Patent Publication Number: US-10308018-B2

Title: Printing apparatus and method of controlling printhead

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a printing apparatus and a method of controlling a printhead. 
     Description of the Related Art 
     In printing apparatuses that cause ink to discharge by heating the ink by energy generation elements (hereinafter referred to as heaters) arranged for discharge ports of a printhead, it is preferable to drive as many heaters as possible simultaneously in order to perform a print by the printhead at a high speed. However, when many heaters are driven simultaneously, current flowing to the wiring increases. As a result, there are cases where a voltage drop due to parasitic resistance of the wiring increases and it is impossible to generate the desired thermal energy in the heaters. Deterioration of image quality occurs because a variation of the thermal energy causes the volume of ink that is discharged to vary. 
     In order to solve such a problem, in Japanese Patent Laid-Open No. H10-44416, for a voltage difference across the two terminals of a heater between when as many heaters as possible are driven simultaneously (all nozzles) and when only one heater (one bit) is driven (single nozzle), a wiring resistance adjustment is conducted in accordance with the distance thereof. Also, in conjunction with an elongation of a substrate (one inch for example) in recent years, heater driving circuit configuration adjustment (employing a source follower configuration) is performed in Japanese Patent Laid-Open No. 2010-155452. 
     By adjusting wiring resistance or making the heater driving circuit (transistor) a source follower configuration as described above, the thermal energy provided to the heaters is fixed even if there is a voltage fluctuation and thereby the volume of ink droplets that is discharged is stabilized. 
     When using a source follower configuration for the heater driving unit, voltage across the heater will be fixed at all times. However, because it is necessary to consider a voltage drop due to wiring resistance in wiring (flexible or printing element substrate) from the main body to the heaters in relation to the heater driving power supply voltage, it is necessary that the voltage across the heater be designed to be lower than the heater driving power supply voltage. In other words, when the voltage applied to the drain of the transistor falls below the gate voltage, the voltage supplied to the heaters will not be fixed. It is thought that heat will be produced proportionally to the voltage loss, and the printhead will heat up more than necessary. Accordingly, this leads to a bottleneck in acceleration because it becomes important to suppress heat within the printhead as much as possible from the perspective of throughput. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a printing apparatus comprising: a plurality of printing elements; driving circuits that have at least one source follower transistor and correspond to each of the plurality of printing elements; and a control unit configured to, in a case where a number of printing elements driven simultaneously does not exceed a predetermined number, perform a first control for driving the at least one source follower transistor by a fixed pulse width irrespective of the number of printing elements driven simultaneously, and, in a case where the number of printing elements driven simultaneously exceed the predetermined number, perform a second control for changing a pulse width to drive the at least one source follower transistor based on the number of printing elements driven simultaneously. 
     According to another aspect of the present invention, there is provided a method for controlling a printhead, the method comprising: the printhead including a plurality of printing elements and driving circuits which have at least one source follower transistor and correspond to each of the plurality of printing elements; in a case where a number of printing elements driven simultaneously does not exceed a predetermined number, perform a first control for driving the at least one source follower transistor by a fixed pulse width irrespective of the number of printing elements driven simultaneously, and, in a case where the number of printing elements driven simultaneously does exceed the predetermined number, perform a second control for changing a pulse width to drive the at least one source follower transistor based on the number of printing elements driven simultaneously. 
     By virtue of the present application invention, it is possible to achieve both keeping voltage across a heater fixed and suppressing deterioration of throughput. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating an example of a configuration of an inkjet printing apparatus according to the present invention. 
         FIG. 2  is a view illustrating an example of a control configuration of the printing apparatus according to the present application invention. 
         FIG. 3  is a view illustrating an example of a configuration of an inkjet printhead. 
         FIG. 4  is a view illustrating an example of a configuration of a heater driving circuit. 
         FIGS. 5A and 5B  are views illustrating examples of a configuration of a voltage converter circuit. 
         FIG. 6  is a view for describing voltage across a heater for a number of simultaneously driven heaters. 
         FIGS. 7A, 7B, and 7C  are views for describing a pulse control method. 
         FIGS. 8A and 8B  are views for describing a relationship of a pulse width and the number of heaters that are driven simultaneously. 
         FIG. 9  is a view for describing a pulse control method. 
         FIGS. 10A, 10B, 10C, and 10D  are views illustrating an example of a configuration of each heater driving circuit. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, description is given regarding an embodiment of the present invention with reference to the figures. 
     Below, more specific descriptions are given in detail for preferred embodiments of the present invention with reference to the attached drawings. However, relative arrangements of configuration elements, and the like, recited in this embodiment are not intended to limit the scope of the invention thereto, unless specifically stated. 
     Note that in this specification, “print” represents forming not only meaningful information such as characters and shapes, but also meaningless information. Furthermore, it is assumed that “print” broadly represents cases in which an image, design, or pattern is formed on a printing medium irrespective of whether or not it is something that a person can visually perceive, and cases in which a medium is processed. 
     Also, it is assumed that “printing medium” broadly represents not only paper used in a typical printing apparatus, but also things that can receive ink such as cloths, plastic films, metal plates, glass, ceramics, wood materials, and hides. 
     Furthermore, similarly to the foregoing definition of “print”, “ink” (also referred to as “liquid”) should be broadly interpreted. Accordingly, it is assumed that “ink” represents liquids that by being applied to a printing medium can be supplied in the forming of images, designs, patterns, or the like, processing of printing mediums, or processing of ink (for example, insolubilization or freezing of a colorant in ink applied to a printing medium). 
     Furthermore, it is assumed that “print element”, unless specified otherwise, means a discharge port and an element that produces energy that is used for discharge of ink and a fluid channel that communicates therewith collectively. 
     Furthermore, it is assumed that “nozzle”, unless specified otherwise, means a discharge port and an element that produces energy that is used for discharge of ink and a fluid channel that communicates therewith collectively. 
     An element substrate for a printhead (a head substrate) used below does not indicate a mere substrate consisting of a silicon semiconductor but rather indicates a configuration in which elements, wiring, and the like are disposed. 
     Furthermore, “on the substrate” means not only simply on top of the element substrate, but also the surface of the element substrate, and the inside of the element substrate in the vicinity of the surface. Also, “built-in” in the present invention does not mean that separate elements are simply arranged as separate bodies on a substrate surface, but rather means that the elements are formed and manufactured integrally on the element board by a semiconductor circuit manufacturing process. 
     For an inkjet printhead (hereinafter referred to as a printhead) having the most important features of the present invention, on a printing element substrate of the printhead, a plurality of printing elements and a driving circuit that drives these printing elements are implemented on the same substrate. Configuration may be taken such that, a plurality of printing element substrates are integrated in a printhead, and these element substrates have a cascade connection structure for example. Accordingly, this printhead is able to achieve a print width that is relatively long. Accordingly, the printhead is used not only in a serial type printing apparatus that is commonly found, but also in a printing apparatus equipped with a full-line printhead whose print width corresponds to the width of the printing medium. Also, the printhead is used in large format printers that use printing mediums of a large size such as A0 and B0 in serial type printing apparatuses. 
     Accordingly, firstly, a printing apparatus in which the printhead of the present invention is used is described. 
     [Device Configuration] 
       FIG. 1  is a view illustrating an example of a configuration of an inkjet method printing apparatus (hereinafter referred to as a printing apparatus) that the present invention can be applied to.  FIG. 1  illustrates an embodiment of a typical inkjet printing apparatus and illustrates a printing apparatus of a scan method. Note that limitation is not made to this configuration and it is possible to apply the present invention to a printing apparatus equipped with a full-line type inkjet printhead (hereinafter referred to as a printhead) for example. Also, the present invention is not limited to this inkjet method and may be applied to a printing apparatus of another method. 
     In  FIG. 1 , the printing apparatus according to the present embodiment is equipped with a feed mechanism, a paper conveyance mechanism, a discharge mechanism, a carriage unit, and the like. In the present embodiment, description of a printing apparatus that performs a print by discharging ink from a discharge port of a printhead  3  to a printing medium based on image information is given as an example. 
     The printhead  3  is mounted as a print unit on a carriage  50  that moves back and forth (main scanning movement). It becomes possible for the printhead  3  to be released by a head set lever  51 . Furthermore, one or a plurality of ink tanks  71  corresponding to each color of ink are removably mounted to the printhead  3 . Also, the printhead  3  comprises a temperature sensor  59  for detecting its own temperature. As a printing medium, various materials may be used such as a print paper, a plastic sheet, cloth, and unwoven fabric if a sheet type image printing is possible. In the description below, a sheet type printing medium is referred to as “a sheet”. Hereinafter, the configuration of the printing apparatus is described for each mechanism. 
     The feed mechanism is configured by attaching a pressing plate  21  for stacking sheets, a feed roller for feeding sheets, a separation roller for separating sheets, a return lever for returning sheets to the stacking position, a moveable side guide  26  for indicating the edge of the sheets, and the like to a feed base  20 . A feed tray for holding stacked sheets P is attached to an exterior covering of the feed base  20  or the printing apparatus. 
     Regarding operation of the feed mechanism, the pressing plate  21  is released by a pressing plate cam (not shown) and the separation roller is released by a control cam in a normal standby state. Furthermore, the return lever returns a sheet P to the stacking position, and also is held in the stacking position such that it blocks the stacking port at a time of stacking so that the sheet P does not enter the interior of the printer. From this state, when the feed operation starts, firstly, the separation roller contacts the feed roller by driving of a motor. Then, the return lever is released and the pressing plate  21  contacts the feed roller. In this state, feeding of the sheet P is started. The sheet P is restricted by the front side separation unit (not shown) arranged in the feed base  20 , and only a predetermined number of sheets P are fed to a nip unit between the feed roller and the separation roller. The fed sheets P are separated by the nip unit and only the top sheet is conveyed (fed). 
     When the sheet P reaches a conveyance roller pair comprising a conveyance roller  36  and a pinch roller  37  described later, the pressing plate  21  is released by a pressing plate cam (not shown) and the separation roller is released by a control cam (not shown). Also, the return lever is returned to the stacking position by a control cam (not shown). At that time, the sheet that has reached the nip unit between the feed roller and the separation roller is returned to the stacking position by the return lever. 
     The paper conveyance mechanism is attached to a chassis  55  comprising bent raised metal plates. The paper conveyance mechanism has a PE sensor (paper edge detection sensor) and the conveyance roller  36  which conveys the sheet P. The conveyance roller  36  is structured by ceramic micro-particles coating the surface of a metal shaft, and is attached to the chassis  55  by pivotally supporting both ends of a metal shaft portion by bearings (not shown). 
     A plurality of pinch rollers  37  that follow a rotation are disposed to contact the conveyance roller  36 . The pinch roller  37  is held in a pinch roller holder  30  and creates conveyance power for the sheet P by being pressed against the conveyance roller  36  by a pinch roller spring (not shown). Here, the rotation shaft of the pinch roller holder  30  is pivotally supported by the bearing of the chassis  55 , and rotates about this rotation shaft. Furthermore, a paper guide flapper and a platen  34  that guide sheets are disposed at an inlet of the paper conveyance mechanism to which the sheet P is conveyed. Also, in the pinch roller holder  30 , a PE sensor lever (not shown) is arranged in order to transmit detection of the leading edge and the trailing edge of the sheet P to the PE sensor (not shown). 
     The platen  34  is positioned attached to the chassis  55 . The paper guide flapper (not shown), fits with the conveyance roller  36 , can centrally rotate a sliding bearing unit (not shown), and is positioned in contact with the chassis  55 . Furthermore, at the downstream sheet conveyance side direction of the conveyance roller  36 , the printhead  3  is arranged as a print unit for printing an image based on image information. 
     The sheet P is conveyed along the top surface of the platen  34  by the pinch roller  37  following the rotation when the conveyance roller  36  rotates by the conveyance motor. A rib which is a conveyance guide surface (vertical direction nominal position) is formed on the platen  34 . The rib manages a gap (distance) between the sheet P and the printhead  3  as well as regulates cockling (corrugation) of the sheet P by cooperating with a discharge mechanism described later. Thereby, image quality degradation due to cockling of the sheet portion printed by the printhead  3  is prevented. Driving of the conveyance roller  36  is performed by a rotational force of a conveyance motor comprising a DC motor being transferred to a pulley  361  arranged on the conveyance roller shaft by a timing belt  541 . 
     The carriage unit has the carriage  50  on which the printhead  3  is mounted and which moves back and forth. The carriage  50  is supported and guided such that it can move back and forth (main scanning) along a guide shaft  52  and a guide rail  54  installed in a direction intersecting (normally perpendicularly to) the direction of conveyance of the sheet P. The guide shaft  52  configures a guidance mechanism for guiding back and forth movement of the carriage  50 . The guide rail  54  also has a function for maintaining a distance (a gap) between the printhead  3  and the sheet P at an appropriate value by supporting a rear end portion of the carriage  50 . The guide shaft  52  is configured by an axis member attached to the chassis  55 , and the guide rail  54  is formed built into part of the chassis  55 . A thin sliding sheet  53  made from SUS or the like is stretched over the part of the guide rail  54  over which the carriage  50  slides, and achieves a reduction of a sliding sound. 
     The carriage unit prevents evaporation of liquid such as ink by causing the printhead  3  to move to a capping position, and performing capping by a cap  61 . A recovery motor  69  operates as a driving source of the cap  61  or the like. Additionally, it performs cleaning of the nozzle surface by a blade  62 . For the blade  62 , cleaning is performed by a blade cleaner  66 . Also, the printhead  3  causes ink to discharge from the nozzle and causes ink clogging to be reduced by operation of a pump  60  equipped within a recovery mechanism  6 . 
     Two discharge rollers are arranged in the discharge mechanism. In each discharge roller, a spur is pressed so as to be able to follow rotation. By causing each discharge roller to rotate in synchronism with the conveyance roller  36 , the sheet P that is printed is ejected to the outside of the main body of the printing apparatus. In the present embodiment, the discharge roller is attached to the platen  34 . The discharge roller of the upstream side of the conveyance direction is configured by arranging a plurality of rubber units (discharge roller rubber) on a metal shaft. A first discharge roller (not shown) is driven by drive from the conveyance roller  36  being transferred via an idler gear. A second discharge roller  41  is configured by an elastic body such as a plurality of elastomers being attached to a resin shaft. The second discharge roller  41  is driven by drive being transferred from the first discharge roller (not shown) via an idler gear. 
     By the foregoing configuration, the sheet P printed on by the printhead  3  in the carriage unit is pinched in the nip unit of each spur and the discharge roller, is ejected to the outside of the main body of the printing apparatus, and is placed on a discharge tray. The discharge tray has a divided structure comprising a plurality of members which are pulled out when used. Also, the discharge tray is formed so as to become higher towards a leading edge, and is formed to have high side edges, and thereby it improves stackability of ejected sheets P and prevents scraping of the printing surface of the sheet P. 
     [Control Configuration] 
       FIG. 2  is a block diagram illustrating a control configuration of the printing apparatus illustrated in  FIG. 1 . 
     As illustrated in  FIG. 2 , a controller  600  is configured by an MPU  601 , a ROM  602 , an application-specific integrated circuit (ASIC)  603 , a RAM  604 , a system bus  605 , an A/D converter  606 , and the like. The ROM  602  stores programs corresponding to various control sequences, particular tables, and other fixed data. The ASIC  603  generates control signals for control of a carriage motor  635 , control of a conveyance motor  636 , and control of the printhead  3 . The ASIC  603  performs control of a signal pulse width described later. The RAM  604  is used as an image data loading region or a work region for execution of programs. The system bus  605  performs reception of data by mutually connecting with the MPU  601 , the ASIC  603 , and the RAM  604 . The A/D converter  606  takes an analog signal as input from a sensor group described below, performs an A/D conversion on it, and supplies the digital signal to the MPU  601 . 
     Also, in  FIG. 2 , a host apparatus  631  is an external information processing apparatus such as a PC which is a supply source of image data. Image data, commands, statuses, and the like are transmitted/received by packet communication via an interface (I/F)  632  between the host apparatus  631  and the printing apparatus. Note, configuration may be taken such that a USB interface is further included separately from the network interface as the interface  632 , and such that bit data or raster data transferred serially from the host can be received. 
     A switch group  610  is configured from a power supply switch  611 , a print switch  612 , a recover switch  613 , and the like. 
     A sensor group  620  is a sensor group for detecting an apparatus state and is configured from a position sensor  621 , a temperature sensor  622 , and the like. Also, a photosensor that detects a remaining amount of ink is arranged. 
     A carriage motor driver  633  is a carriage motor driver that drives the carriage motor  635  in order to cause the carriage  50  to scan back and forth. A conveyance motor driver  634  drives the conveyance motor  636  in order to convey the sheet P. 
     The ASIC  603 , at a time of print scanning by the printhead  3 , transfers data for driving a heating element (heater for ink discharge) to the printhead  3  while directly accessing the storage region of the RAM  604 . In addition, various display units configured by an LCD or an LED are equipped as a user interface in the printing apparatus. 
     [Printhead Configuration] 
     Description is given regarding the details of the printhead  3  according to the present invention by using  FIG. 3 .  FIG. 3  is a view illustrating an example of a configuration of a printing element substrate arranged on the printhead  3  according to the present application invention. 
     An energy generation element array, in which a plurality of heaters  101  that can be energized via the wiring are arrayed, is formed on a silicon semiconductor substrate  110  of the printhead  3 . The wiring can also be applied by either single layer wiring or multilayer wiring. A channel formation member (coating resin material)  111  is arranged on the silicon semiconductor substrate  110 , and a plurality of discharge ports  100  corresponding to each of the plurality of heaters  101  are formed in the channel formation member  111 . The prepared silicon semiconductor substrate  110  is a semiconductor substrate (printing element substrate) of silicon or the like, and the heaters  101  are formed by a material such as tantalum silicon nitride (TaSiN). 
     A bubbling phenomenon occurs within the liquid when thermal energy generated by a heater  101  is applied to a liquid such as ink. Also, an ink droplet from the discharge port  100  is discharged by the bubbling energy. In inkjet printheads of recent years, the density of an array of heaters has been increased and their number has been increased to improve printing density (resolution) and to increase printing speed. 
     Although not illustrated, a driver that drives a respective heater  101 , a shift register of the same number of bits as there are heaters, and a latch circuit that temporarily stores print data outputted therefrom are further arranged on the silicon semiconductor substrate  110 . Note that the shift register is for sending, in parallel, image data inputted serially to each driver. 
     The discharge port  100  is arrayed at a predetermined pitch P in two discharge port lines L 1  and L 2 . Furthermore, the discharge ports  100  of the discharge port line L 1  side and the discharge ports  100  of the discharge port line L 2  side are respectively displaced by a half pitch (P/2) in the direction in which they are arranged. The heater  101  also is arranged at the same pitch as the discharge port  100 . 
     On the silicon semiconductor substrate  110 , a common liquid chamber  112  and a hole shaped ink supply port  500  are formed, and between the silicon semiconductor substrate  110  and the channel formation member  111 , a plurality of ink channels (bubbling chambers)  300  respectively joining the plurality of discharge ports  100  are formed. The channel formation member  111  has a wall of ink channels  300 , and forms the ink channels  300  by contacting the silicon semiconductor substrate  110 . Ink supplied through the common liquid chamber  112  and the ink supply port  500  is introduced into respective ink channels  300  from an ink supply port member  140 . The ink within an ink channel  300  bubbles by generated heat of the heater  101  corresponding to that ink channel  300 , and is discharged from the discharge port  100  corresponding to the ink channel  300  by the bubbling energy. 
     [Heater Driving Circuit] 
       FIG. 4  is an explanatory view of a heater driving circuit  23 . Circuitry of a single heater  101  is represented in  FIG. 4  in order to simplify the explanation. Accordingly, a plurality of heaters  101  and heater driving circuits  23  corresponding thereto are actually disposed on the silicon semiconductor substrate  110  as described above. One terminal of the heater  101  is connected to a source terminal of an NMOS transistor  102 . The other terminal of the heater  101  is connected to a source terminal of a PMOS transistor  103 . Drain terminals of the NMOS transistor  102  and the PMOS transistor  103  are connected to a power supply wiring  104  and a power supply wiring  105  respectively. The output of a voltage converter circuit  106  illustrated in  FIG. 5A  is inputted to the gate terminal of the NMOS transistor  102 . The output of a voltage converter circuit  107  illustrated in  FIG. 5B  is inputted to the gate terminal of the PMOS transistor  103 . Voltages from an external power supply are inputted via input terminals that the silicon semiconductor substrate  110  comprises to the power supply wiring  104  and  105 , and voltages of a high electric potential side VH and a low electric potential side GNDH respectively are applied. These electric potentials are inputted via the input terminals from the outside, and become the voltage VH of a high voltage side wiring and the voltage GNDH of a low voltage side wiring. The voltage GNDH, to use another expression, is a ground voltage. Note, there is a wiring resistance r 1  in the power supply wiring  104  and a wiring resistance r 2  in the power supply wiring  105 . 
     The heater driving circuit  23  is equipped with the two voltage converter circuits  106  and  107 . The voltage converter circuit  106  takes an output signal of a selection circuit  108  as input and outputs to the gate of the NMOS transistor  102 . The voltage converter circuit  107  takes a signal HE  2  as input and outputs to the gate of the PMOS transistor  103 . Note, the circuitry for generation of signals that the voltage converter circuit  106  and the voltage converter circuit  107  take as input is not limited to this form and configuration. 
     A configuration of the voltage converter circuit  106  is illustrated in  FIG. 5A . The voltage converter circuit  106  takes voltages X and GNDH as inputs, and performs a conversion of the amplitude of the input signal. The voltage converter circuit  106  generates a gate voltage for putting the NMOS transistor  102  in an on state based on the voltage X inputted from outside of the silicon semiconductor substrate  110 . The voltage X is a voltage that is different from the power supply voltage GNDH supplied to the drain of the PMOS transistor  103  from outside of the silicon semiconductor substrate  110 . 
     A configuration of the voltage converter circuit  107  is illustrated in  FIG. 5B . The voltage converter circuit  107  takes voltages Y and VH as inputs, and performs a conversion of the amplitude of the input signal. The voltage converter circuit  107  generates a gate voltage for putting the PMOS transistor  103  in an on state based on the voltage Y inputted from outside of the silicon semiconductor substrate  110 . The voltage Y is a voltage that is different from the power source voltage VH supplied to the drain of the NMOS transistor  102 . A gate voltage for putting the PMOS transistor  103  in an on state and a gate voltage for putting the NMOS transistor  102  in an on state are voltages decided based on a power supply VH supplied to the drain of the NMOS transistor  102  or a power supply GNDH supplied to the drain of the PMOS transistor  103 . The selection circuit  108  outputs a signal according to image data to the voltage converter circuit  106 . The voltage converter circuit  106  converts the input signal into a driving voltage (driving signal) of the NMOS transistor  102 . Meanwhile, the voltage converter circuit  107  converts the input signal into a driving voltage (driving signal) of the PMOS transistor  103 . In this way, the voltage converter circuits  106  and  107  are circuits that increase the signal amplitude of inputted signals. 
       FIG. 6  is a graph representing a transition of voltage across a heater according to the number of simultaneously driven heaters. In  FIG. 6 , the ordinate is indicated as the voltage across the heater [V] and the abscissa is indicated as the number of simultaneously driven heaters [bit]. A graph  151  illustrates a conventional voltage across the heater design value. According to the graph  151 , it is possible for the voltage across the heater to be kept fixed for up to the number of simultaneously driven heaters A by the heater driving circuits (hereinafter referred to as voltage compensation circuits) supplying a stable voltage to the heaters  101  described in  FIG. 4 . However, when the number of heaters becomes greater than or equal to the number of simultaneously driven heaters A, the range (hereinafter referred to as a voltage compensation range) in which it is possible to keep the voltage across the heater fixed is exceeded due to a wiring resistance voltage drop. As a result, it becomes impossible to keep the voltage across the heater fixed, and the voltage across the heater becomes lower. The number of simultaneously driven heaters A is decided during design by calculating a worst voltage drop value from a wiring resistance of wiring to the heaters  101  for the printhead  3  on the whole and the current value that flows due to the maximum number of heaters to be driven simultaneously. For this reason, the number of heaters driven simultaneously A will not be exceeded in normal usage, and the voltage across the heater will be kept fixed at all times. 
     However, due to compensate for the worst voltage drop value, the voltage across the heater will be a value that is significantly lower than the heater driving power supply voltage (voltage VH) that is actually being applied. At that time, when the voltage applied to the drain of the transistor falls below the gate voltage, the voltage supplied to the heater will not be fixed. Also, that portion, as loss, will become heat, and it is envisioned that the printhead will heat up more than necessary. 
     Accordingly, in the present invention, as in the graph  152 , the concept of setting the applied voltage (an applied voltage such that, in all envisioned usage environments, a voltage change substantially does not arise) is changed from what was conventional, and heat generation due to voltage loss is reduced by setting applied voltages including those in a region exceeding the voltage compensation range. Also, in the region in which the voltage compensation range is exceeded, a correction of a voltage input period is applied. By this, the objective described above, specifically keeping the voltage across the heater fixed while also suppressing a reduction in throughput due to a temperature rise is resolved. 
     Specifically, a voltage so as to result in B+α is applied to a source follower (the gate of the NMOS transistor  102 ) that is above the heater  101 . Here, for the voltage B+α, a value such that a loss of voltage does not occur is set for the NMOS transistor  102 . By this, regarding the voltage compensation range, it ceases to be possible to keep the voltage across the heater fixed at the number of simultaneously driven heaters A-n as illustrated in the graph  152  of  FIG. 6  when considering a voltage drop due to the resistance of wiring to the heaters  101  for the printhead  3  on the whole. Here, a control region  150  where control of the pulse width for driving the heater  101  is necessary is made to be the region from the number of simultaneously driven heaters A-n to the number of simultaneously driven heaters A which is the minimum number at which the voltage across the heater needs to be kept fixed. Also, in this control region  150 , the amount by which the voltage across the heater deviates from the fixed amount is corrected by pulse width modulation (hereinafter referred to as PWM control). By this, it becomes possible to keep the voltage across the heater fixed up to the number of simultaneously driven heaters A. 
       FIG. 8A  and  FIG. 8B  are for describing a relationship of the number of simultaneously driven heaters and a pulse width. In  FIG. 8A  and  FIG. 8B , the ordinate indicates a main pulse width and the abscissa indicates the number of simultaneously driven heaters. As illustrated in  FIG. 8A , the pulse width is Pc (fixed) when the number of simultaneously driven heaters is less than A-n. When the number of simultaneously driven heaters is greater than or equal to A-n, the pulse width is lengthened in accordance with the number of simultaneously driven heaters increasing. Note that control may be taken so that the pulse width is lengthened stepwise to Pc, Pv 1 , Pv 2  as illustrated in  FIG. 8B . 
     Here, description is given regarding a case of a voltage compensation up to the number of simultaneously driven heaters A. However, rather than being limited to this configuration, configuration may be taken such that the number of simultaneously driven heaters actually increases beyond the number of simultaneously driven heaters A, and PWM control is performed in conjunction with reaching the region outside of the voltage compensation range. 
       FIG. 7A  to  FIG. 7C  are views illustrating one example of a PWM control method. In the present embodiment, the heater driving pulse comprises a first pulse  91  in which the width is controlled depending on the temperature of the printhead  3 , a second pulse  93  in which the width is controlled depending on the number of simultaneously driven heaters, and an interval  92  positioned between the first pulse  91  and the second pulse  93 . In other words, the heater driving pulse here is assumed to be of a double pulse form. Note, although description is given using an example of a double pulse, a single pulse where only the second pulse is present may be used, as necessary. 
     Description is given regarding control when the number of simultaneously driven heaters reaches the control region  150 . In the case of the example of  FIG. 6 , this corresponds to a case where the number of simultaneously driven heaters reaches A-n. In such a case, the first pulse  91  and the interval  92  are modulated in combination with the temperature change as with the change from the heater driving pulse  153  to the heater driving pulse  154  according to the temperature of the printhead  3 , as in  FIG. 7A . In other words, the pulse width of the first pulse  91  increases in accordance with the temperature change. Note, it is possible to detect the temperature of the printhead  3  by the temperature sensor  59  in the case of the example of the device configuration illustrated in  FIG. 1 . 
     Next, the second pulse  93  is modulated in combination with the number of simultaneously driven heaters as with the change from the heater driving pulse  153  to the heater driving pulse  155 , as in  FIG. 7B , in order to correct for the amount that the voltage dropped outside of the voltage compensation range. In other words, as the number of simultaneously driven heaters increases, the pulse width of the second pulse  93  increases. In the present embodiment, although an example in which the pulse width is linked to the number of simultaneously driven heaters is illustrated, limitation is not made to this configuration. For example, configuration may be such that the voltage across the heater is monitored and PWM control is performed when a state in which the voltage is not kept fixed is entered. 
     Finally, as in  FIG. 7C , PWM control so as to achieve the heater driving pulse  156  is performed by combining  FIG. 7A  and  FIG. 7B . 
     Note, in the range in which PWM control is performed, it is necessary to have a pulse width that fits into a discharge cycle (driving cycle) and to maintain at a minimum a voltage level at which the heaters  101  can discharge. Note, the pulse that drives the heater  101  is not limited to a double pulse, and may be a single pulse as illustrated in  FIG. 9 . In the case of  FIG. 9 , only the second pulse  93  is present in the heater driving pulse  901 , and control is such that the pulse width of the second pulse  93  is increased. 
     Furthermore, although in the present invention, using the example of  FIG. 4 , description is given regarding a voltage compensation circuit in which transistors above and below the heater  101  are provided in a source follower configuration, limitation is not made to this. The present invention is valid in circuit configurations having a similar effect such as are illustrated in  FIG. 10A  to  FIG. 10D . In these configurations, since a voltage converter circuit in which though the voltage compensation range is narrower, only one transistor is arranged, there is the advantage that a smaller circuit scale suffices. In  FIG. 10A  to  FIG. 10D , the voltage converter circuit  106  ( 107 ) illustrated in  FIG. 4  is omitted in order to simplify the explanation. 
     In  FIG. 10A , the transistors connected to the two sides of the heater  101  are NMOS transistors  102 . The NMOS transistor  102  connected between the heater  101  and the power supply wiring  104  is a source follower transistor. The NMOS transistor  102  connected between the heater  101  and the power supply wiring  105  is a source-ground connection. 
     In  FIG. 10B , the transistors connected to the two sides of the heater  101  are PMOS transistors  103 . The PMOS transistor  103  connected between the heater  101  and the power supply wiring  105  is a source follower transistor. The PMOS transistor  103  connected between the heater  101  and the power supply wiring  104  is a source-ground connection. 
     In  FIG. 10C , the NMOS transistor  102  is connected between the heater  101  and the power supply wiring  104 . The NMOS transistor  102  connected between the heater  101  and the power supply wiring  104  is a source follower transistor. 
     In  FIG. 10D , the PMOS transistor  103  is connected between the heater  101  and the power supply wiring  105 . The PMOS transistor  103  connected between the heater  101  and the power supply wiring  105  is a source follower transistor. As described above, the driving circuit that drives the heater is equipped with at least one source follower transistor. 
     When the number of heaters that are simultaneously driven exceeds a predetermined number, the voltage across the heater is kept fixed by performing PWM control in conjunction therewith, and the voltage across the heater can be made higher than what was possible conventionally, and voltage loss can be reduced and heating up of the head suppressed. By this, it is possible to achieve both keeping voltage across a heater fixed and suppressing deterioration of throughput. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-208839, filed Oct. 25, 2016, which is hereby incorporated by reference herein in its entirety.