JP2011029463A - Light-emitting diode driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light-emitting diode driving circuit capable of adjusting the luminance of a light-emitting diode by switching the driving current and suppressing an increase of consumed power and a change in luminance due to the variation in characteristic of the light-emitting diode. <P>SOLUTION: During a time period in which a switch circuit of a constant-current circuit 10 is put in an on state and a driving current IL1 is caused to flow in an LED string S1, feedback control is performed in such a manner that a voltage drop VSINK may come close to a reference voltage VP. Meanwhile, during a time period in which the switch circuit of the constant-current circuit 10 is put in an off state and the driving circuit IL1 of the LED string S1 is cut off, the feedback control is suspended. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a light emitting diode driving circuit for driving a light emitting diode used in various lighting devices and display devices.

  Since light emitting diodes (hereinafter referred to as LEDs) have excellent characteristics such as small size, light weight and long life, research and development has been actively performed. In recent years, LEDs have attracted further expectations due to significant progress in technology related to higher brightness, higher efficiency, and lower cost. For example, the field of light sources that have been covered by high-intensity discharge lamps, such as liquid crystal display backlights, can now be replaced with LEDs, and the range of applications is expected to expand in the future.

  Patent Document 1 describes an LED drive circuit that can suppress EMI by adjusting the slope of the rise and fall of the LED drive current.

JP 2009-135138 A

FIG. 24 is a diagram illustrating an example of an LED drive circuit.
A constant drive voltage VOUT is supplied to the n LED groups (LED string S1) connected in series. A constant current circuit 102 is provided in series with the LED string S1, and the drive current IL1 flowing through the LED string S1 is controlled to be constant.

  As shown in FIG. 24, forward voltages (Vled (1), Vled (2),...) Corresponding to the drive current IL1 of the constant current circuit 102 are generated in each LED of the LED string S1. A voltage drop VSINK generated in the constant current circuit 102 is expressed by the following equation.

  The power consumption WL of the constant current circuit 102 is expressed by the following equation as the product of the voltage drop VSINK and the drive current IL1.

  Since the power consumption WL is useless power that does not contribute to the light emission of the LED, it is desirable that the power consumption WL be as small as possible. In order to reduce the power consumption WL, as can be seen from the equation (2), it is necessary to make the drive voltage VOUT as low as possible. However, since the constant current circuit 102 is generally configured using transistors, the constant current operation cannot be maintained at a very low voltage.

  FIG. 25A illustrates a configuration example of the constant current circuit 102. In the constant current circuit 102 shown in FIG. 25A, by inputting a constant voltage Vfix to the gate of the N-type MOS transistor, the drive current IL1 flowing from the drain to the source is controlled to be constant. This constant current characteristic is based on the voltage-current characteristic of an N-type MOS transistor as shown in FIG. When the drain-source voltage of the N-type MOS transistor decreases, the operating point of the N-type MOS transistor deviates from the saturation region, so that the drive current IL1 becomes smaller than a desired value.

  Therefore, in order to reduce the power consumption WL as much as possible while maintaining the luminance of the LED, it is necessary to set the voltage drop of the constant current circuit 102 to an appropriate value.

  However, in general, the forward voltage of an LED has individual variations due to the manufacturing process and variations due to temperature characteristics. Therefore, when a constant drive voltage VOUT is supplied to the LED, the voltage drop VSINK of the constant current circuit 102 connected in series with the LED string S1 is a forward voltage (Vled (1), Vled (2), Fluctuates according to the variation of ...). When the voltage drop VSINK becomes extremely low, the constant current circuit 102 cannot maintain a desired current as described above, and the luminance of the LED is lowered. Further, if the voltage drop VSINK becomes too large, the power loss in the constant current circuit becomes large, and heat generation becomes a problem.

  As a method for avoiding such a problem, for example, a method of controlling the drive voltage according to the voltage drop of the constant current circuit is conceivable. FIG. 26 is a diagram illustrating an example of such an LED drive circuit.

  The LED drive circuit shown in FIG. 26 includes a power supply circuit 101, a constant current circuit 102, and a feedback control circuit 103. The power supply circuit 101 supplies the drive voltage VOUT to the LED string S1. The constant current circuit 102 is provided in the path of the drive current IL1 flowing through the LED string S1, and controls the drive current IL1 to be constant. The feedback control circuit 103 controls the power supply circuit 101 so that the voltage drop VSINK generated in the constant current circuit 102 approaches the reference voltage VP.

  By controlling the drive voltage VOUT so that the voltage drop VSINK of the constant current circuit 102 approaches the reference voltage VP, power consumption is maintained while keeping the LED brightness constant even when the forward voltage of the LED varies. Can be suppressed.

  By the way, when adjusting the brightness of the LED, a method (hereinafter referred to as “PWM dimming”) of switching the LED drive current at a high frequency that is not sensitive to human vision is generally used. In PWM dimming, the average value of the drive current is adjusted by adjusting the ratio (duty ratio) of the on-time of the drive current in one cycle. In human vision, the brightness of light changes according to the average value of the drive current, so that the brightness can be adjusted by adjusting the duty ratio.

  When this PWM dimming is applied to the LED drive circuit shown in FIG. 26, the following problem occurs.

For example, it is assumed that the gate voltage is controlled so that the transistor of the constant current circuit 102 is turned off during the period in which the drive current IL1 is cut off. In this case, the terminal SINK for detecting the voltage drop VSINK is in a floating state with respect to the ground. Usually, since the leakage current of the LED string S1 is larger than the cutoff current of the transistor, when the transistor of the constant current circuit 102 is turned off, the voltage at the terminal SINK rises to the vicinity of the drive voltage VOUT. Since this state is equivalent to the case where the drive voltage VOUT is very high for the feedback control circuit 103, the feedback control circuit 103 generates the feedback control signal SFB so as to extremely decrease the drive voltage VOUT.
Next, when a transition is made from this state to a period in which the drive current IL1 flows, feedback control is started from a state in which the drive voltage VOUT is extremely low at the beginning of the period. When the drive voltage VOUT is low, the voltage drop VSINK is lower than the reference voltage VP, so that the drive current IL1 of the LED string S1 is smaller than the constant value Iconst. As a result, the average value of the current flowing through the LED string S1 decreases, and the luminance decreases.

  The present invention has been made in view of such circumstances, and an object of the present invention is to adjust the luminance of the light emitting diode by switching the driving current, and to increase power consumption and change in luminance due to variations in the characteristics of the light emitting diode. An object of the present invention is to provide a light emitting diode driving circuit capable of suppressing the above.

  A light emitting diode driving circuit according to the present invention is a light emitting diode driving circuit for driving a light emitting diode circuit including one or a plurality of light emitting diodes connected in series, and a voltage supply for supplying a driving voltage to the light emitting diode circuit. A constant current circuit for controlling the drive current to be constant, a voltage drop generated in the constant current circuit, or a cathode voltage of the light emitting diode circuit. When the threshold voltage is lower than 1, the voltage supply circuit is controlled so that the driving voltage is increased, and when the voltage drop or the cathode voltage is higher than the first threshold voltage, When the cathode voltage exceeds a second threshold voltage that is higher than the first threshold voltage, the drive voltage is increased so as to decrease. A feedback control circuit that controls the voltage supply circuit, and the constant current circuit includes a switch circuit that cuts off a path of the drive current according to an input dimming signal, and the feedback control circuit includes: When the switch circuit is turned off, control of the drive voltage according to the voltage drop or the cathode voltage of the constant current circuit is stopped, and when the switch circuit is turned on, the stopped control is resumed.

  According to the light emitting diode driving circuit, the voltage supply circuit controls the driving voltage to increase when a voltage drop generated in the constant current circuit or a cathode voltage of the light emitting diode circuit is lower than a first threshold voltage. The voltage drop or the cathode voltage is higher than the first threshold voltage, or the voltage drop or the cathode voltage exceeds a second threshold voltage higher than the first threshold voltage, The voltage supply circuit is controlled so that the drive voltage decreases. Thereby, feedback control is performed so that the voltage drop generated in the constant current circuit or the cathode voltage of the light emitting diode circuit approaches the voltage range defined by the first threshold voltage and the second threshold voltage. . Further, when the switch circuit of the constant current circuit is turned off according to the dimming signal, the control of the drive voltage according to the voltage drop or the cathode voltage is stopped, and when the switch circuit is turned on, the control is stopped. Control resumes. Accordingly, even if the voltage drop or the cathode voltage fluctuates greatly during the period when the switch circuit is turned off, the influence of the fluctuation does not reach the driving voltage.

Preferably, the feedback control circuit includes a difference between the voltage drop or the cathode voltage generated in the constant current circuit and the first threshold voltage or the second threshold voltage when the switch circuit is in an ON state. Generating a feedback control signal that changes in response to the switching circuit, holding the feedback control signal when the switch circuit transitions from an on state to an off state, and holding the feedback control signal when the switch circuit transitions from an off state to an on state. Release the holding of the control signal. The voltage supply circuit adjusts the drive voltage according to the feedback control signal.
Thus, the feedback control signal generated during the ON state of the switch circuit is used as an initial value at the start of the next ON state across the OFF state. Therefore, the control of the drive voltage is started by the appropriate feedback control signal at the initial stage of the ON state of the switch circuit.

  Preferably, the feedback control circuit holds the feedback control signal after a lapse of a predetermined delay time that can stabilize the voltage drop level after the switch circuit shifts from an off state to an on state. To release.

  The feedback control circuit includes a comparison circuit that compares the voltage drop generated in the constant current circuit or the cathode voltage of the light emitting diode circuit with the first threshold voltage and the second threshold voltage, and the comparison Based on a signal holding circuit that holds an output signal of the circuit at a constant period and an output signal of the comparison circuit held in the signal holding circuit, the voltage drop or the cathode voltage is greater than the first threshold voltage. When the voltage is low, the feedback control signal is changed stepwise or continuously so that the drive voltage increases, and when the voltage drop or the cathode voltage is higher than the second threshold voltage, the drive voltage The feedback control signal is changed stepwise or continuously so that the voltage drop or the cathode voltage is higher than the first threshold voltage. If no lower than the voltage includes a feedback control signal generating circuit for holding the feedback control signal.

  Preferably, the output signal of the comparison circuit and the dimming signal are input, and when the dimming signal indicates the ON state of the switch circuit, the output signal of the comparison circuit is output, and the dimming signal is When indicating an off state of the switch circuit, the switch circuit has a gate circuit that outputs a predetermined signal indicating the off state, and the signal holding circuit holds the signal output from the gate circuit at the certain period, The feedback control signal generation circuit changes or holds the feedback control signal according to the output signal of the comparison circuit held in the signal holding circuit, and the predetermined signal is held in the signal holding circuit. In this case, the feedback control signal is held.

  Preferably, the feedback control signal generation circuit increases or decreases or holds the count value at a certain period according to the output signal of the comparison circuit held in the signal holding circuit, and the signal holding circuit holds the predetermined value. When the signal is held, a counting circuit that holds the count value and a digital-analog conversion circuit that generates an analog signal corresponding to the count value of the counting circuit are included.

  Preferably, when the dimming signal indicating that the switch circuit shifts from the off state to the on state is input, a delay circuit that delays the dimming signal by a predetermined delay time and then inputs the delay circuit to the gate circuit Have.

Preferably, the plurality of constant current circuits for controlling the drive currents flowing through the plurality of light emitting diode circuits supplied with the common drive voltage from the voltage supply circuit to be constant and the plurality of constant current circuits are generated. A plurality of feedback control circuits each generating the feedback control signal according to a difference between the voltage drop or the cathode voltage of the light emitting diode circuit and the first threshold voltage or the second threshold voltage; The voltage supply circuit adjusts the drive voltage in accordance with a signal obtained by combining the plurality of feedback control signals generated in the plurality of feedback control circuits.
Preferably, a signal combining unit that outputs a feedback control signal that raises the drive voltage most among the plurality of feedback control signals generated in the plurality of feedback control circuits, and the voltage supply circuit includes the signal The drive voltage is adjusted according to a feedback control signal output from the synthesis unit.

  Preferably, the voltage supply circuit compares the divided voltage with a reference voltage to generate an error signal, and a voltage dividing circuit that supplies the divided voltage obtained by dividing the drive voltage to the first node. An amplifying circuit; a comparator that generates a switching signal by comparing the error signal and the ramp signal; and a switching transistor that is turned on and off in response to the switching signal. The feedback control circuit includes the voltage drop Alternatively, a feedback control signal for controlling the drive voltage is generated according to the cathode voltage, and the feedback control signal is supplied to the first node through a resistor.

  According to the present invention, during the ON period in which the driving current is supplied to the light emitting diode, the driving voltage of the light emitting diode is controlled according to the voltage drop of the constant current circuit or the cathode voltage of the light emitting diode circuit, and the driving current of the light emitting diode is cut off. By stopping the control of the driving voltage during the off period, an increase in power consumption and a change in luminance at the initial stage of the on period can be suppressed.

It is a figure which shows an example of a structure of the LED drive circuit which concerns on 1st Embodiment. It is a figure which shows the example of the drive current of LED, and the waveform of a light control signal. FIG. 2 is a first diagram illustrating voltage waveforms at various parts when an LED is lit in the LED drive circuit shown in FIG. 1. FIG. 3 is a second diagram illustrating voltage waveforms at various portions when LEDs are lit in the LED drive circuit shown in FIG. 1. FIG. 2 is a first diagram illustrating voltage waveforms at various portions when an LED repeatedly turns on and off in the LED drive circuit shown in FIG. 1. FIG. 4 is a second diagram illustrating voltage waveforms at various parts when the LED is repeatedly turned on and off in the LED drive circuit shown in FIG. 1. An example of the LED drive circuit which added the phase compensation element is shown. It is a figure which shows an example of a structure of the LED drive circuit which concerns on 2nd Embodiment. It is a figure which shows an example of a structure of a logic circuit. It is a figure for demonstrating operation | movement of a delay circuit. It is a figure for demonstrating the count operation | movement of an up / down counter. It is a figure which shows the relationship between the analog signal of a digital-analog converting circuit, and the count value of an up / down counter. FIG. 9 is a first diagram illustrating a voltage waveform of each part at the time of LED lighting in the LED drive circuit shown in FIG. 8. FIG. 9 is a second diagram illustrating a voltage waveform of each part at the time of LED lighting in the LED drive circuit shown in FIG. 8. FIG. 9 is a first diagram illustrating voltage waveforms of respective portions when the LED is repeatedly turned on and off in the LED drive circuit shown in FIG. 8. FIG. 9 is a second diagram illustrating voltage waveforms at various parts when the LED is repeatedly turned on and off in the LED drive circuit shown in FIG. 8. It is a figure which shows an example of a structure of the LED drive circuit which concerns on 3rd Embodiment. It is a figure which shows the other structural example of the LED drive circuit which concerns on 3rd Embodiment. It is a 1st figure which shows an example of the LED drive circuit which concerns on other embodiment. It is a 2nd figure which shows an example of the LED drive circuit which concerns on other embodiment. It is a 3rd figure which shows an example of the LED drive circuit which concerns on other embodiment. It is a 4th figure which shows an example of the LED drive circuit which concerns on other embodiment. It is a 5th figure which shows an example of the LED drive circuit which concerns on other embodiment. It is a 6th figure which shows an example of an LED drive circuit. It is a figure which shows the structural example of a constant current circuit. It is a figure which shows the other example of an LED drive circuit.

<First Embodiment>
FIG. 1 is a diagram showing an example of the configuration of an LED drive circuit according to the first embodiment of the present invention.
The LED driving circuit shown in FIG. 1 includes a switching power supply circuit 20, a switching control circuit 30, a feedback control circuit 40, and a constant current circuit 50.
The circuit block including the switching power supply circuit 20 and the switching control circuit 30 is an example of a voltage supply circuit in the present invention.
The constant current circuit 50 is an example of a constant current circuit in the present invention.
The feedback control circuit 40 is an example of a feedback control circuit in the present invention.

  The switching power supply circuit 20 is a boost converter that boosts the voltage VIN supplied from the DC power supply 10 and generates a drive voltage VOUT. The switching power supply circuit 20 varies the boost ratio according to the control of the switching control circuit 30. In the example of FIG. 1, the switching power supply circuit 20 includes an inductor L1, a transistor M1, a diode D1, and a capacitor C1.

  One terminal of the inductor L1 is connected to the output (voltage VIN) of the DC power supply 10, and the other terminal of the inductor L1 is connected to the ground potential via the transistor M1. The anode of the diode D1 is connected to the common connection node of the inductor L1 and the transistor M1, and the cathode of the diode D1 is connected to the output node of the drive voltage VOUT. A capacitor C1 is connected between the cathode of the diode D1 and the ground potential.

  The transistor M1 is, for example, an N-type MOS transistor, and is turned on or off according to a gate drive signal output from a switching control circuit 30 described later.

  When the transistor M1 is turned on, the DC voltage VIN is applied to the inductor L1, and magnetic energy is stored in the inductor L1. When the transistor M1 is turned off, the magnetic energy stored in the inductor L1 is released as a current, and this current flows to the capacitor C1 via the diode D1. As the transistor M1 is periodically turned on and off, the drive voltage VOUT boosted from the voltage VIN is generated in the capacitor C1.

  The switching control circuit 30 controls the switching operation of the switching power supply unit 20 so that the drive voltage VOUT approaches the target value. In the example of FIG. 1, the switching control circuit 30 includes a ramp waveform signal generator 31, a comparator 32, a reference voltage generator 33, an error amplifier 34, and resistors R1 and R2.

  The resistors R1 and R2 are connected in series between the output of the switching power supply circuit 20 and the ground potential. A feedback voltage VFB obtained by dividing the drive voltage VOUT is generated at a node FB to which the resistors R1 and R2 are commonly connected.

  The error amplifier 34 amplifies the difference between the reference voltage VREF generated in the reference voltage generator 33 and the feedback voltage VFB, and outputs it as an error signal. The error amplifier 34 decreases the output level of the error signal when the feedback voltage VFB increases, and increases the output level of the error signal when the feedback voltage VFB decreases.

  The ramp waveform signal generator 31 outputs a ramp waveform signal (sawtooth waveform signal) that repeats the ramp waveform at a constant period.

  The comparator 32 compares the ramp waveform signal output from the ramp waveform signal generator 31 with the error signal output from the error amplifier 34, and outputs a signal that becomes high level or low level according to the comparison result. That is, the comparator 32 outputs a signal that is at a high level when the error signal is higher than the ramp waveform signal and that is at a low level when the error signal is lower than the ramp waveform signal. The output signal of the comparator 32 is a PWM (pulse width modulation) signal synchronized with the ramp waveform signal, and the ratio (duty ratio) of the high level period to one cycle changes according to the error signal. That is, the lower the error signal, the smaller the duty ratio of the PWM signal.

  The PWM signal output from the comparator 32 is input to the gate of the transistor M1 of the switching power supply circuit 20. The transistor M1 is turned on when the PWM signal is at a high level, and the transistor M1 is turned off when the PWM signal is at a low level.

  When the feedback voltage VFB rises above the reference voltage VREF, the error signal output from the error amplifier 34 decreases, so the duty ratio of the PWM signal output from the comparator 32 decreases and the on-time of the transistor M1 decreases. . When the on-time of the transistor M1 is shortened, the energy stored and released in the inductor L1 for each cycle is decreased, so that the drive voltage VOUT output from the switching power supply circuit 20 is decreased. When the feedback voltage VFB falls below the reference voltage VREF, the drive voltage VOUT rises due to the reverse operation to that described above. Therefore, when the gain of the error amplifier 34 is sufficiently high, the drive voltage VOUT is controlled so that the feedback voltage VFB and the reference voltage VREF are equal.

  The drive voltage VOUT output from the switching power supply circuit 20 is supplied to the LED string S1. The LED string S1 is composed of a plurality of LEDs connected in series with the same polarity.

  The constant current circuit 50 is provided in the path of the drive current IL1 flowing through the LED string S1, and controls the drive current IL1 to a constant current value Iconst. In the example of FIG. 1, the constant current circuit 50 is inserted between the cathode side terminal SINK of the LED string S1 and the ground potential.

  The constant current circuit 50 includes a switch circuit that cuts off the path of the drive current IL1 in accordance with the dimming signal Sc output from the dimming control unit 60. This switch circuit may be realized by, for example, performing a cut-off operation of a transistor for constant current operation, or may be realized by a switch transistor provided independently of a transistor for constant current operation. .

  The dimming control unit 60 generates a dimming signal Sc that has been pulse-width modulated so that the LED string S1 is lit at a desired luminance.

  FIG. 2 is a diagram illustrating examples of waveforms of the dimming signal Sc and the drive current IL1. In the example of this figure, when the dimming signal Sc becomes high level, the driving current IL1 flows, and when the dimming signal Sc becomes low level, the driving current IL1 is cut off. As the duty ratio of the dimming signal Sc (the ratio of the high level period to one cycle) increases, the average value of the drive current IL1 increases, and the luminance of the LED string S1 increases. For example, when luminance data is input from a host device (not shown), the dimming control unit 60 sets the duty ratio of the dimming signal Sc according to the luminance data.

  The feedback control circuit 40 sets the drive voltage VOUT of the switching power supply circuit 20 so that the voltage drop VSINK of the constant current circuit 50 (or the voltage of the terminal SINK (the cathode voltage of the LED string S1)) generated at the terminal SINK approaches the reference voltage VP. Control. That is, the feedback control circuit 40 controls the switching operation (duty ratio) of the transistor M1 so that the drive voltage VOUT increases when the voltage drop VSINK is lower than the reference voltage VP, and the voltage drop VSINK is higher than the reference voltage VP. The switching operation of the transistor M1 is controlled so that the drive voltage VOUT decreases.

  Further, when the switch circuit of the constant current circuit 50 is turned off in response to the dimming signal Sc, the feedback control circuit 40 stops the above-described control of the drive voltage VOUT performed in accordance with the comparison between the voltage drop VSINK and the reference voltage VP. When the switch circuit of the constant current circuit 50 is turned on, the stopped control is resumed.

  As shown in FIG. 1, for example, the feedback control circuit 40 includes a voltage-current conversion amplifier 41, a reference voltage generator 42, a switch circuit SF1, and a resistor R3.

The voltage-current conversion amplifier 41 converts the difference between the voltage drop VSINK of the constant current circuit 50 and the reference voltage VP into a current IADJ and outputs it to the node FB. For example, when the voltage drop VSINK is lower than the reference voltage VP, the voltage-current conversion amplifier 41 increases the polarity (positive polarity) current IADJ flowing from the node FB to the ground potential, and when the voltage drop VSINK becomes higher than the reference voltage VP. Decrease the positive current IADJ (increase the negative current IADJ).

The resistor R3 and the switch circuit SF1 are inserted in series in the current path between the current output terminal of the voltage / current conversion amplifier 41 and the node FB.
The switch circuit SF1 is turned on when the drive current IL flows in response to the dimming signal Sc, and is turned off when the drive current IL1 is cut off.
The resistor R3 is a resistor for limiting the output current IADJ of the voltage / current conversion amplifier 41. Even when the output voltage of the voltage / current conversion amplifier 41 is saturated to the ground potential or the power supply voltage Vcc, the output current IADJ is reduced by the resistor R3. Limited to a predetermined range. By limiting the range of the output current IADJ, the adjustment range of the drive voltage VOUT can be limited.

  When the gain of the error amplifier 34 of the switching control circuit 30 is sufficiently large, the drive voltage VOUT can be generally expressed by the following equation.

In Expression (3), “R1” and “R2” indicate resistance values of the resistors R1 and R2.
When the gain of the voltage-current conversion amplifier 41 is sufficiently large, the output current IADJ is adjusted so that the voltage drop VSINK and the reference voltage VP are substantially equal.

  Here, the operation of the LED driving circuit shown in FIG. 1 having the above-described configuration will be described.

First, the operation for controlling the drive voltage VOUT so that the voltage drop VSINK becomes an appropriate level will be described with reference to FIGS.
FIG. 3 and FIG. 4 are diagrams illustrating voltage waveforms of respective parts when the LED is lit in the LED drive circuit shown in FIG. FIG. 3 shows a case where the voltage drop VSINK is lower than the reference voltage VP, and FIG. 4 shows a case where the voltage drop VSINK is higher than the reference voltage VP.

Referring to FIG. 3, at time t11, the drive voltage VOUT (FIG. 3B) is lower than an appropriate level, and the voltage drop VSINK (FIG. 3C) of the constant current circuit 50 decreases to near zero. is doing. In this case, since the constant current circuit 50 does not function as a constant current source, the drive current IL1 (FIG. 3D) also decreases to near zero. When the voltage drop VSINK becomes lower than the reference voltage VP, the feedback control circuit 40 increases the current IADJ (FIG. 3A) having a positive polarity (from the node FB to the ground potential). When the positive current IADJ increases, the voltage of the node FB decreases, so that the duty ratio of the PWM signal that drives the transistor M1 increases, and the drive voltage VOUT increases. As the drive voltage VOUT increases, the voltage drop VSINK and the drive current IL1 also increase. When the current IADJ increases by “ΔIa”, the drive voltage VOUT increases by “ΔIa × R1” from the relationship of the equation (3).
When the voltage drop VSINK reaches the reference voltage VP at time t12, the feedback control circuit 40 stops increasing the current IADJ and maintains the current IADJ so that the voltage drop VSINK becomes a value close to the reference voltage VP. At this time, the constant current circuit 50 functions as a constant current source, and keeps the drive current IL1 at a substantially constant current value Iconst.

On the other hand, referring to FIG. 4, at time t13, drive voltage VOUT (FIG. 4B) is higher than an appropriate level. In this case, the constant current circuit 50 holds the drive current IL1 (FIG. 4D) at a constant current value Iconst, but its power consumption is unnecessarily large. When the voltage drop VSINK (FIG. 4C) becomes higher than the reference voltage VP, the feedback control circuit 40 decreases the positive current IADJ (FIG. 3A) (increases the negative current IADJ). When the positive current IADJ decreases, the voltage of the node FB increases, so that the duty ratio of the PWM signal that drives the transistor M1 decreases, and the drive voltage VOUT decreases. As the drive voltage VOUT decreases, the voltage drop VSINK and the drive current IL1 decrease.
When the voltage drop VSINK decreases to near the reference voltage VP at time t14, the feedback control circuit 40 stops decreasing the current IADJ and maintains the current IADJ so that the voltage drop VSINK becomes a value close to the reference voltage VP.

  In this way, during the lighting period in which the drive current IL1 flows through the LED string S1, the drive voltage VOUT is controlled so that the voltage drop VSINK is close to the reference voltage VP.

Next, with reference to FIG.5 and FIG.6, operation | movement when LED repeats lighting and extinction is demonstrated.
FIG. 5 and FIG. 6 are diagrams illustrating voltage waveforms of each part when the LED is repeatedly turned on and off in the LED drive circuit shown in FIG. FIG. 5 shows a case where the voltage drop VSINK is lower than the reference voltage VP in the initial state of the LED lighting period, and FIG. 6 shows a case where the voltage drop VSINK is higher than the reference voltage VP in this initial state.

  When the switch circuit of the constant current circuit 50 is in the off state, the switch SF1 of the feedback control circuit 40 is in the off state, and the control for bringing the voltage drop VSINK closer to the reference voltage VP is stopped. At this time, since the current IADJ does not flow, the drive voltage VOUT approaches a constant voltage value “Vr = VREF × (R1 + R2) / R1” determined by the resistors R1 and R2 and the reference voltage VREF from the relationship of Expression (3). To be controlled.

  When the switch circuit of the constant current circuit 50 is switched from OFF to ON, the switch SF1 of the feedback control circuit 40 is also switched from OFF to ON. When the switch SF1 is turned on, feedback control for bringing the voltage drop VSINK closer to the reference voltage VP starts. In the initial state of the feedback control, the voltage value of the drive voltage VOUT is “Vr”.

In the example of FIG. 5, since the voltage value “Vr” in this initial state (time t21) is lower than an appropriate level, the voltage drop VSINK (FIG. 5C) decreases to near zero volts, and the drive current IL1 ( FIG. 5 (D)) hardly flows. When the feedback control of the feedback control circuit 40 is started and the current IADJ (FIG. 5A) increases, the drive voltage VOUT (FIG. 5B) rises accordingly, and the voltage drop VSINK rises. When the voltage drop VSINK rises to near the reference voltage VP (time t22), the constant current operation of the constant current circuit 50 functions normally, and the drive current IL1 is controlled to a constant current value Iconst.
Next, when the dimming signal Sc falls from the high level to the low level (time t23), the switch circuit of the constant current circuit 50 is turned off and the drive current IL1 is cut off. When the drive current IL1 is cut off, the voltage VSINK at the terminal SINK rises to near the drive voltage VOUT. However, since the switch SF1 is turned off at this time and the feedback control of the feedback control circuit 40 is stopped, the drive voltage VOUT approaches the voltage Vr without being affected by the increase in the voltage VSINK (time T24).

On the other hand, in the example of FIG. 6, since the voltage value “Vr” in the initial state (time t25) is higher than an appropriate level, the voltage drop VSINK (FIG. 6C) is higher than the reference voltage VP. In this case, since the constant current circuit 50 normally operates at a constant current from the initial state, the drive current IL1 (FIG. 6D) quickly rises to the constant value Iconst. When feedback control of the feedback control circuit 40 is started and the current IADJ (FIG. 6A) increases to a negative polarity, the drive voltage VOUT (FIG. 6B) decreases accordingly, and the voltage drop VSINK is the reference. The voltage decreases to near the voltage VP (time t26).
Next, when the dimming signal Sc falls from the high level to the low level (time t27), the switch circuit of the constant current circuit 50 is turned off, the drive current IL1 is cut off, and the voltage VSINK at the terminal SINK is changed to the drive voltage VOUT. It rises to near. However, since the feedback control of the feedback control circuit 40 is also stopped in this case, the drive voltage VOUT approaches the voltage Vr without being affected by the increase in the voltage VSINK (time t28).

As described above, according to the LED drive circuit of the present embodiment, the voltage drop VSINK is the reference voltage VP during the period in which the drive current IL1 flows through the LED string S1 when the switch circuit of the constant current circuit 50 is turned on. Feedback control is performed so as to approach. On the other hand, during the period when the switch circuit of the constant current circuit 50 is turned off and the drive current IL1 of the LED string S1 is cut off, the feedback control is stopped.
When the drive current IL1 of the LED string S1 is interrupted, the terminal SINK enters a floating state, and the voltage VSINK generated at the terminal SINK varies greatly. For this reason, when feedback control is activated in this state, the drive voltage VOUT greatly fluctuates. In the present embodiment, by stopping the feedback control by the feedback control circuit 40, the fluctuation of the drive voltage VOUT as described above can be suppressed. Therefore, in the initial state where the drive current IL1 begins to flow, the drive voltage VOUT is prevented from significantly deviating from an appropriate level, the brightness of the LED is drastically decreased, and the power consumption of the constant current circuit 50 is significantly increased. Can be suppressed.

  Further, since the switching control circuit 30 performs control so that the drive voltage VOUT approaches the constant value “Vr” during the cutoff period of the drive current IL1, the drive voltage VOUT is set to a relatively appropriate value in the initial state where the drive current IL1 starts to flow. Can be set to level.

  In the LED drive circuit shown in FIG. 1, a phase compensation element may be provided in a feedback loop by the feedback control circuit 40 in order to maintain the stability of the feedback system. FIG. 7 shows an example of an LED drive circuit to which a phase compensation element is added. A feedback control circuit 40A in the LED drive circuit shown in FIG. 7 is obtained by adding a resistor R4 and a capacitor C2 as phase compensation elements to the feedback control circuit 40 described above. A resistor R4 is inserted in series with the resistor R3, and a capacitor C4 is connected between the common connection node of the resistors R3 and R4 and the ground potential. By adding such a phase delay element, the loop gain in the high frequency region can be suppressed and the control system can be stabilized.

<Second Embodiment>
Next, a second embodiment of the present invention will be described.
FIG. 8 is a diagram illustrating an example of the configuration of the LED drive circuit according to the second embodiment. The LED drive circuit shown in FIG. 8 is obtained by replacing the feedback control circuit 40 in the LED drive circuit shown in FIG. 1 with a feedback control circuit 40B described below, and other components are the same as those of the LED drive circuit shown in FIG. It is.

In the feedback control circuit 40B, the voltage drop VSINK (or the voltage at the terminal SINK (the cathode voltage of the LED string S1)) of the constant current circuit 50 becomes the reference voltage (VL, VH) during the period when the drive current IL1 flows through the LED string S1. The current IADJ is varied so as to approach, and the drive voltage VOUT is controlled to an appropriate level.
That is, when the voltage drop VSINK is lower than the reference voltage VL, the feedback control circuit 40B increases the positive current IADJ so that the drive voltage VOUT increases, and the voltage drop VSINK is higher than the reference voltage VH (VH> VL). In this case, the positive current IADJ is decreased so as to decrease the drive voltage VOUT (the negative current IADJ is increased).

  On the other hand, the feedback control circuit 40B stops the above-described feedback control for bringing the voltage drop VSINK closer to the reference voltage VP and holds the current IADJ during the period in which the drive current IL1 of the LED string S1 is cut off. That is, the feedback control circuit 40B changes the current IADJ according to the difference between the voltage drop VSINK and the reference voltages VL and VH when the switch circuit of the constant current circuit 50 is in the ON state, and the switch circuit of the constant current circuit 50 The current IADJ is held when shifting from the on state to the off state, and the holding of the current IADJ is released when the switch circuit of the constant current circuit 50 shifts from the off state to the on state.

  Further, the feedback control circuit 40B delays the timing for releasing the holding of the current IADJ described above when starting to flow the drive current IL1 of the LED string S1. In other words, the feedback control circuit 40B holds the current IADJ after a predetermined delay time has elapsed after the switch circuit of the constant current circuit 50 shifts from the off state to the on state, so that the level of the voltage drop VSINK can be stabilized. Is released.

For example, as shown in FIG. 8, the feedback control circuit 40B includes comparators 43 and 45, reference voltage generators 44 and 46, a logic circuit 45, a digital-analog conversion circuit 47, a buffer circuit 48, and a resistor R3. Have.
Here, the comparators 43 and 45 are examples of the comparison circuit in the present invention.
The digital-analog conversion circuit 47 is an example of a digital-analog conversion circuit in the present invention.

The comparator 43 compares the reference voltage VH generated in the reference voltage generator 44 with the voltage drop VSINK of the constant current circuit 50, and outputs a signal HI as the comparison result. The comparator 43 outputs a high level signal HI when the voltage drop VSINK is higher than the reference voltage VH, and outputs a low level signal HI when the voltage drop VSINK is lower than the reference voltage VH.
The comparator 45 compares the reference voltage VL generated in the reference voltage generator 46 with the voltage drop VSINK of the constant current circuit 50, and outputs a signal LO as the comparison result. The comparator 45 outputs a high level signal LO when the voltage drop VSINK is lower than the reference voltage VL, and outputs a low level signal LO when the voltage drop VSINK is higher than the reference voltage VL.

The logic circuit 45 generates a digital signal whose value increases or decreases according to the output signals (HI, LO) of the comparators 43 and 45. As shown in FIG. 9, for example, the logic circuit 45 includes a gate circuit 451, a delay circuit UL1, latch circuits 452 and 453, and an up / down counter 454. Gate circuit 451 includes AND circuits U1 and U2.
Here, the gate circuit 451 is an example of a gate circuit in the present invention.
The delay circuit UL1 is an example of a delay circuit in the present invention.
The latch circuits 452 and 453 are examples of the signal holding circuit in the present invention.
The up / down counter 454 is an example of a counting circuit in the present invention.

  The delay circuit UL1 is a circuit that delays and outputs the dimming signal Sc, and delays the rise of the dimming signal Sc from a low level to a high level as shown in FIG. The dimming signal Sc_D (FIG. 10B) output from the delay circuit UL1 is delayed in rising compared to the original dimming signal (FIG. 10A), and the falling is almost the same timing. ing.

  The gate circuit 451 receives the output signals (HI, LO) of the comparators 43 and 45 and the dimming signal Sc_D output from the delay circuit UL1, and when the dimming signal Sc_D is at the high level (of the constant current circuit 50). When the switch circuit is on), the output signals HI and LO are output as they are. On the other hand, when the dimming signal Sc_D is at the low level (when the switch circuit of the constant current circuit 50 is off), the gate circuit 451 sets both the output signals HI and LO to the low level and outputs them.

  In the gate circuit 451, the AND circuit U1 outputs a logical product of the output signal HI and the dimming signal Sc_D to the latch circuit 452, and the AND circuit U2 outputs a logical product of the output signal LO and the dimming signal Sc_D to the latch circuit 453. To do.

  The latch circuit 452 latches the signal output from the AND circuit U1 of the gate circuit 451 in synchronization with the clock signal CLK. The latch circuit 453 latches the signal output from the AND circuit U2 of the gate circuit 451 in synchronization with the clock signal CLK.

The up / down counter 454 increases or decreases the count value DAT according to the signals HI_L and LO_L held in the latch circuits 452 and 453.
FIG. 11 is a diagram for explaining the counting operation of the up / down counter 454. When the signal HI_L (FIG. 11B) held in the latch circuit 452 at the rising edge of the clock signal (FIG. 11A) is at a high level, the up / down counter 454 counts the count value DAT (FIG. 11D). ) Is incremented by one. When the signal LO_L (FIG. 11C) held in the latch circuit 453 at the rising edge of the clock signal is at a high level, the up / down counter 454 decrements the count value DAT by one. On the other hand, when the signals HI_L and LO_L are both low at the rising edge of the clock signal, the up / down counter 454 holds the count value DAT.

The digital-analog conversion circuit 47 converts the count value DAT of the up / down counter 454 into an analog signal.
The buffer circuit 48 inputs the analog signal of the digital-analog conversion circuit 47 with a high impedance, and outputs a voltage VADJ substantially equal to the analog signal with a low impedance. The output of the buffer circuit 48 is connected to the node FB via the resistor R3.

FIG. 12 is a diagram showing the relationship between the analog signal (voltage VADJ) of the digital-analog conversion circuit 47 and the count value DAT of the up / down counter 454.
As shown in FIG. 12, the voltage VADJ changes almost in proportion to the count value DAT. When the count value DAT is minimum (Dmin), the voltage VADJ is minimum (Vmin), and when the count value DAT is maximum (Dmax), the voltage VADJ is maximum (Vmax).
In the initial state of the up / down counter 454, the count value DAT is set to be “Di”. When the count value DAT is “Di”, the digital-analog conversion circuit 47 outputs a voltage VADJ that is substantially equal to the reference voltage VREF.

  When the gain of the error amplifier 34 of the switching control circuit 30 is sufficiently large, the drive voltage VOUT can be generally expressed by the following equation.

  In Expression (4), “R1” to “R3” indicate resistance values of the resistors R1 to R3, respectively.

  Here, the operation of the LED drive circuit shown in FIG. 8 having the above-described configuration will be described.

First, the operation for controlling the drive voltage VOUT so that the voltage drop VSINK becomes an appropriate level will be described with reference to FIGS.
FIG. 13 and FIG. 14 are diagrams illustrating voltage waveforms of respective parts when the LED is lit in the LED drive circuit shown in FIG. FIG. 13 shows a case where the voltage drop VSINK is lower than the reference voltage VL, and FIG. 14 shows a case where the voltage drop VSINK is higher than the reference voltage VH.

Referring to FIG. 13, at time t31, the drive voltage VOUT (FIG. 13B) is lower than an appropriate level, and the voltage drop VSINK (FIG. 13C) of the constant current circuit 50 decreases to near zero. is doing. In this case, since the constant current circuit 50 does not function as a constant current source, the drive current IL1 (FIG. 13D) also decreases to near zero. When the voltage drop VSINK is lower than the reference voltage VL, the comparator 43 outputs a low level signal HI and the comparator 45 outputs a high level signal LO. When the LED is turned on, that is, when the dimming signal Sc is at a high level, the signals HI and LO are input to the latch circuits 452 and 453 through the AND circuits U1 and U2 with the same value. The signals HI and LO input to the latch circuits 452 and 453 are latched by the latch circuits 452 and 453 in synchronization with the clock signal CLK, and input to the up / down counter 454 as the signals HI_L and LO_L. Since the signal HI_L is at a low level and the signal LO_L is at a high level, the up / down counter 454 decrements the count value DAT by 1 every time the clock signal CLK rises. When the count value DAT decreases by one, the voltage VADJ decreases step by step by one voltage step. When the voltage VADJ decreases stepwise, the drive voltage VOUT increases stepwise from the relationship of Equation (4). When the voltage VADJ decreases by “ΔVa”, the drive voltage VOUT increases by “(R1 / R3) × ΔVa”.
When the drive voltage VOUT rises, the voltage drop VSINK rises, and when the reference voltage VL is reached at time t32, the output signals HI and LO of the comparators 43 and 45 both become low levels, and the signals HI_L and LO_L accordingly. Both go low. As a result, the up / down counter 454 holds the count value DAT, and the drive voltage VOUT stops increasing. The voltage drop VSINK is maintained in a range higher than the reference voltage VL and lower than the reference voltage VH. At this time, the constant current circuit 50 functions as a constant current source, and keeps the drive current IL1 at a substantially constant current value Iconst.

On the other hand, referring to FIG. 14, at time t33, the drive voltage VOUT (FIG. 14B) is higher than an appropriate level. In this case, although the constant current circuit 50 holds the drive current IL1 (FIG. 14D) at a constant current value Iconst, the power consumption is unnecessarily large. When the voltage drop VSINK is higher than the reference voltage VH, the comparator 43 outputs a high level signal HI, and the comparator 45 outputs a low level signal LO. At this time, since the dimming signal Sc is at the high level, the signals HI and LO are input to the latch circuits 452 and 453 through the AND circuits U1 and U2 with the same value. The signals HI and LO input to the latch circuits 452 and 453 are latched by the latch circuits 452 and 453 in synchronization with the clock signal CLK, and input to the up / down counter 454 as the signals HI_L and LO_L. Since the signal HI_L is at the high level and the signal LO_L is at the low level, the up / down counter 454 increments the count value DAT by 1 every time the clock signal CLK rises. When the count value DAT increases by 1, the voltage VADJ rises step by step by 1-bit voltage step. When the voltage VADJ increases stepwise, the drive voltage VOUT decreases stepwise from the relationship of the equation (4). When the voltage VADJ increases by “ΔVa”, the drive voltage VOUT decreases by “(R1 / R3) × ΔVa”.
When the voltage drop VSINK decreases in accordance with the decrease in the drive voltage VOUT and decreases to the reference voltage VH at time t34, the output signals HI and LO of the comparators 43 and 45 both become low levels, and accordingly the signals HI_L and LO_L Both go low. As a result, the up / down counter 454 holds the count value DAT, so that the decrease of the drive voltage VOUT is stopped. The voltage drop VSINK is maintained in a range lower than the reference voltage VH and higher than the reference voltage VL.

  As described above, also in the LED drive circuit according to the present embodiment, the drive voltage VOUT is controlled so that the voltage drop VSINK is close to the reference voltage VP during the LED lighting period.

Next, with reference to FIG.15 and FIG.16, operation | movement when LED repeats lighting and extinction is demonstrated.
FIG. 15 and FIG. 16 are diagrams illustrating voltage waveforms of each part when the LED is repeatedly turned on and off in the LED drive circuit shown in FIG. 8, and the LED is driven from the state where the count value DAT is the initial value (Di). The case of starting is shown. FIG. 15 shows a case where the voltage drop VSINK is lower than the reference voltage VL in the initial state, and FIG. 16 shows a case where the voltage drop VSINK is higher than the reference voltage VH in the initial state.

  When the count value DAT is the initial value (Di), the voltage VADJ output from the digital-analog conversion circuit 47 becomes equal to the reference voltage VREF. In this case, the drive voltage VOUT is substantially equal to a constant voltage value “Vr = VREF × (R1 + R2) / R1” from the relationship of the expression (4).

  In the example of FIG. 15, since the voltage value “Vr” is lower than an appropriate level, when the switch circuit of the constant current circuit 50 is turned on at time t41, the voltage drop VSINK (FIG. 15C) decreases to near zero volts. . Since the voltage drop VSINK is low and the constant current circuit 50 does not function as a constant current source, the drive current IL1 (FIG. 15D) hardly flows. From this initial state, when the feedback control of the feedback control circuit 40B is started and the voltage VADJ (FIG. 15A) decreases, the drive voltage VOUT (FIG. 15B) increases accordingly, and the voltage drop VSINK Rises. When the voltage drop VSINK rises to near the reference voltage VL (time t42), the constant current operation of the constant current circuit 50 functions normally, and the drive current IL1 is controlled to a constant current value Iconst.

  Next, when the dimming signal Sc falls from the high level to the low level (time t43), the switch circuit of the constant current circuit 50 is turned off and the drive current IL1 is cut off. When the drive current IL1 is cut off, the voltage VSINK at the terminal SINK rises to near the drive voltage VOUT. On the other hand, when the dimming signal Sc becomes low level, the outputs of the AND circuits U1 and U2 both become low level, so that the signals HI_L and LO_L latched by the latch circuits 452 and 453 both become low level. As a result, the count in the up / down counter 454 is stopped and the count value DAT is held. Since the count value DAT is held, the drive voltage VOUT is also held substantially constant.

  Thereafter, when the dimming signal Sc rises again from the low level to the high level (time t44), the switch circuit of the constant current circuit 50 is turned on and the drive current IL1 flows. When the drive current IL1 flows, the voltage VSINK of the terminal SINK that has risen to the vicinity of the drive voltage VOUT rapidly decreases. On the other hand, when the dimming signal Sc rises from a low level to a high level, the rising signal is delayed by a predetermined delay time by the delay circuit DL1 and then transmitted to the AND circuits U1 and U2. That is, during the period in which the voltage VSINK changes abruptly immediately after the start of driving, the driving voltage VOUT is not controlled and the count value DAT is held. After the delay time of the delay circuit DL1 has elapsed, the signals HI_L and LO_L corresponding to the output signals HI and LO of the comparators 43 and 45 are latched by the latch circuits 452 and 453 and input to the up / down counter 454.

Incidentally, at this time, the count value DAT of the up / down counter 454 is carried over as it is at the last point of the previous LED drive period. The count value DAT is an appropriate value for setting the voltage drop VSINK to be substantially within the target range (VL <VSINK <VH) at the final point of the LED driving period. Since the LED characteristics hardly vary greatly during a slight drive stop period, the count value DAT carried over from the previous LED drive period sets the voltage drop VSINK within an appropriate target range even in the next LED drive period. It is a possible value.
Therefore, immediately after the switch circuit of the constant current circuit 50 is turned on, the LED is driven with an appropriate voltage drop VSINK.

  On the other hand, in the example of FIG. 16, since the voltage value “Vr” is higher than an appropriate level, when the switch circuit of the constant current circuit 50 is turned on at time t45, the constant current circuit 50 immediately starts driving current IL1 (FIG. 16 ( D) is controlled to a constant value Iconst From this initial state, when the feedback control of the feedback control circuit 40B is started and the voltage VADJ (FIG. 16A) rises, the drive voltage VOUT (FIG. B)) decreases, and the voltage drop VSINK decreases to the reference voltage VH (time t46).

  Next, when the dimming signal Sc falls from the high level to the low level (time t47), the switch circuit of the constant current circuit 50 is turned off to cut off the drive current IL1, and the voltage VSINK at the terminal SINK is changed to the drive voltage VOUT. It rises to near. At this time, the count in the up / down counter 454 is stopped and the count value DAT is held. Since the count value DAT is held, the drive voltage VOUT is also held substantially constant.

After that, when the dimming signal Sc rises again from the low level to the high level (time t48), the switch circuit of the constant current circuit 50 is turned on, the drive current IL1 flows, and the voltage VSINK at the terminal SINK rapidly decreases. . After the delay time by the delay circuit DL1 has elapsed from the start of driving, the counting operation of the up / down counter 454 is resumed.
At this time, the count value DAT of the up / down counter 454 is an appropriate value for setting the voltage drop VSINK within the target range (VL <VSINK <VH). Therefore, immediately after the switch circuit of the constant current circuit 50 is turned on, the LED is driven with an appropriate voltage drop VSINK.

As described above, according to the LED drive circuit according to the present embodiment, when the switch circuit of the constant current circuit 50 is turned on and the drive current IL1 flows through the LED string S1, the constant current circuit 50 generates. The voltage VADJ changes according to the difference between the voltage drop VSINK and the reference voltages VH and VL, and the drive voltage VOUT is feedback-controlled so that the voltage drop VSINK falls within an appropriate range (HL <VSINK <VH). On the other hand, when the switch circuit of the constant current circuit 50 shifts from the on state to the off state, the voltage VADJ is held and the feedback control is temporarily stopped. When the switch circuit of the constant current circuit 10 shifts from the off state to the on state again and the drive current IL1 begins to flow through the LED string S1, the holding of the voltage VADJ is released and the feedback control is resumed.
As described above, since the voltage VADJ generated for the feedback control of the feedback control circuit 40B is held in the period during which the drive current IL1 is cut off, when the feedback control is resumed, the voltage drop VSINK is appropriate from the beginning. The drive current IL1 can be passed. Accordingly, it is possible to effectively prevent problems such as the voltage drop VSINK being too low at the start of LED driving and the LED brightness being lowered, or the voltage drop VSINK being too high and causing unnecessary power loss.

  In addition, according to the LED drive circuit according to the present embodiment, the level of the voltage drop VSINK can be stabilized after the switch circuit of the constant current circuit 50 shifts from the on state to the off state and the drive current IL1 starts to flow. After the delay time elapses, the feedback control of the feedback control circuit 40B is resumed. Thereby, feedback control is started in a state where the level of the voltage drop VSINK is unstable, and it is possible to effectively prevent a decrease in luminance of the LED and an increase in power consumption due to the drive voltage VOUT deviating from an appropriate level.

  In addition, according to the LED drive circuit according to the present embodiment, the feedback control system by the feedback control circuit 40B is a discrete system including a digital circuit, so it is necessary to provide a large-capacity capacitor and resistor for phase compensation. The circuit scale can be reduced. In addition, since most of the feedback control circuit 40B can be configured by a digital circuit, the number of circuit elements that need to be adjusted is reduced (for example, only the resistor R3 in the example of FIG. 8), and the manufacturing process can be simplified.

<Third Embodiment>
Next, a third embodiment of the present invention will be described.
FIG. 17 is a diagram illustrating an example of the configuration of the LED drive circuit according to the third embodiment. The LED drive circuit shown in FIG. 17 includes a switching power supply circuit 20 and a switching control circuit 30 similarly to the LED drive circuits shown in FIGS. 1 and 8, and LED strings S1 to Sm and constant current circuits 50-1 to 50. -M and feedback control circuits 40-1 to 40-m.

  The LED strings S1 to Sm are each composed of a plurality of LEDs connected in series, and a common drive voltage VOUT is supplied to the anode side.

  The constant current circuit 50-k (k is an integer from 1 to m) is a circuit similar to the constant current circuit 50 in FIG. 1, for example, and controls the drive current ILk of the LED string Sk to a constant value. The constant current circuit 50-k is provided in the path of the drive current ILk of the LED string Sk, and is connected, for example, between the cathode side terminal of the constant current circuit 50-k and the ground potential. The constant current circuit 50-k includes a switch circuit that cuts off the drive current ILk in accordance with the dimming signal Sck supplied from the dimming control unit 60A.

  The feedback control circuit 40-k is, for example, a circuit similar to the feedback control circuits 40 and 40A in FIGS. 1 and 7 and the feedback control circuit 40B in FIG. 8, and the voltage drop of the constant current circuit 50-k (the voltage at the terminal SINKk). A feedback control signal (current IADJ, voltage VADJ) corresponding to (cathode voltage of LED string Sk) is output. As shown in FIG. 17, since the outputs of the feedback control circuits 40-1 to 40-m are directly connected to the node FB, the combined result of m feedback control signals is input to the node FB.

  As shown in FIG. 17, when a common drive voltage VOUT is supplied to a plurality of LED strings each connected to a constant current circuit, a feedback control circuit is provided for each LED string and the outputs of the feedback control circuits are combined. The drive voltage VOUT of the switching power supply circuit 20 is controlled by the signal. Thereby, the voltage drop of each constant current circuit can be adjusted to an appropriate range.

  In another modification of the LED drive circuit according to the present embodiment, output signals of a plurality of feedback control circuits may be combined with diodes (Ds-1 to Ds-m) as shown in FIG.

The LED drive circuit shown in FIG. 18 is obtained by providing a diode Ds-k between the feedback control circuit 40-k and the node FB in the LED drive circuit shown in FIG. The anode of the diode Ds-k is connected to the node FB, and the cathode of the diode Ds-k is connected to the output of the feedback control circuit 40-k.
As shown in FIG. 18, when the feedback control signals of the feedback control circuits 40-1 to 40-m are synthesized at the node FB via the diodes Ds-1 to Ds-m, the feedback control signal having the lowest output voltage has priority. Is input to the node FB. The feedback control signal with the lowest voltage is a signal that raises the drive voltage VOUT, and that the drive voltage VOUT is controlled according to this feedback control signal means that m constant current circuits (50-1 to 50-50). -M) means that the drive voltage VOUT is feedback-controlled in accordance with the lowest voltage drop VSINK.
Therefore, according to the LED driving circuit shown in FIG. 18, the driving voltage is set such that the lowest voltage drop VSINK among the m constant current circuits (50-1 to 50-m) is higher than the reference voltage (VP, VL). VOUT is controlled. Thereby, since each constant current circuit (50-1 to 50-m) can be normally operated as a constant current source, each LED string S1 to Sm can normally emit light.

  As mentioned above, although several embodiment of this invention was described, this invention is not limited only to embodiment mentioned above, Various modifications are included.

  For example, in the LED drive circuit shown in FIG. 8, the comparators 43 and 45 are provided to compare the voltage drop VSINK with the reference voltages VL and VH, but the present invention is not limited to this. In another embodiment of the present invention, the voltage drop VSINK may be sampled by the clock signal CLK and converted into a digital signal, and the digital signal may be compared with the reference data equivalent to the above comparison operation.

FIG. 19 is a diagram illustrating an example of such an LED drive circuit. The LED drive circuit shown in FIG. 19 is obtained by replacing the feedback control circuit 40B in the LED drive circuit shown in FIG. 8 with a feedback control circuit 40C described below.
The feedback control circuit 40C includes an analog-digital conversion circuit 49, a logic circuit 45A, a digital-analog conversion circuit 47, a buffer circuit 48, and a resistor R3. The analog-digital conversion circuit 49 converts the voltage drop VSINK into digital data DSINK and inputs it to the logic circuit 45A. The logic circuit 45A compares the digital data DSINK with reference data equivalent to the reference voltages VH and VL, processes the comparison result by a circuit similar to the logic circuit 45, and generates a count value DAT. The processing of the count value DAT is the same as that of the feedback control circuit 40B.
Thus, in the present invention, most of the processing can be performed by a digital circuit.

  In other embodiments of the present invention, in contrast to the above, most of the processing can be performed by an analog circuit. For example, a charge pump circuit may be provided instead of the up / down counter 454. Instead of counting up / down the up / down counter 454, the charge pump circuit charges / discharges the capacitor, thereby generating a feedback control signal (current IADJ, voltage VADJ) based on the charge accumulated in the capacitor. Is possible.

  In another embodiment of the present invention, for example, as shown in FIG. 20, the output voltage of the digital-analog conversion circuit 47 may be amplified in the buffer circuit 48. In the LED drive circuit shown in FIG. 20, resistors R5 and R6 are connected in series between the output terminal of the buffer circuit 48 and the ground potential, and the common connection node is connected to the negative input terminal of the buffer circuit 48. . As a result, the output voltage of the digital-analog conversion circuit 47 is amplified by (1 + R5 / R6) times and output as the voltage VADJ.

  In another embodiment of the present invention, as shown in FIG. 21, for example, a current output type digital-analog conversion circuit 49 may be provided in place of the voltage output type digital-analog conversion circuit 47. The digital-analog conversion circuit 49 includes a plurality of current sources, and converts the count value DAT into an analog current by synthesizing the output current of the current source according to the count value DAT. The output current of the digital-analog conversion circuit 49 can be directly output to the node FB without going through the buffer circuit.

In another embodiment of the present invention, the power supply circuit that generates the drive voltage VOUT can be replaced with any power supply other than the step-up switching power supply.
For example, in the LED drive circuit shown in FIG. 22, the drive voltage VOUT is generated by the step-down switching power supply circuit 20A, and in the LED drive circuit shown in FIG. 23, the drive voltage VOUT is generated by the step-up / step-down switching power supply circuit 20B.
The drive voltage VOUT may be generated not only by the switching power supply circuit but by, for example, a linear regulator circuit.

  DESCRIPTION OF SYMBOLS 10 ... DC power supply, 20 ... Switching power supply circuit, 30 ... Switching control circuit, 31 ... Ramp waveform signal generator, 32, 43, 45 ... Comparator, 33, 42, 44, 46 ... Reference voltage generator, 34 ... Error amplifier , 40, 40A to 40E, 40-1 to 40-m ... feedback control circuit, 41 ... voltage-current conversion amplifier, 45, 45A ... logic circuit, 47 ... digital-analog conversion circuit, 48 ... buffer circuit, 49 ... analog- Digital conversion circuit, 50, 50-1 to 50-m ... constant current circuit, S1 to Sm ... LED string, R1 to R6 ... resistor, M1 to M3 ... transistor, SF1 ... switch circuit, L1, L2 ... inductor, D1, D2, Ds-1 to Ds-m ... diode, C1 to C4 ... capacitor

Claims (10)

A light emitting diode driving circuit for driving a light emitting diode circuit including one or a plurality of light emitting diodes connected in series,
A voltage supply circuit for supplying a driving voltage to the light emitting diode circuit;
A constant current circuit that is provided in a path of a drive current flowing through the light emitting diode circuit and controls the drive current to be constant;
When the voltage drop generated in the constant current circuit or the cathode voltage of the light emitting diode circuit is lower than the first threshold voltage, the voltage supply circuit is controlled so that the drive voltage increases, and the voltage drop or the cathode voltage is controlled. Is higher than the first threshold voltage, or when the voltage drop or the cathode voltage exceeds a second threshold voltage higher than the first threshold voltage, the drive voltage is reduced. A feedback control circuit for controlling the voltage supply circuit;
Have
The constant current circuit includes a switch circuit that cuts off a path of the drive current in accordance with an input dimming signal,
The feedback control circuit stops the control of the driving voltage according to the voltage drop or the cathode voltage of the constant current circuit when the switch circuit is turned off, and resumes the stopped control when the switch circuit is turned on. To
Light emitting ion drive circuit. The feedback control circuit is responsive to a difference between the voltage drop generated in the constant current circuit or the cathode voltage and the first threshold voltage or the second threshold voltage when the switch circuit is in an ON state. Generating a feedback control signal that changes, holds the feedback control signal when the switch circuit shifts from the on state to the off state, and holds the feedback control signal when the switch circuit shifts from the off state to the on state. Release the above,
The voltage supply circuit adjusts the drive voltage in response to the feedback control signal;
The light emitting diode drive circuit according to claim 1. The feedback control circuit holds the feedback control signal after a lapse of a predetermined delay time after which the voltage drop or the cathode voltage level can be stabilized after the switch circuit shifts from an off state to an on state. ,
The light emitting diode drive circuit according to claim 2. The feedback control circuit is
A comparison circuit for comparing the voltage drop generated in the constant current circuit or the cathode voltage of the light emitting diode circuit with the first threshold voltage and the second threshold voltage;
A signal holding circuit for holding the output signal of the comparison circuit at a constant period;
Based on the output signal of the comparison circuit held in the signal holding circuit, when the voltage drop or the cathode voltage is lower than the first threshold voltage, the feedback control signal is set so that the drive voltage rises. When the voltage drop or the cathode voltage is higher than the second threshold voltage, the feedback control signal is changed stepwise or continuously so that the drive voltage decreases. When the voltage drop or the cathode voltage is higher than the first threshold voltage and lower than the second threshold voltage, a feedback control signal generation circuit that holds the feedback control signal;
including,
The light emitting diode drive circuit according to claim 2. When the output signal of the comparison circuit and the dimming signal are input, and the dimming signal indicates the ON state of the switch circuit, the output signal of the comparison circuit is output, and the dimming signal is output from the switch circuit. When indicating an off state, it has a gate circuit that outputs a predetermined signal indicating the off state,
The signal holding circuit holds the signal output from the gate circuit at the fixed period,
The feedback control signal generation circuit changes or holds the feedback control signal according to the output signal of the comparison circuit held in the signal holding circuit, and the predetermined signal is held in the signal holding circuit. If you hold the feedback control signal above,
The light emitting diode drive circuit according to claim 4. The feedback control signal generation circuit includes:
Depending on the output signal of the comparison circuit held in the signal holding circuit, the count value is increased / decreased or held at a constant period, and when the predetermined signal is held in the signal holding circuit, the count value is set. A counting circuit to hold;
A digital-analog conversion circuit for generating an analog signal corresponding to the count value of the counting circuit;
including,
The light emitting diode drive circuit according to claim 5. A delay circuit that delays the dimming signal by a predetermined delay time and then inputs it to the gate circuit when the dimming signal indicating that the switch circuit shifts from an off state to an on state is input;
The light emitting diode drive circuit according to claim 6. A plurality of constant current circuits that control each of the driving currents flowing through the plurality of light emitting diode circuits supplied with the common driving voltage from the voltage supply circuit;
The plurality of feedback controls that respectively generate the feedback control signals in accordance with the voltage drop generated in the plurality of constant current circuits or the difference between the cathode voltage and the first threshold voltage or the second threshold voltage. Circuit,
Have
The voltage supply circuit adjusts the drive voltage according to a signal obtained by combining the plurality of feedback control signals generated in the plurality of feedback control circuits;
The light emitting diode drive circuit according to any one of claims 2 to 7. Among the plurality of feedback control signals generated in the plurality of feedback control circuits, a signal synthesis unit that outputs a feedback control signal that raises the drive voltage most,
The voltage supply circuit adjusts the drive voltage according to a feedback control signal output from the signal synthesis unit;
The light emitting diode drive circuit according to claim 8. A voltage dividing circuit for supplying a divided voltage obtained by dividing the drive voltage to a first node; an error amplifying circuit for generating an error signal by comparing the divided voltage with a reference voltage; A comparator that compares the error signal with the ramp signal to generate a switching signal; and a switching transistor that operates on and off in response to the switching signal.
The feedback control circuit generates a feedback control signal for controlling the drive voltage according to the voltage drop or the cathode voltage, and the feedback control signal is supplied to the first node via a resistor;
The light emitting diode drive circuit according to claim 1.