2. The comparator COM1 operates based on the charging voltage of the capacitor C1, and compares the magnitude of the voltages of the + terminal and - terminal. In this case, the voltage of the - terminal (node ​​N3) of the comparator COM1 is lower than the voltage of the + terminal (node ​​N5), the comparator COM1 becomes an open output, and the voltage of the node N2 (the charging voltage of the capacitor C1) is applied to the gate electrode of the NMOS transistor Q1 via the resistors R2 and R1, and the NMOS transistor Q1 turns on. Therefore, the drive current Ik flows to the light emitting elements D1 and D2 of the light source 6 via the NMOS transistor Q1 (via the path kb shown in FIG. 4), and the light source 6 turns on. As a result, no current flows through the resistor Rd, so that when the turn voltage Vt (voltage Vbat) is low, an increase in the input current Iin can be suppressed, and heat generation can be suppressed.

[0068] When the voltage Vbat is no longer applied to the power supply line L1 (when the turn voltage Vt becomes L level), the operation of the drive circuit 12 stops and the drive current Ik is no longer supplied to the light source 6 (light-emitting elements D1, D2), so that the light source 6 is turned off. The operation of the switch control circuit 15 also stops, and the charge stored in the capacitor C2 is discharged to ground via the resistor 45.

[0069] <Simulation results> Fig. 5 shows the simulation results when the turn voltage Vt is high. Fig. 6 shows the simulation results when the turn voltage Vt is low. The upper part of each figure shows the turn voltage Vt, and the lower part shows the drive current Ik for each path. In the drive current Ik, the solid line shows the current flowing through resistor Rd (current of path ka), and the dashed line shows the current flowing through NMOS transistor Q1 (current of path kb).

[0070] 5, the H level of the turn voltage Vt (voltage Vbat) is 12 V (> 10 V). This simulation result also confirms that during the period when the turn voltage Vt is at the H level, a constant current (drive current Ik) flows through the path ka (resistance Rd) (no current flows through the NMOS transistor Q1).

[0071] 6, the H level of the turn voltage Vt (voltage Vbat) is 8 V (<10 V). This simulation result also confirms that during the period when the turn voltage Vt is at the H level, a constant current (drive current Ik) flows through the path kb (NMOS transistor Q1) (no current flows through resistor Rd).

[0072] <<Modifications>> FIG. 7 is a diagram showing a configuration of a modified example of the first embodiment.

[0073] In this modification, a resistor Rd2 is provided.

[0074] The resistor Rd2 is a resistor for adjusting the input current Iin, similar to the resistor Rd. The resistor Rd2 is provided between the resistor Rd and the light source 6 (specifically, between the node N1 and the anode of the light-emitting element D1), and is connected in series to the resistor Rd. The resistor Rd2 corresponds to the "second resistor."

[0075] In this way, by configuring the current adjustment resistors in two stages, even if the input current Iin is below the threshold value when the power supply voltage Vbat is low and the resistor Rd alone is used, the input current Iin can be made to be above the threshold value. Note that in this modified example, the resistor Rd2 is configured with one resistor, but this is not limited thereto, and for example, it may be configured with multiple resistors connected in parallel.

[0076] ===Second embodiment=== FIG. 8 is a diagram showing the configuration of a vehicle lamp 1A including a lighting circuit 10A according to the second embodiment.

[0077] In the lighting circuit 10A of the second embodiment, the resistor Rd and the NMOS transistor Q1 are provided between the light source 6 and the ground. Specifically, the NMOS transistor Q1 and the resistor Rd are connected in parallel, the drain of the NMOS transistor Q1 is connected to the cathode of the light-emitting element D2 of the light source 6, and the source is grounded.

[0078] Further, the lighting circuit 10A includes a switch control circuit 15A and a VDD circuit 16.

[0079] <<VDD circuit 16>> FIG. 9 is a diagram showing an example of the configuration of the VDD circuit 16. The VDD circuit 16 shown in FIG. 9 includes a resistor 61, a Zener diode 62, an NPN transistor 63, and capacitors 64 and 65.

[0080] The resistor 61 and the Zener diode 62 are connected in series. A turn voltage Vt is applied to the resistor 61, and the anode of the Zener diode 62 is grounded. Further, the cathode of the Zener diode 62 is connected to the base of the NPN transistor 63. When the turn voltage Vt exceeds the breakdown voltage of the Zener diode 62, the Zener diode 62 clamps the base voltage of the NPN transistor 63 to the breakdown voltage.

[0081] The turn voltage Vt is applied to the collector of the NPN transistor 63. The NPN transistor 63 outputs a voltage corresponding to the base voltage (breakdown voltage of the Zener diode 62) from the emitter as the voltage VDD. Note that the NPN transistor 63 operates as a so-called emitter follower.

[0082] The capacitors 64 and 65 are noise-cutting capacitors. The capacitor 64 is connected between the resistor 61 and the ground, and the capacitor 65 is connected between the emitter of the NPN transistor 63 and the ground.

[0083] With the above configuration, when the turn voltage Vt is at H level, the NPN transistor 63 turns on and the base voltage is clamped to the breakdown voltage of the Zener diode 62. Therefore, the emitter voltage (voltage VDD) of the NPN transistor 63 becomes a voltage corresponding to the breakdown voltage of the Zener diode 62. On the other hand, when the turn voltage Vt is at L level, the NPN transistor 63 turns off and the emitter voltage (voltage VDD) becomes zero.

[0084] In this way, the VDD circuit 16 generates the voltage VDD while the turn voltage Vt is at H level, and stops generating the voltage VDD while the turn voltage Vt is at L level. Note that the configuration of the VDD circuit 16 is not limited to the above.

[0085] <<Switch control circuit 15A>> The switch control circuit 15A is a circuit for turning on and off the NMOS transistor Q1 in accordance with the voltage level of the turn voltage Vt. As shown in FIG 8, the switch control circuit 15A includes a comparator COM2, resistors R5 and R6, resistors 51, 52, 54 and 55, Zener diodes 53 and 56, and a capacitor C3.

[0086] The voltage VDD generated by the VDD circuit 16 is applied to the positive power supply terminal of the comparator COM2, and the negative power supply terminal is grounded. When the voltage at the - terminal is higher than the voltage at the + terminal, the comparator COM2 outputs an L level (ground level). On the other hand, when the voltage at the - terminal is lower than the voltage at the + terminal, the output of the comparator COM2 becomes high impedance.

[0087] Resistors R5 and R6 are connected in series, a voltage VDD is applied to resistor R5, and resistor R6 is grounded. The connection point between resistors R5 and R6 is connected to the output terminal of comparator COM2 and to the gate of NMOS transistor Q1.

[0088] Resistors 51 and 52 are connected in series between the power supply line L1 and the ground, and their connection point is connected to the negative terminal of comparator COM2.

[0089] A Zener diode 53 is connected in parallel to the resistor 52 and clamps the negative terminal of the comparator COM2 to a predetermined voltage.

[0090] Resistors 54 and 55 are connected in series between the power supply line L1 and the ground, and their connection point is connected to the + terminal of the comparator COM2.

[0091] A Zener diode 56 and a capacitor C3 are each provided in parallel with the resistor 55.

[0092] In addition, the breakdown voltages of each resistor (resistors 51, 52, 54, 55) and Zener diodes 53, 56 are determined so that when the turn voltage Vt (voltage Vbat) is higher than a predetermined level (V2 in Figure 2, here 10 V), the voltage between resistors 51 and 52 (the voltage at the - terminal of comparator COM2) is higher than the voltage between resistors 54 and 55 (the voltage at the + terminal of comparator COM2).

[0093] With the above configuration, when the turn voltage Vt (voltage Vbat) is higher than 10V, the NMOS transistor Q1 is turned off, and the drive current Ik flows to ground via the light source 6 and resistor Rd. This allows the input current Iin to be increased. On the other hand, when the turn voltage Vt (voltage Vbat) is lower than 10V, the output of the comparator COM2 becomes high impedance, and the divided voltage of the voltage VDD by resistors R5 and R6 is applied to the gate of the NMOS transistor Q1, turning the NMOS transistor Q1 on. Therefore, the drive current Ik does not flow through the current adjustment resistor Rd, and heat generation can be suppressed.

[0094] ===Summary=== The lighting circuit 10 and the like according to the present embodiment have been described above.

[0095] The lighting circuit 10 of the first embodiment is applied to a vehicle lamp 1 including light-emitting elements D1 and D2, and includes a drive circuit 12, a resistor Rd, an NMOS transistor Q1, and a switch control circuit 15. When the turn voltage Vt becomes H level (when the power supply voltage Vbat is applied to the power supply line L1), the drive circuit 12 outputs a drive current Ik for driving the light-emitting elements D1 and D2. The resistor Rd is connected in series to the light-emitting elements D1 and D2 in the rear stage of the drive circuit 12, and the NMOS transistor Q1 is connected in parallel to the resistor Rd. When the power supply voltage Vbat is lower than V2 (10V) in FIG. 2, the switch control circuit 15 turns on the NMOS transistor Q1, and when the power supply voltage Vbat is higher than V2 (10V), the switch control circuit 15 turns off the NMOS transistor Q1. As a result, when the power supply voltage Vbat is low, the drive current Ik does not flow through the resistor Rd, so that the input current Iin can be prevented from increasing, and heat generation can be suppressed. In addition, when the power supply voltage Vbat is high, a drive current Ik flows through resistor Rd (the load increases), so that the input current Iin can be increased, and a disconnection of the light source 6 (either light-emitting element D1 or D2) is not erroneously detected by the detection device on the vehicle side.

[0096] In the first embodiment, the resistor Rd and the NMOS transistor Q1 are provided between the output node of the drive circuit 12 and the light source 6 (light-emitting elements D1 and D2). This allows the cathode of the light-emitting element D2 of the light source 6 to be grounded, thereby suppressing the influence of external noise and the like.

[0097] In addition, an NMOS transistor Q1 is used as the transistor (first switch) connected in parallel to the resistor Rd, which makes it possible to reduce the on-resistance compared to a PMOS transistor, thereby further suppressing heat generation.

[0098] The switch control circuit 15 also includes a capacitor C1 connected to the node N1, a charging circuit 20, and an on / off circuit 30. When a power supply voltage Vbat is applied to the power supply line L1, the charging circuit 20 charges the capacitor C1. The on / off circuit 30 operates based on the charging voltage of the capacitor C1, and turns on the NMOS transistor Q1 when the power supply voltage Vbat is lower than 10V, and turns off the NMOS transistor Q1 when the power supply voltage Vbat is higher than 10V. This allows the on / off of the NMOS transistor Q1 to be controlled based on the charging voltage of the capacitor C1.

[0099] Furthermore, when the power supply voltage Vbat is applied to the power supply line L1, the charging circuit 20 charges the capacitor C1 for a predetermined period, and the on / off circuit 30 turns on the NMOS transistor Q1 when the power supply voltage Vbat is lower than V2 after the predetermined period has elapsed. This ensures that the NMOS transistor Q1 is turned on.

[0100] The on / off circuit 30 includes a comparator COM1, a capacitor C2, resistors 42 to 45, and Zener diodes 41 and 46. The voltage Vs of the node N1 is applied to the negative power supply terminal of the comparator COM1, and the voltage of the node N2 (the charging voltage of the capacitor C1) is applied to the positive power supply terminal. The capacitor C2 is connected to the + terminal of the comparator COM1. When the power supply voltage Vbat is lower than 10V, the voltage of the + terminal of the comparator COM1 becomes higher than the voltage of the - terminal, and when the power supply voltage Vbat is higher than 10V, the resistors 42, 43, 44, and 45 and the Zener diodes 41 and 46 output the voltage of the node N5 to the + terminal of the comparator COM1 and the voltage of the node N3 to the - terminal. This allows the output of the on / off circuit 30 (specifically, the comparator COM1) to be switched depending on the magnitude of the power supply voltage Vbat.

[0101] In addition, an NMOS transistor Q2 is provided to discharge the capacitance of the capacitor C1 and the gate electrode of the NMOS transistor Q1 when the power supply voltage Vbat is higher than 10 V. This makes it possible to prevent malfunctions such as the NMOS transistor Q1 turning on due to the charging voltage of the capacitor C1.

[0102] In the lighting circuit 10A of the second embodiment, the resistor Rd and the NMOS transistor Q1 are provided between the light source 6 (light-emitting elements D1, D2) and the ground, which simplifies the configuration of the switch control circuit 15A.

[0103] In the modification of the first embodiment, a resistor Rd2 is connected in series to the resistor Rd, which makes it possible to further increase the input current Iin and make it equal to or greater than the threshold value.

[0104] Moreover, the vehicle lamp 1 is a turn signal lamp, and a voltage Vbat is periodically applied to the power supply line L1. It is effective to apply the lighting circuit 10 to such a vehicle lamp 1.

[0105] The above-mentioned embodiment is for the purpose of facilitating understanding of the present invention, and is not intended to limit the present invention. Furthermore, the present invention can be modified or improved without departing from the spirit of the present invention, and it goes without saying that the present invention includes equivalents thereof. [Explanation of symbols]

[0106] 1,1A Vehicle lighting fixtures 2 Battery 4 Switch 6 light source 10,10A lighting circuit 12 Drive circuit 12a Output Node 13 Control IC 15,15A Switch control circuit 16 VDD circuit 20 Charging circuit 21 Diode 22,24,25 Resistance 23 Capacitor 26 Zener Diode 27 NMOS transistor 30 On / Off Circuit 41,46 Zener diode 42, 43, 44, 45 Resistance 51, 52, 54, 55 Resistance 53,56 Zener diode 61 Resistance 62 Zener Diode 63 NPN transistor 64,65 Capacitor C1, C2, C3 capacitors COM1, COM2 comparator D1.D2 Light emitting element D10 Zener diode L1 Power line Vbat power supply voltage Vt Turn voltage Iin Input current Ik Drive current N1~N5 nodes Rd,Rd2,R1~R6 Resistance Q1, Q2 NMOS transistors

Claims

1. A lighting circuit applied to a vehicle lamp including a light emitting element, a drive circuit that outputs a drive current for driving the light emitting element when a power supply voltage is applied to a power supply line; A first resistor connected in series to the light emitting element; a first switch connected in parallel with the first resistor; a switch control circuit that turns on the first switch when the power supply voltage is lower than a predetermined level, and turns off the first switch when the power supply voltage is higher than the predetermined level; A lighting circuit comprising:

2. 2. The lighting circuit according to claim 1, the first resistor and the first switch are provided between an output node of the drive circuit and the light-emitting element; Lighting circuit.

3. 3. The lighting circuit according to claim 2, The first switch is an NMOS transistor or an NPN transistor. Lighting circuit.

4. 4. The lighting circuit according to claim 3, The switch control circuit includes: a first capacitor connected to a node to which the first switch and the light emitting element are connected; a charging circuit that charges the first capacitor when the power supply voltage is applied to the power supply line; an on / off circuit that operates based on a charging voltage of the first capacitor, and turns on the first switch when the power supply voltage is lower than the predetermined level, and turns off the first switch when the power supply voltage is higher than the predetermined level; A lighting circuit comprising:

5. The lighting circuit according to claim 4, The charging circuit includes: When the power supply voltage is applied to the power supply line, the first capacitor is charged for a predetermined period of time; The on / off circuit is after the predetermined period has elapsed, if the power supply voltage is lower than the predetermined level, turning on the first switch; Lighting circuit.

6. 6. The lighting circuit according to claim 5, The on / off circuit is a comparator having a negative power supply terminal to which the voltage of the node is applied, a positive power supply terminal to which the charging voltage is applied, a non-inverting input terminal, and an inverting input terminal, and controlling the first switch; a second capacitor connected to the non-inverting input terminal; a voltage output circuit that outputs a first voltage to the non-inverting input terminal and a second voltage to the inverting input terminal so that a voltage level of the non-inverting input terminal becomes higher than a voltage level of the inverting input terminal when the power supply voltage is lower than the predetermined level, and so that a voltage level of the non-inverting input terminal becomes lower than a voltage level of the inverting input terminal when the power supply voltage is higher than the predetermined level; A lighting circuit comprising:

7. The lighting circuit according to claim 4, a second switch for discharging the capacitance of the first capacitor and a control electrode of the first switch when the power supply voltage is greater than the predetermined level; A lighting circuit comprising:

8. 2. The lighting circuit according to claim 1, The first resistor and the first switch are provided between the light emitting element and ground. Lighting circuit.

9. 2. The lighting circuit according to claim 1, further comprising a second resistor connected in series with the first resistor; Lighting circuit.

10. The lighting circuit according to any one of claims 1 to 9, The vehicle lamp is a turn signal lamp, The power supply voltage is periodically applied to the power supply line. Lighting circuit.

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