JPH09265650A - Semiconductor laser driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drive a semiconductor laser by a lower power source voltage than a voltage in the forward direction of the semiconductor laser without using an inductance element by providing two switch circuits and a pulse width modulating circuit. SOLUTION: A changeover of the charge/discharge of a capacitor C1 is performed by the switch circuits 1 and 2, and an emitting light quantity of the semiconductor laser 3 is detected by a photodetector 4. The switch circuits 1 and 2 are controlled by the pulse width modulating circuit 5 based on a detecting current of the photodetector 4. Then, the capacitor C1 is connected with a power source 11 in series by the switch circuits 1 and 2, and a voltage charged in the capacitor C1 is added to a voltage Vcc of the power source 11, so as to drive the semiconductor laser 3. Thus, since the capacitor C1 is discharged by one switch circuit, and at this time, the voltage charged in this capacitor is added to the power source voltage Vcc, the higher voltage than the power source voltage Vcc can be obtained. Consequently, the semiconductor laser 3 can be driven by the lower power source voltage than the voltage in the forward direction of the semiconductor laser 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive circuit for an optical disc device such as a compact disc or a mini disc.

[0002]

2. Description of the Related Art Optical disc devices such as compact discs and mini discs using a semiconductor laser as a light source have been known. Many of these optical disk devices are commercially available that use a rechargeable battery or a dry battery as a power source. In order to downsize the battery and downsize the entire device, the electric circuit used in the portable optical disk device is
It is highly desirable to operate at a low power supply voltage.

Therefore, an integrated circuit (IC) has a 1.5
Many devices that operate at a low power supply voltage of [V] or less have been developed and are being rapidly put into practical use.

However, since the forward voltage (voltage between the anode and the cathode) of the semiconductor laser is about 2 [V], the semiconductor laser drive circuit cannot be operated at such a low power supply voltage.

As shown in FIG. 5, the semiconductor laser 21 is
Driven by the output current I ₁ of the drive circuit 23, the emitted light P enters the photodetector 22. The photodetector 22 detects the amount of light emitted from the semiconductor laser 21 as a detection current I ₂ . The detection current I ₂ is fed back to the semiconductor laser 21 via the drive circuit 23, and control is performed to keep the amount of light emitted from the semiconductor laser 21 whose input / output characteristics largely depend on temperature, that is, so-called APC (Automatic Power).
Control) control is performed.

The forward voltage V _OP of the semiconductor laser 21 is 2
It is about [V]. Therefore, the power supply voltage V _CC of the conventional semiconductor laser drive circuit 23 needs to be higher than that. For example, an IC "I" in which such a driving circuit is integrated
The power supply voltage of R3C07 "(Sharp Corporation) is +5
[V] is recommended.

In FIG. 5, a semiconductor laser of the type (cathode common type) in which the package 21a of the semiconductor laser 21 is connected to the cathode terminal of the laser element 21b is used. Here, the reason why the package 21a is connected to GND is generally that the package 21a of the semiconductor laser is made of a conductive metal, and if the potential other than GND is applied, the semiconductor laser is damaged due to a short circuit or the like. Is.

[0008] However, it is important for a portable optical disk device, which is required to be miniaturized, to realize a circuit for driving a semiconductor laser having a forward voltage of about 2 [V] with a power supply voltage lower than the forward voltage. That is, various technologies for that purpose are disclosed.

For example, in the technique disclosed in Japanese Utility Model Publication No. 7-18018, energy is stored in an inductance element when a switching transistor is turned on, and the sum of the voltage due to the energy stored when the switching transistor is turned off and a power supply voltage is applied to a semiconductor laser. are doing. Then, the operation of the switching transistor is feedback-controlled according to the amount of light emitted from the semiconductor laser, so that the semiconductor laser is stably driven even when the power supply voltage changes and becomes lower than the forward voltage of the semiconductor laser.

[0010]

However, the above-described method of storing energy in the inductance element and driving the semiconductor laser requires a coil which is an inductance element. The coil is an element that is difficult to miniaturize, and using the coil means that the portable optical disk device cannot be miniaturized.

The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to drive a semiconductor laser with a power supply voltage lower than the forward voltage of the semiconductor laser without using an inductance element. Another object of the present invention is to provide a semiconductor laser driving circuit.

[0012]

In order to achieve the above object, the present invention provides a photodetector for detecting the amount of light emitted from a semiconductor laser, a capacitor, and a first state in which the capacitor is charged from a power source. A second connecting the capacitor in series with the power supply and adding the stored voltage to the power supply voltage for discharging
And a pulse width modulation circuit that modulates the width of the pulse that controls the switch circuit to the first state or the second state according to the output level of the photodetector. It is characterized by that.

According to the above construction, when the switch circuit is in one state, the capacitor is charged by the power supply voltage, and when the switch circuit is in the other state, the capacitor is discharged. At the time of this discharge, the capacitor is connected in series with the power supply and the stored voltage is added to the power supply voltage, so that a voltage higher than the power supply voltage can be obtained. Therefore, at a power supply voltage lower than the forward voltage of the semiconductor laser,
A semiconductor laser can be driven.

Further, since the ratio of the charge time and the discharge time of the capacitor is controlled by the pulse width modulation circuit according to the output level of the photodetector, the emitted light quantity of the semiconductor laser can be set to a predetermined power. .

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Semiconductor laser drive circuits according to embodiments of the present invention will be described below with reference to FIGS.

FIG. 1 shows a semiconductor laser drive circuit of this embodiment. This drive circuit includes a capacitor C1, a switch circuit 1 attached to both ends of the capacitor C1,
And a switch circuit 2, a capacitor C2 that smoothes the discharge voltage of the capacitor C1, a resistor R1 that limits the drive current of the semiconductor laser 3, a photodetector 4 that detects the amount of light emitted from the semiconductor laser 3, and a detection current of the photodetector 4. A resistor R2 for converting into a voltage and a pulse width modulation circuit 5 are provided.

The semiconductor laser 3 is a cathode common type in which the cathode is connected to the package, and the cathode is connected to the GND.

The switch circuit 1 and the switch circuit 2 are switched by the output digital signal S ₁ of the pulse width modulation circuit 5.

When the signal S ₁ is at low level, the switch circuit 1 and the switch circuit 2 are connected to the contact a side. Since the contact a of the switch circuit 1 is connected to the power supply 11 and the contact a of the switch circuit 2 is connected to GND,
The capacitor C1 is charged according to the power supply voltage V _CC of the power supply 11.

When the signal S ₁ changes from the low level to the high level, the switch circuit 1 and the switch circuit 2 contact the contact a.
To contact b, and the capacitor C1 is discharged.

To the contact b of the switch circuit 1, a capacitor C2 is connected between the switch circuit 1 and GND, and a semiconductor laser 3 is connected via a resistor R1. The power supply 11 is connected to the contact b of the switch circuit 2. At this time,
Since the capacitor C1 and the power supply 11 are connected in series, the sum of the power supply voltage V _CC and the voltage stored in the capacitor C1 is supplied to the contact b of the switch circuit 1.

When the level of the signal S ₁ repeatedly changes,
The capacitor C1 is repeatedly charged and discharged, and the supply and interruption of the voltage to the contact b of the switch circuit 1 are repeated. To smooth the voltage that is repeatedly supplied and cut off,
This is the role of the capacitor C2.

FIG. 2 shows the waveforms of the signal S ₁ and the voltage V ₁ . While the signal S ₁ is at the high level, the voltage is supplied from the capacitor C1 to the capacitor C2 and the resistor R1 via the switch circuit 1. At this time, the voltage V ₁ rises slightly. While the signal S ₁ is at a low level, the voltage supply from the capacitor C1 is cut off, so that the voltage V ₁ stored in the capacitor C2 is supplied to the resistor R1. At this time, the voltage V ₁ decreases slightly.

The semiconductor laser 3 is driven by this voltage V ₁ . That is, due to the difference between the voltage V ₁ and the forward voltage V _OP of the semiconductor laser 3, the current I flowing through the resistor R 1
₁ (I ₁ = (V ₁ −V _OP ) / R ₁ ) is the semiconductor laser drive current.

The sum of the voltages stored as the power supply voltage V _CC and the capacitor C1, since twice the supply voltage V _CC at the maximum, the power supply voltage V _CC is lower than the forward voltage V _OP of the semiconductor laser 3 Also, the semiconductor laser 3 can be driven.

Here, the capacitors C1 and C2
Or the frequency of the signal S ₁ is set so that the fluctuation of the emission power of the semiconductor laser 3 due to the fluctuation V _Z of the voltage V ₁ is at a level that causes no problem for the optical disk device.

FIG. 3 shows an example of the relationship between the ratio of the charging time and the discharging time of the capacitor C1, that is, the duty of the signal S ₁ and the semiconductor laser drive current I ₁ . When the duty is changed to the maximum when the duty is 50%, the current I ₁ decreases.

That is, the emission power of the semiconductor laser 3 can be controlled by changing the duty of the signal S ₁ .

The photodetector 4 has its cathode terminal connected to the power supply 11 and is reverse-biased with the voltage V _CC . The anode terminal is connected to GND via the resistor R2.

The emitted light P of the semiconductor laser 3 is received by the photodetector 4, and a detection current I ₂ proportional to the emitted power flows. The current I ₂ is converted into a voltage V ₂ by the resistor R2 and sent to the pulse width modulation circuit 5.

In the pulse width modulation circuit 5, the voltage V ₂ is input to the non-inverting input terminal of the operational amplifier 6. Operational amplifier 6
Is a buffer circuit in which the inverting input terminal and the output terminal are directly connected, and the output thereof is equal to the voltage V ₂ . Operational amplifier 6
The output terminal of is connected to the inverting input terminal of the operational amplifier 7 through the resistor R3, and the inverting input terminal and the output terminal of the operational amplifier 7 are connected through the resistor R4. The reference voltage source 10 is connected to the non-inverting input terminal of the operational amplifier 7.
The voltage V _ref and the voltage V ₂ of the reference voltage source 10 are differentially amplified,
The output terminal of the operational amplifier 7 has a voltage V ₃ (V ₃ = V _ref ×
(R3 + R4) / R3- V 2 × R4 / R3) are outputted. The voltage V ₃ is input to the inverting input terminal of the comparator 8, and the output signal voltage (triangle wave signal voltage) V _{4 of the} triangular wave generating circuit 9 is input to the non-inverting input terminal of the comparator 8. The output of the comparator 8 is the output signal S ₁ of the pulse width modulation circuit 5 and is connected to the control terminals of the switch circuit 1 and the switch circuit 2.

FIG. 4 shows the waveforms of the voltage V ₃ , the voltage V ₄ and the signal S ₁ . The triangular wave generating circuit 9 outputs a constant triangular wave signal voltage V ₄ . Comparator 8
In the comparison voltage V ₃ and the voltage V _4, the voltage V ₃ higher duration low level than the voltage V _4, the voltage V ₃ lower period than the voltage V ₄ is the digital signals S ₁ of high level is outputted.

Now, when the voltage V ₃ has the value shown by the solid line, the duty of the signal S ₁ is 70%. When the emission power of the semiconductor laser 3 decreases due to the temperature change, the voltage V ₂ also decreases. Then, the voltage V _{3 obtained} by differentially amplifying the reference voltage V _ref and the voltage V ₂ rises and becomes a value shown by a broken line. At this time, the duty of the signal S ₁ becomes 60%,
Since the semiconductor laser drive current I ₁ increases (see FIG. 3), the laser power is corrected.

In this way, the detection current I of the photodetector 4 is
_The semiconductor laser drive current I ₁ can be controlled by changing the duty (pulse width) of the signal S ₁ according to ₂ , so that the emission power of the semiconductor laser can be kept constant.

The emission power of the semiconductor laser is the reference voltage V
Adjusted by changing the value of _ref .

When the duty of the signal S ₁ falls below 50%, the slope of the relationship between the duty and the current I ₁ is reversed (see FIG. 3), so that the value of the voltage V ₃ is V _L by the limiting circuit (not shown). Restrictions are added so that [V] is not exceeded.

In the above configuration, for example, the power supply voltage V _CC
Is 1.5 [V], forward voltage V _OP is 2.3 [V], capacitors C1 and C2 are 100 [μF], and resistor R1 is 3
K [Ω], and the frequency of the triangular wave voltage V ₄ is 20 [kH
z], and the characteristics under this condition are shown in FIGS.

Further, as described above, the value of the capacitor C2 is set so that the fluctuation V _Z of the voltage V ₁ (see FIG. 2) is at a level that does not cause any problem for the optical disk device.
If the frequency of the signal S ₁ (frequency of the triangular wave signal voltage V ₄ ) is sufficiently higher than the reproduction signal frequency of the optical disk device, the capacitor C2 may be omitted. That is, even if the voltage V ₁ is not smoothed, if its fluctuation frequency is sufficiently high (several tens of MH)
z or above) No problem. However, in general, capacitor C
Since it is difficult to charge and discharge 1 at high speed, capacitor C
It is easier to use 2 and to use a low frequency (several tens of kHz).

[0039]

The semiconductor laser drive circuit of the present invention has a structure for adding the power supply voltage and the voltage stored in the capacitor. As a result, the semiconductor laser can be driven even if the power supply voltage is lower than the forward voltage of the semiconductor laser. As a result, the optical disk device can be operated with a low power supply voltage, and the battery used in the portable optical disk device can be downsized.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a semiconductor laser drive circuit according to an embodiment of the present invention.

FIG. 2 shows a signal S ₁ in the semiconductor laser driving circuit.
And is a waveform diagram showing the voltage V _1.

FIG. 3 shows a signal S ₁ in the semiconductor laser driving circuit.
3 is a graph showing the relationship between the duty of the laser diode and the semiconductor laser drive current I ₁ .

FIG. 4 shows a voltage V ₃ in the semiconductor laser drive circuit.
FIG. 6 is a waveform diagram showing a triangular wave signal voltage V ₄ and a signal S ₁ .

FIG. 5 is a configuration diagram for explaining a conventional semiconductor laser drive circuit.

[Explanation of symbols]

 1 switch circuit 2 switch circuit 3 semiconductor laser 4 photodetector 5 pulse width modulation circuit 9 triangular wave generation circuit 11 power supply

Claims (1)

[Claims] 1. A photodetector for detecting the amount of light emitted from a semiconductor laser, a capacitor, a first state in which the capacitor is charged from a power source, and a voltage stored by connecting the capacitor in series with the power source. To a second state in which the switch circuit is switched between a second state in which it is discharged and a second state in which the switch circuit is discharged.
And a pulse width modulation circuit that modulates the width of a pulse to selectively control the semiconductor laser drive circuit.