Patent Publication Number: US-9415584-B2

Title: Liquid discharge head substrate, liquid discharge head, and printing apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid discharge head substrate, a liquid discharge head, and a printing apparatus. 
     2. Description of the Related Art 
     Japanese Patent Laid-Open No. 2010-155452 describes a liquid discharge head substrate that suppresses the influence of the voltage variation of a power supply line which supplies power to a discharge element for discharging a liquid. In this liquid discharge head substrate, transistors are connected to the two terminals of the discharge element. These transistors control a voltage and a current applied to the discharge element. This makes it possible to stably supply power to the discharge element. 
     SUMMARY OF THE INVENTION 
     The present inventors have found that the characteristics of transistors which drive discharge elements may vary, depending on the accuracy of the manufacturing process of a liquid discharge head substrate, among a plurality of liquid discharge head substrates obtained from different wafers or different chips. As a result, power supplied to the discharge elements may vary. Some embodiments of the present invention provide a technique of suppressing variations in the power supplied to the discharge elements among the liquid discharge head substrates. 
     According to some embodiments, a liquid discharge head substrate comprising a discharge unit including a discharge element configured to generate energy for discharging a liquid from an orifice and a discharge control circuit configured to control the discharge element; and a first voltage generation circuit configured to supply, to the discharge control circuit, a first driving voltage for driving the discharge control circuit, wherein the discharge unit includes a first node having a voltage correlated with a voltage to be supplied to the discharge element, and the first voltage generation circuit controls the first driving voltage based on a comparison result of the voltage of the first node and a first reference voltage supplied from outside of the liquid discharge head substrate, is provided. 
     According to some other embodiments, a liquid discharge head substrate comprising a discharge unit including a discharge element configured to generate energy for discharging a liquid from an orifice and a discharge control circuit configured to control the discharge element; and a first voltage generation circuit configured to supply, to the discharge control circuit, a first driving voltage for driving the discharge control circuit, wherein the first voltage generation circuit controls the first driving voltage based on a comparison result of a voltage of one terminal of the discharge element and a first reference voltage supplied from outside of the liquid discharge head substrate, is provided. 
     According to some other embodiments, a liquid discharge head comprising a liquid discharge head substrate and a liquid supply unit, wherein the liquid discharge head substrate comprises: a discharge unit including a discharge element configured to generate energy for discharging a liquid from an orifice and a discharge control circuit configured to control the discharge element; and a first voltage generation circuit configured to supply, to the discharge control circuit, a first driving voltage for driving the discharge control circuit, the discharge unit includes a first node having a voltage correlated with a voltage to be supplied to the discharge element, the first voltage generation circuit controls the first driving voltage based on a comparison result of the voltage of the first node and a first reference voltage supplied from outside of the liquid discharge head substrate; and the liquid supply unit is configured to supply a liquid to the liquid discharge head substrate, is provided. 
     According to some other embodiments, a printing apparatus comprising a liquid discharge head which comprising a liquid discharge head substrate and a liquid supply unit, and a driving unit, wherein the liquid discharge head substrate comprises a discharge unit including a discharge element configured to generate energy for discharging a liquid from an orifice and a discharge control circuit configured to control the discharge element; and a first voltage generation circuit configured to supply, to the discharge control circuit, a first driving voltage for driving the discharge control circuit, the discharge unit includes a first node having a voltage correlated with a voltage to be supplied to the discharge element, the first voltage generation circuit controls the first driving voltage based on a comparison result of the voltage of the first node and a first reference voltage supplied from outside of the liquid discharge head substrate; the liquid supply unit is configured to supply a liquid to the liquid discharge head substrate; and the driving unit is configured to drive the liquid discharge head, is provided. 
     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 block diagram showing the arrangement of a liquid discharge head substrate according to an embodiment of the present invention; 
         FIG. 2  is a circuit diagram showing the arrangement of the liquid discharge head substrate according to the embodiment of the present invention; 
         FIG. 3  is a circuit diagram showing the arrangement of the liquid discharge head substrate according to the embodiment of the present invention; 
         FIG. 4  is a circuit diagram showing the arrangement of the liquid discharge head substrate according to another embodiment of the present invention; 
         FIG. 5  is a chart for explaining the operation of the liquid discharge head substrate in  FIG. 4 ; 
         FIG. 6  is a circuit diagram showing the arrangement of the liquid discharge head substrate according to still another embodiment of the present invention; 
         FIG. 7  is a circuit diagram showing the arrangement of the liquid discharge head substrate according to still another embodiment of the present invention; and 
         FIGS. 8A to 8D  are views showing the arrangements of a liquid discharge head, a printing apparatus, and the control circuit of the printing apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A liquid discharge head substrate according to some embodiments of the present invention will be described with reference to  FIG. 1 .  FIG. 1  is a block diagram schematically showing the arrangement of a liquid discharge head substrate  100  according to an embodiment of the present invention. The liquid discharge head substrate  100  includes a discharge element  101 , a discharge control circuit  102 , and a voltage generation circuit  106 . The discharge element  101  and the discharge control circuit  102  form a discharge unit  105 . The liquid discharge head substrate  100  generally includes the plurality of discharge units  105 . The discharge element  101  discharges a liquid from an orifice by applying energy to the liquid. The discharge element  101  may be a heating element which applies energy to the liquid by generating heat or a piezoelectric element which applies energy to the liquid by deformation. 
     The discharge control circuit  102  controls the operation of the discharge element  101  by changing a voltage applied to the discharge element  101 . The discharge control circuit  102  receives a driving signal from outside of the discharge unit  105 . When the driving signal is at high level, the discharge control circuit  102  applies a voltage to the discharge element  101 . In response to this voltage, the discharge element  101  applies energy to the liquid. Meanwhile, when the driving signal is at low level (for example, 0V), the discharge control circuit  102  applies no voltage to the discharge element  101 . In this case, the discharge element  101  applies no energy to the liquid. 
     The voltage generation circuit  106  receives a reference voltage V ref  input from outside of the liquid discharge head substrate  100  and a monitoring node voltage V m  of the discharge control circuit  102 . The monitoring node voltage V m  is correlated with the voltage applied to the discharge element  101 . Therefore, the voltage generation circuit  106  can check the voltage applied to the discharge element  101  by monitoring the voltage V m . The reference voltage V ref  is supplied, for example, from a liquid discharge apparatus to the liquid discharge head substrate  100 . 
     The voltage generation circuit  106  generates a driving voltage V FB  of the discharge control circuit  102  and supplies it to the discharge control circuit  102 . The discharge control circuit  102  uses the driving voltage V FB  as a driving power supply voltage. The discharge control circuit  102  determines, based on the driving voltage V FB , the voltage to apply to the discharge element  101 . Therefore, the voltage generation circuit  106  can control the amount of current flowing through the discharge element  101  by regulating the value of the driving voltage V FB . More specifically, the voltage generation circuit  106  controls, or regulates, the value of the driving voltage V FB  such that the monitoring voltage V m  and the reference voltage V ref  input from outside of the liquid discharge head substrate  100  become substantially equal to each other. 
     The voltage generation circuit  106  includes a comparison circuit  107  which compares the monitoring voltage V m  and the reference voltage V ref . The voltage generation circuit  106  controls, or regulates, the driving voltage V FB  based on the comparison result of the comparison circuit  107  and supplies it to the discharge control circuit  102 . 
     The effect of the liquid discharge head substrate  100  will now be described. When using a structure described in Japanese Patent Laid-Open No. 2010-155452, the characteristics of transistors in a circuit which drives a discharge element may vary, depending on the accuracy of a process when manufacturing a liquid discharge head substrate, among a plurality of liquid discharge head substrates obtained from different wafers or different chips. When the characteristics of these transistors vary, a voltage applied to the discharge element varies, and thus the amount of current flowing through the discharge element varies accordingly even if the same driving voltage is supplied to each transistor of the plurality of liquid discharge head substrates. As a result, a liquid discharge amount varies among the plurality of liquid discharge head substrates even if they are driven on the same condition. 
     To cope with this, the voltage generation circuit  106  of the liquid discharge head substrate  100  controls, or regulates, the value of the driving voltage V FB  such that the monitoring voltage V m  and the reference voltage V ref  input from outside of the liquid discharge head substrate  100  become substantially equal to each other. Therefore, if the reference voltage V ref  having a predetermined value is supplied to the plurality of liquid discharge head substrates  100 , the monitoring voltage V m  has a predetermined value among the plurality of liquid discharge head substrates  100  irrespective of the characteristics of the transistor in each liquid discharge head substrate  100 . Since the monitoring voltage V m  is correlated with the voltage applied to the discharge element  101 , the currents flowing through the discharge elements  101  also become equal to each other among the plurality of liquid discharge head substrates. As a result, a variation in the liquid discharge amount is suppressed among the plurality of liquid discharge head substrates, increasing a manufacturing yield. 
     A liquid discharge head substrate  200  including an example of a circuit arrangement which implements the function of the liquid discharge head substrate  100  will now be described with reference to  FIG. 2 .  FIG. 2  is a circuit diagram of the liquid discharge head substrate  200  according to this embodiment. In this embodiment, the liquid discharge head substrate  200  includes the plurality of discharge units  105 .  FIG. 2  shows, out of the plurality of discharge units  105 , the three discharge units  105  which are indicated by  105   a ,  105   b , and  105   c , respectively. 
     First, the arrangement and the operation common to each of the discharge units  105   a  to  105   c  will be described. The discharge element  101  which generates energy for discharging the liquid is the heating element and represented as a resistor in the circuit diagram. The piezoelectric element may be used in place of the heating element. The same also applies to other embodiments to be described below. One terminal of the discharge element  101  is connected to a power supply V H  and the other terminal is connected to the discharge control circuit  102 . The discharge control circuit  102  includes a driving transistor  103  and a control circuit  104 . In this embodiment, the driving transistor  103  is formed by, for example, an NMOS transistor. One main electrode of the driving transistor  103  is connected to the discharge element  101 , the other main electrode is connected to ground, and the gate electrode serving as a control electrode is connected to the control circuit  104 . 
     The control circuit  104  of the discharge control circuit  102  receives a driving voltage V HTM  as the driving power supply voltage from the voltage generation circuit  106 . The driving voltage V HTM  corresponds to the driving voltage V FB  in  FIG. 1 . The control circuit  104  also receives a driving signal for controlling the driving transistor  103  from outside of the liquid discharge head substrate  200 . When this driving signal is at high level, the control circuit  104  controls to input the driving voltage V HTM  to the gate electrode of the driving transistor  103 . In this case, the driving transistor  103  is turned on. This passes the current through the discharge element  101 . As a result, the discharge element  101  generates heat and discharges the liquid. When this driving signal is at 0V, the control circuit  104  controls not to input the driving voltage V HTM  to the gate electrode of the driving transistor  103 . Therefore, the driving transistor  103  is turned off and no current flows through the discharge element  101 . A phenomenon in which the driving signal changes to high level and the driving transistor  103  which controls the voltage applied to the discharge element  101  is turned on, thereby operating the discharge element  101  is referred to as switching driving. 
     The arrangement unique to the discharge unit  105   a  will now be described. The discharge unit  105   a  outputs, as the monitoring voltage V m , the voltage of a node  11  in the discharge control circuit  102  to the voltage generation circuit  106 . The node  11  is a portion where the discharge element  101  and the driving transistor  103  are connected to each other. The voltage of the node  11  is correlated with the voltage applied to the discharge element  101 . 
     In this embodiment, the comparison circuit  107  in the voltage generation circuit  106  is formed by, for example, an inverting amplifier circuit. The monitoring voltage V m  is input from the discharge unit  105   a  to the inverting input terminal of the comparison circuit  107  and the reference voltage V ref  is input from outside of the liquid discharge head substrate  200  to the non-inverting input terminal of the comparison circuit  107 . The output from the comparison circuit  107  is fed back, as the driving voltage V HTM , to each discharge unit  105  via the source follower circuit of the voltage generation circuit  106 . One main electrode of this source follower circuit is connected to a power supply V ET  and the other main electrode is connected to ground via the resistor. Since the voltage generation circuit  106  is arranged as described above, the driving voltage V HTM  is supplied to the control circuit  104  of each discharge unit  105  such that the monitoring voltage V m  becomes equal to the reference voltage V ref . 
     The operation of the liquid discharge head substrate  200  will now be described. The discharge unit  105   a  is used as a monitoring unit configured to control the driving voltage V HTM  to be supplied to each discharge unit  105 . Each of the discharge units  105   b  and  105   c  is used as a liquid discharge unit configured to discharge a liquid corresponding to image data. In this embodiment, the discharge unit  105   a  is used only as the monitoring unit and does not discharge the liquid corresponding to the image data. When operating the liquid discharge head substrate  200 , a Hi signal is supplied to the monitoring unit as a driving signal and a pulse signal is supplied to each liquid discharge unit as a driving signal. The Hi signal is always at high level irrespective of the image data. The pulse signal switches between high level and low level in accordance with the image data. In accordance with the image data, the pulse signal changes to high level in a case in which each discharge unit  105  should discharge the liquid and changes to low level (for example, 0V) in other cases. The discharge element  101  of the discharge unit  105   a  is always driven when operating the liquid discharge head substrate  200 . 
     If the driving transistor  103  of the discharge unit  105   a  is ON, a current i 1  flows through the discharge element  101  of the discharge unit  105   a , a current i 2  flows through the driving transistor  103 , and a current i 3  flows from the discharge unit  105   a  to the comparison circuit  107 . In this case, i 1 =i 2 +i 3  holds. The current i 3  flowing through the comparison circuit  107  is much smaller than the currents i 1  and i 2 . Therefore, the currents i 1  and i 2  become substantially equal to each other. Meanwhile, in the liquid discharge unit (for example, the discharge unit  105   b ), the current flowing through the discharge element  101  and the current flowing through the driving transistor  103  become equal to each other if the driving transistor  103  is ON. Variations in the characteristics of the respective elements in the plurality of adjacent discharge units  105  are smaller than those between the wafers or the chips, and thus can be ignored. Therefore, if the common driving voltage V HTM  is input to the respective discharge control circuits  102  of the discharge unit  105   a  and the discharge unit  105   b , the currents flowing through the respective driving transistors  103  of the discharge unit  105   a  and the discharge unit  105   b  become equal to each other. Therefore, it can be regarded that the current flowing through the discharge element  101  of the discharge unit  105   a  and the current flowing through the discharge element  101  of the discharge unit  105   b  are equal to each other. Therefore, as in this embodiment, if the driving voltage V HTM  based on the node  11  in the one discharge unit  105   a  is supplied to the plurality of discharge units  105   a  to  105   c , variations in the currents flowing through the discharge elements  101  of the respective discharge units  105   a  to  105   c  can be ignored. 
     The discharge element  101  is operated by switching driving in the liquid discharge head substrate  200 . However, an arrangement in which, for example, the driving transistor  103  is formed by a PMOS transistor and driven as a source follower circuit may be adopted. In this case, the driving voltage V HTM  has a value decreased by a voltage between the gate and source of the driving transistor  103  from a voltage of the node which connects the discharge element  101  and the driving transistor  103  of the discharge unit  105   a . Further, in this embodiment, the driving transistor  103  is arranged between the discharge element  101  and ground. However, the driving transistor  103  may be arranged between, for example, the discharge element  101  and the power supply V H . 
     Furthermore, the case in which the liquid discharge head substrate  200  includes one driving transistor of the discharge control circuit  102  which controls the discharge element  101  has been described. However, the discharge control circuit  102  may be formed by two driving transistors. In this embodiment,  FIG. 3  is a circuit diagram showing the arrangement of a liquid discharge head substrate  300  when forming the discharge control circuit  102  by the two driving transistors. The liquid discharge head substrate  300  includes the voltage generation circuit  106  and a plurality of discharge units  305 . 
     The driving transistor of each discharge unit  305  is formed by two MOS transistors, namely, the driving transistor  103  using the NMOS transistor and a driving transistor  302  using the PMOS transistor. Each transistor forms a source follower circuit. One terminal of the discharge element  101  is connected to the source of the driving transistor  103 . The other terminal of the discharge element  101  is connected to the source of the driving transistor  302 . The drain of the driving transistor  103  is connected to the power supply V H . The drain of the driving transistor  302  is connected to ground. 
     The gate electrode of the driving transistor  302  receives a constant voltage V cont . In this case, a voltage increased by a voltage between the gate and source of the driving transistor  302  from the constant voltage V cont  is applied to a node  14  between the discharge element  101  and the driving transistor  302 . The control circuit  104  is connected to the gate electrode of the driving transistor  103 . The control circuit  104  receives the driving voltage V HTM  and the driving signal for controlling the driving transistor  103 . In this case, a voltage decreased by the voltage between the gate and source of the driving transistor  103  from the driving voltage V HTM  is applied to the node  11  between the discharge element  101  and the driving transistor  103 . 
     The voltage generation circuit  106  of the liquid discharge head substrate  300  also controls, or regulates, the driving voltage V HTM  such that the monitoring voltage V m  and the reference voltage V ref  supplied from outside of the liquid discharge head substrate  300  become equal to each other. As a result, a voltage across the discharge element  101  of each discharge unit  305  is determined not by the characteristics of the transistors but by the reference voltage V ref  and the constant voltage V cont . Therefore, variations in the voltages applied to the discharge elements  101  among the plurality of liquid discharge head substrates  300  are suppressed. This makes it possible to obtain, in the liquid discharge head substrate  300  using the two driving transistors for the discharge control circuit  102 , the same effect as in the liquid discharge head substrate  200 . 
     In this embodiment, the liquid discharge head substrate  300  adopts the arrangement in which each driving transistor is operated by using the source follower circuit. However, the present invention is not limited to this. The liquid discharge head substrate  300  may adopt, for example, an arrangement in which the two driving transistors undergo switching driving or an arrangement in which driving by the source follower circuit and switching driving are combined. 
     Furthermore, in this embodiment, the monitoring voltage V m  monitors the voltage of the node  11  which connects the discharge elements  101  of the discharge units  105   a  and  305   a , and the driving transistor  103 . However, the present invention is not limited to this. For example, the voltage of a node  12  which connects the driving transistor  103  and the control circuit  104  or a node  13  which connects the voltage generation circuit  106  and the discharge control circuit  102  may be input, as the monitoring voltage V m , to the comparison circuit  107  of the voltage generation circuit  106 . Both the voltages of the node  12  and the node  13  are correlated with the voltage applied to the discharge element  101 . In either case, the voltage generation circuit  106  controls, or regulates, the driving voltage V HTM  such that the monitoring voltage V m  becomes equal to the reference voltage V ref . If each of the discharge units  105   a  and  305   a  only functions as the monitoring unit, the discharge unit may not include the control circuit  104 . In this case, the driving voltage V HTM  is directly input to the gate electrode of the driving transistor  103 . Therefore, the driving transistor  103  is always driven when operating the liquid discharge head substrates  200  and  300  even if the monitoring unit does not receive the driving signal. In this embodiment, the comparison circuit  107  uses the inverting amplifier circuit. However, any circuit arrangement may be adopted as long as feedback of the voltage generation circuit  106  functions so as to equalize the monitoring voltage V m  and the reference voltage V ref  with each other. 
     The arrangement and the operation of a liquid discharge head substrate  400  including another example of a circuit arrangement which implements the function of the liquid discharge head substrate  100  will be described with reference to  FIGS. 4 and 5 .  FIG. 4  is a circuit diagram showing the arrangement of the liquid discharge head substrate  400  according to this embodiment. The liquid discharge head substrate  400  can be the same as the liquid discharge head substrate  200  except that an arrangement of a voltage generation circuit and a switch  452  are included. Therefore, a repetitive description on the components similar to those of the liquid discharge head substrate  200  will be omitted. 
     In the liquid discharge head substrate  400 , a switch  451  and a buffer circuit  402  are connected in series between the inverting input terminal of a comparison circuit  107  and a node  11  of a discharge unit  105   a . A node which connects the buffer circuit  402  and the switch  451  is connected to ground via a holding capacitor  401 . The switch  452  is provided in order to switch between two signals, namely, a monitoring Hi signal and a pulse signal corresponding to the image data, and input the signal to a control circuit  104  of a discharge control circuit  102 . The switch  452  connects the control circuit  104  to either a terminal φA or a terminal φB. A control block  403  is connected to the output portion of the comparison circuit  107 . The control block  403  controls the switch  451  and the switch  452 . Compared to the voltage generation circuit  106 , a voltage generation circuit  406  further includes the holding capacitor  401 , the buffer circuit  402 , the control block  403 , and the switch  451 , and forms a sample-and-hold circuit. 
     The operation of the liquid discharge head substrate  400  according to this embodiment will now be described with reference to  FIG. 5 .  FIG. 5  is a timing chart showing the operation of the liquid discharge head substrate  400  according to this embodiment. First, a case in which the discharge unit  105   a  is used as the monitoring unit configured to control the driving voltage V HTM  to be supplied to each discharge unit  105  will be described. The control block  403  turns on the switch  451  to electrically connect the discharge unit  105   a  with the holding capacitor  401  and the buffer circuit  402 . In this case, the monitoring voltage V m  is input from the discharge unit  105   a  via the buffer circuit  402  to the inverting input terminal of the comparison circuit  107 . Also, the monitoring voltage V m  is held in the holding capacitor  401 . The control block  403  turns on the switch  451  and connects the switch  452  to the terminal φA. This inputs the Hi signal, as a driving signal, to the control circuit  104  of the discharge unit  105   a . Therefore, the discharge unit  105   a  is turned on and operates as the monitoring unit configured to monitor the node of the discharge control circuit  102 . As a result, the voltage generation circuit  406  controls, or regulates, the driving voltage V HTM  such that the monitoring voltage V m  becomes equal to the reference voltage V ref  applied from outside of the liquid discharge head substrate  400 , and supplies the regulated voltage to the control circuit  104  of each discharge unit. 
     A case in which the discharge unit  105   a  is used as a liquid discharge unit for discharging the liquid will now be described. When the monitoring voltage V m  becomes equal to the reference voltage V ref , the control block  403  turns off the switch  451 . This opens between the discharge unit  105   a , and the holding capacitor  401  and the buffer circuit  402 . The control block  403  turns off the switch  451  and connects the switch  452  to the terminal φB. Consequently, the pulse signal corresponding to the image data is input, as the driving signal, to the control circuit  104  of the discharge unit  105   a , and the discharge unit  105   a  functions as the liquid discharge unit which discharges the liquid corresponding to the image data. In this case, the monitoring voltage V m  equal to the reference voltage V ref  and held in the holding capacitor  401  is input to the inverting input terminal of the comparison circuit  107  via the buffer circuit  402 . 
     When using the discharge unit  105   a  as the liquid discharge unit, a current i 3  does not flow from the discharge unit  105   a  to the comparison circuit  107  because the switch  451  is OFF. Therefore, a current i 1  flowing through a discharge element  101  of the discharge unit  105   a  becomes equal to a current i 2  flowing through the driving transistor  103 . As a result, in each discharge unit  105 , a voltage controlled by the reference voltage V ref  is applied to the discharge element  101  when discharging the liquid, making the current i 2  flow. This makes it possible to obtain, in the liquid discharge head substrate  400 , the same effect as in the liquid discharge head substrate  200 . 
     In this embodiment, the pulse signal is input to the terminal φB of the switch  452 . However, an arrangement in which, for example, a 0V-signal is input and the discharge unit  105   a  only operates as the monitoring unit may be adopted. When the monitoring voltage V m  becomes equal to the reference voltage V ref , the control block  403  connects the switch  452  to the terminal φB. In this case, the 0V-signal is input, as the driving signal, to the control circuit  104  of the discharge unit  105   a . This turns off the driving transistor  103 , and the power consumption can be reduced because no current flows through the discharge element  101 . While the switch  452  is connected to the terminal φB, the driving voltage V HTM  obtained when the monitoring voltage V m  and the reference voltage V ref  become equal to each other is supplied to the control circuit  104  of each discharge unit other than the discharge unit  105   a.    
     For example, the switch  452  may have three states, and switch among three signals, namely, the pulse signal, the Hi signal, and the 0V-signal as needed to input the signal to the control circuit  104  of the discharge unit  105   a . This allows the discharge unit  305   a  to function as the liquid discharge unit and the monitoring unit which reduces the power consumption, respectively. 
     In this embodiment, the monitoring voltage V m  monitors the voltage of the node  11 . However, for example, the voltage of a node  12  or a node  13  may be input to the comparison circuit  107  as the monitoring voltage V m , as described above. 
     The arrangement and the operation of a liquid discharge head substrate  600  having another example of a circuit arrangement which implements the function of the liquid discharge head substrate  100  will be described with reference to  FIG. 6 .  FIG. 6  is a circuit diagram showing the arrangement of the liquid discharge head substrate  600  according to this embodiment. The liquid discharge head substrate  600  can be the same as the liquid discharge head substrate  300  except that two voltage generation circuits are included, namely, a voltage generation circuit  106   a  and a voltage generation circuit  106   b . Therefore, a repetitive description on the components similar to those of the liquid discharge head substrate  300  will be omitted. 
     A comparison circuit  107   a  of the voltage generation circuit  106   a  receives a monitoring voltage V ma  which monitors a node  11  of a discharge control circuit  102  in a discharge unit  305   a  and a reference voltage V refa  applied from outside of the liquid discharge head substrate  600 . A comparison circuit  107   b  of the voltage generation circuit  106   b  receives a monitoring voltage V mb  which monitors a node  14  of the discharge unit  305   a  and a reference voltage V refb  applied from outside of the liquid discharge head substrate  600 . 
     The driving transistor in each discharge unit  305  is formed by two transistors, namely, a driving transistor  103  serving as an NMOS transistor and a driving transistor  302  serving as a PMOS transistor. Each transistor forms a source follower circuit. One terminal of a discharge element  101  is connected to the source of the driving transistor  103 . The other terminal of the discharge element  101  is connected to the source of the driving transistor  302 . The drain of the driving transistor  103  is connected to a power supply V H . The drain of the driving transistor  302  is connected to ground. 
     The operation of the liquid discharge head substrate  600  will now be described. The voltage generation circuit  106   a  controls, or regulates, a driving voltage V HTM   _   H  such that the monitoring voltage V ma  becomes equal to the reference voltage V refa , and then outputs the regulated voltage. The voltage generation circuit  106   b  controls, or regulates, a driving voltage V HTM   _   L  such that the monitoring voltage V mb  becomes equal to the reference voltage V refb , and then outputs the regulated voltage. 
     A control circuit  104  is connected to the gate electrode of the driving transistor  103 . The control circuit  104  receives the driving voltage V HTM   _   H  and a driving signal for controlling the driving transistor  103  from outside of the liquid discharge head substrate  600 . The driving voltage V HTM   _   H  is input to the gate electrode of the driving transistor  302 . The monitoring voltages V ma  and V mb , the reference voltages V refa  and V refb , and the driving voltages V HTM H  and V HTM   _   L , respectively, have different values. In this embodiment, assume that V ma &gt;V mb , V refa &gt;V refb , and V HTM   _   H &gt;V HTM   _   L  are satisfied. 
     The voltage generation circuit  106   a  of the liquid discharge head substrate  600  also controls, or regulates, the driving voltage V HTM   _   H  such that the monitoring voltage V ma  and the reference voltage V refa  supplied from outside of the liquid discharge head substrate  600  become equal to each other. The voltage generation circuit  106   b  of the liquid discharge head substrate  600  also controls, or regulates, the driving voltage V HTM   _   L  such that the monitoring voltage V mb  and the reference voltage V refb  supplied from outside of the liquid discharge head substrate  600  become equal to each other. As a result, a voltage across the discharge element  101  of each discharge unit  305  is determined by the reference voltage V refa  and the reference voltage V refb . Therefore, variations in the voltages applied to the discharge elements  101  among the plurality of liquid discharge head substrates  600  are suppressed. This makes it possible to obtain, in the liquid discharge head substrate  600 , the same effect as in the liquid discharge head substrate  300 . 
     In the liquid discharge head substrate  600  according to this embodiment, the two driving transistors control the voltages of the nodes in the two terminals of the discharge element  101 . Furthermore, both of the two driving transistors are controlled by a feedback circuit. This further stabilizes the voltages applied to the two terminals of the discharge element  101  as compared with a case in which only the voltage of the node in one terminal of the discharge element  101  is controlled. As a result, variations in the voltages applied to the discharge element  101  can further be suppressed. 
     In this embodiment, the voltage of the node  11  is used as the monitoring voltage V ma . However, for example, the voltage of a node  12  or a node  13  may be input to the comparison circuit  107   a  as the monitoring voltage V ma , as described above. Also, the voltage of a node  15  which connects, for example, the voltage generation circuit  106   b  and the discharge control circuit  102  may be input, as the monitoring voltage V mb , to the comparison circuit  107   b . The nodes which monitor the monitoring voltage V ma  and the monitoring voltage V mb  may be in any combination. Furthermore, the voltages of the nodes in the two terminals of the discharge element  101  may be monitored. 
     The arrangement and the operation of a liquid discharge head substrate  700  including another example of a circuit arrangement which implements the function of the liquid discharge head substrate  100  will be described with reference to  FIG. 7 .  FIG. 7  is a circuit diagram showing the arrangement of the liquid discharge head substrate  700  according to this embodiment. The liquid discharge head substrate  700  can be the same as the liquid discharge head substrate  600  except that the voltage generation circuit  106  is changed to the voltage generation circuit  406  described in the liquid discharge head substrate  400  and a switch  452  is included. Therefore, a repetitive description on the components similar to those of the liquid discharge head substrates  400  and  600  will be omitted. 
     In the liquid discharge head substrate  700 , a switch  451   a  and a buffer circuit  402   a  are connected in series between the inverting input terminal of a comparison circuit  107   a  and a node  11  of a discharge unit  305   a . A node which connects a buffer circuit  402   a  and the switch  451   a  is connected to ground via a holding capacitor  401   a . A reference voltage V refa  is input from outside of the liquid discharge head substrate  700  to the non-inverting input terminal of the comparison circuit  107   a . A switch  451   b  and a buffer circuit  402   b  are connected in series between the inverting input terminal of a comparison circuit  107   b  and a node  14  of the discharge unit  305   a . A node which connects the buffer circuit  402   b  and the switch  451   b  is connected to ground via a holding capacitor  401   b . A reference voltage V refb  is input from outside of the liquid discharge head substrate  700  to the non-inverting input terminal of the comparison circuit  107   b.    
     A control block  403   a  of a voltage generation circuit  406   a  controls the switch  451   a . A control block  403   b  of a voltage generation circuit  406   b  controls the switch  451   b . The switch  452  is provided in order to switch between two driving signals, namely, a monitoring Hi signal and a pulse signal corresponding to the image data, and input the signal to a control circuit  104  of a discharge control circuit  102 . The signals from the control blocks  403   a  and  403   b  are transmitted to this switch  452  via, for example, a NOR circuit. 
     The operation of the liquid discharge head substrate  700  will now be described. First, a case in which the discharge unit  305   a  is used as a monitoring unit configured to control the driving voltages V HTM   _   H  and V HTM   _   L  to be supplied to each discharge unit  105  will be described. The control blocks  403   a  and  403   b  turn on the switches  451   a  and  451   b  to electrically connect the discharge unit  305   a  with the holding capacitors  401   a  and  401   b  and the buffer circuits  402   a  and  402   b . In this case, the monitoring voltages V ma  and V mb  are input to the inverting input terminals of the comparison circuits  107   a  and  107   b  via the buffer circuits  402   a  and  402   b . In this case, the monitoring voltage V ma  is held in the holding capacitor  401   a  and the monitoring voltage V mb  is held in the holding capacitor  401   b . Also, the switch  452  is connected to a terminal φA in this case. This inputs the Hi signal, as the driving signal, to the control circuit  104  of the discharge unit  305   a . Therefore, the discharge unit  305   a  is turned on and operates as the monitoring unit configured to monitor the node. The voltage generation circuit  406   a  controls, or regulates, the driving voltage V HTM   _   H  such that the monitoring voltage V ma  becomes equal to the reference voltage V refa , and supplies it to the control circuit  104  of each discharge unit. The voltage generation circuit  406   b  controls, or regulates, the driving voltage V HTM   _   L  such that the monitoring voltage V mb  becomes equal to the reference voltage V refb , and supplies it to the gate electrode of the driving transistor  302 . 
     A case in which the discharge unit  105   a  is used as the liquid discharge unit for discharging the liquid corresponding to the image data will now be described. When the monitoring voltages V ma  and V mb  become equal to the reference voltages V refa  and V refb  respectively, the control blocks  403   a  and  403   b  respectively turn off the switch  451   a  and the switch  451   b . Control signals from the control blocks  403   a  and  403   b  turn off the switch  451   a  and the switch  451   b , and connect the switch  452  to a terminal φB by a signal switching circuit using a NOR circuit. This inputs, as the driving signal, the pulse signal corresponding to the image data to the control circuit  104  of the discharge unit  305   a , and the discharge unit  305   a  functions as the liquid discharge unit which discharges the liquid corresponding to the image data. In this case, the monitoring voltages V ma  and V mb  equal to the reference voltages V refa  and V refb  and held in the holding capacitors  401   a  and  401   b  are input, via the buffer circuits  402   a  and  402   b , to the inverting input terminals of the comparison circuits  107   a  and  107   b . Since the control circuit  104  of the discharge unit  305   a  receives the pulse signal, the driving voltage V HTM   _   H  is input to the gate electrode of the driving transistor  103  when the pulse signal changes to Hi. The driving voltage V HTM   _   L  is input to the gate electrode of the driving transistor  302 . 
     The voltage generation circuit  406   a  of the liquid discharge head substrate  700  also controls, or regulates, the driving voltage V HTM   _   H  such that the monitoring voltage V ma  and the reference voltage V refa  supplied from outside of the liquid discharge head substrate  700  become equal to each other. The voltage generation circuit  406   b  of the liquid discharge head substrate  700  also controls, or regulates, the driving voltage V HTM   _   L  such that the monitoring voltage V mb  and the reference voltage V refb  supplied from outside of the liquid discharge head substrate  700  become equal to each other. Since the switch  451   a  and the switch  451   b  are OFF, a current i 3  does not flow from the discharge unit  305   a  to the comparison circuits  107   a  and  107   b . Therefore, a current i 2  flowing through a discharge element  101  has a predetermined value among the respective discharge units  305 . As a result, in the discharge element  101  of each discharge unit  305 , a voltage across the discharge element  101  when discharging the liquid is determined by the reference voltage V refa  and the reference voltage V refb . Therefore, variations in the voltages applied to the discharge elements  101  among the plurality of liquid discharge head substrates  700  are suppressed. This makes it possible to obtain, in the liquid discharge head substrate  700 , an effect obtained by combining the liquid discharge head substrate  400  and the liquid discharge head substrate  600 . 
     The four embodiments according to the present invention have been described above. However, the present invention is not limited to these embodiments. In the liquid discharge head substrate using, for example, two driving transistors, the voltage generation circuit  406  may be used as the voltage generation circuit and the voltage generation circuit  106  may be used as the voltage generation circuit. The respective embodiments described above can be changed and combined as needed. 
     An embodiment of a printing apparatus according to the present invention will be described. An inkjet printing apparatus will be described. A liquid discharge head serving as the printhead of the inkjet printing apparatus includes an inkjet printhead substrate and a liquid supply unit configured to supply ink to the inkjet printhead substrate. The liquid discharge head substrate described in the above-described embodiment can be used as the inkjet printhead substrate. The printing apparatus includes this printhead and a driving unit configured to control this printhead. 
       FIG. 8A  shows the main units of a printhead unit  800  including an inkjet printhead substrate  801  as described above. The printhead unit  800  includes an ink supply port  807 . The discharge element  101  according to the embodiments of the present invention is illustrated as heating units  802 . As shown in FIG.  8 A, the substrate  801  can form the printhead unit  800  by assembling channel wall members  806  for forming fluid channels  805  communicating with a plurality of orifices  804 , and a top plate  803  including the ink supply port  807 . In this case, ink injected from the ink supply port  807  is stored in an internal common ink chamber  808 , and then supplied to each fluid channel  805 . In this state, the substrate  801  and the heating units  802  are driven to discharge ink from the orifices  804 . 
       FIG. 8B  is a view showing the overall arrangement of such a printhead  810 . The printhead  810  includes the printhead unit  800  including the plurality of orifices  804  described above and an ink tank  811  which holds ink to be supplied to this printhead unit  800 . The ink tank  811  is provided detachably from the printhead unit  800  with respect to a boundary line K. The printhead  810  includes an electrical contact (not shown) for receiving an electrical signal from a carriage side when mounted on the printing apparatus shown in  FIG. 8C . The heating units  802  generate heat based on this electrical signal. Fibrous or porous ink absorbers are provided inside of the ink tank  811  to hold ink. 
     It is possible to provide the inkjet printing apparatus capable of achieving high-speed printing and high-resolution printing by attaching the printhead  810  shown in  FIG. 8B  to the main body of the inkjet printing apparatus and controlling a signal given from the main body to the printhead  810 . The inkjet printing apparatus using such a printhead  810  will be described below. 
       FIG. 8C  is a perspective view showing the outer appearance of an inkjet printing apparatus  900  according to the embodiments of the present invention. In  FIG. 8C , the printhead  810  is mounted on a carriage  920  which is engaged with a helical groove  921  of a lead screw  904  rotating in synchronism with forward/reverse rotation of a driving motor  901  via driving force transfer gears  902  and  903 . With this arrangement, the printhead  810  can reciprocally move, by the driving force of the driving motor  901 , in the direction of an arrow a or b along a guide  919  together with the carriage  920 . A paper pressing plate  905  for a printing sheet P conveyed onto a platen  906  by a printing medium feeding apparatus (not shown) presses the printing sheet P against the platen  906  in the carriage moving direction. 
     Photocouplers  907  and  908  are home position detection units configured to confirm the existence of a lever  909  provided in the carriage  920  in a region where the photocouplers  907  and  908  are provided, and perform, for example, switching of the rotation direction of the driving motor  901 . A support member  910  supports a cap member  911  which caps the entire surface of the printhead  810 . A suction unit  912  sucks the inside of the cap member  911  and performs suction recovery of the printhead  810  via an intra-cap opening  913 . A moving member  915  can move a cleaning blade  914  forward and backward. A main body support plate  916  supports the cleaning blade  914  and the moving member  915 . Not only the cleaning blade  914  shown in  FIG. 8C  but also a known cleaning blade can be applied to this embodiment, as a matter of course. Furthermore, a lever  917  is arranged to start sucking in suction recovery and moves along with movement of a cam  918  engaged with the carriage  920 , and a driving force from the driving motor  901  undergoes movement control such as clutch switching by a known transfer unit. A printing control unit (not shown) which gives signals to the heating units  802  provided in the printhead unit  800  or performs driving control of each mechanism of the driving motor  901  or the like is provided on the side of an apparatus main body. 
     The inkjet printing apparatus  900  having the above-described arrangement performs printing on the printing sheet P conveyed onto the platen  906  by the printing medium feeding apparatus while the printhead  810  reciprocally moves over the full width of the printing sheet P. The printhead unit  800  of the printhead  810  uses the inkjet printhead substrate serving as the liquid discharge head substrate according to the above-described embodiments. Therefore, the printhead unit  800  is compact and can achieve high-speed printing. 
     The arrangement of a control circuit configured to perform printing control of the above-described apparatus will now be described.  FIG. 8D  is a block diagram showing the arrangement of the control circuit of the inkjet printing apparatus  900 . The control circuit includes an interface  1000  which receives a printing signal, an MPU (microprocessor)  1001 , a program ROM  1002 , a dynamic RAM (Random Access Memory)  1003 , and a gate array  1004 . The program ROM  1002  stores a control program to be executed by the MPU  1001 . The dynamic RAM  1003  saves various data such as the above-described print signal and print data to be supplied to a printhead. The gate array  1004  controls supply of print data for a printhead  1008 , and also controls data transfer between the interface  1000 , the MPU  1001 , and the RAM  1003 . This control circuit further includes a carrier motor  1010  configured to carry the printhead  1008  and a conveyance motor  1009  configured to convey a printing paper. This control circuit also includes a head driver  1005  which drives the printhead  1008 , and motor drivers  1006  and  1007  configured to drive the conveyance motor  1009  and a carrier motor  1010 , respectively. 
     The operation of the above-described control arrangement will be described. When the print signal is input to the interface  1000 , it is converted into print data for printing between the gate array  1004  and the MPU  1001 . Then, the motor drivers  1006  and  1007  are driven, and the printhead is also driven in accordance with the print data that has transmitted to the head driver  1005 , thereby performing printing. 
     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. 2014-161896, filed Aug. 7, 2014, which is hereby incorporated by reference wherein in its entirety.