JP2011222759A - Led driver circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize LED constant-current driving that consumes less electricity than that of traditional one by eliminating a current limiting resistor which causes a traditional LED driver circuit to waste electricity.SOLUTION: An LED is controlled to keep constant a charge amount flowing through the LED per unit time. The LED is connected to a capacitor through a switch. The constant-current driving is performed by repeating charge and discharge in order to keep a voltage constant during charging the capacitor and to keep a potential of the capacitor constant during discharging the capacitor to the LED. A plurality of LEDs may be driven and controlled by switching according to the number of charge and discharge times. A booster circuit may be used together.

Description

The present invention relates to an LED driving circuit for driving an LED at a constant current, and more particularly to a technique for reducing power consumption when driving an LED at a constant current.

In recent years, LEDs (Light Emitting Diodes) have been used as light emitting elements for illumination and display in many electronic devices. In general, the characteristics of the forward voltage (hereinafter referred to as Vf) and forward current (hereinafter referred to as If) of an LED are such that If hardly flows until Vf becomes a certain value or more, and Vf is a certain value or more. It is known that If reaches a voltage of 1, If flows rapidly. When driving an LED having such If characteristics, when a constant voltage drive is performed by applying a constant voltage to both ends of the LED, the voltage applied to both ends of the LED varies depending on various circumstances. Then, the current that flows through the LED changes abruptly, causing variations in the brightness of the LED, and in the worst case, the maximum rated current of the LED may be exceeded, leading to destruction of the element. For this reason, the LED is generally driven by constant current driving so that the LED current is always constant.

A circuit as shown in FIG. 2 is known as a general method for driving an LED at a constant current (see, for example, Patent Document 1).
In FIG. 2, 21 is an LED, 22 is a current detection resistor that also serves as a protective resistor, and 23 is a MOS transistor that controls the forward current If of the LED by changing the resistance depending on the gate terminal. Reference numeral 24 denotes an LED power source, which serves as a current source for driving the LED 21. The LED power source 24 may be a battery or a power source that is made by boosting the battery voltage, but its configuration is not relevant in this conventional example.
Reference numeral 25 is a reference voltage source for performing constant current control of the forward current If, and is constituted by a known regulator or the like. A comparator 26 compares the reference voltage output from the reference voltage source 25 with the voltage across the current detection resistor 22 and outputs the output to the gate terminal of the MOS transistor 23.

  Next, the operation of the prior art will be described. In the circuit of FIG. 2, the forward current If flowing in the LED 21 is equal to the current flowing in the current detection resistor 22. For this reason, the current value flowing through the LED 21 can be monitored as the voltage value applied to both ends of the current detection resistor 22. This voltage value is compared with the voltage value of the reference voltage source 25 by the comparator 26, the gate voltage of the MOS transistor 23 is controlled so that the current flowing through the LED 21 becomes a desired amount of current, and the ON resistance of the MOS transistor 23 is controlled. By controlling the above, constant current driving of the LED 21 can be realized.

Japanese Utility Model Publication No. 7-39120 (FIG. 7)

However, when the LED is driven at a constant current with the circuit configuration as described above, there are the following problems. In the circuit configuration of FIG. 2, a current equal to the current flowing through the LED 21 flows through the current detection resistor 22 and the MOS transistor 23. For this reason, in particular, the current detection resistor 22 has a problem that a large amount of power is lost as heat and power efficiency is poor. The present invention eliminates such wasteful power and realizes a constant current driving method of LED with high power efficiency.

In order to solve the above problems, the present invention provides:
A power source, an LED, and a capacitor for discharging the LED;
In charge mode,
Charging the capacitor by connecting the power source and the capacitor;
In discharge mode
Switch means for connecting the charged capacitor and the LED, and causing the LED to discharge the charge of the capacitor;
A comparison circuit for comparing the voltage of the capacitor with a predetermined voltage;
A control circuit for controlling the switch means based on the comparison result of the comparison circuit 7 is provided.

The control circuit includes:
In the charging mode, the comparison circuit is
Comparing the capacitor voltage with a first predetermined voltage;
When detecting that the capacitor voltage has reached the first predetermined voltage,
Disconnect the power supply and the capacitor, then switch to discharge mode,
In the discharge mode, the comparison circuit is
Comparing the capacitor voltage with a second predetermined voltage lower than the first predetermined voltage;
If it is detected that the capacitor voltage has reached a second predetermined voltage,
The capacitor and the LED are disconnected and then switched to a charging mode.

A plurality of the capacitors;
A booster circuit for boosting the voltage of the power supply is configured by the plurality of capacitors,
In discharge mode
It is characterized in that the electric charge of the capacitor constituting the booster circuit is discharged to the LED.

A plurality of the LEDs, and
It has a counter that counts the number of times each LED is charged and discharged,
The control circuit charges and discharges these LEDs one by one,
If it detects that the counter has counted a predetermined number of times,
Instead of the LED being driven, charging / discharging driving of another LED is started.

  According to the present invention, constant current control is performed not by detecting current using a resistor but by detecting voltage across the capacitor. Therefore, there is no loss due to resistance, and highly efficient LED driving can be realized.

It is a block diagram which shows 1st Example of this invention. It is a block diagram which shows a prior art. It is an output waveform diagram (timing chart) of the first embodiment of the present invention. 2 is an equivalent circuit in the charging mode of FIG. 1, where (a) represents state I and (b) represents state II. FIG. 3 is an equivalent circuit in the discharge mode of FIG. 1, where (a) represents state III and (b) represents state IV. It is a block diagram which shows 2nd Example of this invention. 6 is an equivalent circuit in the charging mode of FIG. 6, where (a) represents state I and (b) represents state II. 6 is an equivalent circuit in the discharge mode of FIG. 6, where (a) represents state IV and (b) represents state III and state V. FIG. It is an output waveform figure (timing chart) of 2nd Example of this invention. It is a block diagram which shows 3rd Example of this invention. It is an output waveform figure (timing chart) of 3rd Example of this invention. It is a drive algorithm of 3rd Example of this invention.

[First Example: Basic Example]
An embodiment of the present invention will be described with reference to FIGS. 1 and 3-5. FIG. 1 is a configuration diagram of an LED driver according to the present invention, and FIG. 3 is a waveform diagram (timing chart) showing its operation. 4 and 5 are equivalent circuits of the LED driver of FIG. 1, FIG. 4 is a charge mode, and FIG. 5 is an equivalent circuit of a discharge mode.

First, the configuration of the first embodiment will be described with reference to FIG.
Reference numeral 1 denotes an LED, and reference numeral 2 denotes an LED power source, which is the same as the conventional LED 21 and LED power source 24 shown in FIG.
Reference numeral 9 denotes a capacitor serving as a current supply source when driving the LED, and one end thereof is grounded to the ground (GND).

Reference numerals 3, 4, 5, and 6 denote MOS transistors as switches, which are turned on when "H" is input to the gate.
MOS transistors 3 and 4 have their sources connected to terminals (point e) on a different side from the GND-connected terminal of capacitor 9. The drain of the MOS transistor 3 is connected to the anode terminal of the LED 1, and the drain of the MOS transistor 4 is connected to the LED power source 2.
Also,
A control signal c from a control circuit 8 described later is applied to the gate of the MOS transistor 4.
A control signal d from a control circuit 8 described later is applied to the gate of the MOS transistor 3.
Entered,
The MOS transistor 3 is used for opening / closing control between the capacitor 9 and the LED 1.
The MOS transistor 4 is used for opening / closing control between the capacitor 9 and the LED power source 2. The MOS transistors 3 and 4 constitute switch means for selecting a connection destination of the terminal e of the capacitor 9.

The MOS transistors 5 and 6 are connected between an output of a general reference power supply (not shown) that outputs reference voltages VREF1 and VREF2 having different values and a comparison circuit 7 described later.
VREF1 is set as the charge end voltage near the output voltage VLED of the LED power supply 2, and VREF2 is set as the discharge end voltage near GND.
Naturally, VREF1> VREF2.
At the gate of the MOS transistor 6, a control signal a from the control circuit 8 described later is
A control signal b from a control circuit 8 described later is applied to the gate of the MOS transistor 5.
Input and used for selection control of the reference voltages VREF1 and VREF2. The MOS transistors 5 and 6 constitute a selection circuit that selects a comparison partner of the potential at the point e in the comparison circuit 7 described later.

Reference numeral 7 denotes a comparison circuit, which compares the reference voltage VREF1 or VREF2 with the potential at the terminal e of the capacitor 9 and outputs the result g to the control circuit 8. The same circuit as the comparator 26 of the prior art may be used, or another comparison circuit may be used.
A control circuit 8 controls the entire system, inputs a charge / discharge clock signal f and a comparison output g, and outputs control signals a, b, c, and d.
The charge / discharge clock signal f is a signal for controlling the charge timing and the discharge timing, and is supplied from an external circuit (not shown).

Next, the operation of the present embodiment will be described with reference to FIGS. 1 and 3-5.
<Charging mode: I, II>
First, when the charge / discharge clock is L, the charging mode is set, and the control circuit 8 outputs H level to the gate terminals of the switch 6 and the switch 4 made of MOS transistors, and turns on the switch. At this time, the gate levels of the switch 5 and the switch 3 are L, and the switch 5 and the switch 3 remain off.
As a result, the comparison voltage VREF1 for charging is selected as the reference potential for comparison in the comparison circuit 7.
Further, the capacitor 9 is connected to the LED power source 2, while the anode end of the LED 1 is in an open state.
An equivalent circuit is shown in FIG.

The capacitor 9 is charged as time passes, and the potential at the point e of the capacitor 9 rises. When the potential at the point e of the capacitor 9 reaches the value of VREF1, the comparison circuit 7 outputs a signal g for terminating the charging to the control circuit 8 (section I).
The control circuit 8 receives a signal notifying the end of charging of the comparison circuit 7 and outputs L level to the gate of the switch 4 composed of a MOS transistor to stop the charging in order to stop the charging of the capacitor 9 (section). II). An equivalent circuit is shown in FIG.
The reason why the charging is stopped in the section II is that the power of the LED power source 2 is not wasted because it cannot be charged even if the capacitor 9 is further charged.

<Discharge mode: III, IV>
Thereafter, when the charge clock becomes H and the discharge mode is entered, the control circuit 8 outputs an L level to the gate terminal of the switch 6 to turn off the switch, while the switch 5 and the gate terminal of the switch 3 are formed of MOS transistors. On the other hand, the H level is output, and the switch 5 and the switch 3 are turned on.
Thereby, VREF2 for discharge is selected as the reference potential for comparison in the comparison circuit 7. As mentioned above, VREF2 is set lower than VREF1.
The capacitor 9 is connected to the anode end of the LED 1 and disconnected from the LED power source 2.
An equivalent circuit is shown in FIG.

At this time, the electric charge stored in the capacitor 9 is discharged to the LED 1 via the switch 3. The potential of the capacitor 9 decreases with time due to the discharge of electric charge, but when the potential of the capacitor 9 falls below the value of VREF2, the comparison circuit 7 outputs a signal for terminating the discharge to the control circuit 8. To do. The control circuit 8 receives a signal notifying the end of the discharge of the comparison circuit 7 and outputs L level to the gate of the switch 3 made of a MOS transistor to stop the discharge of the capacitor 9 and turns off the switch. An equivalent circuit is shown in FIG.
The reason why the discharge is stopped in the section IV is that the capacitor 9 already has almost no electric charge and the light emission efficiency is low. Therefore, the discharge is stopped to improve the charging efficiency at the next timing.

After that, when the charge / discharge clock becomes L and the charge mode is entered, the control circuit 8 outputs an L level to the gate of the switch 5 to turn off the switch, while charging the capacitor 9 again. Switch 4 is turned on. Thereafter, the LED is similarly driven by repeating charge / discharge for a desired LED lighting time.

Here, the amount of electric charge that flows in the LED 1 by charging and discharging the capacitor once is considered. Since the amount of charge flowing to LED1 is the difference between the amount of charge in the state where the potential of the capacitor 9 is VREF1 and the amount of charge in the state of VREF2, the amount of charge flowing to LED1 is Q and the capacitance of the capacitor 9 is C. , Q is expressed by Equation 1.
<Formula 1>
Q = C × (VREF1-VREF2)

  Since the current is defined by the amount of charge per unit time, the LED can be driven at a constant current by repeatedly charging and discharging the capacitor 9 based on a constant charge / discharge clock. In this circuit configuration, since only the switch 3 made of a MOS transistor is connected in series with the LED, there is no loss like the current detection resistor of the prior art, and the constant current drive with higher efficiency than the conventional circuit configuration. Can be done.

By the way, the sections II and IV are originally useless sections, and when the section reaches VREF1, the charging is stopped and switched to discharging, or when the section reaches VREF2, the discharging is stopped and switched to charging. The control is simpler, and the circuit becomes simple because the charge / discharge clock f is unnecessary.
However, when the above control is performed, there are the following problems.

The capacitance of the capacitor 9, the Vf of the LED 1, the VLED of the LED power source 2, VREF <b> 1, and VREF <b> 2 related to this control are not necessarily constant, but rather vary from product to product. Therefore, the charge / discharge cycle varies from product to product, resulting in variations in the appearance of the illumination.
In order to prevent this, charging / discharging control is performed in synchronization with the charging / discharging clock f having a constant period, so that charging / discharging with a constant period can be achieved and more stable constant current driving can be realized.
The cycle of the charge / discharge clock f is appropriately determined so that it is longer than the cycle determined by the control of only VREF1 and VREF2, and the sections II and IV are not so long.

[Second Embodiment: Constant Current Drive Using Boosted Power Supply]
Subsequently, a second embodiment will be described. In the second embodiment, a booster circuit is used as the LED driving power source.
The LED driving power source naturally needs a voltage equal to or higher than Vf necessary for flowing a desired If to the LED. However, even with a red LED, Vf is about 2.1 V and 1.5 V drive cannot be used as it is in a general analog type timepiece or the like. In addition, the Vf of the blue LED is as high as 3.5 V and cannot be directly driven by a watch battery.
In such a case, for example, the power supply voltage is boosted by a booster circuit, and then the current is suppressed by driving a constant voltage circuit (in the simplest case, resistance division), but the loss in the constant voltage circuit is large. It was useless.
The present embodiment is an example applied to the present invention when a power supply that requires such a booster circuit is used.

A second embodiment of the present invention will be described with reference to FIGS.
FIG. 6 is a block diagram of a second embodiment of the LED driver according to the present invention, and FIG. 9 is a waveform diagram (timing chart) showing its operation. 7 and 8 are equivalent circuits of the LED driver of FIG. 6, FIG. 7 is a charge mode, and FIG. 8 is an equivalent circuit of a discharge mode.

First, the configuration of the second embodiment will be described with reference to FIG.
Reference numeral 1 denotes an LED, which is the same as the LED of the first embodiment. In the second embodiment, the LED power source 2 that is not concerned with the configuration in the first embodiment is composed of MOS transistors 34, 39, 40, 41 as switches and capacitors 32, 33 having the same capacity in the second embodiment. It is a charge pump circuit.
Note that the boost source power supply of the charge pump circuit is the same power supply (VDD) as that of the comparison circuit 7 and the control circuit 8 described later.

A control signal d from the control circuit 8 described later is applied to the gate of the MOS transistor 34.
A control signal i from a control circuit 8 described later is applied to the gate of the MOS transistor 39.
A control signal j from a control circuit 8 described later is applied to the gate of the MOS transistor 40.
A control signal k from a control circuit 8 described later is applied to the gate of the MOS transistor 41.
Entered,
The MOS transistor 34 is used for switching control between the capacitor 33 and VDD.
The MOS transistor 39 is used for switching control between the capacitor 32 and VDD.
The MOS transistor 40 is used for switching control between the capacitor 33 and the capacitor 32.
The MOS transistor 41 is used for switching control between the capacitor 32 and the ground.
The MOS transistor 3 as a switch is used for opening / closing control of the anode terminal of the LED 1 and the output terminal (point e) of the LED power source 2, and a control signal l from a control circuit 8 described later is input to the gate of the MOS transistor. The

The MOS transistors 5 and 6 are connected between an output of a general reference power supply (not shown) that outputs reference voltages VREF1 and VREF2 having different values and a comparison circuit 7 described later.
At the gate of the MOS transistor 6, a control signal a from the control circuit 8 described later is
A control signal b from a control circuit 8 described later is applied to the gate of the MOS transistor 5.
Input and used for selection control of the reference voltages VREF1 and VREF2.

A comparison circuit 7 compares the reference voltage VREF1 or VREF2 with the potential at the terminal c of the capacitor 9 and outputs the result g to the control circuit 8. The same circuit as the comparator 26 of the prior art may be used, or another comparison circuit may be used.
A control circuit 8 controls the entire system. The charge / discharge clock signal f and the comparison output g are input, and control signals a, b, d, g, i, j, k, and l are output. The charge / discharge clock signal f is a signal for controlling the charge timing and the discharge timing, and is supplied from an external circuit (not shown).

<Charging mode: I, II>
First, when the charging clock is L, the charging mode is set, and the control circuit 8 outputs L level to the gate terminals of the MOS transistors 34 and 39 as switches to turn on the switches. At the same time, the control circuit 8 outputs an H level to the gate terminal of the MOS transistor 41 as a switch to turn on the switch. MOS transistors 40 and 3 remain off. At the same time, the control circuit 8 outputs an H level to the gate terminal of the MOS transistor 6 as a switch. As a result, the comparison voltage VREF1 for charging is selected as the reference potential for comparison in the comparison circuit 7. An equivalent circuit is shown in FIG.

In the state I, the capacitors 32 and 33 are charged with time, and when the potential at the point c of the capacitor 33 reaches the value of VREF1, the comparison circuit 7 sends a signal g for terminating the charging to the control circuit 8. Output. At this time, the potential at the point e is equal to the potential at the point c because the capacitors 32 and 33 have the same capacitance.
The control circuit 8 receives a signal notifying the end of charging of the comparison circuit 7 and sets the gate terminals of the MOS transistors 34 and 39 as switches to stop the charging of the capacitors 32 and 33 to the H level. An L level is output to the gate terminal of the MOS transistor 41 to turn off the switch. An equivalent circuit is shown in FIG.
The reason for providing this state II is the same as in the first embodiment.

<Discharge modes: III, IV, V>
Thereafter, when the charge clock becomes H and the discharge mode is entered, the control circuit 8 first outputs an L level to the gate terminal of the MOS transistor 40 as a switch to turn on the switch. At this time, the capacitors 32 and 33 are connected in series, and the potential at the point e, which is the same potential as the point c, is boosted to twice the potential at the point c. An equivalent circuit is shown in FIG. Here, the time required for the state III is a time required until sufficient boosting is performed, and is appropriately determined.

At the same time, the control circuit 8 outputs L level to the gate terminal of the MOS transistor 6 as a switch to turn off the switch, while outputting H level to the gate terminal of the MOS transistor 5 as a switch to turn on the switch. As a result, the comparison voltage VREF2 for charging is selected as the reference potential for comparison in the comparison circuit 7. VREF2 is set lower than VREF1.

Thereafter, the control circuit 8 outputs an H level to the MOS transistor 3 as a switch to turn on the switch, and the anode end of the LED 1 is connected to the point e which is the output end of the LED power source 2. An equivalent circuit is shown in FIG.

At this time, the electric charge stored in the capacitor 32 is discharged to the LED 1 through the MOS transistor 3 as a switch. The potential at the point e decreases with time due to the discharge of charges. At this time, the potential at the point c decreases in the same manner while maintaining one half of the potential at the point e. When the potential at the point c falls below the value of VREF2, the comparison circuit 7 outputs a signal for terminating the discharge to the control circuit 8. The control circuit 8 receives the signal notifying the end of the discharge of the comparison circuit 7 and outputs an L level to the gate terminal of the MOS transistor 3 as a switch to stop the discharge of the charge from the capacitor 32 to the LED 1 (state V). .

After that, when the charge / discharge clock becomes L and the charge mode is entered again, the control circuit 8 outputs H level to the gate terminal of the MOS transistor 40 as a switch, turns off the switch, and cancels the serial connection state of the capacitors 32 and 33. In order to charge the capacitors 32 and 33 again,
L level is output to the gate terminals of the MOS transistors 34 and 39 as switches to turn on the switches,
An H level is output to the gate terminal of the MOS transistor 41 as a switch to turn on the switch,
In order to select the comparison voltage VREF1 as a reference potential for comparison in the comparison circuit 7,
L level is output to the gate terminal of the MOS transistor 5 as a switch to turn off the switch,
An H level is output to the gate terminal of the MOS transistor 6 as a switch to turn on the switch.
Thereafter, the LED is driven by repeating the same charge / discharge for a desired LED lighting time.

The amount of charge Q that flows to the LED 1 by charging and discharging the capacitor once is expressed by Equation 2 where C is the capacitance of the capacitors 32 and 33.
<Formula 2>
Q = C × 2 (VREF1-VREF2)
Since the current is defined by the amount of charge per unit time, the LED can be driven at a constant current by repeatedly charging and discharging the capacitor based on a constant charge / discharge clock.

  Furthermore, in the second embodiment, a constant current drive circuit using a double boosting power supply can be realized by adding one capacitor and three MOS transistors to the first embodiment, so that the LED power supply can be realized with a simple system. Thus, the necessary booster circuit and constant current drive circuit can be realized at the same time, and the scale of the entire system can be reduced.

  Note that the boosted value by the charge pump circuit does not have to be a simple double boost, and may be boosted three times or more. What is necessary is just to select suitably considering the voltage value (VDD) of a pressure | voltage rise original power supply, and Vf of LED1 to drive. However, boosting to an unnecessarily multiple is itself a large power loss and should be set to an appropriate magnification.

  As described above, in the LED driving circuit using the conventional booster circuit, wasteful power consumption is generated such that the boosted LED power supply voltage value is increased to a desired voltage value using a resistor. However, in this embodiment, it is only necessary to adjust VREF1 and VREF2, and thus it is possible to avoid the above-described wasteful power consumption.

[Third embodiment: constant current drive in multiple channels]
Subsequently, a third embodiment will be described. The third embodiment is an embodiment that performs multi-channel driving. The driving of the LED in the electronic device is not limited to driving a single LED as in the first and second embodiments described above, and a plurality of LEDs may be driven simultaneously. In particular, using so-called RGB LEDs that combine red LEDs, blue LEDs, and green LEDs, the amount of light emitted by red, blue, and green LEDs can be controlled to express the color tone by combining the three primary colors of light. is there.
The present embodiment is an application example of the present invention to the case where such multi-channel driving is performed.

A third embodiment of the present invention will be described with reference to FIGS.
FIG. 10 is a configuration diagram of a third embodiment of the LED driver according to the present invention, and FIG. 11 is a waveform diagram (timing chart) representing the operation thereof.

  First, the configuration of the third embodiment will be described with reference to FIG. FIG. 10 shows an example in which the configuration of the first embodiment shown in FIG. 1 is modified for multi-channel driving. Therefore, the same components as those in FIG.

1 is replaced with MOS transistors 51, 52, 53 as switches in FIG. 10 showing the configuration of the third embodiment. .
A control signal d from the control circuit 8 is supplied to the gate of the MOS transistor 51.
A control signal i from the control circuit 8 is supplied to the gate of the MOS transistor 52.
The control signal j from the control circuit 8 is input to the gate of the MOS transistor 53,
The MOS transistor 51 is used for opening / closing control between the capacitor 9 and the LED 61.
The MOS transistor 52 is used for opening / closing control between the capacitor 9 and the LED 62.
The MOS transistor 53 is used for opening / closing control between the capacitor 9 and the LED 63.

Further, the signal d to the gate of the MOS transistor 51 is input to the counter 71, the signal i to the gate of the MOS transistor 52 is input to the counter 72, and the signal j to the gate of the MOS transistor 53 is input to the counter 73. The number of charge / discharge to each LED is counted.
The counter 71 is a counter that counts the number of discharges to the LED 61 in one frame.
The counter 72 is a counter that counts the number of discharges to the LED 62 within one frame.
The counter 73 is a counter that counts the number of discharges to the LED 63 in one frame.
The register 81 indicates the number of discharges to the LED 61 within one frame.
The register 82 indicates the number of discharges to the LED 62 within one frame.
The register 83 is a register that stores the number of discharges to the LED 63 in one frame.
The detection circuit 90 calculates the value of the counter 71 and the value of the register 81,
The values of the counter 72 and the register 82 are
Further, it is detected whether the values of the counter 73 and the register 83 match each other, and the result is output to the control circuit 8 as a signal k.
The values of the registers 81, 82, and 83 are connected to the MPU 100 such as a microcomputer and set by communication.

  As a matter of course, the LED appears bright when the number of times of charging / discharging per unit time is large, and dark when the number is small. Therefore, the color tone and brightness of each LED can be adjusted by defining the number of times of charging / discharging in each LED. This control is made possible by performing control using the counters 71, 72, 73 and the registers 81, 82, 83.

Here, one frame is a unit for controlling the lighting of each of the LEDs 61, 62, and 63 once as shown in FIG.
In addition to the signals used in the first embodiment, the control circuit 8 receives a frame clock l for performing frame control.
The counters 71, 72 and 73 are reset for each frame by the frame clock l.

  When the so-called RGB LED is driven as described above, the red LED, the blue LED, and the green LED correspond to any of the LEDs 61, 62, and 63, respectively, but the description will be continued without being particularly specified here.

  Next, the operation of this embodiment will be described. The operation principle of constant current driving of the LED in the present embodiment is the same as that of the first embodiment, and thus detailed description thereof is omitted. In the first embodiment, the discharge control to the LED is performed only by the MOS transistor 3 as a switch, but in this embodiment, the LED transistors 61, 62, and 63 are used by using the MOS transistors 51, 52, and 53, respectively. .

  In this embodiment, how to discharge the three LEDs 61, 62, 63 will be described with reference to FIG. The driving of the LED is started with the rise of the frame clock l input to the control signal 8. First, as in the first embodiment, when the charging clock is L, the charging mode is set, and the potential at the point e is charged to VREF1 (states I and II). After that, when the charge clock becomes H, the discharge mode is entered. First, the LED 61 is discharged, so that the control circuit 8 operates as a switch MOS until the potential at the point e drops from the value of VREF1 to the value of VREF2. The H level is output to the gate terminal of the transistor 51, and the switch is turned on to discharge (states III and IV). The pulse of the signal d input to the gate signal of the MOS transistor as the switch is also input to the counter 71 and counted as the number of discharges to the LED 61 for each pulse.

When the discharge to the LED 61 is repeated and the number of discharges to the LED 61 coincides with the value of the register 81, the detection circuit 90 recognizes to the control circuit 8 that the discharge to the LED 61 has been achieved a predetermined number of times. The signal k is output so as to be discharged to the LED 62. That is, in the next charge / discharge cycle, in the state III, the control circuit 8 outputs H level to the gate terminal of the MOS transistor 52 as a switch to turn it on, and discharges the LED 62. At this time, the pulse output from the control circuit 8 to the gate of the MOS transistor 52 as a switch is also input to the counter 72 and counted as the number of discharges to the LED 62 for each pulse.

  When the discharge to the LED 62 is repeated and the number of discharges to the LED 62 coincides with the value of the register 82, the detection circuit 90 recognizes that the control circuit 8 has achieved the discharge to the LED 61 a predetermined number of times, and from the next time. The signal k is outputted so as to be discharged to the LED 63. Similarly, in the next charge / discharge cycle, in the state III, the control circuit 8 outputs an H level to the gate terminal of the MOS transistor 53 as a switch to turn it on, and discharges the LED 63. At this time, the pulse output from the control circuit 8 to the gate terminal of the MOS transistor 53 as a switch is also input to the counter 73 and counted as the number of discharges to the LED 63 for each pulse.

  When the discharge to the LED 63 is repeated and the number of discharges to the LED 63 coincides with the value of the register 83, the detection circuit 90 recognizes that the number of discharges to the LED 63 has been achieved for the control circuit 8, and The signal k is output so as to stop the charge / discharge cycle until the next frame ends and the next frame ends.

  Here, the rest period in FIG. 11 will be described. The time required for charging / discharging with LED driving in the present invention varies depending on, for example, temperature and power supply voltage fluctuations. If the charge / discharge time becomes longer than usual, and the pulse driving for the number of times set before the end of the frame cannot be performed, the luminance and color tone cannot be kept constant. For this reason, when driving the LED in this embodiment, even if the charging / discharging time varies long, a pause period during which the pulse driving is not performed during the normal charging / discharging time so that the pulse driving can be performed as many times as desired. It is necessary to prepare in advance.

  The pause period ends, the next frame starts with the rise of the frame clock, the charge / discharge cycle stop state is canceled, and charging of the capacitor 9 is started again. At this time, the counts of the counters 71, 72, 73 are all reset, and the LED 61 starts discharging again. In this way, by controlling the number of discharges of each of R, G, and BLED to be constant for each frame, the desired color tone can be controlled.

  Here, the frame clock l will be described in detail. Consider a case where luminance control is performed in this embodiment. For example, when only RLED is used among RGBLEDs, luminance control is performed by controlling the number of times of RLED pulse driving within a certain time. In the present embodiment, when the minimum luminance is set, the number of times of pulse driving in one frame is set to one. At this time, the LED is turned off from the end of the pulse drive to the next pulse drive after the end of the frame. When the light-off state is sufficiently short, the LED appears to human eyes to have a constant brightness. However, when the light-off state is long (depending on the environment, approximately 10 to 12 mS or more) The eyes will flicker. Therefore, it is desirable that the frame clock has a period in which the flicker does not occur even with the minimum number of drive pulses.

The color tone control algorithm of this embodiment is shown in FIG.
In starting driving of the LED in this embodiment, first, the number of discharges within one frame for the LED 61, LED 62, and LED 63 is set in the register 81, the register 82, and the register 83, respectively (ST10).

Thereafter, as described above, pulse driving is performed on the LED 61 (ST11), and whether the pulse driving is performed a predetermined number of times is compared with the counter 71 and the register 81 (ST12). If the predetermined number of times is not reached (ST12: no) ), The pulse drive for the LED 61 is continued.
When the predetermined number of times has been reached (ST12: yes), the process shifts to pulse driving for the LED 62 (ST21).

In ST21, the LED 62 is pulse-driven, and the counter 72 and the register 82 are compared for a predetermined number of times (ST22). If the predetermined number of times is not reached (ST22: no), the LED 62 is pulse-driven. continue.
When the predetermined number of times has been reached (ST22: yes), the process proceeds to pulse driving for the LED 63 (ST31).

In ST31, pulse driving is performed on the LED 63, and whether the pulse driving is performed a predetermined number of times is compared with the counter 73 and the register 83 (ST32). If the predetermined number of times is not reached (ST32: no), continue.
When the predetermined number of times has been reached (ST32: yes), the process shifts to a pause period in which the LED is not driven (ST40).

The MPU 100 checks whether or not the driving condition (values of the registers 81, 82, and 83) should be changed when the pause period of ST40 ends and one frame ends (ST41). If it is to be changed (ST41: yes), ST10 The number of discharges is reset. If no change is required (ST41: no), the process proceeds to ST42 and it is determined whether or not the driving is terminated. If the driving is continued (ST42: yes), the process returns to ST11 and the driving of the LED 61 is started again. When the driving is finished (ST42: no), the driving is finished as it is.

  As described above, in this embodiment, by setting the number of pulse driving times of the LED 61, LED 62, and LED 63 in one frame, it is possible to perform luminance and color tone control using RGB LEDs or the like. The pulse driving of each LED is all constant current driving based on the first embodiment, and the LED can be driven with the color tone and the luminance kept constant.

In the present embodiment, the following modifications are possible.
<Modification>
(1) Number of counters and registers In FIG. 10, the counters 71, 72, and 73 and the registers 81, 82, and 83 are provided for the LEDs 61, 62, and 63, respectively, but the LEDs 61, 62, and 63 are sequentially driven exclusively. Therefore, there is no problem even if the structure uses one register and one counter in common. In this way, the number of counters and registers can be reduced, leading to downsizing and cost reduction of the driving circuit of this embodiment.
(2) Counter reset timing It has been described that the counters 71, 72, and 73 are reset by the frame clock l, but may be executed when each LED drive is switched. In particular, it is essential in a configuration that provides a counter.
(3) Register setting timing When the register is shared, it is configured so as to be set when each LED drive is switched, like the counter.
(4) Combined use of booster circuit As the LED power source 2, a booster circuit as in the second embodiment can be used. This is particularly effective when driving a blue LED.

1, 21, 61, 62, 63 LED
2, 24 LED power supply 3, 4, 5, 6, 23, 34, 39, 40, 41, 51, 52, 53 MOS transistor switch 7 Comparison circuit 8 Control circuit 9, 32, 33 Capacitor 22 Current detection resistor 25 Reference voltage Source 26 Comparator 71, 72, 73 Counter 81, 82, 83 Register 90 Detection circuit 100 MPU

Claims (4)

A power source, an LED, and a capacitor for discharging the LED;
In charge mode,
Charging the capacitor by connecting the power source and the capacitor;
In discharge mode
Switch means for connecting the charged capacitor and the LED, and causing the LED to discharge the charge of the capacitor;
A comparison circuit for comparing the voltage of the capacitor with a predetermined voltage;
An LED drive circuit comprising a control circuit for controlling the switch means based on a comparison result of the comparison circuit. The control circuit includes:
In the charging mode, the comparison circuit is
Comparing the capacitor voltage with a first predetermined voltage;
When detecting that the capacitor voltage has reached the first predetermined voltage,
Disconnect the power supply and the capacitor, then switch to discharge mode,
In the discharge mode, the comparison circuit is
Comparing the capacitor voltage with a second predetermined voltage lower than the first predetermined voltage;
If it is detected that the capacitor voltage has reached a second predetermined voltage,
The LED driving circuit according to claim 1, wherein the capacitor and the LED are disconnected and then switched to a charging mode. A plurality of the capacitors;
A booster circuit for boosting the voltage of the power supply is configured by the plurality of capacitors,
In discharge mode
3. The LED driving circuit according to claim 2, wherein the LED is configured such that the charge of the capacitor constituting the booster circuit is discharged to the LED. A plurality of the LEDs, and
It has a counter that counts the number of times each LED is charged and discharged,
The control circuit charges and discharges these LEDs one by one,
If it detects that the counter has counted a predetermined number of times,
4. The LED driving circuit according to claim 2, wherein charge / discharge driving of another LED is started instead of the LED being driven.

Images