JPH06152026A - Driving circuit for light emitting element - Google Patents

Abstract

PURPOSE:To provide a sufficient extinction ratio even with the non-linear characteristics of a light emitting element. CONSTITUTION:A driving circuit for a light emitting element is provided with a clip circuit 14, which clips input signals of a certain value or less, a voltage/ current converting circuit 15, which converts the clipped input voltage into current, a light emitting element 11 which is supplied with the converted current as driving current, a pulse width modulation signal forming circuit 19 which generates a pulse width modulation signal based on the input voltage and a switching circuit 16 which switches the driving current to the light emitting element side and to the bypass circuit side based on the pulse width modulation signal. When a voltage of an input voltage equivalent to the minimum driving current to be the non-linear area of the light emitting element or more is inputted, the light emitting element is driven based on the converted current in proportion to the input signal, and when the input voltage is a boundary voltage or less, the light emitting element is driven by the converted current whose pulse width is modulated by the input signal voltage level and a sufficient extinction ratio is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting element drive circuit suitable for driving a light emitting element provided in a printer section of a laser beam printer.

[0002]

BACKGROUND OF THE INVENTION In a printer section (engine section) of a laser beam printer, an image signal read by a scanner section is used as an input signal and converted into an optical signal to form an image on a photoreceptor (latent). A driving circuit for the light emitting element is provided for imaging.

A semiconductor laser diode (LD) or the like is usually used as the light emitting element. This laser diode exhibits the diode characteristics as shown in FIG. 3, and the relationship between the drive current supplied to the laser diode and the light emission amount (light output) PL that is excited by this and emits light is an exponential function characteristic as shown in the figure. (Non-linear characteristic).

On the other hand, in order to drive the light emitting element efficiently, as is well known, a sufficient extinction ratio is required. The extinction ratio is the ratio (= Pmin / P) of the minimum controllable light emission amount (light power) Pmin to the maximum controllable light emission amount Pmax.
max).

[0005]

The laser diode exhibits a nonlinear diode characteristic as shown in FIG. 3, and the extinction ratio in analog drive only in the linear region depends on the diode characteristic of the laser diode used. The extinction ratio cannot be improved without using a wide laser diode. If it can be used in the non-linear region in the same manner as in the linear region by some means, it is possible to obtain an apparently sufficient extinction ratio.

Therefore, the present invention has been made to solve such a conventional problem and proposes a drive circuit for a light emitting element capable of ensuring a sufficient extinction ratio.

[0007]

In order to solve the above-mentioned problems, in the invention described in claim 1, a clipping circuit for clipping a value of an input signal equal to or less than a certain value and the clipped input voltage is converted into a current. A voltage / current conversion circuit, a light emitting element to which this conversion current is supplied as a drive current, a pulse width modulation signal generation circuit for generating a pulse width modulation signal based on the input voltage of the input signal, and its pulse width modulation signal The switching circuit that switches the drive current to the light emitting element side and the bypass circuit side based on, the input voltage corresponding to the minimum drive current in the nonlinear region of the light emitting element is detected in the pulse width modulation signal creation circuit, When the input voltage is higher than the boundary voltage, the light emitting element is driven based on the conversion current proportional to the input signal, and the input voltage is lower than the boundary voltage. When the pressure is characterized in that a pulse width modulated conversion current by the input signal voltage level at the light emitting element is made to be driven.

According to another aspect of the present invention, a clipping circuit that clips a value of the input signal equal to or less than a certain value, a first voltage-current conversion circuit that converts the clipped input voltage into a current, and the conversion current described above. Is supplied as a drive current, a light receiving element for receiving light from the light emitting element,
A current / voltage conversion circuit for converting the monitor current flowing through the light receiving element into a voltage, a differential amplifier for obtaining a differential output between the input signal and the monitor voltage corresponding to the monitor current, and this differential output A second voltage-current conversion circuit that converts the current into a current and supplies the light emitting element as a drive current, a pulse width modulation signal generation circuit that generates a pulse width modulation signal based on the input voltage of the input signal, and It is composed of the light emitting element based on the pulse width modulation signal and a switching circuit for switching the current to the bypass circuit side.In this pulse width modulation signal creating circuit, the input voltage corresponding to the minimum drive current in the non-linear region of the light emitting element. Is detected,
When the input voltage is equal to or higher than the boundary voltage, the light emitting element is driven based on the conversion current proportional to the input signal,
When the input voltage is equal to or lower than the boundary voltage, the light emitting element is driven by the conversion current pulse width modulated by the input signal voltage level at that time.

[0009]

When the invention described in claim 1 is explained,
As shown in FIGS. 1 and 3, in the linear region Wa as the drive current flowing through the laser diode 11, the converted current obtained by converting the input voltage into the current is used as the drive current as in the conventional case.

In the non-linear region Wb, the input voltage is pulse width modulated, and the laser diode 11 is driven by this pulse width modulation current. At this time, as shown in FIG. 4, with reference to the minimum input voltage Vmin in the non-linear region, this input voltage Vm
When the input voltage is Vmin / 2, the minimum drive current Imin is flown for the unit time T, whereas 1/2 of the minimum drive current Imin is used as the drive time at that time when the input voltage becomes Vmin / 2. To do.

When the input voltage becomes Vmin / 4, 1/4 of the driving time T of the minimum driving current Imin is used as the driving time at that time.

If the supply time of the drive current is modulated according to the input voltage as described above, the light emission amount of the laser diode 11 becomes linear with respect to the drive current whose pulse width is controlled.
If the driving current does not flow at all when the input voltage is zero, the extinction ratio of the laser diode 11 can be made infinite in principle.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an example of a drive circuit for a light emitting device according to the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of a drive circuit 10 for a light emitting element according to the present invention, and 11 is a semiconductor laser diode which is a light emitting element. An input signal such as an image signal is supplied to the terminal 13, and the clip circuit 14 clips the input signal at a predetermined input voltage or less.

As will be described later, the clipped input voltage is converted into a current I proportional to the input voltage V by the voltage / current conversion circuit 15 as shown in FIG. 2, and this converted current is converted into a semiconductor laser. Diode 11
Drive current.

When a drive current flows through the laser diode 11, the drive current excites the laser diode 11 to emit light according to the diode characteristics shown in FIG. Since the area Wa is a linear area, the light emission amount (light power) PL is proportional to the drive current. Here, the above-mentioned clip voltage is the input voltage Vmin that provides the minimum current value Imin.

The input signal is further detected by the input voltage detection circuit 18
The input voltage Vmin that is supplied to the input terminal and has the minimum current value Imin is detected. An input voltage equal to or lower than the input voltage Vmin is output from the detection circuit 18 and supplied to the pulse width modulation signal generation circuit 19 in the subsequent stage to generate a pulse width modulation signal corresponding to the input voltage as shown in FIG.

The switching circuit 16 is controlled by the pulse width modulation signal. A bypass circuit 17 is connected to the switching circuit 16 in addition to the laser diode 11, and switching control as described below is performed according to the value of the input voltage.

That is, as shown in FIGS. 2 and 3, when the conversion current when the input voltage is converted into the current, that is, the drive current is within the linear region Wa, the laser diode 11 is switched to the nonlinear region Wb. At some time, switching control is performed according to the pulse width modulation signal, and the drive current I flows to either the laser diode 11 side or the bypass circuit 17 side.

Therefore, with reference to the minimum input voltage Vmin which is the non-linear operation region Wb of the laser diode 11, the minimum drive current Imin is supplied for a unit time T when the input voltage Vmin is applied (see FIG. 4A). For example, when the input voltage becomes Vmin / 2, 1/2 (= Ta) of the minimum drive current Imin with respect to the drive time W is used as the drive time t at that time to flow the drive current Imin (FIG. 4B).

When the input voltage becomes Vmin / 4, 1/4 (= Wa) of the minimum drive current Imin to the drive time T
Is used as the drive time t at that time, and in this case also, Imin is used as the drive current (FIG. 4C). The unit time T is a time given for driving one pixel, and in this example, T = 100 nsec.

When an input signal equal to or lower than the input voltage Vmin which is within the non-linear operation region Wb of the laser diode 11 is applied as described above, instead of supplying a constant drive current Imin to the laser diode 11, the laser diode 11 is supplied. The driving time t is modulated (pulse width modulation) according to the input voltage.

Since the light emission amount of the laser diode 11 has a linear characteristic with respect to the drive current whose pulse width is controlled, the laser diode 11 can be used as a linear characteristic up to the non-linear region Wb. Therefore, if the pulse width modulation is performed so that the drive current I does not flow at all when the input voltage V is zero, the extinction ratio of the laser diode 11 can be made infinitely small in principle.

As described above, since the laser diode 11 is made of a semiconductor, it has temperature characteristics. Figure 5
Shows an example of a temperature characteristic, a curve La is a characteristic curve at 0 ° C., a Lb is a characteristic curve at 25 ° C. (normal temperature), and a Lc is a characteristic curve at 50 ° C.

With the temperature characteristics, when the ambient temperature fluctuates, the same amount of light emission does not occur even if the same drive current is passed. For that purpose, although not shown, a temperature control circuit or a computer may be prepared and controlled so that the same amount of light is emitted when the same drive current is passed.

As another means for obtaining the same amount of light emission for the same level of the input signal despite changes in the ambient temperature, the amount of light emitted by the laser diode is constantly monitored and the same level of the input signal is obtained. Can also be configured as a drive circuit having a feedback loop that produces the same amount of light emission.

FIG. 6 shows an example of a drive circuit 10 for a light emitting element having such a monitor function, which is applied to the embodiment of FIG.

The drive circuit 10 is provided with a photodiode (PD) 12 as a light receiving element for detecting the amount of light emitted from the laser diode 11, as shown in the figure.

The input signal supplied to the terminal 13 is converted into the drive current I in the voltage / current conversion circuit 15. The laser diode 11 is excited by the drive current I, and the light from the laser diode 11 is emitted by the photodiode 12
Photoelectric conversion is performed by, and a monitor current corresponding to the received light amount PL flows through the photodiode 12. The monitor current is converted into a monitor voltage by the current / voltage conversion circuit 25.

The input signal supplied to the terminal 13 is supplied as a reference signal to the differential amplifier 22 together with the monitor voltage, and its differential output is supplied to the second voltage / current conversion circuit 23.
Is converted into a current and is supplied to the adder 24 as a correction current. Therefore, the laser diode 11 is driven by the drive current and the correction current related to the input signal.

By this negative feedback control by the correction current,
Even if the input / output characteristics of the laser diode 11 change due to temperature characteristics, aging, and secular changes, it is possible to realize driving that always corresponds to the input signal.

[0032]

As described above, in the light emitting element drive circuit according to the present invention described in claims 1 and 2, the drive current to the light emitting element is pulsed in the nonlinear area so that the nonlinear area of the light emitting element can be used. It is configured to control the width.

According to this, since it can be used in the non-linear region of the light emitting element in the same manner as in the linear region, a sufficient extinction ratio can be realized and the dynamic range of the input signal that can be handled can be made wider than in the conventional case. It has features that can be greatly improved.

[Brief description of drawings]

FIG. 1 is a system diagram showing an example of a drive circuit for a light emitting element according to the present invention.

FIG. 2 is a characteristic curve diagram of a voltage / current conversion circuit.

FIG. 3 is a characteristic diagram showing a relationship between a drive current of a light emitting element and a light emission amount at that time.

FIG. 4 is an explanatory diagram of pulse width modulation.

FIG. 5 is a characteristic diagram showing input / output characteristics of a light receiving element.

FIG. 6 is a system diagram showing another example of a drive circuit for a light emitting element according to the present invention.

[Explanation of symbols]

 10 Drive Circuit 11 Laser Diode (Light-Emitting Element) 12 Photodiode (Light-Receiving Element) 15, 25 Voltage / Current Conversion Circuit 19 Pulse Width Modulation Circuit 18 Input Voltage Detection Circuit 17 Bypass Circuit

Claims (2)

[Claims] 1. A clipping circuit that clips an input signal below a certain value, a voltage-current conversion circuit that converts the clipped input voltage into a current, and a light-emitting element to which the converted current is supplied as a drive current. A pulse width modulation signal generation circuit that generates a pulse width modulation signal based on the input voltage of the input signal, and a switching circuit that switches the drive current to the light emitting element side and the bypass circuit side based on the pulse width modulation signal. In the pulse width modulation signal generation circuit, an input voltage corresponding to the minimum drive current, which is a non-linear region of the light emitting element, is detected. When the input voltage is equal to or higher than the boundary voltage, the conversion current proportional to the input signal is used. When the light-emitting element is driven by the above, and the input voltage is less than or equal to the boundary voltage, the pulse width is converted by the input signal voltage level at that time. A driving circuit for a light emitting element, wherein the light emitting element is driven by an electric current. 2. A clipping circuit for clipping an input signal below a certain value, a first voltage-current conversion circuit for converting the clipped input voltage into a current, and light emission in which the above-mentioned converted current is supplied as a drive current. An element, a light receiving element for receiving light from the light emitting element, a current / voltage conversion circuit for converting a monitor current flowing through the light receiving element into a voltage, and a differential between the input signal and the monitor voltage corresponding to the monitor current. A differential amplifier for obtaining an output, a second voltage-current conversion circuit for converting the differential output into a current and supplying it to the light emitting element as a drive current, and a pulse based on the input voltage of the input signal. A pulse width modulation signal generation circuit that generates a width modulation signal, and a switching circuit that switches the current to the light emitting element and the bypass circuit side based on the pulse width modulation signal. In this pulse width modulation signal generation circuit, the input voltage corresponding to the minimum drive current which is the non-linear region of the light emitting element is detected, and when the input voltage is higher than the boundary voltage, the conversion current proportional to the input signal is obtained. The light emitting element is driven based on the above, and when the input voltage is less than or equal to the boundary voltage, the light emitting element is driven by a conversion current pulse width modulated by the input signal voltage level at that time. A drive circuit for a light emitting element.