JPH0828528B2 - LED array brightness adjustment method and LED array brightness adjustment resistor resistance value detection device - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention provides a method for adjusting the brightness of an LED print head in which a plurality of LED arrays are arranged, and a method for adjusting the brightness of the LED array.
The present invention relates to a resistance value detection device for a brightness adjustment resistor of an ED array.

[Prior art]

The LED print head has a configuration in which a plurality of LED arrays each driven by a separate IC are arranged as its light source. Normally, one chip IC is 64 units
It is designed to drive an LED array consisting of LEDs, and for example, 38 LED arrays and their corresponding ICs are required to enable printing of A3 width at 8 dots / mm. In order to obtain high-quality prints with such an LED print head, it is essential to eliminate variations in the brightness of each LED array, but it is complicated to accurately standardize the characteristics of each IC chip. It is difficult to be manufactured through various manufacturing processes, and as a result, LEDs under the same conditions
There is a problem that the brightness of the array inevitably varies. As a workaround for this issue, the same LED
There are a method of matching the characteristics of the LEDs and ICs used in the print head, and a method of controlling the drive current for each IC corresponding to each LED array. The former reduces the yield of ICs, and the latter reduces the control system. It is not appropriate because it complicates.

[Problems to be Solved by the Invention]

By the way, in JP-A-62-65486, by adjusting the resistance value of the adjustment resistor externally attached to the IC for each LED array, the gate voltage of the output transistor that causes a current to flow through each unit LED is set to a desired value. LEDs configured to be set to
Array driving ICs have been proposed. When such an IC is used, the brightness of each LED array can be made uniform by selecting the resistance value of the adjustment resistor, even if the characteristics of the IC itself vary. However, the LED
A method for efficiently and accurately selecting the resistance value of the adjusting resistor so that the brightness of the array has a predetermined value has not been proposed yet. Therefore, an object of the present invention is to efficiently and accurately select the resistance value in an LED array in which the brightness is changed by selecting the resistance value of the adjustment resistor externally attached to the IC. To provide.

[Means for Solving the Problems]

In order to solve the above problems, the present invention takes the following technical measures. First, according to the brightness adjusting method for an LED array of the invention described in claim 1 of the present application, the above-mentioned output transistor is provided to a drive circuit element for supplying a drive current to an LED array having a plurality of light emitting regions through the output transistor. A method of adjusting the brightness of the LED array by bias-connecting an external adjustment resistor to the intermediate point between the power supplies and setting the intermediate point to a prescribed voltage. At the point, the control transistor is interposed, and the base potential of the control transistor is controlled so that the brightness of the LED when in the ON state is detected and the brightness is set to a predetermined brightness determined in advance. It is characterized in that the resistance value between the collector and the emitter of the transistor for use is detected and the resistance value of the resistance to be connected to the intermediate point is determined by this. Further, the resistance value detecting device for the brightness adjusting resistor of the LED array according to the invention described in claim 2 of the present application is directed to a drive circuit element for passing a drive current through an output transistor to an LED array having a plurality of light emitting regions. When adjusting the brightness of the LED array by bias-connecting an external adjustment resistor to the intermediate point between the output transistor and the power supply and setting the intermediate point to a predetermined voltage, the resistance value of the adjustment resistor is detected. A resistance value detecting device, which is a brightness detecting means for detecting the brightness of an LED in an ON state, a control transistor whose collector is brought into conduction at the intermediate point and an emitter is connected to a negative power supply, and a collector of the control transistor. There are a collector potential detecting means for detecting a potential, an emitter current detecting means for detecting an emitter current, and a brightness signal from the brightness detecting means. Collector potential control means for controlling the collector potential of the control transistor so as to have a predetermined value which is determined by gaze,
Resistance calculating means for calculating the resistance between the intermediate point and the negative power source from the collector potential and the emitter current detected when the brightness signal reaches a predetermined value.

[Action]

The collector potential control is directly performed by changing the base voltage of the control transistor. Then, the resistance between the collector and the emitter of the control transistor itself substantially changes, so that the collector potential, that is, the potential at the intermediate point changes, and the gate voltage of the output transistor changes. When the gate voltage of the output transistor changes, the current flowing through the LED changes, and the brightness of the LED array changes accordingly. In the present invention, the collector potential of the control transistor is changed so that the sampled brightness has a predetermined value while the brightness of the LED array is sampled. When the brightness of the LED array is set to a predetermined value in this way, the resistance value between the intermediate point and the negative electrode can be calculated from the collector potential and the emitter current at that time. An adjusting resistor having a resistance value detected by the present invention is provided as an external resistor between the intermediate point of the IC and the minus power supply. Then, the gate voltage whose target value is the brightness of the LED array is reproduced. By installing the external adjustment resistors set as above for all LED arrays on the LED print head, even if there are variations in the IC characteristics of each LED array,
You can easily adjust the brightness of all LED arrays to be uniform.

【The invention's effect】

As described above, in the present invention, in order to obtain the luminance of a predetermined LED array, the collector potential of the control transistor that is conducted to the LED array at the time of inspection is controlled while sampling the luminance of the LED array. The resistance value of the external adjustment resistor can be determined.

[Explanation of the embodiment]

Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of the unit drive block 1 of the LED array and the device of the present invention. The drive block means a minimum unit that can be driven in the LED head. In the figure, the part surrounded by the broken line shows the drive block 1 of the LED array. On one side of the drive block 1, a plurality of unit LED arrays 2 ... Which are connected in parallel to the negative power source V _SS are linearly arranged, and the positive terminals of these unit LED arrays 2 ... It is connected to the output terminal of each output transistor 4 in the upper IC3. It should be noted that the output transistor 4 here means that when an ON signal from the logic circuit 5 is input, power is supplied from the positive power source V _DD and the brightness decreases as the gate voltage increases. The function of passing a current through each of the unit LEDs 2 is called. In addition, a positive power supply V
The potential of the intermediate point CT of the internal resistance R ₁ and the external adjustment resistance R ₂ (see FIG. 2) interposed in series between _DD and the minus power supply 6 outside the array is input as the gate voltage. The intermediate point CT is led to the external terminal of the IC. The logic circuit 5 has a data-in terminal.
It has a function of issuing an ON signal to the output transistors 4 ... Corresponding to each unit LED 2 ... According to the data input to D _IN . On the other hand, the resistance value detecting device 7 of the present invention for determining the resistance value of the external adjustment resistor R ₂ is provided with a control transistor 8 for conducting the collector at the intermediate point CT and the emitter at the negative power source 7, respectively. The base of the control transistor 8 is connected to the central processing unit 10 via the D / A converter 11 so as to be controlled by the collector potential control means 9 which will be described later and which is substantially realized by a program. . Further, the luminance signal from the photodiode 12 as the luminance detecting means for sampling the luminance of the LED array is input to the central processing unit 10 via the A / D converter 13 as a control input. Further, the detection signals from the collector potential detector 14 that detects the collector voltage of the control transistor 8 and the emitter current detector 15 that detects the emitter current are A /
It is input to the central processing unit 10 via the D converters 16 and 17, respectively. For the conduction of the collector of the control transistor 8 to the intermediate point CT on the drive block and the supply of predetermined data and power to the logic circuit 5 of the LED array, the prober provided in the test head (not shown) This is performed by contacting a predetermined terminal corresponding to each drive block of the head. The photodiode 12 is also attached to this test head. And this test head
The prober is sequentially brought into contact with a predetermined terminal for each drive block. Further, in FIG. 1, reference numeral 18 is a read-on-memory in which the program is stored,
Reference numeral 19 is a random access memory for recording control data. Next, an example of a control procedure of the collector potential control means 9 and the resistance value calculation means 20 which are substantially realized by a program will be described based on the flowchart of FIG. Here, as an example, the collector potential of the control transistor 8 is variably controlled from 0V to 5V in response to the fact that the gate voltage of the output transistor 4 in the IC is set in the range of 0V to 5V. It should be possible. First, in the initial setting of step 101, as shown in FIG. 4, the initial gate voltage V _{G of} the address a, the low level reference potential of the address b, the high level reference potential of the address c, and the address d of the address d in the memory are set. LED target brightness I
Write initial data to _V respectively. As the initial gate voltage, for example, a user-set gate potential for the target brightness is written. The low-level reference potential and the high-level reference potential have the collector potentials as described above.
Corresponding to the fact that 0V to 5V can be changed, 0 and 5 are written as initial data. Next, in step 102, the data at the address a is read out, and in step 103, the gate voltage, that is, the collector potential of the control transistor 4 is controlled based on the data, and all the unit LEDs on the LED array are turned on. To do. Next, in step 104, the brightness I _V ′ of the LED array is measured, and in step 105, the measured brightness I _V ′ and the address d
Compare the target brightness I _V of. Here, if the measured brightness I _V ′ is larger than the target brightness I _V, it means that the gate voltage is too small.
At 08, the data of address a is enhanced and corrected. In this example, in step 106, the data of the address a is loaded into the address b to increase and update the low-level reference potential data of the address b, and in step 107, the average of the data of the address b and the data of the address c is calculated, And step
In step 108, the result of the above calculation is loaded into the address a. On the other hand, if the measured brightness I _V ′ is smaller than the target brightness I _V in step 105, it means that the gate voltage is too large. Therefore, in steps 109 to 111, the data at the address a is corrected to be low. In this example, step 109
At the same time, the data of the address a is loaded into the address c to lower and update the high level reference potential data of the address c, the average of the data of the address d and the data of the address c is calculated in step 110, and the above calculation is performed in step 111. This is done by loading the result into address a. While repeating such a process, both the high-level reference potential of the address b and the low-level reference potential of the address c approached the gate voltage when the brightness of the LED array was assumed to be the target brightness, and then measured. Brightness _IV 'is the target brightness
Matches I _V. That is, the gate voltage at this time is LED
It is the gate voltage required to make the array emit light at the target brightness. Then, next, in step 112, the collector potential Vc and the emitter current Ie of the control transistor 4 at this time.
Is detected, and the control transistor 8 is detected in step 113.
Calculate the resistance value between the collector and emitter of. This resistance value is the resistance value of the adjustment resistor R ₂ to be externally attached, and as shown in FIG. 2, the external adjustment resistor R ₂ having the resistance value thus determined is interposed between the intermediate point CT and the negative power source 6. When mounted, the gate voltage that should obtain the target brightness is reproduced and LE
The D array 2 will emit light with the target brightness. For all LED arrays on the LED print head, if the target brightness is the same and an external resistor with the resistance value obtained by the above method is connected, even if it is used for each LED array I
Even if there are variations in the characteristics of C, such ICs
It is possible to eliminate the variation in the brightness of each LED array almost completely while using the LED as it is, and it is possible to significantly improve the print quality of the LED print head.

[Brief description of drawings]

FIG. 1 is a circuit diagram schematically showing the configuration of the present invention, FIG. 2 is a circuit diagram of an LED array as a product, and FIG. 3 is a collector potential control means and resistance value calculation means substantially realized by a program. A flow chart for explaining
FIG. 4 is a memory area diagram in the initial state. 1 ... LED array, 3 ... IC, 4 ... Output transistor, 7 ... Resistance detection device, 8 ... Control transistor, 9 ... Collector potential control means, 12 ... Brightness detection means (photodiode) , 14 ... collector potential detecting means (collector potential detector), 15 ... emitter current detecting means (emitter current detector), 20 ... resistance value calculating means, R ₂ ...
… Adjustment resistance.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H01L 33/00 J

Claims (2)

[Claims] 1. An external adjusting resistor is bias-connected to an intermediate point between the output transistor and a power source for a drive circuit element for supplying a drive current to an LED array having a plurality of light emitting regions through an output transistor. A method of adjusting the brightness of an LED array by setting the intermediate point to a predetermined voltage, wherein a control transistor is interposed at the intermediate point before the adjustment resistor is mounted, LE
The brightness of D is detected and the base potential of the control transistor is controlled so that it becomes a predetermined brightness, and the resistance value between the collector and the emitter of the control transistor is detected and A method for adjusting the brightness of an LED array, which is characterized in that the resistance value of the adjustment resistor to be connected to the intermediate point is determined. 2. An external adjusting resistor is bias-connected to an intermediate point between the output transistor and a power source for a drive circuit element for supplying a drive current to the LED array having a plurality of light emitting regions through the output transistor. In adjusting the brightness of the LED array by setting the intermediate point to a predetermined voltage, a resistance value detecting device for detecting the resistance value of the adjusting resistor, the brightness detecting means for detecting the brightness of the LED in the ON state. And a control transistor whose collector is brought into conduction at the intermediate point and whose emitter is connected to the negative power source, collector potential detection means for detecting the collector potential of the control transistor, and emitter current detection means for detecting the emitter current. The collector potential of the control transistor is set so that the brightness signal from the brightness detection means has a predetermined value. A collector potential control means for controlling the resistance, and a resistance calculation means for calculating the resistance between the intermediate point and the negative power source from the collector potential and the emitter current detected when the luminance signal reaches a predetermined value. And a resistance value detecting device for a brightness adjusting resistor of an LED array.