JPS6386584A - Semiconductor laser output control equipment - Google Patents

Abstract

PURPOSE:To prevent the deterioration of a semiconductor laser excluding excessive current flow in the semiconductor laser, by driving the semiconductor laser with the output signal of an error amplifier after sample-hold applying a timing signal, and prohibiting the timing signal by a protection circuit when the output signal of a light detection circuit is smaller than or equal to a specified value. CONSTITUTION:A photo-diode generates a current proportional to the light output of a semiconductor laser 1, and an error amplifier 4 amplifies the difference between the output voltage VM of an amplifier 3 and a reference voltage Vref. This value is subjected to sample-hold by a timing signal in a sample-hold circuit 5. A driving circuit 6 for semiconductor laser flows a current corresponding with the output signal of the sample-hold circuit 5 into the semiconductor laser 1. Thereby the driving current of the semiconductor laser 1 is stabilized as a value corresponding with the reference voltage Vref, and the light output of the semiconductor laser 1 is controlled so as to be constant. When the light output of the semiconductor laser 1 is less than or equal to a specified value, and the output signal of the amplifier 3 is less than or equal to the threshold level VTH, a gate 7 turns OFF, the timing signal is prohibited, and the sampling operation of the sample-hold circuit 5 is interruped.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a semiconductor laser output control device that stabilizes the output of a semiconductor laser used in laser printers, laser facsimile machines, and the like.

(Prior art) Conventionally, as a semiconductor laser output control device, the optical output of the semiconductor laser is detected by a photodetection circuit, and compared with a reference value by a comparator. Based on this result, an up/down counter is incremented or decremented to calculate the count value. There is a known device that causes a current to flow through a semiconductor laser according to the current. It has also been proposed to detect the optical output of a semiconductor laser with a photodetector circuit, amplify the error from the reference value with an error amplifier, sample and hold the output using a timing signal, and drive the semiconductor laser with the output. .

However, in the latter case, if the sample and hold circuit performs a sample when the semiconductor laser is not lit, an excessive current flows through the semiconductor laser and the semiconductor laser deteriorates.

(Objective) An object of the present invention is to provide a semiconductor laser output control device that can eliminate the above-mentioned drawbacks and prevent deterioration of a semiconductor laser.

(Structure) The present invention has a photodetection circuit, an error amplifier, a sample and hold circuit, a semiconductor laser drive circuit, and a protection circuit. amplify the error of

The output signal of this error amplifier is then sampled and held using a timing signal in a sample and hold circuit, and the semiconductor laser is driven by a semiconductor laser drive circuit according to the output signal. Sometimes the above timing signals are inhibited.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 shows the basic circuit configuration of an embodiment of the present invention, and FIG.
The figure is a tamishing chart.

A semiconductor laser 1 is used as a light source in a laser printer or the like, and a photodiode 2 detects the light output emitted from the rear thereof. The photodiode 2 and the amplifier 3 constitute a photodetection circuit. The photodiode 2 generates a current proportional to the optical output of the semiconductor laser 1, and the amplifier 3 converts this current into a voltage. The difference between the output signal VM of the amplifier 3 and the reference voltage Vref is amplified by the error amplifier 4, and sampled and held by the sample and hold circuit 5 according to a timing signal. A corresponding current is passed through the semiconductor laser l. Therefore, the drive current of the semiconductor laser 1 is stabilized to a value corresponding to the reference voltage V ref, and the optical output of the semiconductor laser 1 is controlled to be constant. Further, the semiconductor laser drive circuit 6 modulates (turns on/off) the semiconductor laser 1 with a modulation signal including a video signal, but the modulation signal is passed through the gate 1 to the sample hold circuit 5 as the timing signal (sample signal). Added.

Then, the output signal VM of the amplifier 3 is compared with a predetermined threshold level VTH by a comparator 8 to determine whether it is less than this threshold level VT)1, and the output signal is applied to the gate 7. Therefore, when the optical output of the semiconductor laser 1 is below a predetermined value, the output signal of the amplifier 3 is at the threshold level V.
When the voltage is below TH, the gate 7 is turned off, the second timing signal is inhibited, and the sample and hold circuit 5 is inhibited from sampling. Therefore, when the semiconductor laser 1 is not lit, sampling by the sample and hold circuit 5 is prohibited, so that no excessive current flows through the semiconductor laser 1, and deterioration of the semiconductor laser 1 is prevented.

FIG. 7 specifically shows the circuit of FIG. 5.

The amplifier 3 is composed of an operational amplifier 9 and a resistor 10, and the error amplifier 4 is composed of an operational amplifier 11 and resistors 12-15. Sample hold circuit 5 is sample switch 1
6. The semiconductor laser drive circuit 6 is composed of a hold capacitor 17 and a resistor 18, and the semiconductor laser drive circuit 6 includes a field effect transistor 19.
, a driver 20 and a resistor 21. driver 2
0 turns on/off the semiconductor laser 1 by a modulation signal,
The field effect transistor 19 causes a current to flow through the semiconductor laser 1 in accordance with the output signal of the sample and hold circuit 5 when the semiconductor laser 1 is turned on.

FIG. 1 shows an embodiment of the invention.

In this embodiment, a circuit 23 for determining the presence or absence of the modulation signal and an OR circuit 24 are provided in the circuit shown in FIG.
The signal is applied to the sample and hold circuit 5 via.

The determination circuit 22 includes, for example, a counter 24 as shown in FIG.
and a flip-flop 25, the counter 24 is reset to 1 at the rise of the modulation signal, and the flip-flop 25 is reset at the rise of the modulation signal. When the counter 24 counts a predetermined number of clocks and outputs an output signal, this signal causes the flip-flop 25 to
is set, and the output signal of the flip-flop 25 is outputted to the sample and hold circuit 5 via the OR circuit 23 as a timing signal. Therefore, the sample and hold circuit 5 receives the timing signal when there is a modulation signal (excluding the initial predetermined time by the counter 24), samples the output signal of the error amplifier 4, and controls the output of the semiconductor laser 1.

The determination circuit 22 is a one-shot circuit 2 as shown in FIG.
6 and an AND circuit 27.

The one-shot circuit 26 is triggered by the rising edge of the modulation signal to generate a negative pulse, and this pulse and the modulation signal are ANDed by an AND circuit 27. This AND circuit 27
The output is generated when a modulation signal is input (except when an output pulse is generated from the one-shot circuit 2G), and is output to the sample and hold circuit 5 through the OR circuit 23.

FIG. 4 shows another embodiment of the invention.

In this embodiment, a circuit 28 consisting of a judgment circuit 22 and an OR circuit 23 is provided between the comparator 8 and the gate 7 in the above embodiment, and the output signals of the comparator 8 and the judgment circuit 22 are connected to the OR circuit 23. The timing signal is sent to the gate 7 through the sample hold circuit 5 as in the above embodiment.

(Effects) As described above, according to the present invention, the optical output of the semiconductor laser is detected by the photodetector circuit, the error with the reference value is amplified by the error amplifier, and the output is sampled and held by the sample and hold circuit using a timing signal. In a semiconductor laser output control device that drives a semiconductor laser with a semiconductor laser drive circuit according to its output, a protection circuit is provided that inhibits the timing signal when the output signal of the photodetection circuit is below a predetermined value. Deterioration of the semiconductor laser can be prevented without excessive current flowing through the laser.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIGS. 2 and 3 are block diagrams showing each side of the determination circuit, and FIG. 4 is a block diagram showing another embodiment of the present invention. FIG. 5 is a block diagram showing a basic circuit of an embodiment of the present invention, FIG. 6 is a timing chart of the same circuit, and FIG. 7 is a circuit diagram specifically showing the same circuit. 1... Semiconductor laser, 2, 3... Photo detection circuit,
4...Error amplifier, 5...Sample and hold circuit. 6... Semiconductor laser drive circuit, 7, 8... Protection circuit. Procedural amendment dated November 6, 1985 1. Indication of the case Patent Application No. 232085 of 1988 2. Name of the invention Conductor laser output control device 3. Person making the amendment Relationship with the case Patent applicant name (674) ) Ricoh Co., Ltd.
Address: 4-5-4 Kyodo, Setagaya-ku, Tokyo "
"Detailed Description of the Invention" column and drawings (1) In the specification, page 3, line 13, "tamashing" is corrected to "timing." (2) "Circuit 23 and OR circuit 24" on page 5, line 20
is corrected to [Circuit 22 and OR circuit 23]. (3) Correct "determination circuit 23" in the first line of page 6 to "determination circuit 22." (4) Correct "OR circuit 24" in the second line of page 6 to "OR circuit 23". (5) Correct Figure 7 of the cylinder in the drawing as attached.

Claims (1)

[Claims] A photodetection circuit that detects the optical output of the semiconductor laser, an error amplifier that amplifies the error between the output signal of the photodetection circuit and a reference value, and a sample hold circuit that samples and holds the output signal of the error amplifier using a timing signal. , a semiconductor laser comprising: a semiconductor laser drive circuit that drives the semiconductor laser according to an output signal of the sample and hold circuit; and a protection circuit that inhibits the timing signal when the output signal of the photodetection circuit is below a predetermined value. Output control device.