Failure detection circuit and imaging device

Abstract

To provide a failure detection circuit capable of detecting a light emitting element in which a failure has occurred, and suppressing an increase in circuit scale.SOLUTION: The failure detection circuit (11) includes detection circuit units (D1, D2, and D3) that are provided for the light emitting elements (D1, D2, and D3), respectively, and detect voltages across the corresponding light emitting elements (111a, 111b, and 111c), respectively, the outputs of the detecting circuits (111a, 111b, and 111c) are connected to one of the outputting terminals (2a), and are composed of rectangular waves corresponding to the light-emitting elements (D1, D2, and D3), respectively, and each of the rectangular waves outputs from the outputting terminals (2a) an outputting signal having a different time difference with respect to a flash signal for causing the light-emitting elements to emit light.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a failure detection circuit and an imaging device including the same. [Background technology]

[0002] Driver monitoring systems (hereinafter referred to as DMS) for ensuring the safety of automobile occupants have become widespread. DMSs generally recognize the state of occupants by performing image analysis on images of the occupants captured using infrared light. An imaging device for capturing an image of an occupant, for example, illuminates the occupant as a subject with infrared light emitted from a light source and captures an image of the illuminated occupant using an image sensor.

[0003] The light source provided in the imaging device is made up of multiple infrared light emitting diodes (hereinafter referred to as IR-LEDs) connected in series. If one of the multiple IR-LEDs goes out due to a short circuit, the light source as a whole continues to emit light even though the preset light distribution is not met, so the outage may go unnoticed.

[0004] For example, Patent Document 1 describes a lighting control device that does not detect failures in the IR-LEDs used to capture images of occupants in a DMS, but detects failures in multiple LEDs that make up a vehicle headlamp. In this lighting control device, a Vf detection circuit is connected in parallel to both ends of each of multiple LEDs connected in series. By comparing the forward voltages Vf periodically detected by these Vf detection circuits, a short circuit that has occurred in any of the multiple LEDs is detected. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-011872 Summary of the Invention [Problem to be solved by the invention]

[0006] The conventional technology described in Patent Document 1 acquires the forward voltages of a plurality of light-emitting elements and detects a fault occurring in any of the plurality of light-emitting elements based on the results of comparing these voltages. For this reason, the conventional technology requires providing a plurality of output terminals for outputting detection signals for each light-emitting element to a microcontroller (hereinafter referred to as MCU) that detects a fault in the light-emitting element based on the forward voltage detection signals, which poses a problem of increasing the size of the circuit.

[0007] In particular, the lighting control device described in Patent Document 1 is configured with a relatively large circuit designed to control the lighting of vehicle headlamps, and therefore an increase in circuit size due to the provision of multiple output terminals for outputting detection signals for each light-emitting element is acceptable. However, the circuit that detects failures in the IR-LEDs used to capture images of occupants in the DMS is mounted on a small imaging device. Therefore, it is necessary to suppress the increase in circuit size.

[0008] The present disclosure is intended to solve the above-mentioned problems, and has an object to provide a failure detection circuit that is capable of detecting a light-emitting element in which a failure has occurred and that can prevent an increase in the circuit size. [Means for solving the problem]

[0009] The fault detection circuit of the present disclosure comprises a plurality of detection circuit sections provided for each of a plurality of light-emitting elements, each detecting the voltage across the corresponding light-emitting element, the outputs of the plurality of detection circuit sections being connected to a single output terminal and consisting of a plurality of rectangular waves corresponding to each of the plurality of light-emitting elements, and each of the plurality of rectangular waves outputting an output signal from the output terminal having a different time difference from a flash signal for emitting light to the light-emitting element. [Effects of the Invention]

[0010] According to the present disclosure, the outputs of a plurality of detection circuit units that detect the voltages across the light-emitting elements are connected to a single output terminal, and an output signal consisting of a plurality of rectangular waves corresponding to each of the plurality of light-emitting elements, each of the plurality of rectangular waves having a different time difference from one another with respect to a flash signal for emitting light from the light-emitting element, is output from the output terminal. Since the output signal output from the single output terminal can identify a light-emitting element that has failed, the fault detection circuit according to the present disclosure is capable of detecting a light-emitting element that has failed while preventing an increase in circuit size. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a block diagram showing an example of the configuration of an imaging device according to a first embodiment. [Figure 2] 1 is a circuit diagram showing a configuration example of a failure detection circuit according to a first embodiment. [Figure 3] 3 is a waveform diagram showing voltage waveforms at various points in the failure detection circuit according to the first embodiment. FIG. [Figure 4] FIG. 4A is a waveform diagram showing the voltage waveforms at various points before the light-emitting element D1 is short-circuited, and FIG. 4B is a waveform diagram showing the voltage waveforms at various points after the light-emitting element D1 is short-circuited. [Figure 5] 5A and 5B are waveform diagrams showing voltage waveforms after light-emitting elements D2 and D3 are short-circuited. DETAILED DESCRIPTION OF THE INVENTION

[0012] Embodiment 1 FIG. 1 is a block diagram showing an example of the configuration of an imaging device 1 according to a first embodiment. In FIG. 1, the imaging device 1 is, for example, a device constituting a DMS mounted on a vehicle, and illuminates a vehicle driver with infrared light and captures an image of the illuminated driver. The imaging device 1 is connected to an MCU 2 via a signal line. The MCU 2 controls the imaging process of the imaging device 1 by executing a control program stored in a memory (not shown in FIG. 1). For example, the MCU 2 determines whether a short-circuit fault has occurred in the light source based on a voltage detection signal output from a fault detection circuit 11.

[0013] (Configuration example of imaging device) As shown in FIG. 1, the imaging device 1 includes a fault detection circuit 11, light-emitting elements D1, D2, and D3, a drive circuit 12, and an image sensor 13. The fault detection circuit 11 detects short-circuit faults in the light-emitting elements D1, D2, and D3. The fault detection circuit 11 is connected to an MCU 2 via one output terminal 2a. A voltage detection signal detected by the fault detection circuit 11 is output to the MCU 2 via the output terminal 2a. The fault detection circuit 11 is further connected to a drive circuit 12 via an input terminal 12a and an output terminal 12b. By being connected to the drive circuit 12 via the input terminal 12a and the output terminal 12b, the fault detection circuit 11 forms a closed circuit with the drive circuit 12.

[0014] Light-emitting elements D1, D2, and D3 are IR-LEDs that constitute an infrared light source that emits illumination light. The IR-LED light-emitting elements D1, D2, and D3 are light-emitting diodes that emit near-infrared illumination light. As shown in Figure 1, the light-emitting elements D1, D2, and D3 are connected in series. When an LED current flows through a closed circuit, the light-emitting elements D1, D2, and D3 emit near-infrared light for imaging, generating a forward voltage Vf.

[0015] 1 shows a case where there are three IR-LEDs, but the imaging device 1 is not limited to this. For example, the number of IR-LEDs connected in series in the imaging device 1 may be any number, such as four or more. Furthermore, the light source made up of IR-LEDs may be configured integrally with the imaging device 1, or may be provided externally (for example, detachably) to the imaging device 1 via any connection means (not shown).

[0016] The drive circuit 12 supplies LED current to the light-emitting elements D1, D2, and D3 to cause them to emit light. By passing the LED current through the light-emitting elements D1, D2, and D3, the drive circuit 12 generates a forward voltage Vf between the anode and cathode of each of the light-emitting elements D1, D2, and D3, thereby driving the light-emitting elements D1, D2, and D3. The LED current is a square wave supplied to the light-emitting elements D1, D2, and D3 in synchronization with a flash signal. The flash signal is a square wave sent from the image sensor 13, and the image sensor 13 captures an image of the subject while the light source is illuminating the subject.

[0017] The image sensor 13 is an imaging element that captures an image of a subject. For example, the image sensor 13 is a CMOS (Complementary Metal Oxide Semiconductor) image sensor, which is a solid-state imaging element. The image sensor 13 also outputs a flash signal, which is a square wave, and captures an image of the subject when the flash signal is at a high level, for example.

[0018] (Basic configuration of fault detection circuit) 2 is a circuit diagram showing an example configuration of the fault detection circuit 11. In FIG. 2, the fault detection circuit 11 includes a detection circuit unit 111a, a detection circuit unit 111b, and a detection circuit unit 111c. The outputs of the detection circuit unit 111a, the detection circuit unit 111b, and the detection circuit unit 111c are connected to a single output terminal 2a via diodes D1a, D1b, and D1c, respectively. Since a detection circuit unit is provided for each IR-LED, there are multiple detection circuit units, and the number of detection circuit units may be four or more depending on the number of IR-LEDs. The fault detection circuit 11 outputs a voltage detection signal from the output terminal 2a, which is an output signal consisting of rectangular waves corresponding to the light-emitting elements D1, D2, and D3, each of which has a different time difference from the flash signal.

[0019] In the fault detection circuit 11, the output of the transistor T3a in the detection circuit unit 111a is synchronized with the flash signal, and the outputs of the transistors T3b and T3c in the detection circuit units 111b and 111c have different time differences with respect to the flash signal. As a result, the voltage detection signal output from the output terminal 2a is made up of rectangular waves corresponding to the light-emitting elements D1, D2, and D3, and each of these rectangular waves becomes an output signal having a different time difference with respect to the flash signal. Furthermore, by configuring the detection circuit section corresponding to the light-emitting element in which a failure has occurred to output a ground-level signal, it is possible to detect the light-emitting element in which a failure has occurred by checking the square wave signal that has reached ground level.

[0020] The detection circuit section 111a includes resistance elements R1a, R2a, R3a, R4a, R5a, and R6a, a transistor T1a, a transistor T2a, and a transistor T3a, and a diode D1a. The detection circuit section 111b includes resistor elements R1b, R2b, R3b, R4b, R5b, R6b, and R7b, transistors T1b, T2b, and T3b, diodes D1b and D2b, and capacitors C1b and C2b. The detection circuit section 111c includes resistor elements R1c, R2c, R3c, R4c, R5c, R6c, and R7c, transistors T1c, T2c, and T3c, diodes D1c and D2c, and capacitors C1c and C2c.

[0021] (Detection circuit section 111a) The resistor R1a is a first resistor having one terminal connected to the cathode of the light-emitting element D1 and one terminal connected to the resistor R2a and the gate terminal of the transistor T1a. The resistor R2a is a second resistor having the other terminal connected to the anode of the light-emitting element D1 and the gate and source terminals of the transistor T1a. The forward voltage Vf, which is the voltage across the light-emitting element D1, is resistively divided by the resistors R1a and R2a. The resistors R1a and R2a are provided to divide the forward voltage Vf so that it does not exceed the rated voltage between the gate and source terminals of the transistor T1a. The resistor R2a is provided to stabilize the potential of the gate terminal of the transistor T1a.

[0022] The transistor T1a is a P-channel field-effect transistor, such as a Pch-MOSFET. When an LED current flows through the normal light-emitting element D1, a forward voltage Vf is generated between the anode and cathode of the light-emitting element D1. The forward voltage Vf is resistively divided by the resistor elements R1a and R2a, and the resulting voltage is applied between the gate and source terminals of the transistor T1a. At this time, the potential of the gate terminal of the transistor T1a becomes lower than the potential of the source terminal. As a result, the voltage between the gate and source terminals exceeds the threshold voltage, turning on the transistor T1a.

[0023] The resistor R3a is a third resistor element, one terminal of which is connected to the drain terminal of the transistor T1a, and the other terminal of the resistor R4a is connected to the gate terminal of the transistor T2a. The resistor R4a is a fourth resistor element, the other terminal of which is grounded and connected to the ground potential GND. The resistors R3a and R4a are provided to divide the voltage generated at the drain terminal of the transistor T1a so that it does not exceed the rated voltage between the gate terminal and source terminal of the transistor T2a. The resistor R4a is connected to the ground potential GND so that the potential at the gate terminal of the transistor T2a does not become unstable.

[0024] Transistors T2a and T3a form a two-stage output-side switching element group. Transistor T2a is a first N-channel field-effect transistor, such as an Nch-MOSFET. Transistor T2a has a gate terminal grounded via resistor R4a, a source terminal directly grounded, and a drain terminal connected to power supply Vcc via resistor R5a.

[0025] The transistor T3a is a second N-channel field-effect transistor (e.g., an Nch-MOSFET) serving as an output-side switching element. The gate terminal of the transistor T3a is connected to the drain terminal of the transistor T2a, the source terminal is grounded, and the drain terminal is connected to the power supply Vcc via a resistor R6a. Furthermore, the drain terminal of the transistor T3a is connected to the output terminal 2a via a first diode D1a.

[0026] Transistor T2a turns on in synchronization with the timing when transistor T1a turns on. When transistor T2a turns on, its drain terminal goes low, for example, to ground potential GND. At this time, the potential at the gate terminal of the subsequent transistor T3a also goes low, turning it off. Resistor elements R5a and R6a are provided to pull up the voltage supplied from power supply Vcc. Resistor element R5a is the fifth resistor element, and resistor element R6a is the sixth resistor element.

[0027] The voltage supplied from the power supply Vcc is the power supply voltage for a device, such as the MCU2, that receives the voltage detection signal from the fault detection circuit 11. The potential of the drain terminal of transistor T3a is pulled up to the voltage from power supply Vcc by resistor R6a, i.e., an open drain. When the LED current is high, the voltage detection signal output from the drain terminal of transistor T3a via diode D1a goes high. When the LED current is low, the voltage detection signal output from the drain terminal of transistor T3a via diode D1a goes low. By repeating this operation, the voltage detection signal output from the drain terminal of transistor T3a via diode D1a becomes a square wave synchronized with the LED current. By using diode D1a, the detection circuit unit 111a can output signals independently of the state of light-emitting elements D2 and D3.

[0028] (Detection circuit section 111b) The resistor R1b is a first resistor having one terminal connected to the cathode of the light-emitting element D2 and one terminal connected to the resistor R2b and the gate terminal of the transistor T1b. The resistor R2b is a second resistor having the other terminal connected to the anode of the light-emitting element D2 and the gate and source terminals of the transistor T1b. The forward voltage Vf, which is the voltage across the light-emitting element D2, is resistively divided by the resistors R1b and R2b. The resistors R1b and R2b are provided to divide the forward voltage Vf so that it does not exceed the rated voltage between the gate and source terminals of the transistor T1b. The resistor R2b is provided to stabilize the potential of the gate terminal of the transistor T1b.

[0029] Transistor T1b is a P-channel field-effect transistor, such as a Pch-MOSFET. When an LED current flows through light-emitting element D2 in a normal state, a forward voltage Vf is generated between the anode and cathode of light-emitting element D2. The forward voltage Vf is divided by resistors R1b and R2b, and the resulting voltage is applied between the gate and source terminals of transistor T1b. At this time, the potential of the gate terminal of transistor T1b becomes lower than the potential of the source terminal. As a result, the voltage between the gate and source terminals exceeds the threshold voltage, turning transistor T1b on.

[0030] Resistor R3b is a third resistor element, with one terminal connected to the drain terminal of transistor T1b and the other terminal connected to resistor R4b and the gate terminal of transistor T2b. Resistor R4b is a fourth resistor element, with the other terminal grounded and connected to ground potential GND. Resistors R3b and R4b are provided to divide the voltage generated at the drain terminal of transistor T1b so that it does not exceed the rated voltage between the gate and source terminals of transistor T2b. Resistor R4b is connected to ground potential GND to prevent the potential of the gate terminal of transistor T2b from becoming unstable.

[0031] Capacitor C1b is a first capacitor whose one end is connected to resistors R3b and R4b and whose other end is grounded. Diode D2b is a second diode connected between one end of capacitor C1b and the gate terminal of transistor T2b.

[0032] Transistors T2b and T3b form a two-stage output-side switching element group. Transistor T2b is a first N-channel field-effect transistor, e.g., an Nch-MOSFET. The gate terminal of transistor T2b is grounded via diode D2b, resistor R4b, and capacitor C1b, the source terminal is directly grounded, and the drain terminal is connected to power supply Vcc via resistor R5b. Resistor R7b is connected between the drain terminal of transistor T2b and the gate terminal of transistor T3b. Capacitor C2b is a second capacitor with one end connected to the junction between resistor R7b and the gate terminal of transistor T3b and the other end grounded.

[0033] The transistor T3b is a second N-channel field-effect transistor (e.g., an Nch-MOSFET) serving as an output-side switching element. The gate terminal of the transistor T3b is connected to the drain terminal of the transistor T2b via a resistor R7b, and the source terminal is grounded. The drain terminal of the transistor T3b is connected to the power supply Vcc via a resistor R6b. Furthermore, the drain terminal of the transistor T3b is connected to the output terminal 2a via a diode D1b.

[0034] Transistor T2b turns on in synchronization with the timing when transistor T1b turns on. When transistor T2b turns on, its drain terminal goes low, for example, to ground potential GND. At this time, the potential at the gate terminal of the subsequent transistor T3b also goes low, turning it off. Resistor elements R5b and R6b are provided to pull up the voltage supplied from power supply Vcc. Resistor element R5b is the fifth resistor element, and resistor element R6b is the sixth resistor element.

[0035] The voltage supplied from the power supply Vcc is the power supply voltage of a device, such as the MCU2, that receives the voltage detection signal from the fault detection circuit 11. The potential of the drain terminal of transistor T3b is pulled up to the voltage from the power supply Vcc by resistor R6b, i.e., it is an open drain. When the LED current is high, the voltage detection signal output from the drain terminal of transistor T3b via diode D1b goes high. When the LED current is low, the voltage detection signal output from the drain terminal of transistor T3b via diode D1b goes low. Furthermore, a delay circuit consisting of capacitor C1b, diode D2b, resistor R7b, and capacitor C2b causes a delay time, equal to the time constant of the delay circuit, in the output from the drain terminal of transistor T3b relative to the flash signal. As a result, by repeating the above operation, the voltage detection signal output from the drain terminal of transistor T3b becomes a rectangular wave that is delayed relative to the LED current. Furthermore, by using the diode D1b, the detection circuit section 111b can output a signal independently of the state of the light emitting element D1 or D3.

[0036] (Detection circuit section 111c) The resistor R1c is a first resistor having one terminal connected to the cathode of the light-emitting element D3 and one terminal connected to the resistor R2c and the gate terminal of the transistor T1c. The resistor R2c is a second resistor having the other terminal connected to the anode of the light-emitting element D3 and the gate and source terminals of the transistor T1c. The forward voltage Vf, which is the voltage across the light-emitting element D3, is resistively divided by the resistors R1c and R2c. The resistors R1c and R2c are provided to divide the forward voltage Vf so that it does not exceed the rated voltage between the gate and source terminals of the transistor T1c. The resistor R2c is provided to stabilize the potential of the gate terminal of the transistor T1c.

[0037] Transistor T1c is a P-channel field-effect transistor, such as a Pch-MOSFET. When an LED current flows through light-emitting element D3 in a normal state, a forward voltage Vf is generated between the anode and cathode of light-emitting element D3. The forward voltage Vf is divided by resistors R1c and R2c, and the resulting voltage is applied between the gate and source terminals of transistor T1c. At this time, the potential of the gate terminal of transistor T1c becomes lower than the potential of the source terminal. As a result, the voltage between the gate and source terminals exceeds the threshold voltage, turning transistor T1c on.

[0038] Resistor element R3c is a third resistor element, with one terminal connected to the drain terminal of transistor T1c and the other terminal connected to resistor element R4c and the gate terminal of transistor T2c. Resistor element R4c is a fourth resistor element, with the other terminal grounded and set to ground potential GND. Resistor elements R3c and R4c are provided to divide the voltage generated at the drain terminal of transistor T1c so that it does not exceed the rated voltage between the gate and source terminals of transistor T2c. Resistor element R4c is connected to ground potential GND to prevent the potential of the gate terminal of transistor T2c from becoming unstable.

[0039] The capacitor C1c is a first capacitor whose one end is connected to the resistors R3c and R4c and whose other end is grounded. The diode D2c is a second diode connected between one end of the capacitor C1c and the gate terminal of the transistor T2c.

[0040] Transistors T2c and T3c form a two-stage output-side switching element group. Transistor T2c is a first N-channel field-effect transistor, e.g., an Nch-MOSFET. The gate terminal of transistor T2c is grounded via diode D2c, resistor R4c, and capacitor C1c, the source terminal is directly grounded, and the drain terminal is connected to power supply Vcc via resistor R5c. Resistor R7c is connected between the drain terminal of transistor T2c and the gate terminal of transistor T3c. Capacitor C2c is a second capacitor with one end connected to the junction between resistor R7c and the gate terminal of transistor T3c and the other end grounded.

[0041] The transistor T3c is a second N-channel field-effect transistor (e.g., an Nch-MOSFET) that serves as an output-side switching element. The gate terminal of the transistor T3c is connected to the drain terminal of the transistor T2c via a resistor R7c, and the source terminal is grounded. The drain terminal of the transistor T3c is connected to the power supply Vcc via a resistor R6c. Furthermore, the drain terminal of the transistor T3c is connected to the output terminal 2a via a diode D1c.

[0042] Transistor T2c turns on in synchronization with the timing when transistor T1c turns on. When transistor T2c turns on, its drain terminal goes low, for example, to ground potential GND. At this time, the potential at the gate terminal of the subsequent transistor T3c also goes low, turning it off. Resistor elements R5c and R6c are provided to pull up the voltage supplied from power supply Vcc. Resistor element R5c is the fifth resistor element, and resistor element R6c is the sixth resistor element.

[0043] The voltage supplied from the power supply Vcc is the power supply voltage of a device, such as the MCU2, that receives the voltage detection signal from the fault detection circuit 11. The potential of the drain terminal of transistor T3c is pulled up to the voltage from the power supply Vcc by resistor R6c, i.e., it is an open drain. When the LED current is high, the voltage detection signal output from the drain terminal of transistor T3c via diode D1c goes high. When the LED current is low, the voltage detection signal output from the drain terminal of transistor T3c via diode D1c goes low. Furthermore, a delay circuit consisting of capacitor C1c, diode D2c, resistor R7c, and capacitor C2c causes a delay time of the time constant of the delay circuit in the output from the drain terminal of transistor T3c relative to the flash signal. Therefore, by repeating the above operation, the voltage detection signal output from the drain terminal of transistor T3c becomes a square wave that is delayed relative to the LED current by more than the detection circuit unit 111b. Furthermore, by using the diode D1c, the detection circuit section 111c can output signals individually regardless of the state of the light emitting element D1 or D2.

[0044] FIG. 3 is a waveform diagram showing voltage waveforms at points A, B, C, and D of the fault detection circuit 11 according to the first embodiment. In FIG. 3, the upper diagram shows the voltage waveform of the flash signal with respect to elapsed time. The middle diagram shows the voltage waveforms at points A, B, and C. The lower diagram shows the voltage waveform at point D. As shown in FIG. 2, point A is the connection point between the drain terminal of transistor T3a and diode D1a in the detection circuit unit 111a, point B is the connection point between the drain terminal of transistor T3b and diode D1b in the detection circuit unit 111a, and point C is the connection point between the drain terminal of transistor T3c and diode D1c in the detection circuit unit 111a. Point D is an output point where the outputs of the detection circuit units 111a, 111b, and 111c are connected together via diodes D1a, D1b, and D1c. The voltage waveforms shown in the upper, middle, and lower diagrams of Figure 3 are the results of simulating the operation of the fault detection circuit 11 when LED current is passed through the light-emitting elements D1, D2, and D3, using a simulation model called LTspice (registered trademark).

[0045] As shown in the upper diagram of Figure 3, the voltage waveform of the flash signal for illuminating each of the light-emitting elements D1, D2, and D3 is a square wave synchronized with the LED current. During the short-circuit fault occurrence time TS, for example, even if a short-circuit fault occurs in the light-emitting element D1, the flash signal continues to be supplied.

[0046] 3, the voltage waveform at point A is the output voltage waveform of the detection circuit section 111a. The voltage waveform at point A is a square wave synchronized with the flash signal. The voltage waveform at point B is the output voltage waveform of detection circuit unit 111b. As shown in the middle diagram of Figure 3, the voltage waveform at point B is delayed with respect to the rectangular wave of the flash signal by a delay circuit provided in detection circuit unit 111b, resulting in a rectangular wave adjacent to the rectangular wave at point A. The delay circuit has a time constant set so that the rectangular wave of the voltage at point B does not overlap with the output waveforms of the other detection circuits 111a and 111c. The voltage waveform at point C is the output voltage waveform of detection circuit unit 111c. As shown in the middle diagram of Figure 3, the voltage waveform at point C is further delayed with respect to the flash signal by a delay circuit provided in detection circuit unit 111c, resulting in a rectangular wave adjacent to the rectangular wave at point B. The delay circuit is set with a time constant that prevents the rectangular wave of the voltage at point C from overlapping with the output waveforms of the other detection circuits 111a and 111b. For example, if a short circuit occurs in light-emitting element D1 at short-circuit fault occurrence time TS, the output of detection circuit unit 111a corresponding to light-emitting element D1 becomes ground level, as shown in the middle diagram of Fig. 3. Meanwhile, the outputs of detection circuit units 111b and 111c corresponding to normal light-emitting elements D2 and D3, respectively, remain unchanged.

[0047] When the light-emitting elements D1, D2, and D3 are operating normally, a forward voltage Vf is generated across each of the light-emitting elements D1, D2, and D3 when a rectangular LED current flows. This causes the detection circuits 111a, 111b, and 111c to output rectangular waves as voltage detection signals. The voltage waveform at point D is the output voltage waveform of the output terminal 2a. The voltage detection signal output from the drain terminal of transistor T3a is output via diode D1a, the voltage detection signal output from the drain terminal of transistor T3b is output via diode D1b, and the voltage detection signal output from the drain terminal of transistor T3c is output via diode D1c. By passing through the diodes in this way, the voltage waveform at point D becomes a rectangular wave output individually from the detection circuits 111a, 111b, and 111c, as shown in the lower diagram of FIG. 3. 3, when a short circuit occurs in light-emitting element D1 at short-circuit occurrence time TS, the output of detection circuit unit 111a corresponding to light-emitting element D1 among the voltage waveforms at point D, which are rectangular waves individually output from detection circuit units 111a, 111b, and 111c, becomes ground level, while the outputs of detection circuit units 111b and 111c corresponding to normal light-emitting elements D2 and D3, respectively, remain unchanged. As a result, by checking the voltage waveform at point D, it is possible to detect which of the light-emitting elements D1, D2, and D3 has experienced a short circuit failure.

[0048] Next, the output of the failure detection circuit 11 before and after the short-circuit failure of the light-emitting elements D1, D2, and D3 will be described in detail. FIG. 4A is a waveform diagram showing the voltage waveform of a flash signal, the voltage waveforms at points A, B, and C, and the voltage waveform at point D when light-emitting elements D1, D2, and D3 are in a normal state. In FIG. 4A, the top diagram shows the voltage waveform of the flash signal over time. The middle diagram shows the voltage waveforms at points A, B, and C of fault detection circuit 11. The bottom diagram shows the voltage waveform at point D of fault detection circuit 11. The voltage waveforms shown in the top, middle, and bottom diagrams are the results of a simulation using LTspice of the operation of fault detection circuit 11 when LED current is passed through light-emitting elements D1, D2, and D3.

[0049] When the voltage obtained by dividing the forward voltage Vf by resistors R1a and R2a is applied between the gate and source terminals of transistor T1a, the voltage between the gate and source terminals of transistor T1a exceeds the threshold voltage, and the potential of the gate terminal becomes lower than the potential of the source terminal of transistor T1a. This turns on transistor T1a, and the voltage at point A on the drain terminal side is applied to the gate terminal of transistor T2a, turning on transistor T2a when the potential at point A is high.

[0050] Similarly, when the voltage obtained by dividing the forward voltage Vf by resistors R1b and R2b is applied between the gate and source terminals of transistor T1b, the voltage between the gate and source terminals of transistor T1b exceeds the threshold voltage, and the potential of the gate terminal becomes lower than the potential of the source terminal of transistor T1b. This turns on transistor T1b, and the voltage at point B on the drain terminal side is applied to the gate terminal of transistor T2b, turning on transistor T2b when the potential at point B is high.

[0051] When the voltage obtained by dividing the forward voltage Vf by resistors R1c and R2c is applied between the gate and source terminals of transistor T1c, the voltage between the gate and source terminals of transistor T1c exceeds the threshold voltage, and the potential of the gate terminal becomes lower than the potential of the source terminal of transistor T1c. This turns on transistor T1c, and the voltage at point C on the drain terminal side is applied to the gate terminal of transistor T2c, turning on transistor T2c when the potential at point C is high.

[0052] In the fault detection circuit 11, the open-drain outputs of transistors T3a, T3b, and T3c are connected in parallel via diodes D1a, D1b, and D1c. When light-emitting elements D1, D2, and D3 are normal, square-wave voltage waveforms are obtained at points A, B, and C as shown in the middle diagram of FIG. 4A. A voltage detection signal having a voltage waveform in which high and low levels alternate in synchronization with these voltage waveforms is output from point D of output terminal 2a.

[0053] As shown in the upper diagram of FIG. 4A, the voltage waveform of the flash signal for causing each of the light emitting elements D1, D2, and D3 to emit light is a square wave synchronized with the LED current. In the middle diagram of Figure 4A, the voltage waveform at point A is a square wave synchronized with the flash signal. The voltage waveform at point B is delayed relative to the square wave of the flash signal and is a square wave adjacent to the square wave at point A. The voltage waveform at point C is further delayed relative to the flash signal and is a square wave adjacent to the square wave at point B. As shown in the lower diagram of FIG. 4A, when the light emitting elements D1, D2, and D3 are in a normal state, the voltage waveform at point D is a rectangular wave having different time differences with respect to the flash signal.

[0054] FIG. 4B is a waveform diagram showing the voltage waveform of the flash signal, the voltage waveforms at points A, B, and C, and the voltage waveform at point D when light-emitting element D1 is short-circuited. In FIG. 4B, the top diagram shows the voltage waveform of the flash signal over time. The middle diagram shows the voltage waveforms at points A, B, and C. The bottom diagram shows the voltage waveform at point D. The voltage waveforms shown in the top, middle, and bottom diagrams are the results of a simulation using LTspice to demonstrate the operation of fault detection circuit 11 when LED currents are passed through light-emitting elements D1, D2, and D3.

[0055] If a short circuit occurs in light-emitting element D1, conduction occurs between the anode and cathode of light-emitting element D1, and no forward voltage Vf is generated even if an LED current flows. On the other hand, even if a short circuit occurs in light-emitting element D1, LED current flows through light-emitting elements D2 and D3, which are in normal operation, and a square-wave forward voltage Vf is generated. This causes the detection circuit unit 111a to continue outputting a low-level (ground level) voltage detection signal. In other words, no forward voltage Vf is generated between the anode and cathode of light-emitting element D1. Therefore, no potential difference occurs between the gate terminal and source terminal of transistor T1a, and transistor T1a remains off. At this time, transistor T3a remains on, and a low-level voltage detection signal is output from the detection circuit unit 111a, as shown in FIG. 4B. On the other hand, a forward voltage Vf is generated in the normal light emitting elements D2 and D3, and therefore, as shown in FIG. 4B, square wave voltage detection signals are output from the detection circuit units 111b and 111c.

[0056] The voltage detection signal is output to the MCU2 via the output terminal 2a. The MCU2 determines, based on the voltage detection signal, which of the light-emitting elements D1, D2, and D3 has experienced a short-circuit failure. For example, when the rectangular wave at point A among the rectangular waves at points A, B, and C is at ground level, the MCU2 determines that a short-circuit failure has occurred in the light-emitting element D1 that constitutes the light source provided in the imaging device 1. In this way, the failure detection circuit 11 can determine, based on the voltage detection signal acquired via one output terminal 2a, which light-emitting element D1, D2, and D3 has experienced a failure.

[0057] FIG. 5A is a waveform diagram showing the voltage waveform of a flash signal, the voltage waveforms at points A, B, and C, and the voltage waveform at point D when light-emitting element D2 is short-circuited. In FIG. 5A, the top diagram shows the voltage waveform of the flash signal over time. The middle diagram shows the voltage waveforms at points A, B, and C. The bottom diagram shows the voltage waveform at point D. The voltage waveforms shown in the top, middle, and bottom diagrams are the results of a simulation using LTspice to demonstrate the operation of fault detection circuit 11 when LED currents are passed through light-emitting elements D1, D2, and D3.

[0058] If a short circuit occurs in light-emitting element D2, conduction occurs between the anode and cathode of light-emitting element D2, and no forward voltage Vf is generated even if an LED current flows. On the other hand, even if a short circuit occurs in light-emitting element D2, LED current flows through light-emitting elements D1 and D3, which are in normal operation, and a square-wave forward voltage Vf is generated. This causes the detection circuit unit 111b to continue outputting a low-level voltage detection signal. In other words, no forward voltage Vf is generated between the anode and cathode of light-emitting element D2. Therefore, no potential difference occurs between the gate terminal and source terminal of transistor T1b, and transistor T1b remains off. At this time, transistor T3b remains on, and a low-level voltage detection signal is output from the detection circuit unit 111b, as shown in FIG. 5A. On the other hand, since a forward voltage Vf is generated in the normal light emitting elements D1 and D3, a square wave voltage detection signal is output from the detection circuit units 111a and 111c as shown in FIG. 5A.

[0059] The voltage detection signal is output to the MCU2 via the output terminal 2a. The MCU2 determines, based on the voltage detection signal, which of the light-emitting elements D1, D2, and D3 has experienced a short-circuit failure. For example, when the rectangular wave at point B among the rectangular waves at points A, B, and C is at ground level, the MCU2 determines that a short-circuit failure has occurred in the light-emitting element D2 that constitutes the light source provided in the imaging device 1. In this way, the failure detection circuit 11 can determine, based on the voltage detection signal acquired via one output terminal 2a, which light-emitting element D1, D2, and D3 has experienced a failure.

[0060] FIG. 5B is a waveform diagram showing the voltage waveform of the flash signal, the voltage waveforms at points A, B, and C, and the voltage waveform at point D when light-emitting element D3 is short-circuited. In FIG. 5B, the top diagram shows the voltage waveform of the flash signal over time. The middle diagram shows the voltage waveforms at points A, B, and C. The bottom diagram shows the voltage waveform at point D. The voltage waveforms shown in the top, middle, and bottom diagrams are the results of a simulation using LTspice to demonstrate the operation of fault detection circuit 11 when LED currents are passed through light-emitting elements D1, D2, and D3.

[0061] If a short circuit occurs in light-emitting element D3, conduction occurs between the anode and cathode of light-emitting element D3, and no forward voltage Vf is generated even if LED current flows. On the other hand, even if a short circuit occurs in light-emitting element D3, LED current flows through light-emitting elements D1 and D2, which are in normal operation, and a square-wave forward voltage Vf is generated. As a result, the detection circuit unit 111c continues to output a low-level (ground level) voltage detection signal. In other words, no forward voltage Vf is generated between the anode and cathode of light-emitting element D3. As a result, no potential difference occurs between the gate terminal and source terminal of transistor T1c, and transistor T1c remains off. At this time, transistor T3c remains on, and as shown in FIG. 5B, the detection circuit unit 111c outputs a low-level voltage detection signal. On the other hand, a forward voltage Vf is generated in the normal light emitting elements D1 and D2, and therefore, as shown in FIG. 5B, the detection circuit units 111a and 111b output voltage detection signals having rectangular waves.

[0062] The voltage detection signal is output to the MCU2 via the output terminal 2a. The MCU2 determines, based on the voltage detection signal, which of the light-emitting elements D1, D2, and D3 has experienced a short-circuit failure. For example, when the rectangular wave at point C, among the rectangular waves at points A, B, and C, is at ground level, the MCU2 determines that a short-circuit failure has occurred in the light-emitting element D3 that constitutes the light source provided in the imaging device 1. In this way, the failure detection circuit 11 can determine, based on the voltage detection signal acquired via one output terminal 2a, which light-emitting element D1, D2, and D3 has experienced a failure.

[0063] Furthermore, the switching elements included in each of the detection circuit units 111a, 111b, and 111c may be Pch-MOSFETs and Nch-MOSFETs as described above, but may also be other switching elements.

[0064] As described above, the fault detection circuit 11 according to the first embodiment includes detection circuit units 111a, 111b, and 111c, each provided for one of the light-emitting elements D1, D2, and D3, for detecting the voltage across the corresponding light-emitting element D1, D2, or D3. The outputs of the detection circuit units 111a, 111b, and 111c are connected to a single output terminal 2a, which outputs a rectangular wave corresponding to each of the light-emitting elements D1, D2, or D3, with each rectangular wave having a different time difference relative to a flash signal for emitting light from the corresponding light-emitting element. The output signal output from the single output terminal 2a can identify which of the light-emitting elements D1, D2, or D3 has failed. Therefore, the fault detection circuit 11 can detect the failed light-emitting element without increasing the circuit size.

[0065] In the fault detection circuit 11 according to the first embodiment, light-emitting elements D1, D2, and D3 are connected in series. Each of the detection circuit units 111a, 111b, and 111c includes at least transistors T3a, T3b, and T3c, which output high and low signals depending on whether or not there is a voltage across the light-emitting element. The outputs of the transistors T3a, T3b, and T3c are connected to a single output terminal 2a via diodes D1a, D1b, and D1c, respectively. In the detection circuit unit 111a, the output of T3a is synchronized with the flash signal. In the remaining detection circuit units 111b and 111c, the outputs of the transistors T3b and T3c are synchronized with the flash signal at different times. The output signal output from the single output terminal 2a can identify which of the light-emitting elements D1, D2, and D3 has a fault. Therefore, the fault detection circuit 11 can detect a faulty light-emitting element without increasing the circuit size.

[0066] In the fault detection circuit 11 according to the first embodiment, the transistor T3a, T3b, or T3c included in the detection circuit unit corresponding to the failed light-emitting element among the light-emitting elements D1, D2, and D3 outputs a ground-level signal. By checking that the signal has reached the ground level, it is possible to detect the failed light-emitting element.

[0067] In the fault detection circuit 11 according to the first embodiment, each of the detection circuit units 111a, 111b, and 111c includes resistive elements R1a, R1b, and R1c, resistive elements R2a, R2b, and R2c, resistive elements R3a, R3b, and R3c, resistive elements R4a, R4b, and R4c, resistive elements R5a, R5b, and R5c, resistive elements R6a, R6b, and R6c, transistors T1a, T1b, and T1c, transistors T2a, T2b, and T2c, and transistors T3a, T3b, and T3c. The forward voltage Vf is resistively divided by resistors R1a and R2a and applied between the gate and source terminals of transistor T1a. The voltage generated at the drain terminal of transistor T1a is resistively divided by resistors R3a and R4a and applied between the gate and source terminals of transistor T2a. The gate terminal of transistor T2a is grounded via resistor R4a, the source terminal is directly grounded, and the drain terminal is connected to power supply Vcc via resistor R5a. The voltage generated at the drain terminal of transistor T2a is applied between the gate and source terminals of transistor T3a, and the drain terminal of transistor T3a is pulled up to the voltage supplied from power supply Vcc by resistor R6a. The forward voltage Vf is resistively divided by resistors R1b and R2b and applied between the gate and source terminals of transistor T1b. The resistively divided voltage generated at the drain terminal of transistor T1b by resistors R3b and R4b is applied between the gate and source terminals of transistor T2b. The gate terminal of transistor T2b is grounded via resistor R4b, the source terminal is directly grounded, and the drain terminal is connected to power supply Vcc via resistor R5b. The voltage generated at the drain terminal of transistor T2b is applied between the gate and source terminals of transistor T3b, and the drain terminal of transistor T3b is pulled up to the voltage supplied from power supply Vcc by resistor R6b. The forward voltage Vf is resistively divided by resistors R1c and R2c and applied between the gate and source terminals of transistor T1c. The voltage generated at the drain terminal of transistor T1c is resistively divided by resistors R3c and R4c and applied between the gate and source terminals of transistor T2c. The gate terminal of transistor T2c is grounded via resistor R4c, the source terminal is directly grounded, and the drain terminal is connected to power supply Vcc via resistor R5c. The voltage generated at the drain terminal of transistor T2c is applied between the gate and source terminals of transistor T3c, and the drain terminal of transistor T3c is pulled up to the voltage supplied from power supply Vcc by resistor R6c. The detection circuit units 111b and 111c further include capacitors C1b and C1c, one end of which is connected to resistor elements R3b and R3c and resistor elements R4b and R4c and the other end of which is grounded, diodes D2b and D2c, one end of which is connected between capacitors C1b and C1c and the gate terminals of transistors T2b and T2c, resistor elements R7b and R7c, one end of which is connected to the junction of resistor elements R7b and R7c and the gate terminals of transistors T3b and T3c, and the other end of which is grounded. The output signal output from the single output terminal 2a can identify which of the light-emitting elements D1, D2, and D3 has failed, allowing the failure detection circuit 11 to detect the failed light-emitting element without increasing the circuit size.

[0068] In the fault detection circuit 11 according to the first embodiment, the light emitting element is an IR-LED (infrared light emitting diode). This allows use in fault detection of a device that uses a light source made of an IR-LED. For example, a device that uses a light source made of an IR-LED is the imaging device 1 shown in FIG.

[0069] The imaging device 1 according to the first embodiment includes an image sensor 13 that captures an image of a subject, a light source in which light-emitting elements D1, D2, and D3 that are IR-LEDs for illuminating the subject are connected in series, a drive circuit 12 that supplies LED current to the light-emitting elements D1, D2, and D3 to cause the IR-LEDs to emit light, and a fault detection circuit 11. The imaging device 1 configured in this manner is capable of detecting a faulty light-emitting element and is also capable of suppressing an increase in the circuit size.

[0070] Any of the components of the embodiments may be modified or omitted. [Explanation of symbols]

[0071] 1 imaging device, 2 MCU, 2a output terminal, 11 fault detection circuit, 12 drive circuit, 12a input terminal, 12b output terminal, 13 image sensor, 111a, 111b, 111c detection circuit section, D1, D2, D3 light emitting element, R1a, R1b, R1c, R2a, R2b, R2c, R3a, R3b, R3c, R4a, R4b, R4c, R5a, R5b, R5c, R6a, R6b, R6c, R7b, R7c resistor element, T1a, T1b, T1c, T2a, T2b, T2c, T3a, T3b, T3c transistor, C1b, C1c, C2b, C2c capacitor, D1a, D1b, D1c, D2b, D2c diode.

Claims

1. a plurality of detection circuit units provided for the plurality of light emitting elements, each detecting a voltage across the corresponding light emitting element; outputs of the plurality of detection circuit units are connected to a single output terminal; An output signal is output from the output terminal, the output signal being made up of a plurality of rectangular waves corresponding to the plurality of light-emitting elements, each of the plurality of rectangular waves having a different time difference from each other with respect to a flash signal for emitting light from the light-emitting elements. A fault detection circuit comprising:

2. The plurality of light-emitting elements are connected in series, each of the plurality of detection circuit units includes at least an output-side switching element that outputs a high-level or low-level signal depending on whether or not there is a voltage across the light-emitting element; outputs of the plurality of output-side switching elements are respectively connected to one of the output terminals via first diodes, One of the plurality of detection circuit units has an output of the output-side switching element synchronized with the flash signal, The remaining detection circuit units have outputs of the output-side switching elements with different time differences from each other with respect to the flash signal.

2. The fault detection circuit according to claim 1.

3. The output-side switching element of the detection circuit section corresponding to the light-emitting element in which a failure has occurred outputs a ground level signal.

3. The fault detection circuit according to claim 2.

4. Each of the plurality of detection circuit units a first resistor element, a second resistor element, a third resistor element, a fourth resistor element, a fifth resistor element, a sixth resistor element, a P-channel field effect transistor, a first N-channel field effect transistor, and a second N-channel field effect transistor which is the output side switching element; a voltage obtained by resistively dividing a voltage across the light emitting element by the first resistor element and the second resistor element is applied between a gate terminal and a source terminal of the P-channel field effect transistor; a voltage obtained by resistively dividing a voltage generated on the drain terminal side of the P-channel field effect transistor by the third resistor element and the fourth resistor element is applied between the gate terminal and the source terminal of the first N-channel field effect transistor, the first N-channel field effect transistor has a gate terminal grounded via the fourth resistive element, a source terminal directly grounded, and a drain terminal connected to a power supply via the fifth resistive element; a voltage generated on the drain terminal side of the first N-channel field effect transistor is applied between the gate terminal and the source terminal of the second N-channel field effect transistor; a drain terminal of the second N-channel field effect transistor is pulled up by the sixth resistor element to a voltage supplied from the power supply; The remaining detection circuitry includes: a first capacitor having one end connected to the third resistor element and the fourth resistor element and the other end grounded; a second diode connected between one end of the first capacitor and the gate terminal of the first N-channel field effect transistor; a resistive element connected between the drain terminal of the first N-channel field effect transistor and the gate terminal of the second N-channel field effect transistor; a second capacitor having one end connected to a connection point between the resistive element and the gate terminal of the second N-channel field effect transistor and the other end grounded; 3. The fault detection circuit according to claim 2.

5. The light emitting element is an infrared light emitting diode.

5. The fault detection circuit according to claim 1, wherein:

6. an image sensor that captures an image of a subject; a light source for illuminating the subject, the light source being a plurality of infrared light emitting diodes connected in series; a drive circuit that supplies a current to the light-emitting element to cause the light-emitting element to emit light; and a fault detection circuit according to any one of claims 1 to 4. An imaging device characterized by:

Images