JPH0426589B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a drive circuit for high-speed switching of a laser diode in an optical printer or the like.

(Prior Art) FIG. 2 is a diagram showing the operating principle of a commonly used laser beam printer.

A video signal generator 1 generates a video signal to be printed. 2 is a laser diode, and 3 is a drive circuit for driving the laser diode 2 to turn on. A coupling lens 4 shapes the emitted beam of the laser diode 2. 5 is a scanner motor; 6 is a polygon mirror rotated by the scanner motor 5; 7 is an f-theta lens, and 8 is a photoreceptor drum. The drum surface is scanned by a laser beam that has passed through a polygon mirror 6 and an f-theta lens 7 to form an image. The image formed on the drum surface is obtained as a print by a xerographic process (not shown).

Laser diodes used in such laser beam printers have features such as being small and capable of direct modulation, but because they are semiconductor devices, they are highly temperature dependent. That is, the laser light output greatly changes depending on the temperature. FIG. 3 is a temperature characteristic diagram of the laser light output when the laser diode is driven with a constant current, and shows that the optical output decreases as the temperature increases.

On the other hand, there are two types of factors that cause temperature changes in the laser diode: long-period factors due to environmental temperature changes and temperature rises of the entire printer device, and short-period factors due to self-heating of the laser diode.

One of the causes of self-heating is due to an increase in the temperature of the case of the laser diode, but this can be practically solved by making the heat dissipation fins sufficiently large. Another problem is the temperature rise due to thermal resistance between the device's jacket and case, which is difficult to solve due to the structure of the device.

Figure 4 is a timing chart for explaining the temperature dependence of print output in a laser beam printer, where a is print data and if the laser diode is lit at level "H", the chip temperature of the laser diode is b follows the turning on or off of print data a and changes exponentially. The optical output c has a characteristic distorted with respect to the print data a due to the rise characteristics of the laser diode and the temperature characteristics shown in FIG. 3 corresponding to the chip temperature b. The print d is the threshold Th for the light output c in the xerographic printing process.
This was obtained by setting the value to the level shown by the chain line.

(Problems to be Solved by the Invention) The above-mentioned print data d deviates from the print data a, particularly due to the temperature characteristics of the light output c of the initial lighting source.

FIG. 5A shows an original image corresponding to print data a in FIG. 4, and FIG. 5B shows that the image obtained by print data d in FIG. 4 is distorted. That is, Figure 5 is obtained by scanning from left to right in the same way as Figure 4, and the laser diode is turned off for a short time between positions Y1 and Y2, as in the scanning of range X1. If the light turns on after that, the turn-off time corresponds to time t1-t2 in Fig. 4, and the turn-off position Y1' (corresponding to time t1) almost coincides with the position Y1 of the print data. ,
The subsequent lighting position Y2' (corresponding to time _t2 ) is longer than the print data Y2 by time △T1 due to the characteristics of the light output c.
Only late. In addition, when the light is turned on after being turned off for a long time corresponding to time t3-t4 as in the scanning of the range X2, the position Y4' of the lighting is
It lags behind print data Y4 by time △T2. Since the light output c has a sharp rise when turned on after being turned off for a long time, △T1 > △T2, and since these times △T1 and △T2 are minute, changes in the total size of the print are not a problem. However, position Y2′ is
Since it lags behind Y4' by △T1 - △T2, printing shifts occur and print quality deteriorates.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides that when a binary drive signal connected in series with a laser diode receiving a constant current source is at one level, A first switching element that puts the laser diode into an operating state receives a constant current source in common with the laser diode, and connects the laser diode with the first switching element.
an impedance element connected in parallel with a series circuit with a switching element, an integrating circuit that integrates the level data, and an impedance control section that adjusts and subtracts the impedance of the impedance element according to the data from the integrating circuit. and a second switching element which is connected in series with the impedance element and which activates the impedance element when the drive signal is at the other level.

(Function) According to the present invention, a laser diode and an impedance element are connected in parallel to a constant current source, and current is alternately applied to both elements, and based on the integration result of data related to driving the laser diode, Since the impedance of the impedance element is controlled, the impedance of the impedance element becomes almost equivalent to the impedance of the laser diode. It can be controlled by taking into account the temperature rise depending on the duration of the laser diode's off time, and as the time when the laser diode's off time becomes longer, the impedance of the impedance element is lowered, and the voltage applied to the laser diode when it is on is lowered. By suppressing this, the influence of the duration time on the rise of the laser diode lighting is eliminated, so that printing deviations do not occur.

(Example) Fig. 1 is a laser diode drive circuit diagram showing an embodiment of the present invention, Fig. 6 is a timing chart showing its desired operating characteristics, and Fig. 7 is a timing chart showing an example of its operation. be. 2 is a laser diode, 9 is a constant current source, 10 is a transistor serving as a first switching element, 11 is a transistor serving as an impedance element, 1
2 is a transistor serving as a second switching element. Laser diode 2 is constant current source 9
The transistor 10 is connected in series with the laser diode 2, receives print data a at its base, and is on when the print data a is at the H level. When the laser diode is at the level, it is turned off and the laser diode 2 is turned off. The transistor 11 is
It is connected in parallel with the laser diode 2 and receives a constant current source 9 in common with the laser diode 2, and its impedance characteristics are set to be almost equivalent to the impedance characteristics of the laser diode 2, as will be described later. controlled. In this case, the voltage drop due to impedance during operation of the transistor 11 is controlled to be around 1.8V, which is equivalent to the voltage drop during operation of the laser diode 2, for example. The transistor 12 forms a pair with the transistor 10, and is connected in series with the transistor 11. The transistor 12 is connected in series with the transistor 11, and has its base receiving print data a via an inverter 13. When a is at L level, it is on, and at this time transistor 11 is activated as a load of constant current source 9. When a is at H level, it is off, and at this time, transistor 11 is activated.
deactivate. Note that the resistors 14 and 15 are used to set each transistor 10 and 12 in an active state to speed up the response to print data a.

16 is a bias circuit as an impedance control section, and if the bias circuit 16 controls the impedance of the transistor 11 to be the same as the impedance of the laser diode 2,
When the transistors 10 and 12 are operated alternately according to the print data a, the potential at point h does not change, and therefore almost constant current property is maintained during high-speed switching.

However, as explained in FIG. 4, if the light-off time is long, the rise of the light output during the subsequent lighting will be steep; therefore, if the light-off time is short, during the light-off time, that is, during the operation of the transistor 12. As shown in FIG. 6, the impedance drop of the transistor 11 is made small and the potential of the point h is made relatively high to make the rise of the current i of the laser diode 2 steep. It is preferable to make the rises of both optical outputs equal by lowering the potential of h and slowing the rise of the current i.

Reference numeral 17 denotes an integrating circuit which receives print data a and outputs an integral value e with respect to a characteristic corresponding to chip temperature b shown in FIG. A comparator 18 compares the integrated value e with a predetermined comparison reference potential VR, and sets its output f to H level when it exceeds this, and sets its output f to L level when it falls below it. 19 is an OR circuit,
Upon receiving the output f of the comparator 18 and the print data a,
The logical sum g is output. The bias circuit 16 is
It is controlled to have two values based on the output g. That is, when the output g is at H level, since the light is lit frequently, the transistor 11 is set to high impedance both when the transistor 12 is operating and when it is not operating, so that the point h
is set to a high potential to make the rise of the current i steep. When the output g is at the L level, it is the result of a long turn-off time, so the impedance of the transistor 11 is set to low when the transistor 12 is in operation, and the potential of the point h is set to a low potential, so that the rise of the current i is made gradual. do. In this way, the rise characteristic of the optical output (c in Fig. 4) changes from the rise characteristic of the current i (the 6th
(see i in the figure) to correct printing deviations due to short-term temperature changes of the laser diode 2.

In FIG. 7, the comparison reference potential VR is kept constant and the control amount g of the bias circuit 16 is set to two values, which is generally sufficient, but if higher resolution is required, the comparator 18 It is also possible to provide a plurality of potentials and set a plurality of potentials in place of each comparison reference potential VR, thereby controlling the bias circuit 16 at multiple levels in the same way.

Also, an amplifier may be used in place of the comparator 18 in FIG. By adding the two, the bias circuit 16 is controlled to reflect each minimum value of the curve e in FIG. 7 at each start of lighting, and is controlled with higher precision.

(Effects of the Invention) As described above, according to the present invention, a laser diode and an impedance element are connected to a constant current source, and the impedance of the impedance element is calculated based on the integration result of data related to driving the laser diode. Since the above data is set in relation to the temperature characteristics of the laser diode chip, it is possible to compensate for the temperature of the rise characteristics of the optical output due to the length of the turn-off time. Printing quality is improved without occurrence of printing deviations related to the frequency of lighting.

[Brief explanation of drawings]

FIG. 1 is a laser diode drive circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram of the operating principle of a laser beam printer according to the present invention, FIG. 3 is a temperature characteristic diagram of the optical output of the laser diode, and FIG. The figure is a timing chart for explaining the temperature dependence of print output in a laser beam printer.
B is an explanatory diagram of the deviation of printing due to the characteristics shown in FIG. 4, FIG. 6 is a timing chart showing the desired operating characteristics of the present invention in relation to FIGS. 1 and 4, and FIG.
The figure is a timing chart showing an example of the operation of FIG. 2... Laser diode, 9... Constant current source, 10...
Transistor (first switching element), 11
...Transistor (impedance element), 12...
Transistor (second switching element), 16
...bias circuit (impedance control section), 17
...integrator circuit.

Claims (1)

[Scope of Claims] 1. A first switching element that is connected in series with a laser diode receiving a constant current source and that activates the laser diode when a binary drive signal is at one level; an impedance element that receives a constant current source in common with the diode and is connected in parallel with the series circuit of the laser diode and the first switching element; an integrating circuit that integrates the data at the level; and the integrating circuit. an impedance control section that adjusts or reduces the impedance of the impedance element according to data from the impedance control section; A laser diode drive circuit comprising a switching element and a switching element.