JP2014153780A - Constant current circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a constant current source with better characteristics at a low cost.SOLUTION: In a first bipolar transistor, an emitter is supplied, for example, with a first voltage to a common potential that is a ground potential; a collector is supplied with a second voltage; and a base is supplied with the second voltage through a resistor. In a second bipolar transistor, an emitter is connected with one end of a resistor that is connected with the common potential at the other end; a base is connected with the base of the first bipolar transistor at a contact point; and a collector is connected with a load.

Description

  The present invention relates to a constant current circuit suitable for driving a light source by a semiconductor element.

  In recent years, a method using an LED (Light Emitting Diode) instead of a high-intensity discharge lamp such as a conventional xenon lamp is known as a light source in a scanner device. Since the LED is a semiconductor component, the light source using the LED can obtain excellent responsiveness (dimming function) as compared with a conventional light source by incandescent light emission or discharge light emission, and also significantly saves energy. It has the characteristic that an effect can also be acquired.

  On the other hand, the light receiving element of the scanner device uses a semiconductor component such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) sensor, and can read an image with high resolution. Therefore, the light emission control with relatively high accuracy is required. Therefore, a constant current drive capable of supplying a constant current to each LED used as a light source of the scanner device has already been put into practical use and is generally used.

  FIG. 14 shows a configuration of an example of a constant current circuit according to the prior art that can be applied to driving of an LED. The constant current circuit illustrated in FIG. 14 includes an operational amplifier OP and a transistor Q. The output terminal of the operational amplifier OP is connected to the base of the transistor Q, the emitter is grounded via the resistor R, and is connected to the inverting input terminal of the operational amplifier OP. Further, the reference voltage Vref is input to the non-inverting input terminal of the operational amplifier OP, and an LED, for example, is connected to the collector of the transistor Q as a current load.

In the operational amplifier OP, when the reference voltage Vref is applied to the non-inverting input terminal, the output voltage is adjusted so that the inverting input terminal has an equivalent voltage level. As a result, the voltage across the resistor R is always a constant voltage Vref, and the current I (= V _ref / R) flowing through the resistor R is controlled to a constant current and operates as a constant current source. In this method, the temperature dependency of the transistor Q (mainly the change in the base-emitter voltage V _be ) is absorbed when the output of the operational amplifier OP changes in accordance with the voltage V _be , so that a high temperature tolerance is obtained. It is done.

  Patent Document 1 discloses a configuration in which resistors having different temperature characteristics are used for two transistors of a current mirror type constant current circuit. According to the configuration of Patent Document 1, the circuits operate so as to complement each other's characteristics under various temperature environments by the action of each resistor, and the temperature range corresponding to the operation as a constant current source can be widened. The temperature characteristics of the current mirror type constant current circuit can be greatly improved.

  However, when the LED is driven by a constant current circuit using the operational amplifier OP, there are the following problems.

  When digital dimming for turning on / off an LED at a high speed and controlling the brightness according to the length of the on-time is performed in the configuration as shown in FIG. 14 described above, a pulse-like signal is applied to the non-inverting input terminal of the operational amplifier OP. And the voltage at the inverting input terminal is changed to switch the current I on and off to drive the LED. For this reason, it is necessary to use a highly responsive operational amplifier OP, and there is a problem that the cost may increase.

  That is, in general, the operational amplifier OP tends to increase in cost as the responsiveness increases. This becomes a problem when constructing a system that adjusts the light emission amount of an LED or performs partial lighting switching in an LED array or the like.

  On the other hand, in the case where the reduction of the component cost is more important than the response of the operational amplifier OP, analog dimming is generally used. For example, in the configuration of FIG. 14, the voltage I input to the non-inverting input terminal of the operational amplifier OP is changed in an analog manner to change the current I to obtain the LED driving current.

  In this case, since the response of the operational amplifier OP is low, there is a problem that it is difficult to instantaneously execute partial lighting switching of the light source or change of the light emission amount performed by changing the drive current. Also, in analog dimming, since the amount of light emission is controlled by changing the LED drive current, there is a problem that the LED emission chromaticity may change with the change of the drive current. It was.

  Further, in the scanner device, when the light source is composed of a plurality of columns in which a plurality of LEDs are connected in series, an operational amplifier OP is required for each LED column. Therefore, when more LEDs are used and the number of LED drive trains is increased, there is a problem that the cost increases significantly.

  On the other hand, it is also conceivable that a constant current circuit for supplying a drive current for driving the LED is configured using individual components such as transistors without using the operational amplifier OP. In this case, high-speed response and cost reduction are expected, but there is no means for absorbing the temperature dependence of the output characteristics of the constant current circuit, which may cause the operation to become unstable. was there.

  Furthermore, it is also common to perform drive control of the light source LED using a general-purpose LED driver IC (Integrated Circuit). However, the method using the general-purpose LED driver IC has a problem that the cost is significantly increased as compared with the circuit having the discrete configuration shown in FIG. In other words, general-purpose driver ICs require design and functions that emphasize usability and many protection mechanisms that assume various cases, and it is difficult to avoid the situation where costs increase due to them. is there.

  In Patent Document 1 described above, the problem of temperature dependence in the output characteristics of the constant current circuit can be solved. However, in the configuration of Patent Document 1, it is necessary to prepare a plurality of special resistors having different temperature characteristics, and there is a problem that costs increase.

  The present invention has been made in view of the above, and an object thereof is to realize a constant current circuit with better characteristics at a lower cost.

  In order to solve the above-described problems and achieve the object, the present invention provides a first bipolar transistor to which a first voltage with respect to a common potential is supplied to an emitter, a resistor having one end connected to the common potential, and other resistors. An end is connected to the emitter, the base and the base of the first bipolar transistor are connected at a connection point, and a collector is connected to the load.

  According to the present invention, there is an effect that a constant current source having better characteristics can be realized at a lower cost.

FIG. 1 is a circuit diagram showing a basic configuration example of a constant current circuit according to the first embodiment. FIG. 2 is a circuit diagram showing an example in which N constant current circuits according to the first embodiment are connected in parallel. FIG. 3 is a circuit diagram for explaining a case where an LED array is used as each load according to the first embodiment. FIG. 4 is a circuit diagram showing a configuration of an example of a constant current circuit according to a modification of the first embodiment. FIG. 5 is a circuit diagram showing an example of a constant current circuit using an operational amplifier according to the prior art. FIG. 6 is a circuit diagram illustrating an example of a constant current circuit according to a modification of the first embodiment in which N constant current circuits are connected in parallel. FIG. 7 is a circuit diagram showing an example of a constant current circuit according to the second embodiment. FIG. 8 is a circuit diagram showing an example of a constant current circuit according to a first modification of the second embodiment. FIG. 9 is a circuit diagram illustrating an example of a constant current circuit according to a second modification of the second embodiment. FIG. 10 is a circuit diagram showing a configuration of an example of a light source driving apparatus according to the third embodiment. FIG. 11 is a diagram illustrating a configuration of an example of a scanner device to which the fourth embodiment can be applied. FIG. 12 is a block diagram illustrating an exemplary configuration for performing signal processing in a scanner apparatus to which the fourth embodiment can be applied. FIG. 13 is a diagram illustrating a configuration of an example of a copying machine applicable to the fifth embodiment. FIG. 14 is a circuit diagram showing a configuration of an example of a constant current circuit according to the prior art.

  Exemplary embodiments of a constant current circuit will be described below in detail with reference to the accompanying drawings.

(First embodiment)
FIG. 1 is a circuit diagram showing a basic configuration example of a constant current circuit according to the first embodiment. In FIG. 1, a constant current circuit 100 has two NPN-type bipolar transistors Tv and Ti and resistors Rc and Ri, respectively. Hereinafter, unless otherwise specified, bipolar transistors are simply referred to as transistors.

  In the constant current circuit 100, the base of the transistor Tv and the base of the transistor Ti are connected, and one end of the resistor Rc is connected to the connection point. The other end of the resistor Rc is connected to the collector of the transistor Tv. The emitter of the transistor Tv is connected to the positive electrode of the reference power supply 101. The negative electrode of the reference power supply 101 is grounded. In this example, the reference power supply 101 outputs a voltage Vref with respect to the ground potential.

  A connection point between the collector of the transistor Tv and the resistor Rc is connected to the positive electrode of the power source 102. The negative electrode of the power supply 102 is grounded, for example. The power supply 102 outputs a voltage Vc with respect to the ground potential.

The emitter of the transistor Ti is connected to one end of the resistor Ri, and the other end of the resistor Ri is grounded. The collector of the transistor Ti is connected to a load R _L (current load) not shown.

  The negative electrode of the reference power source 101, the other end of the resistor Ri, and the negative electrode of the power source 102 are not limited to being grounded. That is, the negative electrode of the reference power supply 101, the other end of the resistor Ri, and the negative electrode of the power supply 102 may be connected in common to the same potential.

As illustrated in FIG. 1, the reference power supply 101 is connected to the emitter of the transistor Tv and the voltage Vref is applied, whereby the base of the transistor Tv has the base-emitter voltage _Vbe (hereinafter referred to as the voltage V _be) as the voltage Vref. ) is occurs voltage Vref + _V be, which is plus. Since the bases of the transistor Tv and the transistor Ti are commonly connected, a voltage V _{0 that} is a voltage drop of the voltage V _{be of the} transistor Ti from the base voltage (voltage Vref + V _be ) is generated at the emitter of the transistor Ti. The voltage V ₀ is substantially equal to the voltage Vref when the voltage V _be is approximately equal between the transistor Tv and the transistor Ti.

That is, the voltage V _be of the transistor Tv and Ti and each voltage V _be [Tv] and the voltage V _be [Ti], the voltage V _be [Tv] = When the voltage V _be [Ti], the voltage V ₀ is the following It can be expressed as equation (1).
V ₀ = (V _ref + V _be [Tv]) − V _be [Ti] = V _ref + (V _be [Tv] −V _be [Ti]) (1)

Therefore, a voltage V _0, which is a fixed voltage resulting from the voltage Vref, is generated at the emitter of the transistor Ti, and a current I determined by the resistor Ri connected to the emitter of the transistor Ti and the voltage V ₀ is the collector of the transistor Ti. Will flow toward the emitter. As described above, according to FIG. 1, the constant current circuit capable of controlling the current I flowing through the load _RL connected to the collector of the transistor Ti is configured by the voltage Vref of the reference power supply 101 and the resistor Ri.

On the other hand, the voltage V _be of each transistor Tv and Ti also changes with the temperature change of the components. At this time, if the transistors Tv and Ti are arranged in the vicinity of each other or are thermally coupled, etc., and under similar temperature environments, the fluctuations in the voltage V _be of the transistors Tv and Ti occur in the same manner. As shown in the above equation (1), by using the transistors Tv and Ti whose voltages V _be are substantially equal, the voltage V ₀ is generated by canceling the variation of the voltage V _be of each transistor Tv and Ti. High temperature resistance can be obtained.

As an actual configuration, in order to pass a current from the collector to the emitter in the transistor Tv, for example, the connection point between the emitter of the transistor Tv and the reference voltage 101 is grounded via a resistor R _E (not shown). . Here, the current flowing through the transistor Tv itself because no matter in small and keep the resistance value of the resistor R _E and sufficiently high value, the power consumption is suppressed by the power supply 102. However, the present invention is not limited to this, and the reference power supply 101 may be configured using a current drawing type constant voltage power supply circuit. In this case, the resistor R _E for grounding the emitter is not necessary.

  Further, the constant current circuit 100 needs to operate both the transistors Tv and Ti in a state where the reference voltage Vref is applied to the emitter of the transistor Tv. Therefore, the output voltage Vc of the power supply 102 is selected so that the voltage difference is higher than the reference voltage Vref and the potential difference between the voltage Vc and the reference voltage Vref can operate both the transistors Tv and Ti. .

  Further, in the example of FIG. 1, the collector of the transistor Tv is connected to the resistor Rc, but this is not limited to this example. For example, the collector may be connected to the base of the transistor Tv. Further, the collector may be connected to an arbitrary voltage point.

  Furthermore, in order to satisfy the above-described formula (1), it is preferable to select transistors Tv and Ti that have characteristics closer to each other.

The constant current circuit 100 shown in FIG. 1 can be used in parallel connection. FIG. 2 shows an example in which N constant current circuits 100 ₁ , 100 ₂ ,..., 100 _N are connected in parallel.

In the configuration of FIG. 2, in the parallel connection of the constant current circuits 100 ₁ , 100 ₂ ,..., 100 _N , the voltage V _c is the connection point between the collector of the transistor Tv ₁ and the resistor Rc ₁ and the collector of the transistor Tv ₂ . Connected to the connection point of the resistor Rc ₂ ,... To the connection point of the collector of the transistor Tv _N and the resistor Rc _N. At the same time, the reference power supply 101 is connected in common to the emitters of the transistors Tv ₁ , Tv ₂ ,..., Tv _N.

Although not shown, it is assumed that the path connecting the reference power source 101 and the emitters of the transistors Tv ₁ , Tv ₂ ,..., Tv _N is grounded via a resistor.

Thus, the constant current circuit 100 _1, 100 _2, ..., in 100 _N, each of the transistors Tv _1, Tv ₂ with respect to the reference power source 101, ..., if the emitter of Tv _N are connected in parallel, each transistor Ti ₁ , Ti ₂ ,..., Ti _N emitter voltages V ₁ , V ₂ ,..., V _N are equal to the reference voltage Vref, respectively. Therefore, the constant current circuit 100 _1, 100 _2, ..., in 100 _N, each load R _L1, R _L2, ..., each current flows through _{R LN I 1, I 2,} ..., I N is the reference voltage Vref each resistor Ri _1, Ri _2, ..., it can be determined by the Ri _N, each load R _L1, R _L2, ..., it is possible to respectively constant current driving R _LN.

Here, as illustrated in FIG. 3, for example, an LED array in which a plurality of LED (Light Emitting Diode) elements connected in series is used as each of the loads R _L1 , R _L2, and R _L3. Think. An LED power source V _LED is connected to each LED array, and power is supplied. As described above, the current flowing through each LED array is determined by the reference voltage Vref and the resistors Ri ₁ , Ri ₂ and Ri ₃ .

The LED has large variations in individual forward voltage Vf and temperature dependency in terms of characteristics. That is, when driving a plurality of LED arrays, the voltage at the collector of the transistor driving each LED array (corresponding to the power consumption of the transistor) is often different or fluctuates. Therefore, for example, when a different temperature change occurs for each LED array, the voltage V _be fluctuates in accordance with the temperature change in each transistor that drives each LED array, causing a change in drive current.

In contrast, according to the configuration of the first embodiment, the constant current circuit 100 _1, 100 _2, ..., 100 transistors Tv ₁ and Ti ₁ of _N, transistor Tv ₂ and Ti _2, ..., the transistor Tv _N and the Ti _N pairs each, in accordance with the above equation (1), variation of the voltage V _be of mutual transistor is canceled. Therefore, according to the first embodiment, the constant current circuit 100 _1, 100 _2, ..., in addition to the difference in environmental temperature at 100 _N, the constant current circuit 100 _1, 100 _2, ..., 100 _N is driven A high-resistance constant current circuit can be configured even with respect to variations and fluctuations in the forward voltage Vf in the LED array.

That is, in the example of FIG. 3, even if the constant current circuits 100 ₁ , 100 _2, and 100 ₃ are in different temperature environments and the forward voltage Vf of the LED array to be driven is different, the constant current If the two transistors of the circuits 100 ₁ , 100 _2, and 100 ₃ have the same characteristics, these differences are canceled out according to the above equation (1). Thereby, in each of the transistors Ti ₁ , Ti ₂ and Ti ₃ , the emitter voltages V ₁ , V ₂ and V ₃ become constant regardless of the state of the temperature environment and the like, and accordingly the currents I ₁ , I ₂ and I _3. Is also constant. Therefore, the constant current circuit according to the first embodiment can drive each LED array evenly, and is highly resistant to variations and fluctuations in the forward voltage Vf of each LED array in addition to the temperature environment. Can be operated as

(Modification of the first embodiment)
Next, a modification of the first embodiment will be described. In the constant current circuit 100 according to the first embodiment described above, when switching on / off the supply of the current I to the load R _L at high speed, the operation of at least the transistor Ti of the transistors Tv and Ti can be turned off. And it is sufficient. FIG. 4 shows an example of a configuration of a constant current circuit 100A in which both the transistors Tv and Ti can be turned off according to a modification of the first embodiment.

  The constant current circuit 100A includes transistors Tv and Ti and resistors Rc and Ri, and has the same configuration as the constant current circuit 100 shown in FIG. Distinguish as 100A.

In FIG. 4, the switch SW selects the output of the power supply 103 at one selection input terminal, and selects the ground at the other selection input terminal. Hereinafter, when the output of the power source 103 is selected in the switch SW, the on state is set, and when the ground is selected, the off state is set. In the example of FIG. 4, the constant current circuit 100A enables the output of the power supply 103 that supplies the voltage V _IN to the connection point between the collector of the transistor Tv and the resistor Rc to be turned on / off by the switch SW. Note that the voltage value of the voltage V _IN is selected according to the same conditions as the voltage Vc described above.

When the switch SW is on, the output of the power source 103 is supplied to the connection point between the collector of the transistor Tv and the resistor Rc. In this case, the constant current circuit 100A operates in the same manner as the constant current circuit 100 described with reference to FIG. 1, and the current I is supplied (turned on) to the load R _L. On the other hand, when the switch SW is off, the potential at the connection point between the collector of the transistor Tv of the power source 103 and the resistor Rc is set to the ground potential, and accordingly, the base voltages of the transistors Tv and Ti are also set to the ground potential. Therefore, the transistors Tv and Ti are turned off, and the supply of the current I to the load R _L is stopped (off).

In the modification of the first embodiment, on / off of the supply of the current I to the load _RL is controlled by on / off of the transistors Tv and Ti. Therefore, the on / off response of the supply of the current I to the load _RL with respect to the on / off switching of the switch SW is extremely high.

Switching control of the switch SW can be performed using a PWM (Pulse Width Modulation) signal. For example, the switch SW is configured so that the output of the power supply 103 is selected when the PWM signal is at the H (high) level and the ground is selected at the L (low) level. As described above, the constant current circuit 100A has a high on / off response of the supply of the current I to the load _RL with respect to the on / off switching of the switch SW. Reflected accurately, high-precision control by the PWM signal of the load _RL becomes possible.

  Here, even when the switch SW in FIG. 4 has a simple configuration such as a combination of a power supply pull-up and an open collector or an open drain, the current I to the load is reliably turned off. can do.

Here, for comparison, a conventional constant current circuit will be described. FIG. 5 shows an example of a constant current circuit using an operational amplifier according to the prior art. The output terminal of the operational amplifier OP is connected to the base of the transistor Q2, the emitter of the transistor Q2 is grounded via the resistor Ri, and is connected to the inverting input terminal of the operational amplifier OP. _A predetermined voltage V _A is input via a resistor R to the non-inverting input terminal of the operational amplifier OP. A current load is connected to the collector of the transistor Q2.

Further, the collector of the transistor Q1 is connected to the connection point between the resistor R and the non-inverting input of the operational amplifier OP, and the PWM signal is input to the base of the transistor Q1. The emitter of the transistor Q1 is grounded. In such a configuration, when the PWM signal is at a low level and the transistor Q1 is off, the voltage V _A is input to the non-inverting input of the operational amplifier OP. As a result, the voltage V _A appears at the emitter of the transistor Q2 (point A in FIG. 5), and a current I determined by the voltage V _A and the resistance R flows to the load. On the other hand, when the PWM signal is high level and the transistor Q1 is on, the non-inverting input of the operational amplifier OP is grounded. As a result, the voltage at the emitter of the transistor Q2 becomes zero, and the current flowing through the load also becomes zero.

In the case of this prior art configuration, even when the transistor Q1 is in a saturated state, that is, an on state, a voltage V _Ceo is slightly generated between the collector and the emitter, and this is input to the non-inverting input of the operational amplifier OP. Since the inverting input of the operational amplifier OP follows the voltage level of the non-inverting input, a voltage corresponding to the voltage V _Ceo is generated at the emitter of the transistor Q2, and the current I flowing through the load is not completely zero. As described above, in the configuration according to the conventional technique, there is a restriction that the voltage V _A input to the operational amplifier OP must be completely zero.

On the other hand, in the modification of the first embodiment, the current I to the load is obtained by driving the transistors Tv and Ti which are bipolar transistors. In order for the bipolar transistor to operate, a base voltage higher than the base-emitter voltage V _be is required. For this reason, even if the switch SW has an offset and the output of the switch SW has an offset, the transistors Tv and Ti do not operate if the offset is equal to or lower than the voltage V _be , and the current I to the load is ensured. Can be 0.

Thus, in the modification of the first embodiment, the voltage connected to the other selection input terminal of the switch SW does not necessarily have to be the ground potential, and the base-emitter voltage V of the transistors Tv and Ti. _Any voltage below be may be used.

The constant current circuit 100A according to the modified example of the first embodiment can also be used in parallel connection like the constant current circuit 100 according to the first embodiment described above. FIG. 6 is an example of a constant current circuit in which N constant current circuits 100A ₁ , 100A ₂ ,..., 100A _N are connected in parallel according to a modification of the first embodiment. In FIG. 6, the same reference numerals are given to portions common to the above-described FIG. 4, and detailed description thereof is omitted.

In the configuration of FIG. 6 as well, in the parallel connection of the constant current circuits 100A ₁ , 100A ₂ ,..., 100A _N , the voltage V _IN is the connection point between the transistor Tv ₁ and the resistor Rc ₁ . , And a connection point between the transistor Tv ₂ and the resistor Rc ₂ ,..., And a connection point between the transistor Tv _N and the resistor Rc _N , respectively. At the same time, the reference power supply 101, the transistors Tv _1, Tv _2, ..., are connected to the emitter of Tv _N.

Although not shown, it is assumed that the path connecting the reference power source 101 and the emitters of the transistors Tv ₁ , Tv ₂ ,..., Tv _N is grounded via a resistor.

In the configuration of FIG. 6, by controlling on / off of the switch SW, transistors Tv ₁ and Ti ₁ , transistors Tv ₂ and Ti ₂ ,..., 100A _N of the constant current circuits 100A ₁ , 100A ₂ ,. The transistors Tv _N and Ti _N can be simultaneously controlled on and off. Therefore, the on / off control of the switch SW enables the on / off control of the supply of the currents I ₁ , I ₂ ,..., I _N to the loads R _L1 , R _{L2 ,.}

(Second Embodiment)
Next, a second embodiment will be described. FIG. 7 shows an example of a constant current circuit according to the second embodiment. In FIG. 7, the same reference numerals are given to the portions common to FIG. 6 described above, and detailed description thereof is omitted.

In the second embodiment, the constant current circuit 100B connected in parallel _1, 100B _2, ..., each of the transistors Tv ₁ in 100B _N, Tv _2, ..., the output of the power supply 103 relative to the collector and base of Tv _N Whether or not to supply is controlled independently. This makes it possible to independently control the currents I ₁ , I ₂ ,..., I _N flowing through the loads R _L1 , R _{L2 ,.}

Constant current circuit illustrated in FIG. 7, the constant current circuit 100B _1, 100B _2, ..., the 100B _N, each of the transistors Tv ₁ output of the reference power source 101, Tv _2, ..., in common to the emitter of Tv _N is connected Te, the constant current circuit 100B ₁ the supply of the output of the power supply 103, 100B _2, ..., switch SW _1, SW ₂ to control each 100B _N, ..., SW _N are provided. Each of the switches SW ₁ , SW ₂ ,..., SW _N has a first selection input terminal connected to the output of the power source 103 and a second selection input terminal grounded. Each switch SW _1, SW _2, ..., an output terminal of the SW _N, each resistance _{Rc 1, Rc 2, ...,} Rc each through _N transistors Tv _1, Tv _2, ..., respectively collector and base of Tv _N Connected.

In the example of FIG. 7, for example, the collector and base of the transistor Tv ₁ are connected to the other end of the resistor Rc ₁ . On the other hand, in the above-described FIG. 6, for example, the collector of the transistor Tv ₁ is connected to the base via the resistor Rc _1, which is different from the configuration of FIG.

Although not shown, the path connecting the reference power supply 101 and the emitters of the transistors Tv ₁ , Tv ₂ ,..., Tv _N is assumed to be grounded via a resistor. Further, in each of the switches SW ₁ , SW ₂ ,..., SW _N , the state in which the first selection input terminal is selected is turned on, and the state in which the second selection input terminal is selected is turned off.

In the constant current circuit according to the second embodiment, each of the switches SW ₁ , SW ₂ ,..., SW _N can be controlled independently. For example, on / off of each switch SW ₁ , SW ₂ ,..., SW _N is controlled using individual PWM signals. Thereby, the transistors Tv ₁ and Ti ₁ , the transistors Tv ₂ and Ti ₂ ,..., And the transistors Tv _N and Ti _N can be controlled independently. Therefore, the constant current circuit 100B _1, 100B _2, ..., each load is connected to the _{100B N R L1, R L2,} ..., current I _1, I ₂ flowing in R _LN, ..., on and off of the I _N It can be controlled independently.

In the configuration shown in FIG. 7, the switch SW _1, SW _2, ..., since the SW _N is in the off state, the constant current circuit 100B _1, 100B _2, ..., the operation of 100B _N itself is stopped, a constant current The power consumption of the entire circuit can be suppressed.

(First Modification of Second Embodiment)
Next, a first modification of the second embodiment will be described. FIG. 8 shows an example of a constant current circuit according to a first modification of the second embodiment. In FIG. 8, the same reference numerals are given to the portions common to FIGS. 6 and 7 described above, and detailed description thereof will be omitted.

In the first modification of the second embodiment, the constant current circuit 100C _1, 100C ₂ are connected in parallel, ..., in 100C _N, each transistor Tv _1, Tv _2, ..., with respect to the emitter of Tv _N Whether to supply the output of the reference power supply 101 is controlled independently. This method also enables independent control of the loads R _L1 , R _L2 ,..., R _LN as in the second embodiment described above.

Constant current circuit illustrated in FIG. 8, the constant current circuit 100C _1, 100C ₂ are connected in parallel, ..., switch SW _1, SW ₂ for controlling the supply of the output of the reference power source 101 for each 100C _N, ..., SW _N is provided. Each of the switches SW ₁ , SW ₂ ,..., SW _N has a first selection input terminal connected in common to the output of the reference power supply 101 and a second selection input terminal grounded. Each switch SW _1, SW _2, ..., an output terminal of the SW _N, each transistor Tv _1, Tv _2, ..., are connected to the emitter of Tv _N.

Although not shown, the path connecting the reference power source 101 and the first selection input terminals of the switches SW ₁ , SW ₂ ,..., SW _N is assumed to be grounded via a resistor. Further, in each of the switches SW ₁ , SW ₂ ,..., SW _N , the state in which the first selection input terminal is selected is turned on, and the state in which the second selection input terminal is selected is turned off.

In the constant current circuit illustrated in FIG. 8, each of the switches SW ₁ , SW ₂ ,..., SW _N can be controlled independently. For example, on / off of each switch SW ₁ , SW ₂ ,..., SW _N is controlled using individual PWM signals. Thereby, the transistors Tv ₁ and Ti ₁ , the transistors Tv ₂ and Ti ₂ ,..., And the transistors Tv _N and Ti _N can be controlled independently. Therefore, the constant current circuit 100C _1, 100C _2, ..., each load is connected to _{100C N R L1, R L2,} ..., current current I ₁ flowing through the _{R LN, I 2, ...,} I N on and off Can be controlled independently.

In the configuration of FIG. 8, each of the switches SW _1, SW _2, ..., when SW _N is on, each transistor Tv _1, Tv _2, ..., the reference voltage Vref is supplied to the emitter of Tv _N, each The emitter voltages V ₁ , V ₂ ,..., V _{N of the} transistors Ti ₁ , Ti ₂ ,..., Ti _N are equal to the reference voltage Vref. Therefore, currents I ₁ , I ₂ ,..., I _N determined by the reference voltage Vref and the resistors Ri ₁ , Ri ₂ ,..., Ri _N flow through the loads R _L1 , R _{L2 ,.} .

Meanwhile, the switches _{SW 1, SW 2, ...,} SW N is off state, the transistors Tv _1, Tv _2, ..., the emitter of Tv _N is grounded. In this case, the voltage Vref in equation (1) described above is the ground potential, each transistor Ti _1, Ti _2, ..., the voltage V ₁ of the emitter of _{Ti N, V 2, ...,} V N includes a respective ground potential Will be equal. Therefore, the currents I ₁ , I ₂ ,..., I _N flowing through the loads R _L1 , R _{L2 ,.}

Thus, the output of the reference power source 101, the transistors Tv _1, Tv _2, ..., also by controlling the respective connection on and off for the emitter of Tv _N, each load _{R L1, R L2, ...,} R LN current I _1, I ₂ flowing in, ..., it is possible to independently control the on-off of I _N.

In the configuration shown in FIG. 8, each of the switches SW _1, SW _2, ..., in SW _N is turned off, the transistor Tv ₁ output of the reference power source 101, Tv _2, ..., because it is decoupled from the emitter of Tv _N The load on the reference power supply 101 is reduced.

(Second modification of the second embodiment)
Next, a second modification of the second embodiment will be described. FIG. 9 shows an example of a constant current circuit according to a second modification of the second embodiment. Note that, in FIG. 9, the same reference numerals are given to portions common to FIGS. 6 to 8 described above, and detailed description thereof is omitted.

In the second modification of the second embodiment, the constant current circuit 100D _1, 100D ₂ that are connected in parallel, ..., in 100D _N, transistor Tv ₁ and Ti1, transistors Tv ₂ and Ti _2, ..., the transistor Tv for _N and Ti _N of each set, to independently control the base voltage in each set. Also by this method, independent control of ON / OFF of the currents I ₁ , I ₂ ,..., I _N flowing through the loads R _L1 , R _{L2 ,.} It becomes.

Constant current circuit illustrated in FIG. 9, the constant current circuit 100D _1, 100D ₂ that are connected in parallel, ..., 100D _N buffer amplifier 120 _1, respectively, 120 _2, ..., 120 _N are provided. Each buffer amplifier _{120 1, 120 2, ...,} 120 N , each load RL _1, RL _2, ..., current I _1, I ₂ flowing through RL _N, ..., PWM signal P ₁ for controlling the I _N, P ₂ ,..., P _N are respectively input.

Although not shown, it is assumed that the path connecting the reference power source 101 and the emitters of the transistors Tv ₁ , Tv ₂ ,..., Tv _N is grounded via a resistor.

For example, the buffer amplifier 120 ₁ outputs a high impedance signal or a low level signal in accordance with the input PWM signal P ₁ . As a more specific example, for example, the buffer amplifier 120 ₁ outputs a high impedance signal in a state where the input PWM signal P ₁ is at a high level. On the other hand, the buffer amplifier 120 ₁ outputs a low level signal when the input PWM signal P ₁ is at a low level. The same applies to the other buffer amplifiers 120 _{2 to} 120 _N.

The output of the buffer amplifier 120 ₁ is connected to a connection point between the base of the transistor Tv _{1 and} the base of the transistor Ti ₁ . Similarly, the outputs of the buffer amplifiers 120 _{2 to} 120 _N are connected to the connection point between the transistor Tv ₂ and the transistor Ti ₂ to the connection point between the transistor Tv _N and the transistor Ti _N , respectively.

For example, when the output of the buffer amplifier 120 ₁ is a high impedance signal, the bases of the transistor Tv ₁ and the transistor Ti ₁ are open with respect to the ground potential. That is, the high impedance signal output of the buffer amplifier 120 ₁ has no effect on the constant current circuit 100D ₁ . Therefore, the voltage V ₁ of the emitter of the transistor Ti ₁ is the reference voltage Vref equally Do Re, the load RL _1, the current I ₁ which is determined by the reference voltage Vref and the resistor Ri ₁ flows.

On the other hand, when the output of the buffer amplifier 120 ₁ is a low level signal, the base voltages of the transistors Tv ₁ and Ti _{1 have} the same potential as the low level signal. If the potential of the low level signal is a potential that turns off the transistors Tv ₁ and Ti ₁ , no current flows through the transistor Ti ₁ and the supply of the current I ₁ to the load RL ₁ is also stopped.

Thus, by controlling the voltages at the base connection points of the transistors Tv ₁ and Ti ₁ , the transistors Tv ₂ and Ti ₂ ,..., The transistors Tv _N and Ti _N , the loads R _L1 , R _L2 , ..., the currents I ₁ , I ₂ , ..., I _N flowing through R _LN can be controlled independently.

(Third embodiment)
Next, a third embodiment will be described. The third embodiment is a more specific example in the case where an LED array is used as the load RL of each of the above-described embodiments and each modification of each embodiment. FIG. 10 shows a configuration of an example of a light source driving apparatus according to the third embodiment. In FIG. 10, the same reference numerals are given to the portions common to FIG. 7 described above, and detailed description thereof is omitted.

Here, the constant current circuit according to the second embodiment described with reference to FIG. 7 is used as a constant current circuit for supplying current to each of the loads R _L1 , R _L2 ,..., R _LN . Of course, the third embodiment is not limited to this, and the constant current circuit according to a modification of the first embodiment shown in FIG. 6 or the first embodiment of the second embodiment shown in FIGS. The constant current circuit according to the second modification can be applied.

10, the light source unit 150 includes a plurality of LED arrays 151 _1, 151 _2, ..., and 151 _N, the LED array 151 _1, 151 _2, ..., and an LED power supply 110 common to 151 _N. The LED power supply 110 outputs a voltage V _LED .

Light source driving unit 152, a constant current circuit 100E _1, 100E _2, ..., a constant current circuit 100E _N are connected in parallel, each of the constant current circuit 100E _1, 100E _2, ..., switch SW ₁ corresponding respectively to 100E _N , SW _2, ..., and SW _N, the switches SW _1, SW _2, ..., via the SW _N each constant current circuit 100E _1, 100E _2, ..., a power supply 103 supplies a voltage V _iN to 100E _N, each constant current circuit 100E _1, 100E _2, ..., and a reference power supply 101 supplies a reference voltage Vref in common to 100E _N.

Each constant current circuit 100E _1, 100E _2, ..., 100E _N, respectively load RL _1, RL _2, ..., as RL _{N, 1} each LED array 151 as described above, 151 _2, ..., current I ₁ to 151 _N, I _2, ..., supplies I _N, the LED array 151 _1, 151 _2, ..., driving the 151 _N.

Each constant current circuit _{100E 1, 100E 2, ...,} 100E N , respectively, resistors Rc ₁ and Ri _1, Rc ₂ and Ri _2, ..., and Rc _N and Ri _N, transistor Tv ₁ and Ti _1, the transistor Tv ₂ and Ti _2, ..., a transistor Tv _N and Ti _N, a capacitor C _1, C _2, ..., and C _N.

For example, in the constant current circuit 100E _1, one end of the resistor Rc ₁ is connected to the output end of the switch SW _1, the other end connected to the collector and base of the transistor Tv _1. The base of the transistor Tv _{1 and} the base of the transistor Ti ₁ are connected, and the emitter of the transistor Ti ₁ is grounded through the resistor Ri ₁ . The collector of the transistor Ti ₁ is connected to the cathode of the LED array 151 ₁ that is the load RL ₁ . Further, the connection point between the base of the transistor Tv _{1 and} the base of the transistor Ti ₁ is grounded via the capacitor C ₁ .

Further, a reference power supply 101 is connected in common to the emitters of the transistors Tv ₁ , Tv ₂ ,..., Tv _N. Although not shown, it is assumed that the path connecting the reference power source 101 and the emitters of the transistors Tv ₁ , Tv ₂ ,..., Tv _N is grounded via a resistor. Further, in each of the switches SW ₁ , SW ₂ ,..., SW _N , the state in which the first selection input terminal is selected is turned on, and the state in which the second selection input terminal is selected is turned off.

Each of the switches SW ₁ , SW ₂ ,..., SW _N has a first selection input terminal commonly connected to the output of the power supply 103 and a second selection input terminal grounded. The switches SW ₁ , SW ₂ ,..., SW _N are controlled to be turned on / off by independent PWM signals.

In such a configuration, the switches SW _1, SW _2, ..., by the SW _N to the ON state, as already described, the constant current circuit 100E _1, 100E _2, ..., in 100 _N, the reference voltage and Vref, the resistors Ri _1, Ri _2, ..., current I _1, I ₂ determined by the Ri _N, ..., ₁ I _N each LED array 151, 151 _2, ..., is supplied to 151 _N.

Each of the switches SW ₁ , SW ₂ ,..., SW _N is controlled according to the PWM signal. Each of the constant current circuits 100E ₁ , 100E ₂ ,..., 100E _N is configured with a small number of components such as resistors and transistors, and therefore can respond to an input signal at high speed. Therefore, the amount of light emitted by each LED array 151 ₁ , 151 ₂ ,..., 151 _N can be adjusted.

Each LED array 151 _1, 151 _2, ..., so as to turn off the column according to the lighting unnecessary area of 151 _N, the switches SW _1, SW _2, ..., it is possible to control the SW _N. Thereby, power saving can be achieved.

Further, each of the transistors _{Tv 1, Tv 2, ...,} Tv N, and the transistor Ti _1, Ti _2, ..., in Ti _N, when changing the base voltage at a high speed, the parasitic capacitance existing between the collector and base , the load current I _1, I _2, ..., can lead to behavior abnormalities I _N. In the configuration of FIG. 10, the transistors _{Tv 1, Tv 2, ...,} Tv N, and the transistor Ti _1, Ti _2, ..., the base of Ti _N, capacitor C _1, C _2, ..., through C _N Each of them is grounded to give a time constant for the change in base voltage. Therefore, the transistors Tv _1, Tv _2, ..., Tv _N, and the transistor Ti _1, Ti _2, ..., the influence of the parasitic capacitance between the collector and the base of Ti _N is reduced, the LED array 151 _1, 151 ₂ ,..., 151 _N can be driven more stably.

It is preferable to select the capacitances of the capacitors C ₁ , C ₂ ,..., C _{N so} that the time constant is a practical value.

Further, in the configuration of FIG. 10, the transistors Tv ₁ and Ti ₁ , the transistors Tv ₂ and Ti ₂ ,..., The transistors Tv _N and Ti _N are each in a state where the temperature environment is close to each other. The characteristics of the temperature change of the voltage V _be in the transistor can be brought closer, and the temperature tolerance can be improved.

For example, as a method for bringing the temperature environments of the transistors Tv ₁ and Ti ₁ into a close state, the transistors Tv ₁ and Ti ₁ are arranged close to each other, or the transistors Tv ₁ and Ti ₁ are brought into contact with each other for thermal coupling. A method to plan is considered. Further, as the transistor Tv ₁ and Ti _1, these transistors Tv ₁ and Ti ₁ is also conceivable to use those which are enclosed in the same package. Further, for example, the constant current circuit 100E ₁ itself may be configured by one integrated circuit.

(Fourth embodiment)
Next, a fourth embodiment will be described. In the fourth embodiment, for example, the light source driving apparatus described in the third embodiment converts the reflected light obtained by reflecting the light emitted from the light source by the document into an electrical signal by the light receiving element, thereby converting the document image. This is an example applied to a light source of a scanner device for reading.

  FIG. 11 schematically shows a configuration of an example of a scanner device to which the fourth embodiment can be applied. In FIG. 11, the scanner device includes a contact glass 1 on which a document is placed, a first carriage 6 including a light source 2 for document exposure and a first reflection mirror 3, a second reflection mirror 4 and a third reflection mirror 5. A second carriage 7 including an image sensor 9 such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor) image sensor; a lens unit 8 for forming an image on the image sensor 9; a reading optical system; And a white reference plate 13 for correcting various distortions. The image sensor 9 is mounted on the sensor board unit 10.

Light source 2, ₁ LED array 151 as described above, 151 _2, ..., the 151 _N to PWM driving for emitting light emitted. For example, LED arrays 151 ₁ , 151 ₂ ,..., 151 _N each of which a predetermined number of white LEDs are aligned are arranged in one or more rows in the main scanning direction to constitute the light source 2. The image sensor 9 is formed by, for example, CCD linear image sensors aligned in the main scanning direction.

  During scanning in the book reading mode, the first carriage 6 and the second carriage 7 are moved in the sub-scanning direction indicated by the arrow A by a stepping motor (not shown). The light emitted from the light source 2 is applied to the document placed on the contact glass 1, and the reflected light is applied to the lens unit 8 via the first reflection mirror 3, the second reflection mirror 4, and the third reflection mirror 5. Incident light is imaged on the image sensor 9. The image sensor 9 converts the imaged light into an electrical signal and outputs it.

  On the other hand, in the sheet through mode (automatic document reading mode), after the first carriage 6 and the second carriage 7 move below the sheet through reading slit 15, the document 12 placed in the automatic document feeder 14 is moved by the roller 16. By guiding in the direction indicated by the arrow B, the document is read at the position of the sheet-through reading slit 15.

  The light emitted from the light source 2 is reflected by the white reference plate 13 and forms an image on the image sensor 9 via the first reflection mirror 3, the second reflection mirror 4, the third reflection mirror 5, and the lens unit 8. As described above, by controlling the positions of the first carriage 6 and the second carriage 7, it is possible to perform calibration based on white.

FIG. 12 shows an example configuration for performing signal processing in the scanner apparatus illustrated in FIG. 11. In FIG. 12, the same reference numerals are given to the same parts as those in FIGS. 10 and 11 described above, and detailed description thereof is omitted. The scanner device includes an image sensor 9, a timing clock generation unit 300, an oscillator 301, an analog signal processing unit 310, an A / D conversion unit 311, an I / F (interface) unit 312, a light source driving unit 320, and a light source 2. The light source 2 comprises an LED array 151 _1, 151 _2, ..., the 151 _N.

  The image sensor 9 converts the received light into an analog signal by photoelectric conversion and outputs the analog signal. The analog signal output from the image sensor 9 is supplied to the analog signal processing unit 310, subjected to predetermined processing such as sample hold processing and black level correction, and output. The output of the analog signal processing unit 310 is analog image data converted into a digital signal by the A / D conversion unit 311, and is supplied to an image processing unit (not shown) via the I / F unit 312.

  The timing clock generation unit 300 generates a timing signal to be supplied to each unit of the scanner device based on a reference signal supplied from an oscillator 301 using, for example, a crystal oscillator. For example, the timing clock generation unit 300 receives clock signals used by the image sensor 9, the analog signal processing unit 310, the A / D conversion unit 311, the I / F unit 312 and the light source driving unit 320, and a stepping motor (not shown). Generate and supply these components. At this time, the timing clock generation unit 300 also generates a drive signal for controlling ON / OFF of the switch circuit SW1 in the constant current circuit 200 or the constant current circuit 210 and a drive signal for driving the light source 2.

The light source driving unit 320 includes, for example, the light source driving unit 152 described with reference to FIG. To the light source driving unit 320, LED array 151 _1, 151 _2, ..., may further include a booster circuit for boosting an input voltage (PWM signal) for driving the 151 _N. Here, it is assumed that the LED arrays 151 ₁ , 151 ₂ ,..., 151 _N are driven by the light source driving device according to the third embodiment described above. Not limited to this, the LED arrays 151 ₁ , 151 ₂ ,..., 151 _N are used for the modification of the first embodiment described above, and the method according to each modification of the second embodiment and the second embodiment. Either may be used for driving.

In such a configuration, the switch circuit SW1 of the constant current circuit 200 or 210 is synchronized with the scanning timing in the main scanning direction by the image sensor 9 and the analog signal output from the image sensor 9 by the timing clock generator 300. A drive signal for driving is generated. For example, the drive signal is set so that one cycle of turning on and off the LED arrays 151 ₁ , 151 ₂ ,..., 151 _N or an integral multiple of the cycle coincides with the timing of one scan in the main scanning direction. Is generated. By doing so, it becomes possible to suppress the influence on the read image, such as unevenness in the amount of light and fluctuation in the amount of light.

  In addition, an LED that consumes less power than a high-intensity discharge lamp can be applied to the light source 2, and the above-described embodiments and modifications thereof can be applied as a configuration for driving the LED. It is possible to realize a light source driving system that is functional, high-performance, and low in cost.

  Furthermore, scanner devices are becoming smaller and lower in power consumption. Due to the miniaturization, the distance from the light source 2 to the image sensor 9 is shortened, the incident angle of the reflected lens light is increased, and the light at the end on the reading line is scraped, that is, the transmittance of the lens at the end is reduced. A decreasing phenomenon occurs. One countermeasure is to devise a light source configuration. For example, the amount of light is increased mainly by means of closely mounting the LEDs on the end side or increasing the drive current. However, this method may cause discomfort to the user due to increased glare. For this reason, in the light source 2, it is necessary to take measures such as turning off the light source unnecessary portion (outside the document area) and shortening the lighting time width as much as possible.

  Moreover, the space inside the scanner device becomes narrow due to the miniaturization, and the temperature rise inside the device becomes more remarkable. Since temperature changes cause changes in the characteristics of all semiconductor components, countermeasures are indispensable. Therefore, by applying the above-described embodiments and their modifications to the scanner device, high-speed response control (instant lighting / extinguishing) and high temperature resistance can be realized at lower cost. Become. In addition, since turning off unnecessary portions also contributes to reducing the power consumption of the apparatus, the effect is high.

(Fifth embodiment)
Next, a fifth embodiment will be described. In the fifth embodiment, for example, the light source driving device described in the third embodiment is applied to a copying machine that reads a document image with a scanner unit and forms an image on a sheet using the obtained image data. is there. FIG. 13 shows an example of the configuration of a copying machine 400 applicable to the fifth embodiment.

  In FIG. 13, a contact glass 28 is provided on the upper surface of the copying machine 400. An automatic document feeder (ADF) 501 is provided on the top of the copier 400. The ADF 501 is connected to the copier 400 via a hinge (not shown) or the like so as to open and close the contact glass 28. It is connected. The ADF 501 separates documents one by one from the document tray 502 as a document placement table on which a document bundle composed of a plurality of documents can be placed, and the document bundle placed on the document tray 502 toward the contact glass 28. And a separation / conveying section for conveying.

  The separation conveyance unit conveys and stops the document conveyed toward the contact glass 28 to a reading position on the contact glass 28. At the same time, the separation / conveyance unit carries out the document, which has been read by the scanner unit 40 disposed below the contact glass 28, from the contact glass 28.

  In the example of FIG. 13, the scanner unit 40 includes a light source 29, mirrors 30, 31, 32, a lens unit 35, and an image sensor 36. Light emitted from the light source 29 enters the lens unit 35 through the mirrors 30, 31 and 32. The lens unit 35 forms an image of the incident light on the image sensor 36.

  In the scanner unit 40, the image sensor 36 can be a CCD or a CMOS image sensor. In the fourth embodiment, the light source 29 includes a white LED array as a light emission source. For example, the configuration of the scanner unit 40 can correspond to the configuration of the scanner device described with reference to FIGS. 11 and 12 in the fourth embodiment, and the light source 29 is the light source 2 in FIGS. 11 and 12. It can be set as the same structure.

  The light source 29 is driven to emit light by a light source driving unit (not shown) equivalent to the light source driving unit 320 illustrated in FIG. At this time, any of the methods according to the above-described embodiments and their modifications may be used as the LED array driving method. The lighting cycle of the light source 29 is controlled so that one cycle or an integer multiple of the cycle coincides with the timing of the scanning cycle of the image sensor 36.

  A controller (not shown) drives a paper feed motor (not shown) in a normal rotation direction or in a reverse direction in response to a paper feed start signal from the copying machine 400. When the paper feed motor is driven to rotate forward, the feed roller 503 rotates in the clockwise direction so that the document located at the uppermost position is fed from the bundle of documents and conveyed toward the contact glass 28. When the leading edge of the document is detected by the document set detection sensor 507, the controller drives the paper feed motor in the reverse direction based on the output signal from the document set detection sensor 507. This prevents subsequent documents from entering and prevents separation.

  Further, when the document set detection sensor 507 detects the trailing edge of the document, the controller counts the rotation pulse of the conveying belt motor from this detection point, and when the rotation pulse reaches a predetermined value, the controller By stopping the driving and stopping the feeding belt 504, the document is stopped at the reading position of the contact glass 28. Further, when the trailing edge of the document is detected by the document set detection sensor 507, the controller drives the paper feed motor again, separates the subsequent document as described above, and conveys the document toward the contact glass 28. When the pulse of the paper feed motor from when the original is detected by the original set detection sensor 507 reaches a predetermined pulse, the paper feed motor is stopped and the next original is awaited in advance.

  When the original stops at the reading position of the contact glass 28, the scanner unit 40 of the copier 400 reads the reflected light reflected by the light emitted from the light source 29 by the image sensor 36, thereby reading the original. Done. When this reading is completed, a signal indicating that is input to the controller. When this signal is input, the controller drives the paper feeding motor to rotate forward and transports the document from the contact glass 28 to the discharge roller 505 by the transport belt 504.

  As described above, a document bundle placed on the document tray 502 in the ADF 501 with the image surface of the document facing up is pressed from the top document to the contact glass 28 when the print key on the operation unit is pressed. It is fed to a predetermined position. The fed original is read by the scanner unit 40 via the contact glass 28 and then discharged to the discharge port A (discharge port at the time of reverse document discharge) by the feed belt 504 and the reverse driving roller. Further, when it is detected that there is a next document on the document tray 502, it is fed onto the contact glass 28 in the same manner as the previous document.

  The transfer sheets stacked on the first tray 508, the second tray 509, and the third tray 510 are fed by the first sheet feeding unit 511, the second sheet feeding unit 512, and the third sheet feeding unit 513, respectively, and are vertically conveyed. The unit 514 is transported to a position where it abuts on the photoreceptor 515. The image data obtained by reading the original image by the scanner unit 40 is written on the photosensitive member 515 by the laser from the writing unit 557 and passes through the developing unit 527 to form a toner image.

  The writing unit 557 includes a laser light source 528 and a lens unit 559, and also includes, for example, a mirror unit 560 using a polygon mirror. Laser light emitted from the laser light source 558 is applied to the rotating mirror unit 560 via the lens unit 559. As the mirror unit 560 rotates, the laser beam applied to the mirror unit 560 scans the photoconductor 515 in the main scanning direction, whereby an electrostatic latent image of toner is formed on the photoconductor 515.

  Then, the toner image formed by the electrostatic latent image is transferred onto the photoconductor 515 while the transfer paper is conveyed by the conveyance belt 516 at the same speed as the rotation of the photoconductor 515. Thereafter, the image is fixed by the fixing unit 517 and conveyed to the paper discharge unit 518. The transfer paper conveyed to the paper discharge unit 518 is discharged to a paper discharge tray 519 when the staple mode is not performed.

OP operational amplifier _{Q1, Q2, Ti, Ti 1} , Ti 2, Ti N, R, Rc, Rc 1, Rc 2, Rc N, Ri, Ri 1, Ri 2, Ri N resistors _{R L, R L1, R L2} , R _LN load Tv, Tv ₁ , Tv ₂ , Tv _N transistor SW, SW ₁ , SW ₂ , SW _N switch 2 Light source 9 Image sensor 100, 100 ₁ , 100 ₂ , 100 _N , 100A, 100A ₁ , 100A ₂ , 100A _N , 100B ₁ , 100B ₂ , 100B _N , 100C ₁ , 100C ₂ , 100C _N , 100D ₁ , 100D ₂ , 100D _N , 100E ₁ , 100E ₂ , 100E _N constant current circuit 101 Reference power supply 102, 103 Power supply 151 ₁ , 151 ₂ , 151 ₃ LED array

Japanese Patent No. 4677735

Claims (8)

A first bipolar transistor in which a first voltage with respect to a common potential is supplied to the emitter;
A resistor having one end connected to the common potential;
A constant current circuit comprising: a second bipolar transistor in which the other end of the resistor is connected to an emitter, a base is connected to a base of the first bipolar transistor at a connection point, and a collector is connected to a load. . A plurality of sets of the first bipolar transistor and the second bipolar transistor whose bases are connected to each other at the connection point;
The first voltage is
The constant current circuit according to claim 1, wherein the constant current circuit is commonly supplied to emitters of the plurality of first bipolar transistors. A plurality of the connection points are switched by switching between an on voltage for turning on the first bipolar transistor and the second bipolar transistor and an off voltage for turning off the first bipolar transistor and the second bipolar transistor. The constant current circuit according to claim 2, further comprising: a switching unit that supplies power to the power source. The switching unit is
The constant current circuit according to claim 3, wherein the ON voltage and the OFF voltage are switched for each of the plurality of connection points. The switching device according to claim 2, further comprising a switching unit that switches the first voltage and the common potential for each of the plurality of first bipolar transistors and supplies the first voltage and the common potential to each emitter of the plurality of first bipolar transistors. Constant current circuit. A collector of the first bipolar transistor is connected to the connection point, and a power source that outputs at least an on-voltage that turns on the first bipolar transistor and the second bipolar transistor is connected to the connection point via a resistor. The constant current circuit according to claim 1, further connected. The constant current circuit according to claim 1, wherein the load is an LED. The constant current circuit according to any one of claims 1 to 7, wherein a capacitor is inserted between the connection point and the common potential.

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