Patent Publication Number: US-2007116075-A1

Title: Laser diode driver

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a driver for a laser diode which is used as a light source for data reading, erasing, and writing on CD (Compact Disc), DVD (Digital Versatile Disc) and so on.  
      2. Description of Related Art  
      A laser diode which is used for an optical disc such as CD or DVD is driven using DC current in each of read, erase, and write periods as shown in  FIG. 9  which illustrates a rewritable optical disc as an example. If the light from the laser diode is reflected on a disc surface to return to enter the laser diode, oscillation becomes unstable to cause noise to occur. To avoid this, as shown in  FIG. 10 , there is a technique of superposing high frequency current of several 100 MHz on DC current to change the oscillation mode of the laser diode from a single-mode to a multi-mode to thereby reduce the effect of noise. The superposition of the high frequency current is performed typically during the read period. It may be performed during the read and erase periods as shown in  FIG. 10 , or even during the write period. Further, the superposition of the high frequency current may be performed during a certain period only when focus servo or tracking servo becomes unstable. This is thus the essential feature for a laser diode driver.  
      A laser diode driver of a related art includes a first current source  60  that supplies DC current (e.g. 100 mA) to a laser diode LD, a second current source  61  that supplies high frequency current (e.g. 50 mA peak), and two NMOS transistors  62  and  63  that serve as switches to connect the current from the second current source  61  to either the laser diode LD or a dummy load  65  as shown in  FIG. 11 . In the circuit of  FIG. 11 , if voltage pulses SW 1  and SW 1 ′ with reverse phases to each other are applied to the gates of the NMOS transistors  62  and  63  as shown in  FIGS. 12A and 12B , the current from the second current source  61  flows into the laser diode LD only when the NMOS transistor  62  is ON. Accordingly, a drive current I 3  which flows to the laser diode LD has a waveform as shown in  FIG. 12C , in which the high frequency current of 50 mA peak is added to the DC current of 100 mA. This is described in Japanese Unexamined Patent Application Publication No. 2001-237489.  
      Another laser diode driver of a related art includes a first current source  70  that supplies DC current to a laser diode LD, a second current source  71  that supplies high frequency current, two NMOS transistors  73  and  74  and two PMOS transistors  75  and  76  that respectively have the same channel width and constitute a current mirror as shown in  FIG. 13 . In the circuit of  FIG. 13 , if voltage pulses SW 1  and SW 1 ′ with reverse phases to each other are applied to the gates of the PMOS transistors  75  and  76  as shown in  FIGS. 14A and 14B , when the PMOS transistor  76  is ON, current I 4  which is equal to current I 2  is output from the second current source  71  and then combined with current I 1  from the first current source  70 , so that the current of I 1 +I 4 =I 1 +I 2  flows into the laser diode LD. At this time, the PMOS transistor  75 , the NMOS transistors  73  and  74  are OFF. Then, when the PMOS transistor  75  turns ON, current I 6  which is equal to the current I 2  flows through the path from the second current source  71  through the PMOS transistor  75  and the NMOS transistor  73 . Thus, current I 5  which is equal to the current I 2  flows also to the NMOS transistor  74 , which forms the current mirror with the NMOS transistor  73 . Accordingly, the current I 5  is subtracted from the current I 1  from the first current source  70 , so that the current of I 1 −I 5 =I 1 −I 2  flows into the laser diode LD. At this time, the PMOS transistor  76  is OFF. Therefore, as a result of applying the voltage pulses SW 1  and SW 1 ′ with reverse phases to each other to the gates of the PMOS transistors  75  and  76 , the drive current I 3  of the laser diode LD has a waveform as shown in  FIG. 14C , in which the current I 2  (e.g. 50 mA peak) from the second current source  71  is alternately added to or subtracted from the current I 1  (e.g. 100 mA) from the first current source  70 . This is described in Japanese Unexamined Patent Application Publication No. 2001-237489.  
      The laser diode drivers as shown in  FIGS. 11 and 13 , however, have common problems to be solved. The values of current which flow during each period shown in  FIG. 9  are typically set such that read current&lt;erase current&lt;write current. However, as a result of the superposition of high frequency current on DC current, the peak of the current increases by the amount of the high frequency current. This causes the peak to exceed a prescribed threshold which is determined by the material of an optical disc or the characteristics of a laser diode, which may raise the drawback that erasing is performed during the read period, writing is performed during the erase period, or the like.  
      The laser diode drivers as shown in  FIGS. 11 and 13  also have the drawback that the current consumption is large to cause excessive heating due to the presence of the current flowing from a DC current source and a high frequency current source, bypassing a laser diode LD, into a ground, e.g. the current flowing through the dummy load  65  in the laser diode driver of  FIG. 11  and the current (I 6 ) flowing through the NMOS transistor  73  and the current (I 5 ) flowing through the NMOS transistor  74  in the laser diode driver of  FIG. 13 .  
      Accordingly, a laser diode driver which avoids erroneous operation that erasing is performed during the read period and writing is performed during the erase period or which enables low power consumption to maintain moderate heating is demanded.  
     SUMMARY OF THE INVENTION  
      According to an aspect of the present invention, there is provided a laser diode driver including a DC current source supplying DC current to a laser diode, a high frequency current source connected in parallel with the DC current source and supplying high frequency current to the laser diode, and a circuit capable of changing current of the DC current source when the high frequency current source is operating.  
      According to another aspect of the present invention, a laser diode driver includes a high frequency current source supplying high frequency current to a laser diode and a DC current source supplying DC current to the laser diode. A current value of the DC current can be set differently between a superposition mode when the high frequency current is superposed and a non-superposition mode when the high frequency current is not superposed.  
      According to another aspect of the present invention, a laser diode driver includes a DC current source supplying DC current to a laser diode, a high frequency current source connected in parallel with the DC current source and supplying high frequency current to the laser diode, and a circuit capable of changing current of the DC current source in accordance with operation of the high frequency current source.  
      The laser diode driver of one aspect of the present invention does not have a path for the current supplied from the DC current source and the high frequency current source to flow except for the path from the power supply voltage to the ground through the laser diode, and there is no path which bypasses the laser diode.  
      The laser diode driver according to embodiments of the present invention includes a circuit for changing the current through a DC current source when a high frequency current source is operating, thereby controlling the peak value of the drive current of a laser diode to fall below a prescribed threshold which is determined by the material of an optical disc and the characteristics of the laser diode. This avoids erroneous operation of performing erasing during the read period, writing during the erase period, or the like. Further, there is no path from a power supply voltage to a ground by bypassing the laser diode, and therefore the current supplied from the DC current source and the high frequency current source cannot flow to the ground without passing through the laser diode. Accordingly, all of the current supplied from the DC current source and the high frequency current source contribute to the emission of the laser diode, thereby providing the advantage of preventing high power consumption and excessive heating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a circuit block diagram showing a laser diode driver according to a first and a fourth embodiment of the present invention;  
       FIG. 2  is a detailed circuit diagram showing the laser diode driver according to the first embodiment;  
       FIGS. 3A  to  3 E are views showing the operation of the laser diode driver according to the first embodiment;  
       FIG. 4  is a circuit block diagram showing a laser diode driver according to a second and a third embodiment of the present invention;  
       FIG. 5  is a detailed circuit diagram showing of the laser diode driver according to the second embodiment;  
       FIGS. 6A  to  6 E are views showing the operation of the laser diode driver according to the second embodiment;  
       FIG. 7  is a detailed circuit diagram showing the laser diode driver according to the third embodiment;  
       FIG. 8  is a detailed circuit diagram showing an example of the laser diode driver according to the fourth embodiment;  
       FIG. 9  is a view to describe the drive current of a laser diode;  
       FIG. 10  is a view to describe the drive current of a laser diode onto which high frequency current is superposed;  
       FIG. 11  is a circuit diagram showing a laser diode driver according to a related art;  
       FIGS. 12A  to  12 C are views to describe the operation of a laser diode driver according to a related art;  
       FIG. 13  is a circuit diagram showing another laser diode driver according to a related art; and  
       FIGS. 14A  to  14 C are views to describe the operation of another laser diode driver according to a related art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposed.  
      Exemplary embodiments of the present invention are described hereinafter with reference to the accompanying drawings. In the drawings, the same elements as those described in the related art are denoted by the same reference numerals.  
      Referring first to  FIG. 1 , a laser diode driver according to a first embodiment of the present invention includes a DC current source  102 , a first current setting circuit  101  for setting the current to the DC current source  102 , and a high frequency superposing circuit  20 . The high frequency superposing circuit  20  includes a high frequency current source  202 , a second current setting circuit  201  for setting the current to the high frequency current source  202 , and a superposition controller  203  for controlling a first switching element P 1  and the superposition of high frequency current. In this embodiment, the output of the second current setting circuit  201  is input to the first current setting circuit  101 , so that the current value of the DC current source  102  decreases by the amount proportional to the maximum current flowing through the high frequency current source  202  when high frequency current is superposed.  
      Referring next to  FIG. 2  showing a detailed example of the circuit of  FIG. 1 , the configuration of the laser diode driver is described hereinafter. The PMOS transistor Q 3  serves as the DC current source  102 . The source and drain of the PMOS transistor Q 3  are connected to a power supply voltage VDD and an output terminal T 2 , respectively. The first current setting circuit  101  includes an operational amplifier OP 1 . The inverting input terminal of the operational amplifier OP 1  is connected to a current setting terminal Iset 1  and one end of a resistor R 1 . The non-inverting input terminal of the operational amplifier OP 1  is connected to the drain of a PMOS transistor Q 1 , one end of a resistor R 2 , and the drain of a second switching element P 2 . The output of the operational amplifier OP 1  is connected to the gate of the PMOS transistor Q 1  and the gate of the PMOS transistor Q 3  as the DC current source  102 . The other ends of the resistors R 1  and R 2  are grounded, the sources of the PMOS transistors Q 1  and Q 2  are connected to the power supply voltage VDD, and the drain of the PMOS transistor Q 2  is connected to the source of the second switching element P 2 . The gate of the second switching element P 2  is connected to a super position control terminal T 1 , and the gate of the PMOS transistor Q 2  is connected to the output of the second current setting circuit  201 .  
      The second current setting circuit  201  includes an operational amplifier OP 2 . The inverting input terminal of the operational amplifier OP 2  is connected to a current setting terminal Iset 2  and one end of a resistor R 3 . The non-inverting input terminal of the operational amplifier OP 2  is connected to the drain of a PMOS transistor Q 4  and one end of a resistor R 4 . The output of the operational amplifier OP 2  is connected to the gate of a PMOS transistor Q 4  and the gate of a PMOS transistor Q 5  which serves as the high frequency current source  202 . The other ends of the resistors R 3  and R 4  are grounded, and the source of the PMOS transistors Q 4  is connected to the power supply voltage VDD.  
      The superposition controller  203  includes an oscillator OSC and an OR circuit OR 1 . One input terminal of the OR circuit OR 1  is connected to the super position control terminal T 1 , and the other input terminal is connected to the output of the oscillator OSC. The output terminal of the OR circuit OR 1  is connected to the gate of the first switching element P 1 .  
      The high frequency superposing circuit  20  includes the second current setting circuit  201  and the superposition controller  203 . The source of the PMOS transistor Q 5  as the high frequency current source  202  is connected to the power supply voltage VDD, and the drain is connected to the source of the first switching element P 1 . The drain of the first switching element P 1  is connected to an output terminal T 2 . The output terminal T 2  is connected to the anode of the laser diode LD which serves as a load. The cathode of the laser diode LD is grounded.  
      Referring then to  FIGS. 2 and 3 , the operation of the laser diode driver of this embodiment is described hereinbelow. When the superposition control terminal T 1  is high level (hereinafter referred to as H level), the output S 1  of the OR circuit OR 1  is H level regardless of the output of the oscillator OSC, and the first switching element P 1  formed of a PMOS transistor is OFF. Accordingly, the current from the high frequency current source  202  is blocked by the first switching element P 1  and does not flow into the laser diode LD.  
      Further, when the superposition control terminal T 1  is H level, the second switching element P 2  formed of a PMOS transistor is OFF. If the current Iin 1  is supplied from the current setting terminal Iset 1  to the resistor R 1 , the current of I 1   a =(r 1 /r 2 )*Iin 1  (where r 1  and r 2  indicate the resistance values of the resistors R 1  and R 2 , respectively) flows through the drain of the PMOS transistor Q 1 . If the current ratio of the PMOS transistors Q 1  and Q 3  ( 102 ) is 1:m, the current of I 1 =m*I 1   a  flows from the power supply voltage VDD through the PMOS transistor Q 3  ( 102 ), the output terminal T 2 , and the laser diode LD into the ground, so that the current waveform without the superposition of high frequency current as described in  FIG. 9  is obtained.  
      On the other hand, when the superposition control terminal T 1  is low level (hereinafter referred to as L level), the output S 1  of the OR circuit OR 1  is H level or L level in accordance with the output of the oscillator OSC, and the first switching element P 1  formed of a PMOS transistor turns ON or OFF. If the current Iin 2  is supplied from the current setting terminal Iset 2  to the resistor R 3 , the current of I 2   a =(r 3 /r 4 )*Iin 2  (where r 3  and r 4  indicate the resistance values of the resistors R 3  and R 4 , respectively) flows through the drain of the PMOS transistor Q 4 . If the current ratio of the PMOS transistors Q 4  and Q 5  ( 202 ) is 1:n, the current of I 2 =n*I 2   a  flows from the power supply voltage VDD through the PMOS transistor Q 5  ( 202 ), the first switching element P 1 , the output terminal T 2 , and the laser diode LD into the ground.  
      Further, when the superposition control terminal T 1  is L level, the second switching element P 2  formed of a PMOS transistor is ON, and the PMOS transistors Q 1  and Q 2  are such that the source and the drain are connected in parallel. If the current ratio of the PMOS transistors Q 4  and Q 2  is 1:n/2m, for example, because the current flowing through the PMOS transistor Q 4  when the current of I 2  is flowing through the PMOS transistor Q 5  is I 2 /n, the current I 1   b  flowing through the PMOS transistor Q 2  is (n/2m)*(I 2 /n)=I 2 /2m. At this time, in order to keep a constant voltage to be applied to the non-inverting input terminal of the operational amplifier OP 1 , the current I 1   a  flowing through the PMOS transistor Q 1  decreases by I 2 /2m, and the current flowing though the DC current source  102  (Q 3 ) decreases by m*(I 2 /2m)=I 2 /2.  
      The current waveform at this condition is described hereinafter with reference to  FIGS. 3A  to  3 E. In  FIGS. 3A  to  3 E, i 1  indicates the current value which flows through the DC current source  102  (Q 3 ) when superposition is not performed, and i 2  indicates the average current value which flows through the high frequency current source  202  (Q 5 ) where a duty ratio is assumed to be 1:1. When the superposition control terminal T 1  changes from H level to L level as shown in  FIG. 3A , the waveform of  FIG. 3B  is output as the output S 1  of the OR circuit OR 1 . Further, the current I 1  flowing through the DC current source  102  (Q 3 ) changes from i 1  to i 1 -i 2  as shown in  FIG. 3C . The current I 2  flowing through the high frequency current source  202  (Q 5 ) shown in  FIG. 3D  is superposed thereon, so that the laser diode drive current I 3  has the waveform that the average current is i 1  and the amplitude is 2*i 2  as shown in  FIG. 3E . After that, when the superposition control terminal T 1  changes from L level back to H level, the current I 1  to I 3  return to the original states as shown in  FIGS. 3C  to  3 E.  
      The case where the average current does not change before and after the high frequency superposition when the current ratio of the PMOS transistors Q 4  and Q 2  is 1:n/m is described above. By changing the current ratio of the PMOS transistors Q 4  and Q 2 , the current value to reduce the DC current can be changed in proportion to the maximum current of the high frequency current. For example, if the current ratio is set to 1:2n/m, the current waveform in which the peak current during the high frequency superposition is the same as the current before the high frequency superposition can be obtained.  
      In the laser diode driver according to this embodiment, the output of the second current setting circuit is supplied to the first current setting circuit, so that the current value of the DC current source decreases by the amount proportional to the maximum current flowing through the high frequency current source during switching operation of the first switching element. This controls the peak value of the drive current of the laser diode to fall below a prescribed threshold which is determined by the material of an optical disc and the characteristics of the laser diode, thereby avoiding erroneous operation of performing erasing during the read period, writing during the erase period, or the like. Further, the laser diode driver does not have a path for the current supplied from the DC current source and the high frequency current source to flow except for the path from the power supply voltage through the laser diode to the ground, and there is no path which bypasses the laser diode. Accordingly, all of the current supplied from the DC current source and the high frequency current source contribute to the emission of the laser diode, thereby providing the advantage of preventing high power consumption and excessive heating.  
      Referring now to  FIG. 4 , a laser diode driver according to a second embodiment of the present invention includes a DC current source  102 , a first current setting circuit  101  for setting the current to the DC current source  102 , and a high frequency superposing circuit  20 . The high frequency superposing circuit  20  includes a high frequency current source  202 , a second current setting circuit  201  for setting the current to the high frequency current source  202 , and a superposition controller  203  for controlling a first switching element P 1  and the superposition of high frequency current. In this embodiment, the output of the second current setting circuit  201  is not input to the first current setting circuit  101 , so that the current value of the DC current source  102  decreases at a constant rate when high frequency current is superposed.  
      Referring then to  FIG. 5  showing a detailed example of the circuit of  FIG. 4 , the configuration of the laser diode driver is described hereinafter. The laser diode driver according to the second embodiment has substantially the same configuration as the laser diode driver according to the first embodiment described above. However, the second embodiment is different from the first embodiment in that the gate of the PMOS transistor Q 2  is connected to the output of the operational amplifier OP 1  rather than the operational amplifier OP 2 .  
      Referring further to  FIGS. 5 and 6 A to  6 E, the operation of the laser diode driver of this embodiment is described hereinbelow. The operation when the superposition control terminal T 1  is H level is the same as that in the laser diode driver of the first embodiment, and thus not described herein.  
      When the superposition control terminal T 1  is L level, the current of I 2 =n*I 2   a  flows from the power supply voltage VDD through the PMOS transistor Q 5  ( 202 ), the first switching element P 1 , the output terminal T 2 , and the laser diode LD into the ground, which is the same as in the laser diode driver of the first embodiment.  
      When the superposition control terminal T 1  is L level, the second switching element P 2  formed of a PMOS transistor is ON, and the PMOS transistors Q 1  and Q 2  are such that the source and the drain are connected in common. If the current ratio of the PMOS transistors Q 1  and Q 2  is 1:a, for example, the current I 1   b  flowing through the PMOS transistor Q 2  is a*(I 1 /m). At this time, in order to keep a constant voltage to be applied to the non-inverting input terminal of the operational amplifier OP 1 , the current I 1   a  flowing through the PMOS transistor Q 1  decreases by a*(I 1 /m), and the current flowing though the DC current source  102  (Q 3 ) decreases by m*a*(I 1 /m)=a*I 1 .  
      The current waveform at this condition is described hereinafter with reference to  FIGS. 6A  to  6 E. In  FIGS. 6A  to  6 E, i 1  indicates the current value which flows through the DC current source  102  (Q 3 ) when superposition is not performed, and i 2  indicates the average current value which flows through the high frequency current source  202  (Q 5 ) where a duty ratio is assumed to be 1:1. When the superposition control terminal T 1  changes from H level to L level as shown in  FIG. 6A , the waveform of  FIG. 6B  is output as the output S 1  of the OR circuit OR 1 , and the current I 1  flowing through the DC current source  102  (Q 3 ) changes from i 1  to i 1 −a*i 1  as shown in  FIG. 6C . The current I 2  flowing through the high frequency current source  202  (Q 5 ) shown in  FIG. 6D  is superposed thereon, so that the laser diode drive current I 3  has the waveform that the average current is i 1 −a*i 1 +i 2  and the amplitude is 2*i 2  as shown in  FIG. 6E . After that, when the superposition control terminal T 1  changes from L level back to H level, the current I 1  to I 3  return to the original states as shown in  FIGS. 6C  to  6 E.  
      As described in the foregoing, this embodiment enables the current ratio of the PMOS transistors Q 1  and Q 2  to be variable, thereby allowing the reduction of the DC current at a constant rate regardless of the high frequency current during the superposition of the high frequency current. For example, if the current ratio is set to 1:0.25, the DC current when the superposition of high frequency current is performed can decrease by 25% compared with when the superposition is not performed.  
      In the laser diode driver of this embodiment, the current value of the DC current source decreases at a constant rate during switching operation of the first switching element. This controls the peak value of the drive current of the laser diode to fall below a prescribed threshold which is determined by the material of an optical disc and the characteristics of the laser diode. This avoids erroneous operation of performing erasing during the read period, writing during the erase period, or the like. Further, all of the current supplied from the DC current source and the high frequency current source contribute to the emission of the laser diode as described in the laser diode driver of the first embodiment, thereby providing the advantage of preventing high power consumption and excessive heating.  
      A laser diode driver circuit according to a third embodiment of the present invention is composed of the same circuit blocks as those in the second embodiment described with reference to  FIG. 4 . In the third embodiment, however, the configuration of the first current setting circuit is different in such a way that the current value of the DC current value decreases by a constant value when the high frequency current is superposed.  
      Referring then to  FIG. 7  showing a detailed example of the circuit of the third embodiment, the configuration of the laser diode driver is described hereinafter. The laser diode driver according to the third embodiment is different from the laser diode driver according to the second embodiment only in the configuration of the first current setting circuit  101 . Specifically, the first current setting circuit  101  of this embodiment further has an operational amplifier OP 3  in addition to the components of the first current setting circuit in the laser diode driver of the second embodiment described above. The inverting input terminal of the operational amplifier OP 3  is connected to a current setting terminal Iset 3  and one end of a resistor R 5 , and the non-inverting input terminal of the operational amplifier OP 3  is connected to the drain of a PMOS transistor Q 6  and one end of a resistor R 6 . The output of the operational amplifier OP 3  is connected to the gate of the PMOS transistor Q 6  and the gate of the PMOS transistor Q 2 . The other ends of the resistors R 5  and R 6  are grounded, and the source of the PMOS transistor Q 6  is connected to the power supply voltage VDD.  
      Referring further to  FIG. 7 , the operation of the laser diode driver of this embodiment is described hereinbelow. The operation when the superposition control terminal T 1  is H level is the same as that in the laser diode driver of the first embodiment, and thus not described herein.  
      When the superposition control terminal T 1  is L level, the current of I 2 =n*I 2   a  flows from the power supply voltage VDD through the PMOS transistor Q 5  ( 202 ), the first switching element P 1 , the output terminal T 2 , and the laser diode LD into the ground, which is the same as in the laser diode driver of the first embodiment.  
      When the superposition control terminal T 1  is L level, the second switching element P 2  formed of a PMOS transistor is ON, and the PMOS transistors Q 1  and Q 2  are such that the source and the drain are connected in parallel. If the current of Iin 3  is supplied from the current setting terminal Iset 3  to the resistor R 5 , the current of I 1   c =(r 5 /r 6 )*Iin 3  (where r 5  and r 6  indicate the resistance values of the resistors R 5  and R 6 , respectively) flows to the drain of the PMOS transistor Q 6 . If the current ratio of the PMOS transistors Q 6  and Q 2  is 1:b, for example, the current I 1   b  flowing through the PMOS transistor Q 2  is b*I 1   c . At this time, in order to keep a constant voltage to be applied to the non-inverting input terminal of the operational amplifier OP 1 , the current I 1   a  flowing through the PMOS transistor Q 1  decreases by b*I 1   c , and the current flowing though the DC current source  102  (Q 3 ) decreases by m*b*I 1   c.    
      As described in the foregoing, this embodiment enables the current ratio of the PMOS transistors Q 6  and Q 2  to be variable, thereby allowing the reduction of the DC current always by a constant value regardless of the DC current I 1  during non-superposition or the high frequency current I 2  during the superposition of the high frequency current.  
      In the laser diode driver of this embodiment, the current value of the DC current source decreases by a constant value during switching operation of the first switching element. This controls the peak value of the drive current of the laser diode to fall below a prescribed threshold which is determined by the material of an optical disc and the characteristics of the laser diode. This avoids erroneous operation of performing erasing during the read period, writing during the erase period, or the like. Further, all of the current supplied from the DC current source and the high frequency current source contribute to the emission of the laser diode as described in the laser diode driver of the first embodiment, thereby providing the advantage of preventing high power consumption and excessive heating.  
      The laser diode driver according to the first to the third embodiment of the present invention may be selected appropriately in accordance with the design intention of an optical driver maker as a user of the laser diode driver.  
      A laser diode driver according to a fourth embodiment of the present invention is composed of the same circuit blocks as those in the first embodiment described with reference to  FIG. 1 . In the fourth embodiment, however, the configuration of the first current setting circuit is different in such a way that whether the current of the DC current source changes or not is selectable by an external signal when the high frequency current is superposed.  
      Referring then to  FIG. 8  showing a detailed circuit example of the fourth embodiment, the configuration of the laser diode driver is described hereinafter. The laser diode driver according to the fourth embodiment is different from the laser diode driver according to the first embodiment only in the configuration of the first current setting circuit  101 . Specifically, the first current setting circuit  101  of this embodiment further has an OR circuit OR 2  in addition to the components of the first current setting circuit in the laser diode driver of the first embodiment described above. One input terminal of the OR circuit OR 2  is connected to a DC current control terminal T 3 , and the other terminal is connected to the superposition control terminal T 1 . The output of the OR circuit OR 2  is connected to the gate of the second switching element P 2 .  
      Referring further to  FIG. 8 , the operation of the laser diode driver of this embodiment is described hereinbelow. The operation when the superposition control terminal T 1  is H level is the same as that in the laser diode driver of the first embodiment, and thus not described herein.  
      When the superposition control terminal T 1  is L level, the current of I 2 =n*I 2   a  flows from the power supply voltage VDD through the PMOS transistor Q 5  ( 202 ), the first switching element P 1 , the output terminal T 2 , and the laser diode LD into the ground, which is the same as in the laser diode driver of the first embodiment.  
      If the superposition control terminal T 1  is L level and the DC current control terminal T 3  is also L level, the output of the OR circuit OR 2  is L level, so that the second switching element P 2  formed of a PMOS transistor is ON. Accordingly, the PMOS transistors Q 1  and Q 2  are such that the source and the drain are connected in parallel. In this condition, the current value of the DC current source decreases by the amount proportional to the maximum current flowing through the high frequency current source during the superposition of the high frequency current as in the laser diode driver of the first embodiment described above. On the other hand, when the DC current control terminal T 3  is H level, the output of the OR circuit OR 2  is H level, so that the second switching element P 2  formed of a PMOS transistor is OFF. In this condition, the current value of the DC current source does not change during the superposition of the high frequency current.  
      In the laser diode driver of this embodiment, whether or not to change the current of the DC current source is selectable in accordance with the type of an optical disc by the level of an external signal which is applied to the DC current control terminal T 3  when the high frequency current is superposed. This embodiment may be applied to the laser diode driver according to the second and third embodiments.  
      As described in the foregoing, the laser diode driver according to the embodiments of the present invention includes a circuit for changing the current through the DC current source when the high frequency current source is operating, thereby controlling the peak value of the drive current of the laser diode to fall below a prescribed threshold which is determined by the material of an optical disc and the characteristics of the laser diode. This avoids erroneous operation of performing erasing during the read period, writing during the erase period, or the like. Further, the laser diode driver does not have a path for the current supplied from the DC current source and the high frequency current source to flow except for the path from the power supply voltage through the laser diode to the ground, and there is no path which bypasses the laser diode. Accordingly, all of the current supplied from the DC current source and the high frequency current source contribute to the emission of the laser diode, thereby providing the advantage of preventing high power consumption and excessive heating.  
      Although the above embodiments are described in reference to the case where one DC current source and one high frequency current source are used, it is possible to switch between a plurality of DC current sources and a plurality of high frequency current sources for use in each of the read, erase and write periods. The laser diode driver according to different embodiments may be applied for each period.  
      Further, the above embodiments are described using a rewritable optical disc by way of illustration. However, the present invention may be applied to a write-once optical disc. In addition, it is possible to use the transistors with the conductivity types opposite to those described in the above embodiments or a logic circuit which operates in the same manner.  
      It is apparent that the present invention is not limited to the above embodiment and it may be modified and changed without departing from the scope and spirit of the invention.