JPH04117864A - Optical beam drive circuit for laser printer - Google Patents

Abstract

PURPOSE:To prevent mis-judgment to lighting of a laser from being caused even when an effect of noise is caused in a beam lighting command signal by varying a comparison threshold level with respect to a level change in a video signal. CONSTITUTION:When a VIDEO signal 7 is outputted from a control section 1 in a timing T1, a transistor(TR) 2 in a laser drive circuit 2 compares a level of the VIDEO signal 7 with that of a THL signal 9. When noise is superimposed on a control line leading from the control section 1 to the laser drive circuit 2, since noise is superimposed on both the THL signal 9 and the VIDEO signal 7, a relative level difference between the VIDEO signal 7 and the THL signal 9 is unchanged. Thus, since the presence of lighting commanded by the VIDEO signal 7 is surely judged, the effect is not imposed on a laser drive circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light (laser) beam drive circuit for a laser printer that emits a light beam to form an image on a photoreceptor.

[Conventional technology]

FIG. 10 shows a circuit configuration of a light beam driving device conventionally used in a laser beam printer.

In FIG. 10, reference numeral 101 is a control unit that controls the operations of various circuits within the laser beam printer. 102
is a laser drive circuit that supplies the semiconductor laser 103 with a drive current for laser lighting.

103 is a semiconductor laser that generates laser light.

104 is an operational amplifier, and 105 to 10111 are control lines that transmit control signals sent from the control unit 101 to the laser drive circuit 102.

Operational amplifier 104. Transistor Tr104. Resistance R]
05 to R108 and the capacitor ClO2 constitute a constant current circuit, and the constant current value i [mA] is determined by the voltage level of the control signal on the control line APC108.

Transistor Tr1.01''-Tr103 and resistor R1
02 to R104 constitute a current switching unit that functions as a comparison circuit, and compares the voltage of the video (VIDEO) signal 107 with a fixed voltage to determine whether or not to turn on the semiconductor laser 103. transistor T
r1.02 and Tr103 perform differential operation, and l.
The base voltage of transistor Tr102 is
If it is greater than the base voltage of 103, the constant current i [mA]
flows from the collector to the emitter of the transistor Tr102. As a result, a constant current i [mA] flows through the semiconductor laser 103, turning on the laser.

The base voltage of the transistor Tr102 depends on the VIDEO signal 107, and the base voltage Vth [V] of the transistor Tr103 is determined by the resistance values of the resistors R103 to R104. Note that the capacitor C1,01 and the resistor R1,01 connected in parallel to the semiconductor laser 103 are used to eliminate overshoe 1. and sag of the laser current.

In such a circuit configuration, a video signal for instructing whether or not to turn on the light beam is input from a recording information generation source (not shown), and when the level of this video signal is on level equal to or higher than the above-mentioned fixed voltage, the laser drive current is is the laser emitting element 10
3, the laser is emitted onto the photoreceptor.

[Problems to be Solved by the Invention] However, in the conventional example described above, the reference voltage for determining the on/off level of the video signal, that is, the base voltage of the transistor Tr103 is a fixed value, and therefore has the following drawbacks. there were.

Video (VIDEO) signal and T that instruct laser blinking
If the voltage difference between the base voltages of r103 is small, the VIDEO signal will be affected by external noise on the transmission path, and as a result, the laser will be erroneously lit or the lighting time will not be reproduced correctly.

In particular, when inputting the VIDEO signal from an external device, V
The smaller the pulse width of the IDEO signal, the more susceptible it is to the above-mentioned noise, and the VIDEO signal may not rise sufficiently and may not reach the voltage at the base of the transistor Tr103, making it impossible to turn on the laser.

In view of the above-mentioned points, the present invention provides a light beam drive circuit for a laser printer that is less susceptible to noise and is capable of accurately blinking a laser regardless of the waveform of a beam lighting instruction signal. It's about doing.

[Means for Solving the Problem] To achieve such an object, the present invention externally inputs a beam lighting instruction signal, and compares the level of the input beam lighting instruction signal with a threshold value by a comparison circuit. In a light beam drive circuit of a laser printer that determines whether or not a light beam is emitted from the photoreceptor according to the comparison result, a signal indicating the threshold value to be supplied to the comparison circuit is generated, and the signal is The present invention is characterized by comprising a threshold setting circuit that variably sets the threshold indicated by in response to a change in the level of the beam lighting instruction signal.

[Function] The present invention makes the comparison threshold variable in response to changes in the level of the video signal, so even if the beam lighting instruction signal is affected by noise, the threshold comparison Since the signals used for this change relatively depending on the noise influence, there is no possibility of erroneous determination regarding laser lighting as in the conventional case.

〔Example〕

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

FIG. 1 shows the circuit configuration of an embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes a control section that controls the operation of the laser beam printer. 2 is a laser drive circuit. 3
4 is a semiconductor laser for generating laser light, and 4 is an operational amplifier.

This is a control line that transmits control signals sent from the 5 to 9 controller 1 to the laser drive circuit 2. 10 is a threshold setting circuit related to the present invention.

Operational amplifier 4 and transistor Tr4. A constant current circuit is configured by the resistors R5 to R8 and the capacitor C2, and the constant current value i [mA] is determined by the voltage level of the control signal APC:8 from the control section. Transistor Trl~
Tr3 and resistor R2 constitute a current switching section (comparison circuit), and V is an instruction signal for blinking the laser.
IDEO signal 7 and threshold level signal (T1.IL) 9
It is determined whether or not the above-mentioned constant current value i [mA] is caused to flow through the semiconductor laser 3. In this embodiment, the transistor Tr
When the base voltage of transistor Tr2 is higher than the base voltage of transistor Tr3, the constant current value i [mA] is
Flows from the collector of r2 to the emitter. In other words, a constant current i [mA] flows through the semiconductor laser 3.

The base voltage of the transistor Tr2 depends on the VIDEO signal 7, and the base voltage of the transistor Tr3 is determined by the threshold level signal (T)IL)9.

FIG. 2 shows an example of the circuit configuration of the threshold setting circuit IO.

In FIG. 2, the voltage level VTII[
V] is determined by the resistance values of resistors R21 and R22. The operation of such a circuit will be explained using FIGS. 3(A) and 3(B).

FIG. 3 shows waveforms of various signals in the circuit of FIG. 1.

The TI signal 9 is held and outputted from the control section 1, and when the VIDEO signal 7 is outputted from the control section 1 at the timing shown in FIG.
2, the levels of the VIDEO signal 7 and the TI multiplied signal are compared (see waveforms a and t in FIG. 3).

When noise is added to the control line from the control unit 1 to the laser drive circuit 2, the T)IL signal 9 is
．． VIDEO signal 7 has noise on both, so VID
The relative level difference between the EO signal 7 and the THL signal 9 does not change. Therefore, since it is possible to reliably determine whether or not the lighting is on as instructed by the VIDEO signal 7, the laser drive current is not affected by this.

Further, even if the pulse width of the VIDEO signal 7 is small as shown in waveform a in FIG. 3, accurate level determination can be guaranteed. Note that the symbol Td shown in the laser drive current waveform e in FIG. 3 is V.
This is the response delay time from when the IDEO signal 7 is input until the laser drive current responds, and the response delay time Td is determined by the circuit configuration.

Also, the VIDEO signal 7 can be connected to a TT such as 74AS04.
When transmitting to the laser drive circuit 2 at L (control signal level), the output of the TTI and signal gel in the control section is divided by resistors and the divided voltage output is used as the THL signal, as shown in Fig. 4. It may also be transmitted for reference of differential switching.

A third configuration example of the threshold setting circuit 10 of FIG. 1 is shown in FIG.

This example is an example of creating a threshold level signal from the VIDEO signal, and for this purpose, the NPN transistor Tr31,
A current buffer is provided via a PNP transistor Tr32, and then the voltage of the VIDEO signal is averaged by a resistor R35 and a capacitor C51. Next, transistor Tr33. Current buffering is performed by T, 34, and the average value is transmitted to the laser drive circuit 2 as a TII signal. With such a circuit, the TH
When the L signal is created, high-speed VIDEO is generated as shown in Figure 6.
Even if the signal does not rise completely, the voltage of the threshold level signal THL is lowered, so that waveform deformation effects such as pulse width deterioration are prevented, and the semiconductor laser can be reliably blinked.

FIG. 7 shows a fourth configuration example of the threshold setting circuit IO of FIG. 1. In this example, the VIDEO signal is connected to the NPN transistor Tr51, PNP h transistor T'52
Performs current buffering. Then diode D51
After peak-holding the voltage of the VIDEO signal with a capacitor C52 and buffering the current with transistors Tr53 and Tr54, the voltage is divided by resistors R55 and R56 and transmitted to the laser drive circuit as a THL signal. Note that the resistance values of R55 and R56 are made equal. For example, the threshold setting circuit of this example
As shown in the figure, since the voltage of the threshold level signal THL is lowered even if the signal has not completely risen due to the high speed VIDEO signal, the semiconductor laser can be blinked while preventing the pulse width from deteriorating.

In addition to the examples described above, the following examples can be cited.

1) A signal obtained by inverting the VIDEO signal, which is a laser blinking signal, can also be used as the threshold level signal THL in FIG. In this case, for example, VI at the level of general-purpose TTL
A signal obtained by inverting the DEO signal using an inverter such as 74AS04 is used as a THL signal. Using such a signal, the 9th
As shown in the figure, when the VIDEO signal is instructing lighting (level on), the level of the VIDEO signal becomes higher than the THL signal level. In addition, the threshold signal circuit that generates such a THL signal has a simple circuit configuration, and it is possible to use a gate array in the control section, which can also contribute to reducing the manufacturing cost of the device. .

2) In this embodiment, the threshold setting circuit 10 shown in FIG. 1 is provided in the control section 1, but it goes without saying that it may be provided in the laser drive circuit 2.

[Effects of the Invention] As explained above, according to the present invention, even if the video signal used as a laser lighting instruction signal is affected by noise or has a waveform deformation, it is possible to ensure whether or not the light beam indicated by the video signal is lit. As a result, in addition to preventing malfunctions in the light beam lighting caused by noise and waveform deformation, the laser printer can be installed in locations where noise is likely to occur. can get.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a circuit configuration of an embodiment of the present invention, FIG. 2 is a circuit diagram showing a first configuration example of the threshold setting circuit 10 of FIG. 1, and FIG. 3 is a circuit diagram of the circuit of FIG. A waveform diagram showing signal waveforms, FIG. 4 is a second waveform diagram of the threshold setting circuit 10 of FIG.
A circuit diagram showing a configuration example, FIG. 5 is a third circuit diagram of the threshold setting circuit 10 of FIG.
A circuit diagram showing a configuration example, FIG. 6 is a VID in the threshold setting circuit 10 of FIG.
FIG. 7 is a waveform diagram showing the waveforms of the EO signal and the THL signal.
A circuit diagram showing a configuration example; FIG. 9 is a waveform diagram showing the waveforms of the VIDEO signal and THL signal when another threshold setting circuit according to the embodiment of the present invention is used; FIG. 10 is a circuit diagram of a conventional example. FIG. ioo...Light beam drive circuit, 1.101...Control unit, 2.1.02...Laser drive circuit, 10...Threshold value setting circuit.

Claims (1)

[Claims] 1) A beam lighting instruction signal is externally input, the level of the input beam lighting instruction signal is compared with a threshold value by a comparison circuit, and the light beam is emitted from the photoreceptor according to the comparison result. In a light beam drive circuit of a laser printer that determines the presence or absence of a laser beam, a signal indicating the threshold value is generated to be supplied to the comparison circuit, and the threshold value indicated by the signal is determined based on a level change of the beam lighting instruction signal. 1. A light beam drive circuit for a laser printer, comprising a threshold setting circuit that variably sets a threshold value in response to.