JP5008534B2 - Light emitting element drive circuit - Google Patents

Description

  The present invention relates to a light emitting element driving circuit.

  Semiconductor laser diodes are widely used for writing to and reading from various recording media such as a laser beam printer (LBP), a CD, and a DVD because of their small size and low power consumption. A semiconductor laser diode emits a laser beam when a certain threshold current is exceeded, but in recent years, the threshold value has been lowered and the luminous efficiency has been improved, and the amount of light emission that has conventionally required a driving current of several tens of mA is a driving current of several mA. It has come to be obtained. In LBP, the number of outputs per minute depends on the switching speed of the semiconductor laser diode, and the high definition of the output image depends on the minimum light pulse width that can be output. For this reason, when a conventional drive current of several tens of mA is required, the semiconductor laser diode does not emit light in advance in order to increase the speed and improve the rise time of the current pulse, that is, by passing a current below the threshold, The parasitic capacitance of the semiconductor laser diode is charged in advance.

  Patent Document 1 below describes a laser drive circuit that biases the gate of an output transistor at the time of non-output with a shunt current. Patent Document 2 below describes a laser drive circuit that draws a part of current from a current source and sets a bias current at the time of non-output to a vicinity of a threshold value.

JP 2000-216486 A JP 2003-198047 A

  Patent Documents 1 and 2 both increase the responsiveness at the time of output by biasing in advance using a current obtained by diverting (partially extracting) the gate of the output transistor from the current source at the time of non-output. is there.

  In recent years, the performance of light emitting elements (semiconductor laser diodes) has been improved and the threshold current has decreased to several mA, so that a drive current exceeding a desired light emission amount may flow even at a minimum light emission control voltage. In addition, since the MOS transistor that supplies the drive current has an Early effect, the drive current shows a value larger than the ideal value in the vicinity of the minimum current, and thus a current exceeding the threshold is likely to occur.

  The present invention provides a light emitting element driving circuit capable of reliably performing light emission control of a light emitting element having a small light emission threshold (about 10 mA or less) and capable of correcting distortion of the driving current of the light emitting element due to the early effect of the transistor. With the goal.

A light-emitting element driving circuit according to the present invention is a light-emitting element driving circuit that drives a light-emitting element having a current control unit that controls a main current according to a control voltage, the bias current for subtracting a bias current from the main current source and said by the main current to control the current corresponding to the current or minus the bias current have a control circuit unit for the light emitting element, the control voltage at the time of minimum voltage, than the main current The bias current is larger .

  Light emission control of a light emitting element having a small light emission threshold current can be reliably performed. In addition, it is possible to correct the distortion of the driving current of the light emitting element due to the Early effect of the transistor.

(First embodiment)
FIG. 1 is a diagram showing a conceptual configuration example of a semiconductor laser diode driving circuit (light emitting element driving circuit) according to the first embodiment of the present invention. A case where a semiconductor laser diode is used as the light emitting element will be described as an example.

  The control voltage is input to the current control unit 101. A current control unit (hereinafter also referred to as a constant current value setting unit) 101 controls a value of a main current I1 for setting a drive current for driving the semiconductor laser diode LD according to a control voltage. The main current I1 is a constant current. The bias current source CC1 is a bias current source for subtracting (subtracting) the bias current Inb from the main current I1. The current obtained by subtracting the bias current Inb from the main current I1 is I1-Inb. The current adding unit 102 adds the main current I1 and the negative bias current Inb, and outputs the current I1-Inb to the control circuit unit (hereinafter also referred to as a switching unit or a switching circuit) 103. A sum current I 1 -Inb of a bias current Inb having a polarity opposite to that of the main current I 1 is supplied to the switching circuit 103. When the main current I1 is larger than the bias current Inb having the opposite polarity, the drive current is supplied to the switching circuit 103. The switching circuit (switching unit) 103 controls the light emission of the semiconductor laser diode LD by switching the current I1-Inb or a current corresponding thereto in accordance with the drive signal. The switching circuit 103 has a current amplifier circuit. The current amplifier circuit amplifies the switched current and supplies a drive current to the semiconductor laser diode LD.

  The gain of the current amplifier circuit is n, the control voltage is Vin, the minimum value of the control voltage is Vmin, and the threshold current of the semiconductor laser diode LD is Ith. In this case, the main current I1 = Vin / Rs. Incidentally, Rs is a resistance of a circuit through which the main current Is flows. Since the current ILD = n × (I1−Inb) for driving the light emitting laser diode LD, ILD = n × (Vin / Rs−Inb).

  In order to solve the problem to be solved by the present invention, it is necessary to satisfy the relationship of the drive current ILD <Ith when the control voltage is the minimum value Vmin. Therefore, ILD = n × (Vmin / Rs−Inb) <Ith. Therefore, the bias current Inb having the opposite polarity to the main current I1 needs to satisfy the relationship Inb> Vmin / Rs−Ith / n. Further, the actual problem is noticeable when the threshold current Ith of the light emitting laser diode LD is less than 10 mA in consideration of the performance of the current analog circuit. When the control voltage Vin is the minimum voltage Vmin, the bias current Inb is preferably larger than the main current I1.

  FIG. 2 is a circuit diagram showing a configuration example of the semiconductor laser diode drive circuit according to the present embodiment. The switching circuit 103 includes transistors PM2, PM3, and NM1 to NM4. The transistor PM1 corresponds to the constant current value setting unit 101 in FIG.

  PM1 is a PMOS transistor (P-channel MOS field effect transistor) that supplies the main current I1, and a control voltage Vin is applied to the gate. Inb is a constant current value drawn from the main current I1 by the bias current source CC1. PM2 and PM3 are differential pair PMOS transistors whose sources are commonly connected, and a constant current I1-Inb is supplied to the commonly connected sources. Each gate electrode receives a complementary signal of a drive signal for switching the semiconductor laser diode LD. The drain of the transistor PM3 is connected to the drain and gate of an NMOS transistor (N-channel MOS field effect transistor) NM2 constituting a current mirror and the gate of the NMOS transistor NM3. The drain of the transistor PM2 is connected to the NMOS transistor NM1 whose drain and gate are connected, that is, diode-connected. The NMOS transistor NM4 is M times larger than the transistor NM2 and has a drain connected to the cathode of the semiconductor laser diode LD. BUF is a drive signal buffer, and its output is connected to the gate of the transistor PM2. INV is an inverter that generates an inverted signal of the drive signal, and its output is connected to the gate of the transistor PM3 and the gate of the transistor NM3, and the drain of the transistor NM3 is connected to the gates of the current mirrors NM2 and NM4.

  The switching circuit 103 includes a differential amplifier circuit that switches a current I1-Inb obtained by subtracting the bias current Inb from the main current I1 in accordance with the drive signal. The differential amplifier circuit includes transistors PM2 and PM3. The switching circuit 103 has a current amplification circuit that amplifies the current I1-Isb obtained by subtracting the bias current Inb from the main current I1, and flows the amplified current to the semiconductor laser diode LD. The current amplifier circuit is a current mirror circuit including transistors NM2 and NM4. The operation states of the current mirror circuits NM2 and NM4 are controlled by the transistor NM3 according to the drive signal.

  In FIG. 2, when the drive signal is high level, the gate of the transistor PM2 becomes high level via the buffer BUF, and the gate of the transistor PM3 becomes low level via the inverter INV. Since the output of the inverter INV is connected to the gate of the transistor NM3, the transistor NM3 is turned off, and the current mirror composed of the transistors NM2 and NM4 is turned on. Since the transistors PM2 and PM3 constitute a differential amplifier, the transistor PM2 is turned off and the transistor PM3 is turned on. For this reason, all of the current I1-Inb obtained by subtracting the current Inb of the bias current source CC1 from the current I1 flowing through the transistor PM1 flows through the transistor PM3. The current I1-Inb is supplied from the drain of the transistor PM3 to the transistor NM2, and the current M × (I1-Inb) flows as a drive current for the semiconductor laser diode LD by the transistor NM2 and the M-fold transistor NM4 that forms a current mirror.

  When the drive signal is at a low level, the transistor PM2 is turned on, the transistor PM3 is turned off, and the transistor NM3 is turned on. Since the gates of the current mirrors NM2 and NM4 are set to a low level by the transistor NM3, no current flows through the semiconductor laser diode LD. The drain of the transistor PM2 is connected to a diode-connected transistor NM1 having the same size as that of the current mirror NM2, so that the drain voltages of the transistors PM2 and PM3 when turned on are substantially equal. Thereby, the object property of the differential pairs PM2 and PM3 is improved, and the duty of the drive signal is maintained.

  Switching of the semiconductor laser diode LD is controlled by the high level and low level of the drive signal. The relationship between the drive current value flowing in the semiconductor laser diode LD at that time and the control voltage for setting the drive current value is shown in FIGS.

  FIG. 5 is a diagram showing control voltage-driving current characteristics, and FIG. 6 is an enlarged view of a circled portion in FIG. A characteristic 601 is a characteristic of the semiconductor laser diode driving circuit according to the present embodiment. A characteristic 602 is a characteristic when the bias current source CC1 is not provided. In the characteristic 602, only a drive current exceeding a desired minimum drive current value can be obtained even with the minimum control voltage. The characteristic 601 of the present embodiment is a characteristic having a negative offset current with respect to the characteristic 602, and a desired drive current minimum value can be obtained. The control voltage is a voltage input to the constant current value setting unit 101 that sets the drive current as shown in FIG. 1, and the constant current value, that is, the output voltage corresponding to the dynamic range of the constant current value setting unit 101 is The drive current is controlled. A desired minimum drive current value can be obtained with a control voltage equal to or higher than the minimum control voltage. As a result, a desired drive current range can be obtained within a controllable control voltage range.

  As described above, according to the present embodiment, the semiconductor laser diode LD can be driven without being affected by the Early effect of the transistor that supplies the drive current.

(Second Embodiment)
FIG. 3 is a circuit diagram showing a configuration example of the semiconductor laser diode drive circuit according to the second embodiment of the present invention. In the present embodiment, a gate-grounded PMOS transistor PM4 is inserted between the transistors PM1 and PM2 and PM3 with respect to the first embodiment of FIG. In other respects, the present embodiment is the same as the first embodiment. A bias voltage Vbias is applied to the gate of the transistor PM4 so as to ensure a sufficient source / drain voltage of the transistors PM2 and PM3. The transistor PM4 is connected between the connection node of the transistor PM1 (constant current value setting unit 101) and the bias current source CC1 and the switching unit 103 as a load for keeping the node potential of the sum current I1-Inb constant. In this manner, by inserting the transistor PM4, it is possible to obtain characteristics that do not include the Early effect of the transistor PM1 in the drive current.

(Third embodiment)
FIG. 4 is a circuit diagram showing a configuration example of a semiconductor laser diode drive circuit according to the third embodiment of the present invention. The present embodiment is a case where the present invention is applied to a drive current switching method using a differential method.

  PNP bipolar transistor BP1 has an emitter connected to power supply voltage Vcc, and main current I1 flows through the collector. The NPN bipolar transistor BN1 has a collector connected to the collector of the transistor BP1, and an emitter connected to the ground. The gate and collector of the transistor BN1 are connected to each other. The bias current source CC1 is connected between the collector of the transistor BN1 and the ground, and draws the constant current Inb. The NPN bipolar transistor BN2 has a base connected to the base of the transistor BN1, and an emitter connected to the ground. The size of the transistor BN2 is M times that of the transistor BN1. The buffer BUF buffers and outputs the drive signal. The inverter INV logically inverts the drive signal and outputs it. The emitters of NPN transistor BN3 and NPN transistor BN4 are connected in common, and the commonly connected emitter is connected to the collector of NPN transistor BN2. Transistor BN3 has a collector connected to power supply voltage Vcc through a resistor, and a base connected to the output of buffer BUF. The NPN transistor BN4 has a collector connected to the power supply voltage Vcc via the semiconductor laser diode LD, and a base connected to the output of the inverter INV.

  The transistor BP1 corresponds to the constant current value setting unit 101 in FIG. The transistors BN1, BN2, BN3, and BN4 correspond to the switching circuit 103 in FIG. The switching circuit 103 includes a current amplifier circuit that amplifies a current I1-Inb obtained by subtracting the bias current Inb from the main current I1, and a differential amplifier circuit that switches the amplified current in accordance with a drive signal. The current amplifier circuit is a current mirror circuit composed of transistors BN1 and BN2. The differential amplifier circuit includes transistors BN3 and BN4.

  Since the main current I1 flows through the transistor BP1 and the constant current Inb flows through the bias current source CC1, the current I1-Inb flows through the transistor BN1. The transistors BN1 and BN2 constitute a current mirror, and the size of the transistor BN2 is M times that of the transistor BN1. Therefore, the current M × (I1-Inb) flows through the transistor BN2. If the drive signal is at a low level, the transistor BN4 is turned on, the transistor BN3 is turned off, and a current M × (I1-Inb) flows through the semiconductor laser diode LD. On the other hand, if the drive signal is at a high level, the transistor BN3 is turned on, the transistor BN4 is turned off, and no current flows through the semiconductor laser diode LD. In the first and second embodiments, when the drive signal becomes high level, a current flows through the semiconductor laser diode LD. On the other hand, in the present embodiment, when the drive signal becomes low level, a current flows through the semiconductor laser diode LD.

  In the present embodiment, the same effect as the first and second embodiments can be obtained as the current flowing through the semiconductor laser diode LD.

  As described above, according to the first to third embodiments, a drive current characteristic having a negative bias current can be obtained, control in the vicinity of the minimum current value can be performed, and a 3V single power source which is an advantage of the CMOS process can be obtained. Even if this operation is performed, current control in the vicinity of a desired minimum current can be performed. Furthermore, by inserting the common-gate transistor PM4 between the node of the sum current I1-Inb and the switching circuit 103, it is possible to prevent an increase in current due to the Early effect. Further, the linearity of the drive current in the low luminance region can be improved.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

It is a figure which shows the example of a conceptual structure of the semiconductor laser diode drive circuit by the 1st Embodiment of this invention. 1 is a circuit diagram showing a configuration example of a semiconductor laser diode drive circuit according to a first embodiment. FIG. It is a circuit diagram which shows the structural example of the semiconductor laser diode drive circuit by the 2nd Embodiment of this invention. It is a circuit diagram which shows the structural example of the semiconductor laser diode drive circuit by the 3rd Embodiment of this invention. It is a figure which shows a control voltage-drive current characteristic. It is the elements on larger scale of FIG.

Explanation of symbols

101 Current control unit (constant current value setting unit)
102 current addition unit 103 control circuit unit (switching unit)
I1 Drive current setting main current CC1 Bias current source Inb Constant current value LD Semiconductor laser diode BUF Buffer INV Inverter

Claims (14)

A light emitting element driving circuit for driving a light emitting element having a current control unit for controlling a main current according to a control voltage,
A bias current source for subtracting the bias current from the main current;
A control circuit unit that causes a light emitting element to emit light by controlling a current obtained by subtracting the bias current from the main current or a current corresponding thereto,
During the control voltage is the minimum voltage, it characterized in that from said main current is greater the bias current light emission element driving circuit.   The light emitting element drive circuit according to claim 1, wherein the control circuit unit supplies a current obtained by subtracting the bias current from the main current or a current corresponding thereto to the light emitting element in accordance with a drive signal. Furthermore, the light-emitting element driving circuit according to claim 1, wherein further comprising a load connected between the control circuit unit and the current control unit. 4. The light emitting element driving circuit according to claim 3 , wherein the load is a field effect transistor having a bias voltage applied to a gate. The control circuit unit includes a current amplifier circuit that amplifies a current obtained by subtracting the bias current from the main current, and a differential amplifier circuit that supplies the amplified current to the light emitting element according to the drive signal. light-emitting element driving circuit according to any one of claims 2-4, characterized in. 6. The light emitting element driving circuit according to claim 5 , wherein the current amplifier circuit is a current mirror circuit. A light emitting element driving circuit for driving a light emitting element having a current control unit for controlling a main current according to a control voltage,
A bias current source for subtracting the bias current from the main current;
A control circuit unit that causes a light emitting element to emit light by controlling a current obtained by subtracting the bias current from the main current or a current corresponding thereto,
Wherein said control circuit unit, said amplifies the current obtained by subtracting the bias current from the main current, characterized in that it has a current amplifying circuit for supplying a current in the amplifying to the light emitting element emitting light element driving circuit. The light emitting element driving circuit according to claim 7 , further comprising a load connected between the current control unit and the control circuit unit. 9. The light emitting element driving circuit according to claim 8 , wherein the load is a field effect transistor having a bias voltage applied to a gate. The light-emitting element driving circuit according to claim 7, wherein the current amplifier circuit is a current mirror circuit. The light emitting element driving circuit according to claim 10 , wherein an operation state of the current mirror circuit is controlled in accordance with the driving signal. A light emitting element driving circuit for driving a light emitting element having a current control unit for controlling a main current according to a control voltage,
A bias current source for subtracting the bias current from the main current;
A control circuit unit that causes a light emitting element to emit light by controlling a current obtained by subtracting the bias current from the main current or a current corresponding thereto,
Wherein said control circuit unit, in response to said drive signal, light emission element drive circuit you characterized as having a differential amplifier circuit for supplying a current obtained by subtracting the bias current from the main current to the light emitting element. The light emitting element driving circuit according to claim 12 , further comprising a load connected between the current control unit and the control circuit unit. The light emitting element driving circuit according to claim 13 , wherein the load is a field effect transistor having a bias voltage applied to a gate.

Images