JP5672845B2 - Semiconductor laser driving device and image forming apparatus - Google Patents

Description

  The present invention relates to a semiconductor laser driving device and an image forming apparatus used for a laser printer, a digital copying machine, and the like.

  2. Description of the Related Art An electrophotographic image forming apparatus equipped with a semiconductor laser driving device used in a laser printer, a digital copying machine, or the like performs light amount control in order to keep the light output (light emission amount) of a semiconductor laser constant. For example, in the semiconductor laser driving device disclosed in Patent Document 1, the optical output of the semiconductor laser is detected, and digital data is set so that the optical output of the semiconductor laser becomes a desired optical output based on the detected optical output. A signal is generated, and a driving current of the semiconductor laser is generated by a D / A converter based on the digital data signal.

  When the semiconductor laser is driven with a relatively long rectangular wave pulse drive current, the optical output of the semiconductor laser starts to decrease from the time when the semiconductor laser shifts from the unlit state to the lit state due to self-heating of the semiconductor laser, A steady state is reached after several hundreds of microseconds (this characteristic is referred to as “droop characteristic”). The difference between the initial light output and the light output when the steady state is reached is called the droop amount, and if this is large, density unevenness occurs in the formed image and the image forming quality is lowered.

  An object of the present invention is to provide a semiconductor laser driving device capable of obtaining a constant light output in consideration of the droop characteristic of a semiconductor laser, and an image forming apparatus including the same.

In a first aspect of the present invention, a semiconductor laser driving device that outputs a driving current for driving a semiconductor laser is provided.
This semiconductor laser drive device
A drive current generation circuit that generates a drive current having a magnitude corresponding to a value indicated by the digital data signal and outputs the drive current to the semiconductor laser;
A current correction circuit for correcting the magnitude of the drive current supplied to the semiconductor laser,
The current correction circuit increases or decreases a value indicated by the digital data signal based on a predetermined pattern so that a light output during a lighting period of the semiconductor laser becomes a desired magnitude. Correct the magnitude of the current.

In a second aspect of the present invention, a semiconductor laser driving device is provided.
This semiconductor laser drive device
A switch current generation circuit that generates a switch current having a magnitude corresponding to a value indicated by the first digital data signal and outputs the switch current to the semiconductor laser;
A bias current generation circuit that generates a bias current having a magnitude corresponding to a value indicated by the second digital data signal and outputs the bias current to the semiconductor laser;
A current correction circuit for correcting the magnitude of the switch current generated by the switch current generation circuit according to a third digital data signal,
A current obtained by adding the bias current and the switch current is supplied to the semiconductor laser as a drive current,
The current correction circuit increases or decreases a value indicated by the third digital data signal based on a predetermined pattern so that a light output during a lighting period of the semiconductor laser has a desired magnitude. The magnitude of the switch current is corrected.

In a third aspect of the present invention, a semiconductor laser driving device is provided.
This semiconductor laser drive device
A switch current generation circuit that generates a switch current having a magnitude corresponding to a value indicated by the first digital data signal and outputs the switch current to the semiconductor laser;
A bias current generation circuit that generates a bias current having a magnitude corresponding to a value indicated by the second digital data signal and outputs the bias current to the semiconductor laser;
A current correction circuit for correcting the magnitude of the switch current,
A current obtained by adding the bias current and the switch current is supplied to the semiconductor laser as a drive current,
The current correction circuit, based on a predetermined pattern, by increasing or decreasing the value indicated by the first digital data signal so that the light output during the lighting period of the semiconductor laser becomes a desired value, The magnitude of the switch current supplied to the semiconductor laser is corrected.

  In another aspect of the present invention, there is provided an image forming apparatus including the semiconductor laser driving device according to the first, second, or third aspect of the present invention.

  According to the semiconductor laser driving device of the first aspect of the present invention, when the semiconductor laser driving current having a magnitude corresponding to the value indicated by the digital data signal is generated, the semiconductor laser is turned on based on the predetermined pattern. The magnitude of the drive current is corrected by increasing or decreasing the value indicated by the digital data signal so that the light output during the period becomes a desired magnitude. According to such a configuration, the change in the light output due to the droop characteristic can be suppressed by making the predetermined pattern a pattern that can compensate for the change in the light output due to the droop characteristic.

  According to the second aspect of the present invention, the semiconductor laser driving device generates a switch current having a magnitude corresponding to the value indicated by the first digital data signal, and outputs the switch current to the semiconductor laser. A bias current generation circuit that generates a bias current having a magnitude corresponding to the value indicated by the digital data signal 2 and outputs the bias current to the semiconductor laser, and a semiconductor laser using a current obtained by adding the bias current and the switch current as a drive current In the case of the structure which supplies to, the said effect is show | played. That is, according to the semiconductor laser driving device of the second aspect, the current value of the switch current generated by the switch current generating circuit is controlled in accordance with the third digital data signal, and based on the predetermined pattern The value indicated by the third digital data signal is increased or decreased so that the optical output during the lighting period of the semiconductor laser becomes a desired value, thereby correcting the magnitude of the drive current supplied to the semiconductor laser. The According to such a configuration, the change in the light output due to the droop characteristic can be suppressed by making the predetermined pattern a pattern that can compensate for the change in the light output due to the droop characteristic.

  According to the third aspect of the present invention, the semiconductor laser driving device generates a switch current having a magnitude corresponding to a value indicated by the first digital data signal and outputs the switch current to the semiconductor laser; A bias current generating circuit that generates a bias current having a magnitude corresponding to a value indicated by the second digital data signal and outputs the bias current to the semiconductor laser, and a current obtained by adding the bias current and the switch current is used as a drive current In the case of a configuration for supplying to a semiconductor laser, the above-described effect is exhibited. That is, according to the semiconductor laser driving device of the third aspect, the first digital data signal is increased or decreased so that the light output during the lighting period of the semiconductor laser becomes a desired value based on a predetermined pattern. By decreasing, the magnitude of the drive current supplied to the semiconductor laser is corrected. According to such a configuration, the change in the light output due to the droop characteristic can be suppressed by making the predetermined pattern a pattern that can compensate for the change in the light output due to the droop characteristic.

  In the first to third aspects, the time lapse when the droop occurs and the droop amount are not fixed values, but vary depending on the type of semiconductor laser and the thermal characteristics. Therefore, the predetermined pattern may be set according to the semiconductor laser.

1 is a configuration diagram of an image forming apparatus according to an embodiment of the present invention. It is a schematic block diagram of the semiconductor laser drive device of the 1st Embodiment of this invention. 3 is a timing chart illustrating an operation example of the semiconductor laser driving device according to the first embodiment. It is a schematic block diagram of the semiconductor laser drive device of the 2nd Embodiment of this invention. 6 is a timing chart illustrating an operation example of the semiconductor laser driving device according to the second embodiment. It is a figure which shows the characteristic of the optical output with respect to the drive current in a semiconductor laser. It is a timing chart which shows the droop characteristic of a semiconductor laser, and the example of a correction | amendment ((a) example in which optical output rises with time passage, (b) example in which optical output rises with time passage). It is a schematic block diagram of the semiconductor laser drive device of the 3rd Embodiment of this invention. 12 is a timing chart illustrating an operation example of the semiconductor laser driving device according to the third embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(First embodiment)
First, a first embodiment of the present invention will be described.

  FIG. 1 is a diagram showing an example of use of the semiconductor laser driving device according to the first embodiment of the present invention, and is a configuration example of an electrophotographic image forming apparatus used for a laser printer, a digital copying machine, and the like. Show.

  In the image forming apparatus 1 shown in FIG. 1, laser light emitted from a semiconductor laser LD composed of a laser diode is deflected by a rotary polygon mirror (polygon mirror) 2 that rotates at a high speed at a constant speed, and fθ as an imaging lens. The light passes through the lens 3 and is focused on the surface of the photoreceptor 4. The laser beam deflected by the rotary polygon mirror 2 is exposed and scanned in a direction (main scanning direction) orthogonal to the direction in which the photosensitive member 4 rotates, and performs image signal line-by-line recording. An image (electrostatic latent image) is formed on the surface of the photoreceptor 4 by repeating main scanning at a predetermined cycle corresponding to the rotational speed and recording density of the photoreceptor 4.

  A beam sensor 5 for generating a main scanning synchronization signal is disposed at a position where a laser beam is irradiated in the vicinity of one end of the photosensitive member 4. The image control device 6 outputs a signal for controlling the light amount of the semiconductor laser LD necessary for forming an image to the semiconductor laser driving device 7 that controls the driving of the semiconductor laser LD. The image control device 6 controls the image recording timing in the main scanning direction based on the main scanning synchronization signal and generates a control signal for inputting and outputting the image signal.

  In order to generate the main scanning synchronization signal, there is a forced lighting period by a forced lighting signal, which is a signal for continuously lighting the semiconductor laser LD for a certain period for each line in a non-image area before recording an image. Normally, by performing APC (Automatic Power Control) using the forced lighting period by the forced lighting signal, light intensity correction can be performed for each line or every multiple lines, the ambient temperature rises and the operating current increases. Even in this case, the amount of light when the semiconductor laser LD is turned on can always be accurately controlled. A forced lighting signal, a light emission signal, an APC execution signal, and the like are input from the image control device 6 to the semiconductor laser driving device 7.

  FIG. 2 is a diagram showing an example of the internal configuration of the semiconductor laser driving device 7 in the first embodiment of the present invention.

  As shown in FIG. 2, the semiconductor laser drive device 7 includes a drive current generation circuit (DAC) 11, a control circuit 12, a storage circuit 13, and a switch 14.

  The drive current generation circuit 11 generates a drive current Iop having a current value (magnitude) corresponding to the value indicated by the digital code op_code and outputs it to the semiconductor laser LD. The drive current generation circuit 11 is composed of a current output type D / A converter.

  The storage circuit 13 is composed of a nonvolatile memory or a register circuit, and stores predetermined parameters to be described later for determining a predetermined pattern.

  The control circuit 12 reads a predetermined parameter from the storage circuit 13, generates a digital code op_code based on the read predetermined parameter, the light emission signal ldon, and the clock signal clk, and outputs it to the switch current generation circuit 11. To do. The light emission signal ldon has a "High" or "Low" state, where High is a state in which the semiconductor laser LD is turned on and Low is a state in which the semiconductor laser is turned off. Specifically, the control circuit 12 reads a predetermined parameter from the storage circuit 13, and when the light emission signal ldon changes from low to high, the predetermined pattern read from the storage circuit 13 in synchronization with the input clock signal clk. Based on the above, the digital code op_code is controlled to increase or decrease from a predetermined set value. The predetermined parameter for determining the predetermined pattern includes the number of clocks from the start to the end of the increase / decrease when increasing / decreasing the digital code op_code, and the amount to be increased / decreased in one clock period (hereinafter referred to as “increase / decrease amount in one clock period”) ). The number of clocks from the start to the end of the increase / decrease of the digital code op_code is calculated by dividing the time until the optical output change almost disappears in the droop characteristic (hereinafter referred to as “elapsed time”) by 1 period of the clock signal clk. The increase / decrease amount in one clock period is calculated by dividing the increase / decrease amount ΔIop of the drive current necessary for canceling the droop amount ΔPo by the number of clocks from the increase / decrease start to the end.

  The switch 14 is turned on when the light emission signal ldon input from the image control device 6 is high, and turned off when the light emission signal ldon is low.

  In such a configuration, the drive current Iop output from the drive current generation circuit 11 is not supplied to the semiconductor laser LD when the light emission signal ldon input from the image control device 6 is low, and the light emission signal ldon is high. In some cases, the semiconductor laser LD is supplied. In this manner, output control to the semiconductor laser LD is performed by the light emission signal ldon, and lighting and extinguishing of the semiconductor laser LD are controlled.

  In this embodiment, the drive current generation circuit 11 and the switch 14 form a drive current generation circuit that generates a drive current having a magnitude corresponding to the value indicated by the digital data signal and outputs the drive current to the semiconductor laser. A current correction circuit for correcting the magnitude of the drive current supplied to the semiconductor laser is formed, and the storage circuit 13 forms a storage circuit for storing each parameter. Note that the configuration of the present embodiment is an example of a configuration that realizes the present invention, and the drive current generation circuit, the current correction circuit, and the memory circuit may have different configurations from the present embodiment.

  The drive current generation circuit 11 and the switch 14 (drive current generation circuit), the control circuit 12 (current correction circuit), and the storage circuit 13 (storage circuit) may be integrated in one IC.

  FIG. 3 is a timing chart showing an example of signals of the respective parts having the configuration shown in FIG. 2, and shows an example of correcting a decrease in the optical output P due to the droop characteristic.

  When the emission signal ldon is Low, the drive current Iop is not supplied to the semiconductor laser LD. Therefore, the semiconductor laser LD is turned off and the optical output P is 0 mW. The digital code op_code of the drive current remains the set value.

  When the emission signal ldon changes from Low to High, the drive current Iop is supplied to the semiconductor laser LD, the semiconductor laser LD is turned on, and the optical output P becomes Po (FIGS. 3A, 3D, and 3E). reference). Here, if the droop characteristic is not corrected, the optical output P decreases from Po during the droop elapsed time Tdroop, and finally decreases by the droop amount ΔPo. However, in this embodiment, when the light emission signal ldon is High, the digital code op_code of the drive current is 1 every clock period from the set value in synchronization with the clock signal clk, as shown in FIG. The amount of increase / decrease in the clock period (increase amount in this example) is increased, and as shown in FIG. 3D, the drive current Iop increases accordingly. The increase in the digital code op_code of the drive current stops when the number of clocks from the start to the end of the increase, that is, the elapsed time Tdroop of the droop characteristic elapses.

  The light emission signal ldon and the clock signal clk are asynchronous. Therefore, the timing at which the drive current Iop starts to increase according to the clock signal clk is shifted by a maximum of one clock period after the light emission signal ldon becomes High. However, since the elapsed time Tdroop of the droop characteristic is usually several hundreds μs, whereas the one clock period of the clock signal clk is several hundreds nsec, this deviation is negligible.

  When the emission signal ldon changes from High to Low, the drive current Iop is not supplied to the semiconductor laser LD (see FIG. 3D). Therefore, the semiconductor laser LD is turned off when the optical output P is 0 mW (see FIG. 3E). As shown in FIG. 3 (c), the digital code op_code of the drive current is decreased by a decrease amount of one clock period (a decrease amount in this example) every one clock period in synchronization with the clock signal clk. When it becomes, the decrease stops. If the emission signal ldon = High is input before the set value is reached, the semiconductor laser LD is turned on, and the digital code op_code of the drive current is again the clock signal from the value immediately before the emission signal ldon = High is input. Synchronously with clk, it increases by an increment of one clock period every clock period.

  In FIG. 3, the increase amount of the digital code op_code of the drive current in one clock period is set to the same value in each clock period. However, the increase amount in one clock period may be set to a different value in each clock period. For example, in accordance with the droop characteristic curve of the semiconductor laser LD, the digital code op_code of the drive current is increased sharply in the first half from the start of increase and gradually increased in the second half. In this case, the set value of the increase amount that is different for each clock period is stored in the storage circuit 13.

  In the above description, an example in which the optical output P is reduced due to the droop characteristic and this reduction is corrected has been described. However, the idea of the present embodiment is that the optical output P increases due to the droop characteristic and this increase is corrected. In this case, the drive current Iop may be reduced in order to cancel the droop amount ΔPo, and the idea is the same as described above.

(Second Embodiment)
In the first embodiment, the drive current Iop is increased or decreased to cancel the droop amount ΔPo. However, the drive current Iop is increased or decreased by increasing or decreasing the switch current Isw corresponding to the light emission current Iη of the semiconductor laser LD. Also good. Hereinafter, a configuration for realizing this method will be described.

  FIG. 4 is a diagram showing an internal configuration example of the semiconductor laser driving device 7b according to the second embodiment of the present invention.

  As shown in FIG. 4, the semiconductor laser driving device 7b includes a switch current generation circuit 11A, a bias current generation circuit 11B, a reference switch current generation circuit 11C, a control circuit 12, a storage circuit 13, and a switch 14.

  The switch current generation circuit 11A amplifies a current having a current value (magnitude) corresponding to the value indicated by the digital code sw_code with an amplification factor corresponding to the current value of the reference switch current Iswref output from the reference switch current generation circuit 11C. The switch current Isw is generated and output to the semiconductor laser LD. The bias current generation circuit 11B generates a bias current Ibi having a current value (magnitude) corresponding to the value indicated by the digital code bi_code and outputs it to the semiconductor laser LD. The reference switch current generation circuit 11C generates a reference switch current Iswref having a current value (magnitude) corresponding to the value indicated by the digital code swref_code, and outputs the reference switch current Iswref to the switch current generation circuit 11A. Each of the current generation circuits 11A, 11B, and 11C includes a current output type D / A converter. Here, the set value of the bias current digital code bi_code and the set value of the switch current digital code sw_code are determined when the threshold current Ith and the emission current Iη of the semiconductor laser LD are individually detected, and are continuously lit by one pulse. It does not change during the period. On the other hand, the set value of the digital code swref_code changes so that the change in the light output due to the droop characteristic can be corrected during the continuous lighting period by one pulse.

  The storage circuit 13 is composed of a nonvolatile memory or a register circuit, and stores predetermined parameters to be described later for determining a predetermined pattern.

  The control circuit 12 reads a predetermined parameter from the storage circuit 13, generates a reference switch current digital code swref_code based on the read predetermined parameter, the light emission signal ldon, and the clock signal clk, and generates a reference switch current. Output to the generation circuit 11C. The light emission signal ldon has a "High" or "Low" state, where High is a state in which the semiconductor laser LD is turned on and Low is a state in which the semiconductor laser is turned off. Specifically, when the light emission signal ldon changes from Low to High, the control circuit 12 sets the digital code swref_code to an initial value based on a predetermined pattern read from the storage circuit 13 in synchronization with the input clock signal clk. Control to increase or decrease from. The predetermined parameters for determining the predetermined pattern are the initial value before the start of increase / decrease of the digital code swref_code, the number of clocks from the start / end of increase / decrease when increasing / decreasing the digital code swref_code, and the increase / decrease amount of one clock period. is there. The initial value before the start of increase / decrease of the digital code swref_code is normally set to the center value, and the number of clocks from the start of increase / decrease to the end is calculated by dividing the elapsed time Tdroop of droop characteristics by one cycle of the clock signal clk, and 1 clock The increase / decrease amount of the period is calculated by dividing the increase / decrease amount ΔIsw of the switch current necessary for canceling the droop amount ΔPo by the number of clocks from the increase / decrease start to the end.

  The switch current Isw is generated by amplifying a current having a current value (magnitude) corresponding to the value indicated by the digital code sw_code with an amplification factor corresponding to the magnitude of the reference switch current Iswref. Therefore, the current value of the switch current Isw varies corresponding to the variation ratio of the current value of the reference switch current Iswref, and the relationship ΔIswref / Iswref = ΔIsw / Isw is established. For example, if the reference switch current Iswref increases by 10%, the switch current Isw also increases by 10%. Conversely, if the reference switch current Iswref decreases by 10%, the switch current Isw also decreases by 10%. The reference switch current generation circuit 11C is composed of a D / A converter that has a monotonically increasing and linear characteristic. Therefore, in order to increase the reference switch current Iswref by 10%, the reference switch current digital code swref_code may be increased by 10% from the initial value.

  The switch 14 is turned on when the light emission signal ldon input from the image control device 6 is high, and turned off when the light emission signal ldon is low.

  In such a configuration, the bias current Ibi output from the bias current generation circuit 11B and the reference switch current Iswref output from the reference switch current generation circuit 11C are always constant regardless of whether the semiconductor laser LD is turned off or on. , And the switch current generation circuit 11A. On the other hand, the switch current Isw output from the switch current generation circuit 11A is not supplied to the semiconductor laser LD when the light emission signal ldon input from the image control device 6 is low, and the semiconductor laser when the light emission signal ldon is high. Supplied to LD. In this manner, output control to the semiconductor laser LD is performed by the light emission signal ldon, and lighting and extinguishing of the semiconductor laser LD are controlled.

  In the present embodiment, the switch current generation circuit 11A and the switch 14 form a switch current generation circuit that generates a switch current having a magnitude corresponding to the value indicated by the first digital data signal and outputs the switch current to the semiconductor laser. The bias current generation circuit 11B forms a bias current generation circuit that generates a bias current having a magnitude corresponding to the value indicated by the second digital data signal and outputs the bias current to the semiconductor laser. The reference switch current generation circuit 11C and the control circuit Reference numeral 12 denotes a current correction circuit that corrects the magnitude of the drive current supplied to the semiconductor laser, and the reference switch current generation circuit 11C generates a reference switch current having a magnitude corresponding to the input third digital data signal. A reference switch current generation circuit that outputs to the switch current generation circuit. A control circuit for increasing or decreasing the data signal based on a predetermined pattern and outputting it to the reference switch current generation circuit is provided, and the storage circuit 13 forms a storage circuit for storing each parameter. Note that the configuration of the present embodiment is an example of a configuration that realizes the present invention, and the switch current generation circuit, the bias current generation circuit, the current correction circuit, and the storage circuit may have different configurations from the present embodiment.

  Switch current generation circuit 11A and switch 14 (switch current generation circuit), bias current generation circuit 11B (bias current generation circuit), reference switch current generation circuit 11C and control circuit 12 (current correction circuit), and storage circuit 13 (storage circuit) ) May be integrated in one IC.

  FIG. 5 is a timing chart showing an example of signals of the respective parts having the configuration shown in FIG. 4, and shows an example in the case of correcting a decrease in the light output P due to the droop characteristic.

  The set value of the digital code bi_code of the bias current and the set value of the digital code sw_code of the switch current are determined when the threshold current Ith and the emission current Iη of the semiconductor laser LD are individually detected, and are fixed values during the droop correction. Therefore, it is not particularly shown in FIG.

  When the emission signal ldon is Low, only the bias current Ibi that is always supplied is supplied to the semiconductor laser LD, but the switch current Isw is not supplied. Therefore, the semiconductor laser LD is turned off and the optical output P is 0 mW. The digital code swref_code of the reference switch current remains the initial value.

  When the emission signal ldon changes from Low to High, the switch current Isw is supplied to the semiconductor laser LD, the semiconductor laser LD is turned on, and the optical output P becomes Po (FIGS. 5A, 5E, and 5G). reference). Here, if the droop characteristic is not corrected, the light output P decreases from Po during the elapsed time Tdroop, and finally decreases by the droop amount ΔPo. However, in this embodiment, when the light emission signal ldon is High, the digital code swref_code of the reference switch current according to the clock signal clk is one clock period from the initial value every clock period as shown in FIG. The increase / decrease amount (increase amount in this example) is increased, and the reference switch current Iswref also increases as shown in FIG. 5 (d). Since the switch current Isw is proportional to the reference switch current Iswref, as shown in FIG. 5E, the switch current Isw also increases by an amount corresponding to the increase amount of the reference switch current Iswref. The increase in the digital code swref_code of the reference switch current stops when the number of clocks from the start to the end of the increase, that is, the elapsed time Tdroop of the droop characteristic elapses. When the increase in the reference switch current Iswref stops, the increase in the switch current Isw also stops.

  The light emission signal ldon and the clock signal clk are asynchronous. Therefore, the timing at which the reference switch current Iswref and the switch current Isw start to increase according to the clock signal clk is shifted by a maximum of one clock period after the light emission signal ldon becomes High. However, since the elapsed time Tdroop of the droop characteristic is usually several hundreds μsec, whereas one clock period of the clock signal clk is several hundreds nsec, this variation is negligible.

  When the emission signal ldon changes from High to Low, only the bias current Ibi is supplied to the semiconductor laser LD, and the switch current Isw is not supplied (see FIG. 5E). Therefore, the semiconductor laser LD is turned off when the optical output P is 0 mW (see FIG. 5G). As shown in FIG. 5C, the digital code swref_code of the reference switch current is decreased by a decrease amount of one clock period every clock period in accordance with the clock signal clk, and the decrease stops when the initial value is reached. If the light emission signal ldon = High is input before the initial value is reached, the semiconductor laser LD is turned on, and the digital code swref_code of the reference switch current starts again from the value immediately before the light emission signal ldon = High is input. Increases by 1 clock period increments every clock period according to clk.

  In FIG. 5, the increase amount of one clock period of the digital code swref_code of the reference switch current is the same in each clock period, but the increase amount of one clock period may be set to a different amount in each clock period. For example, in accordance with the curve of the droop characteristic of the semiconductor laser LD, the digital code swref_code of the reference switch current is increased sharply in the first half from the start of increase and gradually increased in the second half. In this case, the set value of the increase amount that is different for each clock period is stored in the storage circuit 13.

  In the above description, an example in which the optical output P is reduced due to the droop characteristic and this reduction is corrected has been described. However, the idea of the present embodiment is that the optical output P increases due to the droop characteristic and this increase is corrected. In this case, the switch current Isw may be decreased to cancel the droop amount ΔPo, and the concept is the same as described above.

  FIG. 6 is a diagram showing an example of characteristics of the semiconductor laser and a set current of the semiconductor laser driving device 7b in the second embodiment of the present invention.

  In the semiconductor laser characteristics of FIG. 6, the threshold current Ith when the semiconductor laser starts to emit light suddenly, and the emission current Iη targeting the region above the threshold current Ith required for the desired laser output is a variation in the manufacturing process. Varies from one semiconductor laser to another. However, since the optical output P of the semiconductor laser increases linearly with respect to the emission current Iη, the emission current Iη and the optical output P in the semiconductor laser are always in a proportional relationship.

  Further, as the set current of the semiconductor laser driving device 7b in the present embodiment, the threshold current Ith and the emission current Iη of the semiconductor laser LD are individually detected, and the bias current Ibi (= Ith) is always supplied regardless of turning off or lighting. It is assumed that the switch current Isw (= Iη) is added at the time of lighting. That is, the optical output P of the semiconductor laser LD is proportional to the switch current Isw. If the switch current Isw is increased by 10%, the optical output P is also increased by 10%. The present invention uses this relationship to correct the droop characteristic and will be described in detail below. Details of a method for individually detecting the threshold current Ith and the emission current Iη of the semiconductor laser LD are described in Japanese Patent No. 4427277.

  FIGS. 7A and 7B are diagrams showing a droop characteristic of the semiconductor laser LD and a correction example using the semiconductor laser driving device 7b in the second embodiment of the present invention. FIG. 7A shows an example in which the light output P decreases with time, and FIG. 7B shows an example in which the light output P increases with time.

  In FIG. 7A, when the light emission signal ldon input from the image control device 6 changes from Low to High, the drive current Iop obtained by adding the switch current Isw to the bias current Ibi flows to the semiconductor laser LD, and the semiconductor laser LD Lights up and the light output P becomes Po. If there is no correction, the optical output P falls from the Po by the droop amount ΔPo during the elapsed time Tdroop due to the droop characteristic of the semiconductor laser LD. In order to set the droop amount ΔPo to 0 mW, the switch current Isw may be increased during the elapsed time Tdroop after the semiconductor laser LD is turned on so as to cancel the droop amount ΔPo. The required increase ΔIsw of the switch current Isw may be ΔIsw = Isw × ΔPo / Po because the optical output P is proportional to the switch current Isw.

  In the example of FIG. 7B, if there is no correction, the light output P increases by the droop amount ΔPo during the elapsed time Tdroop. In order to correct the droop amount ΔPo to 0 mW, the switch current Isw may be decreased during the elapsed time Tdroop after the semiconductor laser LD is lit so as to cancel the droop amount ΔPo. Since the optical output P is proportional to the switch current Isw, the necessary decrease ΔIsw of the switch current Isw may be similarly ΔIsw = Isw × ΔPo / Po.

(Third embodiment)
In the second embodiment, the reference switch current Iswref is increased or decreased to increase or decrease the switch current Isw. However, the switch current Isw may be directly increased or decreased without using the reference switch current Iswref. Hereinafter, a configuration for realizing this method will be described.

  FIG. 8 is a diagram showing an example of the internal configuration of the semiconductor laser driving device 7c in the third embodiment of the present invention.

  As shown in FIG. 8, the semiconductor laser driving device 7c has a switch current generation circuit 11A, a bias current generation circuit 11B, a control circuit 12, a storage circuit 13, and a switch 14.

  The switch current generation circuit 11A generates a switch current Isw having a current value (magnitude) corresponding to the value indicated by the digital code sw_code and outputs it to the semiconductor laser LD. The bias current generation circuit 11B generates a bias current Ibi having a current value (magnitude) corresponding to the value indicated by the digital code bi_code and outputs it to the semiconductor laser LD. Each of the current generation circuits 11A and 11B is configured by a current output type D / A converter. Here, the set value of the digital code bi_code is determined when the threshold current Ith and the light emission current Iη of the semiconductor laser LD are individually detected, and does not change during the continuous lighting period by one pulse. On the other hand, the set value of the digital code sw_code changes so that the change in the light output due to the droop characteristic can be corrected during the continuous lighting period by one pulse.

  The storage circuit 13 is composed of a nonvolatile memory or a register circuit, and stores predetermined parameters to be described later for determining a predetermined pattern.

  The control circuit 12 reads a predetermined parameter from the storage circuit 13, generates a switch current digital code sw_code based on the read predetermined parameter, the light emission signal ldon, and the clock signal clk, and generates a switch current generation circuit. To 11A. The light emission signal ldon has a "High" or "Low" state, where High is a state in which the semiconductor laser LD is turned on and Low is a state in which the semiconductor laser is turned off. Specifically, when the light emission signal ldon changes from Low to High, the control circuit 12 sets the digital code sw_code to the initial value based on a predetermined pattern read from the storage circuit 13 in synchronization with the input clock signal clk. Control to increase or decrease from. The predetermined parameters for determining the predetermined pattern are the number of clocks from the start to the end of increase / decrease of the digital code sw_code and the increase / decrease amount of one clock period. The number of clocks from the start to end of the increase / decrease of the digital code sw_code is calculated by dividing the elapsed time Tdroop of the droop characteristic by one cycle of the clock signal clk, and the increase / decrease amount for one clock period is necessary to cancel the droop amount ΔPo It is calculated by dividing the increase / decrease amount ΔIsw of the switch current by the number of clocks from the increase / decrease start to the end.

  The switch current generation circuit 11A is composed of a D / A converter that has a monotonically increasing and linear characteristic. Therefore, in order to increase the switch current Isw by 10%, the switch current digital code sw_code may be increased by 10% from the set value.

  The switch 14 is turned on when the light emission signal ldon input from the image control device 6 is high, and turned off when the light emission signal ldon is low.

  In such a configuration, the bias current Ibi output from the bias current generation circuit 11B is always supplied to the semiconductor laser LD regardless of whether the semiconductor laser LD is turned off or on. On the other hand, the switch current Isw output from the switch current generation circuit 11A is not supplied to the semiconductor laser LD when the light emission signal ldon input from the image control device 6 is low, and the semiconductor laser when the light emission signal ldon is high. Supplied to LD. In this manner, output control to the semiconductor laser LD is performed by the light emission signal ldon, and lighting and extinguishing of the semiconductor laser LD are controlled.

  In the present embodiment, the switch current generation circuit 11A and the switch 14 form a switch current generation circuit that generates a switch current having a magnitude corresponding to the value indicated by the first digital data signal and outputs the switch current to the semiconductor laser. The bias current generation circuit 11B forms a bias current generation circuit that generates a bias current having a magnitude corresponding to the value indicated by the second digital data signal and outputs the bias current to the semiconductor laser. The control circuit 12 supplies the semiconductor laser with the bias current generation circuit 11B. A current correction circuit for correcting the magnitude of the drive current to be performed is provided, and the storage circuit 13 forms a storage circuit for storing each parameter. Note that the configuration of the present embodiment is an example of a configuration that realizes the present invention, and the switch current generation circuit, the bias current generation circuit, the current correction circuit, and the storage circuit may have different configurations from the present embodiment.

  The switch current generation circuit 11A and the switch 14 (switch current generation circuit), the bias current generation circuit 11B (bias current generation circuit), the control circuit 12 (current correction circuit), and the storage circuit 13 (storage circuit) are integrated into one IC. You may accumulate.

  FIG. 9 is a timing chart showing an example of a signal of each part having the configuration shown in FIG. 8, and shows an example in the case of correcting a decrease in the optical output P due to the droop characteristic.

  Note that the set value of the bias current digital code bi_code and the set value of the switch current digital code sw_code are determined when the threshold current Ith and the emission current Iη of the semiconductor laser LD are individually detected. In particular, the setting value of the digital code bi_code of the bias current is a fixed value during the droop correction, and is not particularly described in FIG.

  When the emission signal ldon is Low, only the bias current Ibi that is always supplied is supplied to the semiconductor laser LD, but the switch current Isw is not supplied. Therefore, the semiconductor laser LD is turned off and the optical output P is 0 mW. The digital code sw_code of the switch current remains the set value.

  When the emission signal ldon changes from Low to High, the switch current Isw is supplied to the semiconductor laser LD, the semiconductor laser LD is turned on, and the optical output P becomes Po (FIGS. 9A, 9D, 9F). reference). Here, if the droop characteristic is not corrected, the light output P decreases from Po during the elapsed time Tdroop, and finally decreases by the droop amount ΔPo. However, in the present embodiment, when the light emission signal ldon is High, the digital code sw_code of the switch current is synchronized with the clock signal clk as shown in FIG. 9C for one clock period from the set value every clock period. The amount of increase / decrease increases (in this example, an increase amount), and the switch current Isw also increases accordingly, as shown in FIG. The increase in the digital code sw_code of the switch current stops when the number of clocks from the start to the end of the increase, that is, the elapsed time Tdroop of the droop characteristic elapses.

  The light emission signal ldon and the clock signal clk are asynchronous. Therefore, the timing at which the switch current Isw starts to increase according to the clock signal clk is shifted by a maximum of one clock period after the light emission signal ldon becomes High. However, since the elapsed time Tdroop of the droop characteristic is usually several hundreds μsec, whereas one clock period of the clock signal clk is several hundreds nsec, this variation is negligible.

  When the emission signal ldon changes from High to Low, only the bias current Ibi is supplied to the semiconductor laser LD, and the switch current Isw is not supplied (see FIG. 9D). Therefore, the semiconductor laser LD is turned off with the optical output P being 0 mW (see FIG. 9F). As shown in FIG. 9C, the digital code sw_code of the switch current decreases by a decrease amount of one clock period every clock period according to the clock signal clk, and stops decreasing when the set value is reached. When the emission signal ldon = High is input before the set value is reached, the semiconductor laser LD is turned on, and the digital code sw_code of the switch current is again the clock signal clk from the value immediately before the emission signal ldon = High is input. In response to this, every clock period increases by an increment of one clock period.

  In FIG. 9, the increase amount of the switch current digital code sw_code in one clock period is the same in each clock period, but the predetermined increase amount in one clock period may be set to a different amount in each clock period. . For example, in accordance with the droop characteristic curve of the semiconductor laser LD, the switch current digital code sw_code is increased sharply in the first half from the start of increase and gradually increased in the second half. In this case, the set value of the increase amount that is different for each clock period is stored in the storage circuit 13.

  In the above description, an example in which the optical output P is reduced due to the droop characteristic and this reduction is corrected has been described. However, the idea of the present embodiment is that the optical output P increases due to the droop characteristic and this increase is corrected. In this case, the switch current Isw may be decreased to cancel the droop amount ΔPo, and the concept is the same as described above.

  Further, FIG. 9 illustrates an example in which the optical output P decreases due to the droop characteristic and this decrease is corrected. However, the present invention can also be applied to the case where the optical output P increases due to the droop characteristic and this increase is corrected. In this case, the switch current Isw may be reduced in order to cancel the droop amount ΔPo, and the concept is the same as in FIG.

  As described above in each of the first, second, and third embodiments, the semiconductor laser LD drive current is generated with a magnitude corresponding to the value indicated by the digital data signal. Further, based on a predetermined pattern, the value indicated by the digital data signal is increased or decreased so that the light output P during the lighting period of the semiconductor laser LD becomes a desired value, so that the magnitude of the drive current is reduced. It is corrected. In the present embodiment, the predetermined parameter for determining the predetermined pattern is set so that the droop amount ΔPo can be canceled during the elapsed time Tdroop of the droop characteristic. Thereby, the change of the light output P during a lighting period is suppressed. Therefore, a desired light output P with no density unevenness can be obtained, and the image forming quality can be improved.

DESCRIPTION OF SYMBOLS 1 Laser unit 2 Polygon mirror 3 Scan lens 4 Photoconductor 5 Beam sensor 6 Image control units 7, 7b, 7c Semiconductor laser drive device 11 Drive current generation circuit 11A Switch current generation circuit 11B Bias current generation circuit 11C Reference switch current generation circuit 12 Control circuit 13 Memory circuit 14 Switch LD Semiconductor laser

JP 2009-164191 A

Claims (23)

A semiconductor laser driving device for outputting a driving current for driving a semiconductor laser,
A drive current generation circuit that generates a drive current having a magnitude corresponding to a value indicated by the digital data signal and outputs the drive current to the semiconductor laser;
A current correction circuit that receives a clock and corrects the magnitude of the drive current supplied to the semiconductor laser;
A storage circuit that stores parameters including the number of clocks from the start to the end of increase / decrease of the digital data signal and the increase / decrease amount of one clock period ;
The current correction circuit increases or decreases a value indicated by the digital data signal based on a predetermined pattern determined by the parameters so that a light output during a lighting period of the semiconductor laser becomes a desired magnitude. By correcting , the magnitude of the drive current is corrected ,
The semiconductor laser drive device an increase of one clock period of the digital data signal, characterized that you set to different values in each clock period.   The current correction circuit is determined so as to obtain a constant light output by compensating for a decrease or an increase in light output when a predetermined drive current is supplied to the semiconductor laser during a lighting period of the semiconductor laser. 2. The semiconductor laser drive device according to claim 1, wherein the magnitude of the drive current is corrected by increasing or decreasing a value indicated by the digital data signal based on the predetermined pattern. 3. The semiconductor laser driving device according to claim 1 , wherein the current correction circuit changes the digital data signal in accordance with a curve of a droop characteristic of the semiconductor laser.   4. The drive current generation circuit controls output of drive current to the semiconductor laser in accordance with a control signal input to control turning on and off of the semiconductor laser. The semiconductor laser driving device according to any one of the above.   5. The semiconductor laser driving device according to claim 1, wherein the driving current generating circuit includes a D / A converter.   6. The semiconductor laser drive device according to claim 1, wherein the drive current generation circuit, the current correction circuit, and the memory circuit are integrated in one IC.   An image forming apparatus comprising the semiconductor laser driving device according to claim 1. A semiconductor laser driving device for outputting a driving current for driving a semiconductor laser,
A switch current generation circuit that generates a switch current having a magnitude corresponding to a value indicated by the first digital data signal and outputs the switch current to the semiconductor laser;
A bias current generating circuit that generates a bias current having a magnitude corresponding to a value indicated by the second digital data signal and outputs the bias current to the semiconductor laser;
A current correction circuit that receives a clock and corrects the magnitude of the switch current generated by the switch current generation circuit according to a third digital data signal;
A storage circuit for storing parameters including an initial value before the start of increase / decrease of the third digital data signal, the number of clocks from the start of increase / decrease to the end, and the increase / decrease amount of one clock period ;
A current obtained by adding the bias current and the switch current is supplied to the semiconductor laser as a drive current,
The current correction circuit is a value indicated by the third digital data signal so that a light output during a lighting period of the semiconductor laser becomes a desired magnitude based on a predetermined pattern determined by the parameters. by increasing or decreasing the to correct the magnitude of said switch current,
The third semiconductor laser driving device to increase one clock period of the digital data signal, characterized that you set to different values in each clock period.   The current correction circuit is determined so as to obtain a constant light output by compensating for a decrease or an increase in light output when a predetermined drive current is supplied to the semiconductor laser during a lighting period of the semiconductor laser. 9. The semiconductor laser driving device according to claim 8, wherein the magnitude of the switch current is corrected by increasing or decreasing the third digital data signal based on the predetermined pattern. 10. The semiconductor laser driving device according to claim 8 , wherein the current correction circuit changes the third digital data signal in accordance with a droop characteristic curve of the semiconductor laser.   11. The switch current generation circuit controls the output of the switch current to the semiconductor laser according to a control signal input to control turning on and off of the semiconductor laser. The semiconductor laser driving device according to any one of the above. The current correction circuit generates a reference switch current having a magnitude corresponding to a value indicated by the input third digital data signal and outputs the reference switch current to the switch current generation circuit;
A control circuit that performs control to increase or decrease a value indicated by the third digital data signal according to a predetermined parameter, and outputs the control switch current generation circuit to the reference switch current generation circuit;
The semiconductor laser driving device according to claim 8, further comprising:   The switch current generation circuit generates the switch current based on the inputted reference switch current, and changes the magnitude of the switch current in accordance with the fluctuation ratio of the magnitude of the reference switch current. The semiconductor laser driving device according to claim 12, wherein the driving device is a semiconductor laser driving device.   14. The semiconductor laser drive device according to claim 12, wherein each of the reference switch current generation circuit, the switch current generation circuit, and the bias current generation circuit is configured by a D / A converter.   15. The semiconductor laser driving device according to claim 8, wherein the switch current generation circuit, the bias current generation circuit, the current correction circuit, and the memory circuit are integrated in one IC.   An image forming apparatus comprising the semiconductor laser driving device according to claim 8. A semiconductor laser driving device for outputting a driving current for driving a semiconductor laser,
A switch current generation circuit that generates a switch current having a magnitude corresponding to a value indicated by the first digital data signal and outputs the switch current to the semiconductor laser;
A bias current generating circuit that generates a bias current having a magnitude corresponding to a value indicated by the second digital data signal and outputs the bias current to the semiconductor laser;
A current correction circuit that receives a clock and corrects the magnitude of the switch current;
A storage circuit that stores a parameter including the number of clocks from the start to the end of increase / decrease of the first digital data signal and the increase / decrease amount of one clock period ;
A current obtained by adding the bias current and the switch current is supplied to the semiconductor laser as a drive current,
The current correction circuit has a value indicated by the first digital data signal so that a light output during a lighting period of the semiconductor laser becomes a desired value based on a predetermined pattern determined by the parameters. By increasing or decreasing, the magnitude of the switch current supplied to the semiconductor laser is corrected ,
The semiconductor laser drive device an increase of one clock period of the first digital data signal, characterized that you set to different values in each clock period.   The current correction circuit is determined so as to obtain a constant light output by compensating for a decrease or an increase in light output when a predetermined drive current is supplied to the semiconductor laser during a lighting period of the semiconductor laser. 18. The semiconductor laser drive device according to claim 17, wherein the magnitude of the switch current is corrected by increasing or decreasing a value indicated by the first digital data signal based on the predetermined pattern. 19. The semiconductor laser driving device according to claim 17 , wherein the current correction circuit changes the second digital data signal in accordance with a curve of a droop characteristic of the semiconductor laser.   The switch current generation circuit controls the output of the switch current to the semiconductor laser in accordance with a control signal input to control turning on and off of the semiconductor laser. The semiconductor laser driving device according to any one of the above.   21. The semiconductor laser driving device according to claim 17, wherein each of the switch current generation circuit and the bias current generation circuit includes a D / A converter.   22. The semiconductor laser driving device according to claim 17, wherein the switch current generation circuit, the bias current generation circuit, the current correction circuit, and the memory circuit are integrated in one IC.   An image forming apparatus comprising the semiconductor laser driving device according to claim 17.

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