JP3601998B2 - Light emitting diode drive circuit - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light emitting diode driving circuit, and more particularly, to a light emitting diode driving circuit that can be driven at low voltage and high speed over a wide temperature range in an optical fiber link or the like that transmits a signal using an optical fiber as a medium.
[0002]
[Prior art]
In recent years, the maintenance of fiber networks has been advanced, and the transmission of signals using optical fibers as media has become widespread. For further widespread use, there is a demand for a low-cost, high-speed, low-power-consumption optical fiber link, and for that purpose, a light-emitting diode drive circuit capable of low-voltage, high-speed drive over a wide temperature range is required.
[0003]
FIG. 6 shows an example of a conventional light emitting diode drive circuit. This light emitting diode drive circuit generally includes a differential amplifier circuit 1 for input and a differential switching circuit 3 for turning on / off a drive current to the light emitting diode 2. The light emitting diode 2 is turned on / off by switching the drive current.
[0004]
[Problems to be solved by the invention]
Although the above-mentioned light emitting diode drive circuit can perform an analog low power supply voltage operation, in order to drive at a high speed, the constant current source of the differential switching circuit 3 is considered in consideration of a voltage drop when the light emitting diode 2 is turned on. It is necessary to design the transistor 7 and the transistors 8 and 9 for switching the light emitting diode 2 not to be saturated. However, the collector potential Vic of the transistor 7 is based on the power supply voltage and has a large temperature characteristic.
[0005]
For example, the collector potential Vic is
Vic = Vcc-IR1-VBE1-VBE2-VBE3
｛However, IR1 = Vt × Ln10 / 300Ω × 1kΩ
Vt = 25.6mV
Vcc: power supply voltage VBE1, VBE2, VBE3: base-emitter voltage of transistors 4, 5, 8
And the cathode potential VCA of the light emitting diode 2 is
VCA = Vcc-Vf
{Vf: voltage drop when light emitting diode 2 is ON (= 1.7V)}
And
The temperature coefficient Tc is
Tc (Vcc) = 0 ppm
Tc (IR1) = k / q × Ln10 / 300Ω × 2 × 1KΩ = 3356 ppm
Tc (VBE) =-3000 ppm
Tc (Vf) =-3000 ppm
｛Where k is Boltzmann's constant q is unit charge of electron｝
It becomes.
That is, the collector potential Vic depends on the power supply voltage, and when the power supply voltage is low, the transistor 7 is saturated depending on the ambient temperature.
[0006]
For example, if Vcc = 2.7 V, Vic = Vcc−IR1−3 × VBE.
Vic = 0.3V, VCA = 1.1V: Ta = 25 ° C.
Vic = 0.03 V, VCA = 0.85 V: Ta = −25 ° C.
Vic = 0.59 V, VCA = 1.36 V: Ta = 75 ° C.
｛However, Ta: ambient temperature｝
It becomes. At such a low temperature, the transistor 7 as the current source of the differential switching circuit 3 is saturated, so that high-speed operation is difficult.
The present invention has been made in view of such a problem, and an object of the present invention is to provide a light emitting diode driving circuit in which a differential switching circuit for driving a light emitting diode operates in a wide temperature range. is there.
[0007]
[Means for Solving the Problems]
A light emitting diode drive circuit according to the present invention includes a current bias circuit that supplies a bias current for compensating for a temperature change, a differential amplifier circuit that is current biased by the current bias circuit and differentially amplifies an input signal, and the differential amplifier. A differential switching circuit for switchingly driving the light emitting diode according to the output of the circuit.
[0008]
The current bias circuit includes a first and second transistor connected to each other, a first resistor connected between the emitter terminals of the first and second transistors, and an emitter terminal of the second transistor. A second resistor connected between the first and second transistors, a current mirror circuit having an input connected to the collector of the first transistor and an output connected to the collector of the second transistor, and an output connected to the base. And a third resistor connected between the base terminal of the first and second transistors, the emitter terminal of the third transistor, and GND, and outputs the collector terminal of the third transistor. With the terminal, the emitter terminal of the third transistor can be set to a potential with a low temperature coefficient.
[0009]
Further, in the differential amplifier circuit, fourth and fifth transistors each having an emitter coupled and each base terminal being a differential input, and an anode terminal connected to the collector terminals of the fourth and fifth transistors. The semiconductor device includes first and second diodes, fourth and fifth resistors connected between cathode terminals of the first and second diodes and GND, and anode terminals of the first and second diodes are output terminals. Thus, the number of constituent elements can be reduced by eliminating the level shift buffer.
[0010]
Further, the current bias circuit has a temperature characteristic of current biasing the differential amplifier circuit so as to compensate for a temperature characteristic of the light emitting diode, thereby avoiding transistor saturation even when a power supply voltage is reduced. Can be.
Further, by controlling the constant current source current in the differential switching circuit by the output current of the current mirror circuit in the current bias circuit, it is not necessary to newly provide a bias circuit for driving the light emitting diode, and the device The number and power consumption can be reduced.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a block diagram showing one embodiment of a light emitting diode drive circuit according to the present invention. The light emitting diode drive circuit 10 of the present embodiment is current biased by the current bias circuit 11, the differential amplifier circuit 13 oscillating at the reference potential with respect to the ground, and switching by the differential amplifier circuit 13. And a differential switching circuit 12 for driving the light emitting diode 2. In addition, 14 is a constant current source. Here, the current bias circuit 11 is configured to supply a bias current for compensating for a temperature change.
[0012]
FIG. 2 is a circuit diagram illustrating an example of the current bias circuit 11. A resistor 23 is connected between the emitter terminals of transistors 21 and 22 which are a common base of a bandgap power supply circuit, and a resistor 24 is connected between the emitter terminal of the transistor 22 and GND. The collector of the transistor 21 is connected to the input of the current mirror circuit 25, and the output of the current mirror circuit 25 is connected to the collector of the transistor 22 and the base of the transistor 26. An emitter terminal of the transistor 26 is connected to GND via a resistor 27, is connected to a common base terminal of the transistors 21 and 22, and has a collector terminal as an output terminal to the differential amplifier circuit 13.
[0013]
The resistance of the current bias circuit 11 and the constant of the transistor size are
By setting the resistance 23 = 600Ω, the resistance 24 = 3600Ω, and the transistor size ratio (transistor 21: transistor 22) = 4: 1, the temperature coefficient Tc of the emitter potential VBG of the transistor 26 becomes
Tc = k / q × Ln4 / 600 × 2 × 3600 ≒ 0
It becomes.
[0014]
FIG. 3 is a circuit diagram illustrating an example of the differential amplifier circuit 13. Each base terminal of the emitter-coupled transistors 30 and 31 is a differential input, each collector terminal is grounded via resistors 32 and 33, and a ground terminal is connected to each collector terminal of the transistors 30 and 31. The shift buffer transistors 34 and 35 are connected. The emitter terminals of the buffer transistors 34 and 35 are output terminals to the differential switching circuit 12. In addition, 40, 41, and 42 are constant current sources.
[0015]
2 and FIG. 3, the collector potential Vic of the constant current source 14 for driving the light emitting diode 2 is Vic = VBG × resistance 32 / resistance 27
Therefore, the temperature characteristic is almost eliminated.
Thus, the collector potential Vic can be determined by the ratio of the resistance 27 of the current bias circuit 11 to the resistance 32 of the differential amplifier circuit 13.
[0016]
When the temperature characteristic of the light emitting diode 2 is taken into consideration, for example, when the temperature coefficient of the voltage Vf of the light emitting diode 2 is -3000 ppm, it is assumed that Vcc = 2.5 V and Vic = 0.4 V are constant.
Vic = 0.4V, VCA = 0.55V, VCE = 0.15V: Ta = 25 ° C.
Vic = 0.4V, VCA = 0.8V, VCE = 0.4V: Ta = −25 ° C.
Vic = 0.4V, VCA = 1.06V, VCE = 0.66V: Ta = 75 ° C.
｛However, VCE = VCA-Vic
: Collector-emitter voltage of transistor 16
It becomes.
[0017]
On the other hand, by adjusting the temperature coefficient of Vic to 6000 ppm, even when Vcc = 2.3 V,
Vic = 0.175 V, VCA = 0.345 V, VCE = 0.17 V: Ta = 25 ° C.
Vic = 0.25V, VCA = 0.6V, VCE = 0.35V: Ta = −25 ° C.
Vic = 0.325V, VCA = 0.855V, VCE = 0.53V: Ta = 75 ° C
It becomes. Thus, even if Vcc is reduced, the saturation of the transistor can be avoided by adjusting the temperature characteristics of Vic.
[0018]
FIG. 4 is a circuit diagram showing an example of the differential amplifier circuit 13 using diodes instead of the level shift buffer transistors 34 and 35. That is, the base terminals of the emitter-coupled transistors 30 and 31 are differentially input, the respective collector terminals are connected to the anodes of the diodes 36 and 37, and the cathode terminals of the diodes 36 and 37 are connected to the resistor 38. , 39, and the collector terminal of each of the transistors 30, 31 is used as an output terminal to the differential switching circuit 12.
[0019]
In the circuit of FIG. 4, power consumption can be reduced and the number of constituent elements can be reduced by eliminating the level shift buffer.
FIG. 5 is a circuit diagram of an example in which the current bias circuit 11 of FIG. 2 and the differential amplifier circuit 13 of FIG. 4 are combined. In this circuit, by using the current of the current mirror 25 of the current bias circuit 11 as the constant current, which is the drive current of the light emitting diode 2, it is not necessary to newly provide a bias circuit for the drive current of the light emitting diode. Power consumption can be reduced.
[0020]
【The invention's effect】
As described above, according to the present invention, the transistor of the constant current source that drives the light emitting diode by the current bias circuit can be stably operated in a wide temperature range, and according to the characteristics of the light emitting diode, By setting the temperature change of the collector potential of the transistor of the constant current source for driving the light emitting diode to an appropriate value, a low voltage and high speed operation in a wide temperature range is realized.
[Brief description of the drawings]
FIG. 1 is a block circuit diagram showing an embodiment of a light emitting diode drive circuit according to the present invention.
FIG. 2 is a circuit diagram showing a current bias circuit of the present embodiment.
FIG. 3 is a circuit diagram illustrating a differential amplifier circuit according to the present embodiment.
FIG. 4 is a circuit diagram showing a differential amplifier circuit according to another embodiment.
FIG. 5 is a circuit diagram showing a light emitting diode drive circuit of the embodiment.
FIG. 6 is a circuit diagram of a conventional light emitting diode drive circuit.
[Explanation of symbols]
2 light emitting diode 10 light emitting diode drive circuit 11 current bias circuit 12 differential switching circuit 13 differential amplifier circuit 21, 22, 26 transistor 23, 24, 27 resistor 25 current mirror circuit 30, 31 emitter coupling transistor 34, 35 Bel shift buffer transistor

Claims (4)

A current bias circuit for supplying a bias current for compensating for a temperature change, a differential amplifier circuit that is current-biased by the current bias circuit and differentially amplifies an input signal, and a switching drive of the light emitting diode by an output of the differential amplifier circuit A current switching circuit, wherein the current bias circuit is connected between first and second transistors whose bases are connected to each other and emitter terminals of the first and second transistors. A first resistor, a second resistor connected between the emitter terminal of the second transistor and GND, a current mirror circuit having an input connected to the collector of the first transistor and an output connected to the collector of the second transistor, A third transistor having an output connected to the base of the current mirror circuit, and first and second transistors. Third resistor and equipped with, characterized in that an output terminal of the collector terminal of the third transistor emitting light diode drive circuit connected between the emitter terminal and the GND of the base terminal and the third transistor of the register. A current bias circuit for supplying a bias current for compensating for a temperature change, a differential amplifier circuit that is current-biased by the current bias circuit and differentially amplifies an input signal, and a switching drive of the light emitting diode by an output of the differential amplifier circuit A differential switching circuit, wherein the differential amplifier circuit includes a fourth and a fifth transistor, each of which has an emitter-coupled base terminal and a differential input, and a fourth and a fifth transistor. comprising a first and a second diode whose anode terminal is connected to the collector terminal of the fifth transistor, and a fourth and fifth resistor connected between the cathode terminal and the GND of the first and second diodes, the 1 and light emission diode driver you characterized in that the anode terminal and the output terminal of the second diode. 3. The light emitting diode drive circuit according to claim 1, wherein the current bias circuit has a temperature characteristic of current biasing the differential amplifier circuit so as to compensate for a temperature characteristic of the light emitting diode. 3. The LED driving circuit according to claim 1, wherein a constant current source current in the differential switching circuit is controlled by an output current of a current mirror circuit in the current bias circuit.

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