JP2001202645A - Laser control circuit for optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser controller with which an abnormal emission power is not outputted without using a switching circuit operating at high speed, when changing an emission power. SOLUTION: An arithmetic and logic unit 5 suppresses the variation of the laser driving current by the transient characteristic of an APC circuit 4 and the emission power by changing a target voltage value so that an intermediate target voltage value is formed between the initial target voltage value and the final target voltage value, as a target voltage value for specifying a target power in the APC circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser control circuit for a semiconductor laser used in an optical disk drive.

[0002]

2. Description of the Related Art As described in Japanese Patent Application Laid-Open No. Hei 5-94633, for example, a semiconductor laser emission power control of a conventional optical disk apparatus is performed between a semiconductor laser and a laser driving circuit.
A switch and a limiting resistor are provided in parallel with the switch. When an abnormality in the output power from the semiconductor laser or an abnormality in the driving current of the laser drive circuit is detected, the switch is turned off, and the current is supplied to the semiconductor laser through the limiting resistor. Is provided.

[0003]

As described above, the control for the abnormal output power and abnormal drive current of the semiconductor laser is performed after detecting the abnormal output power and the abnormal drive current. It is necessary to generate a protection signal for operating the protection circuit
Further, there is a problem that a switch circuit that operates at high speed is required before the semiconductor laser is broken.

According to the present invention, when the output power is changed,
It is an object of the present invention to provide a laser control circuit that does not output abnormal emission power without using a switch circuit that operates at high speed.

[0005]

According to a first aspect of the present invention, there is provided a laser control circuit for an optical disk apparatus, wherein an output power of a semiconductor laser is monitored by a photodetector and converted into a voltage signal. APC to match the voltage value
In a laser control circuit of an optical disk device for supplying power to the semiconductor laser via a circuit, a target voltage setting circuit for setting a target voltage value of the APC circuit and a target voltage value output from the target voltage setting circuit are calculated. An arithmetic circuit, wherein when the emission power of the semiconductor laser is changed, a target voltage value of the target voltage setting circuit before an emission power change of the semiconductor laser is set as an initial target voltage value, and the semiconductor laser Setting the target voltage value of the target voltage setting circuit after the change of the emission power as a final target voltage value, such that an intermediate target voltage value is formed between the initial target voltage value and the final target voltage value. It is characterized in that it is configured to change the value.

According to a second aspect of the present invention, there is provided a laser control circuit for an optical disk device according to the first aspect, further comprising a first voltage detection circuit for detecting a monitor voltage value output from the photodetector, and the arithmetic circuit When the output voltage of the semiconductor laser is changed, a detection voltage value obtained by detecting the monitor voltage value of the photodetector by the first voltage detection circuit, and when the semiconductor laser outputs a target output power And comparing the detected voltage value with the detected voltage value of the first voltage detecting circuit, and correcting the target voltage value of the target voltage setting circuit when they do not coincide with each other.

According to a third aspect of the present invention, in the laser control circuit of the optical disk device according to the first aspect, the arithmetic circuit changes the target voltage value of the target voltage setting circuit based on calibration data. Characterized in that it is configured to control the output power of

According to a fourth aspect of the present invention, a laser control circuit for an optical disk device according to the first aspect stores a calibration result between a target voltage of the target voltage setting circuit and a detection voltage of the first voltage detection circuit. A memory circuit, wherein the arithmetic circuit sets a final target voltage value and an intermediate target voltage value of the target voltage setting circuit with respect to a target emission power when changing the emission power of the semiconductor laser; It is characterized in that the final target voltage value and the intermediate target voltage value are determined from the calibration result stored in a storage circuit.

According to a fifth aspect of the present invention, there is provided a laser control circuit for an optical disk device according to the first aspect, wherein the arithmetic circuit is set to the target voltage setting circuit when the emission power of the semiconductor laser is changed. When the difference between the target voltage value and the final target voltage value is equal to or smaller than a predetermined value, the intermediate target voltage value is not set.

According to a sixth aspect of the present invention, in the laser control circuit of the optical disk apparatus according to the first aspect, the first voltage detection circuit is configured to control the operation circuit by the first voltage detection circuit when changing the emission power of the semiconductor laser. A monitor voltage value of the photodetector is detected, and when the detected voltage value of the first voltage detection circuit is abnormal, a command is issued to stop the operation of the APC circuit.

According to a seventh aspect of the present invention, there is provided a laser control circuit for an optical disk device according to the first aspect, further comprising a second voltage detecting circuit for detecting an output voltage value of the APC circuit, wherein the arithmetic circuit is provided with the semiconductor circuit. When changing the output power of the laser, the output voltage value of the APC circuit is detected by the second voltage detection circuit, and when the detection voltage value of the second voltage detection circuit is abnormal, the APC circuit is detected. Characterized in that it is configured to instruct the stop of the operation.

According to an eighth aspect of the present invention, there is provided a laser control circuit for an optical disk device according to the first aspect, further comprising a second voltage detecting circuit for detecting an output voltage value of the APC circuit, A current limiter for limiting a current value flowing into the semiconductor laser between the semiconductor lasers, wherein the operation circuit stops the operation of the APC circuit when a detection voltage value of the second voltage detection circuit is abnormal; It is characterized by having comprised so that it may perform.

According to a ninth aspect of the present invention, in the laser control circuit of the optical disk device according to the eighth aspect, the arithmetic circuit changes a limit value of the current limiter according to a target voltage value of the target voltage setting circuit. It is characterized by having such a configuration.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to FIGS. (Embodiment 1) FIGS. 1 and 2 show (Embodiment 1) of the present invention.

Semiconductor laser 10 in optical disk drive
The laser control circuit that changes the emission power according to the amount of current flowing into the laser is configured as follows. In FIG. 1, reference numeral 1 denotes a light receiving element (photodetector) for converting light energy into electric energy, and is arranged to detect light energy of a semiconductor laser 10 to be controlled. Reference numeral 2 denotes an IV converter that changes a current value generated in the light receiving element 1 into a voltage value. In this embodiment, the light receiving element 1
And the IV converter 2 constitute a photodetector 3.

Reference numeral 4 denotes an APC (Auto Power Control) circuit which controls an output voltage value in accordance with an input voltage difference between the photodetector 3 and the target voltage setting circuit 8, and eliminates two input voltage differences. Thus, the output voltage value is determined.

Reference numeral 5 denotes an arithmetic circuit constituted by a microcomputer, which performs arithmetic processing described later on a digital signal input from the outside. Reference numeral 6 denotes a first voltage detection circuit, which converts an input voltage value into a digital value like an A / D converter, for example. The target voltage setting circuit 8 outputs a voltage according to the input digital signal such as a D / A converter.

Reference numeral 9 denotes a storage circuit which stores and reads out data such as a semiconductor memory, for example, and stores data necessary for the operation of the arithmetic circuit 5. Semiconductor laser 1
0 is supplied with a laser drive current from the APC circuit 4,
By changing the amount of current, the output power changes. The output power of the semiconductor laser 10 is converted into a current amount by the light receiving element 1, and the current amount is converted into a voltage value by the IV converter 2. That is, the output power of the semiconductor laser 10 can be monitored by the monitor voltage value output from the photodetector 3.

The monitor voltage and the target voltage set by the target voltage setting circuit 8 are input to the APC circuit 4. The APC circuit 4 increases the output laser drive current when the monitor voltage is lower than the target voltage. On the other hand, if it is larger, the laser drive current is automatically controlled so as to decrease, and a laser drive current that finally matches the monitor voltage and the target voltage is output.

The case where the semiconductor laser 10 is turned on with the reproduction power from the light-off state will be described in comparison with the conventional case. In the conventional method, as shown in FIG. 2A, the target voltage value of the target voltage setting circuit is set to an initial target voltage value (= 0).
To the final target voltage value (the target voltage value when the semiconductor laser 10 is turned on with the target emission power). At this time, since the voltage difference between the monitor voltage and the target voltage is large,
The laser drive current output from the APC circuit 4 increases rapidly.

In response to the increase in the laser drive current, the output power from the semiconductor laser 10 increases and reaches the target output power. However, since there is a delay in following the monitor voltage with respect to the change in the laser drive current (hereinafter, referred to as “transient characteristics of the APC circuit”), an emission power higher than the target emission power is temporarily output.

At this time, since the beam is applied to the optical disc arranged at the focal length of the semiconductor laser 10, there is a risk of damaging the optical disc. Furthermore, since a large driving current flows into the semiconductor laser 10, the semiconductor laser 10 itself may be destroyed.

On the other hand, in this embodiment, FIG.
As shown in (b), when the semiconductor laser 10 is turned on with the reproduction power from the light-off state, the initial target voltage value (= 0) which is the target voltage value of the target voltage setting circuit 8 at the time of the light-off, and the target reproduction power emission. For example, seven intermediate target voltage values are provided for the final target voltage value that is the target voltage value at the time. The number of the intermediate target voltage values is not limited to this.

The final target voltage value and the intermediate target voltage value are
From the relationship between the predetermined output power and the target voltage value,
It is determined by the operation of the operation circuit 5. Then, the output voltage of the target voltage setting circuit 8 is changed to each intermediate target voltage value and final target voltage value according to the set value input from the arithmetic circuit 5.

When the semiconductor laser 10 is changed from the off state to the reproducing power emission state, first, the target voltage value of the target voltage setting circuit 8 is changed from the initial target voltage value to the closest intermediate target voltage value. At this time, since the difference between the monitor voltage value of the photodetector 3 input to the APC circuit 4 and the target voltage value of the target voltage setting circuit 8 is small, abnormal fluctuation and emission of the laser drive current due to the transient characteristics of the APC circuit. Abnormal power emission can be suppressed.

By sequentially updating the target voltage value to an intermediate target voltage value in a direction approaching the final target voltage value,
Finally, the emission power of the semiconductor laser 10 can be stably changed to the target reproduction power.

There is no limit to the number of intermediate target voltage values.
It can be set so that abnormal fluctuations in the laser drive current and the output power due to the transient characteristics of the circuit are suppressed, and the output power of the semiconductor laser 10 is quickly converged to the target output power.

In FIG. 2B, the change width of the intermediate target voltage value is constant. However, the change width of the intermediate target voltage value does not need to be constant, and the laser drive current and the emission due to the transient characteristics of the APC circuit. It is possible to suppress abnormal fluctuation of the power and set the emission power of the semiconductor laser 10 to quickly converge to the target emission power.

In FIG. 2B, the update time interval of the target voltage value is constant. However, the update time interval of the target voltage value does not need to be constant, and the laser drive current and the emission due to the transient characteristics of the APC circuit. Suppress abnormal fluctuation of power, and
The output power of the semiconductor laser 10 can be set so as to quickly converge to the target output power.

Further, the configuration of the arithmetic circuit 5 will be described in detail. When the semiconductor laser 10 is turned on with the reproduction power from the off state, the final target voltage value of the target voltage setting circuit 8 that outputs the target reproduction power is determined by the relationship between the predetermined emission power and the target voltage value. Thus, it is determined by the operation of the arithmetic circuit 5. However, a target reproduction power may not be output due to a voltage offset of a circuit, a variation in components, or the like. The output power at that time is monitored by the photodetector 3, the monitor voltage value output from the photodetector 3 is detected by the first voltage detection circuit 6, and the detected voltage value is input to the arithmetic circuit 5. .

In the arithmetic circuit 5, the first detected voltage value detected when the input detection voltage value is output at the target reproduction power is output.
Is compared with a known voltage value of the voltage detection circuit 6 of FIG. If the detected voltage value is smaller than the voltage value corresponding to the target reproduction power, the arithmetic circuit 5 raises the target voltage value to the target voltage setting circuit 8 by a predetermined specified voltage value. Change settings.

Conversely, when the detected voltage value is higher than the voltage value corresponding to the target reproduction power, the setting is changed so that the target voltage value is lowered by a predetermined voltage value. Even if there is a voltage offset of the circuit or variation in components, by correcting the target voltage value output from the target voltage setting circuit 8, it is possible to output the emission power close to the target reproduction power from the semiconductor laser 10.

(Embodiment 2) The above (Embodiment 1)
Then, the target voltage value output from the target voltage setting circuit 8 is corrected by a predetermined specified voltage value so that the target reproduction power is output as much as possible even if there is a voltage offset of the circuit or variation of components. By doing so, the actual emission power from the semiconductor laser 10 is brought closer to the target reproduction power. In this (Embodiment 2), the actual emission power from the semiconductor laser 10 can be made closer to the target reproduction power. it can.

The arithmetic circuit 5 of this (Embodiment 2)
Only the part relating to the output correction of the target voltage setting circuit 8 in the first embodiment is configured as follows. For example, when the semiconductor laser 10 is turned on with the reproduction power from the off state, the final target voltage value of the target voltage setting circuit 8 that outputs the target reproduction power is determined by a predetermined emission power and target voltage value. Is determined by the operation of the arithmetic circuit 5. However, a target reproduction power may not be output due to a voltage offset of a circuit, a variation in components, or the like. Therefore, the arithmetic circuit 5 of this embodiment executes a calibration operation between the output power and the target voltage value of the target voltage setting circuit 8 every time the system power supply of the optical disk device is switched to the ON state. It is configured to eliminate the influence of the voltage offset of the circuit and the variation of the components based on the result.

The calibration of the relationship between the output power and the target voltage value is realized by calibrating the relationship between the detection voltage detected by the first voltage detection circuit 6 and the target voltage. The procedure is as follows. The detection voltage of the first voltage detection circuit 6 is input to the arithmetic circuit 5 and monitored, and the set value for the target voltage setting circuit 8 is changed so that the detected voltage value matches a predetermined value. The target voltage value input to the APC circuit 4 is changed by changing the target voltage value.
The calibration of the emission power and the target voltage value is performed at at least one point, and this calibration data is stored in the arithmetic circuit 5 and the storage circuit 9.
To memorize. The calibration data is a set value set for the target voltage setting circuit 8 from the arithmetic circuit 5 when the detected voltage value of the first voltage detection circuit 6 matches a predetermined value.

During the operation of the optical disk device, when the semiconductor laser 10 is turned on with the reproducing power from the light-off state,
The arithmetic circuit 5 reads the calibration data stored in the storage circuit 9, and determines the final target voltage value and the intermediate target voltage value for the target reproduction power by an approximate expression derived from the calibration data.

For example, when there are two points of calibration data of the target voltage values A and B for the predetermined powers A and B as shown in FIG. 3, the final target voltage with respect to the target reproduction power is obtained by linear approximation of the calibration data. And the intermediate target voltages a and b can be determined.

However, when the difference between the initial target voltage value and the final target voltage value is small, the fluctuation of the output power due to the transient characteristics of the APC circuit is small. Therefore, the convergence speed of the output power to the target output power is taken into consideration. , The intermediate target voltage value is not set.

As described above, the arithmetic circuit 5 collects the calibration data every time the system power supply of the optical disk device is turned on, and controls the target voltage setting circuit 8 based on the calibration data. The target reproduction power can be output even if there is a variation in parts.

(Embodiment 3) FIG. 4 shows (Embodiment 3).
The optical disc device of the present embodiment is different from the above-mentioned (Embodiment 1) or (Embodiment 2) only in the APC circuit 4 and the arithmetic circuit 5.

The arithmetic circuit of the third embodiment monitors the detection voltage of the first voltage detection circuit 6 when the semiconductor laser 10 is turned on with the reproduction power from the off state. When the detected voltage value input to the arithmetic circuit 5 deviates from the detected voltage value corresponding to the target reproduction power by a predetermined value or more, it is determined that the semiconductor laser 10 is emitting abnormal light.
A is applied to the switch 12 of the APC circuit 4 from the arithmetic circuit 5 by A
Generate a PC stop signal. Note that in FIG.
Is represented by an equivalent circuit. Specifically, when the APC stop signal is input to the APC circuit 4, the driving of the semiconductor laser 10 is stopped immediately.

In the above specific example, the arithmetic circuit 5 is the first
The APC operation of the APC circuit 4 is stopped by monitoring the detection voltage of the voltage detection circuit 6 described above, but this can be similarly realized by the following configuration.

In this case, a second voltage detecting circuit 7 for detecting the output voltage of the APC circuit 4 is provided as shown in FIG. The second voltage detection circuit 7 converts an input voltage value into a digital value like an A / D converter, for example. The detection voltage of the second voltage detection circuit 7 is input to the arithmetic circuit 5 and monitored. If the detected voltage value inputted to the arithmetic circuit 5 exceeds a certain voltage value, the APC circuit 4
It is determined that an abnormally large current flows into the semiconductor laser 10 from the
Generate a stop signal. When an APC stop signal is input to the APC circuit 4, the APC is stopped immediately.

(Embodiment 4) FIG. 5 shows (Embodiment 4).
The current limiter 11 is provided between the APC circuit 4 and the semiconductor laser 10 in the above (Embodiment 4), and the arithmetic circuit 5 changes the current limiter 11 according to the target voltage output from the target voltage setting circuit 8. The only difference is that you control

Specifically, when the semiconductor laser 10 is turned on with the reproduction power from the off state, an abnormally large current flows from the APC circuit 4 into the semiconductor laser 10 due to the transient characteristics of the APC circuit, and the semiconductor laser 10 is turned off. A current limiter 11 is provided between the APC circuit 4 and the semiconductor laser 10 because there is a risk that the optical disc disposed at the focal length may be damaged due to destruction or abnormal light emission.

The current limiter 11 limits the value of the current flowing through the semiconductor laser 10, so that even if the output of the APC circuit 4 is abnormal, protection of the semiconductor laser 10 from destruction and emission power limitation can be realized.

The current limiter 11 comprises, for example, a transistor 13 and a variable voltage source 14 for setting a reference voltage value. The variable voltage source 14 is controlled by the arithmetic circuit 5.
The output voltage of the APC circuit 4 is equal to the reference voltage + V _BE (base−
If the voltage exceeds the emitter-emitter voltage, a part of the current flows into the transistor 13, and the current flowing into the semiconductor laser 10 does not exceed a certain value.

The operating state of the current limiter 11 is determined by monitoring the detected voltage of the second voltage detecting circuit 7 by the arithmetic circuit 5, and when the detected voltage value is equal to or higher than the reference voltage + V _BE , If it is less than that, it is determined to be in the stopped state. When the current limiter 11 is operating, the arithmetic circuit 5 generates an APC stop signal to the APC circuit 4. When an APC stop signal is input to the APC circuit 4, the APC is stopped immediately.

The above embodiments are not limited to the case where the reproducing power is changed in an optical disk apparatus which reproduces different types of optical disks by changing the reproducing power, in addition to the change from the unlit state to the reproducing power. For example, in a recordable optical disk device, from reproduction to recording,
Alternatively, the same control operation can be performed when the power is changed when switching from recording to reproduction. Further, the same control operation can be performed not only when the emission power of the semiconductor laser 10 is increased but also when it is decreased.

[0050]

As described above, according to the laser control circuit of the optical disk apparatus of the present invention, when the output power from the semiconductor laser is changed, the target voltage value of the APC circuit is updated stepwise, Abrupt fluctuations in the laser drive current and abnormal output power output due to the transient characteristics of the above.

Further, since the calibration of the target voltage value and the emission power of the APC circuit is performed before the semiconductor laser is turned on, the target emission power generated by the voltage offset of the circuit, the variation of parts, and the change with time are compared with the actual emission power. An error with the output power can be corrected.

Further, by monitoring the laser drive current and the emission power, when the laser drive current abnormality and the emission power abnormality due to the transient characteristics of the APC circuit are detected, the operation of the APC circuit can be stopped.

Further, between the semiconductor laser and the APC circuit,
By providing the current limiter, it is possible to prevent the abnormal drive current from flowing into the semiconductor laser.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a laser control circuit of an optical disc device according to a first embodiment of the present invention;

FIG. 2 is a characteristic diagram showing a general control method and an output power control state in the embodiment.

FIG. 3 is an explanatory diagram of calibration of an APC circuit target voltage value and emission power and determination of a final target voltage value and an intermediate target voltage value in the laser control circuit of the optical disc device according to the second embodiment of the present invention.

FIG. 4 is a configuration diagram of a laser control circuit of the optical disc device according to the third embodiment of the present invention.

FIG. 5 is a configuration diagram of a laser control circuit of the optical disc device according to (Embodiment 4) of the present invention;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light receiving element 2 IV converter 3 photodetector 4 APC (Auto Power Control) circuit 5 arithmetic circuit 6 first voltage detection circuit 7 second voltage detection circuit 8 target voltage setting circuit 9 storage circuit 10 semiconductor laser 11 Current limiter 12 Switch 13 Transistor 14 Variable voltage source

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Kazuo Manabe, Inventor Kazuo Manabe, 8-8 Koshinmachi, Takamatsu City, Kagawa Prefecture F-term (reference) 5D119 AA33 AA34 BA01 DA01 DA05 FA05 HA12 HA44 HA68 5F073 BA05 EA15 EA27 FA01 GA02 GA12

Claims (9)

[Claims] An output power of a semiconductor laser is monitored by a photodetector and converted into a voltage signal, and power is supplied to the semiconductor laser via an APC circuit so that the monitored voltage value matches a target voltage value. A laser control circuit for an optical disc device, comprising: a target voltage setting circuit that sets a target voltage value of the APC circuit; and a calculation circuit that calculates a target voltage value output from the target voltage setting circuit. When changing the emission power of the laser, the target voltage value of the target voltage setting circuit before the change of the emission power of the semiconductor laser is set as the initial target voltage value, and the target voltage setting circuit after the change of the emission power of the semiconductor laser is changed. With the target voltage value as the final target voltage value, the intermediate target voltage value is formed between the initial target voltage value and the final target voltage value. Laser control circuitry configured optical disc device so as to vary the target voltage value. A first voltage detection circuit for detecting a monitor voltage value output from the photodetector, wherein the operation circuit changes the first voltage after changing an emission power of the semiconductor laser. A detection voltage value obtained by detecting a monitor voltage value of the photodetector by a detection circuit; and a first voltage value detected when the semiconductor laser outputs a target emission power.
2. The laser control circuit for an optical disk device according to claim 1, wherein the target voltage value of the target voltage setting circuit is corrected when the detected voltage values of the target voltage setting circuit do not match. 3. The laser for an optical disk device according to claim 1, wherein said arithmetic circuit is configured to control the emission power of said semiconductor laser by changing a target voltage value of said target voltage setting circuit based on calibration data. Control circuit. 4. A storage circuit for storing a calibration result between a target voltage of said target voltage setting circuit and a detection voltage of said first voltage detection circuit, wherein said arithmetic circuit changes emission power of said semiconductor laser. For the target output power at the time,
When setting the final target voltage value and the intermediate target voltage value of the target voltage setting circuit, the final target voltage value and the intermediate target voltage value are determined from the calibration result stored in the storage circuit. A laser control circuit for an optical disk device according to claim 1. 5. A method according to claim 1, wherein when the output power of said semiconductor laser is changed by said arithmetic circuit, a difference between an initial target voltage value and a final target voltage value set in said target voltage setting circuit is equal to or less than a predetermined value. 2. A laser control circuit for an optical disk device according to claim 1, wherein said intermediate target voltage value is not set. 6. The first voltage detection circuit detects a monitor voltage value of the photodetector by the first voltage detection circuit when changing the emission power of the semiconductor laser. If the detected voltage value is abnormal, the APC
2. The laser control circuit for an optical disk device according to claim 1, wherein the laser control circuit is configured to instruct the operation of the circuit to stop. 7. A semiconductor device comprising: a second voltage detection circuit for detecting an output voltage value of the APC circuit; wherein the arithmetic circuit is adapted to detect the output voltage value of the semiconductor laser by changing the output power of the semiconductor laser. 2. The laser for an optical disk device according to claim 1, wherein an output voltage value of the APC circuit is detected, and when the detected voltage value of the second voltage detection circuit is abnormal, a command to stop the operation of the APC circuit is issued. Control circuit. 8. A semiconductor device comprising: a second voltage detection circuit for detecting an output voltage value of the APC circuit; and a current limiter for limiting a current value flowing into the semiconductor laser between an output of the APC circuit and the semiconductor laser. 2. The laser control circuit according to claim 1, wherein the arithmetic circuit is configured to stop the operation of the APC circuit when the detected voltage value of the second voltage detection circuit is abnormal. 9. The laser control circuit according to claim 8, wherein the arithmetic circuit is configured to change a limit value of the current limiter according to a target voltage value of the target voltage setting circuit.