Device for driving a laser diode and an electrophotography type image forming apparatus using the same

An apparatus has a plurality of semiconductor cells as current switchings for driving a laser diode which exposes to light a photosensitive member so as to effect image recording. The semiconductor cells have each different characteristics, or same characteristics respectively. In response to the light amount signal corresponding to light quantity of the laser diode, a selector selects one or more of the semiconductor cells so as to supply a suitable driving current turned on or off later with an image signal. As a result, no fluctuations in the pulse duty ratio and the rise time are present in the entire driving current active range, thus improving the quality of a recorded image. The effect of selection by the selector is held for a certain recording time, whereby malfunction due to noises or the like can be avoided, and stable actions of the laser diode are ensured.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for driving a laser diode, and 
more particularly, to an electrophotography type image forming apparatus 
using the device. The image forming apparatus includes not only a printer, 
but also a facsimile and a copying machine, etc. 
2. Description of the Related Art 
A typical construction of an image forming apparatus called a laser beam 
printer is illustrated in FIG. 1. An image signal (VDATA signal) 101 
inputs into a laser unit 102, which emits a laser beam 103 ON-OFF 
modulated in response to the VDATA signal. A motor 104 rotates a rotary 
polygon mirror 105 at a constant speed, to deflect the laser beam 103, 
generating a laser beam 107. An image formation lens 106 focuses the laser 
beam 107 on a photosensitive drum 108. Thus, the laser beam 107 modulated 
with the image signal 101 is scanned horizontally (in a direction of main 
scanning) on the photosensitive drum 108. A beam detector 109 has a 
photoelectric conversion element 110 (e.g. a photodiode), and generates a 
horizontal synchronizing signal (BD signal) 111 which serves for an image 
write timing. A latent image formed on the photosensitive drum 108 is 
visualized as a toner image by a developing device (not shown). The toner 
image is transferred to a printing paper 112 by a transfer printing device 
(not shown). 
The actions of these respective parts will be described. The laser unit 102 
generates a laser beam 103 modulated in response to the image signal 101 
supplied. The VDATA signal 101 is produced by a controller (not shown) 
disposed in the laser beam printer. The modulated laser beam 103 is 
deflected by the polygon mirror 105 having a plurality of mirror surfaces 
which makes a constant-speed rotation and which is driven by the motor 
104. The deflected laser beam 107 is scanned with a constant-speed on the 
photosensitive drum 108. 
The laser beam 107 is also focused on the photosensitive drum 108 by the 
image formation lens 106. When the photosensitive drum 108 is rotated at a 
constant speed and the laser beam 107 is scanned at a constant speed, the 
latent image based on the VDATA signal 101 is formed on the photosensitive 
drum 108. At this time, the laser beam 107 is incident on the 
photoelectric conversion element 110 of the beam detector 109 fixed in the 
neighborhood of a starting point for scanning. This incident induces the 
generation of BD signal 111 from the beam detector 109. In synchronism 
with the BD signal 111, the controller generates VDATA signal 101 
corresponding to one scanning operation, thereby limiting the image 
position in the direction of main scanning on the photosensitive drum 108. 
Next, a signal for forming an image will be described by reference to FIG. 
2. BD signal 111 is a synchronizing signal in the main scanning direction, 
as above-mentioned. FIG. 2 shows an output timing in the main scanning 
direction (i.e. horizontal direction) with respect to the printing paper 
112. When image signal 101 is generated at a time of t.sub.1 after the 
rise of BD signal 111, an image is formed in a position at a distance of 
D.sub.1 from the left end of the printing paper 112. As above-described, 
it is needless to say that the laser beam 107 does not directly form an 
image on the printing paper 112, but is merely focused on the 
photosensitive drum 108 so as to create a latent image. 
The image signal 101 is generated by an image processing unit (not shown), 
such as an image processor, which is different from the controller for 
controlling the sequence of image formation. The controller masks the 
image signal 101 by means of an image mask signal 113 so that no exposure 
will occur even if the image processing unit turn on the image signal 101 
within outside of the image range (i.e. areas other than D.sub.2 of FIG. 
2). As aforementioned, the BD signal 111 is a signal to be generated when 
the laser beam 107 scans on the beam detector 109. Hence, the controller 
needs to light the laser forcibly at a time when the laser beam 107 is 
expected to scan on the beam detector 109. A signal for this 
force-lighting is an unblanking signal 114 (shown in FIG. 2). 
The above-mentioned mask signal 113 and unblanking signal 114 are generated 
by counting a system clock 124 as shown in FIG. 3. 
An explanation will now be described with reference to FIG. 3. BD signal 
111 from the beam detector 109 is shaped by a waveform shaping circuit 123 
into a pulse waveform comparable to that of one pulse of the system clock 
124. This shaped BD signal serves to clear a main scanning counter 122. 
The main scanning counter 122 counts up while synchronizing with the 
system clock 124, and is cleared every time a pulse of the BD signal 
inputs. Therefore, the current scanning position of the laser beam can be 
recognized by knowing the counter value of the main scanning counter 122. 
A register 115 for generating an unblanking start signal and a register 116 
for generating an unblanking end signal latched unblanking start data or 
unblanking end data, respectively, via data lines 127, 128. Strobe pulses 
125, 126 are trigger pulses for latching these data into the two registers 
115, 116, respectively. The contents (i.e. data) latched in the registers 
115, 116 are compared with those contents of the main scanning counter 122 
by comparators 117, 118. As a result, the unblanking start signal 129 
outputs from a gate 119, and the unblanking end signal 130 outputs from a 
gate 120, to a flip-flop 121. The flip-flop 121 creates an unblanking 
signal 114 from these signals 129, 130, as shown in FIG. 4. 
The image mask signal 113 can also be generated by a circuit of the same 
structure as that of the aforementioned circuit for generating the 
unblanking signal 114 illustrated in FIG. 3. 
In the above description regarding FIG. 1, it has been described that the 
laser unit 102 is driven with ON-OFF driving by the image signal 101 in 
order to simplify explanation. In fact, however, the image signal 101 
needs to take a logical product and a logical sum relative to the image 
mask signal 113, the unblanking signal 114, and a laser force lighting 
signal 131 until the image signal 101 reaches the laser unit 102. By these 
actions, the image signal 101 can be formed only within the image range 
D.sub.2 shown in FIG. 2. The resulting laser lighting signal 132 is 
supplied as a drive signal for the laser unit 102. The laser force 
lighting signal 131 is a signal with which the controller turns on the 
laser forcibly. 
Next, an automatic power control (APC) will be described. The relationship 
between an electric current supplied to a laser chip and an optical output 
produced from it differs from each of chips, and also varies depending on 
the heat evolution of the chip itself. Thus, a constant current control 
with mere open loop cannot emit laser light. It is necessary to monitor 
the optical output of the laser and control the laser driving current in 
response to the level of optical output so that the desired optical output 
level will be obtained. This type of control is called the APC. 
Moreover, details of the APC will be explained. FIG. 6 shows the structure 
of a laser control circuit. This laser control circuit consists of a 
current stabilizer 133, a switching circuit 135, and an amplifier 138. The 
current stabilizer 133 is a voltage-current converter, which flows an 
electric current I.sub.1 corresponding to a light amount signal 134 (i.e. 
an APC voltage) from the controller. A circuit for switching this current 
with a laser lighting signal 132 is the switching circuit 135 as above 
mentioned. Responsive to this action of the switching circuit 135, a laser 
diode 136 emits light. The quantity of this laser light emission is taken 
out by a photodiode 137 as a current volume, and this current volume is 
converted by a resistor 140 into a voltage volume. The quantity of light 
emission taken out as the voltage volume is amplified by the amplifier 138 
to become an emission volume signal 139 representing an intensity of the 
light emitted from the laser diode 136. The controller raises the level of 
the light amount signal 134 in response to the level of the emission 
volume signal until the desired value is obtained, while monitoring the 
emission volume signal 139. 
FIG. 7 illustrates a control procedure for the above-described APC 
operation. This control is performed in the following manner: First, the 
laser force lighting signal 131 shown in FIG. 5 is made active, and then 
the emission volume signal 139 of FIG. 6 is monitored (S1). If the value 
of the emission volume indicated by the signal 139 is lower than the 
desired value, the level of the light amount signal 134 is raised by one 
step (S2). If the value of the emission volume is higher than the desired 
value, on the contrary, the level of the light amount signal 134 is 
lowered by one step (S3). If the quantity of light emission is identical 
with the desired value, the APC operation is concluded. During this APC 
operation, a laser beam scans portions corresponding to an arrows shown in 
FIG. 8 in a positional relationship with the printing paper 112. 
The APC is performed first of all in the image formation procedure, and 
also in case a plurality of sheets are printed continuously, the APC 
should be performed in a period corresponding to a space between a 
preceding sheet and a succeeding sheet. 
Alternatively, as shown in FIG. 9, there is a method of the APC by which 
the APC may be performed outside the image range. This method is carried 
out in cases where the light quantity (light intensity) level by each of 
scanning lines should be ensured, or in cases where the generation of 
latent image lines between sheets as shown in FIG. 8 adversely affects 
image formation or the like. According to this method, the aforementioned 
unblanking period is used to generate the unblanking signal 114 by each of 
scanning lines, as shown in FIG. 10, whereby the laser lighting signal 132 
is generated. Thus, the emission volume signal 139 arisen out at the 
starting point of unblanking as illustrated in FIG. 10. 
In the above-described prior art examples, however, the switching circuit 
135 for the laser diode 136 shown in FIG. 6 consists of a pair of current 
drivers. Hence, if the entire driving current active range (10 mA to 120 
mA) of the laser diode 136 is covered, the switching circuit 135, 
especially in the small-current range, is influenced by an internal device 
parasitic capacity of the integrated circuit (IC), thus posing the 
following disadvantages: 
(a) The pulse duty ratio varies in the small-current range. 
(b) The rise time is delayed in the small-current range. 
These deteriorations become notability, particularly, in proportion to the 
speeding up of video data (laser lighting signal 132) which is a digital 
signal. This will exert a direct adverse effect on half-tone reproduction 
and a smoothing process. Consequently, there arise problems such that 
image quality is remarkably lowered. 
In order to solve the above-mentioned problems associated with the prior 
art, it is an object of the present invention provide an image forming 
apparatus which enables stable driving of a laser diode free from 
fluctuations in the pulse duty ratio and the rise time over the entire 
driving current active range of the laser diode. 
SUMMARY OF THE INVENTION 
To meet the above object, one aspect of the present invention provides; an 
image forming apparatus having a light source for generating a light beam 
modulated with an image signal, the apparatus comprising: a plurality of 
semiconductor cells for switching a driving current flowing into the light 
source; and selection means for selecting one or more of the plurality of 
semiconductor cells. 
Another aspect of the present invention provides; a light quantity 
controller (light intensity controller) having a light source for 
generating a light beam modulated with an information signal, the device 
comprising: 
a plurality of semiconductor cells for switching a driving current flowing 
into the light source; and 
selection means for selecting one or more of the plurality of semiconductor 
cells. 
According to a feature of the invention, the selection means may select one 
or more of the plurality of semiconductor cells in response to the driving 
current flowing into the light source. 
According to a feature of the invention, each of the plurality of 
semiconductor cells may have different characteristics corresponding to 
different driving currents flowing into the light source, and the 
selection means may select some one semiconductor cell having 
characteristics responsive to the driving current flowing into the light 
source from among the plurality of semiconductor cells. 
According to the present invention, the selection means selects one of a 
plurality of semiconductor cells for switching a driving current for the 
light source whose characteristics are different individually (e.g. 
optimized in advance individually) according to the different values of 
the driving current flowing into the laser diode. Alternatively, the 
present invention uses a plurality of identical semiconductor cells (e.g. 
whose characteristics are optimized previously according to a 
predetermined value of driving current flowing into the laser diode), and 
the selection means selects one or plural of the semiconductor cells in a 
selected combination according to the value of the driving current flowing 
into the laser diode. 
Thus, the present invention enables stable driving of a laser diode free 
from fluctuations in the pulse duty ratio and the rise time over the 
entire driving current active range of the laser diode. As a result, an 
image forming apparatus can be accomplished which involves an optimized 
smoothing process, improved half-tone reproducibility, and high image 
quality. 
Furthermore, the results of selection by the selection means are retained 
during control of recording for a recording medium. Consequently, wrong 
operations due to noises or the like can be avoided, and the drive cell 
during the control of recording for the recording medium is fixed. 
Moreover, it becomes possible to perform stable driving of a laser diode 
without causing fluctuations in the pulse duty ratio and the rise time 
over the entire driving current active range of the laser diode. 
The above and other objects, effects, features and advantages of the 
present invention will become more apparent from the following description 
of embodiments thereof taken in conjunction with the accompanying drawings 
.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Embodiments of the present invention will now be described in detail below 
with reference to the accompanying drawings. 
&lt;First embodiment&gt; 
FIG. 11 shows the outlined circuit diagram for a laser diode driving 
circuit in accordance with a first embodiment of the present invention. In 
the instant embodiment, an APC voltage 207 as explained with respect to 
the prior art is analog-to-digital converted to generate a light amount 
signal. In response to the value of the light amount signal, a decoder 210 
selects one of a plurality of semiconductor cells 201 to 206 on driving 
stages and uses it, thereby accomplishing an optimal drive for a laser 
diode 213. Since an explanation for an electrophotographic type recording 
apparatus is the same as in the aforementioned prior art, the present 
embodiment focuses on an explanation for the end-driving stage of the 
laser diode. 
The end-driving stage of the laser diode has six output stage cells 201 to 
206 with optimized characteristic properties, including speed 
characteristics, ringing response characteristics, linearity 
characteristics and the like. These cells 201 to 206 have those 
characteristics optimized in driving current ranges, 0 to 20 mA, 20 to 40 
mA, 40 to 60 mA, 60 to 80 mA, 80 to 100 mA, and 100 to 120 mA, 
respectively. 
First, the light amount signal (APC voltage) 207 from a controller (not 
shown), which is comparable to the light amount signal 134 shown in FIG. 
6, enters an analog-to-digital (A/D) converter 209 via a low pass filter 
(LPF) 208. The APC level of the light amount signal A/D converted by the 
A/D converter is decoded into one of the corresponding digital values 
preset by a decoder of decoder/selector 210. Based on this digital value 
decoded, the selector of decoder/selector 210 elects one of the 
above-mentioned cells 201 to 206. 
Image signal (VDATA) 211 ON-OFF modulated based on the video signal, which 
image signal is comparable to the image signal 101 shown in FIG. 5, passes 
through a time delay circuit (TIME D) 212 for correcting the pulse duty 
ratio, whereby the pulse duty ratio of the ON-OFF modulated image signal 
211 is corrected. 
Then, the signal 211 with a corrected pulse duty ratio is put out from the 
time delay circuit 212 as a logical product signal with respect to a 
selection signal of the selector 210 so as to be output as a gate input to 
field effect transistors (FET) 214 to 219 in charge of the ON-OFF control 
of the transistors 201 to 206 driving the laser diode 213. For this 
purpose, the selector 210 has a logical circuit similar to that of FIG. 5. 
On the other hand, in order to supply an appropriate driving current to the 
laser diode 213 in response to the voltage volume of the light amount 
signal 207, a constant-current source current I.sub.APC is determined by a 
voltage-current converter (V-I converter) 220 in response to that voltage 
volume. Symbol RC1 denotes a resistor for determining a reference voltage 
to be used in the voltage-current conversion by the converter 220. As a 
result, a driving current flows into the laser diode 213 via one of the 
output stage cells 201 to 206 which has been selected according to the 
current range for driving the laser diode 213 and whose characteristics 
mentioned above have been optimized. 
The instant embodiment adopts a construction in which the pulse duty ratio 
of the ON-OFF modulated image signal 211 is corrected by passing this 
signal through the time delay circuit 212 for pulse duty ratio correction. 
This construction is not necessarily indispensable, for example, the image 
signal 211 may enter directly into the decoder/selector circuit 210. 
This embodiment also adopts a layout of six output stage cells 201 to 206, 
however the present invention is not limited thereto. If cells with a 
wider current range are used, the invention may be constructed of a 
slightly smaller number of cells. If cells with a narrower current range 
are used, the invention may be constructed of a slightly larger number of 
cells. 
&lt;Modification of the first embodiment&gt; 
FIG. 12 shows a circuit structure as an example of the modification of the 
circuit in accordance with the first embodiment shown in FIG. 11. Numeral 
221 denotes a control signal (MODE) for instructing the rejection of the 
change of selection about the cells 201 to 206 and for retaining the 
aforementioned selection signal shown in FIG. 11, and this signal 221 is 
supplied to the decoder/selector 210. This control signal 221 is fixed at 
least to some time equivalent to the image data recording time in the 
sub-scanning range shown in FIG. 2. In order to supply an appropriate 
driving current to the laser diode 213 in response to the voltage volume 
of the light amount signal 207, a constant-current source current 
I.sub.APC is determined by the voltage-current converter (V-I converter) 
220 in response to the voltage volume of the light amount signal 207. 
Symbol RC1 denotes a resistor for determining a reference voltage to be 
used in the voltage-current conversion by the converter 220. Since the 
selection signal is held as stated above, malfunction due to noises, etc. 
can be avoided, and the employable driving cell is fixed during the 
control of recording onto the recording medium. Furthermore, a driving 
current flows into the laser diode 213 via one cell selected from among 
the output stage cells 201 to 206 whose characteristics have been 
optimized beforehand according to the range of current for driving the 
laser diode 213. 
FIG. 13 shows a circuit structure as another modification of the circuit 
shown in FIG. 11. In this case, output stage cells 201 to 206 are each 
constituted of the same cell with common characteristics. Their 
characteristics, including speed characteristics, ringing response 
characteristics, and linearity characteristics, etc. are optimized 
beforehand in the driving current range of 0 to 20 mA. Numeral 210 denotes 
only a decoder, which uses one or more of cells optionally selected with 
combination from among plurality of the cells 201 to 206 in response to 
the value of the driving current to be supplied to the laser diode 213. 
&lt;Second embodiment&gt; 
FIG. 14 shows the outlined circuit diagram for structure of a laser diode 
driving circuit in accordance with a second embodiment of the present 
invention. In the instant embodiment, a plurality of output stage cell 
selection signals S1, S2, S3 from a controller (not shown) select one of 
driving stage semiconductor cells 201 to 206 through a selector 210, 
thereby accomplishing an optimal drive of laser diode 213. This embodiment 
focuses on an explanation for portions different from the first 
embodiment. The output stage cell selection signals S1, S2, S3 are usually 
generated by the controller (not shown) for controlling the printer 
engine. When the control device determines the aforementioned APC voltage 
(207), then the device determines the output stage cell selection signals 
S1, S2, S3 by table look-up or the like according to the value of the APC 
voltage determined. Since the APC voltage corresponds to the driving 
current for the laser diode, some one of the six driving cells can be 
selected with the 3-bit digital signals S1 to S3. 
&lt;Third embodiment&gt; 
FIG. 15 shows the outlined circuit diagram for a laser diode driving 
circuit in accordance with a third embodiment of the present invention. In 
the instant embodiment, an output stage cell selection signal as serial 
data (SIN) 231 from a controller (not shown) becomes a parallel signal in 
a serial-parallel data converter 230. This parallel signal selects one of 
driving stage semiconductor cells 201 to 206 through the selector 210 as 
described in the second embodiment, thereby accomplishing an optimal drive 
of laser diode 213. 
This embodiment focuses on an explanation for portions different from the 
second embodiment. The parallel signal for the output stage cell selection 
is usually generated by serial-parallel data converter 230 based on the 
serial data (SIN) 231 from the controller (not shown) for controlling the 
printer engine, and a synchronizing clock (SCLK) for this serial data. 
Therefore, the present embodiment enables control without need to increase 
selection signals, if there are more cells of which selection should be 
made. 
The present invention has been described in detail with respect to 
preferred embodiments, and it will now be obvious that changes and 
modifications may be made without departing from the invention in its 
broader aspects, and it is our intention, therefore, in the appended 
claims to cover all such changes and modifications as fall within the true 
spirit of the invention.