JPH02218183A - Mos integrated circuit for driving led - Google Patents

Abstract

PURPOSE:To specify the output current values per integrated circuit by a method wherein a mirror circuit is composed of the first and second constant current circuits while the second detecting element is provided with a reference element and a multitude of controlling elements which are selected by a selecting circuit. CONSTITUTION:A mirror circuit is composed of the first constant current circuit comprising a voltage comparator 6a, a resistor 5a and the second current circuit comprising a MOS transistor 4b, another voltage comparator 6b, another resistor 5c while a detecting element in the second constant circuit is provided with a reference element 5c and a multitude of adjusting elements 5d, 5e which are selected by a selecting circuit 8. Accordingly, even if the values of the reference detecting element 5c are subjected to any dispersion, the current running in the voltage decline element 5b generating the reference voltage in the first constant current is kept at specified value with high precision so that a current controlling element 4a may be controlled by the first constant current circuit while the driving current of a light emitting diode 3 may be kept at the specified value. Through these procedures, the driving current values for integrated circuit can be easily stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a MOS integrated circuit for driving LEDs of an LED array head used as a printing light source of an electrophotographic printer, for example.

[Conventional technology]

FIG. 3 is a configuration diagram of a conventional LED driving MOS integrated circuit disclosed in, for example, Japanese Unexamined Patent Publication No. 198872/1983.

In order to drive several dozen LEDs (light emitting diodes), the LED array head uses several dozen integrated circuits with the configuration shown in Figure 3, and one integrated circuit has three
There are 2 to 64 drive circuits, and the LEDs are driven in pairs.

In FIG. 3, an LED driving MOS integrated circuit l has light emitting diodes 33 to 3c (
This is a circuit for controlling power supplied to an LED (hereinafter referred to as an LED) so that its brightness is maintained at a predetermined value, and is formed as an integrated circuit on a single substrate.

This integrated circuit 1 is provided with a MOS transistor 4 connected in series with the LEDs 3a to 3c as a current limiting element for the purpose of limiting the current value flowing through the LEDs 33 to 3c, and the MOS transistor 4 is further provided with a resistor. 5 to the positive terminal of the series power supply 2. Therefore, the required LED drive current limited by the MOS transistor 4 flows through the resistor 5, across which the LED
Detection voltage (1) is generated at a level according to the magnitude of the drive current flowing through 3a to 3c. The detected voltage V is the voltage comparator 6
is applied to the ten input terminals.

Terminal voltage of DC power supply 2 ■. In order to extract the required constant level reference voltage regardless of the level of
A drain electrode of a depletion type MOS (Rangistaph) commonly connected to the electrodes is connected to the positive electrode of the DC power supply 2 via a resistor 8.

Drain current! A voltage vr generated across the resistor 8 by flowing through the resistor 8 is applied to one input terminal of the voltage comparator 6.

Therefore, from the output of the voltage comparator 6, the output voltage ■ corresponds to the difference between the detected voltage ■ and the reference voltage ■.

is output, and this output voltage VG causes the transistor 4 to
The degree of conductivity is controlled. As a result, when Vl<vl, the output voltage VG increases in the positive direction, the drain current of the transistor 4 increases, and the voltage drop across the resistor 5 increases. That is, the value of the detection voltage ■1 increases. On the contrary, V,
>V, the output voltage ■. increases in the negative direction and the drain current of transistor 4 decreases, so resistor 5
The voltage drop at is reduced. That is, the detection voltage V1 decreases. In this way, the conductivity of the transistor 4 is controlled so that V, = V, and as a result, the reference voltage ■,
A current corresponding to the level of L E flows through transistor 4.
The current flows through D 3 a to 3 c, and L and ED 3 a to 3 c are driven with a constant current.

A switching transistor 9 is provided between each gate of the transistor 4 and the ground so as to selectively drive each LED 3a, 3b, and 3c, and each switching transistor 9 operates according to a signal from a control '11 circuit 10. is controlled, thereby controlling the light emission of each LED.

[Problem to be solved by the invention]

The conventional LED driving integrated circuit is constructed as described above, and the resistors 5.8 are simultaneously formed of diffused resistors or polysilicon resistors, so there is little variation in resistance value within one integrated circuit. However, due to manufacturing conditions, variations in each integrated circuit occur by ±35% or more, and therefore output current variations in each integrated circuit also vary to the same degree or more, that is, a difference of more than twice as much.

On the other hand, since the image quality of a printer depends on the uniformity of the light output of the LED head, it is necessary to make the light output uniform (for example, ±20% or less) in the LED head. Important factors include an LED driving integrated circuit, an LED array head, and a focusing lens (not mentioned).

Therefore, in order to make the optical output uniform, it is necessary to at least suppress the variation in the drive power supply for each integrated circuit to a low value of, for example, ±5% or less. To achieve this, it was necessary to measure the output power of each integrated circuit and classify them into ranks, which was the biggest factor in increasing cost.

Further, the light emitting output of the LED depends on the temperature, and the output value decreases as the temperature rises, so it is necessary to control the LED drive current according to the temperature. In the conventional integrated circuit (1), a reference voltage V is generated using a depletion type MOS transistor (7) in order to control the temperature of the drive current, but the value of Variations occur due to manufacturing conditions that cannot be used, and any LED
Since the drive current value cannot be controlled, the yield of usable integrated circuits has been low and they have become expensive.

This invention was made in order to solve the above-mentioned problems, and it is possible to easily improve the accuracy of the output current value of each integrated circuit, and to reduce the manufacturing cost of integrated circuits and LED array heads. The purpose is to provide the following.

[Means to solve the problem]

An integrated circuit according to the present invention includes a current control element connected in series with a light emitting diode, a detection element for detecting a current value flowing through the light emitting diode, and a current control element connected in series with the light emitting diode, and a current control element connected in series with the light emitting diode. a first constant current circuit comprising a drive circuit that operates in response to a detection voltage of the light emitting diode and drives a current control element so that the drive current flowing through the light emitting diode is maintained at a predetermined value; A second constant current circuit connected to the reference voltage input section of the circuit and having the same configuration as the first constant current circuit constitutes a mirror circuit, and the detection element in the second constant current circuit is connected to the reference element and a plurality of detection elements. It has adjusting elements, and these adjusting elements are selected by a selection circuit.

[Effect]

In the integrated circuit of the present invention, the load of the current control element in the second constant current circuit can be adjusted, so even if there is variation in the value of the reference detection element, the voltage drop that produces the reference voltage of the first constant current circuit can be reduced. The current flowing through the element becomes a predetermined value with high precision, and the current control element of the first constant current circuit is controlled by the first constant current circuit, and the output current value of the first constant current circuit, that is, the value of the light emitting diode. The drive current is maintained at a predetermined value.

〔Example〕

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

FIG. 1 shows the basic circuit configuration of the integrated circuit of the present invention, in which 4a to 4c are current control elements and MOS transistors with the same performance.
) consists of a transistor.

63 to 6b are circuits for driving the current control elements, and are composed of voltage comparators having the same configuration. 5a is a current detection element, which is a resistor (
Resistor RIN), 5b is a voltage drop element for obtaining the first reference voltage and is a load resistor (resistance RIN) similar to resistor 5a, 5c is used as a current detection element and resistor 5
This is a reference resistor (resistance RE) having almost the same value as a. Voltage comparator 5a and resistor 5a are each connected to MOS transistor 4. in FIG. Voltage comparator6. A first constant current circuit corresponding to the resistor 5 and having the voltage at point B as a reference voltage is formed,
A switch 9 drives and controls the LED 3. Also M
OS) The transistor 4b, the voltage comparator 6b, and the resistor 5C form a second constant current circuit that uses the voltage Vrl as the second reference voltage, and forms a mirror circuit with the first constant current circuit described above. As a result, the current I flowing through the load resistor 5C. and the driving current I2 of the LED 3 can be made almost the same. Note that this integrated circuit is supplied with voltage VDD from an external DC power supply (not shown).

Elements 5d, 5e, 7d, 7e and 8 will be described later.

Next, the operation will be explained. First, in the second constant current circuit, resistors 5a and 5b are connected to a predetermined reference voltage V11.
By connecting a resistor 5C with approximately the same value as , we obtain . At this time, the gate voltage VGI is applied to the MOS transistor 4C. MOS) A gate voltage VG is applied to the MOS transistor 4b formed to have the same performance as the transistor 4C.
Current flowing through load resistor 5b when I is applied! ,teeth,
Current value of the second constant current circuit■. The reference voltage at point B of the voltage comparator 6a of the first constant current circuit becomes ■, and becomes the same value as ■.

The current (2) flowing through the resistor 5a, which is a current detection element in the first constant current circuit serving as a reference voltage, is determined by the drive current I2 of the LED 3 by the resistor 5C of the integrated circuit.

Variations in the resistance values of the resistors 5a and 5b in the integrated circuit shown hereAs mentioned above, the variation in the resistance values of the resistors 5a and 5b occurs by more than 35% for each integrated circuit with respect to the circuit design value, but individual variations within one integrated circuit. is extremely small. Therefore, in order to reduce the variation in current value from one integrated circuit to another, it is necessary to make the resistor 5c highly accurate.

Methods for increasing the precision of the resistor 5c include trimming and attaching it externally, but this requires a manufacturing process and is expensive.

Therefore, in this embodiment, by utilizing the relationship between the load resistance value of the second transistor 4C and the drive current I2 (shown in FIG. 2), a resistor for adjusting the resistance value is connected in parallel to the resistor 5c. It is set up. As is clear from FIG. 2, the drive current I!
can be stably controlled within a range of, for example, 20 to 30% depending on the load resistance value.

That is, 5d and 5e are resistors for adjusting the resistance value, and have a resistance value that is, for example, about 10 times the resistance value of the resistor 5c. Resistors 5d and 58 are connected in parallel to resistor 5c via switching elements (transistors in this example) 7d17e. 8 is a selection circuit (launch circuit in this example) which receives the latch signal LATCH. , data d, , d, are stored in the switching elements 7d and 7e.
on/off control.

Therefore, the value of the load resistance of the transistor 4C is four types: both switching elements 7d and 7e are on, both are off, and one of switching elements 7d and 7d is on, and the load resistance is determined by control data d, , d, The value can be adjusted in four steps, and variations in the resistance value of the reference resistor 5C can be corrected, so variations in the drive current of the integrated circuit can be minimized (and any drive current value can be set). Therefore, as the resistor 5C, there is no need to use a highly accurate resistor through trimming, external attachment, etc.

In this embodiment, the number of adjusting resistors is two, but if the number is three or more, the number of adjustment stages can be increased, and thus the adjustment accuracy can be improved.

Furthermore, the light emitting output of the LED depends on the temperature, and as the output value decreases as the temperature rises, it is necessary to control the LED drive current according to the temperature, but as shown in Figure 1, the reference voltage Vrl
By inputting the signal from the outside of the integrated circuit, it is possible to control the drive current values of a plurality of integrated circuits to be constant and to obtain an arbitrary drive current value.

〔Effect of the invention〕

As described above, in the LED driving MOS integrated circuit of the present invention, it is extremely easy to make the drive current value constant for each integrated circuit, and by using a configuration in which the drive current value is controlled externally, multiple In the LED array head using the integrated circuit, uniform and stable light output can be obtained, and the load of the second constant current circuit can be adjusted, so the above effects can be obtained without incurring high costs. This makes it possible to provide an inexpensive integrated circuit and LED array head.

[Brief explanation of the drawing]

FIG. 1 shows an LED driving MO according to an embodiment of the present invention.
A circuit diagram showing the S integrated circuit, Fig. 2 is a diagram showing the relationship between drive current and load resistance value, and Fig. 3 is a conventional MO for LED driving.
FIG. 2 is a circuit diagram showing an S integrated circuit. 3...Light emitting diode (LED), 4...Current control element (MOS) transistor), 5a, 5c...-Detection element (resistor), 5b--Voltage drop element (resistor), 5
d, 5e--adjustment resistor, 6a, 6b--circuit for driving the current control element (voltage comparator), 7d, 7e--switching element, 8--adjustment circuit. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] a first current control element connected in series with the light emitting diode; a first detection element connected in series with the light emitting diode for detecting a current value flowing through the light emitting diode; a first drive circuit that operates in response to the detection voltage of the light emitting diode and a first reference voltage and drives the first current control element so that the drive current flowing through the light emitting diode is maintained at a predetermined value. In the LED driving MOS integrated circuit having one constant current circuit, a second current connected in series with the voltage drop element for controlling the current flowing from the DC power source to the voltage drop element that generates the first reference voltage. It operates in response to a detection voltage from a control element and a second detection element connected to a DC power source and through which a predetermined current flows, and a second reference voltage, and the current flowing through the voltage drop element is maintained at a predetermined value. a second constant current circuit comprising a second drive circuit for driving the second current control element, and the first constant current circuit and the second constant current circuit constitute a mirror circuit. , wherein the second detection element has a reference element and a plurality of adjustment elements, and these adjustment elements are selected by a selection circuit.
integrated circuit.