JPH0635189B2 - Drive circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a load driving circuit, for example, an LED driving MOS integrated circuit of an LED array head used as a printing light source of an electrophotographic printer.

[Conventional technology]

FIG. 2 is a block diagram of a conventional LED driving MOS integrated circuit disclosed in, for example, Japanese Patent Laid-Open No. 60-198872.

Since the LED array head drives thousands of LEDs (light emitting diodes), thousands of integrated circuits having the configuration shown in FIG. 2 are used.
There are 64 to 64 drivers, which drive the LEDs in pairs.

In FIG. 2, the LED driving MOS integrated circuit 1 includes light emitting diodes 3a to 3c driven by a DC power supply 2.
This is a circuit for controlling the electric power supplied to (hereinafter referred to as LED) so that the brightness thereof maintains a predetermined value, and is formed as an integrated circuit on a single substrate.

In this integrated circuit 1, a MOS transistor 4 connected in series with the LEDs 3a to 3c is provided as a current limiting element for the purpose of limiting the value of the current flowing through the LEDs 3a to 3c, and the MOS transistor 4 further includes a resistor 5. It is connected to the positive electrode of the DC power supply 2 via the. Therefore, the required LED drive current limited by the MOS transistor 4 flows through the resistor 5, and LE is connected across the resistor 5.
The detection voltage V ₁ having a level according to the magnitude of the drive current flowing through D3a to 3c is generated. The detection voltage V ₁ is applied to the + input terminal of the voltage comparator 6.

The drain electrode of the depletion type MOS transistor 7 whose gate electrode is commonly connected to the source electrode is connected via the resistor 8 in order to take out a reference voltage of a required constant level regardless of the level of the terminal voltage V ₀ of the DC power supply 2. DC power supply 2
Is connected to the positive electrode of.

The voltage V _r generated across the resistor 8 due to the drain current I _D flowing through the resistor 8 is applied to the − input terminal of the voltage comparator 6.

Therefore, the output of the voltage comparator 6 outputs the output voltage V _G according to the difference between the detection voltage V ₁ and the reference voltage V _r ,
The output voltage V _G controls the conductivity of the transistor 4. As a result, when V ₁ <V _r , the output voltage V
_G increases in the positive direction to increase the drain current of the transistor 4 and increase the voltage drop across the resistor 5. That is, the value of the detection voltage V ₁ increases. Conversely, when V ₁ > V _r , the output voltage V _G increases in the negative direction and the transistor 4
, The voltage drop across resistor 5 is reduced. That is, the detection voltage V ₁ decreases. In this way, the conductivity of the transistor 4 is controlled so that V ₁ = V _r, and as a result, a current corresponding to the level of the reference voltage V _r flows through the transistor 4 to the LEDs 3 a to 3 c and the LEDs 3 a to 3 c. 3c is 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 of the LEDs 3a, 3b, 3c, and the operation of each switching transistor 9 is controlled according to a signal from the control circuit 10. The light emission of each LED is controlled by this.

[Problems to be Solved by the Invention]

Since the conventional LED driving integrated circuit is configured as described above and the resistors 5 and 8 are simultaneously formed of diffused resistors or polysilicon resistors, there is little variation in resistance value within one integrated circuit. However, since the variation of each integrated circuit is ± 35% or more depending on the manufacturing conditions, the variation of the output current of each integrated circuit is about the same or more, that is, the difference of two times or more.

On the other hand, since the image quality of the printer depends on the uniformity of the light output of the LED head, it is necessary to make the light output of the LED head uniform (for example, ± 20% or less). Such factors include an LED array chip and a converging lens, which have not been described, in addition to the LED driving integrated circuit. Therefore, in order to make the light output uniform, at least the variation of the driving power source for each integrated circuit is, for example, ± 5%.
It is necessary to keep the value as low as possible. In order to achieve this, it is necessary to measure the output power supply for each integrated circuit and classify it into ranks, which is the biggest factor in cost increase.

Further, the light emission output of the LED depends on the temperature, and the output value decreases as the temperature rises. Therefore, it is necessary to control the LED drive current according to the temperature. In the conventional integrated circuit (1), the reference voltage V _r is generated using the depletion type MOS transistor (7) in order to control the temperature of the drive current, but the value of V _r is constant in the integrated circuit. Since variations occur due to manufacturing conditions that cannot be used and the LED drive current value control cannot be performed arbitrarily, the yield of usable integrated circuits is poor and the cost is high.

The present invention has been made in order to solve the above problems, and the output current value to the LED, which is a load for each drive circuit configured by an integrated circuit, can be easily and accurately made and configured by the integrated circuit. It is an object of the present invention to provide a low-priced product that can reduce the manufacturing cost of an LED array head that is a drive circuit and a load.

[Means for Solving the Problems]

A drive circuit configured by an integrated circuit according to the present invention connects a current control element to a light emitting diode that is a load, and includes a light emitting diode that is a load and a detection element for detecting a current value flowing in the light emitting diode that is the load. A drive circuit which is connected and which operates in response to a reference value and a detection result from the detection element to control the current control element so that the drive current flowing through the light emitting diode as a load is maintained at a predetermined value. A mirror circuit is configured by one constant current circuit and a second constant current circuit which is connected to the reference value input section of the first constant current circuit and has the same configuration as the first constant current circuit. .

[Action]

The drive circuit configured by the integrated circuit according to the present invention is
The current flowing through the reference value calculating element that generates the reference value of the first constant current circuit by the second constant current circuit becomes a predetermined value, and the current control element of the first constant current circuit is changed by the first constant current circuit. The output current value of the first constant current circuit is controlled, that is, the drive current of the light emitting diode which is the load is maintained at a predetermined value.

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

FIG. 1 shows a basic circuit configuration of an LED driving MOS integrated circuit 11 which is a load driving circuit of the present invention.
The current control elements a to 4c are composed of MOS transistors having the same performance. Reference numerals 6a and 6b are circuits for controlling the current control element and are composed of voltage comparators having the same configuration. 5
Reference numeral a is a current detection element, which is a resistor (resistor RIN) formed of a diffusion resistance or polysilicon resistance, and 5b is a voltage drop element which is a reference value calculation element for obtaining the first reference voltage. Similar load resistor (resistor RIN), 5
c is a resistor (resistor R _E ) used as a current detecting element and having substantially the same value as that of the resistor 5a, and each of them is highly accurate by trimming or the like. The MOS transistor 4a, the voltage comparator 6a, and the resistor 5a correspond to the MOS transistor 4, the voltage comparator 6, and the resistor 5 of FIG. 2, respectively, and the first point is the voltage at the point B as a reference voltage which is a reference value. The constant current circuit is formed, and the LED 9 is driven and controlled by the switch 9. Further, the MOS transistor 4b, the voltage comparator 6b,
The resistor 5C forms a second constant current circuit in which the voltage V _r1 is the second reference voltage which is the second reference value, and the resistor 5C forms a mirror circuit configuration with the first constant current circuit described above. Thus, the current I ₀ flowing through the load resistor 5c and the drive current I _{2 of the} LED 3 can be made substantially the same. In addition, this integrated circuit 1
1, a voltage V _DD from an external DC power source (not shown)
Is given.

Next, the operation will be described. First, in the second constant current circuit, a high-precision resistor 5c, which is almost equivalent to the resistors 5a and 5b, is connected to a predetermined reference voltage V _r1 , and a predetermined constant current value I ₀ Is getting At this time, the MOS transistor 4c is applied with the gate voltage V _G1 . When the gate voltage V _G1 is applied to the MOS transistor 4b formed with the same performance as the MOS transistor 4c, the current I ₁ flowing through the load resistor 5b becomes the same value as the current value I _{0 of} the second constant current circuit. B reference voltage V of the voltage comparator 6a of the constant current circuit 1
_r2 is Becomes Therefore, in the first constant current circuit using V _r2 as the reference voltage, the current I flowing through the resistor 5a which is the current detecting element.
₂ is Then, the drive current I _{2 of the} LED ₃ is. It will be determined by the precision resistor 5c of the integrated circuit.

As described above, the variation of the resistance values of the resistors 5a and 5b in the integrated circuit shown here is ± 35% or more with respect to the circuit design value for each integrated circuit. Variation is extremely small. Therefore, in order to reduce the variation in the current value of each integrated circuit, it is only necessary to improve the accuracy of the resistor 5c by performing the trimming. Further, as a method of improving the accuracy of the resistor 5c, there is a method of externally attaching in addition to trimming. As highly accurate external resistors, variations within ± 1% are commercially available at low cost, and variations in the drive current of the integrated circuit can be achieved by simply attaching one external resistor as a resistor 5c to one integrated circuit. It can be made extremely small and an arbitrary drive current value can be obtained.

Further, the light emission output of the LED depends on the temperature, and it is necessary to control the LED drive current according to the temperature because the output value decreases as the temperature rises. However, as shown in FIG. 1, the reference voltage V _r1 is applied to the integrated circuit. It is possible to control the driving current values of a plurality of integrated circuits to be constant and to obtain arbitrary driving current values by adopting a configuration of inputting from the outside.

〔The invention's effect〕

As described above, in the drive circuit configured by the integrated circuit of the present invention, it is extremely easy to make the drive current value constant for each drive circuit, and the drive current value is controlled externally. In an LED array head that is a load using a drive circuit configured by a plurality of integrated circuits,
It is possible to provide an LED array head that is a drive circuit and a load that can obtain a uniform and stable light output and that is configured by an inexpensive integrated circuit.

[Brief description of drawings]

FIG. 1 shows an MO for driving an LED according to an embodiment of the present invention.
FIG. 2 is a circuit diagram showing an S integrated circuit, and FIG. 2 is a circuit diagram showing a conventional LED driving MOS integrated circuit. 3 ... Light emitting diode (LED), 4 ... Current control element (MOS transistor), 5a, 5c ... Detection element (resistor), 5b ... Voltage drop element (resistor), 6a, 6b
...... A circuit (voltage comparator) that controls the current control element. In the drawings, the same reference numerals indicate the same or corresponding parts.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location H01L 33/00 J 7514-4M

Claims (2)

[Claims] 1. A first current control element connected to the load to control a current flowing from a power source, and a first detection connected to the load to detect a value of a current flowing to the load. A first constant current circuit including an element and a first control circuit that operates in response to a detection result of the first detection element and a first reference value to control the first current control element. And a reference value calculation element that generates the first reference value,
A second current control element connected to the reference value calculation element for controlling a current flowing through the reference value calculation element, a second detection element connected to a power supply, and flowing a predetermined current, and the second detection element. And a second constant current circuit configured to operate in response to a detection result from the element and a second reference value to control the second current control element. Drive circuit. 2. A patent in which a mirror circuit is constituted by the first constant current circuit and the second constant current circuit, and the second detection element and a second reference value are externally connected. The drive circuit according to claim 1.