JP2010109006A - Circuit for driving light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To generate a minimum step-up voltage with a simple circuit configuration and to drive light emitting elements such as LEDs connected in parallel with one another at the same luminance. <P>SOLUTION: The output voltage VOUT of a step-up part 40 is supplied to LED lines 1a, 1b, and the LED lines 1a, 1b are driven at constant currents by constant current parts 10a, 10b connected in series to thereto, respectively. The voltages of nodes N1a, N1b are detected by voltage detection parts 20a, 20b, and output to nodes N3a, N3b, respectively. The node N3a, N3b are connected to a node N4, in common, and a lower voltage within the voltages of the nodes N3a, N3b is output from the node N4. The voltage of the node N4 is applied to a voltage control part 30, and a control voltage VC in accordance with the difference from a source voltage of a PMOS 33 is generated to control an output voltage VOUT of the step-up part 40. By this feedback loop, the output voltage VOUT is controlled so that the lower voltage within the voltages of the nodes N1a, N1b is set equal to the source voltage of the PMOS 33. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light emitting element driving circuit that drives a plurality of light emitting elements connected in parallel so as to have substantially the same luminance.

  In a liquid crystal display (hereinafter referred to as an LCD) of a portable device such as a cellular phone, a light emitting element such as a light emitting diode (hereinafter referred to as an LED) is used as a backlight. A cellular phone or the like often includes a booster circuit for turning on an LED due to power supply restrictions. Also, it is common to drive a plurality of LEDs connected in parallel. In such a case, it is necessary to cause a plurality of LEDs to emit light with the same luminance, and for this purpose, it is necessary to drive each LED with the same constant current.

  However, since the forward voltage varies among LEDs (particularly white LEDs), it is necessary to determine the output voltage of the booster circuit assuming the worst state. For this reason, the LED is driven at a voltage higher than necessary, and wasteful power is often consumed. On the other hand, the output voltage of the booster circuit is low, and there is a case where sufficient luminance cannot be obtained with some LEDs and image quality deteriorates.

  In order to avoid such a problem, the following patent document 1 detects the LED having the lowest potential by comparing the terminal potential of each LED driven by a constant current circuit with an LED terminal monitoring circuit. The LED lighting control device is configured such that the analog signal selection circuit selects the voltage state of the corresponding LED based on the above and controls the output voltage of the booster circuit based on the selected voltage state.

  With such a configuration, the booster circuit performs the boosting operation based on the LED having the lowest voltage state among the plurality of LEDs, so that the plurality of LEDs can be driven reliably and at the minimum necessary voltage. It can be driven.

JP 2005-116199 A

  However, the LED lighting control device of Patent Document 1 is based on the LED terminal monitoring circuit for detecting the LED having the lowest voltage state among the plurality of LEDs, and the corresponding LED based on the detection result of the LED terminal monitoring circuit. An analog signal selection circuit for selecting the voltage state is required. In particular, the LED terminal monitoring circuit has a number of voltage comparators for detecting the lowest LED terminal potential, and a logic for identifying the LED having the lowest terminal voltage based on the comparison result of these voltage comparators. It consists of a circuit. For this reason, when the number of driven LEDs increases, there is a problem that the scale of the LED terminal monitoring circuit becomes extremely large. An object of the present invention is to provide a light emitting element driving circuit capable of generating a minimum boosted voltage with a simple circuit configuration and driving light emitting elements connected in parallel with the same luminance.

In order to achieve the above object, a light emitting element driving circuit according to the present invention is a light emitting element driving circuit for driving a plurality of light emitting elements connected in parallel, and is connected in series between each light emitting element and a common potential. A plurality of constant current portions that pass a constant current to the light emitting element, and a detection node for outputting a detection voltage, respectively, and detects a voltage at a connection point between the light emitting element and the constant current portion. A plurality of voltage detection units that output the detection voltage from the detection node, a common node to which the detection nodes of the voltage detection units are connected in common, and a difference between a reference voltage and the voltage of the common node A voltage control unit that outputs a control voltage; and a boosting unit that boosts an input voltage according to the control voltage and supplies it as a drive voltage for the plurality of light emitting elements,
The drive voltage output from the boosting unit is controlled by the control voltage so that the voltage of the common node becomes equal to the reference voltage.

  Here, each of the constant current units has a first transistor and a resistor connected in series between the light emitting element and a common potential, and the current flowing through the resistor becomes the constant current. Each voltage detection unit includes a second transistor having a source electrode connected to the detection node and a drain electrode connected to the common potential; and configured to control a conduction state of the first transistor. The second transistor is configured to control a conduction state so that a voltage based on a voltage at a connection point between the light emitting element and the constant current unit is output from the source electrode, and the voltage control unit includes a source A third transistor in which the electrode is pulled up, the drain electrode is connected to the common potential, a reference voltage is applied to the gate electrode and the reference voltage is output from the source electrode; and the reference voltage and the common voltage It can be constituted by an operational amplifier for outputting the control voltage in accordance with the difference between the voltage of the node.

  The reference voltage is set to the same value as a voltage obtained by adding a voltage applied to the on-resistance of the first transistor to a voltage generated in the resistor when the constant current flows through the resistor of the constant current portion. Can do. The light emitting element can be a white LED.

According to the present invention, a plurality of constant current portions that allow a constant current to flow in light emitting elements connected in parallel, and a plurality of voltages that are detected from voltages at connection points between the light emitting elements and the constant current portions and output from the respective detection nodes. A voltage control unit that outputs a control voltage corresponding to a difference between a reference voltage and a voltage of a common node to which a detection node of each voltage detection unit is commonly connected, and an input voltage according to the control voltage And a boosting unit that boosts the voltage and supplies the driving voltage to the light emitting element.
Thus, the lowest voltage among the voltages of the plurality of detection nodes is output to the common node, and the drive voltage output from the boosting unit is controlled so that the voltage of the common node becomes equal to the reference voltage. Therefore, the present invention has an effect that it is possible to generate the minimum necessary driving voltage with a simple circuit configuration and to drive the light emitting elements connected in parallel with the same luminance.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
As shown in FIG. 1, the LED driving circuit of the present invention is configured such that the same constant currents Ia and Ib are caused to flow through the LED rows 1a and 1b in which a plurality of white LEDs are connected in series as light emitting elements. The white LED is driven to emit light with the same luminance.

  The boosted output voltage VOUT is commonly supplied to the anode side of each LED row 1a, 1b, and the cathode side is connected to nodes N1a, N1b, respectively. Further, the nodes N1a and N1b are connected to a common potential via constant current portions 10a and 10b for flowing constant currents Ia and Ib, respectively.

  The constant current units 10a and 10b have the same circuit configuration, and as shown in the constant current unit 10a, an N channel type MOS transistor (hereinafter referred to as a drain) connected to a node N1a and a source connected to a common potential via a resistor 12a. And an operational amplifier (hereinafter referred to as OP) 13a. The reference voltage REF1 is applied to the non-inverting input terminal + of the OP 13a, and the inverting input terminal − is connected to the source of the NMOS 11a. The output terminal of OP13a is connected to the gate of NMOS 11a.

  The nodes N1a and N1b are further connected to voltage detectors 20a and 20b for detecting the voltages of these nodes N1a and N1b. The voltage detection units 20a and 20b have the same circuit configuration, and as shown in the voltage detection unit 20a, a P-channel MOS having a gate connected to the node N1a, a drain connected to a common potential, and a source connected to the node N2a It has a transistor (hereinafter referred to as PMOS) 21a. The node N2a is connected to the power supply potential VDD via the pull-up PMOS 22a. Note that a bias voltage VB1 for supplying a small constant current is applied to the gate of the PMOS 22a.

  The non-inverting input terminal + of OP23a is further connected to the node N2a, and the output terminal of OP23a is connected to the gate of the PMOS 24a. The drain of the PMOS 24a is connected to the common potential, and the source is connected to the node N3a. The node N3a is connected to the inverting input terminal − of the OP 23a, and the same voltage as that of the node N2a is output from the node N3a. That is, the voltage detector 20a constitutes a source follower type voltage follower circuit. Similarly, the same voltage as that of the node N2b is output from the node N3b of the voltage detection unit 20b. The nodes N3a and N3b of the voltage detectors 20a and 20b are commonly connected to the node N4, and the lowest voltage among the voltages of the nodes N3a and N3b is output from the node N4. The voltage control unit 30 is connected to the node N4.

  The voltage control unit 30 includes PMOSs 31, 32, 33 and OP34, and outputs a control voltage VC corresponding to the difference between the voltage of the node N4 and the reference voltage REF (specifically, the source voltage of the PMOS 33). is there. The node N4 is connected to the power supply potential VDD via the pull-up PMOS 31, and is also connected to the non-inverting input terminal + of OP34. The inverting input terminal − of the OP 34 is connected to the power supply potential VDD via the pull-up PMOS 32 and is connected to the common potential via the PMOS 33. Note that a reference voltage REF2 higher than the reference voltage REF1 in the constant current unit 10 is applied to the gate of the PMOS 33. Further, the output terminal of OP34 is connected to the control terminal of the booster 40.

  The booster 40 boosts the input voltage VIN (usually a power supply voltage) based on the control voltage VC supplied from the voltage controller 30, and generates a variable output voltage VOUT. The step-up unit 40 includes, for example, a step-up unit such as a pulse width modulator (PWM modulator) that changes the pulse width of the clock signal with the control voltage VC supplied from the voltage control unit 30 and a switching regulator that uses the modulated clock signal. It can be configured with a circuit. The variable output voltage VOUT generated by the booster 40 is supplied to the anode side of the LED strings 1a and 1b.

Next, the operation will be described.
The output voltage VOUT from the booster 40 is applied to the anode side of the LED strings 1a and 1b, and drive currents Ia and Ib flow through these LED strings 1a and 1b, respectively.
In OP13a of the constant current unit 10a, the conduction state of the NMOS 11a is controlled by the output signal of the OP13a, and the voltage generated in the resistor 12a is fed back to the inverting input terminal −. From the imaginary short-circuit principle of the OP having the feedback circuit, the voltage at the inverting input terminal − of the OP 13a (that is, the voltage generated at the resistor 12a) becomes equal to the voltage at the non-inverting input terminal + (that is, the reference voltage REF1). Therefore, if the resistance value of the resistor 12a is R, the current flowing through the resistor 12a is REF1 / R.

  Since the LED string 1a, the NMOS 11a, and the resistor 12a are connected in series, the drive current Ia flowing through the LED string 1a is also REF1 / R. On the other hand, since the constant current unit 10b has the same circuit configuration as the constant current unit 10a, the drive current Ib flowing through the LED array 1b is also REF1 / R. Accordingly, since the white LEDs constituting the LED array 1a and the LED array 1b are all driven by the same current, they emit light with substantially the same luminance. However, the voltage applied to each white LED varies depending on variations in the characteristics of the elements. For this reason, in general, the voltage drops due to the LED strings 1a and 1b are different, and the voltages generated at the nodes N1a and N1b are not the same.

  The voltage of the node N1a is applied to the gate of the PMOS 21a on the input side of the voltage detection unit 20a that constitutes the voltage follower circuit. As a result, the same voltage as the node N2a is output to the node N3a on the output side of the voltage detection unit 20a (that is, the source of the PMOS 24a). Similarly, the voltage of the node N1b is applied to the gate of the PMOS 21b on the input side of the voltage detection unit 20b constituting the voltage follower circuit, and the same voltage as the node N2b is output to the node N3b on the output side of the voltage detection unit 20b. Is done.

  The nodes N3a and N3b are connected to the common potential via the PMOSs 24a and 24b, respectively, and are commonly connected to the node N4. For this reason, the voltage at the node N4 is lowered to the lower one of the voltages at the nodes N3a and N3b. Therefore, the voltage at the node N4 is the lower voltage of the nodes N1a and N1b.

  The voltage of the node N4 is applied to the non-inverting input terminal + of OP34 of the voltage control unit 30, and the level of the source voltage of the PMOS 33 (the reference voltage REF2 to the threshold voltage Vt of the PMOS 33) applied to the inverting input terminal − of OP34. Compared to the shifted voltage). As a result, the control voltage VC corresponding to the difference between the voltage of the node N4 and the source voltage of the PMOS 33 is output from the output terminal of OP34 and applied to the booster 40.

  The booster 40 boosts the input voltage VIN according to the control voltage VC, and outputs an output voltage VOUT corresponding to the control voltage VC. The output voltage VOUT is commonly supplied to the anode side of the LED strings 1a and 1b, and the LED strings 1a and 1b are driven with the same constant current by the constant current units 10a and 10b.

  Here, for example, when the voltage of the node N4 is higher than the source voltage of the PMOS 33, the control voltage VC output from the voltage control unit 30 acts to lower the output voltage VOUT of the boosting unit 40. As a result, the voltages at the nodes N1a and N1b are lowered while the drive currents Ia and Ib flowing through the LED strings 1a and 1b are kept constant. The voltages at nodes N1a and N1b are output to nodes N3a and N3b through voltage detectors 20a and 20b, respectively. A voltage corresponding to the lower one of the voltages of the nodes N1a and N1b is output to the node N4.

  On the other hand, when the voltage of the node N4 is lower than the source voltage of the PMOS 33, the control voltage VC output from the voltage control unit 30 acts to raise the output voltage VOUT of the boosting unit 40. As a result, the voltages at the nodes N1a and N1b rise while the drive currents Ia and Ib flowing through the LED strings 1a and 1b are kept constant.

  By such a feedback loop, the voltage of the node N4 is controlled to be equal to the source voltage of the PMOS 33. Here, the voltage at the node N4 is the lower of the voltages at the nodes N2a and N2b. Therefore, in this LED drive circuit, the lower one of the voltages at the nodes N1a and N1b is controlled to be equal to the source voltage of the PMOS 33.

FIG. 2 is an explanatory diagram of the operation of FIG. FIG. 2 shows the relationship between the drive voltage (output voltage VOUT) supplied to the LED strings 1a and 1b and the drive current ILED (Ia and Ib) when feedback by the control voltage VC is not performed.
When the output voltage VOUT is increased from 0, the drive current ILED is constant current I (= REF1 /) set by the constant current units 10a and 10b from the point where the total value of forward voltages of the LEDs connected in series is exceeded. Until the output voltage VOUT rises.

  When the drive current ILED reaches the constant current I, the drive current ILED does not change even if the output voltage VOUT is increased further. In this state, the voltage applied to the LED strings 1a and 1b has a value corresponding to the characteristics of the white LEDs constituting the LED strings 1a and 1b, and is independent of the output voltage VOUT. That is, the difference voltage between the output voltage VOUT and the voltage applied to the LED strings 1a and 1b becomes the voltage of the nodes N1a and N1b.

  In order for the constant current units 10a and 10b to operate normally (that is, to allow the constant current I to flow through the LED strings 1a and 1b), the voltage at the nodes N1a and N1b is at least the voltage Vtn applied to the on-resistance of the REF1 + NMOSs 11a and 11b. It is necessary to be above. However, if a voltage higher than that is applied, the excess voltage is consumed as wasted power by the NMOSs 11a and 11b. Therefore, by setting the lower one of the voltages of the nodes N1a and N1b to the minimum necessary voltage (that is, REF1 + Vtn) at which the constant current units 10a and 10b operate normally, wasteful power consumption can be reduced. Can be suppressed.

  As described above, the feedback of the LED strings 1a and 1b → constant current units 10a and 10b → voltage detection units 20a and 20b → voltage control unit 30 → boost unit 40 → LED strings 1a and 1b causes the nodes N1a and N1b to The lower one of the voltages is controlled to be equal to the reference voltage REF2. Therefore, by setting the reference voltage REF2 to the minimum necessary voltage (that is, REF1 + Vtn) at which the constant current units 10a and 10b operate normally, wasteful power consumption can be suppressed.

  As described above, in the LED drive circuit of this embodiment, the output side nodes N3a and N3b of the source follower type voltage detection units 20a and 20b are commonly connected to the input side node N4 of the voltage control unit 30. . As a result, the LED driving circuit generates light sources such as white LEDs connected in parallel with a simple circuit configuration and generates the minimum boosted voltage, and drives the light emitting elements with almost the same brightness while suppressing wasteful power consumption. There is an advantage that you can.

In addition, this invention is not limited to the said embodiment, The following various deformation | transformation are possible.
(A) Although FIG. 1 shows a configuration in which two LED rows 1a and 1b are driven, an arbitrary number of LED rows 1 can be driven in parallel. In that case, what is necessary is just to provide the constant current part 10 and the voltage detection part 20 by the number of the LED rows 1. Moreover, although LED row | line 1a, 1b is comprised by connecting several LED in series, a single LED may be sufficient.

(B) The circuit configurations of the voltage detection units 20a and 20b and the voltage control unit 30 are merely examples, and circuits having the same function can be used. For example, a high resistance may be used in place of the pull-up PMOSs 22a, 22b, 31, 32.
Alternatively, the PMOSs 21a, 21b, 22a, and 22b of the voltage detection units 20a and 20b may be deleted, and the non-inverting input terminals + of the OPs 23a and 23b may be directly connected to the nodes N1a and N1b, respectively. As a result, the voltages at the nodes N1a and N1b are output to the nodes N3a and N3b, respectively.
(C) A light emitting element is not limited to white LED. The present invention can be similarly applied to current-driven light-emitting elements such as organic electroluminescence.

(D) The configuration of the booster 40 is not limited to that illustrated. Any configuration may be used as long as it generates the output voltage VOUT according to the control voltage VC output from the voltage control unit 30.

It is a circuit diagram which shows the LED drive circuit which concerns on embodiment of this invention. It is operation | movement explanatory drawing of FIG.

Explanation of symbols

1a, 1b LED string 10a, 10b Constant current part 11a, 11b NMOS
12a, 12b Resistors 20a, 20b Voltage detectors 24a, 24b, 32, 33 PMOS
30 Voltage Controller 34 Operational Amplifier 40 Booster

Claims (4)

A light emitting element driving circuit for driving a plurality of light emitting elements connected in parallel,
A plurality of constant current portions connected in series between each of the light emitting elements and a common potential, and allowing a constant current to flow through the light emitting elements;
A plurality of voltage detection units each having a detection node for outputting a detection voltage, detecting a voltage at a connection point between the light emitting element and the constant current unit and outputting the detection voltage from the detection node;
A common node to which detection nodes of the respective voltage detection units are connected in common;
A voltage controller that outputs a control voltage corresponding to a difference between a reference voltage and a voltage of the common node;
A booster that boosts an input voltage in accordance with the control voltage and supplies the input voltage as a drive voltage for the plurality of light emitting elements;
The light emitting element driving circuit, wherein the driving voltage output from the boosting unit is controlled by the control voltage so that the voltage of the common node becomes equal to the reference voltage. Each of the constant current portions includes a first transistor and a resistor connected in series between the light emitting element and a common potential, and the first current is set so that a current flowing through the resistor becomes the constant current. Configured to control the conduction state of the transistor;
Each of the voltage detection units includes a second transistor having a source electrode connected to the detection node and a drain electrode connected to the common potential, and is connected to a voltage at a connection point between the light emitting element and the constant current unit. Configured to control a conduction state of the second transistor such that a voltage based on the second transistor is output from the source electrode;
The voltage control unit includes a third transistor in which a source electrode is pulled up, a drain electrode is connected to the common potential, a reference voltage is applied to a gate electrode, and the reference voltage is output from the source electrode; An operational amplifier that outputs the control voltage according to a difference between a voltage and a voltage of the common node;
The light-emitting element driving circuit according to claim 1.   The reference voltage is set to the same value as a voltage obtained by adding a voltage applied to the on-resistance of the first transistor to a voltage generated in the resistor when the constant current flows through the resistor of the constant current unit. The light emitting element drive circuit according to claim 2, wherein   The light emitting element driving circuit according to claim 1, wherein the light emitting element is a white light emitting diode.

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