JP2602932Y2 - LED drive circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for driving an LED (light emitting diode), and more particularly to a driving circuit for driving a plurality of LEDs.
The present invention relates to an LED drive circuit that causes an ED to emit light at a constant luminance.

[0002]

2. Description of the Related Art Generally, when a plurality of LEDs are arranged and used as a surface light source, it is required that each LED emits light at a constant luminance. For example, FIG.
FIG. 2 is a conceptual diagram of a bar code reader that uses a light source as a light source. A plurality of LEDs 2 are arranged on a substrate 1, these are arranged in a housing 3 of the bar code reader, and connected to a drive circuit 4 to emit light. The emitted light illuminates the bar code 6 through the lens 5, and the reflected light is received by the CCD line sensor 7. The CCD line sensor 7 outputs an electric signal of a level based on a difference in reflected light from each of a black bar and a white bar of the bar code, and the electric signal is processed by the processing circuit 8 to read the bar code. Is made possible. As described above, when a plurality of LEDs are arrayed and used as a planar light source, if the brightness of each LED is not constant, the illuminance of the illuminated barcode will not be uniform, and thus the black bar In some cases, there is a variation in the amount of light reflected by the white line and the white bar, causing an error in reading by the CCD line sensor.

Therefore, in order to keep the light emission luminance of the LED constant even when the power supply voltage fluctuates, a constant current circuit is conventionally provided in the LED drive circuit.
By making the current supplied to D constant, a plurality of LEDs emit light uniformly and uniformly. However, this constant current circuit requires a constant current for all currents supplied to a plurality of LEDs, so that the circuit configuration becomes large-scale, and a small-sized hand-held bar code reader as shown in FIG. Is not preferred.

[0004] For this reason, conventionally, there has been proposed a circuit in which an operational amplifier is used to make an LED constant current. FIG.
Shows an example thereof, and a plurality of LEDs, here, LED strings LED1, LED2 in which two LEDs are respectively connected in series.
LED2 is connected to current control transistors TR1 and T
By connecting to the collector and emitter of R2 and supplying a constant voltage to each base of these transistors TR1 and TR2, the supply current of each LED1 and LED2 is made constant. Here, the constant voltage supply circuit is an operational amplifier O
P, and the input voltage VIN and the emitter voltage V1 of one transistor TR1 are positive-phase input and negative-phase input, respectively. The emitters of the transistors TR1 and TR2 have resistors R respectively.
1 and R2 are connected.

According to this configuration, since the positive-phase input terminal and the negative-phase input terminal of the operational amplifier OP are imaginary short, VIN = V1, and the currents I11 and I12 passed through the LEDs 1 and 2 are made equal. LED1, LED2
Are made equal in light emission luminance. In the example of FIG.
Although the LED 1 and the LED 12 are connected by an LED string, the same applies to a case where each LED 12 is configured by one LED.

[0006]

As described above, in a driving circuit using an operational amplifier, one LED (or LE) is used.
When a failure occurs in column D (hereinafter the same), a current larger than the current set for the other LED may flow.
That is, although the detailed reason will be described later, when one of the LEDs is turned off (open state) at the time of failure or maintenance, the output voltage of the operational amplifier is increased to near the power supply voltage, and this voltage and the other LED are turned on. A large current determined by the transistor and the resistor connected to the other LED flows into the other LED, whereby the luminance of the LED fluctuates and the bar code reading performance is deteriorated.
The ED may be damaged. The purpose of this invention is
It is an object of the present invention to provide an LED drive circuit that can stabilize the light emission luminance of a plurality of LEDs and prevent a large current from being supplied to the other LED even when one LED is turned off. .

[0007]

Means for Solving the Problems The present invention of the LED drive circuit connects two or more LED in two rows, connects the current control element for controlling the driving current of each LED to the respective LED in each column and, an operational amplifier for controlling the respective current control element is provided, one of the inputs a constant voltage to the input terminal, the voltage is generated based on the other of the current flowing through one of said current control device to an input terminal of the operational amplifier And the output of the operational amplifier as the control input of each of the current control elements.
And the LED driving circuit configuration, the respective current control elements are constituted by bipolar transistors, respectively the same standard, each connecting the LED between a power supply of the collectors and one potential, each emitter and other potential respectively connected a resistor between the power supply, mutual together directly connected to each base, connects the output terminal of the operational amplifier and the connection point via a resistor, and the one to the other input terminal of the operational amplifier Generated based on transistor emitter current
It is configured to input the generated voltage . In this case, the resistance value of the resistor connected between the output terminal of the operational amplifier and the connection point of the base of the transistor is set to be sufficiently larger than the resistance value of the resistor connected to the emitter of the transistor.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. Figure 1 is a circuit diagram of an embodiment of the LED drive circuit of the present invention shows an example applied to the LED drive circuit as the bar code reader light source shown in FIG. 6, for example. The transistors TR3 and TR4 and the resistors R4 to R7 constitute a switch circuit unit. When an on / off signal for causing the LED to emit light is input to a drive signal input terminal IN connected to the base of the transistor TR3, the power supply voltage is reduced. It is configured to output VCC to a power supply line connected to the collector of the transistor TR4. In this power supply line, a plurality of (two in this case) LEDs are connected in tandem, ie, two LED rows, that is, LED1 and LED2 are connected in parallel, and each LED1 and LED2 has a base connected to each other. Current control transistor TR directly connected to
1 and TR2 are connected to each other, and each emitter is grounded via resistors R1 and R2, respectively.

On the other hand, a constant voltage circuit is connected to the power supply line. This constant voltage circuit section has a voltage regulator IC for holding the voltage supplied to the power supply line at a predetermined voltage, and stabilizes the output voltage VO. Further, the output voltage VO of this voltage regulator IC is
8, R9, and the divided voltage VIN is supplied to the positive-phase input terminal of the operational amplifier OP. A resistor R3 is connected in series to the output terminal of the operational amplifier OP.
The output of the operational amplifier OP is supplied to the bases of the transistors TR1 and TR2 via the resistor R3. Further, the emitter of one transistor TR1 is connected to the opposite-phase input terminal of the operational amplifier OP.

According to this configuration, the drive signal input terminal IN
When an ON signal is input to transistors TR3 and TR3
4 is turned on, the power supply voltage VCC is supplied to the power supply line. Then, a constant voltage VO is output from the voltage regulator IC, and the voltage VIN divided by the resistors R8 and R9 is input to the positive-phase input terminal of the operational amplifier OP. Then, a voltage V2 is output from the output terminal of the operational amplifier OP, and the voltage V2 is input to the bases of the transistors TR1 and TR2 via the resistor R3. By controlling the collector / emitter currents of these transistors TR1 and TR2, the LED strings LED1 and LE1 are controlled.
The currents I1 and I2 flowing through D2 are controlled. At this time, if transistors TR1 and TR2 have the same characteristics and resistors R1 and R2 have the same resistance value,
Currents I1, I2 flowing through ED columns LED1, LED2
Are equal to each other, and each LED row LED1, LED2
Are emitted with uniform brightness.

That is, in the operational amplifier OP, the positive-phase input terminal and the negative-phase input terminal are imaginary short-circuited.
VIN and the current I1 of I1 ≒ VIN / R2 is LED
Flow to column LED1. Assuming that the resistance R1 = R2, the transistors TR1 and TR2 have a common base and the base-emitter voltage VB of each transistor.
Since E is equal, I2 = I1 depending on the current mirror configuration.
Thus, a current equal to that of LED1 flows through LED row LED2. As a result, an equal and constant current flows through LED1 and LED2, and the brightness of LED1 and LED2 can be kept constant and uniform.

For example, as shown in parentheses, the resistance value and the voltage value in FIG.
= R2 = 33Ω, VB of each transistor TR1, TR2
If E = 0.6V, the base voltage V3 of the transistors TR1 and TR2 is V3 = V1 + VBE, so that V3 = 0.5V + 0.6V = 1.1V. Also, Thus, I1 = I2 ≒ 15.2 mA.

Here, one LED row, for example, LED1
Is turned off and the circuit is opened. At this time, the circuit of FIG. 1 becomes the equivalent circuit of FIG. The transistor TR1 is equivalent to the diode D1 of Vf = 0.6V. The operational amplifier OP tries to set V1 to VIN due to an imaginary short.
Since the resistor R3 is connected in series to the output of P, even if the output voltage V2 of the operational amplifier OP rises close to the power supply voltage VCC, the rise of V1 to VIN is suppressed as shown in the following equation. . When resistance R3 = 10KΩ,
Even if the output voltage of the operational amplifier OP becomes VCC (= 10V), V1 is suppressed to 0.031V.

That is, since V1 is the voltage across the resistor R1 due to V2, V1 = (V2−Vf) × R1 / (R3 + R1) = (V2−Vf) × 33Ω / (10KΩ + 33Ω) = (10-0. 6) × 33/10033 = 0.031 Therefore, the base voltage V3 of the transistor TR2 is V3 = V1 + Vf = 0.031 + 0.6 = 0.631V, which is lower than that in the normal state. The I2 flowing through is reduced. In other words,
No overcurrent flows through the LED array LED2 at all.

On the other hand, FIG. 2 shows an equivalent circuit when the other LED row LED2 is turned off and the circuit is opened.
(B). The transistor TR2 is equivalent to the diode D2 of Vf = 0.6V. The operational amplifier OP tries to set V1 to VIN by imaginary short circuit. However, since the base of the transistor TR1 is grounded via the diode and the resistor R1, the output voltage V of the operational amplifier OP is
2 rises close to VCC, V1 remains at VIN (0.5
V). Therefore, even if the maximum output voltage of the operational amplifier is 10 V, the base voltage V3 of the transistor TR1 is 0.63V.

That is, since V3 is a voltage obtained by adding Vf to the voltage between both ends of the resistor R2 by V2, V3 = (V2−Vf) × R2 / (R3 + R2) + Vf = (10−0.6) × 33 / 10033 + 0.6 = 0.63V in addition, V1 becomes V1 = V3-VBE = 0.63-0.6 = 0.03 V. Therefore, the transistor TR1 allows the LED
The current I1 flowing through the column LED1 is significantly reduced.
From the above, when one of the LED arrays LED1 or LED2 is turned off for some reason, the other LE1 is turned off.
The current in column D is reduced to prevent the other LED from being damaged by the overcurrent.

Here, a case is shown in which the conventional LED driving circuit shown in FIG. 7 is compared with the circuit of the present invention described above. In a normal state, since the positive-phase input terminal and the negative-phase input terminal of the operational amplifier OP are imaginary short, V1 = VIN, and a current of I1 = VIN / R1 flows to the LED1. At this time, if R1 = R2, the transistor TR1
, I12 = I11
, A constant and uniform current flows through each of the LEDs 1 and 2.

Here, when one of the LEDs 1 is turned off, the equivalent circuit shown in FIG. 8 is obtained. The operational amplifier OP is V
A voltage of V2 is output so that 1 becomes VIN. A current I11 'of V1 / R1 = 0.5 / 33 = 15.2 mA flows through R1, and a similar current flows between the base and the emitter of the transistor TR1 which is equivalently a diode D11. At this time, the base-emitter voltage VBE (TR1) of the transistor TR1 is
Since the current is larger than the base-emitter voltage VBE (TR2) of R2, VBE (TR1)> VBE (T
R2).

Now, VBE (TR1) = 0.8 V, VBE
If (TR2) = 0.6V, V2 = V1 + VBE (TR1) = 0.5V + 0.8V = 1.3V, and I12 '= (V2-VBE (TR2)) / R2 = (1.
3−0.6) / 33Ω = 21 mA, which is larger than the initial set current. Therefore, the current of the other LED 2 increases and the brightness increases,
If the light emission of LED2 is continued in this state, LED2
May have a shorter lifespan. Note that the other LED2
Is cut off, I1 does not change and its breakage is prevented.

Here, in the present invention, LED 1 and LED 2
It is important that the bases of the current control transistors TR1 and TR2 are directly connected. For example, as shown in FIG. 3, even when the configuration is such that the resistors R3A and R3B are interposed on the output side of the operational amplifier OP, the bases of the current control transistors TR1 and TR2 of the LED1 and the LED2 are not directly connected. Consider the case of circuits individually controlled by the output of the operational amplifier OP. In this circuit, when the LED 1 is turned off, the equivalent circuit of FIG. 4 is obtained. The operational amplifier OP sets V1 to V2 so that it becomes VIN.
To increase the voltage. However, since R3A = 10 KΩ is connected, V1 does not reach 0.5V even if V2 is raised to near the power supply voltage of the operational amplifier OP.

On the other hand, when V2 is raised to near the power supply voltage VCC (10V), the transistor TR2 becomes a simple emitter follower circuit, and the current I2 becomes Then, an overcurrent flows to the other LED 2 and the LED 2 is damaged. However, when LED2 is turned off, I1 does not change and its breakage is prevented.

In the embodiment of the present invention shown in FIG.
When the off signal is input to the drive signal input terminal IN, the transistors TR3 and TR4 are turned off, so that the power supply VCC is not supplied to the power supply line.
Power is not supplied to the anode side of LED1 and LED2. Therefore, leakage current in each of the LEDs 1 and 2 can be reliably prevented, which is effective in reducing power consumption.

In the present invention, as shown in FIG. 5, when it is desired to emit a specific LED 3 with a drive current different from that of the other LEDs 1 and 2, the operational amplifier OP1 for controlling the current of the LEDs 1 and 2 is different from the operational amplifier OP1. A different operational amplifier OP2 may be used to drive the current controlling transistor TR5 of the specific LED3. That is, by making the value of the resistor R10 connected to the emitter of the current control transistor TR5 different, the LED3 can emit light with a current different from that of the LED1 and the LED2. Also, here, the operational amplifier O
The resistor R3 'is connected to the output terminal of P2. In this case, LED1 and LED2 emit light with the same drive current,
Other LEDs 3 can emit light with different drive currents. Even in this case, even if one of LED1 and LED2 is turned off, it is needless to say that the other LED2 or LED1 can be prevented from being damaged. Although the above embodiment shows an example in which the present invention is applied to a light source LED of a bar code reader, the present invention is similarly applied to an LED driving circuit for forming a surface light source using a plurality of LEDs. It goes without saying that you can do it.

[0024]

As described above, according to the present invention, each of the LEDs connected in two rows is a bipolar device as a current control element.
And connect these bipolar transistors
An output terminal of the operational amplifier for controlling a driving current of each LED by motor, the base of each bipolar transistor mutually
Since a resistor is connected between the direct connection node and the output voltage of the operational amplifier when the output voltage of the operational amplifier is increased when the LED of one column is turned off, the resistance of the LED of the other column is increased even if the output voltage of the operational amplifier is increased. It is possible to suppress an increase in current and effectively prevent the other LED from being shortened or damaged. Also , since the bases of the bipolar transistors are directly connected to each other,
With the current mirror configuration, the current flowing through each LED is made equal, and a uniform and constant current is caused to flow, so that light emission with uniform and constant brightness can be performed. In this case, by making the resistance of the resistor connected between the output terminal of the operational amplifier and the base of the bipolar transistor sufficiently larger than the resistance of the resistor connected to the emitter, one of the LEDs can be turned off. An increase in base voltage can be more effectively prevented, and an increase in current can be suppressed.

[Brief description of the drawings]

FIG. 1 is an overall circuit diagram of an LED driving circuit according to an embodiment of the present invention.

FIG. 2 illustrates a case where one of the LEDs is turned off and the other L
FIG. 3 is an equivalent circuit diagram for explaining a current value of the ED.

FIG. 3 is a circuit diagram in a case where the base of the current control element is not directly connected in the present invention.

FIG. 4 is an equivalent circuit diagram for explaining a current value of one LED in a circuit of FIG. 3 when one LED is turned off;

FIG. 5 is a circuit diagram of a main part of another embodiment of the present invention.

FIG. 6 is a schematic configuration diagram of a bar code reader using an LED as a light source.

FIG. 7 is a partial circuit diagram of an example of a conventional LED drive circuit.

8 is an equivalent circuit diagram for explaining a current value of one LED in the circuit of FIG. 7 when one LED is turned off.

[Explanation of symbols]

 LED1, LED2 Light-emitting diode array (LED array) OP operational amplifier TR1, TR2 Transistor (current control element) R1-R3 Resistance 2 LED 4 LED drive circuit 5 Lens 6 Bar code 7 CCD line sensor 8 Signal processing circuit

────────────────────────────────────────────────── (5) References JP-A-63-240317 (JP, A) JP-A-63-305837 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03K 17/78 H01L 33/00 G06K 7/10

Claims (2)

(57) [Scope of request for utility model registration] 1. A method according to claim 1, wherein two or more LEDs are connected in two rows, and the driving current of each LED is controlled by each of the LEDs in each row .
Connect that current control element, an operational amplifier for controlling the respective current control element is provided, to enter the constant voltage to one input terminal of the operational amplifier, based on the other of the current flowing through one of said current control device to an input terminal In the LED drive circuit having a configuration in which a voltage generated by the operational amplifier is input and an output of the operational amplifier is set as a control input of each of the current control elements,
The current control elements are bipolar transistors of the same standard.
And a collector between each collector and one potential power supply.
Each LED is connected, and each emitter and power supply of other potential
Connect a resistor between them and connect each base directly to each other.
Connection point and the output terminal of the operational amplifier.
And the other input of the operational amplifier.
The output terminal is based on the emitter current of the one transistor.
LE inputting a voltage generated based on the
D drive circuit. 2. A method resistance value of the resistor connected between the base of the connection point of the output terminal and the respective Tran <br/> register of said operational amplifier, said than the resistance value of the resistor connected to the emitter of each transistor 2. The LE according to claim 1 , wherein the LE is set sufficiently large.
D drive circuit.