JP4653782B2 - LED lighting control device - Google Patents

Description

  The present invention relates to a lighting control device capable of stably lighting an LED even when the LED is turned on / off for dimming.

  Since the advent of LEDs that can emit white light, the number of cases where LEDs are used in lighting devices has increased. When an LED is used in a lighting device, there is a limit to the amount of light obtained from one LED, so that a necessary amount of light is obtained by using a large number of LEDs. When the LED is turned on by passing a current, a forward drop voltage is generated between the terminals of the LED, which is larger than that of a normal diode. For this reason, when too many LEDs are connected in series, the voltage to be supplied between the start end and the end of the LED array becomes high. Therefore, normally, a plurality of LED rows with a reduced number of series are connected in parallel to ensure the necessary number of LEDs while keeping the supply voltage low.

  Since the amount of light emitted from the LEDs depends on the magnitude of the current, it is necessary to control the currents flowing through the LED rows to be substantially the same value in order to light the plurality of LED rows uniformly. However, when a voltage of the same magnitude is simply supplied to each LED string connected in parallel, there is a large variation in the forward drop voltage from element to element, resulting in a phenomenon in which the current magnitude varies depending on the LED string. . For this reason, in recent devices for lighting the LED, in order to make the currents of the respective LED strings uniform, independent constant current circuits are connected in series to the respective LED strings. And the structure which supplies the stable voltage from the DC-DC converter circuit with respect to the serial body (plural) of the LED row | line | column and a constant current circuit is taken. (For example, see Patent Document 1 and Patent Document 2)

FIG. 2 shows an example of the configuration of a recent LED lighting control device, which has the following configuration.
In FIG. 2, reference numeral 1 denotes an external supply of an unstable power supply voltage via the high potential side input terminal 2a and the low potential side input terminal 2b, and the output terminal 3 (high potential side) A DC-DC converter circuit configured to supply a stable voltage to a connected load circuit, and 4 is a control circuit for feedback-controlling the operation of the DC-DC converter circuit 1. The low-potential side output terminal of the DC-DC converter circuit 1 is connected to the ground as a reference potential point together with the low-potential side input terminal 1b, and the ground is handled as an actual low-potential side output terminal. Is omitted.

  The output terminal 3 of the DC-DC converter circuit 1 is connected to one end of the output voltage smoothing capacitor C1, and the other end of the capacitor C1 is connected to the ground. The output terminal 3 of the DC-DC converter circuit 1 is further connected to one end (starting end) of a first LED row LA1, a second LED row LA2, and a third LED row LA3 in which a plurality of LEDs are connected in series. Yes. The other ends (terminations) of the LED rows LA1, LA2, LA3 are connected to the ground via constant current circuits CS1, CS2, CS3, respectively.

  The connection point between the first LED array LA1 and the constant current circuit CS1 is connected to the first series detection terminal ST1 of the control circuit 4. The connection point between the second LED array LA2 and the constant current circuit CS2 is connected to the second series detection terminal ST2, and the connection point between the third LED array LA3 and the constant current circuit CS3 is connected to the third series detection terminal ST3. Has been. The drive signal output terminal DRV of the control circuit 4 is connected so as to supply a signal generated there to the DC-DC converter circuit 1.

  Here, the control circuit 4 is configured by diodes D1, D2, and D3 having anodes connected in common, and a resistor R1 connected between the common connection point of the anodes and the power supply line (VDD). A detection circuit 5 is provided. The cathode of the diode D1 of the detection circuit 5 is connected to the first series detection terminal ST1, similarly, the cathode of the diode D2 is connected to the second series detection terminal ST2, and the cathode of the diode D3 is connected to the third series detection terminal ST3. Each is connected. The common connection point of the anode is connected to the inverting input terminal (−) of the error amplifier AMP as the detection point P1 of the detection circuit 5.

  The non-inverting input terminal (+) of the error amplifier AMP is connected to the reference voltage source VR so as to receive the supply of the reference value signal, and the output terminal of the error amplifier AMP is connected to the input terminal of the PWM comparator not shown. Yes. Here, since it is assumed that the DC-DC converter circuit 1 is a general PWM control switching regulator, the control circuit 4 for driving and controlling the DC-DC converter circuit 1 is not shown. However, it is assumed that an oscillation circuit, a PWM comparator, and the like that a control circuit of a PWM control system should normally have are configured therein. A signal processing circuit 6 is configured by the error amplifier AMP, a PWM comparator (not shown), an oscillation circuit, and the like, and the drive signal generated here is supplied to the DC-DC converter circuit 1 via the drive signal output terminal DRV. Shall.

In the circuit of FIG. 2, the voltages at the start ends of the LED rows LA1 to LA3 are the same. However, since the forward voltage drop generated between the start and end of each LED row LA1 to LA3 is different, the voltages appearing at the first to third series detection terminals ST1 to ST3 are not the same.
Here, it is assumed that the forward voltage drop of the third diode array LA3 is the largest. In this case, the voltage appearing at the position of the third series detection terminal ST3 is the lowest. Then, the voltage at the position of the common connection point of the anodes of the diodes D1 to D3 in the detection circuit 5 becomes substantially the same value as the voltage of the third series detection terminal ST3 having the lowest voltage value.

  The common connection point of the anode is connected to the inverting input terminal (−) of the error amplifier AMP as the detection point P1 of the detection circuit 5. On the other hand, the non-inverting input terminal (+) of the error amplifier AMP is connected to the reference voltage source VR so as to receive the supply of the reference value signal. Therefore, the error amplifier AMP generates an error signal corresponding to the difference between the voltage value of the third series detection terminal ST3 and the voltage value of the reference value signal, and supplies the error signal to a PWM comparator not shown. Thereafter, in the control circuit 4, a drive signal is generated by a general pulse width modulation operation and supplied to the DC-DC converter circuit 1 from the drive signal output terminal DRV. As a result, an output voltage that enables lighting of the third LED row LA3 having the largest voltage drop amount is supplied from the DC-DC converter circuit 1 to each of the LED rows LA1 to LA3.

  Here, what is actually controlled by the control circuit 4 is the voltage value of the third series detection terminal ST3 having the lowest voltage value. The voltage value of the third series detection terminal ST3 is equal to the inter-terminal voltage of the constant current circuit CS3 connected in series to the third LED row LA3. In general, a constant current circuit requires a voltage between terminals that is equal to or greater than a predetermined value when controlling the current. Normally, when the voltage between the terminals is lower than that, the operation of the constant current circuit becomes unstable, and the current cannot be controlled to be constant at a specified value. Therefore, the DC-DC converter circuit is set by setting the reference value signal supplied from the reference voltage source VR to the error amplifier AMP to a value higher than the minimum value of the inter-terminal voltage that enables stable operation of the constant current circuit CS3. A voltage that enables a stable operation is supplied from 1 to the constant current circuit CS3.

  In terms of circuit configuration, the voltage appearing at the position of the third series detection terminal ST3 is equal to the voltage between the terminals of the constant current circuit CS3, and is the lowest among the three series detection terminals. In other words, the voltage between the terminals of the constant current circuits CS1 and CS2 is higher than the voltage between the terminals of the constant current circuit CS3. For this reason, if the constant current circuit CS3 connected to the LED string having the largest voltage drop (ie, LA3) is in a state where stable operation is possible, all other constant current circuits are also capable of stable operation. It will be that.

Note that the higher the inter-terminal voltage of the constant current circuit, the greater the power loss generated in the constant current circuit. Therefore, the value of the reference value signal supplied from the reference voltage source VR to the error amplifier AMP is set to a value that is slightly higher than the minimum value of the inter-terminal voltage that enables stable operation of the constant current circuit CS3. In this way, a useless high voltage is prevented from being supplied between the terminals of the constant current circuit, in other words, a necessary minimum voltage is supplied between the terminals of the constant current circuit, At the same time as enabling stable operation, it is possible to reduce the power loss generated there.
JP 2002-008409 A Special table 2005-537669

  The lighting device is usually provided with a light control means for adjusting the light emission amount. For example, the constant current circuits CS1 to CS3 in FIG. 2 that can be turned on and off according to an external signal are used. Then, a dimming control circuit that changes the time ratio between the current supply period and the pause period is installed, and the dimming control circuit is configured to supply a signal for turning on and off the operation to each of the constant current circuits CS1 to CS3. For example, the lighting control device shown in FIG. In addition, as an operation method for generating a current supply period and a rest period for each of the LED rows LA1 to LA3, two methods can be considered: a method in which all the columns are simultaneously generated and a method in which the timing is shifted for each LED row.

First, in the circuit shown in FIG. 2, the case where the current supply period and the rest period for the LED strings are generated simultaneously is considered.
In the idle period, the constant current circuits CS1 to CS3 are turned off, and the voltages of the respective series detection terminals ST1 to ST3 rise all at once. At this time, the voltage value of each of the series detection terminals ST1 to ST3 exceeds the range that can be processed inside the signal processing circuit 6, the signal in the signal processing circuit 6 becomes saturated, and the operation of the control circuit 4 is practically performed. Stops. When the supply period is started in this state, the constant current circuits CS1 to CS3 are turned on, and the voltages of the respective series detection terminals ST1 to ST3 are simultaneously reduced. However, the control circuit 4 cannot immediately resume the operation, and the restart of the voltage supply from the DC-DC converter circuit 1 to each of the LED rows LA1 to LA3 is delayed. Also during this time, current flows into each of the LED rows LA1 to LA3, so that the voltage between the terminals of the smoothing capacitor C1 decreases.

  In the circuit having the configuration as shown in FIG. 2, generally, the output voltage of the DC-DC converter circuit 1 (voltage between terminals of the smoothing capacitor C1) simultaneously realizes stabilization of operation of the constant current circuits CS1 to CS3 and reduction of loss. Therefore, the voltage between the terminals of the constant current circuits CS1 to CS3 is a value that is slightly higher than the “minimum value of the voltage between terminals that enables stable operation of the constant current circuit” (hereinafter referred to as the operation minimum voltage). Set to When the voltage between the terminals of the smoothing capacitor C1 decreases in the circuit having such a specification, the voltage between the terminals of the constant current circuits CS1 to CS3 also decreases. For example, the voltage between the terminals of the constant current circuit CS3 is lower than the minimum operating voltage. It will be lower.

  Then, the operation of the constant current circuit CS3 becomes unstable, and the current flowing therethrough (1) decreases the value following the decrease in the output voltage of the DC-DC converter circuit 1 (= inter-terminal voltage of C1). {Period from t1 to t2 in FIG. 3} (2) Following the inversion increase of the output voltage due to the resumption of the operation of the control circuit 4 and the DC-DC converter circuit 1, and increasing the value {Period from t2 to t3 in FIG. }, (3) finally stabilize at a specified current value {after t3 in FIG. 3}. Such a phenomenon in which the current changes is more likely to appear in a constant current circuit connected to an LED string having a high forward drop voltage. Although depending on the conditions, such a phenomenon usually appears only in a few LED strings, such as a constant current circuit connected to the LED string having the highest forward drop voltage.

  Then, a phenomenon occurs in which an average value within a current supply period of a specific LED row (in this example, the third LED row LA3) becomes smaller than other LED rows. As can be seen from the current waveform in FIG. 3, this phenomenon becomes more prominent as the current supply period is shortened for dimming. For this reason, when the supply period is shortened for dimming, there is a problem in that a difference in the amount of light emission occurs depending on the LED array, and uneven brightness tends to occur on the light emitting surface. In addition, since the voltage between the terminals of the smoothing capacitor C1 fluctuates with the on / off operation of the constant current circuits CS1 to CS3, the smoothing capacitor C1 may make a sound depending on the setting of the cycle (frequency) of the on / off operation. .

  On the other hand, when the current supply period and the rest period are generated by shifting the timing for each LED string, the control circuit 4 sets the voltages of the series detection terminals ST1 to ST3 connected to the constant current circuits CS1 to CS3 in the supply period. Accordingly, the output voltage of the DC-DC converter circuit 1 is controlled. Here, the constant current circuit CS1 connected to the first LED row LA1 having the lowest forward drop voltage shifts from the supply period to the rest period, and immediately thereafter, the third LED row having the highest forward drop voltage. Assume that the constant current circuit CS3 connected to LA3 has shifted from the idle period to the supply period. In order to simplify the setting situation, the constant current circuit CS2 is assumed to be in a rest period throughout.

  When the constant current circuit CS1 is in the supply period, the output voltage of the DC-DC converter circuit 1 is set and controlled to a value such that the voltage at the first series detection terminal ST1 is equal to the reference value signal. After that, when the constant current circuit CS1 shifts to the idle period, and immediately after that the constant current circuit CS3 shifts from the idle period to the supply period, the output voltage of the DC-DC converter circuit 1 next becomes the third series detection terminal ST3. The voltage is set and controlled so that the voltage is equal to the reference value signal.

  However, when the constant current circuit CS3 shifts from the pause period to the supply period, the output voltage of the DC-DC converter circuit 1 (voltage between terminals of C1) is still based on the forward drop voltage of the first LED row LA1. The size of the voltage source VR is approximately equal to the sum of the reference value signals. Then, since the forward voltage drop of the third LED array LA3 is higher than that of the first LED array LA1, the voltage of the third series detection terminal ST3 becomes lower than the reference value signal of the reference voltage source VR. For this reason, the control circuit 4 supplies a drive signal for increasing the output voltage to the DC-DC converter circuit 1 in an attempt to make the voltage of the third series detection terminal ST3 equal to the reference value signal.

  However, at the beginning of the supply period, the voltage between the terminals of the constant current circuit CS3 may be lower than the minimum operating voltage due to the difference in forward voltage drop between the first LED array LA1 and the third LED array LA3. Actually, in the circuit specification that simultaneously realizes the stabilization of the operation of the constant current circuit and the reduction in loss, the voltage between the terminals of the constant current circuit CS3 is usually lower than the minimum operation voltage. Then, the current flowing through the constant current circuit CS3 becomes a value lower than the specified current value at the start of the supply period, and thereafter changes so as to approach the specified current value as the output voltage increases. However, the constant current circuit CS3 connected to the LED string having the highest forward voltage drop transitions from the supply period to the rest period, and immediately thereafter, the constant current circuit CS1 connected to the LED string having the lowest forward voltage drop. Is shifted from the idle period to the supply period, the voltage level relationship is reversed, so that the voltage across the constant current circuit CS1 does not become lower than the minimum operating voltage.

Therefore, even when the current supply period and the rest period are generated by shifting the timing for each LED array, the specific LED array (specifically, the third LED array LA3 having the highest forward drop voltage) A phenomenon occurs in which the average value within the current supply period is smaller than that of the other LED strings. As a result, there is a problem that when the supply period is shortened for dimming, there is a difference in the amount of light emission depending on the LED array, and uneven brightness tends to occur on the light emitting surface.
Therefore, the present invention is less likely to cause uneven luminance on the light emitting surface even when the circuit specification is to simultaneously realize the stabilization of the operation of the constant current circuit and the reduction in loss, and the light control range can be increased, and Another object of the present invention is to provide an LED lighting control device that can easily prevent a capacitor from squealing due to fluctuations in output voltage.

To solve the above problems, the present invention provides a plurality of constant current circuits respectively connected in series to a plurality of LED strings, a converter circuit for supplying an output voltage controlled by the LED strings and the constant current circuit, and a converter In an LED lighting control device for stably lighting an LED, comprising: a control circuit that feedback-controls the operation of the converter circuit so that the output voltage of the circuit is stabilized at a control target value;
An output detection terminal for receiving an output voltage detection signal; a reference value signal for setting a control target value for the output voltage; and a drive signal supplied to the converter circuit in accordance with the output voltage detection signal and the reference value signal A signal processing circuit for generating; a reference value signal generating circuit capable of changing the magnitude of the reference value signal; and a detection circuit for detecting an operating state of each of the plurality of constant current circuits; Here, the reference value signal generation circuit is
Turns on when the detection result of the detection circuit indicates that the operation of the constant current circuit is unstable, and switches from on to off when the operation of all the constant current circuits indicates a stable state. A switch that is charged when the switch is on; and a resistor that is connected between the switch and that limits the charging current of the capacitor; Generating a reference value signal that is set to a value that enables stable operation of the constant current circuit.

  According to the configuration of the present invention, when the constant current circuit is turned on / off for dimming, the voltage across the terminals of the smoothing capacitor provided on the output side of the DC-DC converter circuit hardly decreases. Even when the constant current circuit is turned on and off at different timings, the voltage between the terminals is lower than the value that enables stable operation when the specific constant current circuit enters the supply period. Can be reduced. As a result, the dimming range can be increased, and even if the supply period is shortened for dimming, it is possible to make it difficult to cause uneven brightness on the light emitting surface. In addition, the fluctuation of the voltage between the terminals of the smoothing capacitor can be adjusted by the reference value signal generation circuit for both the fluctuation range and the period, so that the noise of the smoothing capacitor can be easily prevented.

  A constant current circuit is connected in series to each of the plurality of LED strings, and a series body of the LED string and the constant current circuit is connected in parallel to the output terminal of the DC-DC converter circuit. A resistor circuit for detecting the output voltage is further connected to the output terminal of the DC-DC converter circuit. Then, a control circuit that feedback-controls the operation of the converter circuit is provided so that the output voltage of the DC-DC converter circuit is stabilized at the control target value, and its drive signal output terminal is connected to the DC-DC converter circuit to detect output. Connect the terminal to the resistor circuit. Here, the control circuit includes a detection circuit, a reference value signal generation circuit, and a signal processing circuit.

The detection circuit is connected to a connection point between each LED array and the constant current circuit via a series detection terminal, and detects the operation state of the constant current circuit from the voltage.
The reference value signal generation circuit includes a switch, a capacitor, and a current limiting resistor. When the signal supplied from the detection circuit indicates that the operation of at least one constant current circuit is unstable, the switch is turned on. When the signal supplied from the detection circuit indicates that the operation of all the constant current circuits is stable, the switch is turned off, and the reference value signal is obtained from the voltage across the capacitor charged through the switch. It is set as the structure which obtains.
The signal processing circuit receives the output detection signal from the resistance circuit and the reference value signal from the reference value signal generation circuit, generates a drive signal according to the two signals, and converts the drive signal to the DC-DC converter circuit. It is set as the structure supplied.

  The signal supplied from the detection circuit to the reference value signal generation circuit has a relatively low signal level when the operation of the constant current circuit indicates unstable operation, and the operation of all the constant current circuits is stable. The signal level is relatively high when it is in a state indicating that the Therefore, a reference voltage source that generates a threshold signal and a hysteresis comparator that compares the threshold signal and the signal from the detection circuit are provided inside the reference value signal generation circuit, and the switch is turned on and off by the output signal of the hysteresis comparator. Let

  Specifically, when the signal supplied from the detection circuit becomes lower than the lower threshold value of the hysteresis comparator, the switch is turned on by the output signal of the hysteresis comparator, and the signal supplied from the detection signal is changed to the upper side of the hysteresis comparator. When it becomes higher than the threshold value, the switch is turned off by the output signal of the hysteresis comparator. Then, a reference value signal is obtained from the voltage between the terminals of the capacitor charged by this operation. The control circuit drives the converter circuit so that the reference value signal and the output voltage detection signal have the same value.

  Here, the threshold signal is supplied from the detection circuit when the operation of the last one constant current circuit shifts from an unstable state to a stable state and the operations of all the constant current circuits become stable. It is assumed that the lower threshold value of the hysteresis comparator is set to a value slightly higher than the value of the signal to be output.

FIG. 1 shows the configuration of the LED lighting control device according to the present invention that can increase the light control range and make it difficult to cause uneven brightness on the light emitting surface.
The circuit of FIG. 1 includes resistors R3 and R4 for detecting the output voltage of the DC-DC converter circuit 1, a control circuit 7 further including a reference value signal generation circuit 8 and an output detection terminal FB. Except for the configuration of the control circuit 7 and the resistors R3 and R4, the other circuit portions have the same configuration in the circuits of FIGS.

In FIG. 1, the control circuit 7 is configured as follows.
A detection circuit 5 is provided that includes diodes D1, D2, and D3 each having an anode connected in common, and a resistor R1 connected between the common connection point of the anodes and a power supply line (VDD). The cathode of the diode D1 of the detection circuit 5 is connected to the first series detection terminal ST1, and similarly, the cathode of the diode D2 and the cathode of the diode D3 are connected to the second series detection terminal ST2 and the third series detection terminal ST3. Each is connected. The common connection point of the anodes is connected to the inverting input terminal (−) of the hysteresis comparator COM as the detection point P1 of the detection circuit 5.

  The non-inverting input terminal (+) of the hysteresis comparator COM is connected to a reference voltage source VTH that generates a threshold signal, and the output terminal of the hysteresis comparator COM is connected to a gate as a control terminal of the switching transistor Q1. The drain of the switching transistor Q1 is connected to the power supply line (VDD), and the source is connected to one end of the capacitor C2 via the current reducing resistor R2. The other end of the capacitor C2 is connected to the ground as a reference potential point. The hysteresis comparator COM, the reference voltage source VTH, the switching transistor Q1, the resistor R2, and the capacitor C2 constitute a reference value signal generation circuit 8.

  A connection point between one end of the capacitor C2 and the resistor R2 is connected to a non-inverting input terminal (+) of the error amplifier AMP constituting the signal processing circuit 6 as a circuit contact for generating a reference value signal. A connection point between one end of the capacitor C2 and the resistor R2 is further connected to a discharge circuit DIS for discharging the accumulated charge of the capacitor C2. The inverting input terminal of the error amplifier AMP is connected to a connection point between resistors R3 and R4 provided outside the control circuit 4 via an output detection terminal FB. The resistors R3 and R4 are connected in series between the output terminal 3 and the ground as a resistance detection circuit for the output voltage of the DC-DC converter circuit 12. The output terminal of the error amplifier AMP is connected to the input terminal of a PWM comparator (not shown).

The circuit of FIG. 1 having such a configuration stably lights each LED row by the following operation.
At the time of start-up, the voltage between the terminals of the smoothing capacitor C1 is initially zero, and the voltage at each of the series detection terminals ST1 to ST3 is also zero. Naturally, the voltage of the signal supplied from the detection point P1 of the detection circuit 5 to the inverting input terminal (−) of the hysteresis comparator COM is also zero. At this time, since the signal supplied from the detection circuit 5 is lower than the threshold signal, the hysteresis comparator COM turns on the switching transistor Q1. Then, a charging current flows into the capacitor C2 via the switching transistor Q1 and the resistor R2, and the voltage between the terminals of the capacitor C2 increases.

  As the voltage across the capacitor C2 increases, the value of the reference value signal supplied from the reference value signal generation circuit 8 to the error amplifier AMP also increases. Then, the error amplifier AMP generates a signal corresponding to the difference between the reference value signal and the detection signal of the output voltage of the DC-DC converter circuit 1 and supplies the signal to the PWM comparator. Accordingly, the control circuit 7 outputs the output voltage from the drive signal output terminal DRV to the DC-DC converter circuit 1 so that the detection signal of the output voltage of the DC-DC converter circuit 1 and the reference value signal have the same magnitude. A drive signal that raises the voltage is supplied. As long as the charging of the capacitor C2 of the reference value signal generation circuit 8 and the increase in the voltage between the terminals continue, the output voltage of the DC-DC converter circuit 1 also continues to increase.

  The output voltage of the DC-DC converter circuit 1 eventually increases beyond the value of the forward voltage drop of the LED rows LA1 to LA3. Then, a current starts to flow through each of the LED rows LA1 to LA3, and a voltage is generated between the terminals of the constant current circuits CS1 to CS3. This current and the voltage between terminals increase as the output voltage of the DC-DC converter circuit 1 increases. Each of the constant current circuits CS1 to CS3 operates so that the current flowing therethrough becomes constant at a specified value in order from the point where the voltage between the terminals exceeds the minimum operation value.

  Here, for convenience, it is assumed that the forward voltage drop of the third LED array LA3 is the highest among all the LED arrays. In this case, when the voltage between the terminals of the constant current circuit CS3 connected to the third LED row LA3 exceeds the minimum operation voltage, all the constant current circuits CS1 to CS3 perform a stable operation. Thereafter, the voltage at the detection point P1 (anode common connection point) of the detection circuit 5 reaches the lower threshold value of the hysteresis comparator COM, but the hysteresis comparator COM does not change the state of the output signal. Accordingly, the voltage across the capacitor C2, the output voltage of the DC-DC converter circuit 1, the voltage across the constant current circuit CS3, and the voltage at the third series detection terminal ST3 continue to rise while the switching transistor Q1 remains on. become.

  When the voltage at the third series detection terminal ST3 further rises, the voltage at the detection point P1 of the detection circuit 5 eventually reaches the upper threshold value of the hysteresis comparator COM. Incidentally, the upper threshold value has a magnitude obtained by adding a predetermined hysteresis width to the threshold signal supplied from the reference voltage source VTH. Then, the hysteresis comparator COM inverts the state of the output signal and turns off the switching transistor Q1. Thereby, the rise of the voltage between the terminals of the capacitor C2 is stopped, and the rise of the output voltage of the DC-DC converter circuit 1 is also stopped. At this time, all the constant current circuits CS1 to CS3 are in a state capable of stable operation, and all the LED rows LA1 to LA3 are stably lit with a specified current value.

  After the switching transistor Q1 is turned off, the capacitor C2 is discharged little by little through the discharge circuit DIS, and the voltage between the terminals is gradually reduced. As the inter-terminal voltage of the capacitor C2 decreases, the output voltage of the DC-DC converter circuit 1, the inter-terminal voltage of the constant current circuit CS3, and the voltage of the third series detection terminal ST3 gradually decrease. When the voltage at the third series detection terminal ST3 further decreases, the voltage at the detection point P1 of the detection circuit 5 eventually reaches the lower threshold value of the hysteresis comparator COM. Incidentally, the lower threshold value is set by a threshold signal supplied from the reference voltage source VTH. Specifically, the voltage between the terminals of the constant current circuit CS3 remaining at the end of circuit startup is the minimum operating voltage. It becomes a value slightly higher than the voltage appearing at the detection point P1 at the time of becoming. For this reason, even when the voltage at the detection point P1 of the detection circuit 5 reaches the lower threshold value of the hysteresis comparator COM, all the constant current circuits CS1 to CS3 continue to operate stably.

  When the voltage at the detection point P1 reaches the lower threshold value, the hysteresis comparator COM inverts the state of the output signal and turns on the switching transistor Q1. When the switching transistor Q1 is turned on, the capacitor C2 is charged again. The voltage between the terminals of the capacitor C2, the output voltage of the DC-DC converter circuit 1, the voltage between the terminals of the constant current circuit CS3, and the voltage of the third series detection terminal ST3 are as follows. Turns up. After that, the circuit of FIG. 1 supplies each constant current circuit (CS1 to CS3) with a voltage higher than the minimum operating voltage while repeating charging and discharging of the capacitor C2, and stably lights each LED row (LA1 to LA3). I will let you.

  By the way, the forward voltage drop of the LED changes not only with the current flowing therethrough but also with the temperature. When the LED rows LA1 to LA3 are lit for a long time, the forward drop voltage changes due to heat generation, and the inter-terminal voltages of the constant current circuits CS1 to CS3 also change. For this reason, when the output voltage of the DC-DC converter circuit 1 after startup is kept constant, the voltage between the terminals of the constant current circuits CS1 to CS3 increases, and the power loss may increase. Therefore, the circuit of FIG. 1 intentionally repeats charging / discharging of the capacitor C2 after startup, and periodically resets the value of the reference value signal. As a result, the output voltage of the DC-DC converter circuit 1 is such that the voltage between the terminals of the constant current circuits CS1 to CS3 is higher than the minimum operating voltage even if the forward voltage drop of the LED strings LA1 to LA3 changes after startup, and It will be readjusted to a value that can suppress an increase in power loss.

  Also in the circuit of FIG. 1, the output voltage (voltage between terminals of C1) of the DC-DC converter circuit 1 fluctuates with the charge / discharge operation of the capacitor C2. However, since it is not necessary to reset the reference value signal frequently, the cycle (frequency) of the charge / discharge cycle of the capacitor C2 is the cycle (frequency) of the on / off operation of the constant current circuits CS1 to CS3 for dimming. It is set very long compared to. Further, the fluctuation range of the output voltage basically depends on the hysteresis width of the hysteresis comparator COM and can be easily adjusted. Accordingly, in the circuit of FIG. 1, the smoothing capacitor C1 can be easily prevented from making noise.

  The LED lighting control device according to the present invention in FIG. 1 sets and controls the output voltage of the DC-DC converter circuit 1 to such a value that a voltage higher than the minimum operating voltage can be supplied between the terminals of the constant current circuits CS1 to CS3. This is the same as the conventional circuit of FIG. However, the LED lighting control device of FIG. 1 detects the output voltage of the DC-DC converter circuit 1, performs feedback control so that the output voltage is stabilized at the control target value, and a reference value signal for setting the control target. Is different from the conventional circuit in that the constant current circuits CS1 to CS3 with respect to the output voltage are set according to the operating state, specifically, the inter-terminal voltage.

  In the circuit of FIG. 1, when the current supply period and the rest period for the LED rows LA1 to LA3 are simultaneously generated for dimming, the voltages of the respective series detection terminals ST1 to ST3 are generated during the rest period. Rises all at once. However, inside the control circuit 7, the switching transistor Q <b> 1 is only maintained in the OFF state, and basically the voltage across the capacitor C <b> 2 does not fluctuate due to factors other than the discharge circuit DIS. Therefore, the signal in the signal processing circuit 6 is maintained in a processable range even during the pause period, and when entering the supply period, the control circuit 7 immediately performs an operation according to the output voltage of the DC-DC converter circuit 1. It can be carried out.

  Further, if the discharge amount of the capacitor C2 by the discharge circuit DIS is set to be small, and the period of the charge / discharge cycle of the capacitor C2 is set to be much longer than the cycle of the on / off operation of the constant current circuits CS1 to CS3, The amount of decrease in the voltage between the terminals of the capacitor C2 and the output voltage of the DC-DC converter circuit 1 (voltage between the terminals of C1) generated during the period can be suppressed to be small. For this reason, even if the voltage between the terminals of the smoothing capacitor C1 is slightly lowered when the current supply period starts, the operation of the constant current circuit is unstable when the supply period is shortened compared to the conventional LED lighting control device. It is hard to become.

  In addition, except for the case where the suspension period is entered at a special timing (for example, immediately before the capacitor C2 is recharged), the constant current circuit (specifically, when the transition is made from the suspension period to the supply period) The terminal voltage of CS3) does not become lower than the minimum operating voltage. For this reason, compared with the conventional LED lighting control apparatus, the magnitude | size (average value of a supply period) which flows through each LED row LA1-LA3 during a supply period can be made into substantially the same magnitude | size, and a light emission surface Is less likely to cause uneven brightness.

  The same applies to the case where the current supply period and the rest period are generated by shifting the timing for each LED string, and the constant current circuit CS1 is activated at a special timing (for example, timing immediately before the capacitor C2 is recharged). The voltage between the terminals of the constant current circuit CS3 does not become lower than the minimum operation voltage except when the constant current circuit CS3 shifts from the supply period to the idle period and immediately after that the constant current circuit CS3 shifts from the idle period to the supply period. For this reason, luminance unevenness is less likely to occur on the light-emitting surface as compared with the conventional LED lighting control device.

  In the above description of the embodiment of the LED lighting control device according to the present invention, it is assumed that a switching regulator type power source is applied to the DC-DC converter circuit 1. However, as long as the output voltage is a feedback-controlled power supply, for example, a power supply of a series regulator system or another system may be used. The same applies to the control circuit 6. In the description of the embodiment, the PWM control method is assumed, but a control circuit of a PFM control method or other control methods may be used. Incidentally, when the DC-DC converter circuit 1 is a series regulator system, the internal configuration of the signal processing circuit 6 can be simplified. For example, the output terminal of the error amplifier AMP is connected to the DC-DC via the drive signal output terminal DRV. Connected to the converter circuit 1.

  In the description of the above embodiment, it is assumed that the capacitor C2 of the reference value signal generation circuit 8 is discharged via the discharge circuit DIS. However, when utilizing a phenomenon in which a minute current flows into the input terminal of an actual error amplifier, the discharge circuit DIS can be omitted. Further, when an increase in power loss occurring in the constant current circuit is allowed and the reference value signal is not reset during operation, the discharge circuit DIS is omitted from the circuit of FIG. 1, and the hysteresis comparator COM is replaced with a normal comparator. It is also possible. However, if the discharge circuit DIS is omitted, a means for initializing the voltage between the terminals of the capacitor C2 may be necessary when the LED lighting control device is started or stopped.

Furthermore, in the above description, the lower threshold value of the hysteresis comparator COM is set by the reference voltage source VTH. However, a circuit configuration in which the upper threshold value of the hysteresis comparator COM is set by the reference voltage source VTH may be used. Absent.
In the circuit of FIG. 1, the current limiting resistor R2 is connected between the switching transistor Q1 and the capacitor C2, but the drain of the switching transistor Q1 is connected to the current source instead of the power supply line VDD. Alternatively, when the switching transistor Q1 has a constant current supply function, the resistor R2 can be omitted.
Of course, the number of LED rows may be three or more, and the circuit configuration can be changed without departing from the scope of the present invention.

The circuit diagram which shows the Example of the LED lighting control apparatus by this invention. The circuit diagram which shows the Example of the conventional LED lighting control apparatus. The output voltage waveform of the conventional DC-DC converter circuit and the current waveform of the constant current circuit connected to the LED string having the highest forward voltage drop.

Explanation of symbols

1: DC-DC converter circuit 2a, 2b: input terminal 3: output terminal 5: detection circuit 6: signal processing circuit 7: control circuit (present invention)
8: Reference value signal generation circuit AMP: Error amplifier C1: Smoothing capacitor C2: Capacitor COM: Hysteresis comparators CS1-CS3: Constant current circuit DIS: Discharge circuit DRV: Drive signal output terminal FB: Output detection terminals LA1-LA3: LED array P1: detection point Q1: switching transistor R2: current limiting resistors R3, R4: resistors ST1 to ST3 of output detecting resistor circuit: series detection terminal VTH: reference voltage source (for generating threshold signal)

Claims (4)

A plurality of constant current circuits connected in series to a plurality of LED strings, a converter circuit for supplying an output voltage controlled by the LED strings and the constant current circuit, and an output voltage of the converter circuit being a control target value In an LED lighting control device for stably lighting an LED, comprising a control circuit that feedback-controls the operation of the converter circuit so as to be stable at
The control circuit is
An output detection terminal for receiving the output voltage detection signal;
A reference value signal for setting a control target value of the output voltage;
A signal processing circuit for generating a drive signal supplied to the converter circuit in response to the output voltage detection signal and the reference value signal;
A reference value signal generation circuit capable of changing the magnitude of the reference value signal;
A detection circuit for detecting an operating state of each of the plurality of constant current circuits;
Comprising
Here, the reference value signal generating circuit is
Turns on when the detection result of the detection circuit indicates that the operation of the constant current circuit is unstable, and turns from on to off when the operation indicates that the operation of all the constant current circuits is stable. A switch to be switched,
A capacitor that is charged when the switch is on;
A resistor connected between the switch and the capacitor to limit a charging current of the capacitor;
And a reference value signal that sets the output voltage to a value that enables stable operation of all constant current circuits from the charging voltage of the capacitor. When the operation of all the constant current circuits is stable, the detection circuit supplies a relatively high level signal to the reference value signal generation circuit, and the operation of the at least one constant current circuit is unstable. A relatively low level signal is supplied to the reference value signal generation circuit in the state,
The reference value signal generating circuit is
One input terminal is supplied with a signal from the detection circuit, the other input terminal is supplied with a threshold signal, and a signal from the detection circuit is set to a lower threshold value set by the threshold signal When the signal is lower than the threshold value, a signal for turning on the switch is output, and when the signal from the detection circuit becomes higher than the upper threshold value set by the threshold value signal and the hysteresis width, the switch is turned off. A hysteresis comparator configured to output a signal;
The switch that is turned on and off by the output signal of the hysteresis comparator,
The threshold signal is supplied from the detection circuit when the operation of the last one constant current circuit shifts from an unstable state to a stable state and the operations of all the constant current circuits become stable. 2. The LED lighting control device according to claim 1, wherein the LED lighting control device is set to a value slightly higher than a signal value.   3. The LED lighting control device according to claim 1, wherein the detection circuit detects an operation state of the constant current circuit from a voltage at a connection point between the LED array and the constant current circuit. 4. The detection circuit comprises:
A resistor with one end connected to the voltage supply point;
A detection point connected to the other end of the resistor;
The same number of diodes as the LED string connected between the connection point of the LED string and the constant current circuit and the detection point;
Comprising
A relatively high value signal is generated at the detection point when all the constant current circuits are in a stable operating state, and a relatively low value is generated when the at least one constant current circuit is in an unstable operating state. The LED lighting control device according to claim 3, wherein the signal is generated.

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