JP5036766B2 - LED drive circuit - Google Patents

Description

  The present invention relates to an LED driving circuit for driving light emission of a light emitting diode (hereinafter referred to as “LED”).

  Conventionally, discharge lamps such as cold cathode fluorescent lamps have been used as backlights for liquid crystal displays for computers and liquid crystal televisions. On the other hand, recently, LEDs having features such as low power consumption, long life, and small size are attracting attention, and a discharge lamp as a light source is replaced with an LED.

  In addition, as a prior art which performs dimming control with respect to a discharge lamp, there exist some which were shown by the following patent document 1, for example. In Patent Document 1, while the OFF function for stopping the operation of the drive circuit is provided, a function for setting the OFF function to be invalid is added when the OFF function is not required. A discharge lamp lighting device is disclosed in which the setting is switched based on a duty ratio of a PWM (Pulse Width Modulation) signal that is a dimming signal.

JP 2005-216842 A

  By the way, in the case of configuring an LED drive circuit that drives an LED to emit light, a dimming signal for controlling the light emission of the LED may be supplied from the outside. In this case, the dimming signal is prepared by the supplier manufacturer of the LED driving circuit, and the LED driving circuit is provided with a signal input terminal called an external dimming terminal.

  In addition, as a dimming signal for controlling the light emission of the LED, there is a specification for inputting the PWM signal as described above, and there is a specification for inputting a DC signal. In the case of a specification for inputting a DC signal, the amount of light emission is controlled according to the level of the DC signal input to the external dimming terminal, but this DC signal is usually prepared by the supplier manufacturer.

  On the other hand, in the case of a specification in which a DC signal is input to the external dimming terminal, it is necessary for the supplier manufacturer to prepare a level adjustment function for adjusting the level of the DC signal, which increases the burden on the supplier manufacturer. For this reason, there is also an LED drive circuit having specifications that provide dimming control by providing only a variable resistance element at a supplier manufacturer and connecting the variable resistance element to an external dimming terminal.

  That is, in recent LED drive circuits, there are at least three types of signal input modes, such as when a DC signal and a PWM signal are input to the external dimming terminal, or when a variable resistance element is connected to the external dimming terminal. . Therefore, the manufacturer of the LED drive circuit needs to prepare an LED drive circuit that can cope with these various signal input modes.

  On the other hand, since it is an LED drive circuit, suppressing flickering during dimming control is a conventional technical problem that must be overcome regardless of the signal input mode. In response to this technical problem, it is not preferable to simply prepare a circuit unit individually corresponding to each signal input mode because the circuit scale increases.

  The present invention has been made in view of the above, and in an LED drive circuit to which various dimming signals are input, flickering during dimming control can be reduced while suppressing an increase in circuit scale. An object is to provide an LED driving circuit.

  In order to solve the above-described problems and achieve the object, an LED drive circuit according to the present invention is a signal determination that determines the type of a dimming signal input from an external dimming terminal in an LED drive circuit that drives an LED to emit light. A circuit, a filter circuit for removing a noise component of the input signal, a filter function switching means for switching between valid / invalid of the function of the filter circuit based on a determination result of the signal determination circuit, and passing the filter circuit A comparator circuit that compares the dimming signal with a reference signal and outputs a signal based on the comparison result as a burst signal for LED dimming.

  According to the present invention, the type of the dimming signal input from the external dimming terminal is determined, and the filter circuit function is switched between valid / invalid based on the determination result. There is no need to provide individually corresponding circuit portions. For this reason, the LED drive circuit which can reduce the flicker at the time of light control can be provided, suppressing the increase in a circuit scale.

  In the LED drive circuit according to the present invention, the filter function switching means effectively controls the function of the filter circuit when the type of the dimming signal is determined to be a DC signal, and the dimming signal When the type is determined to be a PWM signal, the function of the filter circuit is controlled to be invalid.

  According to the present invention, it is possible to suppress jitter that occurs according to the characteristics of a dimming signal input from an external dimming terminal.

  The LED driving circuit according to the present invention includes a triangular wave signal generating circuit that generates a triangular wave signal, and the triangular wave signal generating circuit outputs the generated triangular wave signal as the reference signal to the comparator circuit. .

  According to the present invention, since only a triangular wave signal needs to be output as a reference signal to the comparator circuit, a simpler circuit configuration can be achieved.

  The LED driving circuit according to the present invention includes a triangular wave / DC signal generation circuit that generates a triangular wave signal and a DC signal, and the triangular wave / DC signal generation circuit is configured to generate the triangular wave based on a determination result of the signal determination circuit. One of the signal and the DC signal is output to the comparator circuit as the reference signal.

  According to the present invention, it is possible to output a suitable reference signal corresponding to the characteristics of the dimming signal input from the external dimming terminal to the comparator circuit.

  In the LED driving circuit according to the present invention, the triangular wave / DC signal generation circuit outputs a triangular wave signal when the dimming signal input to the external dimming terminal is a DC signal, and the external dimming terminal In the case where the dimming signal input to is a PWM signal, a DC signal is output.

  When the dimming signal input to the external dimming terminal is a PWM signal, the signal waveform may become dull before reaching the comparator circuit. In this case, if the reference signal is a triangular wave signal, the duty ratio of the input dimming signal (PWM signal) may slightly differ from the duty ratio of the LED dimming burst signal output from the comparator circuit. On the other hand, according to the present invention, when the dimming signal input to the external dimming terminal is a PWM signal, a DC signal is output as a reference signal. Therefore, LED dimming faithful to the input dimming signal is performed. Burst signal can be output.

  The LED drive circuit according to the present invention further includes a converter that generates a DC voltage to be applied to the LED using a DC voltage or an AC voltage as a power input, and that drives the LED at a constant current based on the dimming signal. It is characterized by that.

  According to the present invention, the LED can be driven at a constant current. Also, in the present invention, if the converter is a boost converter, the input voltage can be boosted, so that even if the input voltage is lower than the voltage applied to the LED, the dimming control for the LED is possible. . In the present invention, if the converter is an AC / DC converter, it is possible to cope with the case where the input power supply is an AC power supply.

  In the LED driving circuit according to the present invention, when a variable resistance element is connected to the external dimming terminal, a variable DC voltage is applied to the input terminal of the filter circuit by changing a resistance value of the variable resistance element. And a bias circuit that operates to perform the above operation.

  According to the present invention, in addition to a signal input mode in which a DC signal and a PWM signal are input to the external dimming terminal, it is possible to cope with a signal input mode in which a variable resistance element is connected to the external dimming terminal. . That is, the present invention can provide an LED drive circuit that can cope with various signal input modes.

  According to the LED drive circuit of the present invention, in the LED drive circuit to which various dimming signals are input, it is possible to reduce flicker during dimming control while suppressing an increase in circuit scale. It is done.

FIG. 1 is a diagram illustrating a functional configuration of the LED drive circuit according to the first embodiment of the present invention. FIG. 2 is a graph showing the relationship between the capacitor capacity and the jitter time width when the filter circuit is composed of capacitors. FIG. 3 is a diagram illustrating an example of a circuit configuration of the first embodiment that embodies the function of the LED drive circuit illustrated in FIG. 1. FIG. 4 is a diagram illustrating a burst signal generated using a triangular wave signal as a reference signal when the input dimming signal is a PWM signal. FIG. 5 is a diagram showing an example of a circuit configuration of the second embodiment that embodies the function of the LED drive circuit shown in FIG. FIG. 6 is a diagram illustrating a functional configuration of the LED drive circuit according to the third embodiment of the present invention. FIG. 7 is a diagram illustrating an example of a circuit configuration of the third embodiment that embodies the function of the LED drive circuit illustrated in FIG. 5. FIG. 8 is a diagram illustrating a signal input mode in which a variable resistance element is connected to the external dimming terminal.

  Hereinafter, an LED drive circuit according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In addition, this invention is not limited by embodiment shown below.

Embodiment 1 FIG.
FIG. 1 is a diagram illustrating a functional configuration of the LED drive circuit according to the first embodiment of the present invention. As shown in FIG. 1, the LED drive circuit according to the first embodiment includes a filter circuit 16, a comparator circuit 18, a signal determination circuit 20, a triangular wave signal generation circuit 26a, a DC signal generation circuit 26b, and a signal switching unit 26c. Composed.

  Next, the operation of the LED drive circuit in Embodiment 1 shown in FIG. 1 will be described. The external dimming terminal 6 receives the above-described PWM signal, DC signal, or the like. This dimming signal is input to the filter circuit 16 and the signal determination circuit 20. The signal determination circuit 20 determines the type of the input dimming signal, that is, whether the dimming signal is a PWM signal or a DC signal. The filter circuit 16 has a function of removing noise components (ripple noise) of the input dimming signal. This function is switched based on the output of the signal determination circuit 20, and specifically, switching control is performed as follows.

  First, when the dimming signal input from the external dimming terminal 6 is a DC signal, the function of the filter circuit 16 is controlled to be effective (ON). On the other hand, when the dimming signal input from the external dimming terminal 6 is a PWM signal, the function of the filter circuit 16 is controlled to be invalid (OFF).

  When a DC signal is input to the external dimming terminal 6, ripple noise is often superimposed on a signal line connecting the external dimming terminal 6 and the comparator circuit 18. The conventional LED driving circuit has a problem that jitter is generated in the output (burst signal output) of the comparator circuit due to the influence of the ripple noise. Therefore, in the LED drive circuit of the first embodiment, the filter circuit 16 is inserted at an arbitrary position between the external dimming terminal 6 and the comparator circuit 18 in order to suppress the influence of the ripple noise. When the filter circuit 16 is inserted, the influence of ripple noise is suppressed, so that the jitter of the burst signal output is reduced.

  FIG. 2 is a graph showing the relationship between the capacitor capacity and the jitter time width when the filter circuit 16 is composed of a capacitor. In FIG. 2, a curve K1 plotted with black rhombus marks shows a case where the dimming signal is a DC signal, and a curve K2 plotted with black square marks shows a case where the dimming signal is a PWM signal. Increasing the capacitor capacity is equivalent to making the filter circuit 16 work more effectively.

  As shown in FIG. 2, the filter circuit 16 acts positively on the DC signal as the capacitor capacity is increased, but acts negatively on the PWM signal. Therefore, in the LED drive circuit of the first embodiment, the signal determination circuit 20 is provided, and the type of the input dimming signal is determined based on the determination result of the signal determination circuit 20.

  FIG. 3 is a diagram illustrating an example of a circuit configuration of the first embodiment that embodies the function of the LED drive circuit illustrated in FIG. 1. In FIG. 3, the LED drive circuit 10 is provided with a power supply terminal 4, an external dimming terminal 6, and an output terminal 8 that is an LED connection terminal, and the external power supply 1 is connected to the power supply terminal 4. LED group 2 is connected. In FIG. 3, the LED group 2 has a configuration in which a plurality of LEDs are connected in series. However, the LED group 2 is not necessarily a plurality and may be a single LED. Moreover, the structure by which several LED was connected in parallel may be sufficient.

  In FIG. 3, the LED drive circuit 10 includes the filter circuit 16, the comparator circuit 18, and the signal determination circuit 20 illustrated in FIG. 1, and is provided between the signal determination circuit 20 and the filter circuit 16. A switch circuit 22 is provided. A buffer circuit 24 and a triangular wave / DC signal generation circuit 26 are provided in this order between the signal determination circuit 20 and the comparator circuit 18, and between the comparator circuit 18 and the output terminal 8. A diode 32, a resistance element 36, a switch circuit 38, and the like are provided.

  In the configuration of FIG. 3, the switch circuit 22 functions as a filter function switching unit that switches between valid / invalid of the filter circuit 16. The triangular wave / DC signal generation circuit 26 is a circuit unit that implements the functions of the triangular wave signal generation circuit 26a, the DC signal generation circuit 26b, and the signal switching unit 26c in FIG. Further, the diode 32, the resistance elements 34 and 36, the switch circuit 38, and the like are circuit elements for dimming control of the LED group 2 by applying a burst signal, which is the final output of the LED drive circuit 10, to the LED group 2. Or it is a circuit part.

  Next, the operation of the LED drive circuit 10 shown in FIG. 3 will be described. First, when a DC signal is input to the external dimming terminal 6, the transistor Tr1 of the signal determination circuit 20 is turned off while the transistor Tr2 of the switch circuit 22 is turned on, so that the capacitor C1 of the filter circuit 16 is connected to GND. Connected. At this time, since the transistor Tr3 and the transistor Tr4 of the buffer circuit 24 are turned OFF, the triangular wave / DC signal generation circuit 26 generates a triangular wave signal having a period according to the time constants of the capacitor C2 and the resistance element R1. The comparator circuit 18 receives the triangular wave signal generated by the triangular wave / DC signal generation circuit 26 and the dimming signal (in this case, the DC signal) input through the filter circuit 16 to dimm the LED group 2. A burst signal is generated. This burst signal is applied to the LED group 2 by turning on / off the switch circuit 38, and the LED group 2 is PWM-driven.

  The operation when a PWM signal is input to the external dimming terminal 6 is as follows. Since the signal determination circuit 20 is configured as a differentiation circuit, the transistor Tr1 is turned on when the PWM signal rises and falls, but the time constant due to the capacitor C3 and the resistance element R2 of the signal determination circuit 20 is PWM. Since it is set to be larger than the signal period, the terminal voltage of the capacitor C3 maintains a substantially constant value. As a result, since the transistor Tr2 of the switch circuit 22 is turned off, the capacitor C1 of the filter circuit 16 is not connected to GND, and the function of the filter circuit 16 becomes invalid. At this time, since the transistor Tr3 and the transistor Tr4 of the buffer circuit 24 are kept in the ON state, the triangular wave / DC signal generation circuit 26 generates a DC signal having a DC voltage according to the voltage dividing resistance of the resistance elements R3 and R4 as an input Operates as a circuit. The comparator circuit 18 generates a burst signal based on the DC signal generated by the triangular wave / DC signal generation circuit 26 and the dimming signal (in this case, the PWM signal) input through the filter circuit 16. The subsequent operation is the same as that for the DC signal.

  In the LED drive circuit of the first embodiment, it has been described that the triangular wave / DC signal generation circuit 26 generates a DC signal when a PWM signal is input from the external dimming terminal 6, but the external dimming terminal Similarly to the case where a DC signal is input from 6, a triangular wave signal may be generated and output. As shown in FIG. 4, even if the triangular wave signal is a reference signal, a burst signal equivalent to the input dimming signal can be generated.

  However, when the dimming signal (input dimming signal) input from the external dimming terminal is a PWM signal, the signal waveform may become dull before reaching the comparator circuit 18 depending on the circuit configuration. In this case, if the reference signal is a triangular wave signal, the duty ratio of the PWM signal, which is the input dimming signal, and the duty ratio of the LED dimming burst signal output from the comparator circuit 18 may slightly differ. Therefore, when the input dimming signal is a PWM signal, it is preferable to use the DC signal as a reference signal. By using the DC signal as a reference signal, a burst signal faithful to the input dimming signal can be generated.

  As described above, according to the LED drive circuit of the first embodiment, the type of the dimming signal input from the external dimming terminal is determined, and the validity / invalidity of the filter circuit function is switched based on the determination result. Since the configuration is adopted, it is possible to obtain an effect that it is not necessary to provide a circuit unit individually corresponding to each dimming signal.

  Moreover, according to the LED drive circuit of Embodiment 1, even if it is the LED drive circuit of the specification into which both a PWM signal and DC signal are input from one external dimming terminal, it is separately for every dimming signal. Therefore, there is no need to provide a circuit unit corresponding to the above, and thus an effect that a circuit configuration in which an increase in circuit scale is suppressed can be obtained.

  Moreover, according to the LED drive circuit of Embodiment 1, even if it is the LED drive circuit of the specification into which both a PWM signal and DC signal are input from one external dimming terminal, according to the kind of dimming signal Since a suitable circuit is selected, even if any dimming signal is input, the jitter can be reduced and the flicker at the time of dimming control can be reduced.

Embodiment 2. FIG.
FIG. 5 is a diagram showing an example of the circuit configuration of the second embodiment that embodies the function of the LED drive circuit shown in FIG. 1, and the same or equivalent parts as those in the first embodiment (FIG. 3) are denoted by the same reference numerals. The description is omitted. As shown in FIG. 5, the LED drive circuit of the second embodiment has a boost converter 12 that is a DC / DC converter for driving the LED group 2 at a constant current, and outputs the boost converter 12 to the LED group 2. It is set as the structure to apply. In FIG. 5, the controller 30 provided in the boost converter 12 is a controller for detecting and controlling a current flowing in the LED group 2 (hereinafter referred to as “LED current”). The F / B terminal of the controller 30 is connected to one end (the cathode side in the example of FIG. 5) of the LED group 2 through the resistance element 36 and is also connected to the output terminal of the comparator circuit 18 through the diode 32. ing.

  Next, the operation of the LED drive circuit according to the second embodiment shown in FIG. 5 will be described. When the controller 30 is monitoring the LED current, if a voltage higher than a certain threshold is applied to the F / B terminal of the controller 30, the LED current becomes the same as the state exceeding the set value. 30 performs control to stop the boosting operation of the boosting converter 12. For example, when a burst signal (PWM signal) having a period of 200 Hz is applied to the F / B terminal, the LED group 2 is turned off when the PWM signal is “Hi”, and the LED group 2 is turned on when it is “Lo”. As described above, by operating the boost converter 12 based on the burst signal that is the output of the comparator circuit 18, the constant current drive for the LED group 2 can be performed. The operation of other circuit units is the same as that of the first embodiment, and a description thereof is omitted.

  As described above, in the LED drive circuit of the second embodiment, when the dimming control is performed on the LED group, the LED current is monitored, and the LED group is driven at a constant current based on the monitored LED current. Even when various dimming signals are input from the external dimming terminal, more precise dimming control can be performed.

  Further, according to the LED drive circuit of the second embodiment, since the input voltage can be boosted by the boost converter, even if the input voltage is lower than the voltage applied to the LEDs, the dimming control for the LED group Can be obtained.

  In the LED driving circuit according to the second embodiment, a configuration including a boost converter as a DC / DC converter that uses a DC voltage as a power input and boosts and outputs the input DC voltage is illustrated. However, the configuration is limited to this configuration. Is not to be done. For example, a step-down converter can be used instead of a step-up converter, and an AC / DC converter that converts an AC voltage input into a DC voltage and outputs the AC voltage can be used instead. .

Embodiment 3 FIG.
FIG. 6 is a diagram illustrating a functional configuration of the LED drive circuit according to the third embodiment of the present invention, and FIG. 7 is a diagram illustrating an example of a circuit configuration that embodies the function of the LED drive circuit illustrated in FIG. is there. 6 and FIG. 7, the same or equivalent parts as those in FIG. 1 and FIG. In the LED drive circuit of the third embodiment, as shown in FIGS. 6 and 7, a bias circuit 14 is provided between the external dimming terminal 6 and the filter circuit 16. The bias circuit 14 corresponds to a signal input mode in which a variable resistance element is connected to the external dimming terminal 6.

  Next, the operation of the LED drive circuit according to the third embodiment will be described with reference to FIGS. FIG. 8 is a diagram illustrating a signal input mode in which a variable resistance element is connected to the external dimming terminal.

In the case of a signal input mode in which the variable resistance element 46 is connected to the external dimming terminal 6, in the example shown in FIG. 8, the resistance values of the resistance elements 40, 42, and 44 in the bias circuit 14 and the resistance of the variable resistance element 46. The voltage V ₁ determined by the relationship with the value becomes the input voltage (DC voltage) to the filter circuit 16. Further, by changing the resistance value of the variable resistance element 46, the input voltage to the filter circuit 16 can be changed. Other circuit operations are the same as those in the first and second embodiments, and the description thereof is omitted.

  As described above, according to the LED drive circuit of the third embodiment, the bias drive circuit is provided between the external dimming terminal and the filter circuit in the LED drive circuit of the first and second embodiments. In addition to the effects of the first and second aspects, the signal input mode in which the variable resistance element is connected to the external dimming terminal can also be obtained.

  In the LED driving circuit of the third embodiment, as shown in FIG. 7, as a configuration example in which the bias circuit 14 is provided between the external dimming terminal 6 and the filter circuit 16, the second embodiment shown in FIG. Although the example applied to the LED driving circuit has been disclosed, it can also be applied to the LED driving circuit of the first embodiment shown in FIG. 2, and the effects of the first and third embodiments can be obtained.

  As described above, the LED drive circuit according to the present invention can reduce flicker during dimming control while suppressing an increase in circuit scale in an LED drive circuit to which various dimming signals are input. Useful as.

DESCRIPTION OF SYMBOLS 1 External power supply 2 LED group 4 Power supply terminal 6 External dimming terminal 8 Output terminal 10 LED drive circuit 12 Boost converter 14 Bias circuit 16 Filter circuit 18 Comparator circuit 20 Signal determination circuit 22 Switch circuit 24 Buffer circuit 26 Triangular wave / DC signal generation circuit 26a Triangular wave signal generation circuit 26b DC signal generation circuit 26c Signal switching means 30 Controller 32 Diode 34, 36, 40, 42, 44, R1 to R4 Resistive element 38 Switch circuit 46 Variable resistance element C1 to C3 Capacitor Tr1 to Tr4 transistor

Claims (7)

In an LED drive circuit that drives an LED to emit light,
A signal determination circuit for determining the type of dimming signal input from the external dimming terminal;
A filter circuit for removing noise components of the input signal;
Filter function switching means for switching between valid / invalid of the function of the filter circuit based on the determination result of the signal determination circuit;
A comparator circuit that compares the dimming signal passed through the filter circuit with a reference signal and outputs a signal based on the comparison result as a burst signal for the LED dimming;
An LED drive circuit comprising: the filter function switching means connected between the signal determination circuit and the filter circuit. The filter function switching means is
When the type of the dimming signal is determined to be a DC signal, the function of the filter circuit is effectively controlled,
The LED drive circuit according to claim 1, wherein when the type of the dimming signal is determined to be a PWM signal, the function of the filter circuit is controlled to be invalid. A triangular wave signal generation circuit for generating a triangular wave signal is provided,
The LED driving circuit according to claim 1, wherein the triangular wave signal generation circuit outputs the generated triangular wave signal as the reference signal to the comparator circuit. A triangular wave / DC signal generation circuit for generating a triangular wave signal and a DC signal,
2. The triangular wave / DC signal generation circuit outputs either the triangular wave signal or the DC signal to the comparator circuit as the reference signal based on a determination result of the signal determination circuit. Or the LED drive circuit of 2. The triangular wave / DC signal generation circuit includes:
When the dimming signal input to the external dimming terminal is a DC signal, a triangular wave signal is output,
The LED drive circuit according to claim 4, wherein a DC signal is output when the dimming signal input to the external dimming terminal is a PWM signal.   6. A converter for generating a DC voltage to be applied to the LED using a DC voltage or an AC voltage as a power supply input and driving the LED at a constant current based on the dimming signal. The LED driving circuit according to any one of the above.   A bias circuit that operates to apply a variable DC voltage to the input terminal of the filter circuit by changing a resistance value of the variable resistance element when a variable resistance element is connected to the external dimming terminal; The LED driving circuit according to claim 1, wherein:

Images