JP5897938B2 - LED drive device - Google Patents

Description

  The present invention relates to an LED drive device that drives an LED (light emitting element), and more particularly to an LED drive device that maintains the accuracy of the current value even when the set value of the LED drive current is reduced.

  LEDs, particularly white LEDs, are used, for example, as backlights for mobile phone displays powered by lithium ion batteries, as backlights for displays in small personal computers, and the like.

  For example, as shown in FIG. 4, an LED drive device that drives an LED used for such a backlight is normally configured with an LED load circuit 10 in which a predetermined number of LEDs are connected in series and a set luminance. The LED drive circuit 20A that supplies a drive current to each LED in response to this, and the reference voltage generation circuit 30C that generates the reference voltage V2 and outputs the reference voltage V2 to the LED drive circuit 20A are provided.

  The LED drive circuit 20A has an NMOS transistor MN1 that supplies a drive current to the LED string of the LED load circuit 10, a reference voltage V2 input to the non-inverting input terminal, a source voltage V1 of the transistor MN1 input to the inverting input terminal, and an output. A signal at the terminal is output to the gate of the transistor MN1, and the operational amplifier OP1 that negatively controls the transistor MN1 so that the source voltage V1 becomes a voltage corresponding to the reference voltage V2, and between the source of the transistor MN1 and the ground The resistor R2 is connected.

  The reference voltage generation circuit 30C inputs the reference voltage V5 to the NMOS transistor MN3, the non-inverting input terminal, the source voltage Va of the transistor MN3 to the inverting input terminal, and outputs the signal at the output terminal to the gate of the transistor MN3. An operational amplifier OP2 for performing negative feedback control of the transistor MN3 so that the source voltage Va becomes a voltage corresponding to the reference voltage V5, and a brightness adjusting variable resistor Ra connected between the source of the transistor MN3 and the ground. The transistors MP1 and MP2 constituting the current mirror circuit so as to output a current I2 obtained by mirroring the drain current I1 of the transistor MN3, and a resistor R1 connected between the drain of the transistor MP2 and the ground, and the transistor MP2 The reference voltage described above at the common connection point of the drain of the resistor and the resistor R1 2 is adapted to generate.

となる。ここで、電圧Ｖ１は、

で求められる。ＶoffはオペアンプＯＰ１のオフセット電圧である。また、電圧Ｖ２は、

で求められる。ここで、

とする。ｎ３は抵抗Ｒ１とＲ２の抵抗比、ｎ２はトランジスタＭＰ１とＭＰ２のアスペクト比である。この結果、電流Ｉ_LEDは、式（１）〜（５）から、

となる。 In this LED driving device, the current I _LED flowing through the LED of the LED load circuit 10 is:

It becomes. Here, the voltage V1 is

Is required. Voff is an offset voltage of the operational amplifier OP1. The voltage V2 is

Is required. here,

And n3 is a resistance ratio of the resistors R1 and R2, and n2 is an aspect ratio of the transistors MP1 and MP2. As a result, the current I _LED is obtained from the equations (1) to (5).

It becomes.

In the equation (6), the values of the voltages Va, Voff, the resistance R2, and the ratios n2, n3 are fixed values. By adjusting the resistance value of the variable resistance Ra, the current I _LED that flows through the _LED is controlled. The brightness can be adjusted.

However, at this time, the term “Voff / R2” in the equation (6) is a fixed error factor. When the value of the resistor Ra is increased to reduce the current I _LED , this error factor “Voff / R2”. ”Is relatively large, resulting in deterioration in accuracy when realizing a small current I _LED .

  As an LED driving device that may be able to solve this problem, there is one shown in FIG. 5 (see, for example, Patent Document 1). This LED device includes a reference voltage generation circuit 30D in which the resistor R1 is replaced by an NMOS transistor MN4 having a fixed bias Vg applied to the gate in the LED driving device of FIG. 4, and the resistor R2 is connected to a plurality of parallel NMOS transistor groups MN2A. The replaced LED drive circuit 20B is configured, and in this LED drive circuit 20B, on / off of each transistor of the transistor group MN2A is controlled by the current control circuit 21.

  5, the total resistance value of the transistor group MN2A can be adjusted by switching the number of transistors to be turned on in the transistor group MN2A corresponding to the resistor R2 in FIG. 4 by the current control circuit 21. Therefore, if the total resistance value is adjusted in accordance with the adjustment of the variable resistor Ra, it is possible to prevent deterioration in accuracy due to the error factor.

JP 2010-135379 A

However, the LED driving device of FIG. 5 adjusts the number of transistors that are turned on in the transistor group MN2A composed of a plurality of transistors. In this method, since there is a boundary of switching bits, the total resistance value is adjusted linearly. In addition to the fact that the current I _LED flowing through the _LED cannot be linearly adjusted, the transistor group MN2A requires a large number of transistors, and a current control circuit that controls on / off of the transistors in the transistor group MN2A There is a problem that the configuration of 21 is complicated.

  An object of the present invention is to reduce the error factor included in the drive current when the drive current for driving the LED is reduced, to avoid a decrease in accuracy of the drive current, and to adjust the current flowing through the LED linearly. It is providing the made LED drive device.

To achieve the above object, according to a first aspect of the present invention, there is provided a first transistor for supplying a drive current from a drain to an LED load circuit, and a second transistor connected to a source of the first transistor. And a first operational amplifier that performs negative feedback control so that the first voltage appearing at the source of the first transistor becomes a voltage corresponding to the second voltage, and the value of the drive current flowing through the LED load circuit is set. A variable resistor, a third transistor for passing a second current corresponding to the first current flowing through the variable resistor, a drain connected to the third transistor and the drain, and the second voltage applied to the common connection point comprising a fourth transistor for generating, and a control section for controlling the third said to generate a voltage of the second and fourth transistor which varies in response to the first current, the control unit When said second current is changed, characterized by holding said second voltage by cooperation between said fourth transistor substantially constant.
According to a second aspect of the present invention, in the LED drive circuit according to the first aspect, the third voltage generated by the control unit controls the gates of the second and fourth transistors in common. And
According to a third aspect of the present invention, in the LED drive circuit according to the first or second aspect, the control unit is a fifth transistor in which a gate and a drain through which a third current corresponding to the first current flows are connected. The third voltage is generated at the drain of the fifth transistor .
According to a fourth aspect of the present invention, in the LED drive circuit according to the first or second aspect , the control unit performs negative feedback control of the fourth transistor so that the second voltage becomes a fixed fourth voltage. characterized Rukoto such from the second operational amplifier to be.

  According to the present invention, when the drive current for driving the LED is reduced, the error factor included in the drive current is also reduced, so that a reduction in the accuracy of the drive current can be avoided. Further, since the change in on-resistance of the second transistor is a linear change, the resolution of the set value of the drive current can be increased. Furthermore, the configuration is simplified.

It is a circuit diagram of the LED drive device of the 1st Example of this invention. It is a circuit diagram of the LED drive device of the 2nd Example of this invention. It is a circuit diagram of the LED drive device of the 3rd Example of this invention. It is a circuit diagram of the conventional LED drive device. It is a circuit diagram of another conventional LED drive device.

<First embodiment>
FIG. 1 shows an LED driving apparatus according to a first embodiment of the present invention. The LED driving device includes an LED load circuit 10 in which a predetermined number of LEDs are connected in series, an LED driving circuit 20 that drives each LED in accordance with a set luminance, and a reference voltage V2 to generate the LED driving circuit 20. And a reference voltage generation circuit 30 that outputs the reference voltage.

  The LED drive circuit 20 has an NMOS transistor MN1 that supplies a drive current to the LED string of the LED load circuit 10, a reference voltage V2 input to the non-inverting input terminal, a source voltage V1 of the transistor MN1 input to the inverting input terminal, and an output. A signal at the terminal is output to the gate of the transistor MN1, and the operational amplifier OP1 that negatively controls the transistor MN1 so that the source voltage V1 becomes a voltage corresponding to the reference voltage V2, and between the source of the transistor MN1 and the ground The drive current setting NMOS transistor MN2 is connected.

  The reference voltage generation circuit 30 inputs the reference voltage V5 to the non-inverting input terminal of the NMOS transistor MN3, inputs the source voltage Va of the transistor MN3 to the inverting input terminal, and outputs the signal of the output terminal to the gate of the transistor MN3. An operational amplifier OP2 for performing negative feedback control of the transistor MN3 so that the source voltage Va becomes a voltage corresponding to the reference voltage V5, and a brightness adjusting variable resistor Ra connected between the source of the transistor MN3 and the ground. The transistors MP1 and MP2 constituting the current mirror circuit so as to output a current I2 obtained by mirroring the drain current I1 of the transistor MN3, the NMOS transistor MN4 connected between the drain of the transistor MP2 and the ground, and the drain of the transistor MN3 Transistor MN2 in response to current I1 It is composed of a control unit 31 which generates a voltage V3 for controlling the gate of MN4, reference voltage V2 which is the common connection point of the drains of the transistor MN4 of the transistor MP2 is adapted to generate.

  In relation to the claims, the transistors MN1, MN2, MP2, and MN4 correspond to the first, second, third, and fourth transistors, respectively. The voltages V1, V2, V3, and V4 correspond to the first, second, third, and fourth voltages, respectively.

となる。ここで、Ｒds２はトランジスタＭＮ２のオン抵抗である。電圧Ｖ１は、

で求められる。ＶoffはオペアンプＯＰ１のオフセット電圧である。また、電圧Ｖ２は、

で求められる。Ｒds４はトランジスタＭＮ４のオン抵抗である。ここで、

とする。ｎ１はトランジスタＭＮ２とＭＮ４のアスペクト比、ｎ２はトランジスタＭＰ１とＭＰ２のアスペクト比である。 In this LED driving device, the current I _LED flowing through the LED of the LED load circuit 10 is:

It becomes. Here, Rds2 is the on-resistance of the transistor MN2. The voltage V1 is

Is required. Voff is an offset voltage of the operational amplifier OP1. The voltage V2 is

Is required. Rds4 is the on-resistance of the transistor MN4. here,

And n1 is an aspect ratio of the transistors MN2 and MN4, and n2 is an aspect ratio of the transistors MP1 and MP2.

となる。 As a result, the current I _LED is obtained from the equations (11) to (15).

It becomes.

In Expression (16), the values of Va, Voff, n1, and n2 are fixed values. By adjusting the resistance value of the variable resistor Ra, the current I _LED that flows through the _LED is controlled to adjust its luminance. Can do. The term “Voff / Rds2” in equation (16) becomes an error factor in the same manner as “Voff / R2” in equation (6), and causes a reduction in accuracy of the current I _LED .

However, in this embodiment, the gate voltage V3 of the transistor MN2 is controlled by the control unit 31, and when the value of the variable resistor Ra is set to a large value in order to reduce the current I _LED , the currents I1 and I2 become small. Since the voltage V3 decreases and the value of the on-resistance Rds2 of the transistor MN2 increases, the current I _LED decreases and the LED brightness decreases. Further, the ratio of the value is small influence of the error factors "Voff / Rds2" contained in the current I _LED is relatively small, avoids degradation in accuracy of current I _LED, to reduce the set value of the current I _LED However, the accuracy can be kept high. Further, since the change is a linear change, the resolution of the set value of the current I _LED can be increased.

At this time, the gate voltage V3 of the transistor MN4 also decreases and the on-resistance Rds4 also increases. Therefore, even if the current I2 decreases, the decreasing tendency of the reference voltage V2 is reduced or stabilized. The same applies to the voltage V1 linked to the reference voltage V2. When the voltage V3 decreases, as described above, the LED brightness decreases due to the decrease in the current I _LED due to the increase in the on-resistance Rds2 of the transistor MN2. That is, the change in luminance is made solely by the change in the on-resistance Rds2 of the transistor MN2.

<Second embodiment>
FIG. 2 shows an LED driving apparatus according to the second embodiment. In this embodiment, the control unit 31 in the LED driving device of FIG. 1 is connected to the PMOS transistor MP3 that is current-mirror connected to the transistor MP1, and the gate and drain that generate the voltage V3 according to the output current I3 of the transistor MP3. The reference voltage generation circuit 30A configured with the NMOS transistor MN5 is used. In relation to the claims, the transistor MN5 corresponds to a fifth transistor.

In the present embodiment, in order to decrease the LED current I _LED , the current I3 decreases when the resistance value of the variable resistor Ra is increased to decrease the current I1. For this reason, the drain voltage V3 of the transistor MN5 decreases . Therefore, the transistors MN2 and MN4 increase the on-resistances Rds2 and Rds4 of the transistors MN2 and MN4 in the same manner as described with reference to FIG. The decreasing tendency of the reference voltage V2 is reduced.

<Third embodiment>
FIG. 3 shows an LED driving device of the second embodiment. In this embodiment, the non-inverting input terminal of the control unit 31 in the LED driving device of FIG. 1 is connected to the common connection terminal of the drain of the transistor MP2 and the drain of the transistor MN4, and the fixed voltage V4 is input to the inverting input terminal. A reference voltage generation circuit 30B configured by an operational amplifier OP3 whose output terminal is connected to the gates of the transistors MN2 and MN4 is used.

In this embodiment, in order to reduce the current I _LED of the _LED , if the resistance value of the variable resistor Ra is increased to reduce the current I1, the current I2 also decreases. For this reason, the voltage V2 determined by the product of the current I2 and the on-resistance Rds4 of the transistor MN4 tends to decrease, but the operational amplifier OP3 and the transistor MN4 are subjected to negative feedback control so that V2 = V4. As a result, the output voltage V3 of the operational amplifier OP3 decreases, and the on-resistances Rds2 and Rds4 of the transistors MN2 and MN4 increase, and the same operation as described in FIG. 1 is performed. The reference voltage V2 is fixed at the voltage V4.

10: LED load circuit 20, 20A, 20B: LED drive circuit, 21: current control circuit 30, 30A, 30B, 30C, 30D: reference voltage generation circuit, 31: control unit

Claims (4)

A first transistor for supplying a driving current from the drain to the LED load circuit, a second transistor connected to the source of the first transistor, and a first voltage appearing at the source of the first transistor are A first operational amplifier that performs negative feedback control so that the voltage corresponds to the second voltage; a variable resistor that sets a value of a drive current that flows through the LED load circuit; and a first current that flows through the variable resistor. A third transistor through which a second current flows, a fourth transistor that has a drain connected in common to the third transistor and generates the second voltage at a common connection point thereof, and corresponds to the first current. generating a third voltage varying Te and a control unit for controlling said second and fourth transistors,
The control unit holds the second voltage substantially constant by cooperating with the fourth transistor when the second current is changed . The LED driving circuit according to claim 1,
The LED drive circuit, wherein the third voltage generated by the control unit controls the gates of the second and fourth transistors in common. The LED drive circuit according to claim 1 or 2,
The control unit includes a fifth transistor in which a gate and a drain through which a third current corresponding to the first current flows is connected, and the third voltage is generated at a drain of the fifth transistor. LED drive circuit characterized by the above. The LED drive circuit according to claim 1 or 2 ,
Wherein the control unit, the LED driving circuit in which the second voltage is characterized Rukoto a second operational amplifier for negative feedback controlling the fourth transistor so that the fourth voltage fixed.

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