JPH05129665A - Led drive circuit - Google Patents

Abstract

PURPOSE:To obtain an LED drive circuit which can reduce cost and uniform the light quantity of an LED. CONSTITUTION:LED on-signals a-n from outside are inputted to PNP-type transistors 4a-4n via the respective base resistors with the result that a constant current flows from a constant current power source 3 by ON of the PNP-type transistors 4a-4n to turn off LEDs 1a-1n, or that a constant current flows from the constant current power source to the LEDs 1a-1n by OFF of the PNP-type transistors 4a-4n to turn on the LEDs 1a-1n.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED drive circuit capable of uniformly displaying the light quantity of a light emitting diode (LED) used in an operation panel or the like.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram showing a conventional LED drive circuit. In the figure, 1a to 1n are LEDs (light emitting diodes), 6 is a power source, 7a to 7n are current limiting resistors, the anodes of the LEDs 1a to 1n are commonly connected to the power source 6, and the LEDs 1a are also connected. ~ 1n cathode for each LED
Is connected to one end of each of the current limiting resistors 7a to 7n. 8a to 8n are ICs for open collectors, and LED ON signals a to n output as control signals from a control unit (not shown) are input to the input terminals of the ICs 8a to 8n for open collectors. It is input corresponding to 8n. I for each open collector
The outputs of C8a to 8n are connected to the other ends of the current limiting resistors 7a to 7n, respectively.

The conventional LED drive circuit is constructed as described above, and FIG. 5 is an explanatory diagram for obtaining the operating point from the load characteristics of the LED. First, the operating point of the LED will be described.

In the figure, a forward current-forward voltage characteristic curve A for obtaining the forward current of the LED in proportion to the voltage applied to the LED in the forward direction and the current limiting resistor 7a.
The intersection point P at which the load characteristic B when 7n is connected to the power supply 6 intersects is the operating point P of the LED, and this operating point leads to L
The drive voltage VF and drive current IF of the ED can be obtained.

Next, the operation of the LED drive circuit will be described.
For example, when only the LED 1a is caused to emit light, the LED ON signal a of logic level L is output from the control section to the open collector IC 8a, and the output of the open collector IC 8a becomes L, so that the voltage VF applied from the power source 6 The LED 1a emits light by passing a drive current IF. Further, when turning off the emitted LED 1a, the LED ON signal a of the logic level H is sent from the control unit to the open collector I.
It is output to C8a and the output of the open collector IC 8a becomes H. Therefore, based on the voltage applied from the power source 6, LE
The drive current does not flow to D1a and the light is turned off.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the ED drive circuit, there is a problem in that the characteristics of the forward current-forward voltage of each LED vary, so that the operating point P varies, the current flowing through each LED varies, and the light amount of each LED varies.

Further, in order to eliminate the variation in the light amount of each LED, the LED is selected or the current limiting resistance is finely adjusted to make the drive current of the LED constant. There was a problem that the cost would increase.

The present invention has been made to solve the above problems, and an object of the present invention is to obtain an LED drive circuit which can reduce the cost and make the light amount of the LED uniform.

[0009]

An LED drive circuit according to the present invention comprises a group of light emitting elements connected in series in the forward direction, a constant current power source for supplying a constant current to the group of light emitting elements, and a group of the above light emitting element groups. A switch element group that is connected in parallel to each light emitting element and that turns on or off the corresponding light emitting element based on a control signal from the outside is provided.

[0010]

In the present invention, the switch element connected in parallel to each light emitting element is controlled by an external control signal, and the amount of light is supplied to each light emitting element by the constant current supplied from the constant current power source. Make it uniform.

[0011]

FIG. 1 is an LED drive circuit diagram showing an embodiment of the present invention, in which a to n and 1a to 1n are the same as the conventional one. In the figure, 2 is each LED whose one end is connected in series.
A pull-up resistor connected to the terminal ends of 1a to 1n (the anode side of the LED 1n) and the other end connected to a constant current source (described later) so that the base current of each transistor (described later) flows stably. In addition, the collector voltage of each transistor is compensated. Reference numeral 3 denotes a constant current power supply, and one end of each of the LEDs 1a to 1n connected in series has a starting end (LED 1a).
The cathode side) and the other end is connected to the pull-up resistor 2 to supply a constant current. 4a to 4n are each P
The NP type transistor is connected in parallel to each of the LEDs 1a to 1n, the collector is connected to the cathode of the LED, and the emitter is connected to the anode of the LED. 5a to 5n are base resistors, and the base resistors 5a to 5n are
One end of 5n is connected in series corresponding to each of the transistors 4a to 4n, and the other end is connected in series corresponding to the LED on signals a to n from the control unit.

The LED drive circuit of the present invention is constructed as described above, FIG. 2 is an equivalent circuit diagram when all the LEDs of FIG. 1 are turned on, and FIG. 3 is an LED of FIG. It is an explanatory view when is turned on, and the operation of this circuit will be described.

In FIG. 1 and FIG. 2, all LEDs 1a
When lighting up to 1n, L output from the control unit
ED on signals a to n are all in open state (H level of IC output for open collector or N grounded emitter)
It corresponds to the collector output when the PN type transistor is turned off) and outputs to each base 5a to 5n connected to the base of each PNP type transistor 4a to 4n. Then, the PNP transistors 4a to 4n are turned off, and as a result, an equivalent circuit as shown in FIG.
The EDs 1a to 1n flow constantly due to the constant current supplied from the constant current power supply 3. Therefore, the values of the currents flowing through the individual LEDs 1a to 1n do not fluctuate, so that
The light amount of D1a to 1n can be stabilized.

Further, in FIGS. 1 and 3, when only the LED 1a is turned off and the other LEDs 1b to 1n are turned on,
The LED on signal a output from the control unit is set to L level, and the other LED on signals b to n are opened and output to the base resistors 5a to 5n connected to the PNP transistors 4a to 4n. Then, the base current flows only in the PNP type transistor 4a to be turned on, and the other PNP type transistors 4b to 4n are turned off, and as a result, a circuit as shown in FIG. 3 is obtained. The base current of the PNP transistor 4a is determined by the voltage generated from the constant current power source 3 and the base resistor 5b, and the PNP can be selected by appropriately selecting both of them.
Transistor 4a can be turned on in the saturation region.

That is, the voltage between the anode and the cathode of the LED 1a is the ON voltage Vcsa of the PNP transistor 4a.
t (generally, this value is sufficiently smaller than the voltage between the anode and the cathode generated when the LED supplies a current necessary for sufficient light emission), the LED 1a becomes unlit, and the other LEDs 1b ˜1n constantly flow by the constant current supplied from the constant current power source 3 through the PNP transistor 5a, and can stabilize the light amount of each of the LEDs 1b to 1n.

When the PNP transistor corresponding to the LED at an arbitrary position connected in series with the constant current power source 3 as a power source is turned on, that is, when the base current of the arbitrary PNP transistor is passed, the branch is made. The current flowing through the LEDs of the next and subsequent stages decreases, and as a result, the current becomes equal to the current flowing through all the LEDs to be turned on, which may cause variations in the light amount of the LEDs.
By selecting a P-type transistor with a large value of DC current amplification factor hfe (hfe = collector current ÷ base current), the constant current from the constant current power source 3 is branched (base current) to make it as small as possible. Can solve the problem. For example, there are many transistors in which hfe has a value of 100 or more, and hfe = 100.
When the transistor of is used, the base current value branching to the transistor is very small compared to the constant current value from the constant current power source flowing to the collector (1 / the collector current).
100), the variation with respect to the light amount of the LED can be ignored.

Further, in the above description, the PNP type is used as the transistor for controlling lighting / non-lighting of each LED.
A PN type or other active device may be used.

As described above, since a constant current is supplied to each LED, there is no variation in the light amount of each LED and stable driving is possible.

[0019]

As described above, according to the present invention, a constant current can be supplied to a light emitting element even if each light emitting element is turned on or off. Therefore, the forward current-forward voltage characteristic of the light emitting element can be improved. The amount of light can be made uniform without being affected by variations.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is an equivalent circuit when all the LEDs in FIG. 1 are turned on.

FIG. 3 is an explanatory diagram when one LED of FIG. 1 is turned off and the other LEDs are turned on.

FIG. 4 is a circuit diagram showing a conventional LED drive circuit.

FIG. 5 is an explanatory diagram for obtaining an operating point based on a load characteristic of an LED.

[Explanation of symbols]

 1a to 1n LED 2 Pull-up resistor 3 Constant current power source 4a to 4n PNP transistor 5a to 5n Base resistor

Claims (1)

[Claims] 1. A light emitting element group connected in series in a forward direction, a constant current power supply for supplying a constant current to the light emitting element group, and each of the light emitting elements of the light emitting element group are connected in parallel to each other and externally connected. And a switch element group for turning on or off a corresponding light emitting element based on a control signal from the LED drive circuit.