Drive circuit for semiconductor light-emitting device

Abstract: A drive circuit for supplying a constant drive current to the anode terminal of a semiconductor light emitting device, includes a current supply source connected to the anode terminal for supplying a bias current to the semiconductor light emitting device. Since the drive circuit is formed so as to supply the bias current to the anode terminal of the semiconductor light emitting device, a charge current for charging a conjunction capacitance can be reduced independently on the time period of turning off the semiconductor light emitting device. As a result, a spike noise can be removed in the laser current waveform at its rise-up part, and a reduction of its rise-up speed can be restricted, both of which are due to a dependent upon the time period of turning off the semiconductor device.

Description: The present invention relates to a drive circuit for semiconductor light-emitting device. More specifically, this invention relates to a drive circuit for semiconductor light-emitting device which is applicable to a laser drive integrated circuit (IC) for high-speed switching used for, for example, magneto-optical disks and laser beam printers. Fig. 1 shows a laser drive circuit categorized as a cathode-common type circuit as known in the prior art. In Fig. 1, a component 1 is a base current source for determining the drive current to the laser semiconductor, which is ordinarily controlled so that the output voltage which is the quantity of laser light may be regulated to be a designated value by monitoring the quantity of laser light from the laser light-emitting device by means of a photodiode not shown. Components 2 and 3 form a current mirror circuit composed of a couple of P-channel MOS transistors, where both of the drain and source terminals of the PMOS transistor 2 are connected to the base current source 1. The value of the drain current to the PMOS transistor 2 is reflected (mirrored) to the value of the output current from the PMOS transistor 3. The drain terminal of the PMOS transistor 3 is connected to the terminal shorted both to the collector and base of the NPN transistor 4. The base of the NPN transistor 5 is connected to the terminal shorted both to the collector and base of the NPN transistor 4. With these NPN transistors 4 and 5, a current mirror circuit is formed. By making the ratio of the area of the emitter of the NPN transistor 4 to the area of the emitter of the NPN transistor 5 is 1:N, the value of the output current from the emitter terminal common to the transistors 4 and 5 may be obtained so as to be (1+N) times as large as the value of the drain current to the PMOS transistor 3. A component 7 is a laser diode, and its cathode is connected to the ground level point (GND) 9, while its anode is connected the common-emitter terminal of the NPN transistors 4 and 5. The N-Channel MOS transistor 6 is a switching transistor which is turned on when the high-level signal is applied to the control signal input terminal, and to which the current supplied from the PMOS transistor 3 is led. In this state, a current is not supplied to the short-circuit terminal to the collector and base of the NPN transistor 4, and thus the current mirror circuit formed by the NPN transistors 4 and 5 is turned off. Therefore, the drive current for the laser diode 7 is zero. In addition, as the NMOS transistor 6 is turned off when the low-level signal is applied to the input terminal 10, the output current from the PMOS transistor 3 drives the laser diode 7 by driving the current mirror circuit composed of the NPN transistors 4 and 5. And then, the light emission from the laser diode 7 is switched in a high speed by turning the NMOS transistor 6 on and off in a high speed. Fig. 2 shows another example of the semiconductor laser drive circuit as known in the prior art. In Fig. 2, the cathode of the laser diode 30 is connected to the lower electric potential terminal 22 in the circuit, and its anode is connected to the current mirror circuit 24. The input terminal to the current mirror circuit 24 is connected to the output terminal from the constant-current circuit 23. The emitter of the NPN transistor 25 which is switched by the control signal supplied from outside the circuit is connected to the lower electric potential terminal 22, and its base is connected to the control signal input terminal 12 and its collector is connected to the output terminal from the constant-current circuit 23. In the circuit shown in Fig. 2, the semiconductor laser diode 30 emits laser light in responsive to the current supplied to itself. Its light emission process is stated as below. At first, when the voltage applied to the control signal input terminal 32 is VH, the NPN transistor 25 is turned on and the current supplied by the constant-current source 23 is led to the NPN transistor 25 and is not supplied to the current mirror circuit 24 and hence as the current is not also supplied to the laser diode 30, and the laser light is not emitted. Next, when the voltage applied to the control signal input 32 is V L , the NPN transistor 25 is turned off and the current supplied by the constant-current source 23 is led to the current mirror circuit 24 and hence, as the current is also supplied to the laser diode 30, and the laser light is emitted from the laser diode 30. At this time, the electric potential at the base of the current mirror circuit 24 is the sum of the voltage V F defined between the anode and the cathode of the laser diode 30 and the voltage V BE(ON) defined between the emitter and the base of the current mirror circuit 24. However, in the prior art shown in Fig. 9, as the charge current for charging the junction capacitance in the laser diode, the following disadvantages are found. (1)When the laser diode is turned on after a long period of time while which the laser diode is turned off, spike noises occur at the rise-up part of the waveform of the current supplied to the anode terminal of the laser diode. (2) The rise-up speed of the current when the laser diode is turned on after a longer period of time while which the laser diode is turned off is slower than the rise-up speed of the current when the laser diode is turned on after a shorter period of time while which the laser diode is turned off. The factors leading to the above disadvantages (1) and (2) are described below. Fig. 3 shows an equivalent circuit of the laser diode 7. In Fig. 3, A is the anode terminal, K is the cathode terminal, D is the junction part of the laser diode, and C is the junction capacitance of the laser diode. In case that the laser drive circuit shown in Fig. 9 drives the laser diode 7, the current is led to the laser diode junction part D when the voltage V F defined between the terminals of the laser diode rises up to about 1 to 1.5V. That is, the drive current in the laser drive circuit is at first used for charging the junction capacitance C, and when V F rises up to the voltage with which the laser light can be emitted from the laser diode, the fraction of the current used for charging the junction capacitance C is reduced and the effective current is led to the junction part D of the laser diode. Figs. 4A-4C show waveforms of currents supplied into the laser diode shown in Figs. 1 and 2. The waveform (Fig. 4A) in Fig. 4 refers to the current Icj led to the junction capacitance C, the waveform (Fig. 4B) refers to the current Ild led to the laser diode junction part D, and the waveform (Fig. 4C) refers to the current ILD led to the anode terminal A. In the time domain, the current ILD led to the anode terminal A is the sum of Icj and Ild, and hence, the peak of the waveform of the current Icj used for charging the junction part D makes a noise onto the current ILD led to the anode terminal A. Next, the factor leading to the above disadvantage (2) is described below. Figs. 5A-5D show waveforms of currents and voltage with the turn-off time period of the laser diode shown in Figs. 1 and 2 changed. In this figure, the waveform (Fig. 5A) refers to the laser drive current ILD led to the anode terminal A of the laser diode 7, the waveform (Fig. 5B) refers to the voltage V F , the waveform (Fig. 5C) refers to the charge current Icj for charging the laser diode junction capacitance C, and the waveform (Fig. 5D) refers to the current Ild led to the junction part D of the laser diode. Individual waveforms show differences in the time-domain behavior in responsive to the change of the turn-off time period of the drive current of the laser diode. The pulse P1 in the waveform in Fig. 5A is a step response of the laser drive current when the laser diode is driven after a long time Toff1 during which the laser diode has been turned off. That is, Toff1 is far grater than Toff2. As shown in the waveform in Fig. 5B, while the time period Toff1 in which the laser diode is turned off in a long time, the electric char stored in the laser diode junction capacitance C is almost discharged with a small quantity of Off current, and hence, VF goes down to the GND level. Therefore, the additional current is required for charging the laser diode junction capacitance C when the laser diode is turned on next, and the charge current flows in the waveform in Fig. 5C. Eventually, the current led to the anode terminal A takes a step response shaped in the pulse P1 containing a noise in its rise-up part. As for the pulse P2 formed after a short period Toff2 after the fall-down of the pulse P1, as the laser diode is turned on before the electric charge stored in the laser diode junction capacitance C is not fully discharged, the voltage V F does not fall down to the GND level at the beginning of the pulse P2 as shown in the waveform in Fig. 5B, and the charge current shown by Fig. 5C flows a little. Therefore, the current ILD led to the anode terminal A does not contain noises. In addition, as the pulse P2 does not need the current Icj, the rise-up speed of the pulse P2 is faster than the pulse P1. In summing up, in the laser drive circuit in the prior art shown in Fig. 1, after a long period of time while the laser diode is turned off, a spike noise is contained in the laser current waveform at its rise-up part, and its rise-up speed is restricted. These disadvantages described above are found in another prior art shown in Fig. 2. Parasitic capacitors are formed at the output terminal of the constant-current circuit 23, the collector of the switching NPN transistor 25, and the collector and base of the current mirror circuit 24, and individual parasitic capacitors are charged by the output current from the constant-current circuit 23 when the switching NPN transistor 25 is transferred from the turn-off state to the turn-on state. And then, a designated quantity of current is led to the current mirror circuit 24 when the electric potential at the base of the current mirror circuit 24 rises up from V CE(SAT) to V F + V BE(ON) , and finally, the laser diode 30 emits laser light with its intensity determined in responsive to the current led to the laser diode. Due to the time constant developed in the time domain behavior for charging the above described capacitances, the laser diode 30 emits laser light with a delayed time after the voltage applied to the control input terminal 32 changes from V H to V L . Especially, in case of reducing the output current from the constant-current circuit 23 in order to reduce the output power of laser light, the delayed time for emitting laser light is remarkable and can not be ignored for attaining the high-speed switching response of the laser light. Document EP-A-0 262 713 discloses a driving circuit according to the preamble of claim 1 for supplying a constant driving current to a semiconductor light emitting device, comprising input means for inputting a control signal, constant driving current generating means for generating the constant driving current, and switching means for switching a supply of said constant driving current to the light emitting device in accordance with the control signal inputted by the input means. For the purpose of reducing energy dissipation due to a DC bias current supplied in order to reduce switching transients, a second switching means and a second constant current source are provided for supplying a bias current (pedestal current) during a time period a short time before until a short time after the application of the control signal. Thereby, a laser-threshold current is exceeded during the combined application. Other driving circuits for driving a semiconductor light emitting device are described in the US-A-4 748 633 and the Journal of lightwave Technology, Vol. 6, No. 3, March 1988, pages 475-479. It is therefore an object of the present invention to provide a driving circuit which is capable of removing noise from its drive current due to switching transients. This object is achieved by a driving circuit as claimed in claim 1. The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. Fig. 1 is a circuit diagram showing a prior art drive circuit for laser diodes of a cathode common type; Fig. 2 is a circuit diagram showing another embodiment; Fig. 3 is an equivalent circuit of the laser diode; Figs. 4A-4C are waveforms showing currents supplied into the laser diode shown in Figs. 1 and 2; Figs. 5A-5D are waveforms showing currents supplied into the laser diode shown in Figs. 9 and 10; Fig. 6 is a circuit diagram showing a first embodiment of the present invention; Fig. 7 is a diagrammatical picture showing a multi-stage emitter follower circuit using NPN transistors as another embodiment of the level shift circuit 27 shown in Fig. 6; Fig. 8 is a diagrammatical picture showing a multi-stage emitter follower circuit using NMOS transistors as another embodiment of the level shift circuit 27 shown in Fig. 6; Fig. 9 is a circuit diagram showing a modification of the embodiment shown in Fig. 6; Fig. 10 is a circuit diagram showing the fourth embodiment of the present invention; Fig. 11 is a circuit diagram showing the fifth embodiment of the present invention; and Fig. 12 is a waveform showing voltages and currents measured at each node in the fifth embodiment. Fig. 6 shows a first embodiment of the present invention. Following embodiments are preferably structured by IC. In Fig. 6, a component 21 is a power supply line, a component 22 is a ground line, a component 23 is a constant-current circuit, a component 24 is a current mirror circuit, a component 25 is an NPN transistor for switching, a component 26 is a current source, a component 27 is a level shift circuit using diodes, a component 28 is a BicMOS inverter, a component 29 is an inverter, a component 30 is a semiconductor laser diode, and a component 32 is an input terminal for control signals. Next, what is described is an operation of the circuit in the first embodiment. When the voltage applied to the control signal input terminal 32 is V H , the output from the inverter 28 is lower voltage, and therefore, the output from the inverter 29 is high voltage designated V OH . The NPN transistor 25 is turned on and the current supplied by the constant-current source 23 is led to the NPN transistor 25 and is not supplied to the current mirror circuit 24, and therefore, as the current is not supplied to the laser diode 30, the laser light is not emitted. At this time, as the difference of electric potentials between both end terminals of the level shift circuit 27 is not found, the diode is in a zero-biased state and the current does not flow in the diode. Next, in case that the voltage applied to the control signal input terminal 32 is V L , the output from the inverter 29 is lower voltage, and therefore, the output from the inverter 25 is high voltage. Therefore, in order to turn off the NPN transistor 25 for switching from its turn-on state, the output current supplied by the constant-current circuit 23 begins to be supplied to the current mirror circuit 24, and at the same time, as the output from the inverter 28 is high voltage, this voltage is transferred through the level shift circuit 27 to the electric potential of the base of the current mirror circuit 24, and thus the base potential is suddenly raised. The above base potential has the value V CH - n V DF which is subtracted the value n V DF from the output of the inverter 28, in which n V DF is obtained by multiplying the number (n) of the diodes serially connected in the level shift circuit 27 to the forward potential (V DF ) of the diode. In order that the value V CH - n V DF has somewhat lower than the value V BE(ON) + V F in the prior art (see Fig. 2), the value of n or the junction area of the diode must be adjusted. As compared with the time necessary for charging the parasitic capacity by using the output current of the constant current circuit 23 of the prior art shown in Fig. 2 and then causing the base potential of the current mirror circuit 24 to be the value V BE(ON) + V F , in the present embodiment, the above time is very short. Accordingly, it becomes possible to switch at a high speed the laser light emission. When the control input terminal is at low potential, the output potential of the level shift circuit is smaller than the value V BE(ON) + V F as described above. Therefore, since the diodes in the level shift circuit is at a zero-biased state or reverse-biased state when the output current of the constant current circuit 23 flows into the current mirror circuit 24 and the desired current is supplied to the laser diode 30, the current does not flow into the laser diode 30, it never effect on the setting of the driving current to the laser diode 30 by the current source 26. Figs. 7 and 8 are embodiments which modified the embodiment shown in Fig. 6. Namely, it may be possible to use an n-stage of emitter follower as shown in Fig. 7 or an n-stage of a source follower as shown in Fig. 8. A third embodiment shown in Fig. 9 is fundamentally the same as one in Fig. 6, except that an ordinary inverter is used as the inverter 28 in this embodiment. Namely, any ordinary drivers which has a lower output impedance may be used as the inverter 28. In the embodiment shown in Fig. 9, the current source 26 shown in Fig. 6 is removed and the current source is directly connected as the constant current circuit 23. Fig. 10 shows another embodiment of the present invention. In Fig. 10, a cathode of a laser diode 30 is connected to a lower potential point 22 in a circuit, an anode of the diode is connected to a current mirror circuit 24 of which an input is connected to an output of a constant current circuit 23. The lower electric potential point 22 is connected to an emitter of an NPN transistor 25 which can be switched by a control signal, a base of the NPN transistor 25 is an output of a level shift circuit 27 of which an input is connected to an output of a voltage driver. In operation, an output of an inverter 28 is at a high potential when a control input terminal is at a lower potential V L . Therefore, an output of an inverter 29 and the output of the level shift circuit 27 are turned into a lower potential. As a result, the switching transistor 25 is turned on to draw an output current of the constant current source 23. Accordingly, since any current can not be supplied into the laser diode 30, it does not emit light. Next, when a control input terminal 32 is turned into the high potential V H , the output of the inverter 28 is turned into the lower potential. As a result, the switching transistor 25 switched from its turned-on state to its turned-off state, so that the output current of the constant current source 23 begins to flow into the current mirror circuit 24. The output of the inverter 29 becomes at the high potential just after the current flowing. Thus the level shift circuit 27 becomes at a conductive state and the output current of one is supplied to the current mirror circuit 24. As a result, a base potential of the current mirror circuit 24 is suddenly raised, and then the current mirror circuit 24 gradually supplies a current to the laser diode 30. When the magnitude of the supplied current become larger than a predetermined threshold value, the laser diode begins to emit the light. This embodiment also has the same effects as that in the embodiment described above. However, it takes the inverter 29 and the level shift circuit 27 a certain time td to obtain the maximum value of the output current of the level shift circuit 27 from when the control input terminal 32 changes from the low potential to the high potential and the switching transistor 25 switches from the turned-on state to the turned-off state. Accordingly, if the laser driver is driven at a higher frequency, it may be necessary to take account of the time td. Next, another embodiment of the present invention will be explained, in which a delay element is inserted between an input terminal of the voltage driver switched by the control input and an input terminal of an active element switched by the control input, the effect of an operation delay time of the voltage driver and the level regulation means is eliminated by operating the voltage driver before the active element is switched, it is, therefore, possible to minimize the time from when the active element is switched from the turned-on state to the turned-off state to when the current mirror circuit 24 begins to supply a current to the laser diode. Fig. 11 shows a fifth embodiment of the present invention in which like elements corresponding to those of the above embodiment are identified by the same reference numerals, and the explanation of the like elements will be abbreviated. In this embodiment, a delay element 39 is inserted between an input of an NPN switching transistor and the control input terminal. In operation, when the control input terminal 32 is at a high potential V H , an output of the delay element 39 is also at a high potential. Thus the switching transistor 25 is at the turned-on state and the output current of the constant current source 23 flows thereinto. At this time, since the output of the inverter 29 is at the low potential, the level shift circuit 27 does not provide the output current. Therefore, any current are not supplied to the current mirror circuir 24 and thus the laser diode 30, as result, the laser diode does not emit the light. Next, referring to Fig. 12, the operation when the control input terminal 32 changes to the low potential will be explained. In the figure, each symbols indicate as follows; A: a waveform of the control signal input terminal; B: a base waveform of the switching transistor 25; C: an input waveform of the level shift circuit 27; D: a cathode current of the laser diode 30; and d: delay time of the inverter 29. When the control input terminal changes from the high potential to the low potential, an input waveform of the level shift circuit 27 changes to the high potential after the delay time of the inverter 29 elapses. The base of the switching transistor changes to the low potential after the delay time of the delay element 39, its collector current decrease along with change in the base current. At this time, since the output of the level shift circuit 27 is already at the high potential, the current is supplied at a high speed from the output of the level shift circuit 27 to the input of the current mirror circuit 24, and thus the base potential of the current mirror circuit 24 is also raised at a high speed. When the base potential of the current mirror circuit 24 rises just before the value which can supply a desired current to the laser diode 30, the level shift circuit 27 become apart from a forward bias, and thus it does not supply the output current. Therefore, the current supplied to the input of the current mirror circuit 24 is only the output current of the constant current source 23, in which the laser diode 30 is driven by the constant current with the set value. By using such the delay element 39, it is possible to decrease very much the time from when the switching transistor 25 switches from the turned-on state to the turned-off state to when the set current is supplied to the laser diode 30, and it is also possible to drive it at a high speed. Since it can charge at a high speed a parasitic capacitance accompanying such as a semiconductor emitting element and a current mirror circuit for driving it just before the turned-on state of the current mirror circuit, by using the voltage driver and the level regulating means, it is possible to allow the semiconductor light emitting device to emit the light and stop emitting it at a high speed without effecting on the setting of the current flowing to the semiconductor light emitting device. The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.

Claims: A driving circuit for supplying a constant drivinq current to a semiconductor light emitting device (30) , comprising: a) input means (32) for inputting a control signal; b) constant driving current generating means (23) for generating said constant driving current; and c) switching means (24) for switching a supply of said constant driving current to said semiconductor light emitting device (30) in accordance with said control signal inputted by said input means (32) , characterized by d) supplying means (27, 28; 27, 29) for supplying a current to said switching means (24) in response to an input of said control signal representing a turn-on before said semiconductor light emitting device (30) is actually driven. A driving circuit according to claim 1, characterized in that said semiconductor light emittig device is a semiconduc-tor laser device (30) . A driving circuit according to claim 1, characterized in that said switching means (24) comprises a transistor. A driving circuit according to claim 1, characterized in that said switching means (24) comprises a current mirror circuit having a plurality of transistors. A driving circuit according to claim 4, characterized in that said switching means further comprises means (25) for switching said current mirror circuit in accordance with said control signal inputted by said input means (32) . A driving circuit according to claim 1, characterized in that said supplying means is a level shifting circuit (27) . A driving circuit according to claim 6, characterized in that said level shifting circuit (27) comprises one or more diodes. A driving circuit according to claim 6, characterized in that said level shifting circuit (27) comprises one or more emitter follower transistors. A driving circuit according to claim 6, characterized in that said level shifting circuit (27) comprises one or more source follower transistors. A driving circuit according to claim 1, characterized in that said control signal inputted by said input means (32) is supplied to both said supplying means (27) and said switching means (24) ; and that an inverter (29) is provided between said input means (32) and said switching means (24) . A driving circuit according to claim 10, characterized by a delay circuit (39) between said input means (32) and said switching means (24) . A driving circuit according to claim 1, characterized in that said driving circuit is an integrated circuit.

Inventors: OGAWA KATSUHISA, SAKURAGI TAKAMASA, OGAWA, KATSUHISA, SAKURAGI, TAKAMASA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H01S3
IPC codes: H03G1, H05B33, H01L33, H01S5