JP2006258959A - Led driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain a long life of a light emitting diode (LED) by relaxing a reverse voltage to the LED in a matrix shape connection circuit. <P>SOLUTION: An LED driving circuit comprises at least; a first LED; a second LED; a power supply; a first driving circuit which connects the power supply and the first LED; a second driving circuit which connects the power supply and the second LED; and a ground side driving circuit which is common for the first LED and the second LED. When the first driving circuit is OFF and the second driving circuit is ON, a first compensation voltage of a smaller voltage than the operation voltage of the first LED is applied to the first LED, and when the second driving circuit is OFF and the first driving circuit is ON, a second compensation voltage of a smaller voltage than the operation voltage of the second voltage is applied to the second LED in the LED driving circuit. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an LED driving circuit. Specifically, the present invention relates to a circuit for driving LEDs used in a display or the like and connected in a matrix.

  In recent years, LEDs have been used as light sources for displays due to advantages such as high brightness and long life. The LEDs of red, green, blue, etc. constituting the display are connected in a matrix, and the light emission of each LED is controlled by time division control to provide an image.

Among LEDs that make up a display, blue LEDs or green LEDs are made of GaN-based compound semiconductors, so the operating voltage is high. When a voltage is applied for light emission, a reverse voltage is applied to other LEDs due to the problem of the circuit configuration of the matrix connection. Cost and burden. As a result, other LEDs may be deteriorated.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to reduce the reverse voltage to other LEDs in a matrix-like connection circuit and to extend the life of the LEDs.

In order to achieve the above object, the present invention has the following configuration. That is,
At least a first LED, a second LED, a power source, a first drive circuit connecting the power source and the first LED, a second drive circuit connecting the power source and the second LED, and the first 1 LED and a ground side drive circuit common to the second LED, wherein the first drive circuit is off and the second drive circuit is on, A first compensation voltage lower than the operating voltage of the first LED is applied to the first LED, the second drive circuit is off, and the first drive circuit is on In some cases, the LED driving circuit is characterized in that a second compensation voltage lower than the operating voltage of the second LED is applied to the second LED.

  With the above configuration, when the first drive circuit is turned on for the purpose of light emission and a reverse voltage is applied to the second LED due to the voltage applied to the first LED, a voltage is applied to the second LED in the forward direction. Is applied. That is, a forward voltage is applied in a direction that cancels the reverse voltage. Since the forward voltage is lower than the operating voltage of the second LED, no light is emitted. Therefore, by applying a forward voltage equal to or lower than the operating voltage to the second LED, the reverse voltage can be relaxed and the burden can be reduced without emitting light from the second LED. Therefore, the lifetime of the LED can be extended.

Hereinafter, the components of the LED drive circuit according to the present invention will be described in detail.
(1st LED, 2nd LED, 3rd LED)
The first LED, the second LED, or the third LED is an LED constituting a display, and a blue LED, a green LED, and a red LED are used. Each LED operates by being supplied with a voltage from the power supply side. The blue LED and the green LED are made of a GaN-based compound semiconductor, and the operating voltage is 3.0 to 3.5V. The red LED is made of a GaAs-based or GaP-based compound semiconductor and has an operating voltage of 1.8 to 2.5V.
Each LED is connected in a matrix, and light emission of each LED is controlled by a switching element such as an FET or a transistor. Blue, green, or red LEDs are arranged in a predetermined order on the display, and an image is provided on the display by controlling the switching circuit in a time-sharing manner.

(First drive circuit, second drive circuit, third drive circuit, ground side drive circuit)
The first drive circuit, the second drive circuit, or the third drive circuit is connected between the power source and the first LED, the second LED, or the third LED, and controls each LED. The configuration of each drive circuit is not particularly limited. For example, a switching element such as an FET or a transistor is used.
Each LED is connected in a matrix, and the light emission of each LED is controlled by controlling the first drive circuit, the second drive circuit, or the third drive circuit in a time-sharing manner.

The ground side drive circuit is connected to the cathode side of each LED, and controls the light emission of each LED together with the first drive circuit, the second drive circuit, or the third drive circuit. Although the configuration of the ground side drive circuit is not particularly limited, for example, a switching element such as an FET or a transistor is used. A constant current circuit can also be used.
When both the first drive circuit and the ground side drive circuit are on, current flows from the power source to the first drive circuit, the first LED, the ground side drive circuit, and the ground, and the first LED emits light To do. Similarly, the second LED or the third LED emits light when both the second drive circuit or the third drive circuit and the ground side drive circuit are on.
On the other hand, when the first drive circuit is on and the ground side drive circuit is off, no current flows between the first LED and the ground, so the first LED does not emit light. For this reason, the voltage applied to the first LED from the power supply is the reverse voltage of the second LED.
FIG. 4 is a circuit diagram of a conventional LED driving circuit 200. When the FET 3a is on and the transistor 5a is off, 5V is applied as it is from the power source 2 to the connection point A1. This directly becomes the reverse voltage of the LED 4b, and places a burden on the LED 4b.

(First compensation voltage, second compensation voltage, third compensation voltage)
The first compensation voltage, the second compensation voltage, or the third compensation voltage applies a forward voltage lower than the operating voltage of the LED in order to relax the reverse voltage applied to the LED. By applying a forward voltage lower than the operating voltage, the reverse voltage can be relaxed without causing the LED to emit light.
The timing for applying each compensation voltage is not particularly limited. Therefore, a voltage may be applied constantly, or a compensation voltage may be applied to an LED connected to another off drive circuit when at least one drive circuit is on.
Each power source (voltage source) of the first compensation voltage, the second compensation voltage, or the third compensation voltage may be provided with a separate power source, or a power source used for driving the LED may be diverted. Good.

With reference to FIG. 1, a description will be given of a reverse voltage mitigating action when a compensation voltage is applied to an LED when two LEDs are connected to the ground side drive circuit.
When the first drive circuit (FET 3a) is on and the ground side drive circuit (transistor 5a) is off (when the first LED (LED 4a) does not emit light), the supply voltage (5V) remains at the connection point A1. Applied to the reverse voltage of the second LED (LED 4b). Here, when the second compensation voltage (2.5 V) is applied to the second LED as a forward voltage equal to or lower than the operating voltage, the reverse voltage can be relaxed without causing the second LED to emit light. Similarly, when the second drive circuit is on and the ground side drive circuit is off, the reverse voltage applied to the first LED is alleviated.
On the other hand, when the first drive circuit is on and the ground-side drive circuit is on (when the first LED emits light), a voltage obtained by subtracting the operating voltage of the first LED from the supply voltage at the connection point A1 It takes. This voltage is applied as a reverse voltage to the second LED. Here, when the second compensation voltage is applied to the second LED as a forward voltage equal to or lower than the operating voltage, the reverse voltage can be relaxed without causing the second LED to emit light. Similarly, when the second drive circuit is on and the ground side drive circuit is on, the reverse voltage applied to the first LED is reduced.
This is because the voltage value of the difference between the reverse voltage applied to the first LED and the first compensation voltage is smaller than the operating voltage value of the first LED, or the reverse voltage applied to the second LED. When the voltage value of the difference between the second compensation voltage and the second compensation voltage is smaller than the operating voltage value of the second LED, it means that the reverse voltage is relaxed without causing the LED to emit light.

Specifically, the operating voltage of the LED used in the LED drive circuit is 3.0V to 3.5V for a blue LED or a green LED, and the power supply voltage supplied to the LED is 5V. In this case, a compensation voltage of 2.0V to 2.8V is used.
When the first drive circuit is on and the ground side drive circuit is off (when the first LED does not emit light), 5 V is applied as a reverse voltage from the connection point A1 to the second LED. Since the compensation voltage of 2.0V to 2.8V is applied to the LED of No. 2 in the forward direction, the voltage of 2.2V to 3.0V obtained by subtracting 2.0V to 2.8V from 5V is the second voltage as the reverse voltage. It will be applied to the LED. In addition, the rated value of the reverse voltage resistance of the normal LED is about 5V, and the reverse voltage is relaxed to about half.
On the other hand, when the first drive circuit is on and the ground-side drive circuit is on (when the first LED emits light), the reverse voltage applied from the connection point A1 to the second LED is as follows. .
Although 5 V is supplied from the power supply, 3.0 to 3.5 V is applied as the operating voltage of the LED, so the reverse voltage at the connection point A1 is 1.5 V to 2.0 V. However, since a forward voltage of 2.0 V to 2.8 V is applied to the second LED as a compensation voltage, the reverse voltage applied to the second LED is (1.5 V to 2.0 V) − ( 2.0V to 2.8V). That is, the reverse voltage with respect to the second LED is relaxed or eliminated.

When the LED used in the LED driving circuit is a red LED, the operating voltage is about 2.0V, and the power supply voltage supplied to the LED is 4.5V. In this case, a compensation voltage of 2.0V to 2.8V is used.
When the first drive circuit is on and the ground side drive circuit is off (when the first LED does not emit light), 4.5V is applied as a reverse voltage from the connection point A1 to the second LED. Since the compensation voltage 2.0V to 2.8V is applied to the second LED in the forward direction, the voltage 1.7V to 2.5V obtained by subtracting 2.0V to 2.8V from 4.5V is the reverse voltage. As a result, the second LED is applied. That is, the reverse voltage is relaxed.
On the other hand, when the first drive circuit is on and the ground-side drive circuit is on (when the first LED emits light), the reverse voltage applied from the connection point A1 to the second LED is as follows. .
Although 4.5V is supplied from the power source, 2V is applied as the operating voltage of the first LED, so the reverse voltage at the connection point A1 is 2.5V. However, since a forward voltage of 2.0 V to 2.8 V is applied as a compensation voltage to the second LED, the reverse voltage applied to the second LED is 2.5 V− (2.0 V to 2 .8V). That is, the reverse voltage is relaxed or eliminated.

Next, with reference to FIG. 3, a description will be given of the action of relaxing the reverse voltage when the number of LEDs connected to the ground side drive circuit is three and the compensation voltage is applied to the LEDs.
When one drive circuit is on and the remaining drive circuits are off, the reverse voltage relaxation action is as follows. For example, the first drive circuit (FET 3a) is turned on, the ground side drive circuit (transistor 5a) is turned on, the first LED (LED 4a) emits light, and the second drive circuit (FET 3b) and the third drive circuit ( When the FET 3x) is off, a reverse voltage is applied to the second LED (LED 4b) and the third LED (LED 4x). Therefore, a forward voltage lower than the operating voltage of the LED is applied to the second LED (LED 4b) and the third LED (LED 4x) from the power source 6b and the power source 6x, thereby relaxing the reverse voltage.
The first drive circuit (FET 3a) is turned on, the ground side drive circuit (transistor 5a) is turned off, and the first LED (LED 4a) does not emit light, so that the second drive circuit (FET 3b) and the third drive circuit (FET 3x) ) Is off, a reverse voltage is applied to the second LED (LED 4b) and the third LED (LED 4x). Therefore, a forward voltage lower than the operating voltage of the LED is applied to the second LED (LED 4b) and the third LED (LED 4x) from the power source 6b and the power source 6x, thereby relaxing the reverse voltage.

When the two drive circuits are on and the remaining drive circuits are off, the reverse voltage mitigation action is as follows. For example, the first drive circuit (FET 3a) and the second drive circuit (FET 3b) are turned on, the ground side drive circuit (transistor 5a) is turned on, and the first LED (LED 4a) and the second LED (LED 4b) emit light. When the third drive circuit (FET 3x) is off, a reverse voltage is applied to the third LED (LED 4x). Therefore, a forward voltage lower than the operating voltage of the third LED (LED 4x) is applied to the third LED (LED 4x) from the power source 6x, thereby relaxing the reverse voltage.
The first drive circuit (FET 3a) and the second drive circuit (FET 3b) are on, the ground side drive circuit (transistor 5a) is off, and the first LED (LED 4a) and the second LED (LED 4b) do not emit light. Even when the third drive circuit (FET 3x) is off, a reverse voltage is applied to the third LED (LED 4x). Therefore, a forward voltage lower than the operating voltage of the third drive circuit (LED4x) is applied to the third LED (LED4x) from the power supply 6x, thereby relaxing the reverse voltage.

FIG. 1 shows a circuit diagram of an LED drive circuit 1 used in a display which is an embodiment of the present invention. The LED driving circuit 1 is connected to a power source 2 by an FET 3 (referring to an FET 3a, an FET 3b, and so on) that controls the LED 4 (referring to LEDs 4a, LED 4b, LED 4c, and LED 4d). A transistor 5 for controlling a current flowing from the cathode side to the ground side (referred to as a transistor 5a and a transistor 5c, hereinafter the same) is connected. Further, a power source 6 (referred to as a power source 6a and a power source 6b; the same applies hereinafter) for applying a compensation voltage is connected to the anode side of the LED 4. The resistor 7 (refers to the resistor 7a and the resistor 7b. The same applies hereinafter) is connected between the power source 6 and the anode of the LED 4. This is because the wiring from the drain of the FET 3 to each LED 4 is long and electromagnetically floats, so that the impedance of the wiring is lowered and stabilized by connecting the resistor 7. The resistor 8 connected between the cathode side of the LED 4 and the collector side of the transistor 5 is connected to prevent an overcurrent from flowing through the LED 4 when the FET 3 and the transistor 5 are on. A constant current circuit may be inserted in place of the resistor 8.
The FET 3 and the transistor 5 are controlled by a control circuit (not shown). In accordance with a predetermined procedure, the control circuit turns on or off the light emission of the LED by time-sharing the FET 3 or the transistor 5 and provides an image on the display.

Next, voltage applied to the LED 4b and the LED 4d when the LED 4a emits light will be described.
First, the circuit operation for causing the LED 4a to emit light is as follows. The control circuit turns on the FET 3a and the transistor 5a. As a result, a current flows to the ground through the power source 2, the FET 3a, the LED 4a, the resistor 8, and the transistor 5a, and the LED 4a emits light.
At this time, a voltage obtained by subtracting the operating voltage of the LED 4a from the supply voltage 5V of the power source 2 is applied to the connection point A1. When a blue LED is used as the LED 4a, the operating voltage is about 3.5V, so 1.5V is applied to the connection point A1. This becomes a reverse voltage with respect to LED4b, and will place a burden. However, since 2.5V is applied as a forward voltage to the LED 4b from the power source 6b, the reverse voltage applied to the LED 4b is 1.5V−2.5V = −1V, and the reverse voltage applied to the LED 4a is eliminated. . Further, since the forward voltage applied to the LED 4b is an LED operating voltage of 3.5 V or less, no light is emitted.
On the other hand, when the LED 4c is not emitting light, that is, when the FET 3a is on and the transistor 5c is off, the voltage applied to the LED 4d is as follows. Since the transistor 5c is off and the LED 4c is not emitting light, the supply voltage 5V of the power supply 2 is directly applied to the connection point A2. That is, 5V is applied as a reverse voltage from the connection point A2 to the LED 4d side. Here, since 2.5V is applied to the LED 4d as a forward voltage from the power source 6b, the reverse voltage is relaxed to 5V-2.5V = 2.5V.

As described above, the reverse voltage is eliminated or alleviated by applying a forward voltage lower than the operating voltage to the LED. That is, the reverse voltage applied to the second LED when the first LED is emitting light is eliminated. In addition, the reverse voltage applied to the second LED is relaxed when the first LED is not emitting light.
Therefore, the deterioration of the LED can be prevented and the life can be extended.

FIG. 2 shows a circuit diagram of an LED drive circuit 10 used in a display which is another embodiment of the present invention. The same elements as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.
As a power source for supplying the compensation voltage, the existing power source 2 is divided by the resistors 12 and 13, and the voltage is supplied to the LED 4a using the operational amplifier 11a. Thereby, the compensation voltage can be supplied to the LED 4a using the existing power source 2. By using the operational amplifier 11a, a voltage can be supplied as a voltage source with a low output impedance, and the state where the wiring from the drain of the FET 3 to each LED 4 is electromagnetically floated is lowered by reducing the impedance of the wiring together with the resistor 7. It can be made.

  Similar to the first embodiment, also in the LED drive circuit 10, the reverse voltage to other LEDs can be reduced or eliminated. Therefore, the deterioration of the LED can be prevented and the life can be extended. Furthermore, since the existing power supply 2 can be used in the second embodiment, it is not necessary to separately prepare a power supply for supplying a compensation voltage.

FIG. 3 shows a circuit diagram of an LED drive circuit 100 used in a display which is another embodiment of the present invention. The same elements as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.
The light emission is controlled by using three LEDs connected to the ground side drive circuit. The LED 4x and the FET 3x for controlling the LED 4x are newly connected between the power source 2 and the connection point Ax. Further, a power source 6x for applying a forward voltage to the LED 4x is connected.

In the LED drive circuit 100, when at least one drive circuit on the power supply side is on, a forward voltage is applied to the LED connected to the other off drive circuit, and the reverse voltage applied to the LED is relaxed. .
That is, for example, when the FET 3a is on, the transistor 5a is on and the LED 4a emits light, and the FET 3b and the FET 3x are off, a reverse voltage is applied to the LED 4b and LED 4x. Therefore, a forward voltage lower than the operating voltage of the LED is applied to the LED 4b and LED 4x from the power source 6b and the power source 6x to alleviate the reverse voltage.
Even when the FET 3a is on, the transistor 5a is off and the LED 4a does not emit light, and the FET 3b and the FET 3x are off, a reverse voltage is applied to the LED 4b and the LED 4x. Therefore, a forward voltage lower than the operating voltage of the LED is applied to the LED 4b and LED 4x from the power source 6b and the power source 6x to alleviate the reverse voltage.

When a plurality of LEDs emit light, for example, when the FET 3a and FET 3b are on, the transistor 5a is on and the LEDs 4a and 4b emit light, and the FET 3x is off, the LED 4x has a lower operating voltage than the power supply 6x than the LED 4x. Apply forward voltage and relax reverse voltage.
Even when the FET 3a and FET 3b are on, the transistor 5a is off, the LED 4a and LED 4b do not emit light, and the FET 3x is off, a forward voltage lower than the operating voltage of the LED 4x is applied to the LED 4x from the power source 6x. To do.

As described above, as in the first embodiment, even if three LEDs are connected to the ground side drive circuit, the forward voltage is applied to the LEDs connected to the off drive circuit, thereby reducing the reverse voltage. Can do.
In addition, by using the LED drive circuit 100, an LED drive circuit with three colors of a blue LED, a green LED, and a red LED can be realized. In this case, the power supply voltage of the blue LED and the green LED may be set to 5V, and the power supply voltage of the red LED may be set to 4V.

  The present invention is not limited to the description of the embodiments and examples of the invention described above. Various modifications are also included in the present invention as long as those skilled in the art can easily conceive without departing from the scope of the claims.

FIG. 1 shows an LED drive circuit used in the first embodiment. FIG. 2 shows an LED drive circuit used in the second embodiment. FIG. 3 shows an LED driving circuit used in the third embodiment. FIG. 4 shows a conventional LED driving circuit.

Explanation of symbols

1 10 100 200 LED drive circuit 2 6 Power supply 3a 3b 3x FET
4a 4b 4x LED
5a 5c transistor

Claims (5)

At least a first LED, a second LED, a power source, a first drive circuit connecting the power source and the first LED, a second drive circuit connecting the power source and the second LED, and the first An LED drive circuit comprising one LED and a ground side drive circuit common to the second LED,
When the first drive circuit is off and the second drive circuit is on, a first compensation voltage having a voltage smaller than the operating voltage of the first LED is applied to the first LED. And
When the second drive circuit is off and the first drive circuit is on, a second compensation voltage having a voltage smaller than the operating voltage of the second LED is applied to the second LED. ing,
An LED driving circuit characterized by that.   In the LED driving circuit, a voltage value of a difference between the reverse voltage applied to the first LED and the first compensation voltage is smaller than an operating voltage value of the first LED, and the second LED 2. The LED drive circuit according to claim 1, wherein a voltage value of a difference between a reverse voltage applied to the LED and the second compensation voltage is smaller than an operating voltage value of the second LED. . At least a first LED, a second LED, a third LED, a power source, a first drive circuit connecting the power source and the first LED, and a second drive connecting the power source and the second LED LED drive comprising: a circuit, a third drive circuit connecting the power source and the third LED, and a ground side drive circuit common to the first LED, the second LED, and the third LED A circuit,
A first compensation voltage is applied to the first LED when the first drive circuit is off and at least one of the second drive circuit and the third drive circuit is on. ,
A second compensation voltage is applied to the second LED when the second drive circuit is off and at least one of the first drive circuit and the third drive circuit is on. ,
A third compensation voltage is applied to the third LED when the third drive circuit is off and at least one of the first drive circuit and the second drive circuit is on. ,
An LED driving circuit characterized by that.   The LED driving circuit according to claim 1, wherein the first LED and the second LED are made of a GaN-based compound semiconductor.   5. The LED drive circuit according to claim 1, wherein a value of the first compensation voltage, the second compensation voltage, or the third compensation voltage is 2.0 V to 2.8 V. 6. .

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