JPH0880635A - Led printing head driving circuit - Google Patents

Abstract

PURPOSE: To provide an LED printing head driving circuit, by which the output voltage developed from a control voltage generating circuit can be accurately controlled and, at the same time, the rate of change of the increment of output current can be smoothed over the whole range of the output current. CONSTITUTION: A switching means 16 selects a reference voltage value within the range of the reference voltage VDD generated by a reference voltage generating source 17 so as to supply the selected reference voltage to a digital-to-analog converter 15. In the digital-to-analog converter 15, on the basis of the supplied reference voltage value, the reference voltage having the range V--V+ narrower than the range of the reference voltage is set. The digital-to-analog converter 15 sets increments in the narrow ranged reference voltage V--V+ so as to generate gate voltage VG in correspondence to the increment. The gate voltage VG controls a transistor TR in order to control the quantity of light emitted from an LED 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LE mounted on an LED print head used in a printer or a facsimile.
An LED print head drive circuit that supplies a drive current to D, in particular, L that drives an LED on the print head with a light amount set by controlling the magnitude of the drive current.
The present invention relates to an ED print head drive circuit.

[0002]

2. Description of the Related Art An LED print head is generally constructed by arranging a plurality of chips in which a plurality of LEDs are arranged in line along a print line. The drive circuit selectively supplies a drive current to the LEDs corresponding to the print dots to cause the LEDs to emit light, thereby performing line printing line by line. In this type of print head, it is necessary to equalize the light amount of each LED aligned to a predetermined target value in order to obtain uniform print quality in each row.

As is well known, since the light quantity of the LED is determined according to the magnitude of the current flowing through the LED, the correction of the light quantity of each LED is usually performed by adjusting the current quantity flowing through the LED. In a conventional LED printhead drive circuit, an FET transistor is interposed between the current source and the LED, and the gate voltage VG applied to the FET transistor is changed to change the magnitude of the drive current supplied from the current source to the LED. Have control. FIG. 5 shows a conventional example of a gate voltage generating circuit as a control voltage generating circuit for generating a gate voltage VG. This gate voltage generation circuit obtains a desired gate voltage VG = VDD · Rs / (Rb + Rs) by combining a resistor Rb and a resistor group Rs composed of a plurality of resistors and dividing the power supply voltage VDD. Resistor group Rs that influences the voltage value of the gate voltage VG
The resistance value of is set by grounding one or more appropriate resistors among the m resistors R1 to Rm connected in parallel.

[0004]

However, the resistor Rs used in the above-mentioned conventional LED print head drive circuit is used.
Since (s = 1 ... m) is a resistance having a resistance value based on a general standard, an arbitrary resistance value cannot be used. Therefore, it is difficult to accurately adjust the value of the combined resistance Rs to a desired value, and the accuracy of the partial pressure becomes rough. In addition, trying to set a better value for the partial pressure would require much more resistance.

It is also conceivable to use a D / A converter in the gate voltage generating circuit. The D / A converter outputs a voltage corresponding to the input digital data, and can output a voltage having an optimum voltage value from the equidistant steps within the range of the supplied reference voltage. However, if you set equal intervals in such a wide range,
The smaller the drive current flowing from the transistor, the more 1
The rate of change in one step (the rate of change in voltage when one step changes) becomes large, and the adjustment accuracy in the low drive current region deteriorates. For example, the reference voltage 5V is 100
In the case of equal division, the rate of increase is 100% when one increment is added from 0.05V to 0.1V, whereas the rate of increase is approximately 1% when one increment is corrected from 4.95V to 5V. Is.

Depending on the individual characteristics of the LEDs, the required current may differ for each type of LED even when they emit the same amount of light. Therefore, if the change rate of one step is greatly different as described above, the LED light amount can be corrected with a change rate of 1% in an LED that requires a large drive current region, while the same LED light amount is driven with a small amount. For an LED that can be achieved in the current amount region, a situation occurs in which the light amount of the LED can be corrected only at a larger rate of change.

The present invention has been made in view of the above circumstances, and an LED capable of accurately adjusting the output voltage from the control voltage generating circuit and smoothing the rate of change in increments of the output current over the entire range. An object is to provide a print head drive circuit.

[0008]

In order to achieve the above object, according to the invention of claim 1, a transistor for supplying a driving current from a power source to an LED and a magnitude of the driving current are defined. An LE having a control voltage generating circuit for generating a control voltage applied to a transistor, and driving an LED on a print head with a preset light quantity by controlling the magnitude of a drive current by changing the control voltage.
In the D print head driving circuit, the control voltage generating circuit switches the predetermined reference voltage based on the setting data of the driving current of the LED and outputs the predetermined reference voltage to the set value data within the range of the reference voltage value. And a D / A converter that outputs a voltage based on the voltage.

According to a second aspect of the invention, in addition to the configuration of the first aspect of the invention, binary data for setting the output voltage value of the D / A converter and the switching means are controlled. Dedicated storage means for temporarily storing the switching data for switching.

According to a third aspect of the invention, in addition to the configuration of the second aspect of the invention, an LED print head drive having a storage element for holding the binary data and the switching data to be supplied to the storage means. An LED printhead with circuitry is provided.

[0011]

According to the invention described in claim 1, the switching means is L
A reference voltage is selected based on the set value data about the drive current of the ED, and this reference voltage is supplied to the D / A converter. The D / A converter converts the input data within the range of the supplied reference voltage into an analog voltage and generates a control voltage. The control voltage is supplied to the transistor and controls the magnitude of the drive current supplied to the LED.
Thereby, the light quantity of the LED is adjusted.

According to the invention of claim 2, D /
Binary data for setting the output voltage value of the A converter and switching data for controlling the switching unit can be quickly supplied from the storage unit.

Further, according to the third aspect of the invention, the binary data and the switching data are stored in the storage element, and these data are supplied to the LED print head drive circuit during the operation of the LED print head. Optimal data is stored for each LED printhead.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. As shown in FIG. 1, an LED print head drive circuit 11 that drives the LED 10 is connected to the LED 10 mounted on the LED print head. LED10 is
The light is turned on with the amount of light according to the magnitude of the output current from the drive circuit 11.

The LED print head drive circuit 11 has five
A power supply 12 for outputting a current voltage V0 of V, and this power supply 12
And a transistor TR interposed between the LEDs 10 to control the magnitude of the drive current I. The transistor TR is, for example, a P-channel FET, and passes a predetermined drive current I according to the magnitude of the gate-source voltage. Therefore, as the gate voltage VG increases, the gate-source voltage decreases and the drive current I passing through decreases.

The transistor TR includes a switch element 13
A gate voltage generation circuit 14 as a control voltage generation circuit is connected via. The switch element 13 is on / off controlled so as to supply the generated gate voltage VG to the transistor TR only when the LED 10 corresponds to a print dot. This control is performed by the control signal CTL created based on the print data input to the LED print head.

The gate voltage generating circuit 14 generates a gate voltage VG having a preset voltage value to generate a transistor TR.
Supply to. The gate voltage VG is an m-bit D that outputs a voltage based on the input m-bit binary data.
It is output from the / A converter 15. The D / A converter 15 outputs a voltage in a voltage range V− to V + defined by the upper limit and lower limit reference voltages V + and V−. The output voltage corresponds to the value of the input m-bit binary data, and the step width is 1/2 ^m .

A switching means 16 is connected to a pair of reference voltage input terminals of the D / A converter 15 via operational amplifiers OP1 and OP2. The switching means 16 uses a plurality of resistors and a plurality of switching elements to provide two reference voltages (upper limit reference voltage and lower limit reference voltage) having a predetermined difference in the range of the reference voltage VDD supplied from the reference voltage generation source 17. ) To D / A
Supply to the converter 15. By setting the upper limit and lower limit reference voltages V + and V- arbitrarily by the setting in the switching means 16, it becomes possible to change the output voltage range V- to V + of the D / A converter 15.

In the switching means 16 of this embodiment, the six resistors R1 to R6 are the reference voltage generating source 17 and the GND1.
8 are connected in series. V from the terminal of the resistor R1
1 = VDD · (R2 + R3 + R4 + R5 + R6) / Rs
(Here, Rs = R1 + R2 + R3 + R4 + R5 + R
The voltage of 6) is taken out, and V2 =
The voltage of VDD · (R3 + R4 + R5 + R6) / Rs is taken out. From the terminal of the resistor R3, V3 = VDD. (R
4 + R5 + R6) / Rs voltage is taken out and the resistance R4
The voltage of V4 = VDD. (R5 + R6) / Rs is taken out from the terminal. And from the terminal of the resistor R5, V5
= VDD · R6 / Rs is taken out, and a voltage of V6 = 0V is taken out from the terminal of the resistor R6. Resistance R1
Is connected to the reference voltage terminal V + of the D / A converter 15 via the first switch element S1, and the terminal of the resistor R2 is connected to the reference voltage terminal V- via the second and third switch elements S2 and S3, respectively. Connected to V +. The terminal of the resistor R3 is connected to the reference voltage terminals V− and V + via the fourth and fifth switch elements S4 and S5, respectively, and the terminal of the resistor R4 is connected to the sixth and seventh switch elements S6 and S7.
Are connected to the reference voltage terminals V− and V +, respectively. Further, the terminal of the resistor R5 is connected to the reference voltage terminals V− and V− through the eighth and ninth switch elements S8 and S9, respectively.
Is connected to +, and the terminal of the resistor R6 is the tenth switch element S.
It is connected via 10 to the reference voltage terminal V−. The switching means 16 turns on / off these ten switch elements S1 to S10 based on the data input from the control means 19.
By controlling the off state and arbitrarily combining the six resistors R1 to R6, the voltage VDD from the reference voltage generation source 17 can be obtained.
Is divided and two reference voltages are obtained.

For example, as shown in Table 1, when only the first and second switch elements S1 and S2 are turned on, the voltage V1 output from the first switch element S1 is the reference voltage terminal V + of the D / A converter 14. And the voltage V2 output from the second switch element S2 is supplied to the D / A converter 1
4 reference voltage terminal V−. Therefore, from the D / A converter 14, the output voltage range V2 to V1 is 2 ^m.
A voltage value specified by m-bit binary data input from among the individual pieces is selectively output. Hereinafter, similarly, the output voltage range shown in Table 1 can be set.

[0022]

[Table 1] Here, in this embodiment, as the gate voltage VG decreases and the gate-source voltage of the transistor TR increases, that is, as the drive current flowing through the LED 10 increases, the output voltage range V− to V + expands. Since the resistance values of the resistors R1 to R6 are set, it is possible to smooth the rate of change in increments of the output current. That is, the voltage from the reference voltage generation source 17 is not equally divided by using resistors having the same resistance value, but as the output voltage VG increases, the setting range of the output voltage narrows as shown in FIG. The resistance value is set to. Since the output of the D / A converter 15 is stepped at equal intervals within the range of the upper limit / lower limit reference voltage, if the voltage is directly set from the reference voltage generation source 17 as in the conventional case, as described above, D
As the output voltage VG of the / A converter increases and the drive current decreases, the rate of change of the drive current increases (dotted line in FIG. 2). As a result, the light amount of the LED changes greatly in the range where the driving current is small. On the other hand, if the resistance value is set so that the set range of the output voltage (the range of the upper and lower reference voltages) becomes narrower as the output voltage VG becomes larger, the step becomes finer in the region where the driving current is small, and as a result, the entire range The rate of change in the LED light amount can be equalized over the entire range (solid line in FIG. 2). Therefore, the rate of change in the low drive current region can be reduced, and fine light amount control can be performed.

Next, the operation of the present embodiment will be described. First, the switch element 13 is turned on, the transistor TR is turned on, and the LED 10 is turned on in a state where voltages are respectively output from the power source 12 and the reference voltage source 17. At this time, the input data to the switching means 16 and the D / A converter 15 is set to a predetermined value. In this state, the light quantity of the LED 10 is measured. Then, the gate voltage VG to be set is determined according to the amount of light obtained by the measurement. Then, the switching means 16 and the m-bit data are adjusted according to the set voltage, and the light amount of the LED 10 is adjusted to an arbitrary target value. Thus, in this embodiment, the range in which the gate voltage VG is changed is narrowed down in advance, and the switching means 16 is switched to switch the D / A.
The output voltage range of the converter 15 is set. Then, D
For the A / A converter 15, m binary data are determined from the upper bits, and the gate voltage VG is determined in steps of 2 ^m.
To obtain a desired amount of light emission.

As described above, according to the present invention, the upper and lower reference voltages supplied to the D / A converter 15 are changed to change the output voltage range. Therefore, the step can be made fine without increasing the number of bits of the D / A converter 15, and as a result, the gate voltage VG can be adjusted with high accuracy. Further, the range of the upper limit / lower limit reference voltage is changed according to the gate current VG. LED by this
A fine adjustment of the light intensity of 10 is achieved.

The m-bit binary data for setting the output voltage of the D / A converter 15 and the switching data for controlling the switching means 16 are used every time the LED 10 is turned on. Therefore, the LED print head drive circuit 11 is provided with a storage means such as a latch for storing these data.
In this case, the quick operation of the drive circuit 11 is ensured. Further, when the drive circuit 11 is mounted on the LED print head, if the print head is provided with a storage element such as a ROM that holds data in the storage means of the LED print head, the drive circuit always outputs appropriate data for each print head. 11 and the data can be easily changed by changing the ROM.

In actual printing, the switching element 13 is turned on / off to control the emission of the corresponding LED 10. Then, since the light emission amount of each LED 10 is adjusted with the same configuration, uniform printing is performed.

FIG. 3 shows a second embodiment of the present invention. This gate voltage generation circuit 20 is a 9-bit D / A converter 2
1 is provided. The switching means 22 connected to the D / A converter 21 includes two resistors Ra and Rb connected in series between the reference voltage generation source 23 and the GND 24. The resistance value ratio is set to Ra: Rb = 1.3: 1.2. A voltage of VDD = 5.0V is supplied from the reference voltage generation source 23, and as a result, a voltage of 2.4V is extracted from the intermediate point 25 between the resistors Ra and Rb and a voltage of 0V is extracted from GND.

The voltage extracted from the terminal at the midpoint 25 between the resistors Ra and Rb is supplied to the reference voltage terminal V- of the D / A converter 21 via the switch element SWa. Similarly, the voltage extracted from GND is the switching element SWb.
Is supplied to the reference voltage terminal V- of the D / A converter 21 via. The voltage from the reference voltage generation source 23 is directly input to the reference voltage terminal V + of the D / A converter 21.
Therefore, the switching means 22 sets the output voltage range of the D / A converter 21 to 5.0 V to 2.4 V and sets the gate-source voltage of the transistor TR to 0 V to 2 when the switch element SWa is turned on. It can be set within the range of 0.6V. When the switch element SWb is turned on,
The output voltage range of the D / A converter 21 is 5.0V to 0
Set to V. At this time, the gate-source voltage of the transistor TR can be set within the range of 0V to 5.0V. The switch elements SWa and SWb are on / off controlled by the control means 26 based on the data input from the latch as the storage means. The control means sets H when the MSB of the data stored in the latch is “1”, for example.
Is output and the switch element SWb is turned on. The control signal is inverted and input to the switch element SWa. Therefore, when one switch element is on, the other switch element is off.

The 9-bit binary data input to the D / A converter 21 is supplied from a latch as a storage means. This latch may be a latch common to the latch that stores the switching data supplied to the control means 26.

According to the LED print head drive circuit of the second embodiment, when the required gate-source voltage is high and therefore the gate voltage VG is low, the switch element SWa is turned off and the switch element SWb is turned on. gate voltage V to a range of 5.0V~0V were increments of 2 ⁹
Output G. As a result, the light amount of the LED 10 is changed corresponding to the gate-source voltage 0V to 5.0V of the transistor TR. On the other hand, when the required gate-source voltage is low and therefore the gate voltage VG is high, the switch element SWa is turned on and the switch element SWb is turned on.
Turn off the outputs of the gate voltage VG was 2 ⁹ increments the scope of 5.0V～2.4V. At this time, the light quantity of the LED 10 is 0V to the gate-source voltage of the transistor TR.
Changed corresponding to 2.6V. Therefore, it is possible to smooth the output current change rate by reducing the step in the low drive current region of the LED 10.

FIG. 4 shows a third embodiment of the present invention. The gate voltage generating circuit 30 is an 8-bit D / A converter 3
1 is provided. The switching means 32 connected to the D / A converter 31 includes three resistors Rc, Rd, and Re connected in series between the reference voltage generation source 33 and the GND 34.
The resistance value ratio is Rc: Rd: Re = 1.8: 1.2:
It is set to 2.0. VDD from the reference voltage source 33
= 5.0V voltage is supplied, resulting in resistors Rc and R
A voltage of 3.2 V is applied from the terminal 35 in the middle of d to the resistance Rd.
The voltage of 2.0V from the terminal 36 between the
A voltage of 0V is taken out from D.

The third embodiment differs from the second embodiment described above in that the adjustment range of the gate-source voltage of the transistor is raised by 1.8 V in consideration of the characteristics of the transistor in advance. is there. That is, according to the LED print head drive circuit of the third embodiment, when the required gate-source voltage is high and therefore the gate voltage VG is low, the switch element SWa is turned off and the switch element SWb is turned on. The range from 2V to 0V is 2 ⁸
The chopped gate voltage VG is output. By this, L
The light amount of the ED 10 is changed in accordance with the gate-source voltage of the transistor TR, which is 1.8V to 5.0V. on the other hand,
When the required gate-source voltage is low and therefore the gate voltage VG is high, the switch element SWa is turned on and the switch element SWb is turned off, and the voltage is 3.2 V or more.
The range of 2.0V to output the 2 ⁸ increments the gate voltage VG. At this time, the light amount of the LED 10 is
It is changed corresponding to the gate-source voltage of 1.8V to 3.0V. By raising the 1.8V, the output voltage of the D / A converter can be made finer without increasing the number of bits of the D / A converter.

[0033]

According to the first aspect of the present invention, since the reference voltage that determines the output voltage range is changed when the D / A converter performs D / A conversion, the input data LSB 1 bit of the D / A converter is changed. The change (step) of the output voltage corresponding to can be changed. Therefore, it is possible to make the steps fine without increasing the number of bits of the D / A converter. Therefore, the LED light amount can be finely adjusted with higher accuracy.

According to the invention of claim 2, D /
It is possible to rapidly supply the binary data for the A converter and the switching data for the switching means, and to operate the LED print head drive circuit quickly and reliably.

Further, according to the invention of claim 3, L
Since the ED print head has the storage element for storing the binary data and the switching data, it is possible to always send appropriate data to the LED print head drive circuit for each LED print head, and to change the data such as changing the stored content. Can be easily achieved by.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an LED print head drive circuit according to a first embodiment of the present invention.

FIG. 2 is a graph showing a relationship between an output voltage of a D / A converter and a driving current change rate of a transistor.

FIG. 3 is a circuit configuration diagram of a gate voltage generation circuit of an LED print head drive circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit configuration diagram of a gate voltage generation circuit of an LED print head drive circuit according to a third embodiment of the present invention.

FIG. 5 is a circuit configuration diagram of a gate voltage generation circuit of a conventional LED print head drive circuit.

[Explanation of symbols]

10 LED 11 LED print head drive circuit 12 Power supply 14, 20, 30 Gate voltage generation circuit as control voltage generation circuit 15, 21, 31 D / A converter 16, 22, 32 Switching means 17, 23, 33 Reference voltage generation source TR transistor VDD Reference voltage VG Gate voltage as control voltage

Claims (3)

[Claims] 1. A transistor for supplying a driving current from a power source to an LED, and a control voltage generating circuit for generating a control voltage applied to the transistor for defining the magnitude of the driving current. In an LED printhead drive circuit that drives an LED on a printhead with a preset amount of light by controlling the magnitude of a drive current, the control voltage generation circuit, based on setting data about the drive current of the LED, An LED print head drive circuit comprising: a switching unit that switches and outputs a predetermined reference voltage; and a D / A converter that outputs a voltage based on the set value data within a range of a reference voltage value. 2. A dedicated storage unit for temporarily storing binary data for setting an output voltage value of the D / A converter and switching data for controlling the switching unit. The LED print head drive circuit according to claim 1. 3. The LED print head having the LED print head drive circuit according to claim 2, further comprising a storage element for holding the binary data and the switching data supplied to the storage means.