JP3083639B2 - Light emitting element drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for driving a light emitting element such as a light emitting diode at a high speed pulse.

[0002]

2. Description of the Related Art FIG. 2 shows an example of a drive circuit for causing a light emitting diode 21 to perform a pulse operation.

The drive circuit includes a bias circuit 20 for applying a bias voltage to the light emitting diode 21 and an inverter 22 for controlling the bias voltage according to an input signal. The bias circuit 20 has a configuration in which a diode 27 and resistors 25, 24, and 26 are connected in series from the power supply side of the voltage Vcc, and the resistor 26 is connected to the ground. The output terminal of the inverter 22 is connected to the connection point B between the resistors 24 and 26,
Is connected to a power supply and a connection point of the resistors 25 and 24.

When the input signal of the inverter 22 is at a low level, the voltage at the connection point B is at a high level. At this time, the light emitting diode 21 is turned off and does not emit light. When the voltage applied to both ends of the light emitting diode 21 in the forward direction is V _OFF , V _OFF is given by the following equation.

V _OFF ≒ V _D + b × (V _CC −V _BH −V _D ) b = R ₂₅ / (R ₂₄ + R ₂₅ ) where V _D is the voltage across diode 27 and V
_BH is the voltage at node B. R ₂₄ and R ₂₅ are resistance values of the resistors 24 and ₂₅ , respectively.

On the other hand, when the input signal of the inverter 22 is at a high level, the voltage at the connection point B is at a low level. At this time, the light emitting diode 21 is turned on,
Emits light. Assuming that a forward current flowing through the light emitting diode 21 is I _ON , I _ON is given by the following equation.

[0007] _{I ON ≒ (V CC -V BL} -V ') / R 24 V' = a × V ON -c × V D c = R 24 / R 25 a = (R 24 + R 25) / R 25 where V _ON is a voltage applied to both ends of the light emitting diode 21 in the forward direction, and V _BL is a voltage at the connection point B.

As described above, the light emitting diode 21 is turned on or off according to the level of the input signal of the inverter 22, and emits light in a pulsed manner.

[0009]

However, the above-described conventional configuration has a problem that it is difficult to increase the response speed of light emission.

That is, when the time constant of the circuit is τ when the light emitting diode 21 is in the off state, τ is given by the following equation.

Τ ≒ C _j × (1 / R ₂₄ + 1 / R ₂₅ ) ^-1 where τ is a junction capacitance of the light emitting diode 21.

When τ is reduced, the response is improved. Therefore, R ₂₄ or R ₂₅ may be reduced. However, when R ₂₄ is reduced, the current consumption increases, and when R ₂₅ is reduced, V _OFF decreases. Therefore, there is a problem that it is difficult to increase the response speed of light emission without increasing current consumption and without reducing V _OFF .

[0013]

In order to solve the above-mentioned problems, a light emitting element driving circuit according to the present invention has a diode and a resistor connected in series. In a light emitting element driving circuit that turns on or off a diode in accordance with a voltage level to control a bias voltage between both ends of the light emitting element to a high level or a low level to turn on or off light emission,
The above-mentioned diode is characterized by being constituted by a plurality of diodes connected in series.

[0014]

According to the above arrangement, the diode and the resistor are connected in series, the light emitting element is connected to both ends thereof, and the diode is turned on or off according to the voltage level of the input signal. In the driving circuit of the light emitting element which turns on or off the light emission by controlling the bias voltage to the high level or the low level, the light emission is turned off because the diode is constituted by a plurality of diodes connected in series. The resistance value of the above resistor can be set small without lowering the bias voltage at that time. Thus, the time constant of the circuit can be reduced, so that light emission of the light emitting element can be turned on or off at high speed.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG.

Light-emitting diode 1 (light-emitting element) of the present embodiment
The drive circuit comprises a bias circuit 10 for applying a bias voltage to the light emitting diode 1 and an inverter 2 for controlling the bias voltage according to an input signal.

The bias circuit 10 has a configuration in which a diode 7, a Schottky barrier diode 8, and resistors 5, 4, and 6 are connected in series from the power supply side of the voltage Vcc, and the resistor 6 is connected to the ground. Have been. The output terminal of the inverter 2 is connected to a connection point B between the resistors 4 and 6. The light emitting diode 1 is connected to a power supply and a connection point between the resistors 5 and 4.

In the above configuration, the Schottky barrier diode 8 and the diode 7 are connected to the inverter 2
When the input signal is low, the light emitting diode 1
Are provided to prevent a decrease in bias voltage applied in the forward direction.

When the input signal of the inverter 2 is at a low level, the voltage at the connection point B is at a high level. At this time, the light emitting diode 1 is turned off and does not emit light.
The voltage applied to both ends of the light emitting diode 1 in the forward direction is V
_{When turned off} , V _OFF is given by the following equations (1) and (2).

V _OFF ≒ (V _D1 + V _D2 ) + b ′ × (V _CC −V _BH − (V _D1 + V _D2 )) (1) b ′ = R ₅ / (R ₄ + R ₅ ) (2) Here And V _D1 and V _D2 are voltages across Schottky barrier diode 8 and diode 7, respectively.
_BH is the voltage at node B. R ₄ and R ₅ are resistance values of the resistors 4 and 5, respectively.

On the other hand, when the input signal of the inverter 2 is at a high level, the voltage at the connection point B is at a low level.
At this time, the light emitting diode 1 is turned on and emits light.

Assuming that a forward current flowing through the light emitting diode 1 is I _ON , I _ON is given by the following equation.

I _ON ≒ (V _CC −V _BL −V ′) / R ₄ (3) V ′ = a ′ × V _ON −c ′ × (V _D1 + V _D2 ) (4) c ′ = R ₄ / R ₅ (5) a ′ = (R ₄ + R ₅ ) / R ₅ (6) where V _ON is a voltage applied to both ends of the light emitting diode 1 in the forward direction, and V _BL is a connection point. B voltage.

As described above, the light emitting diode 1 is turned on or off according to the level of the input signal of the inverter 2, and emits light in a pulsed manner.

Here, the response speed of the drive circuit having the Schottky barrier diode 8 of this embodiment and the response speed of the conventional drive circuit will be compared.

Since the conventional drive circuit corresponds to a circuit in which the Schottky barrier diode 8 of the drive circuit of the present embodiment is short-circuited, the voltage applied to both ends of the light emitting diode 1 in this case and the light emission are applied. The forward currents V _OFF ′ and I _ON ′ flowing through the diode 1 can be obtained by setting V _D1 in Expressions (1) and (2) to 0, respectively.

In these drive circuits, the voltage applied to both ends of the light emitting diode 1 in the forward direction and the forward current flowing through the light emitting diode 1 are made equal.
When the light emitting diode 1 is driven, that is, when it is driven so as to satisfy V _OFF = V _OFF 'I _ON = I _ON ', when R ₄ = R ₄ ', R ₅ <R ₅ ' always holds. Here, R ₄ ′ and R ₅ ′ are the resistance values of the resistors 4 and 5 when the Schottky barrier diode 8 is short-circuited, respectively.

Therefore, according to the drive circuit having the Schottky barrier diode 8 of this embodiment,
Compared to having no drive circuit Schottky barrier diode 8, without reducing the V _OFF, R
₅ can be made smaller. That is, the time constant can be shortened. Thereby, the response speed increases. Further, even if the junction capacitance of the light emitting diode 1 is increased, a faster response speed than before can be obtained. In order to improve the responsiveness of light emission, it is desirable to use the Schottky barrier diode 8, but a normal diode may be used.

If a so-called speed-up capacitor is connected in parallel with the resistor 4, the light emitting diode 1 can be driven at a higher speed.

Further, when a drive circuit is manufactured using a lead frame, if the light emitting diode 1 using a P-type semiconductor substrate is die-bonded to a frame portion connected to a power supply terminal, three terminals (power supply terminal, ground terminal) are provided. The drive circuit and the light emitting diode 1 can be accommodated in a package of the input terminal and the input terminal. As a result, an inexpensive and compact drive circuit I with a light emitting diode 1 is provided.
C (integrated circuit) can be realized.

In the above embodiment, the light emitting diode 1 is described as the light emitting element. However, the present invention can be applied to a laser diode or the like.

[0032]

As described above, in the light emitting element driving circuit of the present invention, since the diode is constituted by a plurality of diodes connected in series, it is possible to reduce the bias voltage when light emission is turned off. The resistance value of the above resistor can be set small. As a result, the time constant of the circuit can be reduced, so that light emission of the light emitting element can be turned on or off at high speed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a light emitting diode driving circuit according to the present invention.

FIG. 2 is a circuit diagram of a conventional light emitting diode driving circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light emitting diode (light emitting element) 2 Inverter 2 5 Resistance 7 Diode 8 Schottky barrier diode

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 33/00 H01S 5/00-5/50

Claims (1)

(57) [Claims] A diode and a resistor are connected in series, a light emitting element is connected to both ends thereof, and the diode is turned on or off in accordance with a voltage level of an input signal, thereby providing a bias voltage at both ends of the light emitting element. A high-level or low-level light emitting element drive circuit for turning light emission on or off, wherein the diode is composed of a plurality of serially connected diodes. .