JPH07337077A - Brushless motor drive circuit - Google Patents

Abstract

PURPOSE:To obtain a brushless motor drive circuit which can detect the positional signal of a rotor accurately by comparing the voltage at the end of a stator coil with a predetermined potential for each phase. CONSTITUTION:When the voltage at the end of each stator of a brushless motor 106 is compared with a predetermined potential in order to detect the positional signal of a rotor, a pulse generation circuit 1 generates a pulse of predetermined width in the vicinity of rise and fall of the positional signal. The pulse is fed, as a time constant switching signal, to a low-pass filter 104 in order to switch the time constant thereof to a larger one. Consequently, the noise components are removed from the drive signal in the vicinity of rise and fall of the positional signal. This constitution realizes accurate positional detection of the rotor in a brushless motor and enhances the rotational accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device for recording information on a recording layer of an optical disk or reproducing / erasing previously recorded information by using a light spot obtained by focusing light of a semiconductor laser with a lens. The present invention relates to a brushless motor drive circuit.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a conventional brushless motor drive circuit. A conventional technique will be described with reference to FIG. In FIG. 5, 101 is an optical disk, and 102.
Is an optical head that irradiates an optical spot on the optical disc 101 and detects reflected light from the optical disc 101 to obtain a reproduction signal. Reference numeral 106 rotates the optical disc 101 at a constant linear velocity with respect to the irradiation position of the optical spot on the optical disc 101. A brushless motor 103 is a drive voltage generation circuit for generating a drive voltage from a reproduction signal from the optical head 102, 104
Is a low pass filter, 105 is a low pass filter 104
Drive circuits for supplying a drive current to the brushless motor 106 according to the output level of the brushless motor 106, 107, 108, 109 comparing coil end voltage generated in each stator coil of the brushless motor 106 with a predetermined potential.
A comparator for detecting the position signal of the internal rotor, 110 is a timing generation circuit for generating a timing signal for energizing each stator coil of the brushless motor 106 from the position signals output by the comparators 107, 108, 109, 1
Reference numeral 11 is a waveform generation circuit for generating a conduction waveform to each stator coil of the brushless motor 106 from the timing signal, 1
Reference numerals 12, 113, 114, 115, 116, and 117 denote transistors that switch the energization to each stator coil according to the output waveform from the waveform generation circuit 111.

The operation of the brushless motor drive circuit configured as above will be described below. First optical head 1
Reference numeral 02 illuminates a light spot on the recording layer of the optical disc 101, detects the amount of light reflected from the optical disc 101, and obtains a reproduction signal. The reproduction signal reproduced from the optical head 102 is
It is input to the drive voltage generation circuit 103. Then, the drive voltage generation circuit 103 extracts a predetermined clock signal from the reproduced signal, detects a phase difference and a speed difference between the extracted predetermined clock signal and the reference clock signal, and generates a drive voltage. The generated drive voltage is smoothed by the low-pass filter 104, and the drive circuit 105 causes the low-pass filter 10 to operate.
Drive current according to the output level of 4 brushless motor 1
Supply to 06.

FIG. 6 is a waveform diagram of each part of the brushless motor drive circuit of FIG. In combination with FIG.
The timing of energizing each stator coil of the brushless motor 106 will be described in detail. Brushless motor 10
When 6 rotates the optical disk 101 at a constant linear velocity with respect to the irradiation position of the light spot of the optical head 102, the voltage generated at the end of the stator coil of the brushless motor 106 is first detected.

FIG. 6A is a voltage waveform at each stator coil end of the brushless motor 106. Comparator 107, 10
Reference numerals 8 and 109 compare the coil end voltage generated in each phase u, v, w of the stator coil inside the brushless motor 106 with a predetermined potential, and the binary signals u, v, as shown in FIG.
detect w. The detected binary signals u, v, w are input to the timing generation circuit 110 as position signals of the rotor inside the brushless motor 106, and generate six types of timing signals as shown in FIG. 6-c. This timing signal is input to the waveform generation circuit 111, and the waveform generation circuit 111 generates a conduction waveform to each stator coil as shown in FIG. 6-d based on the timing signal. Transistor 1
12, 113, 114, 115, 116, and 117 are turned on and off by the energization waveform generated by the waveform generation circuit 111, and the drive current supplied from the drive circuit 105 is passed through each stator coil, whereby the optical disk 10 is driven.
1 was rotated at a predetermined rotation speed.

[0006]

However, in the conventional brushless motor drive circuit, the voltage waveform generated at each stator coil end has a substantially sinusoidal induced voltage component and a voltage drop component due to the current supplied to the stator coil. Are superposed and contain a lot of noise components. Therefore, this noise component causes mutual induction between each phase of the stator coil, and the rise and fall of the position signal of the rotor inside the brushless motor that detects by comparing the coil end voltage generated in each phase with a predetermined potential. As shown in FIG. 6, there is a problem that noise is generated in the vicinity and the rotation accuracy of the brushless motor is deteriorated.

In view of the above points, the present invention compares the coil end voltage generated in each phase of the stator coil with a predetermined potential to detect a position signal, and drives the position signal in the vicinity of rising and falling. An object of the present invention is to provide a brushless motor drive circuit that accurately detects the position of a rotor inside a brushless motor and improves the rotation accuracy of the brushless motor by reducing noise superposition of voltage.

[0008]

In order to solve the above-mentioned problems, a brushless motor drive circuit of the present invention comprises an optical head for irradiating a light spot on an optical disc, detecting reflected light from the optical disc to obtain a reproduction signal, and an optical disc. A brushless motor that rotates the optical disc at a constant linear velocity with respect to the irradiation position of the light spot above, and a predetermined clock signal is extracted from the reproduction signal from the optical head, and the extracted clock signal and the reference clock signal Drive voltage generation means for detecting a phase difference or speed difference to generate a drive voltage, a low-pass filter for smoothing the drive voltage, and a drive circuit for supplying a drive current to the brushless motor according to the output level of the low-pass filter. And comparing the coil end voltage generated in each phase of the stator coil of the brushless motor with a predetermined potential, Comparing means for detecting the position signal of the rotor inside the ladderless motor, timing generating means for generating a timing signal based on the output of the comparing means, and a waveform for energizing each phase of the stator coil by the timing signal. Waveform generating means, switching means for switching the energization of each phase of the stator coil by the output of the waveform generating means, pulse generating means for generating a predetermined pulse based on the timing signal, and the pulse generating means And a time constant switching means for switching the time constant of the low-pass filter according to the output of.

Further, the time constant switching means is a cycle measuring means for measuring the cycle of the position signal of the rotor inside the brushless motor detected by the comparator, and a time constant selecting means for selecting a time constant according to the output of the cycle measuring means. And a constant selection means.

Further, the brushless motor drive circuit of the present invention includes: an optical head for irradiating an optical spot on the optical disc to detect a reflected light from the optical disc to obtain a reproduction signal; and an irradiation position of the optical spot on the optical disc. A brushless motor that rotates the optical disk at a constant linear velocity, and a predetermined clock signal is extracted from the reproduction signal from the optical head, and the phase difference or speed difference between the extracted clock signal and the reference clock signal is detected to set the drive voltage. Drive voltage generating means for generating, drive voltage holding means for holding the output level of the drive voltage generating means, and drive for the brushless motor according to the output level of the drive voltage generating means or the output level of the drive voltage holding means A drive circuit that supplies a current, a coil end voltage generated in each phase of the stator coil of the brushless motor, and a position Comparing means to detect the position signal of the rotor inside the brushless motor, timing generating means for generating a timing signal based on the output of the comparing means, and the stator coil of the stator coil by the timing signal. Waveform generating means for generating an energization waveform for each phase, switching means for switching the energization of each phase of the stator coil by the output of the waveform generating means, and generating a predetermined pulse based on the timing signal It is provided with pulse generating means and drive voltage switching means for switching the input to the drive circuit from the output of the drive voltage generating means to the output of the drive voltage holding means by the output of the pulse generating means.

[0011]

According to the first aspect of the present invention, the position signal of the rotor inside the brushless motor is detected by comparing the coil end voltage generated in each phase of the stator coil of the brushless motor with a predetermined potential. The detected position signal is input to the timing generation circuit as a binary signal, and the waveform generation circuit generates a timing signal for generating a conduction waveform. At the same time, the timing signal is also input to the pulse generation circuit, and this pulse generation circuit generates a pulse having a predetermined width near the rising and falling edges of the binary signal detected by the comparator. The pulse having the predetermined width is input to the low-pass filter, and the pulse is switched to make the time constant of the low-pass filter larger than that in the normal time.

As a result, noise components contained in the drive signal near the rising and falling edges of the binary signal are removed,
The position of the rotor inside the brushless motor can be accurately detected, and the rotation accuracy of the brushless motor can be improved.

According to the second configuration, the cycle measuring means measures the cycle of the binary signal output from the comparator, and the time constant selecting means selects the time constant according to the cycle measured by the cycle measuring means. , Set the time constant of the low pass filter.

Thus, when the open loop gain characteristic of the brushless motor drive circuit changes depending on the reproduction position on the optical disk, the time constant of the low pass filter can be selected according to the noise level of the drive voltage.

With the third configuration, the comparator detects the position signal of the rotor inside the brushless motor by comparing the coil end voltage generated in each phase of the stator coil of the brushless motor with a predetermined potential. The detected position signal is input to the timing generation circuit as a binary signal, and the waveform generation circuit generates a timing signal for generating a conduction waveform. At the same time, the timing signal is also input to the pulse generation circuit, and this pulse generation circuit generates a pulse having a predetermined width near the rising and falling edges of the binary signal detected by the comparator. This predetermined width pulse is
It is input to the drive voltage holding circuit, and in the drive voltage holding circuit,
The pulse generation circuit holds the drive voltage level immediately before the generation of a pulse having a predetermined width. Then, while the pulse having the predetermined width is generated, the input to the drive circuit is switched from the output of the drive voltage generation circuit to the output of the drive voltage holding circuit.

As a result, the noise component contained in the drive signal near the rising and falling edges of the binary signal can be completely removed, the position of the rotor inside the brushless motor can be accurately detected, and the rotation accuracy of the brushless motor can be improved. Can be improved.

[0017]

EXAMPLE A first example of the present invention will be described below. FIG. 1 is a block diagram of a brushless motor drive circuit showing a first embodiment of the present invention. The brushless motor drive circuit of this embodiment is different from the conventional brushless motor drive circuit shown in FIG. 5 in that a connection point between a resistor 118 and a capacitor 119 that newly forms a time constant inside the pulse generation circuit 1 and the low-pass filter 104. And the analog switch 2 connected to the resistor 3 to switch the time constant of the low-pass filter 104.

The operation of the brushless motor drive circuit shown in FIG. 1 will be described with reference to the signal waveform diagram of FIG.
First, the optical head 102 irradiates a light spot on the recording layer of the optical disc 101, detects the amount of light reflected from the optical disc 101, and obtains a reproduction signal. The reproduction signal reproduced from the optical head 102 is input to the drive voltage generation circuit 103.
Then, the drive voltage generation circuit 103 extracts a predetermined clock signal from the reproduced signal, detects a phase difference and a speed difference between the extracted predetermined clock signal and the reference clock signal, and generates a drive voltage. The generated drive signal is smoothed by the low-pass filter 104, and the drive circuit 105 supplies the drive current according to the output level of the low-pass filter 104 to the brushless motor 106.

On the other hand, the timing of energizing each stator coil of the brushless motor 106 is determined as follows.
The comparators 107, 108 and 109 are the brushless motor 1
Coil end voltage generated at each stator coil end of 06 (see FIG.
-A) is compared with a predetermined potential, and as shown in FIG.
The value signals u, v, w are detected. This binary signal u, v, w
Is input to the timing generation circuit 110 as a rotor position signal inside the brushless motor 106. Then, the timing generation circuit 110 generates six kinds of timing signals as shown in FIG. 2D for generating the energization waveform. The timing signal is input to the waveform generation circuit 111 and also to the pulse generation circuit 1.
In this pulse generation circuit 1, the zero cross point of the coil end voltage generated in each stator coil is predicted based on the timing signal, and the Lo cross point shown in FIG.
A pulse "select signal" having a predetermined width of w level is generated.

The pulse "select signal" having a predetermined width is used as a time constant switching signal by the low pass filter 1.
It is input to 04. The low-pass filter 104 has an analog switch 2 therein, and the analog switch 2 is driven by a time constant switching signal "select signal", and the "select signal" is turned on at "High" and turned off at "Low". .

In the coil end voltage waveform generated in each stator coil of the brushless motor 106, the voltage drop component due to the current supplied to the coil is superimposed on the induced voltage component having a substantially sinusoidal shape, and contains a lot of noise components. There is. And
Time constant switching signal "select signal" is "Low"
In this section, one of the coil end voltages of each stator coil end of the brushless motor 106 is located in the vicinity of the zero cross point, and during this period, the time constant of the low-pass filter 104 for smoothing the drive voltage is set larger than that in the normal time. It works to lower the cutoff frequency of the filter.

Thus, when the position signal of the rotor inside the brushless motor is detected by comparing the coil end voltage generated in each stator coil with a predetermined potential, the drive voltage near the rising and falling positions of the position signal. The noise component included in the brushless motor can be removed and the rotor position inside the brushless motor can be accurately detected, and the rotation accuracy of the brushless motor can be improved.

Next, a second embodiment of the present invention will be described. FIG. 3 is a block diagram of a brushless motor drive circuit showing a second embodiment of the present invention. The components newly added to the block diagram of the brushless motor drive circuit of the first embodiment of FIG. 1 will be mainly described.

A newly added 8 is a cycle measuring circuit for measuring the cycle of the binarized coil end voltage, and 9 is a cycle measuring circuit 8
Is a time constant selection circuit that is connected to the pulse generation circuit 1 and switches the select signal of the pulse generation circuit 1 based on the output of the cycle measurement circuit 8 to select the time constant of the low-pass filter 104. Reference numerals 5, 7 and 4, 6, 10 are analog switches for selecting a resistor and a time constant inside the low-pass filter 104.
Is connected to the resistor 118, and the series circuit of the resistor 5 and the analog switch 4 and the series circuit of the resistor 7 and the analog switch 6 are the same as the series circuit of the resistor 3 and the analog switch 2, respectively. Switch 10
Are connected in parallel to the series circuit. Reference numeral 11 is an OR gate, and 12, 13, 14, and 15 are inverters, and the "select signal 1", "select signal 2", and "select signal 3" output from the time constant selection circuit 9 are output to the inverters 12, 13, and 14. Is input to the OR gate 11 via the inverters 12, 13, 1
The outputs of 4 drive the analog switches 2, 4 and 6, and the output of the OR gate 11 drives the analog switch 10 via the inverter 15.

The operation of the brushless motor drive circuit configured as described above will be described below. When the optical disc 101 is rotated at a constant linear velocity with respect to the irradiation position of the light spot on the optical disc 101, the number of rotations of the optical disc 101 decreases as the irradiation radial position on the optical disc moves in the outer peripheral direction.

Then, the cycle of the clock signal extracted by the drive voltage generation circuit 103 is constant, whereas the one rotation cycle of the optical disk 101 is reduced, so that the loop open loop gain of the brushless motor drive circuit is increased. . Therefore, the noise component included in the drive voltage is also amplified, and when binarizing the coil end voltage, noise is likely to occur near the rising and falling edges of the binary signal. Therefore, the cycle measuring circuit 8 first measures the cycle of the binarized signals output from the comparators 107, 108 and 109. At this time, the binarized signal for which the cycle measurement is performed may be any of u, v, and w shown in FIG. Then, the output of the cycle measuring circuit 8 is input to the time constant selecting circuit 9.

In the time constant selection circuit 9, the time constant switching signal "select signal" shown in FIG. 2B is changed to "sel" based on the output of the cycle measuring circuit 8 and the output of the pulse generating circuit.
ect signal 1 "," select signal 2 "," sel "
ct signal 3 ", and outputs" High "to the other lines. Analog switches 2, 4
6 through the inverters 12, 13 and 14
ct signal 1 "," select signal 2 "," selec "
Driven by the t signal 3 ", the inverters 12, 13, 1
4 outputs are ON when "High" and 0 when "Low"
FF is done.

Resistor 11 inside low-pass filter 104
The resistance values of 8, 3, 5, and 7 are R <R1 <R2 <, respectively.
There is a relationship of R3, and a predetermined resistance value is selected according to the number of rotations of the optical disc 101, and one of the analog switches 2, 4 and 6 is turned on and off so as to select the time constant.
Then, as the open loop gain of the brushless motor drive circuit increases, the time constant of the low-pass filter 104 is increased and the cut-off frequency of the low-pass filter is lowered according to the rotation speed of the optical disc 101.

As a result, even when the open loop gain characteristic of the brushless motor drive circuit changes with respect to the irradiation radial position of the light spot on the optical disc 101,
The time constant of the low-pass filter can be selected to an appropriate value according to the noise level due to the change in the open loop gain characteristic.

Next, a third embodiment of the present invention will be described. FIG. 4 is a block diagram of a brushless motor drive circuit showing a third embodiment of the present invention. The components newly added to the block diagram of the brushless motor drive circuit of the first embodiment of FIG. 1 will be mainly described.

A newly added 16 is a drive voltage holding circuit, 1
Reference numeral 8 is an analog switch, and this drive voltage holding circuit 1
A parallel circuit of 6 and the analog switch 18 is interposed between the drive voltage generation circuit 103 and the drive circuit 105 instead of the low pass filter. Further, the drive voltage holding circuit 16 is
It has analog switches 17 and 19, a hold capacitor 20 and an inverter 21 inside, and holds the hold capacitor 2
0 is connected to the intermediate connection point of the analog switches 17 and 19 connected in series, and the analog switches 17 and 19 are the inverter 21 to which the "select signal" of the pulse generating circuit 1 and this "select signal" are input, respectively.
It is driven by the output signal of. Alternatively, the analog switch 18 is driven by the “select signal” of the pulse generation circuit 1.

The operation of the brushless motor drive circuit configured as described above will be described below. In the third embodiment, unlike the first embodiment, the pulse "select signal" having a predetermined width generated near the zero cross point of the coil end voltage (FIG. 2-a) is the analog switch 18 and the drive voltage holding circuit 16. Analog switch 17,1
9 and the analog switches 17, 18 and 19 are
When the select signal "is" High ", it is turned on and" Lo
It is turned off by w ".

A pulse "select signal" having a predetermined width
Outputs "Low" near the zero crossing point of the coil end voltage generated at each stator coil end, and outputs "High" in other sections. "Select signal" is "
In the “High” section, the analog switch 17 inside the drive voltage holding circuit 16 is turned on, and the drive voltage output from the drive voltage generation circuit 103 is held by the hold capacitor 20.
Hold by. At this time, the analog switch 18
Is on, and the output of the drive voltage generation circuit 103 is input to the drive circuit 105.

On the other hand, in the section where the "select signal" is "Low", the analog switches 17 and 18 are turned off,
The input to the drive circuit 105 is switched from the output of the drive voltage generation circuit 103 to the hold level of the hold capacitor 20. That is, in the vicinity of the zero-cross point of the coil end voltage generated at each stator coil end, the drive circuit 105 supplies the drive current corresponding to the DC hold level held by the hold capacitor 20 to the brushless motor 1.
Supply to 06.

As a result, when the coil end voltage generated in each stator coil is compared with a predetermined potential to detect the position signal inside the brushless motor, the rise of the position signal,
It is possible to completely remove the noise component contained in the drive voltage near the falling edge, accurately detect the position of the rotor inside the brushless motor, and improve the rotation accuracy of the brushless motor.

[0036]

As described above, according to the present invention, an optical head for irradiating an optical spot on an optical disc and detecting reflected light from the optical disc to obtain a reproduction signal, and an irradiation position of the optical spot on the optical disc. On the other hand, a brushless motor that rotates the optical disk at a constant linear velocity and a predetermined clock signal are extracted from the reproduction signal from the optical head, and the phase difference or speed difference between the extracted clock signal and the reference clock signal is detected. Driving voltage generating means for generating a driving voltage, a low-pass filter for smoothing the driving voltage, a driving circuit for supplying a driving current to the brushless motor according to an output level of the low-pass filter, and a stator coil of the brushless motor. The coil end voltage generated in each phase of is compared with a predetermined potential to determine the position signal of the rotor inside the brushless motor. Outputting means, timing generating means for generating a timing signal based on the output of the comparing means, waveform generating means for generating a conduction waveform to each phase of the stator coil by the timing signal, and the waveform generating Switching means for switching the energization to each phase of the stator coil by the output of the means, pulse generating means for generating a predetermined pulse based on the timing signal, output of the pulse generating means when the low-pass filter By providing the time constant switching means for switching the constant, it is possible to remove the noise component contained in the drive voltage near the rise and fall of the position signal, and to accurately detect the position of the rotor inside the brushless motor, The rotation accuracy of the brushless motor can be improved.

Further, there are provided cycle measuring means for measuring the cycle of the position signal of the rotor inside the brushless motor output from the comparator, and time constant selecting means for selecting a time constant according to the output of the cycle measuring means. Thus, even when the loop open loop gain characteristic of the brushless motor drive circuit changes depending on the reproduction position on the optical disc, the time constant of the low pass filter can be selected to an appropriate value according to the noise level.

Further, with respect to the optical head which irradiates a light spot on the optical disc and detects the reflected light of the optical disc to obtain a reproduction signal, and the irradiation position of the light spot on the optical disc,
A brushless motor that rotates the optical disk at a constant linear velocity, and a predetermined clock signal is extracted from the reproduction signal from the optical head, and the phase difference or speed difference between the extracted clock signal and the reference clock signal is detected to set the drive voltage. Drive voltage generating means for generating, drive voltage holding means for holding the output level of the drive voltage generating means, and drive for the brushless motor according to the output level of the drive voltage generating means or the output level of the drive voltage holding means A drive circuit that supplies a current; a comparison unit that compares a coil end voltage generated in each phase of the stator coil of the brushless motor with a predetermined potential to detect a position signal of a rotor inside the brushless motor; Timing generating means for generating a timing signal based on the output of the comparing means, and the timing signal Waveform generating means for generating an energization waveform to each phase of the stator coil, switching means for switching the energization to each phase of the stator coil by the output of the waveform generating means, and a predetermined pulse based on the timing signal And a drive voltage switching means for switching the input to the drive circuit from the output of the drive voltage generating means to the output of the drive voltage holding means by the output of the pulse generating means. It is possible to completely remove the noise component contained in the drive voltage near the rising and falling edges of the signal, accurately detect the position of the rotor inside the brushless motor, and improve the rotation accuracy of the brushless motor.

[Brief description of drawings]

FIG. 1 is a block diagram of a brushless motor drive circuit according to a first embodiment of the present invention.

FIG. 2 is a signal waveform diagram in the first embodiment of the present invention.

FIG. 3 is a block diagram of a brushless motor drive circuit according to a second embodiment of the present invention.

FIG. 4 is a block diagram of a brushless motor drive circuit according to a third embodiment of the present invention.

FIG. 5 is a block diagram of a conventional brushless motor drive circuit.

FIG. 6 is a signal waveform diagram in a conventional brushless motor drive circuit.

[Explanation of symbols]

1 pulse generation circuit 2,4,6,10,17,18,19 analog switch 3,5,7 resistor 8 period measuring circuit 9 time constant switching circuit 11 OR gate 12,13,14,15,21 inverter 16 drive voltage Holding circuit 20 Hold capacitor 101 Optical disc 102 Optical head 103 Drive voltage generating circuit 104 Low pass filter 105 Drive circuit 106 Brushless motor 107, 108, 109 Comparator 110 Timing generating circuit 111 Waveform generating circuit 112, 113, 114, 115, 116, 117 Transistor

Claims (3)

[Claims] 1. An optical head for irradiating an optical spot on an optical disc to detect reflected light from the optical disc to obtain a reproduction signal,
A brushless motor that rotates the optical disc at a constant linear velocity with respect to the irradiation position of the light spot on the optical disc,
A driving voltage generating unit that extracts a predetermined clock signal from a reproduction signal from the optical head, detects a phase difference or a speed difference between the extracted clock signal and a reference clock signal, and generates a driving voltage; A low-pass filter for smoothing, a drive circuit for supplying a drive current to the brushless motor according to the output level of the low-pass filter, a coil end voltage generated in each phase of the stator coil of the brushless motor, and a predetermined potential. Comparing means for comparing and detecting the position signal of the rotor inside the brushless motor; timing generating means for generating a timing signal based on the output of the comparing means; and for each phase of the stator coil by the timing signal. Waveform generating means for generating a current-carrying waveform and output of the waveform generating means are used to energize each phase of the stator coil. Brushless motor drive having switching means for switching, pulse generating means for generating a predetermined pulse based on the timing signal, and time constant switching means for switching the time constant of the low-pass filter by the output of the pulse generating means circuit. 2. The time constant switching means includes a cycle measuring means for measuring a cycle of a position signal of a rotor inside a brushless motor output from a comparator, and a time constant for selecting a time constant according to the output of the cycle measuring means. The brushless motor drive circuit according to claim 1, further comprising a selection unit. 3. An optical head for irradiating a light spot on the optical disc to detect reflected light from the optical disc to obtain a reproduction signal,
A brushless motor that rotates the optical disc at a constant linear velocity with respect to the irradiation position of the light spot on the optical disc,
Drive voltage generating means for generating a drive voltage by extracting a predetermined clock signal from the reproduction signal from the optical head and detecting a phase difference or speed difference between the extracted clock signal and the reference clock signal; and the drive voltage generating means. Drive voltage holding means for holding the output level of the brushless motor, a drive circuit for supplying a drive current to the brushless motor in accordance with the output level of the drive voltage generating means or the output level of the drive voltage holding means, and the brushless motor Comparing means for detecting the position signal of the rotor inside the brushless motor by comparing the coil end voltage generated in each phase of the stator coil with a predetermined potential, and a timing signal is generated based on the output of the comparing means. Timing generating means, and waveform generating means for generating an energization waveform to each phase of the stator coil by the timing signal Switching means for switching the energization to each phase of the stator coil by the output of the waveform generating means, pulse generating means for generating a predetermined pulse based on the timing signal, and the driving by the output of the pulse generating means. A brushless motor drive circuit comprising: a drive voltage switching means for switching an input to the circuit from an output of the drive voltage generating means to an output of the drive voltage holding means.