JPS6377173A - Circuit for driving light emitting diode - Google Patents

Abstract

PURPOSE:To increase the maximum operating frequency of a light emitting diode by providing means for controlling the charging/discharging of an equivalent capacity of the diode at the anode or cathode side of the diode. CONSTITUTION:A coil 26 is connected as the collector of a transistor (TR) 22 for supplying a prebias current, and connected to the collector of a TR 2 for driving an LED as a whole bias current supplying circuit 25, i.e., the cathode of the LED1. The emitter of the TR 22 is grounded through a prebias current limiting resistor (R) 24. R21 for applying the bias of the base and a diode 23 for a base bias constant voltage are connected to the base of the TR 22 so that the base voltage of the TR 22 becomes constant and the TR 22 always becomes ON. Thus, a constant bias current always flows to the LED 1 to shorten the rising and falling times of the TR 2, thereby enhancing the maximum operating frequency of the LED.

Description

[Detailed Description of the Invention] [Summary] In order to reduce the dullness of the light emitting diode, a circuit configured to control the charging and discharging of the equivalent capacity of the light emitting diode is added to the conventional light emitting diode drive circuit. The rise and fall times of the light emitting diode drive circuit are shortened and the maximum operating frequency of one light emitting diode is increased.

[Industrial application field]

The present invention relates to a light emitting diode drive circuit used in a digital optical transmission device, and more particularly to a light emitting diode drive circuit used in an ultra high-speed digital optical transmission device.

Conventionally, digital optical transmission devices using laser diodes (hereinafter referred to as LDs) have been used to perform ultra-high-speed digital optical transmissions, but recently, single light emitting diodes (hereinafter referred to as LEs) have been used as digital optical transmission devices.
For example, with the change in the material of LED elements from gallium aluminum arsenide to indium gallium aluminum i1, the capacitance between LED terminals has decreased, making it possible to use LEDs for considerably high-speed digital optical transmission. Digital optical transmission equipment is now in use.

Furthermore, LEDs are cheaper than LDs,
The maximum LED operating frequency in conventional LED drive circuits is also 70 to 80' (MH) per 3 (dB) LED operating bandwidth.
It used to be 300 to 400 (MHz).
It has improved to.

Since there is a tendency for further improvement in the future, it is necessary to use an optical transmission device using LEDs for ultra-high-speed digital optical transmission.

[Conventional technology]

FIG. 5 is a diagram showing the configuration of a conventional LED drive circuit.

In FIGS. 5(a) and 5(b), 51 is an LED, 52 is an LED driving transistor, 53 is an LED overcurrent limiting resistor, 54 is a current switch circuit, 55 is a constant current source, 5
6 represents an overcurrent protection resistor, and 57 represents a resistor.

FIG. 5(a) shows a collector drive circuit that lights up and extinguishes the LED 51 through the switching operation of the LED drive transistor. The LED 51 is connected as a load on the collector side of the LED drive transistor 52, and the LED
The emitter of the driving transistor 52 is grounded via an LED overcurrent limiting resistor 53.

The conventional LED drive circuit using this collector drive circuit is
By keeping the power supply voltage VCC constant, changing the input signal V1N, and performing a switching operation of the LED driving transistor 52, the current flowing between VccGRD is controlled, and the LED 51 is turned on and off.

To explain in more detail, when the input signal VIN is at a high level, the LED driving transistor 52 is turned on, and V
A current flows between cc and GRD, causing the LED 51 to emit light, and when the input signal VIN is at a low level, the LED driving transistor 52 is turned off, so that no current flows between VCC and GRD, and the LED 51 turns off.

Another example of a conventional drive circuit is shown in FIG. 5(b).
Some devices use a current switch circuit 54 to turn on and off the LED 51.

This will be explained in detail below.

The LED 51 is connected as a collector load of an LED driving transistor 52(a), and a resistor 57 is connected as a collector load of an LED driving transistor 52(b). LED driving transistor 52 (al(b
) are connected to a common terminal and grounded via a constant current source 55 and an overcurrent protection resistor 51.

The method of emitting and extinguishing the LED 51 by the current switch circuit 54 is to compare the reference voltage V ref and the input signal v1N, and if +Vr@f is higher than ■, a current flows through the LED driving transistor 52 (a) and the LED 51 is turned off. Emits light. Conversely, when VIN is higher than V□, current flows through the LED driving transistor 52(b), so the LED 5
1 was quenched.

[Problem that the invention seeks to solve]

However, in both cases of FIGS. 5(a) and 5(b), the equivalent circuit of the LED is as shown in FIG.
ED62, a resistor R63, and an inductance L64 are connected in series, and a capacitance C61 is connected in parallel with them. Therefore, for charging and discharging the capacitance C61.

As shown in FIG. 7(b), the rise and fall (particularly the fall) of the optical waveform are rounded with respect to the input signal shown in FIG. 7(a). Time 1.
As a result, the maximum operating frequency of the LED has been limited.

Therefore, an object of the present invention is to provide a circuit that reduces the rise and fall times of the LED drive circuit, thereby reducing the bluntness of the rise and fall of the output waveform, and increasing the maximum operating frequency of the LED. This is what I did.

[Means for solving problems]

FIG. 1 is a diagram showing the basic configuration of the present invention.

In FIG. 1, 1 is an LED, 2 is an LP and D driving means for driving the LEDI, 3 is an LED overcurrent limiting means for restricting current exceeding a certain value from flowing through the LEDI, and 4 is an LED and charging/discharging control means for controlling charging/discharging of the equivalent capacity of the battery.

That is, the LED drive circuit according to the present invention has two functions in addition to conventionally used LED drive circuits.

LED on either the anode or cathode side of the LED
It is configured to be connected to charge/discharge control means 4 for controlling charge/discharge of the equivalent capacity of the battery.

[Effect]

The LED drive circuit according to the present invention is configured as described above, and in addition to conventionally used LED drive circuits, it is possible to fill the equivalent capacitance of the LEDI on either the anode side or the cathode side of the LEDI.・By connecting a circuit that can control discharge, Figure 7 (C)
(As shown in d+, it is now possible to control the charging and discharging of the output waveform, and the rise time t of the LED drive circuit,
, the fall time tf is shortened.

〔Example〕

Hereinafter, one embodiment of the present invention will be explained in detail with reference to FIG.

FIG. 2 shows one embodiment of the present invention.

In FIG. 2, the same reference numerals as in FIG. 1 indicate the same objects.

In FIG. 2, 21 is a base bias application resistor, 22
is a pre-bias current supply transistor.

23 is a base bias constant voltage diode, 24 is connected as a collector load of the pre-bias current supply transistor 22, and also serves as a bias current supply circuit 2.
5 as a whole is connected to the end of the LED driving transistor 2, that is, to the cathode side of the LED 1. The emitter of the pre-bias current supply transistor 22 is grounded via a pre-bias current limiting resistor 24. The base bias applying resistor 21 and the base bias constant voltage diode 23 are connected to the pre-bias current supply transistor 2.
2, the base voltage of the pre-bias current supply transistor 22 becomes constant, the pre-bias current supply transistor 22 is always on, and a constant bias current constantly flows through the LEDI.

To explain in more detail, 2. For example, if the maximum current flowing through the LED 1 is 100100 (for example, 100100), when the input signal VIN is at a high level, the maximum current 90 (mA) is caused to flow through the LED driving transistor 2. A bias current of 10 (mA) is made to flow through the pre-bias current supply transistor 22, which is always on regardless of whether the driving transistor 2 is on or off.On the contrary, when the LED driving transistor 2 is in the off state At the time of
In this case, a bias current of 10 (mA) flows through the pre-bias current supply transistor 22 which is always on.
It will only flow.

FIG. 3 is a diagram showing another embodiment of the present invention.

In this embodiment, the conventional current switch circuit 54 shown in FIG. 5(b) is used to cause the LEDI to emit and extinguish light, which is similar to the embodiment shown in FIG. 2.

As the collector load of the transistor, the connected L
A similar result can be obtained by connecting the bias current supply circuit 25 to the cathode side of the EDI.

FIG. 4 is a diagram showing still another embodiment of the present invention. In this embodiment, as shown in the figure, the charge/discharge control means include
It uses an emitter follower circuit.

Hereinafter, an embodiment using an emitter follower circuit will be described in detail with reference to FIG.

In FIG. 4, the same reference numerals as in FIG. 5 indicate the same objects. In the figure, 41 is an emitter follower circuit, 42 is a level exchange circuit, 43 is a gate connected to the output signal MA, and a collector is a current switch circuit 54.
It is connected to and grounded. The emitter is connected to the cathode side of the LED 51 included in the current switch circuit 54, and is connected to the power supply voltage V! via a resistor. It is connected to E. By configuring as above, LED
By passing current through the emitter follower circuit 41 to the cathode side of the LED 51 to eliminate the rounding that occurs during discharge when the LED 51 is extinguished, the discharge time of the LED is significantly shortened and the LED
51 accent is reduced.

〔Effect of the invention〕

As explained above, in addition to the conventionally used LED drive circuit, the present invention provides a charging/discharging circuit that controls the charging and discharging of the equivalent capacity of the LED on either the anode or cathode side of the LED. A discharge control means is connected. For this reason.

It is now possible to control the rise and fall of the signal waveform, which shortens the rise and fall times of the LED drive circuit, reducing the rise and fall of the output waveform.

This has the advantage that the falling edge is reduced and the maximum operating frequency of the LED is increased.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of the present invention. FIG. 2 is a diagram showing one embodiment of the present invention using a bias current supply circuit. FIG. 3 is a diagram showing another embodiment in which a current switch circuit is connected. FIG. 5 is a diagram showing still another embodiment using an emitter follower circuit. FIG. 5 is a configuration diagram of a conventional LED drive circuit, and FIG.
Figure 5 shows an example using a collector drive circuit (bl shows an example using a current switch circuit) Figure 6 shows an equivalent circuit of an LED. Figure 7 shows the output time relative to the input signal. (a) is an input signal waveform; (b) is a conventional output signal waveform; (C) is an output signal waveform using a bias current supply circuit; (d) is an output signal waveform using an emitter follower circuit; In the figure: 1...LED, 2...LED driving means, 3-...L
ED overcurrent limiting means, 4.--Charging/discharging control means. 21--Resistor for applying base bias, 22--Transistor for supplying pre-bias current, 23--Diode for base-bias constant voltage, 24--Resistor for limiting pre-bias current, 25--Bias current supply circuit. 41--Emitter follower circuit, 54--Current switch circuit, 61--Capacitance. Electricity〕Trillion Voltage A Law/$ Rito p Monthly Hara Hara, #) Origin'$ 1 Figure) 7th electric power j1 and 10th time surprise - Funeral T: 1 implementation of the present inventionj
Column ￥ 2 power> To switch times 1 and 葎粟 rτ another example cc Emi 1. ．． Tahoro 7 circuit T! Example of use τ FU 4121 (α) @ Hidaka 1 (b) Power〉 Tosui 7 times 7 minutes γ This Conventional example Conventional LED, driving date and time a-Fig. 15

Claims (1)

[Scope of Claims] 1. A light emitting diode (1) and a light emitting diode drive means (2) for driving the light emitting diode (1), and a switching operation of the light emitting diode drive means (2) causes the light emitting diode ( In the light emitting diode drive circuit for emitting and extinguishing light as described in 1), charging and discharging of the equivalent capacitance of the light emitting diode (1) included in the light emitting diode drive circuit is performed on either the anode or cathode side of the light emitting diode (1). A light emitting diode drive circuit characterized in that a charge/discharge control means (4) is connected thereto. 2. The charge/discharge control means (4) is comprised of a bias current supply circuit that constantly supplies current to the equivalent capacitance of the light emitting diode (1). Light emitting diode drive circuit. 3. The charge/discharge control means (4) controls the light emitting diode (1) when the light emitting diode (1) is turned off.
2. The light emitting diode drive circuit according to claim 1, wherein the light emitting diode drive circuit is constituted by a discharge circuit that discharges the charge accumulated in the equivalent capacitance of the light emitting diode drive circuit.