JP2010062327A - Light emitting element drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element drive circuit having an operating mode allowing fault detection with low current consumption. <P>SOLUTION: The circuit includes a current control circuit 14 controlling a current Ic for driving a light emitting element 13 connected between an external power supply 11 and an output terminal 12 and a fault detection circuit 15 monitoring the voltage of the output terminal 12 to detect the turned-off state of the light emitting element 13 to output a result of detection to the outside. The current control circuit 14 drives the light emitting element 13 with a first current Ic1 in a first operation mode to turn on and off the light emitting element 13 to display a pattern and drives the light emitting element 13 with a second current Ic2 lower than the first current Ic1 in a second operating mode to detect a fault in the light emitting element 13. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a drive circuit for a light emitting element.

Display devices using light emitting elements, especially LEDs (Light Emitting Diodes), are actively used mainly as outdoor display devices, taking advantage of their low power consumption and long life.
The LED display device includes a current control circuit that drives each of the plurality of LEDs with a constant current, and a failure detection circuit that detects a failure of the LED (see, for example, Patent Document 1).

  The LED display device disclosed in Patent Document 1 includes a display that displays an arbitrary pattern by light emission and non-light emission of each light emitting element of a light emitting element unit in which a plurality of light emitting elements are arranged, and a pattern that is displayed on the display. The display control means includes a light-emitting element unit in which a plurality of light-emitting elements are arranged, and the light-emitting element unit controls light emission and non-light emission of each light-emitting element unit according to pattern information transmitted from the display control means. A light emitting element control means, a communication control means for transmitting pattern information from the display control means to the light emitting element control means, and a failure detection means for detecting a failure of the light emitting element unit by a current value flowing through the power supply line of the light emitting element unit. is doing.

  Failure detection of the light emitting element unit by the failure detection means is performed by comparing the current value flowing through the power supply line when a specific pattern of light is emitted by the light emission element control means with a predetermined value.

However, the LED display device disclosed in Patent Document 1 drives the LED with the same current as when a pattern is displayed on the LED when a failure of the LED is detected.
Therefore, since much more current flows than the current sufficient to detect a failure of the LED, there is a problem that wasteful power is consumed.

In particular, if all the LEDs are turned on at the same time in order to detect some unlit LEDs, the reliability of the LEDs may be impaired due to the heat generated by the LEDs.
JP 2005-141094 A

  The present invention provides a drive circuit for a light emitting element having an operation mode in which failure detection can be performed with low current consumption.

  A drive circuit for a light-emitting element according to one embodiment of the present invention includes a current control circuit that controls a current that drives a light-emitting element connected between an external power supply and an output terminal, and monitors the voltage of the output terminal, A failure detection circuit for detecting a non-lighting state of the light emitting element and outputting the detection result to the outside, wherein the current control circuit displays a pattern by turning on and off the light emitting element. In this case, the light emitting element is driven with a first current, and the light emitting element is driven with a second current smaller than the first current in the second operation mode in which failure detection of the light emitting element is performed. It is characterized by that.

  According to the present invention, a drive circuit for a light emitting element having an operation mode capable of detecting a failure with a small current consumption can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings.

  An embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing a drive circuit of the light emitting device of this embodiment.

  As shown in FIG. 1, the drive circuit 10 of the light emitting element of this embodiment is a drive circuit having a constant current output of a type that sinks current (sink type), and is connected between an external power supply 11 and an output terminal 12. A current control circuit 14 for controlling the current Ic for driving the light emitting element 13 to be detected, and a failure detection circuit for monitoring the voltage at the output terminal 12 to detect the non-lighting state of the light emitting element 13 and outputting the detection result to the outside. 15.

  The current control circuit 14 drives the light emitting element 13 with the first current Ic1 and detects a failure of the light emitting element 13 in the first operation mode in which the light emitting element 13 is turned on and off to display a pattern. In the operation mode, the light emitting element 13 is driven with a second current Ic2 smaller than the first current Ic1.

  Furthermore, the drive circuit 10 of the light emitting element includes a display control unit 16 that controls operations of the current control circuit 14 and the failure detection circuit 15 in accordance with the first operation mode and the second operation mode. .

  The light emitting element drive circuit 10 has a plurality of, for example, 16 output terminals 12. Each output terminal 12 is connected to the cathode of the light emitting element 13. The anodes of the respective light emitting elements 13 are commonly connected to the external power supply 11.

The external power supply 11 is a power supply that supplies a drive current Ic to each light emitting element 13, and is also called an LED power supply.
The light emitting element 13 is a visible LED having a light emitting layer made of, for example, an InGaAlP-based material or a GaN-based material.

  The current control circuit 14 includes a differential amplifier 17, a MOS transistor 18 incorporated in a feedback loop of the differential amplifier 17, a constant current circuit 19 that supplies a current corresponding to the current of the MOS transistor 18 to each light emitting element 13, It has.

  The current control circuit 14 performs feedback control so that the feedback voltage Vfb obtained by flowing the current of the MOS transistor 18 through the reference resistor Rref is equal to the reference voltage Vref, and varies the reference voltage Vref according to the operation mode. The first current Ic1 or the second current Ic2 is set.

  The reference voltage Vref includes a first voltage V1 for setting the first current Ic1 and a second voltage V2 lower than the first voltage V1 for setting the second current Ic2, and depends on the operation mode. The first voltage V1 or the second voltage V2 is selected. Selection of the first voltage V1 or the second voltage V2 is performed by the switch 20.

The first voltage V1 and the second voltage V2 of the reference voltage Vref can be obtained using, for example, constant voltage characteristics of a PN junction diode or a Zener diode. The second voltage V2 can also be obtained by dividing the first voltage V1 by resistance.
The switch 20 is, for example, a double throw single pole switch circuit obtained by alternately turning on and off two MOS transistors connected in parallel.

When the current flowing through the reference resistor Rref is Ic0, the feedback voltage Vfb is expressed by Ic0 × Rref.
Since the differential amplifier 17 operates so that the feedback voltage Vfb is equal to the reference voltage Vref, Ic0 = Vref / Rref.

The constant current circuit 19 is a circuit for increasing the current Ic0 of the MOS transistor 18, and is a current mirror circuit driven by the current Ic0 of the MOS transistor 18, for example. There are as many current mirror circuits as the number of output terminals 12.
A current corresponding to Ic0 flows on the secondary side of each current mirror circuit, and becomes a drive current Ic of each light emitting element 13.

  When the reference voltage Vref is the first voltage V1, the driving current Ic of the light emitting element 13 is set to the first current Ic1, and when the reference voltage Vref is the second voltage V2, the driving current Ic of the light emitting element 13 is the second current Ic1. The current Ic2 is set.

  In the first operation mode, the first current Ic1 is normally about 20 to 50 mA. However, in the second operation mode, the second current Ic2 is about 1 mA because the forward current may flow slightly. No problem.

  The failure detection circuit 15 includes an open detection circuit 21 (OOD: Output Open Detector) that detects an open state of the light emitting element 13, a short detection circuit 22 (OSD: Output Short Detector) that detects a short state of the light emitting element 13, And a multiplexer 23 that takes in the detection result of the open detection circuit 21 and the detection result of the short detection circuit 22 and outputs the inspection result according to the inspection mode.

The open detection circuit 21 includes, for example, a comparator having a positive input terminal connected to the output terminal 12 and a negative input terminal connected to the open detection voltage.
When the light emitting element 13 is in an open state, the current flowing into the constant current circuit 19 is stopped, so that the potential of the output terminal 12 becomes approximately 0V. The comparator compares with an open detection voltage, for example, 0.3 V, and outputs an “H” level when the potential of the output terminal 12 is lower than the open detection voltage.

The short detection circuit 22 includes, for example, a comparator having a positive input terminal connected to the output terminal 12 and a negative input terminal connected to the short detection voltage.
When the light emitting element 13 is short-circuited, the voltage drop corresponding to the forward voltage (about 2 to 4 V) of the light emitting element 13 is eliminated, so that the potential of the output terminal 12 is increased accordingly. The comparator compares with a short detection voltage, for example, 3.0 V, and outputs “H” level when the potential of the output terminal 12 is higher than the short detection voltage.

  That is, the failure detection circuit 15 determines that the light emitting element 13 is operating normally when the potential of the output terminal 12 is larger than the open detection voltage and smaller than the short detection voltage, and the open detection circuit 21 and the short detection circuit 22 L ”level is output.

When the potential of the output terminal 12 becomes lower than the open detection voltage, the open detection circuit 21 outputs “H” level.
When the potential of the output terminal 12 becomes higher than the short detection voltage, the short detection circuit 22 outputs “H” level.

  The multiplexer 23 takes in the detection result of the open detection circuit 21 and the detection result of the short detection circuit 22 as a 16-bit parallel signal, and outputs the inspection result corresponding to the inspection mode as a 16-bit serial signal.

  The display control unit 16 includes, for example, a latch 24 that stores a pattern for turning on / off each light-emitting element 13, a serial register that turns on / off each light-emitting element 13, and a shift register 24 that captures inspection results, a shift register 24, And a control circuit 26 for controlling the operation of the latch 25.

  When the constant current output enable signal Qe enters the first operation mode, the control circuit 26 sends a command to the switch 20 of the current control circuit 14 to select the first voltage V1 as the reference signal Vref. Thereby, the light emitting element 13 is driven by the first current Ic1.

  Next, the control circuit 26 sends a command to the multiplexer 23 and the shift register 24, and the shift register 25 takes in the serial data Sin for turning on / off each light emitting element 13 according to the serial data transmission clock Sck, and the latch circuit 24. Is stored as a pattern for turning on / off each light emitting element 13 in accordance with the latch signal Slat. Each light emitting element 13 is turned on or off according to the pattern stored in the latch circuit 24.

  When the constant current output enable signal Qe enters the second operation mode, the control circuit 26 sends a command to the switch 20 of the current control circuit 14 to select the second voltage V2 as the reference signal Vref. Thereby, the light emitting element 13 is driven by the second current Ic2.

  Next, the control circuit 26 sends a command to the multiplexer 23 and the shift register 24, and outputs the open detection result and the short detection result from the multiplexer 23 to the outside as the serial data Sout via the shift register 25.

  Thus, in the first operation mode in which the light emitting element 13 is turned on / off to display a pattern, the light emitting element 13 is driven by the first current Ic1 and the failure detection of the light emitting element 13 is detected. Sometimes it is possible to drive the light emitting element 13 with a second current Ic2 that is smaller than the first current Ic1.

FIG. 2 is a diagram showing a semiconductor integrated device (driver IC) incorporating a drive circuit 10 for a light emitting element.
As shown in FIG. 2, in the semiconductor integrated device 30, the current control circuit 14, the failure detection circuit 15, and the display control unit 16 are monolithically formed on the semiconductor substrate 31.

  The semiconductor integrated device 30 receives a constant current output enable signal terminal (Oe) to which a constant current output enable signal is input and serial data indicating which one of the plurality of light emitting elements 12 is turned on and which is turned off. Serial data input terminal (Sin), latch signal input terminal (Slat) to which a latch signal is input, clock signal input terminal (Sck) to which a serial data transfer clock signal is input, serial data from which serial data is output An output terminal (Sout), a power input terminal (Vdd), a ground terminal (GND), and a resistor external terminal (Rext) are provided.

  The reference resistance Rref of the current control circuit 14 is externally connected via a resistance external terminal (Rext). Thereby, the user can freely set the first current Ic1 according to the brightness required for the light emitting element 13.

  As described above, the current control circuit 14 of the drive circuit 10 of the light emitting element of this embodiment has the first voltage V1 for setting the first current Ic1 and the second voltage Ic2 for setting the second current Ic2. The switch 20 includes a second voltage V2 lower than the first voltage V1 and a switch 20 that selects the first voltage V1 or the second voltage V2 according to the operation mode.

As a result, the light emitting element 13 is driven with the first current Ic1 in the first operation mode, and the light emitting element 13 is driven with the second current Ic2 smaller than the first current Ic1 in the second operation mode. can do.
Accordingly, a drive circuit for a light emitting element having an operation mode capable of detecting a failure with a small current consumption can be obtained.

  In particular, when shipping inspection of an LED panel in which a large number of light emitting elements 13 are incorporated, or when maintenance is performed after the LED panel is installed at night, it is possible to prevent the occurrence of adverse effects such as excessive glare.

Although the case where the light emitting element 13 is an LED has been described here, an OLED (Organic Light Emitting Diode) may be used.
Although the case where the transistor is the MOS transistor 18 has been described, it may be a bipolar transistor.

FIG. 3 is a circuit diagram showing the main part of the drive circuit of the light emitting element according to Example 2 of the present invention. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, description thereof will be omitted, and different portions will be described.
The difference of the present embodiment from the first embodiment is that the second voltage for setting the second current can be varied in the second operation mode.

That is, the current control circuit 41 of the drive circuit 40 of the light emitting element of this embodiment converts the digital signal Din corresponding to the second current Ic2 into an analog voltage and outputs it as the second voltage V2. It has.
The digital signal Din corresponding to the second current Ic2 is input to the digital / analog converter 42 from the outside, for example.

The digital / analog converter 42 is, for example, an 8-bit digital / analog converter and can finely adjust the second current Ic2 in 256 steps.
By detecting the failure of the light emitting element 13 without being affected by noise or the like by finely adjusting the second current Ic2 in accordance with the IV characteristic difference depending on the type of the light emitting element 13 and the IV characteristic fluctuation. Is possible.

As described above, the current control circuit 41 of the drive circuit 40 of the light emitting element of the present embodiment converts the digital signal Din corresponding to the second current Ic2 into an analog voltage and outputs it as the second voltage V2. An analog converter 42 is provided.
Accordingly, the second current Ic2 can be finely adjusted according to the IV characteristic difference and the IV characteristic variation depending on the type of the light emitting element 13, and therefore, the light emitting element 13 is not affected by noise or the like. There is an advantage that a failure can be detected.

  Here, the case where the second voltage V2 is generated by the digital-analog converter 42 has been described. However, when the gradation of the light emitting element 13 is modulated by PWM (Pulse Width Modulation), the PWM waveform of the first voltage V1 is changed. The second voltage V2 can also be generated using a digital-to-analog converter for generation. In that case, the switch 20 becomes unnecessary.

FIG. 4 is a circuit diagram showing a drive circuit of a light emitting device according to Example 3 of the present invention. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, description thereof will be omitted, and different portions will be described.
This embodiment differs from the first embodiment in that the reference resistance is made variable according to the operation mode.

  That is, the current control circuit 51 of the drive circuit 50 of the light emitting element of the present embodiment sets the first resistor R1 for setting the first current Ic1 and the second current Ic2 as the reference resistor Rref. A second resistor R2 having a higher resistance value than the first resistor R1 and a switch 52 for selecting the first resistor R1 or the second resistor R2 according to the operation mode are provided.

  Thus, the first resistor R1 is selected as the reference resistor Rref in the first operation mode, and the second resistor R2 is selected as the reference resistor Rref in the second operation mode.

  As described above, the current control circuit 51 of the drive circuit 50 of the light emitting element of the present embodiment includes the first resistor R1 as the reference resistor Rref and the second resistor R2 having a resistance value higher than the first resistor R1. The first resistor R1 or the second resistor R2 is selected according to the operation mode.

  This eliminates the need for the second voltage V2 and has an advantage of reducing the current consumption of the circuit that generates the second voltage V2.

1 is a circuit diagram showing a drive circuit of a light emitting element according to Example 1 of the invention. 1 is a diagram showing a semiconductor integrated device having a drive circuit for a light emitting element according to Embodiment 1 of the present invention. The circuit diagram which shows the principal part of the drive circuit of the light emitting element which concerns on Example 2 of this invention. FIG. 6 is a circuit diagram showing a drive circuit of a light emitting element according to Example 3 of the invention.

Explanation of symbols

10, 40, 50 Drive circuit 11 External power supply 12 Output terminal 13 Light emitting element (LED)
14, 41, 51 Current control circuit 15 Failure detection circuit 16 Display control unit 17 Differential amplifier 18 MOS transistor 19 Constant current circuit 20, 52 Switch 21 Open detection circuit 22 Short detection circuit 23 Multiplexer 24 Latch 25 Shift register 26 Control circuit 30 Semiconductor integrated device 31 Semiconductor substrate 42 DA converter Ic1 First current Ic2 Second current Vref Reference voltage Vfb Feedback voltage V1 First voltage V2 Second voltage Rref Reference resistor R1 First resistor R2 Second resistor

Claims (5)

A current control circuit for controlling a current for driving a light emitting element connected between the external power supply and the output terminal;
A failure detection circuit that monitors the voltage of the output terminal, detects a non-lighting state of the light emitting element, and outputs the detection result to the outside;
Comprising
The current control circuit drives the light emitting element with a first current and detects a failure of the light emitting element in a first operation mode in which the light emitting element is turned on and off to display a pattern. A drive circuit for a light emitting element, wherein the light emitting element is driven with a second current smaller than the first current in the mode. The current control circuit includes a differential amplifier, a transistor incorporated in a feedback loop of the differential amplifier, and a constant current circuit that flows a current corresponding to the current of the transistor to the light emitting element,
The feedback voltage obtained by causing the current of the transistor to flow through the reference resistor is feedback-controlled so as to be equal to the reference voltage, and the reference voltage or the reference resistor is varied according to the operation mode, and the first voltage is changed. The drive circuit of the light emitting element according to claim 1, wherein a current or the second current is set.   The reference voltage includes a first voltage for setting the first current and a second voltage lower than the first voltage for setting the second current, and according to the operation mode, The drive circuit of the light emitting device according to claim 2, wherein the first voltage or the second voltage is selected.   The reference resistor includes a first resistor for setting the first current and a second resistor having a higher resistance value than the first resistor for setting the second current, and the operation mode The drive circuit of the light emitting element according to claim 2, wherein the first resistor or the second resistor is selected according to the above.   4. The drive circuit for a light-emitting element according to claim 3, wherein the second voltage is a voltage obtained by converting a digital signal corresponding to the second current into an analog voltage by a digital / analog converter. .

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