JP2009064324A - Reference voltage circuit, drive circuit, print head, and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reference voltage generating circuit, a drive circuit, a print head, and an image forming apparatus which compensate minus temperature dependency of LED light emission power and temperature dependency of a reference resistor in a driver IC and are capable of arbitrarily setting a desired temperature coefficient and an output voltage value. <P>SOLUTION: A reference voltage generating circuit 39 comprises a regulator circuit 71, a resistor 81, and a temperature-sensitive resistor 82, wherein a power supply terminal of the regulator circuit 71 is connected to a power supply VDD, and an output terminal is connected to one end of the resistor 81, while a ground terminal is connected to the other end of the resistor 81. The ground terminal of the regulator circuit 71 is connected to one end of the temperature-sensitive resistor 82 and connected to a reference voltage output terminal Vref. The other end of the temperature-sensitive resistor 82 is connected to the ground. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention selects a group of driven elements, for example, a row of LEDs in an electrophotographic printer using a light-emitting diode (hereinafter referred to as LED) as a light source, a row of heating resistors in a thermal printer, and a row of display devices in a display device. In particular, the present invention relates to a reference voltage generation circuit for driving cyclically, a drive circuit having the reference voltage generation circuit, and further to a print head and an image forming apparatus having such a drive device.

  In the following description, the light emitting diode is abbreviated as LED (Light Emitting Diode), the monolithic integrated circuit is abbreviated as IC (Integrated Circuit), the N channel MOS (Metal Oxide Semiconductor) transistor is abbreviated as NMOS, and the P channel MOS transistor is abbreviated as PMOS. Further, the same names are used for the signal terminal names and the signal names input to and output from the signal terminal names. Each of the electrostatic latent image formed on the photosensitive drum by the light emission of each light emitting element or the toner image after development or transferred onto the print medium is referred to as a dot.

  Similarly, each light emitting element corresponding to the dot is called a dot. The LED head taken up in this document is a general name of a unit in which a light emitting element and its driving element are arranged. When the LED head is applied only to a printer device, it is called an LED print head. In the following description, it is assumed that the group of driven elements is an LED array used in an electrophotographic printer.

  In a conventional image forming apparatus, for example, an electrophotographic printer, an electrostatic latent image is formed by selectively irradiating a charged photosensitive drum according to print information, and toner is attached to the electrostatic latent image. Development is performed to form a toner image, and the toner image is transferred to a sheet and fixed. As such an electrophotographic printer, one using an LED as a light source is known. An LED head used in such a printer includes an LED array chip in which a plurality of LED elements are arranged, and a driver IC that drives the LED array chip.

  The LED head includes a reference voltage generation circuit that generates a reference voltage, and a reference voltage generated from the reference voltage generation circuit and a drive current for driving the LED element are determined by a resistor disposed in the driver IC. ing. The resistor is created by using a semiconductor process technology. As a material of the resistor element, polysilicon, an impurity diffusion resistor or the like is generally used, and is monolithically integrated in the driver IC.

  In the electrophotographic printer having the above configuration, the temperature dependence of the light emission power of the LED element has a negative temperature coefficient, and it is known that the light emission power decreases as the junction temperature of the LED array chip increases. Yes.

  As an example, in an LED made using a GaAsP substrate, it is about −0.6% / ° C., and that using an AlGaAs substrate is −0.25% / ° C., infrared using a GaAs substrate. Depending on the composition of the compound semiconductor used, the emission wavelength, etc., such as −1% / ° C. for LEDs, there is a difference in the temperature dependence of the emission power.

  As described above, the LED element driving means is provided in the LED head. However, in order to compensate for the negative temperature coefficient of the LED light emission power, the temperature coefficient of the LED driving current value can be configured as a positive one. desirable. Since the drive current output value from the driver IC is determined by the resistor arranged in the driver IC and the output voltage value of the reference voltage generation circuit, the temperature coefficient of the resistor (usually having a positive value) is taken into consideration. Therefore, it is necessary to give a positive temperature characteristic to the output voltage of the reference voltage generation circuit.

  As described above, the LED head needs to maintain the light emission power at a predetermined value even if there is a temperature fluctuation accompanying LED driving, and can drive to compensate for the temperature dependence of the light emission power of the LED element described above. There is a need to have a method. As a circuit provided with such a temperature compensation circuit, for example, there is one disclosed in Japanese Patent Laid-Open No. 10-332494 (Patent Document 1). Hereinafter, it demonstrates using drawing.

  FIG. 12 is a circuit diagram showing a driving circuit of the LED head, and FIG. 13 is a circuit diagram showing a reference voltage generating circuit disclosed in Patent Document 1. FIG. 12 shows the main part of the driver IC, shows the connection relationship between the LED drive circuit and its peripheral circuits, and shows one LED element (typically, dot 1).

  In FIG. 12, a portion G1 surrounded by a broken line is a prebuffer circuit, and an AND circuit 42, a PMOS transistor 43, and an NMOS transistor 44 are arranged in the prebuffer circuit G1. G0 is an inverter circuit, and LT1 is a latch circuit. A portion 36 surrounded by an alternate long and short dash line is a control voltage generation circuit, and one circuit is provided for each driver IC chip.

  Reference numeral 51 denotes an operational amplifier which is described in the figure as a potential whose output voltage is Vcont. The potential is a control voltage applied to the LED driving transistor Tr1 in order to adjust the driving current of the LED element LD1. Reference numeral 53 denotes a resistor, and the resistance value is described as Rref in the figure. A PMOS transistor 52 is configured such that the gate length is the same as that of the LED driving transistor Tr1.

  VREF is a reference voltage input terminal which is connected to the inverting input terminal of the operational amplifier 51 and receives a reference voltage Vref generated from a reference voltage generation circuit described later. A circuit comprising an operational amplifier 51, a PMOS transistor 52, and a resistor 53 constitutes a feedback control circuit, and the current (Iref) flowing through the resistor 53, that is, the current flowing through the PMOS transistor 52 does not depend on the power supply voltage (VDD). The configuration is determined only by the voltage Vref and the value Rref of the resistor 53.

  That is, since the potential of the inverting input terminal and the potential of the non-inverting input terminal are controlled to be substantially equal by the operation of the operational amplifier 51, the potential of the non-inverting input terminal of the operational amplifier 51 is substantially equal to the reference voltage Vref. The current Iref flowing through the resistor 53 is given as Iref = Vref / Rref.

  As described above, the LED driving transistor Tr1 and the PMOS transistor 52 are configured to have the same gate length, and the gate potential is equal to Vcont when the LED is driven, and the PMOS transistor 52 and the LED driving transistor Tr1 are driven. Operates in the saturation region and has a current mirror relationship.

  As a result, the drive current value of the LED element LD1 is proportional to the current Iref flowing through the resistor 53, and this reference current Iref is proportional to the reference voltage Vref input to the VREF terminal. Therefore, the LED drive current is determined by the reference voltage Vref. It is possible to adjust the values collectively.

  FIG. 13 shows a reference voltage generation circuit 37 for generating the reference voltage Vref. In FIG. 13, PMOS transistors 61 to 63 of the same size whose source terminals are connected to the power supply VDD are connected to their gate terminals to form a current mirror circuit, and the drain output of the PMOS transistor 61 is a resistor connected in series. The emitter of the NPN bipolar transistor 64 is connected to the ground, and the base thereof is connected to the connection point of the resistors 66 and 67.

  On the other hand, the drain of the PMOS transistor 62 of the current mirror is connected to the collector of the NPN bipolar transistor 65, the emitter of the NPN transistor 65 is connected to the ground, and the base thereof is connected to the collector of the NPN bipolar transistor 64. The drain of the PMOS transistor 63 is connected to the ground via the resistor 68. Here, the emitter area of the NPN bipolar transistor 65 is set to N times the emitter area of the NPN bipolar transistor 64. Further, the connection point between the drain of the PMOS transistor 63 and the resistor 68 becomes the output of the reference voltage generation circuit 37, which is described as Vref in FIG. 13, and the voltage value at the terminal becomes the reference voltage Vref.

  As disclosed in Patent Document 1, an output voltage having a positive temperature coefficient with respect to temperature is obtained from the reference voltage generation circuit 37 shown in FIG. This will be briefly described below. Note that the resistance values of the resistors 66, 67, and 68 shown in the figure are denoted as R1, R2, and R3, respectively.

Assuming that the base current is negligible with respect to the collector currents of the bipolar transistors 64 and 65 in the reference voltage generating circuit 37 of FIG. 13, the output voltage Vref of the reference voltage generating circuit 37 is
Vref = (R3 / R2) (kT / q) ln (N)
Given in. Here, k is the Boltzmann constant, T is the absolute temperature, q is the charge of the electron, and ln () is a natural logarithmic function.
Here, the temperature coefficient Tc of Vref is
Tc = (1 / Vref) x (ΔVref / ΔT)
, The temperature coefficient of the Vref voltage is given by 1 / T, and the value at room temperature (about 300 K) is about + 0.33% / ° C. (see Patent Document 1, page 16).

In an LED element made of a GaAlAs base material used for an LED head, the temperature dependence of the light emission power is approximately −0.25% / ° C., while the temperature coefficient of the reference resistance in the driver IC configured by the CMOS process is about + 0.1% / ° C.
The LED temperature is substantially equal to the temperature of the driver ICs arranged adjacent to each other, and the LED elements are arranged to be substantially equal by arranging the LED chips and the above-described reference voltage generation circuit on the ground wiring formed on the printed wiring board. can do.
For this reason, in order to compensate the decrease in the light emission power accompanying the increase in the LED temperature, the reference voltage Vref is
− (− 0.25−0.1) = ＋ 0.35% / ℃
It suffices to give a temperature coefficient of a certain degree. This value is substantially equal to the temperature coefficient of the reference voltage generation circuit 37 described above.

  Next, a conventional example disclosed in another document disclosing a temperature compensation circuit (Patent Document 2: Japanese Patent Laid-Open No. 2000-159472) will be described. FIG. 14 is a circuit diagram showing a reference voltage generating circuit disclosed in Patent Document 2. In FIG. In FIG. 14, the reference voltage generation circuit 38 is provided with a regulator circuit 71, diodes 72 and 73, and resistors 74 and 75.

  The first terminal of the regulator circuit 71 is a power supply terminal connected to the power supply VDD, the second terminal is an output terminal connected to the anode of the diode 72, and the third terminal of the regulator circuit 71 is a ground terminal connected to the ground. Connected. The cathode of the diode 72 is connected to the anode of the diode 73, the cathode of the diode 73 is connected to one end of the resistor 74, and the other end of the resistor 74 is connected to the ground via the resistor 75. The connection midpoint of the resistors 74 and 75 is connected to the reference voltage Vref terminal.

In FIG. 14, the output voltage of the regulator circuit 71 is denoted by Vo, the forward voltage of the diodes 72 and 73 is denoted by Vf, the cathode potential of the diode 73 is denoted by Vk, and the resistance values of the resistors 74 and 75 are denoted by R1 and R2, respectively. Than this,
Vk = Vo − 2 × Vf
Vref = R2 x Vk / (R1 + R2) = R2 x (Vo-2 x Vf) / (R1 + R2)
The following relational expression is obtained. In these equations, since the forward voltage of the diodes 72 and 73 decreases at a rate of about −2 mV / ° C. with respect to the temperature rise, the reference voltage value Vref has a characteristic that increases almost linearly with respect to the temperature rise. I know that there is.

Here, let us find the temperature coefficient of the reference voltage Vref. Since the temperature dependency of the resistors 74 and 75 is small and the temperature dependency of the regulator circuit 71 itself is small, these temperature coefficients can be ignored, and the temperature coefficient Tc of the reference voltage Vref in the circuit of FIG.
Tc = (1 / Vref) x (ΔVref / ΔT) = 2 / (Vo-2 x Vf) x (-ΔVf / ΔT)
It can be asked.

Here, the temperature coefficient and the reference voltage value are obtained by actually applying numerical values.
(Numerical example 1)
As typical numerical values, the forward voltage Vf of the diodes 72 and 73 is 0.6 V, the temperature coefficient of the forward voltage is −2 mV / ° C., the output voltage value of the regulator circuit 71 is Vo = 2.5 V, and the numerical value is R1 = 0. And applying
Tc = (1 / Vref) × (ΔVref / ΔT) = 2 / (2.5−2 × 0.6) × (2mV / ° C)
= +0.31 [% / ℃]
Get. The output voltage Vref at this time is
Vref = 2.5−2 × 0.6 = 1.3 [V]
It is.

(Numerical example 2)
As another case, let us calculate the case where Vo = 1.87V as the output voltage value of the regulator circuit 71. At this time, the output voltage Vref is
Vref = 1.87−2 × 0.6 = 0.67 [V]
At this time, the temperature coefficient is
Tc = 2 x (2 mV / ° C) /0.67 = 0.6 [% / ° C]
Thus, although a temperature coefficient approximately twice that of the previous case was obtained, it can be seen that the obtained output voltage Vref is reduced to about ½.
As described above, the temperature coefficient of the LED drive IC alone is about -0.1% / ° C, and when combined with the reference voltage generation circuit, the temperature coefficient of the LED drive current is + 0.5% / ° C, which depends on the temperature. It can be seen that it is suitable for the temperature compensation of an LED having a property of −0.5% / ° C.

(Numerical example 3)
As yet another case, let us calculate the output voltage value of the regulator circuit 71 when Vo = 1.56V. At this time, the output voltage Vref is
Vref = 1.56−2 × 0.6 = 0.36 [V]
At this time, the temperature coefficient is
Tc = 2 x (2 mV / ° C) /0.36 = 1.1 [% / ° C]
Thus, although a temperature coefficient approximately twice that of the previous case was obtained, it can be seen that the obtained Vref output voltage is further reduced to about ½.

In addition, as described above, the temperature coefficient of the LED driving IC alone is about −0.1% / ° C., and the temperature coefficient of the LED driving current when combined with the reference voltage circuit is + 1% / ° C. It can be seen that it is suitable for temperature compensation of LEDs in which becomes −1% / ° C.
Japanese Patent Laid-Open No. 10-332494 JP 2006-159472 A

  The LED head needs to be able to maintain the light emission power at a predetermined value even if there is a temperature fluctuation associated with LED driving, and it is necessary to have a driving method that can compensate for the temperature dependence of the light emission power of the LED. However, as described above, since the temperature dependence of LEDs varies, a temperature compensation circuit capable of obtaining a predetermined temperature coefficient with a simple configuration has never been desired.

  For example, in the reference voltage generation circuit (FIG. 13) disclosed in Patent Document 1, the temperature coefficient can only take a value inversely proportional to the absolute temperature, and is used for temperature compensation of LEDs having various temperature dependencies. There was a problem that could not.

  In addition, in the reference voltage generation circuit disclosed in Patent Document 2 (FIG. 14), it is theoretically possible to perform temperature compensation for an LED having a temperature dependency of −0.5% / ° C. or −1% / ° C. Although it is possible, if an attempt is made to set a large temperature coefficient, the reference voltage that can be output becomes a minute voltage value such as Vref = 0.67 V or Vref = 0.36 V, and matches the desired value of the LED driver IC. It was difficult.

  As another method, the number of diodes used in the circuit shown in FIG. 14 is increased from two series connections to three or four series connections, and the output voltage of the regulator circuit is increased at the same time. Although there is a problem that the number of necessary diode elements increases and the cost increases, the required output voltage of the regulator circuit also increases to a value close to the power supply voltage VDD. Therefore, the desired regulation characteristics cannot be obtained, and in some cases, an output voltage exceeding the power supply voltage is required.

  The present invention compensates for the negative temperature dependence of LED light emission power and the temperature dependence of the reference resistance in the driver IC, and is a reference voltage that can arbitrarily set a desired temperature coefficient and output voltage value. An object is to provide a generation circuit, a drive circuit, a print head, and an image forming apparatus.

  In order to solve the above problems, a reference voltage generation circuit of the present invention is a reference voltage generation circuit that outputs a reference voltage for adjusting a drive current for driving a driven element, and outputs a predetermined current from an input voltage. A predetermined current output means and a compensation resistor circuit for obtaining the reference voltage from the predetermined current output from the predetermined current output means and compensating for temperature dependence of the driven element are provided. It is.

  The reference voltage generation circuit of the present invention is a reference voltage generation circuit that outputs a reference voltage for adjusting a drive current for driving a driven element, and a predetermined voltage output unit that outputs a predetermined voltage from an input voltage; A compensation circuit for compensating for temperature dependence of the driven element is provided.

The drive circuit of the present invention includes a reference voltage generation circuit that outputs a reference voltage from an input voltage, and is a drive circuit that adjusts a drive current for driving a driven element by the reference voltage, the reference voltage generation circuit Is provided with predetermined voltage output means for outputting a predetermined voltage from the input voltage, and a compensation circuit for compensating for temperature dependence of the driven element.

  The drive circuit of the present invention includes a reference voltage generation circuit that outputs a reference voltage from an input voltage, and is a drive circuit that adjusts a drive current for driving a driven element by the reference voltage, the reference voltage generation circuit Is a predetermined current output means for outputting a predetermined current from the input voltage, and a compensation resistor for obtaining the reference voltage from the predetermined current output from the predetermined current output means and compensating for temperature dependence of the driven element. And a circuit.

  Further, the print head of the present invention is a print head having a reference voltage generating circuit for outputting a reference voltage from an input voltage, and having a drive circuit for adjusting a drive current for driving a driven element by the reference voltage, The reference voltage generation circuit includes a predetermined voltage output unit that outputs a predetermined voltage from the input voltage, and a compensation circuit that compensates for temperature dependence of the driven element.

  Further, the print head of the present invention is a print head having a reference voltage generating circuit for outputting a reference voltage from an input voltage, and having a drive circuit for adjusting a drive current for driving a driven element by the reference voltage, The reference voltage generating circuit obtains the reference voltage from the predetermined current output means for outputting a predetermined current from the input voltage, and the predetermined current output from the predetermined current output means, and the temperature dependence of the driven element. A compensation resistor circuit for compensation is provided.

  The image forming apparatus according to the present invention is an image forming apparatus having a reference voltage generating circuit for outputting a reference voltage from an input voltage, and having a driving circuit for adjusting a driving current for driving a driven element by the reference voltage. The reference voltage generation circuit includes a predetermined voltage output means for outputting a predetermined voltage from the input voltage, and a compensation circuit for compensating for temperature dependence of the driven element.

  Furthermore, an image forming apparatus according to the present invention is an image forming apparatus having a reference voltage generating circuit that outputs a reference voltage from an input voltage, and a drive circuit that adjusts a drive current for driving a driven element by the reference voltage. The reference voltage generation circuit obtains the reference voltage from the predetermined current output from the predetermined current output means, the predetermined current output means for outputting a predetermined current from the input voltage, and the temperature of the driven element. A compensation resistor circuit for compensating the dependency is provided.

  According to the present invention having the above configuration, it is possible to set a large temperature coefficient without increasing the number of elements, and it is possible to arbitrarily set a desired temperature coefficient and an output value of a reference voltage.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the element which is common in each figure. FIG. 1 is a block diagram showing an electrophotographic printer according to the present invention, and FIG. 2 is a time chart showing the operation of the electrophotographic printer shown in FIG.

  In FIG. 1, reference numeral 1 denotes a print control unit including a microprocessor, a ROM, a RAM, an input / output port, a timer, and the like. The print control unit 1 is disposed inside the printer print unit, and includes control signals SG1, The entire printer is sequence-controlled by a video signal (one-dimensionally arranged dot map data) SG2 or the like, and a printing operation is performed.

  When the print instruction is received by the control signal SG1, the print controller 1 first detects whether or not the fixing device 22 including the heater 22a is within the usable temperature range by the fixing device temperature sensor 23, and the temperature range. If not, the heater 22a is energized to heat the fixing device 22 to a usable temperature. Next, the development / transfer process motor (PM) 3 is rotated via the driver 2, and at the same time, the charging voltage power supply 25 is turned on by the charge signal SGC to charge the developing device 27.

  The presence / absence and type of paper (not shown) set is detected by the paper remaining amount sensor 8 and the paper size sensor 9, and paper feeding suitable for the paper is started. Here, the paper feed motor (PM) 5 can be rotated in both directions via the driver 4, and the paper is set in advance until it is first reversed and detected by the paper inlet sensor 6. Send only the amount. Subsequently, the sheet is rotated forward to convey the sheet into a printing mechanism inside the printer.

  1 and 2, the print control unit 1 transmits a timing signal SG3 (including a main scanning synchronization signal and a sub-scanning synchronization signal) to the host controller when the paper reaches a printable position. The video signal SG2 is received from the host controller. The video signal SG2 edited for each page in the host controller and received by the print control unit 1 is transferred to the LED head 19 as the print data signal HD-DATA. The LED head 19 has a plurality of LEDs arranged for printing one dot (pixel) on a line.

  When the print control unit 1 receives the video signal SG2 for one line, the print control unit 1 transmits a latch signal HD-LOAD to the LED head 19 to hold the print data signal HD-DATA in the LED head 19. Further, the print control unit 1 can print the print data signal HD-DATA held in the LED head 19 even while the next video signal SG2 is being received from the host controller. HD-CLK is a clock signal for transmitting the print data signal HD-DATA to the LED head 19.

  Transmission / reception of the video signal SG2 is performed for each print line. Information printed by the LED head 19 is formed into a latent image as a dot with an increased potential on a photosensitive drum (not shown) charged to a negative potential. Then, in the developing unit 27, the toner for image formation charged to a negative potential is sucked to each dot by an electric suction force to form a toner image.

  Thereafter, the toner image is sent to the transfer unit 28, and on the other hand, the transfer high voltage power supply 26 is turned on to a positive potential by the transfer signal SG4, and the transfer unit 28 passes through the interval between the photosensitive drum and the transfer unit 28. Transfer the toner image on top. The sheet onto which the toner image has been transferred is brought into contact with a fixing device 22 having a built-in heater 22a and conveyed, and the toner image is fixed on the sheet by the heat of the fixing device 22. The sheet on which the toner image is fixed is further conveyed and discharged from the printer printing mechanism through the sheet discharge sensor 7 to the outside of the printer.

  In response to detection by the paper size sensor 9 and the paper inlet sensor 6, the print control unit 1 applies a voltage from the transfer high-voltage power supply 26 to the transfer device 28 only while the paper passes through the transfer device 28. When printing is completed and the paper passes through the paper discharge sensor 7, the application of the voltage to the developing device 27 by the charging high-voltage power supply 25 is finished, and at the same time, the rotation of the developing / transfer process motor 3 is stopped. Thereafter, the above operation is repeated.

  Next, the LED head 19 will be described. FIG. 3 is a view showing the structure of the LED head according to the present invention. In the description of the present embodiment, as an example, an LED head capable of printing at a resolution of 600 dots per inch on an A4 size paper will be taken and its specific configuration will be described. In this embodiment, the total number of LED elements is 4992 dots, and 26 LED arrays are arranged to constitute this, and each LED array includes 192 LED elements.

  In FIG. 3, CHP1 to CHP26 are LED arrays, and descriptions of CHP3 to CHP25 are omitted. IC1 to IC26 are driver ICs arranged corresponding to CHP1 to CHP26, and drive the LED arrays CHP1 to CHP26, respectively. Each driver IC is composed of the same circuit, and adjacent driver ICs are connected in cascade.

  As described above, in the LED head shown in FIG. 3, 26 LED arrays (CHP1 to CHP26) and 26 driver ICs (IC1 to IC26) for driving the LED array face each other on a printed wiring board (not shown). 192 LED elements can be driven per driver IC chip, and 26 of these chips are connected in cascade so that print data input from the outside can be transferred serially.

  The configuration of the LED head shown in FIG. 3 will be described in the following order. Each of the driver ICs IC1 to IC26 is composed of the same circuit, and is connected in cascade with the adjacent driver IC. The driver IC receives the clock signal HD-CLK, shift register circuit 31 that performs shift transfer of print data, a latch circuit 32 that latches an output signal of the shift register circuit 31 using a latch signal (hereinafter referred to as HD-LOAD), An AND circuit 34 that inputs an output signal from the latch circuit 32 and the inverter circuit 33 to obtain a logical product, and an LED drive circuit 35 that supplies a drive current from the power supply VDD to the LED element (CHP1 or the like) by the output signal of the AND circuit 34. And a control voltage generation circuit 36 for generating a command voltage so that the drive current of the LED drive circuit 35 is constant. HD-STB-N is a strobe signal that is input to the inverter circuit 33.

  Reference numeral 39 is a reference voltage generation circuit, and its output is connected to the control voltage generation circuit 36 of IC1 to IC26 to supply a predetermined reference voltage Vref. The HD-DATA, HD-CLK, HD-LOAD, and HD-STB-N signals are sent from the print control circuit 1 during printing.

  FIG. 4 is a circuit diagram showing a simplified configuration of the LED head shown in the block diagram of FIG. As shown in FIG. 4, the print data signal HD-DATA is input to the LED head 19 together with the clock signal HD-CLK. In the printer, the bit data for 4992 dots is supplied to the flip-flop circuits FF1, FF2,. . , FF4992 is sequentially transferred through the shift register.

  Next, a latch signal HD-LOAD is input to the LED head 19, and the bit data is stored in each latch circuit LT1, LT2,. . , And latched in LT4992. Subsequently, the light emitting elements LD1, LD2,... Are generated by the bit data and the print drive signal HD-STB-N. . , LD 4992 corresponding to high (high) level dot data is lit. In FIG. 4, G0 is an inverter circuit, G1, G2,. . , G4992 are pre-buffer circuits, Tr1, Tr2,. . Tr4992 is a switch element, and VDD is a power source.

  FIG. 5 is a diagram for explaining the LED driving essential part of the driver IC in FIG. 4 and shows the connection relationship between the LED driving circuit and its peripheral circuits. In FIG. The area around the drive circuit) is described. As described above, the LED drive current value is determined by the reference current value generated inside the driver IC. In the following, an outline of the operation will be described by entering the IC. The circuit configuration shown in FIG. 5 is the same as that shown in FIG. 12 described as the prior art.

  In FIG. 5, a portion G1 surrounded by a broken line is a prebuffer circuit, and an AND circuit 42, a PMOS transistor 43, and an NMOS transistor 44 are arranged in the prebuffer circuit G1. G0 is an inverter circuit, and LT1 is a latch circuit. A portion 36 surrounded by an alternate long and short dash line is a control voltage generation circuit, and one circuit is provided for each driver IC chip.

  Reference numeral 51 denotes an operational amplifier which is described in the figure as a potential whose output voltage is Vcont. The potential is a control voltage applied to the LED driving transistor Tr1 in order to adjust the LED driving current. Reference numeral 53 denotes a resistor, and the resistance value is described as Rref in the figure. A PMOS transistor 52 is configured such that the gate length is the same as that of the LED driving transistor Tr1.

  VREF is a reference voltage input terminal which is connected to the inverting input terminal of the operational amplifier 51 and receives a reference voltage Vref generated from a reference voltage generation circuit described later. A circuit comprising an operational amplifier 51, a PMOS transistor 52, and a resistor 53 constitutes a feedback control circuit, and the current (Iref) flowing through the resistor 53, that is, the current flowing through the PMOS transistor 52 does not depend on the power supply voltage (VDD). The configuration is determined only by the voltage Vref and the value Rref of the resistor 53.

  That is, since the potential of the inverting input terminal and the potential of the non-inverting input terminal are controlled to be substantially equal by the operation of the operational amplifier 51, the potential of the non-inverting input terminal of the operational amplifier 51 is substantially equal to the reference voltage Vref. The current Iref flowing through the resistor 53 is given as Iref = Vref / Rref.

  As described above, the LED driving transistor Tr1 and the PMOS transistor 52 are configured to have the same gate length, and the gate potential is equal to Vcont when the LED is driven, and the PMOS transistor 52 and the LED driving transistor Tr1 are driven. Operates in the saturation region and has a current mirror relationship.

  As a result, the drive current value of the LED element LD1 is proportional to the current Iref flowing through the resistor 53, and this reference current Iref is proportional to the reference voltage Vref input to the VREF terminal, so that the LED driving is performed by the reference voltage Vref. It is possible to adjust the current value collectively. The resistor 53 is created by using a semiconductor process technology. Generally, polysilicon, an impurity diffused resistor, or the like is used as a material for the resistor element, and is monolithically integrated in the driver IC.

  FIG. 6 is a circuit diagram showing the configuration of the reference voltage generating circuit in the first embodiment, which is a circuit indicated by 39 in FIG. In FIG. 6, the reference voltage generation circuit 39 includes a regulator circuit 71, a resistor 81, and a temperature sensitive resistor 82. The first terminal of the regulator circuit 71 is a power supply terminal and is connected to the power supply VDD, the second terminal is an output terminal and connected to one end of the resistor 81, and the third terminal of the regulator circuit 71 is a ground terminal and is a resistor 81 is connected to the other end. The third terminal of the regulator circuit 71 is connected to one end of the temperature-sensitive resistor 82 and is connected to the reference voltage output terminal Vref. The other end of the temperature sensitive resistor 82 is connected to the ground.

  Any regulator circuit 71 can be used as long as a predetermined output voltage Vo can be obtained irrespective of the power supply voltage VDD applied to the power supply terminal. In this embodiment, the regulator circuit 71 desirably has a temperature coefficient of the output voltage Vo of substantially zero. Specifically, for example, a CMOS voltage regulator S-817 series manufactured by Seiko Instruments Inc. can be used. This CMOS voltage regulator has a very low temperature coefficient of output voltage of about 100 ppm / ° C. and a low current consumption of several μA. Of course, the regulator circuit 71 is not limited to this specific example, and various circuits can be used.

  The temperature-sensitive resistor 82 has a characteristic that the temperature coefficient of the resistance value becomes a positive value, and the resistance value increases substantially linearly as the ambient temperature increases. A thermistor is generally used as an element whose resistance value varies with temperature. However, the thermistor exhibits a negative temperature dependency in which the resistance value decreases exponentially with an increase in ambient temperature. Cannot be used.

  As the temperature sensing element of this embodiment, products such as KOA square plate type temperature sensitive chip resistor LP73 series, LA73 series, LT73 series, or Panasonic electronic device company square plate type temperature sensitive chip resistor ERA series, etc. The temperature-sensitive resistor is formed by forming a metal alloy film made of platinum, titanium, or the like as a temperature detection film on an alumina substrate using a thin film or thick film wiring technique.

  For example, it is well known that when platinum is used as the temperature sensitive resistance film, the resistance value of the element increases substantially linearly with respect to the temperature rise, and the temperature coefficient becomes + 0.385% / ° C. It is well known that the standard has been established as Japanese Industrial Standard JIS C1604. By using various alloy films as the temperature sensitive resistance film, it is easy to prepare various temperature coefficients, and by selecting the type from the above products, + 0.1% / ° C to + 0.5% / ° C A desired temperature coefficient can be given in the range of.

  FIG. 7 is a diagram for explaining the operation of the first embodiment, and corresponds to FIG. In FIG. 7, resistance values of the resistors 81 and 82 are represented by R0 and R1, respectively. The output voltage generated between the second terminal and the third terminal of the regulator circuit 71 is Vo, the current flowing through the resistor 81 is Io, the current flowing through the third terminal of the regulator circuit 71 is Iss, and the current flowing through the temperature-sensitive resistor 82 is I1. It expresses.

At this time, the current Iss is negligibly small compared to the current Io, so that I1 = Io + Iss≈Io
It is. Also, Io = Vo / R0
Therefore, the output voltage Vref of the reference voltage generation circuit 39 is Vref = I1 × R1≈ (R1 / R0) × Vo.
It becomes. Here, since the temperature coefficient of the resistor 81 is substantially zero and the temperature coefficient of the output voltage of the regulator circuit 71 is also substantially zero, the temperature coefficient of the output voltage Vref can be a value substantially equal to the temperature coefficient of the temperature-sensitive resistor 82. .

  As described above, since the temperature coefficient of the temperature-sensitive resistor 82 can be selected relatively arbitrarily, by setting it to + 0.33% / ° C., the temperature dependence of the LED element made of the GaAlAs base material is compensated. A suitable temperature coefficient can be obtained. This temperature coefficient is a numerical value equivalent to that obtained from the configuration shown in FIG. 13 or FIG. 14 described as the prior art.

  In this embodiment, the regulator circuit 71 can obtain a predetermined voltage regardless of the input power supply voltage, and the temperature coefficient of the output voltage is almost zero. Further, since the temperature coefficient of the temperature-sensitive resistor 82 can be selected relatively arbitrarily, setting it to + 0.33% / ° C. is a suitable temperature coefficient for compensating the temperature dependence of the LED element made of the GaAlAs base material. Can be obtained.

  When trying to set a large temperature coefficient in the configuration of the prior art, as described with reference to FIG. 14, it is necessary to increase the number of diodes connected in series, which increases the number of desired elements and increases the cost. In the configuration of 1, the number of elements does not increase, and the cost of the LED head using it can be kept low.

In addition to it,
Vref≈ (R1 / R0) × Vo
As is clear from the relational expression, the output voltage Vref lower than the output voltage Vo of the regulator circuit 71 can be obtained (R1 / R0 <1) by arbitrarily setting the ratio of the resistance values R1 and R0. It is also possible to obtain an output voltage Vref higher than the output voltage Vo of the regulator circuit 71 (R1 / R0> 1), and the output voltage can be set relatively freely.

  Next, a reference voltage generating circuit according to the second embodiment will be described. FIG. 8 is a circuit diagram showing a reference voltage generating circuit according to the second embodiment. In FIG. 8, the reference voltage generation circuit 40 according to the second embodiment includes a constant current diode 83 and a temperature sensitive resistor 82. The first terminal of the constant current diode 83 is an anode terminal connected to the power supply VDD, and the second terminal is a cathode terminal connected to one end of the temperature sensitive resistor 82. The temperature sensitive resistor 82 is the same as that of the first embodiment, and one end thereof is connected to the reference voltage output terminal and the other end is connected to the ground.

  As the constant current diode 83, for example, a diode having characteristics such as F-102 in CRD (Current Regulate Diode) series manufactured by Ishizuka Electronics can be used. This product has a characteristic that when a voltage value applied between the anode terminal and the cathode terminal is equal to or higher than a predetermined value, a predetermined current value flows regardless of the voltage value.

Next, the operation of the second embodiment will be described. FIG. 9 is an operation explanatory diagram for explaining the operation of the second embodiment, and corresponds to FIG. In FIG. 9, the resistance value of the temperature-sensitive resistor 82 is represented as R1, and the current value of the current flowing through the constant current diode 83 is represented as Io. The output voltage Vref of the reference voltage generation circuit 40 is Vref = Io × R1
It becomes.

  Here, ideally, the temperature coefficient of the current Io is substantially zero, whereas the temperature coefficient of the temperature-sensitive resistor 82 can be selected relatively arbitrarily, so it is set to + 0.33% / ° C. Thus, it is possible to obtain a temperature coefficient suitable for compensating the temperature dependence of the LED element made of the GaAlAs base material. This temperature coefficient is a numerical value equivalent to that obtained from the configuration shown in FIG. 13 or FIG. 14 described as the prior art.

  In the reference voltage generation circuit 40 of the second embodiment, the constant current diode 83 can obtain a predetermined output current regardless of the applied power supply voltage VDD, and the temperature coefficient of the output current can be ignored. In addition, since the temperature coefficient of the temperature sensitive resistor 82 can be selected relatively arbitrarily, by setting it to + 0.33% / ° C., a temperature suitable for compensating the temperature dependence of the LED element made of the GaAlAs base material. A coefficient can be obtained.

  When trying to set a large temperature coefficient in the configuration of the prior art, as described with reference to FIG. 14, it is necessary to increase the number of diodes connected in series, which increases the number of desired elements and increases the cost. In the configuration of 2, the number of elements does not increase, and the cost of the LED head using it can be kept low.

  Next, a reference voltage generating circuit according to the third embodiment will be described. FIG. 10 is a circuit diagram showing a reference voltage generating circuit according to the third embodiment. In FIG. 10, the reference voltage generation circuit 41 according to the third embodiment includes a regulator circuit 71, a diode 84, a resistor 85, and a temperature sensitive resistor 82. The first terminal of the regulator circuit 71 is a power supply terminal connected to the power supply VDD, and the second terminal is an output terminal connected to the anode of the diode 84.

  The cathode terminal of the diode 84 is connected to one end of the resistor 85, and the other end of the resistor 85 is connected to the output terminal Vref. The third terminal of the regulator circuit 71 is a ground terminal and is connected to the output terminal Vref. This third terminal is also connected to one end of the temperature sensitive resistor 82. The other end of the temperature sensitive resistor 82 is connected to the ground. The temperature sensitive resistor 82 is the same as that of the first and second embodiments.

  Next, the operation of the third embodiment will be described. FIG. 11 is a diagram for explaining the operation of the third embodiment, and corresponds to FIG. In FIG. 11, the resistance value of the resistor 85 is represented as R0, the resistance value of the temperature sensitive resistor 82 is represented as R1, the forward voltage applied to the diode 84 is represented as Vf, and the output voltage of the regulator circuit 71 is represented as Vo. Further, the current flowing through the resistor 85 is represented as Io, the current flowing through the temperature sensitive resistor 82 is represented as I1, and the current at the ground terminal of the regulator circuit 71 is represented as Iss.

The voltage applied to both ends of the resistor 85 is Vo−Vf, and the current value of the current I0 flowing through the resistor 85 is I0 = (Vo−Vf) / R0.
It is. Since the ground current Iss of the regulator circuit 71 is negligibly small compared to the current I0, the current I1 flowing through the temperature-sensitive resistor 82 is I1 = Iss + I0≈I0.
From this, the output voltage Vref is
Vref = I1 * R1 = (R1 / R0) * (Vo-Vf)
Can be obtained as

  Since the temperature dependence of the resistor 85 is small and the temperature dependence of the regulator circuit 71 itself is small, these temperature coefficients can be ignored, and the temperature coefficient of the output voltage Vref is equal to (Vo -Vf) equal to the sum of the temperature coefficients. Since the temperature dependence of the forward voltage of the diode 84 has a negative dependence of about −2 mV / ° C., the temperature coefficient of (Vo−Vf) is a positive value, and the temperature coefficient of the output voltage Vref described above is It has a positive value exceeding the temperature coefficient of the temperature sensitive resistor 82 itself.

  Further, the temperature coefficient can be adjusted by changing the setting of the output voltage Vo of the regulator circuit 71. In addition, in the third embodiment, compared with the conventional circuit shown in FIG. 14, the output voltage is output by simply multiplying the Vo voltage by the forward voltage Vf and multiplying it by (R1 / R0). Therefore, by changing the resistance ratio (R1 / R0), the output voltage Vref can be set to a desired value independently of the setting of the temperature coefficient of the output voltage Vref, and the number of parts is increased. The design advantage of obtaining an arbitrary output voltage Vref with an arbitrary temperature coefficient is obtained.

  As described above, in the first to third embodiments of the present invention, the case where the present invention is applied to an LED head in an electrophotographic printer using an LED as a light source as a drive circuit has been described. The present invention can also be applied to an organic EL head using an EL element, and can also be applied to driving a heating resistor in a thermal printer and a display element column in a display device.

1 is a block diagram showing an electrophotographic printer according to the present invention. It is a time chart which shows operation | movement of an electrophotographic printer. It is a figure which shows the structure of a LED head. It is a circuit diagram which simplifies and shows the structure of a LED head. It is a circuit diagram which shows the LED drive principal part of driver IC. FIG. 3 is a circuit diagram illustrating a configuration of a reference voltage generation circuit according to the first embodiment. FIG. 3 is a circuit diagram illustrating the operation of the first embodiment. FIG. 6 is a circuit diagram illustrating a configuration of a reference voltage generation circuit according to a second embodiment. FIG. 6 is a circuit diagram for explaining an operation of the second embodiment. FIG. 6 is a circuit diagram illustrating a configuration of a reference voltage generation circuit according to a third embodiment. FIG. 10 is a circuit diagram illustrating the operation of the third embodiment. It is a circuit diagram which shows the drive circuit of a LED head. 10 is a circuit diagram showing a reference voltage generating circuit disclosed in Patent Document 1. FIG. 10 is a circuit diagram showing a reference voltage generating circuit disclosed in Patent Document 2. FIG.

Explanation of symbols

19 LED head 39, 40, 41 Reference voltage generating circuit 71 Regulator circuit 81, 85 Resistor 82 Temperature sensitive resistor 83 Constant current diode LD1, LD2,..., LD4992 LED element Vref Reference voltage

Claims (12)

In a reference voltage generation circuit that outputs a reference voltage for adjusting a drive current for driving a driven element,
Predetermined voltage output means for outputting a predetermined voltage from the input voltage;
And a compensation circuit for compensating for temperature dependence of the driven element. The reference voltage generating circuit according to claim 1, wherein the compensation circuit has a temperature sensitive resistor. 2. The reference voltage generation circuit according to claim 1, further comprising a circuit capable of increasing a temperature coefficient of the compensation circuit as the temperature rises. The reference voltage generation circuit according to claim 3, wherein the first circuit includes a diode, and the second circuit includes a temperature-sensitive resistor. In a reference voltage generation circuit that outputs a reference voltage for adjusting a drive current for driving a driven element,
A predetermined current output means for outputting a predetermined current from the input voltage;
A reference voltage generating circuit comprising: a compensation resistor circuit that obtains the reference voltage from the predetermined current output from the predetermined current output means and compensates for temperature dependency of the driven element. The reference voltage generating circuit according to claim 5, wherein the compensation resistor circuit has a temperature sensitive resistor. In a drive circuit that has a reference voltage generation circuit that outputs a reference voltage from an input voltage and adjusts a drive current that drives a driven element by the reference voltage,
The reference voltage generation circuit includes:
Predetermined voltage output means for outputting a predetermined voltage from the input voltage;
A drive circuit comprising a compensation circuit for compensating for temperature dependence of the driven element. In a drive circuit that has a reference voltage generation circuit that outputs a reference voltage from an input voltage and adjusts a drive current that drives a driven element by the reference voltage,
The reference voltage generation circuit includes:
Predetermined current output means for outputting a predetermined current from the input voltage;
A drive circuit comprising: a compensation resistor circuit that obtains the reference voltage from the predetermined current output from the predetermined current output means and compensates for temperature dependence of the driven element. In a print head having a reference voltage generating circuit for outputting a reference voltage from an input voltage, and having a drive circuit for adjusting a drive current for driving a driven element by the reference voltage,
The reference voltage generation circuit includes:
Predetermined voltage output means for outputting a predetermined voltage from the input voltage;
A print head comprising a compensation circuit for compensating for temperature dependence of the driven element. In a print head having a reference voltage generating circuit for outputting a reference voltage from an input voltage, and having a drive circuit for adjusting a drive current for driving a driven element by the reference voltage,
The reference voltage generation circuit includes:
Predetermined current output means for outputting a predetermined current from the input voltage;
A print head comprising: a compensation resistor circuit that obtains the reference voltage from the predetermined current output from the predetermined current output means and compensates for temperature dependence of the driven element. In an image forming apparatus having a reference voltage generation circuit for outputting a reference voltage from an input voltage, and having a drive circuit for adjusting a drive current for driving a driven element by the reference voltage,
The reference voltage generation circuit includes:
Predetermined voltage output means for outputting a predetermined voltage from the input voltage;
An image forming apparatus, comprising: a compensation circuit that compensates for temperature dependence of the driven element. In an image forming apparatus having a reference voltage generation circuit for outputting a reference voltage from an input voltage, and having a drive circuit for adjusting a drive current for driving a driven element by the reference voltage,
The reference voltage generation circuit includes:
Predetermined current output means for outputting a predetermined current from the input voltage;
An image forming apparatus, comprising: a compensation resistor circuit that obtains the reference voltage from the predetermined current output from the predetermined current output unit and compensates for temperature dependence of the driven element.

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