JP7388991B2 - constant current circuit - Google Patents

Description

The present invention relates to a constant current circuit used, for example, to supply power for driving a light emitting element.

2. Description of the Related Art As a lighting device using a light emitting element such as an LED as a light source, there is a device that lights up a plurality of light emitting elements, such as a turn signal of a vehicle. In this type of lighting device, a drive circuit using a DC/DC converter is widely used as a circuit for driving a light emitting element. As a conventional example of a drive circuit, for example, as shown in Patent Document 1, a light emitting device using a step-up DC/DC converter is disclosed in order to obtain sufficient voltage to light a plurality of LEDs.

In recent years, so-called sequential turn lamps in which a plurality of LEDs are turned on in sequence have become popular in turn signals for vehicles. As a drive circuit for realizing a sequential turn lamp, for example, as disclosed in Patent Document 2, Patent Document 3, Non-Patent Document 1, etc., a circuit that generates a signal for sequential turn and drives a plurality of LEDs. is proposed. In the drive method of Patent Document 3, it is necessary to operate a plurality of drive units in parallel to perform sequential turns, resulting in high cost. On the other hand, as in Non-Patent Document 1, cost reduction is possible by providing a switch in parallel with the LED and implementing a sequential turn by turning the switch on and off.

Patent No. 6146984 Japanese Unexamined Patent Publication No. 51-36092 JP 2017-74803 Publication

Texas Instruments, TPS92661-Q1 data sheet "High-Brightness LED Matrix Manager for Automotive Headlight Systems", [online], February 2016, [Retrieved June 4, 2020], Internet <http://www .tij.co.jp/jp/lit/gpn/TPS92661-Q1>

When a sequential turn lamp is driven by a conventional drive circuit, depending on the specifications of the lighting device, a problem arises in that the ripple current during the lighting drive becomes large and the device requirements cannot be met.

SUMMARY OF THE INVENTION An object of the present invention is to provide a constant current circuit that can reduce ripple current when driving a load, such as when sequentially lighting up a plurality of light emitting elements, and can stably supply current.

The present invention includes a first drive voltage generation circuit that outputs a first drive voltage, and a second drive voltage generation circuit that outputs a second drive voltage that is a smaller voltage than the first drive voltage. , a first switch that turns on and off the supply of the first drive voltage, a second switch that is connected in series with the first switch and turns on and off the supply of the second drive voltage, and a second switch that turns on and off the supply of the second drive voltage; an inductor, one end of which is connected to the connection point between the switch and the second switch, and the other end of which is connected to the load; and the first switch and the a control circuit that controls on/off of a second switch, the control circuit being an error amplifier that outputs an error signal proportional to the difference between the detected voltage and a reference voltage that indicates the target amount of the output current. a reference waveform generation circuit that outputs a reference signal synchronized with a clock that turns on and off the first switch and the second switch; and a comparator that outputs a comparison signal that indicates a comparison result between the error signal and the reference signal. When the comparison signal indicates that the output current is less than the target amount, the first switch is turned on, and when the comparison signal indicates that the output current is greater than the target amount, the second switch is turned on. A constant current circuit includes a driver that controls turning on a switch, and performs feedback control so that the detected voltage and the reference voltage match.

The present invention includes a first drive voltage generation circuit that outputs a first drive voltage, and a second drive voltage generation circuit that outputs a second drive voltage that is a smaller voltage than the first drive voltage. , a first switch that turns on and off the supply of the first drive voltage; a diode whose cathode is connected to the first switch and whose anode is connected to the output end of the second drive voltage generation circuit; An inductor having one end connected to a connection point between the first switch and the diode and the other end connected to a load, and an inductor that turns the first switch on and off according to a detected voltage proportional to the output current flowing to the load. a control circuit for controlling the first output current; a reference waveform generation circuit that outputs a reference signal synchronized with a clock for turning on and off a switch; a comparator that outputs a comparison signal indicating a comparison result between the error signal and the reference signal; A driver that controls turning on the first switch when the current is low and turns off the first switch when showing that the output current is greater than the target amount. The present invention also provides a constant current circuit that performs feedback control so that the detected voltage and the reference voltage match.

Further, the present invention provides the above constant current circuit, wherein the control circuit includes a load control circuit that turns on and off the load, and a load control circuit that controls the first drive voltage and the second drive voltage according to the state of the load. Provided is a constant current circuit having an output voltage control circuit that performs control to increase or decrease the output voltage.

Further, the present invention provides the above constant current circuit, wherein the second drive voltage generation circuit sets the target of the second drive voltage using a voltage proportional to the load voltage applied to the load. A constant current circuit is provided that includes a reference voltage generation circuit that generates a second reference voltage.

Further, the present invention provides the constant current circuit described above, wherein the control circuit is a load control circuit that turns on and off the load, and controls to increase or decrease the first drive voltage depending on the state of the load. Provided is a constant current circuit having an output voltage control circuit.

Further, the present invention provides any of the above constant current circuits, wherein the first drive voltage generation circuit and the second drive voltage generation circuit are each configured by a step-up DC/DC converter. Provides a current circuit.

According to the present invention, it is possible to provide a constant current circuit that can reduce ripple current when driving a load, such as when sequentially lighting up a plurality of light emitting elements, and can stably supply current.

FIG. 2 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to the first embodiment. FIG. 7 is a characteristic diagram showing an example of waveforms of an output voltage and an output current in a constant current circuit of a comparative example. FIG. 2 is a characteristic diagram showing an example of waveforms of output voltage and output current in the constant current circuit of the present embodiment. FIG. 7 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to a second embodiment. FIG. 7 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to a third embodiment. FIG. 7 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to a fourth embodiment. FIG. 3 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit of a comparative example.

EMBODIMENT OF THE INVENTION Hereinafter, an embodiment (hereinafter referred to as "this embodiment") specifically disclosing a constant current circuit according to the present invention will be described in detail with reference to the drawings.

(Background leading to this embodiment)
Assume that a sequential turn lamp is driven by combining conventional techniques. As a comparative example, below is an example of a drive circuit that drives a sequential turn lamp with a configuration including a step-up DC/DC converter, a step-down constant current circuit, and a lighting circuit in which a switch is provided in parallel with the light emitting element. show.

FIG. 7 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit of a comparative example. The drive circuit of the comparative example includes a switching power supply circuit 5E including a step-up DC/DC converter and a step-down constant current circuit, and supplies drive power for lighting the light emitting elements to a lighting circuit 6 using an LED.

The switching power supply circuit 5E includes a control circuit 100E, a boost circuit 101 forming a step-up DC/DC converter, and an output circuit 103E forming a step-down constant current circuit. The control circuit 100E outputs a drive control signal that drives and controls the switching transistor M11 of the booster circuit 101 and the switches M31 and M32 of the output circuit 103E, respectively. The booster circuit 101 includes an inductor L11, a rectifying diode D1, and a switching transistor M11, and boosts an input voltage V _IN to output a drive voltage V _OUT1 . The output circuit 103E is composed of a synchronous rectification step-down switching regulator including an inductor L3 and switches M31 and M32. The switches M31 and M32 are connected in series, the drive voltage V _OUT1 is supplied to the first end of the switch M31, the inductor L3 is connected to the connection point between the second end of the switch M31 and the first end of the switch M32, and the inductor L3 is connected to the connection point between the second end of the switch M31 and the first end of the switch M32. The second end of is grounded. The output circuit 103E outputs a constant drive current _ILED based on the drive voltage _VOUT1 supplied from the booster circuit 101, and supplies drive power to the lighting circuit 6.

The lighting circuit 6 includes an LED 61 as a light emitting element and a switching transistor 62 connected in parallel to the LED 61, and is configured by connecting a plurality of sets of the LED 61 and the switching transistor 62 in series. The lighting circuit 6 implements a sequential turn lamp by turning on/off the plurality of LEDs 61 at predetermined timings to turn them off/on under the control of the switch control circuit 30.

In the switching power supply circuit 5E, the booster circuit 101 boosts the input voltage V _IN to the drive voltage V _OUT1 by controlling the on/off time ratio (duty ratio) of the switching transistor M11, and controls the step-down constant current circuit in the subsequent stage. Power is supplied to the output circuit 103E of. The output circuit 103E outputs a constant drive current I _LED by controlling the on/off time ratio (duty ratio) of the switches M31 and M32, and supplies the drive current I _LED to the LED 61 of the lighting circuit 6. .

The drive current I _LED is converted into a detection voltage V _R3 by the current detection resistor R3 and inputted to the error amplifier 312 in the control circuit 100E, and the error amplifier 312 amplifies the error with the reference voltage V _REF3 of the voltage source 314 and provides feedback. A voltage V _FB3 is generated. Then, the comparator 332 compares the triangular wave V _TRI output from the triangular wave generation circuit 331 with the feedback voltage V _FB3 , and an output signal based on the comparison result of the comparator 332 is input to the driver 340. The driver 340 outputs a drive control signal to the gates of the switches M31 and M32 based on the output signal of the comparator 332, and turns on/off the switches M31 and M32. When the feedback voltage V _FB3 is smaller than the triangular wave V _TRI , the switch M32 is turned on and the switch M31 is turned off, providing the driving current I _LED while consuming the energy of the inductor L3. When the feedback voltage V _FB3 is greater than the triangular wave V _TRI , the switch M31 is turned on and the switch M32 is turned off, and the driving current I _LED is provided while storing energy in the inductor L3. As described above, feedback is applied so that the detection voltage V _R3 , which is proportional to the value of the drive current I _LED , matches the reference voltage V _REF3 , and the drive current I _LED is fixed so that I _LED = V _REF3 /R3. The current is controlled and the LED 61 is driven with a constant current. Note that the switches M31 and M32 may be switched on so that they are not turned on at the same time in order to avoid deterioration of the transistors due to through current, and are turned on after a dead time in which both the switches M31 and M32 are turned off.

Problems with the drive circuit of the comparative example will be explained. The output voltage V _SW of the output circuit 103E is approximately equal to the drive voltage V _OUT1 when the switch M31 is on and the switch M32 is off, and is approximately 0V when the switch M31 is off and the switch M32 is on. On the other hand, since the load voltage V _LED applied to the load LED hardly changes, a large voltage difference occurs across the inductor L3 when the switches M31 and M32 turn on/off, and the drive current I _LED flowing to the LED via the inductor L3 is , the current decreases significantly when the output voltage _VSW becomes approximately 0V. The decrease in the drive current _ILED directly corresponds to a ripple current, and the inventors of the present invention have discovered that the ripple current increases. To reduce ripple current, it is necessary to make the inductor larger or increase the switching frequency. However, the former has the problem of increased cost, and the latter may not be selected due to EMC problems.

In view of the above circumstances, the present embodiment shows a configuration example of a constant current circuit capable of reducing ripple current and a driving circuit of a lighting device including the constant current circuit.

(First embodiment)
FIG. 1 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to the first embodiment. The drive circuit of this embodiment includes a switching power supply circuit 5 including a step-up DC/DC converter and a step-down constant current circuit, and provides light emission to a lighting circuit 6 constituting a sequential turn lamp using an LED as a light source. Supplies driving power for lighting the element.

The switching power supply circuit 5 includes a control circuit 100, two boost circuits 101 and 102 forming a step-up DC/DC converter, and an output circuit 103 forming a step-down constant current circuit. The control circuit 100 outputs a drive control signal that drives and controls the switching transistor M11 of the booster circuit 101, the switching transistor M21 of the booster circuit 102, and the switches M31 and M32 of the output circuit 103, respectively.

The control circuit 100 includes an error amplifier 112, a voltage source 114, a slope compensation circuit 131, a comparator 132, an oscillator (OSC) 133, a flip-flop 134, and a gate driver 140 as a first boost control section that drives the switching transistor M11. Further, as a second boost control unit that drives the switching transistor M21, it includes an error amplifier 212, a voltage source 214, a slope compensation circuit 231, a comparator 232, an oscillator (OSC) 233, a flip-flop 234, and a gate driver 240. The control circuit 100 also includes an error amplifier 312, a voltage source 314, a triangular wave generation circuit 331, a comparator 332, a driver 340, a switch control circuit 30, and a switching transistor 62 as an output control unit that drives the plurality of LEDs 61 of the lighting circuit 6. have The error amplifiers 112, 212, and 312 of the control circuit 100 are configured by, for example, a gm amplifier. The control circuit 100 is constituted by, for example, an integrated control IC.

The control circuit 100 is connected to phase compensation circuits 113, 213, and 313 that perform phase compensation for each feedback loop of the first boost control section, the second boost control section, and the output control section. The phase compensation circuit 113 has a phase compensation capacitor C _FB1 and a phase compensation resistor R _FB1 , and is connected between the output end of the error amplifier 112 and the ground. The phase compensation circuit 213 has a phase compensation capacitor C _FB2 and a phase compensation resistor R _FB2 , and is connected between the output end of the error amplifier 212 and ground. The phase compensation circuit 313 has a phase compensation capacitor C _FB3 and a phase compensation resistor R _FB3 , and is connected between the output end of the error amplifier 312 and the ground. For stable operation of these phase compensation circuits 113, 213, and 313, it is desirable that appropriate time constants be set by phase compensation capacitors C _{FB1 to 3} and phase compensation resistors R _{FB1 to 3,} respectively.

The first booster circuit 101 includes an inductor L11, a rectifying diode D1, a switching transistor M11, a resistor R11 for inductor current detection, an output capacitor C11, and feedback resistors R _FB11 and R _FB12 . The first booster circuit 101 boosts the input voltage V _IN and outputs the first drive voltage V _OUT1 . A first boost control section using the first boost circuit 101 functions as a first drive voltage generation circuit. The switching transistor M11 has a gate serving as a control end connected to an output end of the gate driver 140, and is controlled to be turned on/off by the gate driver 140. A drain serving as a first end of the switching transistor M11 is connected to the power source of the input voltage V _IN via the inductor L11, and is also connected to the anode of the rectifying diode D1. A source serving as a second end of the switching transistor M11 is grounded via a resistor R11, a connection point between the source and the resistor R11 is connected to a slope compensation circuit 131, and a first detection signal proportional to the current flowing through the inductor L11 is connected to the slope compensation circuit 131. Voltage _VR1 is input to slope compensation circuit 131. The cathode of the rectifying diode D1 is connected to the switch M31 of the output circuit 103 and grounded via the output capacitor C11, and the feedback resistors R _FB11 and R _FB12 are connected to the ground. The connection point between the feedback resistors R _FB11 and R _FB12 is connected to the inverting input terminal of the error amplifier 112, and a voltage proportional to the first drive voltage V _OUT1 is input to the error amplifier 112.

The second booster circuit 102 includes an inductor L21, a rectifying diode D2, a switching transistor M21, a resistor R21 for inductor current detection, an output capacitor C21, and feedback resistors R _FB21 and R _FB22 . The second booster circuit 102 boosts the input voltage V _IN and outputs the second drive voltage V _OUT2 . A second boost control section using the second boost circuit 102 functions as a second drive voltage generation circuit. The second drive voltage V _OUT2 is lower than the first drive voltage V _OUT1 and is sufficiently higher than 0V. That is, the second drive voltage V _OUT2 is set to a voltage slightly lower than the load voltage V _LED applied to the LED of the load. The configuration of the second booster circuit 102 is similar to the first booster circuit 101 described above, and detailed description thereof will be omitted. The switching transistor M21 has a gate serving as a control end connected to an output end of the gate driver 240, and is controlled to be turned on/off by the gate driver 240. A connection point between the source as the second end of the switching transistor M21 and the resistor R21 is connected to the slope compensation circuit 231, and a second detection voltage V _R2 proportional to the current flowing through the inductor L21 is input to the slope compensation circuit 231. Ru. The connection point between the feedback resistors R _FB21 and R _FB22 is connected to the inverting input terminal of the error amplifier 212, and a voltage proportional to the second drive voltage V _OUT2 is input to the error amplifier 212.

Output circuit 103 includes a first switch M31, a second switch M32, and an inductor L3. The first switch M31 is a high potential side switch, and is also referred to as a high side switch, an upper switch, or the like. The second switch M32 is a low potential side switch, and is also referred to as a low side switch, a lower side switch, or the like. The output circuit 103 outputs a constant drive current I _LED based on the first drive voltage V _OUT1 and the second drive voltage V _OUT2 , and supplies drive power to the lighting circuit 6 as a load.

The switches M31 and M32 are configured by MOSFETs, for example, and are connected in series with each other. In the illustrated example, the switch M31 is configured by a PchMOS transistor, and the switch M32 is configured by an NchMOS transistor. The first end of switch M31 is connected to the output end of first booster circuit 101, the first end of switch M32 is connected to the output end of second booster circuit 102, and the second end of switch M31 is connected to the output end of switch M32. The second end is connected. The switches M31 and M32 have their respective gates as control ends connected to the driver 340 of the control circuit 100, and are controlled to be turned on/off by the driver 340. The first switch M31 turns on and off the supply of the first drive voltage V _OUT1 . The second switch M32 turns on and off the supply of the second drive voltage V _OUT2 . One end of the inductor L3 is connected to the connection point between the switch M31 and the switch M32, and the other end of the inductor L3 becomes the output end of the output circuit 103 and is connected to the LED 61 at the upper end of the lighting circuit 6.

The lighting circuit 6 includes an LED 61 as a light emitting element serving as a load of the drive circuit, and a switching transistor 62 connected in parallel to the LED 61, and a plurality of sets of the LED 61 and the switching transistor 62 are connected in series. configured. The output circuit 103 is connected to the anode side (upper end) of the plurality of LEDs 61 connected in series, one end of the current detection resistor R3 is connected to the cathode side (lower end), and the other end of the current detection resistor R3 is grounded. A connection point between the cathode side of the LED 61 and the current detection resistor R3 is connected to an inverting input terminal of the error amplifier 312, and a detection voltage _VR3 proportional to the value of the drive current _ILED is input to the error amplifier 312. The plurality of switching transistors 62 have respective gates connected to the switch control circuit 30 and are controlled to be turned on/off by the switch control circuit 30. The switch control circuit 30 functions as a load control circuit that turns on and off the plurality of LEDs 61 of the lighting circuit 6 as a load. The lighting circuit 6 realizes a sequential turn lamp by turning on/off the plurality of LEDs 61 at predetermined timing to turn them off/on. The switch control circuit 30 outputs a control signal to the gate of each switching transistor 62 for controlling the lighting of the sequential turn lamp, such as lighting a plurality of LEDs 61 in sequence to display flowing light.

In the first boost control section of the control circuit 100, the output terminal of the error amplifier 112 is connected to the first input terminal of the comparator 132, and the output terminal of the slope compensation circuit 131 is connected to the second input terminal of the comparator 132. The output terminal of the comparator 132 is connected to the R input terminal of the flip-flop 134 , the oscillator 133 is connected to the S input terminal of the flip-flop 134 , and the Q output terminal of the flip-flop 134 is connected to the input terminal of the gate driver 140 . Similarly, in the second boost control section of the control circuit 100, the output terminal of the error amplifier 212 is connected to the first input terminal of the comparator 232, and the output terminal of the slope compensation circuit 231 is connected to the second input terminal of the comparator 232. Ru. The output terminal of the comparator 232 is connected to the R input terminal of the flip-flop 234, the oscillator 233 is connected to the S input terminal of the flip-flop 234, and the Q output terminal of the flip-flop 234 is connected to the input terminal of the gate driver 240.

In addition, in the output control section of the control circuit 100, the output terminal of the error amplifier 312 is connected to the second input terminal of the comparator 332, the triangular wave generation circuit 331 is connected to the first input terminal of the comparator 332, and the output terminal of the comparator 332 is connected to the input terminal of driver 340. The triangular wave generation circuit 331 is an example of a reference waveform generation circuit that generates a triangular or sawtooth reference signal synchronized with a clock that turns on and off the switches M31 and M32, and outputs a triangular wave as the reference signal.

Note that the first boost control section, the second boost control section, and the output control section of the control circuit 100 are not limited to the above-mentioned configurations, and may be configured using other known duty controllers. For example, as a reference waveform generation circuit that generates a reference signal, various alternative configurations are possible, such as replacing a triangular wave generation circuit with a sawtooth wave generation circuit, or replacing a flip-flop with a flip-flop or latch circuit other than an SR flip-flop. be. Further, in the lighting circuit 6, the drive current _ILED flowing through the LED 61 is passed to the ground via the current detection resistor R3, but it may be returned to the input voltage _VIN .

Next, the operation of the drive circuit of this embodiment will be explained. In the first step-up control section of the control circuit 100, the error amplifier 112 amplifies the error between the voltage proportional to the first drive voltage V _OUT1 and the reference voltage V _REF1 output by the voltage source 114 to generate a first feedback. A voltage V _FB1 is generated and output to the comparator 132. The slope compensation circuit 131 superimposes a slope signal on a first detection voltage V _R1 that is proportional to the current flowing through the inductor L11, and outputs it to the comparator 132 as a slope voltage V _SLP1 . The comparator 132 compares the first feedback voltage V _FB1 and the slope voltage V _SLP1 , and when the slope voltage V _SLP1 reaches the first feedback voltage V _FB1 , an off signal (high level ) is output. That is, when the current flowing through the inductor L11 reaches a peak current corresponding to the first feedback voltage _VFB1 , the off signal input to the R input terminal of the flip-flop 134 is asserted. A periodic signal output from the oscillator 133 is input to the S input terminal of the flip-flop 134. The output signal at the Q output end of the flip-flop 134 transitions to high level (on level) at the positive edge of the periodic signal at the S input end, and goes to low level (off level) when the off signal at the R input end is asserted. Transition. Based on the output of the flip-flop 134, a drive signal is supplied from the gate driver 140 to the gate of the switching transistor M11, and the switching transistor M11 is controlled on/off.

The operation of the second boost control section of the control circuit 100 is also similar to that of the first boost control section. That is, when the slope voltage V _SLP2 reaches the second feedback voltage V _FB2 , that is, when the current flowing through the inductor L21 reaches the peak current according to the second feedback voltage V _FB2 , the R input terminal of the flip-flop 234 The off signal input to the output signal goes high and is asserted. The output signal at the Q output terminal of the flip-flop 234 transitions to high level (on level) at the positive edge of the periodic signal at the S input terminal, and goes to low level (off level) when the off signal at the R input terminal is asserted. ). Based on the output of the flip-flop 234, a drive signal is supplied from the gate driver 240 to the gate of the switching transistor M21, and the switching transistor M21 is controlled on/off.

In the output control section of the control circuit 100, the error amplifier 312 outputs a detection voltage (detection voltage of output current) V _R3 proportional to the value of the drive current I _LED of the lighting circuit 6 as a load and a voltage source 314. The third feedback voltage V _FB3 as an error signal is generated by amplifying the error with the reference voltage (output current reference voltage) V _REF3 and output to the comparator 332 . Here, the reference voltage V _REF3 is a voltage that indicates the target amount of the drive current I _LED as the output current. The comparator 332 compares the third feedback voltage V _FB3 and the triangular wave V _TRI as a reference signal output from the triangular wave generating circuit 331, and outputs a comparison signal indicating the comparison result. The comparator 332 outputs a comparison signal of a high level when the third feedback voltage V _FB3 is greater than the triangular wave V _TRI , and outputs a comparison signal of a low level to the driver 340 when vice versa. In this embodiment, a high level comparison signal indicates that the drive current I _LED as the output current is less than the target amount, and a low level comparison signal indicates that the drive current I _LED is greater than the target amount.

The driver 340 outputs a drive control signal to the gates of the switches M31 and M32 based on the output signal of the comparator 332, that is, based on whether the drive current _ILED , which is the output current, is less than or more than the target amount. Drive on/off. When the third feedback voltage V _FB3 is smaller than the triangular wave V _TRI , the switch M32 is turned on and the switch M31 is turned off, and the driving current I _LED is provided while consuming the energy of the inductor L3. When the third feedback voltage V _FB3 is greater than the triangular wave V _TRI , the switch M31 is turned on and the switch M32 is turned off, and the driving current I _LED is provided while storing energy in the inductor L3. Through the above operation, feedback is applied so that the detection voltage V _R3 , which is proportional to the value of the drive current I _LED , matches the reference voltage V _REF3 , and the drive current I _LED is controlled at a constant current. Note that if the switches M31 and M32 are turned off at the same time, the power supplied to the inductor L3 is cut off, the output voltage V _SW decreases and reaches a negative voltage, and the second drive voltage V _OUT2 is applied to both ends of the switch M32. Since the current flows while the above voltage is applied, there is a risk that the switch M32 may be damaged due to heat generation. In order to avoid this, the switches M31 and M32 may be switched so that they are simultaneously turned on and then turned off.

Here, examples of operating waveforms in the drive circuits of the present embodiment and the comparative example shown in FIG. 7 will be shown, and the effects of the present embodiment will be described.

FIG. 2 is a characteristic diagram showing an example of the waveforms of the output voltage and output current in the constant current circuit of the comparative example. FIG. 3 is a characteristic diagram showing an example of the waveforms of the output voltage and output current in the constant current circuit of this embodiment. In FIGS. 2 and 3, the output voltage V _SW , drive voltages V _OUT1 , V _OUT2 , load voltage V _LED , drive current I _LED , and peak current values I _{LED_H} , I _{LED_L} , I _{LED_H2} , and I _{LED_L2} in the drive circuit are schematically shown. Each is shown by the waveform shown in .

In the comparative example of FIG. 2, the output voltage V _SW of the output circuit 103E is approximately equal to the drive voltage V _OUT1 when the switch M31 is on and the switch M32 is off, and is approximately 0V when the switch M31 is off and the switch M32 is on. becomes. On the other hand, the load voltage V _LED applied to the LED of the load does not substantially change due to the characteristics of the diode. Therefore, when the output voltage VSW becomes approximately 0V, the driving current _ILED flowing to the LED through the inductor _L3 is V=L*dI/dt (where, L: inductance of the inductor L3, dI/dt: driving The current decreases by a larger amount than the current I (the unit time change rate of _{the LED} ). Therefore, the drive current I _LED has a large difference between the drive current maximum value I _{LED_H} and the drive current minimum value I _{LED_L} , and the ripple current becomes large. For example, in the above equation, if L3 is 22 μH, V _LED is 28 V, and dt is 0.6 μs, the drive current I _LED will decrease by 760 mA. This directly corresponds to ripple current.

In the present embodiment of FIG. 3, the output voltage V _SW of the output circuit 103 is approximately equal to the first drive voltage V _OUT1 when the switch M31 is on and the switch M32 is off, and when the switch M31 is off and the switch M32 is on. At the time of , it becomes approximately equal to the second drive voltage V _OUT2 . The second drive voltage V _OUT2 is set lower than the load voltage V _LED , and the output voltage V _SW varies within a small range. The voltage difference between the second drive voltage V _OUT2 and the load voltage V _LED is set to a value within a predetermined range such that the ripple current amount of the drive current I _LED as an output current is sufficiently small. In this case, the difference between the drive current maximum value I _{LED_H2} and the drive current minimum value I _{LED_L2} becomes small in the drive _current I LED flowing to the LED via the inductor L3.

The amount of ripple current in the comparative example is expressed by the following equation (1), where T is the switching period.

The amount of ripple current in this embodiment is expressed by the following equation (2), where T is the switching period.

したがって、比較例と本実施形態のリプル電流の比は、次の（３）式で表される。

Therefore, the ratio of ripple currents between the comparative example and this embodiment is expressed by the following equation (3).

In this way, in this embodiment, the ripple current can be significantly reduced compared to the comparative example. For example, in equation (3), if V _OUT1 is 36V, V _OUT2 is 27V, and V _LED is 28V, the amount of ripple current is reduced to 1/7.

As described above, in the configuration of this embodiment, by raising the voltage when the switch M32 is on from 0V to _VOUT2 , the ripple current can be reduced without increasing the size of the inductor or increasing the switching frequency. can be reduced.

(Second embodiment)
FIG. 4 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to the second embodiment. A switching power supply circuit 5A in the drive circuit of the second embodiment includes a controller 350A in addition to the configuration of the switching power supply circuit 5 of the first embodiment shown in FIG. Components similar to those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

The controller 350A provided in the control circuit 100A functions as an output voltage control circuit, and includes a variable voltage source 114A as a first reference voltage generation circuit, a variable voltage source 214A as a second reference voltage generation circuit, The operation of the switch control circuit 30A as a load control circuit is controlled. In order to turn on/off the LED 61 of the lighting circuit 6, the controller 350A sends a control signal to the switch control circuit 30A to turn off/on the switching transistor 62. In accordance with the lighting/extinguishing of the LED 61, the variable voltage source 114A and the variable voltage source 214A are controlled to increase/lower the voltages of the first reference voltage V _REF1 and the second reference voltage V _REF2 . The output voltages of the booster circuit 101 and the second booster circuit 102 are raised/lowered. Note that the controller 350A may perform the above control based on a control signal input from an external controller or the like.

That is, when the LED 61 is turned on, the reference voltages V _REF1 and V _REF2 are increased, and the first drive voltage V _OUT1 of the output of the first booster circuit 101 and the second drive voltage of the output of the second booster circuit 102 are increased. Increase the voltage V _OUT2 . Further, when the LED 61 is turned off, the voltages of the reference voltages V _REF1 and V _REF2 are lowered, and the first drive voltage V _OUT1 of the output of the first booster circuit 101 and the second drive voltage of the output of the second booster circuit 102 are lowered. Try to lower the voltage V _OUT2 .

More specifically, the boosted voltages of the first booster circuit 101 and the second booster circuit 102 are varied according to the number of LEDs to be lit in the sequential turn lamp, and the first drive voltage V _OUT1 and the second drive voltage VOUT1 are changed. Change the drive voltage V _OUT2 . In this manner, in the second embodiment, control is performed to increase or decrease the first drive voltage V _OUT1 and the second drive voltage V _OUT2 depending on the load state such as lighting/extinguishing of the LED 61 and the number of lit LEDs 61. I do. The minimum necessary load voltage V _LED changes depending on the number of LEDs lit, and the necessary boost voltages by the first boost circuit 101 and the second boost circuit 102 differ. Therefore, by changing the boosted voltage according to the lighting state of the LED, it is possible to further reduce the ripple current.

For example, when using a lighting circuit 6 in which 8 LEDs are connected in series as a load, and the voltage across each LED is 3.5V, the load voltage V required to light all 8 _LEDs is 28V. . At this time, by adjusting the first drive voltage V _OUT1 to 36V and the second drive voltage V _OUT2 to 27V, for example, the ripple current when eight lights are turned on can be reduced. In addition, when lighting six LEDs, the required load voltage V _LED is 21V. For example, by adjusting the first drive voltage V _OUT1 to 29V and the second drive voltage V _OUT2 to 20V, it is possible to ripple current can be further reduced.

In the configuration of FIG. 4, the output voltage of the DC/DC converter is changed by controlling the voltage values of the reference voltages V _REF1 and V _REF2 , but the feedback resistors R _FB11 , R _FB12 , R _FB21 , and R _FB22 The output voltage may be changed by controlling the resistance value, or a general power supply circuit capable of obtaining an appropriate output voltage may be used instead of the DC/DC converter. Furthermore, although the driving current _ILED flowing through the LED 61 is flowing to the ground via the current detection resistor R3, it may be returned to the input voltage _VIN .

In this way, in the second embodiment, the ripple current can be further reduced by changing the boosted voltages of the first booster circuit 101 and the second booster circuit 102 according to the lighting/extinguishing of the LED. .

(Third embodiment)
FIG. 5 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to the third embodiment. The switching power supply circuit 5B in the drive circuit of the third embodiment partially changes the configuration of the switching power supply circuit 5A of the second embodiment shown in FIG. 4, and includes a circuit that generates the second reference voltage V _REF2 . This is a modification with a different configuration. Note that it is also possible to apply the configuration of the third embodiment to the switching power supply circuit 5 of the first embodiment shown in FIG. Components similar to those in the first and second embodiments are designated by the same reference numerals, and description thereof will be omitted.

A controller 350B provided in the control circuit 100B functions as an output voltage control circuit, and controls the operation of the variable voltage source 114B and the switch control circuit 30B as a load control circuit. The reference voltage generation circuit 214B that generates the second reference voltage V _REF2 has a diode D22 and feedback resistors R _FB23 and R _FB24 connected in series to the ends of the plurality of LEDs 61 of the lighting circuit 6 on the output circuit 103 side (anode side). Consists of a connected voltage generation circuit. The reference voltage generation circuit 214B generates a voltage proportional to the load voltage V _LED applied to the LED 61 of the lighting circuit 6 serving as a load as the second reference voltage V _REF2 . The anode of a diode D22 is connected to the connection point between the output end of the output circuit 103 and the LED 61, feedback resistors R _FB23 and R _FB24 are connected in series to the cathode of the diode D22, and one end of the feedback resistor R _FB23 is grounded. . The connection point between the feedback resistors R _FB23 and R _FB24 is connected to the non-inverting input terminal of the error amplifier 212 in the control circuit 100B, and a voltage proportional to the load voltage V _LED is input to the error amplifier 212.

As can _be seen from the above equation (3), the closer V _OUT2 and V _LED are _, the smaller the ripple current can be. is desirable. In order to realize this, in the third embodiment, the reference voltage V _REF2 is a voltage obtained by resistance-dividing a voltage obtained by lowering the load voltage V _LED at the upper end of the LED 61 by one diode. Thereby, even if the load voltage V _LED fluctuates by turning on/off the LED, it is possible to follow this change and output an output voltage slightly lower than V _LED as the second drive voltage V _OUT2 .

In this way, in the third embodiment, the ripple current can be further reduced by adjusting the boosted voltage of the second booster circuit 102 to a value close to V _LED according to the load voltage V _LED .

(Fourth embodiment)
FIG. 6 is a circuit diagram showing the configuration of a drive circuit including a constant current circuit according to the fourth embodiment. The switching power supply circuit 5C in the drive circuit of the fourth embodiment is a modification in which the configuration of the switching power supply circuit 5B of the third embodiment shown in FIG. 5 is partially changed and the switch M32 is replaced with a diode D3. . Components similar to those in the first, second, and third embodiments are designated by the same reference numerals, and description thereof will be omitted.

In the output circuit 103C, the anode of the diode D3 is connected to the output end of the second booster circuit 102, and the second end of the switch M31 and the cathode of the diode D3 are connected. One end of the inductor L3 is connected to the connection point between the switch M31 and the diode D3, and the other end of the inductor L3 becomes the output end of the output circuit 103C and is connected to the LED 61 at the upper end of the lighting circuit 6.

In the output circuit 103C, by replacing the switch M32 with a diode D3, the driver 340C of the control circuit 100C outputs a drive control signal to the gate of the switch M31 to turn on/off the switch M31. In the configuration including the switch M32, the drive control signal output to the gate of the switch M32 needs to be higher than the second drive voltage V _OUT2 , so a booster circuit and a bootstrap circuit (not shown) are included in the switching power supply circuit. It may be necessary. In the fourth embodiment, only the switch M31 needs to be driven, and a booster circuit and a bootstrap circuit are not required, so the driver 340C can be simplified.

As described above, in this embodiment, two DC/DC converters as booster circuits are provided to generate the first drive voltage V _OUT1 and the second drive voltage V _OUT2 , and the switch element of the output circuit is turned on/off. Reduce the voltage difference that occurs in the inductor when it is turned off. As a result, in the constant current circuit constituting the output circuit of the drive circuit, the ripple current of the constant current output can be reduced without increasing the switching frequency or increasing the inductance. This embodiment is suitable for an application example in which a step-up DC/DC converter and a step-down constant current converter are combined to obtain a constant current output, for example, in a drive circuit for a sequential turn lamp configured with a plurality of light emitting elements. , it becomes possible to reduce the output ripple current.

Note that the drive circuit for the light emitting element according to the present embodiment is not limited to the combination of a step-up circuit and a step-down type constant current circuit, and can generate a drive voltage using another type of constant voltage circuit such as a step-down type, for example. Any device having a constant current circuit and a constant current circuit as an output circuit can be similarly applied. That is, the constant current circuit of this embodiment can be applied to a light emitting element drive circuit, etc. as a circuit that inputs the output voltages of two drive voltage generation circuits to obtain a constant output current.

Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is clear that those skilled in the art can come up with various changes or modifications within the scope of the claims, and these naturally fall within the technical scope of the present invention. Understood. Further, each component in the above embodiments may be arbitrarily combined without departing from the spirit of the present invention.

The present invention has the effect of reducing ripple current when driving a load such as when lighting a plurality of light emitting elements in sequence, and making it possible to provide a stable current supply, for example, when driving a plurality of light emitting elements such as a sequential turn lamp. This is useful for constant current circuits installed in drive circuits and the like.

5, 5A, 5B, 5C: Switching power supply circuit 6: Lighting circuit 30, 30A, 30B: Switch control circuit 61: LED
62: Switching transistors 100, 100A, 100B, 100C: Control circuits 101, 102: Boost circuits 103, 103C: Output circuits 112, 212, 312: Error amplifiers 113, 213, 313: Phase compensation circuits 114, 214, 314: Voltage Sources 114A, 114B, 214A: Variable voltage sources 131, 231: Slope compensation circuits 132, 232, 332: Comparators 133, 233: Oscillators 134, 234: Flip-flops 140, 240: Gate driver 214B: Reference voltage generation circuit 331: Triangular wave Generation circuits 340, 340C: Drivers 350A, 350B: Controllers C11, C21: Output capacitors D1, D2: Rectifying diodes D3, D22: Diodes L3, L11, L21: Inductors M11, M21: Switching transistors M31, M32: Switch R3 : Current detection resistor R11, R21: Resistor R _FB11 , R _FB12 , R _FB21 , R _FB22 , R _FB23 , R _FB24 : Feedback resistor

Claims (6)

a first drive voltage generation circuit that outputs a first drive voltage;
a second drive voltage generation circuit that outputs a second drive voltage that is a smaller voltage than the first drive voltage;
a first switch that turns on and off the supply of the first drive voltage;
a second switch that is connected in series with the first switch and turns on and off the supply of the second drive voltage;
an inductor having one end connected to a connection point between the first switch and the second switch, and the other end connected to a load;
a control circuit that controls on/off of the first switch and the second switch according to a detected voltage proportional to the output current flowing to the load,
The control circuit includes:
an error amplifier that outputs an error signal proportional to the difference between the detected voltage and a reference voltage that indicates the target amount of output current;
a reference waveform generation circuit that outputs a reference signal synchronized with a clock that turns on and off the first switch and the second switch;
a comparator that outputs a comparison signal indicating a comparison result between the error signal and the reference signal;
When the comparison signal indicates that the output current is less than the target amount, the first switch is turned on, and when the comparison signal indicates that the output current is greater than the target amount, the second switch is turned on. It has a driver that controls turning on,
performing feedback control so that the detected voltage and the reference voltage match;
Constant current circuit. a first drive voltage generation circuit that outputs a first drive voltage;
a second drive voltage generation circuit that outputs a second drive voltage that is a smaller voltage than the first drive voltage;
a first switch that turns on and off the supply of the first drive voltage;
a diode whose cathode is connected to the first switch and whose anode is connected to the output end of the second drive voltage generation circuit;
an inductor having one end connected to a connection point between the first switch and the diode and the other end connected to a load;
a control circuit that controls on/off of the first switch according to a detected voltage proportional to the output current flowing to the load,
The control circuit includes:
an error amplifier that outputs an error signal proportional to the difference between the detected voltage and a reference voltage that indicates the target amount of output current;
a reference waveform generation circuit that outputs a reference signal synchronized with a clock that turns on and off the first switch;
a comparator that outputs a comparison signal indicating a comparison result between the error signal and the reference signal;
The first switch is controlled to be turned on when the comparison signal indicates that the output current is less than the target amount, and the first switch is turned on when the comparison signal indicates that the output current is greater than the target amount. It has a driver that controls turning off,
performing feedback control so that the detected voltage and the reference voltage match;
Constant current circuit. The control circuit includes:
a load control circuit that turns on and off the load;
an output voltage control circuit that controls increasing or decreasing the first drive voltage and the second drive voltage according to the state of the load;
The constant current circuit according to claim 1 or 2. The second drive voltage generation circuit includes:
a reference voltage generation circuit that generates a second reference voltage that is a target of the second drive voltage using a voltage proportional to the load voltage applied to the load;
The constant current circuit according to claim 1 or 2. The control circuit includes:
a load control circuit that turns on and off the load;
an output voltage control circuit that controls to increase or decrease the first drive voltage according to the state of the load;
The constant current circuit according to claim 4. The first drive voltage generation circuit and the second drive voltage generation circuit are each configured by a step-up DC/DC converter,
A constant current circuit according to any one of claims 1 to 5.

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