LED driver circuit

Abstract

An LED driver circuit includes a first terminal to which a current path of the switch device is connected at one end thereof; a second terminal to which the current path of the switch device is connected at another end thereof, the switch device and a battery being connected in series between the first terminal and the second terminal; a detection circuit that periodically detects a current flowing to the first terminal and outputs a detection signal responsive to a result of the detection at a first node; a comparison circuit that compares a detection voltage responsive to the detection signal with a threshold voltage and outputs a comparison result signal responsive to a result of the comparison; and a control circuit that controls a current detection operation of the detection circuit and controls driving of the LED lamp based on the comparison result signal.

Claims

1. An LED driver circuit that controls driving of an LED lamp in response to an on/off state of a mechanical switch device, comprising: a first terminal to which a current path of the switch device is connected at one end thereof; a second terminal to which the current path of the switch device is connected at another end thereof, the switch device and a battery being connected in series between the first terminal and the second terminal; a detection circuit that periodically detects a current flowing to the first terminal and outputs a detection signal responsive to a result of the detection at a first node; a comparison circuit that compares a detection voltage responsive to the detection signal with a threshold voltage and outputs a comparison result signal responsive to a result of the comparison; and a control circuit that controls a current detection operation of the detection circuit and controls driving of the LED lamp based on the comparison result signal, wherein the control circuit determines that the switch device is in an on state and the current path is conductive between the one end and the another end if the comparison result signal indicates that the detection voltage is equal to or higher than the threshold voltage, and determines that the switch device is in an off state and the current path is interrupted between the one end and the another end if the comparison result signal indicates that the detection voltage is lower than the threshold voltage, wherein the detection circuit comprises: a first switch element that is connected to the first terminal at one end thereof and is turned on and off under the control of the control circuit; a detection capacitor that is connected to another end of the first switch element at one end thereof and to the first node at another end thereof; a detection resistor that is connected to the first node at one end thereof and to the second terminal at another end thereof; and a discharge resistor that is connected in parallel with the detection resistor and the detection capacitor between another end of the first switch element and the second terminal, the discharge resistor being connected to the another end of the first switch element at one end thereof and to the second terminal at another end thereof, the control circuit controls the first switch element to periodically switch on and off, and the detection circuit outputs the detection signal at the first node.

2. The LED driver circuit according to claim 1, wherein the detection circuit further comprises: a second switch element that is connected in series with the discharge resistor between the another end of the first switch element and the second terminal, and the control circuit controls the second switch element to switch off when the control circuit turns on the first switch element, and controls the second switch element to switch on when the control circuit turns off the first switch element.

3. The LED driver circuit according to claim 1, further comprising: a peak hold circuit that holds a peak voltage of the detection signal and outputs the held peak voltage as the detection voltage at a second node.

4. The LED driver circuit according to claim 3, wherein the peak hold circuit comprises: a holding diode that is connected to the first node at an anode thereof and to the second node at a cathode thereof; and a holding capacitor that is connected between the second node and the second terminal.

5. The LED driver circuit according to claim 4, wherein the comparison circuit comprises: a voltage divider circuit that outputs a divided voltage as the threshold voltage, the divided voltage being obtained by dividing a voltage between the first terminal and the second terminal; and a comparator that receives the detection voltage and the threshold voltage, compares the detection voltage with the threshold voltage, and outputs the comparison result signal in response to a result of the comparison.

6. The LED driver circuit according to claim 5, wherein the voltage divider circuit comprises: a first voltage divider resistor that is connected to the first terminal at one end thereof and to a voltage dividing node at another end thereof; and a second voltage divider resistor that is connected to the voltage dividing node at one end thereof and to the second terminal at another end thereof, and the voltage divider circuit outputs a voltage at the voltage dividing node as the threshold voltage.

7. The LED driver circuit according to claim 3, wherein the threshold voltage is set to be higher than the detection voltage, which is the peak voltage of the detection signal that is output from the detection circuit as a result of the control circuit periodically turning on and off the first switch element, in a state where the switch device is in the off state and a leak current is flowing in the current path.

8. The LED driver circuit according to claim 7, wherein the threshold voltage is set to be lower than the detection voltage, which is the peak voltage of the detection signal that is output from the detection circuit as a result of the control circuit periodically turning on and off the first switch element, in a state where the switch device is in the on state.

9. The LED driver circuit according to claim 1, wherein the switch device and the battery are connected in series between the first terminal and the second terminal, the battery being connected to the first terminal on the side of a positive electrode thereof and to the second terminal on the side of a negative electrode thereof.

10. The LED driver circuit according to claim 4, wherein the detecting capacitor has a capacitance greater than a capacitance of the holding capacitor.

11. The LED driver circuit according to claim 1, wherein a leak current flows in the current path of the switch device despite the switch device being in the off state when the switch device is wetted with water.

12. The LED driver circuit according to claim 1, wherein the LED driver circuit is mounted on a motorcycle, the LED lamp is a headlamp or a turn signal of the motorcycle, and the switch device is a handle switch of the motorcycle that is manipulated by a user to control driving of the LED lamp.

13. The LED driver circuit according to claim 1, further comprising: a power supply circuit that is connected to the first terminal and supplies electric power to the control circuit based on a current input from the first terminal, the power supply circuit operates on the current input from the first terminal and supplies electric power to the control circuit based on the current input from the first terminal when the switch device is in the on state or when the switch devices is in the off state and a leak current flows in the current path, and the control circuit operates on the electric power supplied from the power supply circuit and drives the LED lamp.

14. The LED driver circuit according to claim 1, wherein the first switch element is a pMOS transistor that is connected to the first terminal at a source thereof and to the one end of the detection capacitor at a drain thereof and has a gate voltage controlled by the control circuit.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram showing an example of a configuration of an LED driver system 1000 according to a first embodiment.

(2) FIG. 2 is a waveform diagram showing an example of operation waveforms of an LED driver circuit 100 shown in FIG. 1.

(3) FIG. 3 is a waveform diagram showing another example of operation waveforms of the LED driver circuit 100 shown in FIG. 1.

(4) FIG. 4 is a diagram showing an example of a configuration of an LED driver system 2000 according to a second embodiment.

DESCRIPTION OF EMBODIMENTS

(5) Embodiments of the present invention will be described with reference to the drawings.

First Embodiment

(6) An LED driver system 1000 (FIG. 1) according to a first embodiment includes a battery B, a mechanical switch device SW connected to the battery B, an LED lamp 101 and an LED driver circuit 100 that controls driving of the LED lamp 101 in response to the on/off state of the mechanical switch device SW.

(7) The LED driver system 1000 is mounted on a motorcycle, for example. In that case, the LED lamp 101 is a headlamp or a turn signal of the motorcycle, for example. Furthermore, in that case, the switch device SW is a handle switch of the motorcycle that is manipulated by the user for controlling driving of the LED lamp 101.

(8) If the mechanical switch device SW is wetted with water, for example, a leak current flows in a current path of the switch device SW even if the switch device SW is in the off state.

(9) The LED driver circuit 100 includes a first terminal T1, to which the current path of the switch device SW is connected at one end thereof, and a second terminal T2, to which the current path of the switch device SW is connected at another end thereof (FIG. 1).

(10) The switch device SW and the battery B are connected in series between the first terminal T1 and the second terminal T2. In the example shown in FIG. 1, the current path of the switch device SW is connected to the first terminal T1 at one end thereof and to a positive electrode of the battery B at another end thereof, and the battery B is connected to the second terminal T2 at a negative electrode thereof.

(11) That is, the switch device SW and the battery B are connected in series between the first terminal T1 and the second terminal T2 in such a manner that the battery B is connected to the first terminal T1 on the side of the positive electrode and to the second terminal T2 on the side of the negative electrode.

(12) In the example shown in FIG. 1, the second terminal T2 is grounded.

(13) The LED driver circuit 100 includes a detection circuit DC that periodically detects the current flowing to the first terminal T1 and outputs a detection signal SX responsive to the detection result at a first node N1. The LED driver circuit 100 further includes a comparison circuit CC that compares a detection voltage VZ responsive to the detection signal SX with a threshold voltage Vth and outputs a comparison result signal So responsive to the comparison result.

(14) The LED driver circuit 100 further includes a peak hold circuit HC that holds a peak voltage of the detection signal SX and outputs the held peak voltage as the detection voltage VZ at a second node N2. The LED driver circuit 100 further includes a control circuit CON that controls the current detection operation of the detection circuit DC and controls driving of the LED lamp 101 based on the comparison result signal So.

(15) The LED driver circuit 100 further includes a power supply circuit SC that is connected to the first terminal T1 and supplies electric power to the control circuit CON based on a current input from the first terminal T1.

(16) In the example shown in FIG. 1, the detection circuit DC includes a first switch element Q1 that is connected to the first terminal T1 at one end thereof and is turned on and off under the control of the control circuit CON, and a detection capacitor CX that is connected to another end of the first switch element Q1 at one end thereof and to the first node N1 at another end thereof.

(17) The detection circuit DC further includes a detection resistor RX that is connected to the first node N1 at one end thereof and to the second terminal T2 at another end thereof, and a discharge resistor RY that is connected to the another end of the first switch element Q1 at one end thereof and to the second terminal T2 at another end thereof.

(18) As shown in FIG. 1, the first switch element Q1 is a pMOS transistor that is connected to the first terminal T1 at a source thereof and to the one end of the detection capacitor CX at a drain thereof, for example. A gate voltage of the pMOS transistor is controlled by a control signal (a gate signal SG1 output from a pre-driver circuit PC) output from the control circuit CON. That is, the pMOS transistor is turned on and off under the control of the gate signal SG1.

(19) The detection resistor RX is a resistor for detecting a current IX that flows to the detection capacitor CX.

(20) The discharge resistor RY is connected in parallel with the detection resistor RX and the detection capacitor CX between the another end of the first switch element Q1 and the second terminal T2. The discharge resistor RY is a resistor for discharging the detection capacitor CX.

(21) The detection circuit DC configured as described above outputs the detection signal SX at the first node N1. That is, the detection signal SX is a voltage at the first node N1.

(22) If the first switch element Q1 is turned on when the switch device SW is in the off state and a leak current is flowing in the current path of the switch device SW or when the switch device SW is in the on state and a current is flowing in the current path of the switch device SW (that is, when a current is flowing from the first terminal T1), for example, the current IX flows from the first terminal T1 to the detection capacitor CX, and the detection capacitor CX is charged.

(23) If the first switch element Q1 is then turned off, the detection capacitor CX is discharged through the discharge resistor RY.

(24) If the first switch element Q1 is turned on when the switch device SW is in the off state and no leak current is flowing in the current path of the switch device SW (that is, when no current is flowing from the first terminal T1), the current IX does not flow from the first terminal T1 to the detection capacitor CX, and the detection capacitor CX is not charged.

(25) As described above, the peak hold circuit HC holds the peak voltage of the detection signal SX and outputs the held peak voltage as the detection voltage VZ at the second node N2.

(26) As shown in FIG. 1, for example, the peak hold circuit HC includes a holding diode DZ that is connected to the first node N1 at an anode thereof and to the second node N2 at a cathode thereof, and a holding capacitor CZ that is connected between the second node N2 and the second terminal T2.

(27) The capacitance of the detection capacitor CX described above is set to be greater than the capacitance of the holding capacitor CZ.

(28) A peak voltage of the voltage of the detection signal SX (a voltage VX between the opposite ends of the detection resistor RX) is held in the holding capacitor CZ via the holding diode DZ.

(29) When the switch device SW is in the on state, for example, the voltage VZ between the opposite ends of the holding capacitor CZ is approximately equal to the voltage of the battery B. On the other hand, when the switch device SW is in the off state and a leak current is flowing in the current path of the switch device SW, the voltage VZ between the opposite ends of the holding capacitor CZ is reduced, since the current IX flowing to the detection capacitor CX is low.

(30) As described above, the comparison circuit CC compares the detection voltage VZ responsive to the detection signal SX with the threshold voltage Vth and outputs the comparison result signal So responsive to the comparison result.

(31) For example, the comparison circuit CC compares the detection voltage VZ with the threshold voltage Vth, and outputs a comparison result signal So at a High level if the detection voltage VZ is equal to or higher than the threshold voltage Vth. On the other hand, if the comparison circuit CC compares the detection voltage VZ with the threshold voltage Vth, and the detection voltage VZ is lower than the threshold voltage Vth, the comparison circuit CC outputs a comparison result signal So at a Low level.

(32) For example, as shown in FIG. 1, the comparison circuit CC includes a voltage divider circuit RD that divides the voltage between the first terminal T1 and the second terminal T2 and outputs the divided voltage as the threshold voltage Vth, and a comparator COMP that receives the detection voltage VZ and the threshold voltage Vth, compares the detection voltage VZ with the threshold voltage Vth and outputs the comparison result signal So responsive to the comparison result.

(33) As shown in FIG. 1, the voltage divider circuit RD includes a first voltage dividing resistor RD1 that is connected to the first terminal T1 at one end thereof and to a voltage dividing node ND at another end, and a second voltage dividing resistor RD2 that is connected to the voltage dividing node ND at one end and to the second terminal T2 at another end thereof.

(34) The voltage divider circuit RD outputs a voltage at the voltage dividing node ND as the threshold voltage Vth.

(35) The comparison circuit CC further includes an output resistor Ro that is connected between the first terminal T1 and an output of the comparator COMP, and a protective resistor RA that is connected between the second node N2 and an input of the comparator COMP.

(36) Although the comparison circuit CC shown in FIG. 1 is formed with the comparator COMP, the comparison circuit CC may be formed with other circuits capable of comparing the voltages, such as a transistor.

(37) As described above, the control circuit CON controls the current detection operation of the detection circuit DC and controls driving of the LED lamp 101 based on the comparison result signal So.

(38) The control circuit CON controls the first switch element Q1 to periodically switch on and off.

(39) If the comparison result signal So indicates that the detection voltage VZ is equal to or higher than the threshold voltage Vth, the control circuit CON determines that the switch device SW is in the on state and the current path of the switch device SW is conductive between the one end and the another end thereof.

(40) In this case, the control circuit CON supplies a drive current to the LED lamp 101, for example, such that the LED lamp 101 illuminates.

(41) On the other hand, if the comparison result signal So indicates that the detection voltage VZ is lower than the threshold voltage Vth, the control circuit CON determines that the switch device SW is in the off state and the current path of the switch device SW is interrupted between the one end and the another end thereof.

(42) In this case, the control circuit CON supplies no drive current to the LED lamp 101, for example, such that the LED lamp 101 does not illuminate.

(43) The threshold voltage Vth is set to be higher than the detection voltage VZ, which is the peak voltage of the detection signal SX that is output from the detection circuit DC as a result of the control circuit CON periodically turning on and off the first switch element Q1, in the state where the switch device SW is in the off state and a leak current is flowing in the current path of the switch device SW.

(44) Thus, if the comparison result signal So indicates that the detection VZ is equal to or higher than the threshold voltage Vth, the control circuit CON can determine that the switch device SW is in the on state and the current path of the switch device SW is conductive between the one end and the another end thereof.

(45) Furthermore, the threshold voltage Vth is set to be lower than the detection voltage VZ, which is the peak voltage of the detection signal SX that is output from the detection circuit DC as a result of the control circuit CON periodically turning on and off the first switch element Q1, in the case where the switch device SW is in the on state.

(46) Thus, if the comparison result signal So indicates that the detection voltage VZ is lower than the threshold voltage Vth, the control circuit CON can determine that the switch device SW is in the off state and the current path of the switch device SW is interrupted between the one end and the another end thereof.

(47) The LED driver circuit 100 includes the pre-driver circuit PC that controls the gate signal SG1 of the pMOS transistor (that is, drives the first switch element Q1) in response to a control signal for controlling the first switch element Q1 output from the control circuit CON. The pre-driver circuit PC may be omitted. That is, the control circuit CON may output the gate signal SG1 by itself to control the first switch element Q1.

(48) The LED driver circuit 100 further includes an interface circuit IC that processes the comparison result signal So output from the comparison circuit CC and outputs the resulting signal So to the control circuit CON. That is, the control circuit CON receives the comparison result signal So from the comparison circuit CC via the interface circuit IC. The interface circuit IC may be omitted.

(49) As described above, in order to detect the on/off state of the switch device SW with higher reliability, the LED driver circuit 100 includes the peak hold circuit HC. However, if the LED driver circuit 100 can detect the on/off state of the switch device SW based on whether a pulse signal is fed back or not, the peak hold circuit HC can be omitted.

(50) As described above, the power supply circuit SC is connected to the first terminal T1 and supplies electric power to the control circuit CON based on the current input from the first terminal T1.

(51) The power supply circuit SC operates on the current input from the first terminal T1 when the switch device SW is in the on state or when the switch device SW is in the off state and a leak current is flowing in the current path of the switch device SW. The power supply circuit SC supplies electric power to the control circuit CON based on the current input from the first terminal T1.

(52) The control circuit CON operates on the electric power supplied from the power supply circuit SC and drives the LED lamp.

(53) Furthermore, the control circuit CON outputs a pulse signal to the pre-driver circuit PC to periodically switch the first switch element Q1 in the detection circuit DC.

(54) The pulse signal preferably has a frequency of 10 to 200 Hz and an on-duty of approximately 1% to 10%, for example. That is, the frequency of the switching of the first switch element Q1 by the control circuit CON is 10 to 200 Hz, and the on-duty of the first switch element Q1 is approximately 1% to 10%.

(55) Next, an example of an operation of the LED driver circuit 100 configured as described above will be described with reference to FIGS. 2 and 3.

(56) As an example, FIG. 2 shows waveforms in a case where the switch device SW transitions from the on state to the off state, and no leak current flows in the current path of the switch device SW.

(57) The power supply circuit SC operates on the current input from the first terminal T1 when the switch device SW is in the on state. The power supply circuit SC supplies electric power to the control circuit CON based on the current input from the first terminal T1.

(58) The control circuit CON periodically switches on and off the first switch element Q1 in the detection circuit DC (until a time t2 in FIG. 2).

(59) For example, when the first switch element Q1 is turned on at a time t1, the current IX flows to the detection capacitor CX, and the detection capacitor CX is charged. As a result, the voltage VX between the opposite ends of the detection resistor RX (the voltage of the detection signal SX) increases.

(60) The peak hold circuit HC holds the peak of the voltage VX between the opposite ends of the detection resistor RX and outputs the held peak voltage as the detection voltage VZ at the second node N2.

(61) When the switch device SW is in the on state, the voltage VZ between the opposite ends of the holding capacitor CZ is approximately equal to the voltage of the battery B.

(62) The comparison circuit CC compares the detection voltage VZ with the threshold voltage Vth, and outputs the comparison result signal So at the High level because the detection voltage VZ is equal to or higher than the threshold voltage Vth.

(63) Since the comparison result signal So indicates that the detection voltage VZ is equal to or higher than the threshold voltage Vth, the control circuit CON determines that the switch device SW is in the on state and the current path of the switch device SW is conductive between the one end and the another end thereof.

(64) In this case, the control circuit CON supplies the drive current to the LED lamp 101, for example, such that the LED lamp 101 illuminates.

(65) Then, at the time t2 in FIG. 2, a user turns off the switch device SW. As a result, the power supply circuit SC stops operating, and the control circuit CON also stops operating. Thus, supply of the drive current to the LED lamp 101 is stopped, and the LED lamp 101 is turned off.

(66) When the switch device SW is in the off state, and no leak current flows in the current path of the switch device SW, the current IX does not flows to the detection capacitor CX, and therefore, the voltage VZ between the opposite ends of the holding capacitor CZ is zero.

(67) FIG. 3 shows waveforms in a case where the switch device SW transitions from the on state to the off state, and a leak current flows in the current path of the switch devise SW.

(68) As described above, the power supply circuit SC operates on the current input from the first terminal T1 when the switch device SW is in the on state. The power supply circuit SC supplies electric power to the control circuit CON based on the current input from the first terminal T1.

(69) The control circuit CON periodically switches on and off the first switch element Q1 in the detection circuit DC (until a time t2 in FIG. 3). The operation until the time t2 shown in FIG. 3 is the same as the operation shown in FIG. 2.

(70) At the time t2 in FIG. 3, the user turns off the switch device SW. Then, for example, the switch device SW is wetted with water, and a leak current flows in the current path of the switch device SW despite the switch device SW having been turned off.

(71) In this state where the switch device SW is in the off state and a leak current flows in the current path of the switch device SW, the power supply circuit SC operates on the current input from the first terminal T1. And the power supply circuit SC supplies electric power to the control circuit CON based on the current input from the first terminal T1.

(72) The control circuit CON periodically switches on and off the first switch element Q1 in the detection circuit DC (from the time t2 in FIG. 3 onward).

(73) For example, if the first switch element Q1 is turned on at times t3 and t5, the current IX flows to the detection capacitor CX, and the detection capacitor CX is charged. As a result, the voltage VX between the opposite ends of the detection resistor RX (the voltage of the detection signal SX) increases.

(74) The current IX flowing when the leak current is flowing is lower than the current IX flowing when the switch device SW is in the on state. Therefore, the increase of the voltage VX between the opposite ends of the detection resistor RX (the voltage of the detection signal SX) is also smaller than that at the time when the switch device SW is in the on state.

(75) The peak hold circuit HC holds the peak of the voltage VX between the opposite ends of the detection resistor RX and outputs the held peak voltage as the detection voltage VZ at the second node N2.

(76) In the state where the switch device SW is in the off state, and the leak current is flowing in the current path of the switch device SW, the voltage VZ between the opposite ends of the holding capacitor CZ is reduced, since the current IX flowing to the detection capacitor CX is reduced.

(77) The comparison circuit CC compares the detection voltage VZ with the threshold voltage Vth, and outputs the comparison result signal So at the Low level when the detection voltage VZ becomes lower than the threshold voltage Vth (at a time t4 in FIG. 3).

(78) Since the comparison result signal So indicates that the detection voltage VZ is lower than the threshold voltage Vth, the control circuit CON determines that the switch device SW is in the off state and the current path of the switch device SW is interrupted between the one end and the another end thereof.

(79) In this case, the control circuit CON supplies no drive current to the LED lamp 101, for example, such that the LED lamp 101 does not illuminate.

(80) As described above, the LED driver circuit 100 according to this embodiment can detect the on/off state of the switch device SW with higher reliability even if the switch device SW is a relatively cheap mechanical switch device SW with low waterproofness and is wetted with water, and a leak current flows in the switch device SW. The LED driver circuit 100 can turn on the LED lamp in response to the on/off state of the switch device.

(81) As described above, an LED driver circuit according to an aspect of the present invention is an LED driver circuit that controls driving of an LED lamp in response to an on/off state of a mechanical switch device, and the LED driver circuit includes: a first terminal to which a current path of the switch device is connected at one end thereof; a second terminal to which the current path of the switch device is connected at another end thereof, the switch device and a battery being connected in series between the first terminal and the second terminal; a detection circuit that periodically detects a current flowing to the first terminal and outputs a detection signal responsive to a result of the detection at a first node; a comparison circuit that compares a detection voltage responsive to the detection signal with a threshold voltage and outputs a comparison result signal responsive to a result of the comparison; and a control circuit that controls a current detection operation of the detection circuit and controls driving of the LED lamp based on the comparison result signal.

(82) If the comparison result signal indicates that the detection voltage is equal to or higher than the threshold voltage, the control circuit determines that the switch device is in the on state, and the current path is conductive between the one end and the another end. On the other hand, if the comparison result signal indicates that the detection voltage is lower than the threshold voltage, the control circuit determines that the switch device is in the off state, and the current path is interrupted between the one end and the another end.

(83) In this way, the on/off state of the switch device can be detected even if the switch device is wetted with water and a leak current occurs.

(84) The LED driver circuit according to the present invention does not require adjustment of the detection timing nor any expensive element, such as a Hall device. In addition, the LED driver circuit according to the present invention can be used with a relatively cheap mechanical switch device (such as a switch device with low waterproofness), and the cost can be reduced.

(85) In short, the LED driver circuit according to the present invention can be manufactured with reduced cost and can reduce the possibility of erroneous detection of the on/off state of the switch device caused by a leak current when the switch device is wetted with water.

(86) The LED driver circuit according to this embodiment can detect with higher reliability the on/off state of the switch device, which is used by the user to operate the LED lamp such as a headlamp or a turn signal, and can turn on the LED lamp in response to the on/off state of the switch device.

Second Embodiment

(87) An example of a configuration of an LED driver circuit according to a second embodiment, which differs from the LED driver circuit according to the first embodiment in the configuration of the detection circuit, will be described. FIG. 4 is a circuit diagram showing an example of a configuration of an LED driver system 2000 according to the second embodiment. In FIG. 4, the same reference symbols as those in FIG. 1 denote the same components as those in the first embodiment, and redundant descriptions thereof will be omitted.

(88) The LED driver system 2000 (FIG. 4) according to the second embodiment includes the battery B, the mechanical switch device SW connected to the battery B, the LED lamp 101 and an LED driver circuit 200 that controls driving of the LED lamp 101 in response to the on/off state of the mechanical switch device SW.

(89) The LED driver circuit 200 according to the second embodiment differs from the LED driver circuit 100 according to the first embodiment in the configuration of the detection circuit DC.

(90) The detection circuit DC includes the first switch element Q1 that is connected to the first terminal T1 at one end thereof and is turned on and off under the control of the control circuit CON, the detection capacitor CX that is connected to another end of the first switch element Q1 at one end thereof and to the first node N1 at another end thereof, the detection resistor RX that is connected to the first node N1 at one end thereof and to the second terminal T2 at another end thereof, the discharge resistor RY that is connected to the another end of the first switch element Q1 at one end thereof and to the second terminal T2 at another end thereof, and a second switch element Q2 that is connected in series with the discharge resistor RY between the another end of the first switch element Q1 and the second terminal T2.

(91) In short, this detection circuit DC differs from the detection circuit DC in the first embodiment in that the detection circuit DC further includes the second switch element Q2. In the example shown in FIG. 4, the second switch element Q2 is an nMOS transistor that is connected in series with the discharge resistor RY between the another end of the first switch element Q1 and the second terminal T2 and has a gate voltage controlled by the control circuit CON.

(92) When the control circuit CON turns on the first switch element Q1 by using the first gate signal SG1, the control circuit CON turns off the second switch element Q2 by using a second gate signal SG2 via the pre-driver circuit PC, for example.

(93) On the other hand, when the control circuit CON turns off the first switch element Q1 by using the first gate signal SG1, the control circuit CON turns on the second switch element Q2 by using the second gate signal SG2 via the pre-driver circuit PC.

(94) As a result of this operation of the second switch element Q2, the detection capacitor CX is discharged only when the first switch element Q1 is in the off state. Thus, the efficiency of charging and discharging of the detection capacitor CX can be improved.

(95) The remainder of the configuration of the LED driver circuit 200 is the same as that of the LED driver circuit 100 shown in FIG. 1.

(96) The remainder of the operational characteristics of the LED driver circuit 200 configured as described above is the same as that of the LED driver circuit 100 according to the first embodiment.

(97) That is, as with the LED driver circuit according to the first embodiment, the LED driver circuit according to the second embodiment can be manufactured with reduced cost and can reduce the possibility of erroneous detection of the on/off state of the switch device caused by a leak current when the switch device is wetted with water.

(98) The LED driver circuit according to this embodiment can detect with higher reliability the on/off state of the switch device, which is used by the user to operate the LED lamp such as a headlamp or a turn signal, and can turn on the LED lamp in response to the on/off state of the switch device.

(99) The LED driver circuits according to the above embodiments have been described with regard to a case where the on/off state of the switch device used by the user to operate the LED lamp such as a headlamp or a turn signal of a motorcycle, as an example. However, the embodiments are not limited to such a case.

(100) Although embodiments of the present invention have been described, these embodiments are shown as examples and are not intended to limit the scope of the present invention. These embodiments can be implemented in other various forms, and various omissions, replacements or modifications are possible without departing from the spirit of the present invention. These embodiments and modifications thereof are included in the scope and spirit of the present invention and are included in the scope of the present invention set forth in the claims and equivalents thereof.