JPH01310580A - Drive circuit for light emitting element - Google Patents

Abstract

PURPOSE:To enable a light emitting diode to be stabilized an intensity of emitted light through a wide range of temperature by a method wherein a control circuit composed of a drive switch element and a control switch element which makes the drive switch element ON or OFF is provided inside a power circuit of the light emitting diode. CONSTITUTION:A drive transistor Tr1 is provided inside a power circuit of an LED composed of a power source E, a light emitting diode LED and a resistor R1, and concurrently a control circuit 2 composed of a control transistor Tr2, a diode array 1 consisting of two or more diodes D1-D3 connected with each other in a forward direction and a resistor R2 is provided in parallel with the power source E. The above components are so structured as to make switching of the drive switch element Tr1 corresponding to a potential of a connecting point a of the diode array 1 with the resistor R2 inside the control circuit 2, and the control voltage of the drive switch element Tr1 is made to change proportionally through temperature. By these processes, a light emitting diode can be stabilized in intensity of emitted light in a wide range of temperature as compensating the temperature change without using a special temperature compensating element.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a light emitting element driving circuit that provides temperature compensation means when driving a light emitting element.

In general, in light emitting devices such as light emitting diodes, the recombination of excess carriers during light emission is dependent on temperature, so there is a problem that the light emission intensity decreases as the temperature rises. .

Therefore, conventionally, a special thermistor is installed in the power supply circuit of the light emitting diode, which exhibits negative resistance characteristics up to a certain temperature and positive resistance characteristics above that critical temperature. The drive current flowing through the light emitting diode is increased by the amount that the thermistor's resistance decreases, suppressing the drop in light emission intensity, and when the temperature exceeds the critical temperature, the thermistor's resistance increases rapidly to protect the light emitting diode from excessive current. A light emitting element driving circuit has been developed (recognized in Japanese Utility Model Publication No. 62-25838).

However, in such a device, the thermistor directly controls the driving current of the light emitting diode to compensate for the temperature, so the resistance of the thermistor becomes too small, causing an excessive current to flow through the light emitting diode. In order to prevent destruction, the critical temperature (approximately 80 degrees Celsius) is
It is necessary to use a thermistor with special temperature-resistance characteristics whose characteristics change with the In addition, the operating temperature range in which the light emitting intensity of the light emitting diode is actually stable is 20 to 80 °C.
It is narrow, about ℃.

The present invention has been made in consideration of the above points, and it is possible to effectively compensate the temperature of the light emitting element over a wide temperature range and increase the luminous intensity of the light emitting element without using any special temperature compensation element. An object of the present invention is to provide a light emitting element drive circuit that can stably hold the light emitting element.

! In order to achieve the object, the present invention provides a driving switch element in the power supply circuit of the light emitting element, which can proportionally control the current flowing through the light emitting element depending on the magnitude of the control voltage when turned on, and also provides a switch command. The drive switch element is turned on according to the

A control circuit consisting of a control switch element for turning off is provided, and means is provided in the control circuit to vary the control voltage of the drive switch element proportionally depending on the temperature.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In the light emitting element drive circuit according to the present invention, as shown in FIG.
A drive transistor Tri is provided in the LED power supply circuit consisting of the LED, and a control transistor Tri is provided in parallel with the DC power source E. Multiple diodes D1 to D in the forward direction
A diode array for temperature compensation to which 3 is connected 1. A control circuit 2 consisting of a resistor R2 is provided, and the driving transistor Tri is switched in accordance with the potential at a point a, which is a connection point between the diode array 1 and the resistor R2 in the control circuit 2. has been done.

A diode array l for temperature compensation is arranged near the LED so as to be directly affected by the heat of the LED.

Moreover, on the base side of control 1 to transistor Tr2.

An input circuit 3 is provided for turning on and off the jtl* official I-transistor Tr2 in response to an externally supplied LED lighting/lighting-off command S.

However, in the light emitting element drive circuit configured in this way, when the external command S given to the input circuit 3 becomes a low level, which is an LED lighting command, the control 1 to transistor Tr
2 turns on, thereby supplying the base current to the driving transistor 1-tri, which turns on and turns LE
D will light up.

At this time, since the diode array l for temperature compensation is provided in the control circuit 2, each diode D1 in the diode array 1 increases as the temperature of the LED increases.
~The resistance of D3 decreases, thereby driving I/transistor T!・The base potential of point 1, that is, the potential of point a, increases proportionally.

Therefore, as the temperature of the LED increases, the driving 1
The current IF flowing through the helan sister Tr'l increases, and thereby the drive current Io flowing through the LED increases, and the light emission intensity of the LED can be held almost constant without decreasing.

Here, when the emission intensity of the LED is Pw, Pw father I
The relationship o holds true.

Now, the power supply of DC power supply E is set to 7 days, the collector saturation voltage of control transistors 1 to Tr2 is V (Esa), each forward voltage of diodes D1 to D3 is vF, and the number of diodes installed in diode array 1 is n, and when the base-collector voltage of the driving transistor Tri is v8., it is obtained as follows: 0=1F×α.

Therefore, the temperature change aIo/F3T of the drive current Io, which is proportional to the light emission intensity Pw of the LED, is given by the following equation.

here, Therefore, aIo/21T>O.

Therefore, by changing the number of diodes installed in diode array 1, in equation l), r+-)VF
/ '; The term jT and the term n-Vp are heard, and the LED
It becomes possible to finely adjust the temperature coefficient 2+Io/aT of the drive current (2)0, and as a result, it becomes possible to maintain the light emission intensity of the LED almost constant over a wide temperature range.

In Figure 2, assuming E=5V, R1=l 50Ω, 5 r
It shows the temperature characteristics of the optical output PO of the ED taken out by the optical fiber cable of n. Here, the difference between the maximum value and the minimum value of the optical output Po with respect to the temperature T is 2 dB or less.

In addition, Fig. 3 and Fig. 4 show the IF and V at that time.
The temperature characteristics of F are shown respectively.

In addition, Fig. 5 shows E''5V, R1= ]-50Q when a resistor is provided instead of the diode array l.
, shows the temperature characteristics of the optical output Po of the LED taken out through a 5 m optical fiber cable. here,
The difference between the maximum value and the minimum value (direct) of the optical output PO with respect to the temperature T is 3 dB or more.

In addition, since a parasitic capacitance exists in an LED, the parasitic capacitance is charged while the driving current 0 flows through the LED and it emits light. The LED continues to emit light for a while due to the discharge charge of the capacitor, and therefore, the shortening of the pulse period of the LED lighting/lighting out command S in the input circuit 3 is limited by the discharge time of the parasitic capacitance of the LED. It has become.

Therefore, in the light emitting element drive circuit according to the present invention, a speed-up circuit in which a resistor R5 and a capacitor C are connected in parallel is provided in the input circuit 3.

Therefore, at the rise of the pulse of the LED lighting/lighting-off command S, the charging current of the capacitor C is transferred to the control transistor Tr.
The base of 2 is strongly overdriven, and at the falling edge of the pulse, the discharge current of the capacitor C or the reverse base current will equivalently reduce the storage time of the parasitic capacitance connected in parallel with the LED.

Therefore, it is effectively suppressed that the LED continues to be lit due to the discharge current of the parasitic capacitance when the LED is turned off, and the frequency response characteristics according to the LED lighting and lighting-off commands S are improved, and the LED is turned on.

It becomes possible to turn off the light at high speed.

Further, FIG. 6 shows another embodiment of the present invention, in which, unlike the case of FIG. 1, the driving transistor T
By using an npn type transistor as r 1 and a pnp type transistor as the control transistor T r 2, when the lighting/lighting out command S in the input circuit 3 becomes high level, the control transistor T r 2 is activated. The transistor T r 1 is turned on and the LED is driven to light up.

In this circuit configuration, Io"IFXα given as.

Therefore, the temperature coefficient aIo/of the LED drive current ■0 is
θT is expressed as follows.

That is, in this case as well, by changing the number of diodes installed in the diode array 1, the terms of n-a VF / a T and n-VF are heard in equation (4), and the LED drive current Io temperature coefficient F3 I
o / B T can now be finely adjusted,
As a result, it becomes possible to maintain the light emission intensity of the LED almost constant over a wide temperature range.

Note that the light emitting element driving circuit according to the present invention is not limited to the above-mentioned embodiments, but is shown in FIGS. 1 and 6.
In each of the circuit configurations shown in the figure, instead of using diode array 1, the same effect can be achieved by providing a thermistor whose resistance value decreases as the temperature rises at the same location. .

In this case, compared to the conventional case where the thermistor is provided directly in the power supply circuit of the LED, the resistance value of the thermistor becomes smaller, but if the drive current Iθ of the ED becomes too large, the LED may be destroyed. There will be no more problems.

Also, from the relationship in equation (2), 0Vc1- sa
By changing t/c)T, it becomes possible to finely adjust the temperature coefficient aTo/c)T of the LED drive current Io.

However, in each circuit configuration of FIGS. 1 and 6,
Instead of using diode array 1.

A similar effect can be produced by using a Darlington-connected control transistor Tr2 and setting 9Vcv=sat/ )T to an appropriate value.

(1) In the light-emitting element drive circuit according to the present invention, a driving switch element is included in the power supply circuit for the light-emitting element, which can proportionally control the current flowing through the light-emitting element depending on the magnitude of the control voltage when turned on. In addition, a control circuit consisting of a control switch element that turns on and off the drive switch element in accordance with a switch command is provided, and the control circuit controls the control voltage of the drive switch element according to the heat generation temperature of the light emitting element. This device uses a means to proportionally change the temperature of the light emitting element over a wide temperature range without using any special temperature compensation elements, and stabilizes the light emission intensity of the light emitting element. It has the excellent advantage that it can be maintained at a high temperature and that the circuit can be effectively integrated into an IC.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing one embodiment of the light emitting element driving circuit according to the present invention, FIG. 2 is a diagram showing an example of the temperature T-light output Po characteristic of the LED in the same embodiment, and FIG. 3 is a diagram showing the same embodiment. FIG. 4 is a diagram showing the T-VF characteristics in the example, and FIG. 5 is the temperature T- of LED when a low resistance is provided instead of the diode array. A diagram showing an example of the optical output Po characteristic, and FIG. 6 is an electric circuit diagram showing another embodiment of the present invention.

Claims (1)

[Claims] 1. A driving switch element is provided in the power supply circuit of the light emitting element, which can proportionally control the current flowing through the light emitting element depending on the magnitude of the control voltage when it is turned on, and a driving switch element is provided in the power supply circuit of the light emitting element. A control circuit consisting of a control switch element that turns on and off the drive switch element according to the temperature is provided, and the control circuit takes means for proportionally changing the control voltage of the drive switch element depending on the temperature. A light emitting element drive circuit characterized by: 2. The description of item 1 above, characterized in that a diode array for temperature compensation is provided in the control circuit consisting of the control switch element as a means for proportionally changing the control voltage of the drive switch element depending on the temperature. light emitting element drive circuit. 3. According to the above item 1, characterized in that a thermistor for temperature compensation is provided in the control circuit consisting of the control switch element as a means for proportionally changing the control voltage of the drive switch element depending on the temperature. Light emitting element drive circuit. 4. The light emitting element drive circuit according to item 1 above, characterized in that a Darlington-connected control switch element is used as a means for proportionally changing the control voltage of the drive switch element depending on temperature.