JPH01124011A - Led driving circuit - Google Patents

Abstract

PURPOSE:To allow the title circuit to be built in an optical transmitter as a compact circuit by controlling a transistor (TR) circuit so that a voltage applied to an LEd goes high at a high peripheral temperature or goes low at a low peripheral temperature. CONSTITUTION:The TR circuit is connected in series between the LED and a power supply to supply a constant voltage from the power supply to the LED. When the peripheral temperature is raised, respective terminal voltages of plural diodes D1-Dn in an LED temperature compensating circuit 4 are dropped, the base potential of a TR Tr3 is dropped and a difference between the base potential and a voltage determined by a Zener diode ZD is reduced. Since the base current of a TR Tr2 in an LED protection circuit 3 is increased as the result, a terminal voltage V0 is boosted and the drop of output emitted from the LED can be compensated. Namely, the voltage applied to the LED goes high at a high peripheral temperature or goes low at a low temperature. Consequently, the LED driving circuit can be built in the optical transmitter as a compact circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an LED driving circuit used in an optical transmitter used in optical digital communication.

[Conventional technology]

Conventionally, an LED driving circuit used in an optical transmitter in optical digital communication is a circuit in which a current limiting resistor R and a transistor tr are connected in series between the LED and a power source, as shown in FIG. High (formerly GH) or low (L)
ow), the transistor tr is turned on/off (ONl
oFF) This is a method that turns on/off (ONLOFF) the current flowing to the LED, and the supplied power supply voltage is often 5 ± 0.25 V, which is shared with other digital devices or digital ICs. The degree varies.

[Problem that the invention seeks to solve]

On the other hand, the characteristics of LEDs include that the light emitting output of the LED has a negative temperature coefficient as shown in FIG. 6, and that it is weaker against surge currents than other digital ICs. For the former problem, as shown in Fig. 7(a), a thermistor R5 having a negative temperature coefficient in the current limiting resistance is used.
etc., and for the latter problem, the seventh
As shown in Figure (b), a surge absorber S8 or the like was attached externally, or a simple CR filter was built inside the optical transmitter. In addition, to deal with fluctuations in the power supply voltage, a power supply stabilizing circuit dedicated to the transmitter was provided on the outside as shown in FIG. 7(C). However, in the past, there was no effective way to compactly combine the above three measures into a single circuit, and the equipment became larger.

[Means for solving problems]

The present invention has been made to solve the above problems, and in an LED drive circuit in which a transistor circuit is connected in series between an LED and a power source, and a constant voltage is supplied from the power source to the LED, a fixed resistor is used. and a resistive element having a negative temperature coefficient, the transistor circuit is controlled based on the terminal voltage of the resistive element obtained by applying a voltage to a series circuit of the resistive element and the resistive element having a negative temperature coefficient,
This device is characterized in that the voltage supplied to the LED is high when the ambient temperature is high and low when the ambient temperature is low.

Embodiments of the present invention will be described in detail below with reference to FIG.

The LED drive circuit of the present invention includes the present circuit 1 and the power supply stabilization circuit 2.
, and an LED protection circuit 3, which are connected in parallel. This circuit 1 has a current limiting resistor R, an LED, and a transistor Tr4 connected in series to provide a constant voltage. is on. In the power supply stabilizing circuit 2, a Zener diode Z is connected between the emitter side of the transistor TP3 and the ground, a resistor R2 is connected between the emitter side and the positive potential, and a resistor R and a plurality of negative temperature coefficients are connected on the base side. The LED temperature compensation circuit 4, which is formed by connecting diodes D, , D, , . It is connected to the base side of the transistor Tr2 of the circuit 3. L.E.
D protection circuit 3 consists of Darlington-connected transistors T
□, Tr2, resistor R1, and capacitor C1. Capacitor C3 is connected between the base side of transistor Tr2 and ground, and resistor R is connected between the base side of transistor Tr2 and a positive potential. Next, the LE of the present invention
The operation of the D drive circuit will be explained. LED protection circuit 3
is a circuit whose purpose is to protect the LED of this circuit l from surge current, and if the input voltage suddenly rises, C
The base potential of transistor Tr2 rises with the time constant of ,,R, and the emitter output potential of transistor T,1 becomes C+,R.
It rises slowly in a time determined by the + time constant. The power supply stabilization circuit 2 has the terminal voltage of this circuit 1 ■. This is a circuit for keeping constant the LED temperature compensation circuit 4, which is made up of a resistor R4 and diodes D5, D2, ...D, l having a plurality of negative temperature coefficients connected in series, and a resistor R3. A voltage divided by resistance is applied to the base of the transistor Trl, and a control current inversely proportional to the difference between this base potential and the voltage determined by the Zener diode ZD flows to the base of the transistor Tr2 in the LED protection circuit 3. Therefore, the terminal voltage ■.

As soon as the voltage increases, a commensurate current decrease occurs, so the terminal voltage ■ can always be kept constant. Furthermore, as described in the section on the prior art, the light emitting output of the LED has a negative temperature coefficient as shown in FIG. 6, and as the ambient temperature rises, the light emitting output of the LED decreases. LED
The temperature compensation circuit 4 is for compensating for this. When the ambient temperature rises, multiple diodes D1, D2,...
...Each terminal voltage of D□ V□, ■. 2...■, I decreases, the base potential of the transistor Trff decreases,
The difference between this base potential and the voltage determined by the Zener diode ZD becomes smaller. Then, the base current of the transistor Tr2 in the LED protection circuit 3 increases, so that the terminal voltage (2) increases. increases to compensate for the decrease in the light emitting output of the LED.

2 to 4 are LED driving circuit diagrams showing other embodiments, in which a thermistor T or a Zener diode Z is used instead of the diode in the LED temperature compensation circuit 4, as shown in FIGS. 2 and 3.

It is also possible to use FIG. 4 is an example of an LED drive circuit in which the transistor is changed from npn type to pnp type.

〔Effect of the invention〕

As explained above, the LED drive circuit of the present invention connects a transistor circuit in series between the LED and the power supply, and supplies a constant voltage from the power supply to the LED. The transistor circuit is controlled based on the terminal voltage of the resistive element obtained by applying a voltage to a series circuit with the resistive element, and the voltage supplied to the LED is increased when the ambient temperature is high and when it is low. Since it is made to be low, the light emission output is stable against fluctuations in power supply voltage and ambient temperature, and it can be built into the optical transmitter itself as a compact circuit that can simultaneously realize LED protection.

[Brief explanation of the drawing]

Figures 1 to 4 are LED drive circuits of the present invention, Figure 5 is a conventional LED drive circuit, Figure 6 is a relationship between LED light output and temperature, and Figure 7 (a) is LED temperature compensation. FIG. 7(b) is a conventional surge current absorption circuit diagram, and FIG. 7(C) is a conventional power supply stabilization circuit diagram. 1 - Wood circuit, 2 - Power supply stabilization circuit, 3 - LED protection circuit, 4 - LED temperature compensation circuit. Patent applicant Furukawa Electric Co., Ltd. Figure 1 O Figure 2 Figure 7

Claims (1)

[Claims] In an LED drive circuit that connects a transistor circuit in series between the LED and a power supply and supplies a constant voltage from the power supply to the LED, voltage is applied to a series circuit of a fixed resistor and a resistance element with a negative temperature coefficient. controlling the transistor circuit based on the terminal voltage of the resistance element obtained by
An LED drive circuit characterized in that the voltage supplied to the LED is high when the ambient temperature is high and low when the ambient temperature is low.