Patent Publication Number: US-7896469-B2

Title: Head substrate, printhead, head cartridge, and printing apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a head substrate, printhead, head cartridge, and printing apparatus. Particularly, the present invention relates to a head substrate prepared by forming, on the same substrate, an electrothermal transducer for generating heat energy necessary to print, and a driver circuit for driving the electrothermal transducer, a printhead using the head substrate, a head cartridge using the printhead, and a printing apparatus. 
     2. Description of the Related Art 
     The electrothermal transducers (heaters) and driver circuits of a printhead mounted in a conventional inkjet printing apparatus are formed on the same substrate by a semiconductor process technique as disclosed in, for example, U.S. Pat. No. 6,290,334. There has already been proposed a substrate on which an ink supply channel for supplying ink is arranged on the substrate and heaters are arrayed at positions opposite to each other near the ink supply channel. 
       FIG. 6  is a view showing the layout of a head substrate used in a conventional inkjet printhead. As a method of driving a printhead of this type, time-divisional driving is put into practical use. A maximum power capable of simultaneously driving heaters has an upper limit. According to the time-divisional driving, a plurality of heaters are divided into M heater blocks each of N heaters, and N heaters of each heater block are simultaneously driven. 
     In  FIG. 6 , a substrate  100  is formed by integrating, by a semiconductor process technique, heaters and driver circuits for driving them. A heater &amp; driver array  101  is an array of heaters and drivers. The driver includes a driver transistor which serves as a driving element. An ink supply channel  102  supplies ink from the back surface of the substrate. Each shift register (S/R)  103  temporarily stores print data. Each latch circuit  104  latches print data stored in the corresponding shift register (S/R)  103  at once. Each decoder  105  selects a desired heater block of the heater &amp; driver array  101 . Each input circuit block  106  includes a buffer circuit for inputting digital signals to the shift register  103  and decoder  105 . The decoder receives a block selection signal as a control signal. Signal lines  107  transmit signals from the shift register  103  and decoder  105  to select individual segments in the heater &amp; driver array  101 . Each contact pad  110  is used to input/output an electrical signal from/to outside the substrate. 
       FIG. 7  is a circuit diagram showing an equivalent circuit corresponding to one segment (one heater) of the heater &amp; driver array  101  which are integrated on the head substrate shown in  FIG. 6  and drive heaters for discharging ink. 
     In a head substrate layout as shown in  FIG. 6 , the contact pads  110 , input circuit blocks  106 , decoders  105 , shift registers  103 , and latch circuits  104  are arranged at the ends of the head substrate  100  in a longer side direction. In this layout, the signal lines  107  are provided along the longer side direction of the head substrate  100 . 
     In  FIG. 7 , an AND circuit  201  calculates the logical product of two input signals. The AND circuit  201  receives a block selection signal which is sent from the decoder  105  to select heaters of each block, and a print data signal which is transferred to the shift register  103  and latched by the latch circuit  104 . Based on the logical product, each segment can be selectively turned on. An inverter circuit  202  buffers an output from the AND circuit  201 . A VDD power supply line  203  serves as the power supply of the inverter circuit  202 . An inverter circuit  204  buffers an output from the inverter circuit  202 . A VH power supply line  205  is used for supplying a voltage to be applied to a heater. A driver transistor  207  serves as a switching element for switching between supplying a current and not supplying the current, to a heater  206 . A VHTM power supply line  208  serves as a power supply for supplying power to the inverter circuit  204  functioning as a buffer, thereby applying a gate voltage to the driver transistor  207 . A voltage conversion circuit  209  converts the voltage of an output signal from the AND circuit  201  into a voltage VHTM for driving the driver transistor  207 . The voltage conversion circuit  209  incorporates a level converter  210  which converts a voltage to the voltage VHTM. 
       FIG. 8  is an equivalent circuit diagram of a circuit corresponding to one bit of the shift register  103  and latch circuit  104  which temporarily store print data. 
     In  FIG. 8 , print data DATA is input to the shift register in synchronism with a clock CLK, and the input print data is latched in synchronism with a latch signal LT. When a heat enable signal HE is input, a print data signal is output from the latch circuit to the AND circuit  201  while the heat enable signal is enabled. 
       FIG. 9  is a timing chart for explaining a series of operations from receiving print data in the shift register  103  to driving the heater  206  by supplying a current to it. 
     In  FIG. 9 , print data is supplied to a data pad (not shown) in synchronism with the clock CLK input to a clock pad (not shown). The shift register  103  temporarily stores the print data. The latch circuit  104  latches the print data in synchronism with the latch signal LT supplied to a latch pad (not shown). Then, the logical product of a block selection signal for selecting heaters of a desired block, and a print data signal held in accordance with the latch signal LT is calculated. A heater current (current VH) flows in synchronism with the heat enable signal HE, which directly determines a current driving time, and the logical product. 
     Printing is performed by repeating the series of operations for respective blocks. 
       FIG. 10  is a view showing connection of power supply wiring lines in the head substrate shown in  FIG. 6 . 
     In  FIG. 10 , power supply pads VH  130 ,  132 ,  134 , and  136  supply voltages to be applied to heaters. Ground pads GND  131 ,  133 ,  135 , and  137  correspond to the power supply pads. Wiring lines  140  are divided to independently supply power from the power supply pads VH to respective blocks. Wiring lines  141  are divided to feed back power from the blocks to the ground pads GND. These wiring lines will be called VH power supply wiring lines and GND wiring lines. 
     Segments including heaters and driver transistors arranged on the head substrate are divided into  16  groups A to P. Power is independently supplied and fed back to and from each group in order to keep power loss constant by making uniform the wiring resistances of the VH power supply wiring lines and GND wiring lines which are connected to the respective groups. The widths of the wiring lines are adjusted to have the same resistance value. Each group is comprised of segments (including heaters), respectively belonging to different time-divisionally driven blocks. 
     A head substrate on which ink supply channel arrays are staggered is proposed in, e.g., Japanese Patent Publication Laid-Open No. 2006-88648. 
     However, according to the power supply wiring connection as shown in  FIG. 10 , the wiring becomes longer as the longer side of the chip (head substrate) becomes longer. In addition, as the group division count increases, the widths of wiring lines independently connected to respective groups become narrower, and the wiring resistance tends to rise as a whole. The increase in wiring resistance causes so-called power loss because power, which should be originally consumed by heaters, is consumed by the wiring to a certain degree. If the original power supply voltage is increased to compensate for the power loss, this adversely affects the durable service life of heaters. Further, heat generated by power consumption by the wiring raises the temperature of the printhead itself, adversely affecting the ink discharge characteristic. 
     As for a head substrate on which ink supply channel arrays are staggered, the above reference (Japanese Patent Publication Laid-Open No. 2006-88648) does not disclose a specific layout of circuits on the head substrate. A circuit layout effectively utilizing a head substrate with a limited area is required. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is conceived as a response to the above-described disadvantages of the conventional art. 
     For example, a head substrate according to this invention is capable of increasing the layout efficiency, reducing power loss, and reducing the substrate area by effectively utilizing the area of a head substrate on which ink supply channels are staggered. 
     According to one aspect of the present invention, preferably, there is provided a rectangular head substrate used in an inkjet printhead having a printing element array of printing elements which print by discharging supplied ink, and a driving element array of driving elements which drive the printing elements, the head substrate comprising: a plurality of ink supply channels having a predetermined length along a longer side direction of the head substrate; a plurality of element arrays which are arranged on at least one side of each of the plurality of ink supply channels, and each of which has the printing element array and the driving element array; and a signal line which is provided along the longer side direction of the head substrate and transmits a signal to the plurality of element arrays, wherein the element arrays and the signal line are provided in an order named from the plurality of ink supply channels toward a longer side of the head substrate, plural pairs of the ink supply channels and the element arrays corresponding to the respective ink supply channels are arrayed in a staggered manner in the longer side direction of the head substrate, and another building element, of the head substrate, electrically connected to two adjacent element arrays is arranged in an area surrounded by the signal line and every other element array out of the plurality of element arrays arranged in the staggered manner. 
     According to another aspect of the present invention, preferably, there is provided a printhead using a head substrate described above. 
     According to still another aspect of the present invention, preferably, there is provided a head cartridge integrating the above printhead and an ink tank containing ink to be supplied to the printhead. 
     According to still another aspect of the present invention, preferably, there is provided a printing apparatus using the above printhead. 
     The invention is particularly advantageous since a power supply pad is arranged in an area formed when ink supply channels and corresponding element arrays are staggered, and the power supply pad supplies power to an element array adjacent to the power supply pad. The area can be effectively utilized, and the distance between the pad and the element array can be shortened. Hence, the wiring resistance for power supply can be suppressed to reduce power loss. 
     Since the area which is free on a conventional head substrate can be effectively utilized, the head substrate can be efficiently utilized, contributing to downsizing the head substrate. 
     Since the power supply line need not be made thick, the layout area can be reduced, contributing to downsizing the head substrate. 
     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 schematic perspective view showing the outer appearance of the structure of an inkjet printing apparatus as a typical embodiment of the present invention; 
         FIG. 2  is a block diagram showing the arrangement of the control circuit of the printing apparatus; 
         FIG. 3  is a perspective view showing the outer appearance of the structure of a head cartridge IJC which integrates an ink tank and printhead; 
         FIG. 3A  is a plan view showing the ink discharge surface of a printhead  3 ; 
         FIG. 4  is a view showing the layout of a head substrate according to an embodiment of the present invention; 
         FIG. 4A  is an equivalent circuit diagram showing the detailed arrangement of a group  101 G including a heater array, driver array, and the like; 
         FIG. 4B  is an enlarged view of part of  FIG. 4  showing a more specific layout of two groups  101 G 1  and  101 G 2  and their periphery; 
         FIG. 5  is a view showing another layout of the head substrate according to the embodiment of the present invention; 
         FIG. 5A  is an enlarged view of part of  FIG. 5  showing a more specific layout of three groups  101 G 1 ,  101 G 2 , and  101 G 3  and their periphery; 
         FIG. 5B  is a view showing the layout of a head substrate using a through-hole electrode; 
         FIG. 5C  is a sectional view showing the form of backside mounting using a through-hole electrode formed in a substrate; 
         FIG. 5D  is a view showing a layout when a VHT buffer is arranged in a free area formed by a staggered array; 
         FIG. 5E  is an equivalent circuit diagram for explaining in more detail a converted voltage generator and driver array in  FIG. 5D ; 
         FIG. 6  is a view showing the layout of a head substrate according to a conventional art; 
         FIG. 7  is a circuit diagram showing an equivalent circuit corresponding to one segment of the heater &amp; driver array  101  which is integrated on the head substrate shown in  FIG. 6  and drives heaters for discharging ink; 
         FIG. 8  is an equivalent circuit diagram of a circuit corresponding to one bit of a shift register  103  and latch circuit  104  which temporarily store print data; 
         FIG. 9  is a timing chart for explaining a series of operations from receiving print data in the shift register  103  to driving the heater  206  by supplying a current to it. 
         FIG. 10  is a view showing connection of power supply wiring lines in the head substrate shown in  FIG. 6 ; and 
         FIG. 11  is a view showing another layout of the head substrate as a comparative example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. The same reference numerals denote the same parts, and a description thereof will not be repeated. 
     In this specification, the terms “print” and “printing” not only include the formation of significant information such as characters and graphics, but also broadly includes the formation of images, figures, patterns, and the like on a print medium, or the processing of the medium, regardless of whether they are significant or insignificant and whether they are so visualized as to be visually perceivable by humans. 
     Also, the term “print medium” not only includes a paper sheet used in common printing apparatuses, but also broadly includes materials, such as cloth, a plastic film, a metal plate, glass, ceramics, wood, and leather, capable of accepting ink. 
     Furthermore, the term “ink” (to be also referred to as a “liquid” hereinafter) should be extensively interpreted similar to the definition of “print” described above. That is, “ink” includes a liquid which, when applied onto a print medium, can form images, figures, patterns, and the like, can process the print medium, and can process ink (e.g., can solidify or insolubilize a coloring agent contained in ink applied to the print medium). 
     The term “printhead substrate (head substrate)” in the description not only includes a simple substrate made of a silicon semiconductor, but also broadly includes a substrate with elements, wiring lines, and the like. 
     The expression “on a substrate” not only includes “on an element substrate”, but also broadly includes “on the surface of an element substrate” and “inside of an element substrate near its surface”. The term “built-in” in the present invention not only includes “simply arrange separate elements on a substrate surface”, but also broadly includes “integrally form and manufacture elements on an element substrate by a semiconductor circuit manufacturing process or the like”. 
     &lt;Description of Inkjet Printing Apparatus (FIG.  1 )&gt; 
       FIG. 1  is a schematic perspective view showing the outer appearance of the structure of an inkjet printing apparatus  1  as a typical embodiment of the present invention. 
     In the inkjet printing apparatus (to be referred to as a printing apparatus hereinafter), as shown in  FIG. 1 , a carriage  2  supports a printhead  3  for printing by discharging ink according to the inkjet method. A transmission mechanism  4  transmits a driving force generated by a carriage motor M 1  to the carriage  2 , and the carriage  2  can reciprocate in directions indicated by an arrow A. In printing, a print medium P such as print paper is fed via a paper feed mechanism  5  and conveyed to a print position. At the print position, the printhead  3  prints by discharging ink to the print medium P. 
     To maintain a good state of the printhead  3 , the carriage  2  moves to the position of a recovery device  10 . The recovery device  10  intermittently performs a discharge recovery operation for the printhead  3 . 
     The carriage  2  of the printing apparatus  1  supports not only the printhead  3 , but also an ink cartridge  6  which contains ink to be supplied to the printhead  3 . The ink cartridge  6  is detachable from the carriage  2 . 
     The printing apparatus  1  shown in  FIG. 1  can print in color. For this purpose, the carriage  2  supports four ink cartridges which respectively contain magenta (M), cyan (C), yellow (Y), and black (K) inks. The four ink cartridges are independently detachable. 
     The carriage  2  and printhead  3  can achieve and maintain a predetermined electrical connection by properly bringing their contact surfaces into contact with each other. The printhead  3  selectively discharges ink from a plurality of orifices and prints by applying energy in accordance with print data. In particular, the printhead  3  according to the embodiment employs an inkjet method of discharging ink by using heat energy. For this purpose, the printhead  3  comprises an electrothermal transducer for generating heat energy. Electric energy applied to the electrothermal transducer is converted into heat energy. Ink is discharged from orifices by using a change in pressure upon growth and shrinkage of bubbles due to film boiling generated by applying the heat energy to ink. The electrothermal transducer is arranged in correspondence with each orifice, and ink is discharged from a corresponding orifice by applying a pulse voltage to a corresponding electrothermal transducer in accordance with print data. 
     As shown in  FIG. 1 , the carriage  2  is coupled to part of a driving belt  7  of the transmission mechanism  4  which transmits the driving force of the carriage motor M 1 . The carriage  2  is slidably guided and supported along a guide shaft  13  in the directions indicated by the arrow A. The carriage  2  reciprocates along the guide shaft  13  by normal rotation and reverse rotation of the carriage motor M 1 . 
     The printing apparatus  1  has a platen (not shown) facing the orifice surface of the printhead  3  having orifices (not shown). The carriage  2  supporting the printhead  3  reciprocates by the driving force of the carriage motor M 1 . At the same time, the printhead  3  receives print data to discharge ink and print on the entire width of the print medium P conveyed onto the platen. 
     &lt;Control Arrangement of Inkjet Printing Apparatus (FIG.  2 )&gt; 
       FIG. 2  is a block diagram showing the control arrangement of the printing apparatus shown in  FIG. 1 . 
     As shown in  FIG. 2 , a controller  600  comprises a MPU  601 , ROM  602 , ASIC (Application Specific Integrated Circuit)  603 , RAM  604 , and system bus  605 . The ROM  602  stores a program corresponding to a control sequence, a predetermined table, and other permanent data. The ASIC  603  generates control signals for controlling the carriage motor M 1 , a conveyance motor M 2 , and the printhead  3 . The RAM  604  is used as an image data expansion area, a work area for executing a program, and the like. The system bus  605  connects the MPU  601 , ASIC  603 , and RAM  604  to each other, and allows exchanging data. 
     In  FIG. 2 , a computer (or an image reader, digital camera, or the like)  610  serves as an image data source and is generally called a host apparatus. The host apparatus  610  and printing apparatus  1  transmit/receive image data, commands, status signals, and the like via an interface (I/F)  611 . 
     A carriage motor driver  640  can drive the carriage motor M 1  for reciprocating the carriage  2  in the directions indicated by the arrow A. A conveyance motor driver  642  drives the conveyance motor M 2  for conveying the print medium P. 
     The ASIC  603  transfers print data DATA of a printing element (heater for ink discharge) to the printhead while directly accessing the storage area of the RAM  604  in printing and scanning by the printhead  3 . 
     The ink cartridge  6  and printhead  3  is separable from each other, as described in  FIG. 1 , but may also be integrated into an exchangeable head cartridge. 
       FIG. 3  is a perspective view showing the outer appearance of the structure of the head cartridge IJC which integrates the ink tank and printhead. In  FIG. 3 , a dotted line K indicates the boundary between an ink tank IT and a printhead IJH. The head cartridge IJC has an electrode (not shown) to receive an electrical signal supplied from the carriage  2  when the head cartridge IJC is mounted on the carriage  2 . The electrical signal drives the printhead IJH to discharge ink, as described above. 
     In  FIG. 3 , reference numeral  500  denotes an ink orifice array. The ink tank IT has a fibrous or porous ink absorber for holding ink. 
       FIG. 3A  is a plan view showing the ink discharge surface of the printhead  3 . 
       FIG. 3A  shows an arrangement for discharging one kind of ink. To discharge a plurality of inks, the same arrangements as that shown in  FIG. 3  are arranged by the number of inks in the carriage moving direction. 
     As shown in  FIG. 3A , two ink supply channel arrays  31  and  32  are parallel-arranged along the longer side direction on the printhead  3 . Each of the ink supply channel arrays  31  and  32  has ink supply channels  45  arrayed at predetermined intervals in the longer side direction of the printhead  3 . Ink orifices  43  are formed at predetermined pitches on the two sides of each ink supply channel  45 . Of the ink orifices, ink orifices positioned on the inner side of the printhead with respect to the shorter side direction of the printhead  3  form a first orifice array  30 . Of the ink orifices, ink orifices positioned on the outer side of the printhead along the shorter side direction of the printhead  3  form a second orifice array  33 . 
     In  FIG. 3A , the first orifice array  30  looks straight. However, the ink orifices of the first orifice array need not always be formed straight. The ink orifices of the first orifice array are staggered every ink supply channel depending on the interval between the ink supply channel arrays  31  and  32  with respect to the shorter side direction of the printhead  3 . 
     As described above, the ink supply channels of the printhead  3  according to the embodiment are staggered. Since the ink orifices are formed on the two sides of each ink supply channel, they are also staggered every ink supply channel. 
     &lt;Layout of Head Substrate&gt; 
     The layout of a head substrate assembled into the printhead mounted in the printing apparatus having the above-described arrangement will be described. 
     As a comparative example, a layout of a rectangular head substrate will be explained. 
       FIG. 11  is a view showing the layout of the head substrate as the comparative example. 
     In this layout, pads  110  for electrical connection to the outside of the head substrate, input circuit blocks  106 , shift registers  103 , latch circuits  104 , decoders  105 , and the like are arranged at the ends of the head substrate in the longer side direction. This layout can suppress an increase in the size of the head substrate in the shorter side direction. Signal lines extending from the shift registers and latch circuits, and signal lines extending from the decoders are provided along the longer side direction of the head substrate. 
     In the example shown in  FIG. 11 , ink supply channels  102  are divided for respective groups and arranged at positions offset from each other (this arrangement is called a staggered array). The staggered array is employed especially when the substrate needs to be made thin or when the chip becomes very narrow, in order to increase the mechanical strength of the head substrate. To the contrary, segment groups (element arrays) each including heaters (heater array) and driver transistors (driver array) need not always be staggered. In terms of the wiring and the arrangement of a diffusion layer formed in the head substrate, the segment groups are preferably parallel-arranged successively in the longer side direction of a chip, as shown in  FIG. 6 . 
     However, on the head substrate as shown in  FIG. 11  on which sets of ink supply channels and groups  101 G are staggered, a free area is created at a portion surrounded by two or three groups  101 G and the signal line  107 , as indicated by each broken line in  FIG. 11 . The free area cannot be effectively utilized and is unwanted. 
     To efficiently utilize such a free area, the embodiment proposes a new layout for the head substrate to arrange the building elements of the head substrate in the free area. 
       FIG. 4  is a view showing the layout of the rectangular head substrate integrated into the printhead  3 . 
     In  FIG. 4 , the same reference numerals as those in  FIGS. 6 and 11  denote the same parts, and a description thereof will not be repeated. Only a characteristic arrangement of the layout shown in  FIG. 4  will be explained. 
     In the layout shown in  FIG. 4 , the ink supply channels  102 , groups  101 G, and signal lines  107  are arranged in the order named from the ink supply channels  102  toward the longer side of the head substrate (along the shorter side direction). 
     According to the embodiment, as is apparent from comparisons between  FIG. 4 , and  FIG. 10  showing the conventional art and  FIG. 11  showing the comparative example, a power supply pad VH  130  and a ground pad GND  131  corresponding to the power supply pad VH are arranged in each area which remains free on a conventional head substrate. Further, wiring lines are arranged to independently supply power to respective groups of a heater &amp; driver array  101  from the power supply pad VH  130  and ground pad GND  131  which are arranged in the area which was used to be free on the conventional head substrate. 
       FIG. 4A  is an equivalent circuit diagram showing the detailed arrangement of the group  101 G including a heater array, driver array, and the like. 
     In  FIG. 4A , a block selection signal and print data are input to an AND circuit  201  functioning as a heater selection circuit. When these two signals become active, an output from the AND circuit  201  becomes active. The circuit arrangement of the heater array and driver array in the group  101 G in the embodiment is the same as that shown in  FIG. 7 . 
     A voltage conversion circuit  209  shown in  FIG. 7  converts the voltage amplitude of an output signal from the AND circuit  201  to have a voltage VHTM higher than a voltage represented by the voltage amplitude VDD of an output from the AND circuit  201 . The converted signal is supplied to the gate of a driver transistor  207  functioning as a driving element. As a result, a current is supplied to a heater  206  connected to the driver transistor  207  to which the voltage is applied at its gate, thereby driving the heater  206 . One terminal of the heater  206  is connected to a VH power supply line  140  connected to a heater power supply pad VH  130 . The source terminal of the driver transistor  207  is connected to a GNDH power supply line  141  connected to a ground pad GND  131 . 
       FIG. 4B  is an enlarged view of part of  FIG. 4  showing a more specific layout of two groups  101 G 1  and  101 G 2  and their periphery. 
     The power supply lines  140  and  141  hatched in  FIG. 4B  are connected to the heater power supply pad VH  130  and ground pad GND  131 , respectively. These wiring lines are formed from interconnection layers of a metal such as Al, and formed on the driver transistor and heater selection circuit. The block selection signal and print data signal are input to each heater segment by providing the signal lines around the heater power supply pad VH  130  and ground pad GND  131 . 
     According to the embodiment, the power supply pad VH and ground pad GND can be arranged in a free area formed by the staggered array on the conventional head substrate. In addition, wiring lines can be individually connected to the two adjacent groups  101 G 1  and  101 G 2 , as shown in  FIG. 4B , shortening the wiring length. 
     The connection of wiring lines from the power supply pad VH and ground pad GND is not limited to the arrangement shown in  FIG. 4 . The same effects can also be achieved by, e.g., a connection arrangement as shown in  FIG. 5 . 
       FIG. 5  is a view showing another layout of the head substrate integrated into the printhead  3 . 
     Also in  FIG. 5 , the same reference numerals as those in  FIGS. 6 and 11  denote the same parts, and a description thereof will not be repeated. 
     In  FIG. 5 , VH and GNDH wiring lines to two adjacent groups are designed to have almost the same wiring length from VH and GNDH pads for the two adjacent groups. The layout as shown in  FIG. 5  can make almost uniform the resistances of the wiring lines extending from the VH and GNDH pads to two adjacent groups. The wiring length can be shortened to reduce power loss. Wiring lines can be made uniform between groups to minimize the negative influence of power loss. 
       FIG. 5A  is an enlarged view of part of  FIG. 5  showing a more specific layout of three groups  101 G 1 ,  101 G 2 , and  101 G 3  and their periphery. Also in  FIG. 5A , the same reference numerals as those in  FIG. 4A  denote the same parts, and a description thereof will not be repeated. 
     Also in  FIG. 5A , the hatched power supply lines  140  and  141  are connected to the heater power supply pad VH  130  and ground pad GND  131 , respectively. These wiring lines are formed from interconnection layers of a metal such as Al, and formed on the switching element and heater selection circuit. The block selection signal and print data signal are input to each heater segment by providing the signal lines around the heater power supply pad VH  130  and ground pad GND  131 . 
     When a through-hole electrode is employed in each of the layouts shown in  FIGS. 4 and 5 , it allows electrical connection to the power supply pad VH and ground pad GND from the back surface of the substrate. This contributes to further increasing the layout efficiency of the entire head substrate, and downsizing the head substrate. 
       FIG. 5B  is a view showing the layout of the head substrate using the through-hole electrode. 
     As shown in  FIG. 5B , one terminal of the heater  206  is connected to the VH power supply line  140  connected to the heater power supply pad VH  130  formed from a through-hole electrode. The source terminal of the driver transistor  207  is connected to the GNDH power supply line  141  connected to the ground pad GND  131  formed from a through-hole electrode. The through-hole electrode means an electrode which connects an electrode to the back surface of a substrate via a hole extending through the substrate. 
       FIG. 5C  is a sectional view showing the form of so-called backside mounting in which a wiring line extends to the back surface of a substrate via a through-hole electrode formed in the substrate, and is connected to a member for connecting an external wiring line such as a flexible cable substrate. 
     As shown in  FIG. 5C , the heater power supply pad VH  130  is connected to a through-hole electrode  501 , and connected via the through-hole electrode  501  to another pad  502  formed on the back surface of the substrate  100 . The pad  502  has a bump  503 . The pad  502  is connected via the bump  503  to a wiring line  505  provided on a flexible cable  510 . In this way, the heater power supply pad VH  130  is connected via the through-hole electrode  501  to the flexible cable substrate  510  serving as an external wiring line. An insulating member  504  is inserted between the substrate  100  and the flexible cable substrate  510 . 
     This layout allows connecting a power supply wiring line to the back surface of a substrate and directly to an external electrode. This contributes to further decreasing wiring resistance, and greatly enhancing the effects of the present invention. 
     Other than the illustrated power supply pad and ground pad, a VHT buffer, voltage conversion circuit, and the like may also be arranged in the free area. 
     A converted voltage generator serving as a circuit which internally generates a voltage for driving a driver transistor is made up of a VHT buffer, and a dividing resistor unit which generates the gate voltage of a transistor serving as the buffer. 
       FIG. 5D  is a view showing a layout when the VHT buffer is arranged in a free area formed by the staggered array. The same reference numerals as those described above denote the same parts, and a description thereof will not be repeated. Only a characteristic arrangement of the layout shown in  FIG. 5D  will be explained. 
     In  FIG. 5D , reference numeral  109  denotes a VHT buffer; and  111 , a dividing resistor unit. 
     In this layout, the VHT buffer  109  can be arranged in a free area formed by the staggered array. One VHT buffer can apply a voltage to two adjacent groups  101 Ga and  101 Gb. 
       FIG. 5E  is an equivalent circuit diagram for explaining in more detail the converted voltage generator and driver array in  FIG. 5D . The same reference numerals as those described above denote the same parts, and a description thereof will not be repeated. 
     In this layout, the converted voltage generator is divided into the dividing resistor unit  111  and VHT buffer  109 . The VHT buffer  109  is arranged in a free area formed by the staggered array, and applies the voltage VHTM to two adjacent groups. 
     The VHT buffer  109  shown in  FIG. 5E  operates as a circuit using a source follower. The VHT buffer  109  includes an n-MOS transistor  222  serving as a buffer, and a resistor  225  serving as the load of the source follower. The dividing resistor unit  111  includes two dividing resistors  221  for determining the gate voltage of the n-MOS transistor  222 . A VHT power supply line  223  is the source of the VHTM power supply voltage in the converted voltage generator. A VSS line  211  provides the GND potential of these circuits. 
     The operation of this circuit arrangement will be explained in more detail. A voltage applied to the gate of the n-MOS transistor  222  is set to a desired value by the dividing resistors  221 , determining the voltage VHTM of an output from the source follower. The voltage VHTM is applied to the groups  101 Ga and  101 Gb. 
     In each of the groups  101 Ga and  101 Gb, the voltage conversion circuit  209  converts a signal from the heater selection circuit by converting the amplitude voltage VDD of the signal output pulse of the AND circuit  201  into the voltage VHTM for driving the gate of the driver transistor. The gate of the driver transistor  207  is driven at a voltage higher than one represented by the signal output amplitude of the AND circuit  201 . Thus, the ON resistance of the driver transistor decreases, and the heater driving energy efficiency increases. 
     This layout can reduce the influence of a voltage drop by the wiring resistance, the influence of noise which comes into the wiring line, and the like, as compared with a case where VHT buffers are arranged at one portion on a substrate and supply power to groups via wiring lines. 
     Note that the total number of segment groups including heaters and driver transistors is 16 in the above description, but the present invention is not limited to this. The effects of the present invention can be similarly obtained regardless of the number of segment groups. 
     In the above-described embodiments, droplets discharged from the printhead are ink, and the liquid contained in the ink tank is ink. However, the content is not limited to ink. For example, the ink tank may also contain a process liquid which is discharged to a print medium in order to improve the fixing characteristic and water repellency of a printed image and improve the print quality. 
     In the above-described embodiments, high print density and high resolution can be achieved by, of inkjet printing methods, a method of changing the ink state by heat energy generated by a means (e.g., electrothermal transducer) for generating heat energy to discharge ink. 
     In addition, the inkjet printing apparatus according to the present invention may also take the form of an image output apparatus for an information processing apparatus such as a computer, the form of a copying apparatus combined with a reader or the like, and the form of a facsimile apparatus having transmission and reception functions. 
     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. 2006-328852, filed Dec. 5, 2006, which is hereby incorporated by reference herein in its entirety.