Improved power supply for individual control of power delivered to integrated drive thermal inkjet printhead heater resistors

Abstract: A circuit (10') for controlling the power applied to the heater resistor (RH) of a thermal inkjet printer printhead. The inventive circuit (10') includes a first transistor (Q1) having a first terminal connected to the heater resistor (RH), a second terminal connected to the return path (KR) for the heater resistor and a simple circuit (R I , D1) for maintaining a constant voltage at a third terminal of the transistor (Q1). In a particular embodiment, the circuit for maintaining a constant voltage at the third terminal of the transistor (Q1) includes a diode (D1) connected between the second and third terminals and a resistor (R I ) connected between a second source of current and the third terminal of the transistor (Q1). In the illustrative embodiment, the transistor (Q1) is a bipolar NPN transistor and the anode of the diode (D1) is connected to the base terminal thereof. In the best mode, the diode (D1) is fabricated by connecting the base and collector terminals of a second transistor fabricated on a substrate with the first transistor (Q1). The invention provides a simple, low cost, reliable system for controlling the power applied to the heater resistor of a thermal inkjet printhead which consumes little power.

Description: BACKGROUND OF THE INVENTION Field of the Invention : The present invention relates to thermal inkjet printer technology. More specifically, the present invention relates to systems and techniques for energizing heater resistors within an inkjet printhead to expel ink. Description of the Related Art : Thermal inkjet printers are currently used for a wide variety of high speed, high quality printing applications. These printers include a thermal inkjet printhead. The thermal inkjet printhead includes one or more ink-filled channels communicating with an ink supply chamber or cartridge at one end and having an opening at the opposite end, referred to as a nozzle. A heater resistor is located in the channel at a predetermined distance underneath the nozzle. The resistors are individually addressed with a current pulse to momentarily vaporize the ink to form a bubble. The bubble expels an ink droplet towards a recording medium such as paper. By energizing heater resistors in different combinations as the printhead moves across the paper, an inkjet printer prints different characters on the paper. The heater resistors within the printhead are addressed through flexible conductors that connect the resistors to control circuitry within the thermal inkjet printer. In many prior systems, each resistor was connected directly to a flexible conductor. However, inasmuch as resolution of the printed characters is improved by adding nozzles, the drive for greater print quality has created an associated increase in the number of heater resistors in a printhead. This caused an associated increase in the number of conductors required to address the individual heater resistors. To minimize the number of conductors required, many resistors were connected to a common return line. Thus, the conventional printhead had one conductor per resistor and a common return. With as many as 10 - 13 resistors per common return, the cumulative current in the return was, in many cases, so high as to cause a significant voltage drop and associated power dissipation in the return line. This lowered the voltage and power delivered to the heater resistor. Hence, because of the resistance of the power and return conductors of a thermal inkjet printhead, the power delivered to the individual elements was a function of the number of the elements energized. Since, optimum print quality requires precise control of the energy supplied to the heater resistor, losses in the return line were adversely affecting the operation of the system. This effect was minimized by energizing only one element per power/return pair. In these systems, external power transistors were activated in sequence to provide drive current to the heater resistors to be fired during a print cycle. However, the provision of a separate transistor per resistor was expensive. In addition, this technique required a large number of external connections to the printhead and a considerable amount of power was lost in the control element used to sequence the transistors. The interconnect problem was mitigated somewhat by numerous decoding schemes. One such scheme is that of U. S. Patent No. 3,852,563, entitled THERMAL PRINTING HEAD, issued December 3, 1974 to J. H. Bohorquez. A more sophisticated multiplexing scheme was developed by which logic circuitry comprising active elements (transistors) were added to the printhead. In any event, the loss elements were the trace (the conductor from the resistor to the contact to the external circuitry), the heating element, and the return are all loss elements. Nonetheless, a problem remained in delivering a correct voltage to the heating element notwithstanding changes in the circuitry surrounding the element. U. S. Patent No. 5,083,137 entitled ENERGY CONTROL CIRCUIT FOR A THERMAL INK-JET PRINTHEAD, issued January 21, 1992 to Badyal et al., discloses a system for addressing the problem by controlling the power to each heating element individually. A measurement resistor is added and used to measure the current through the heater resistor. By regulating the power delivered to the element, the energy may be delivered to the element independent of the losses in the power and return lines. However, this method has several disadvantages. First, a considerable amount of additional circuitry is required in order to control the current through each heater resistor. This is costly in manufacturing time and space on the substrate. In addition, the measurement resistor and the other control elements are lossy. It is the object of the present invention to provide a more efficient, less expensive circuit for individually controlling the power applied to a heater resistor in the printhead of an inkjet printer. This object is achieved by a circuit according to claim 1 or claim 2. In a preferred embodiment, the transistor is a bipolar NPN transistor and the anode of the diode is connected to the base terminal thereof. In the best mode, the diode is fabricated by connecting the base and collector terminals of a second transistor fabricated on a substrate with the first transistor. This mode provides best matching of operational parameters of the diode and the transistor. The inventive circuit provides a simple, low cost, reliable system for controlling the power applied to the heater resistor of a thermal inkjet printhead which consumes little power. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a conventional energy control circuit for the heater resistor of a thermal inkjet printhead implemented in metal-oxide semiconductor (MOS) technology. Fig. 2 is a schematic diagram of a second conventional energy control circuit for the heater resistor of a thermal inkjet printhead implemented in bipolar semiconductor technology. Fig. 3 is a simplified schematic diagram of conventional circuits for controlling the energy applied to the heater resistor of thermal inkjet printheads. Fig. 4 is a simplified schematic diagram of an energy control circuit for the heater resistor of a thermal inkjet printhead constructed in accordance with the present teachings. Fig. 5 is a schematic diagram of the current source I s of the energy control circuit for the heater resistor of a thermal inkjet printhead constructed in accordance with the present teachings. Fig. 6 is a schematic diagram of an alternative embodiment of an energy control circuit for the heater resistor of a thermal inkjet printhead constructed in accordance with the present teachings which shows how multiple current sources can be used to set the programming current I 1 . DESCRIPTION OF THE INVENTION Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. The novel and advantageous design of the present invention is best illustrated with a review of typical conventional designs. Fig. 1 is a schematic diagram of a conventional energy control circuit for the heater resistor of a thermal inkjet printhead implemented in metal-oxide semiconductor technology. Fig. 2 is a schematic diagram of a second conventional energy control circuit for the heater resistor of a thermal inkjet printhead implemented in bipolar semiconductor technology. The operation of the circuits of Figs. 1 and 2 are described in detail in the above-referenced U. S. Patent No. 5,083,137 entitled ENERGY CONTROL CIRCUIT FOR A THERMAL INK-JET PRINTHEAD, issued January 21, 1992 to Badyal et al. In both circuits, an address decoder 12 allows for the selection of a particular heater resistor circuit by address signals provided in a manner well known in the art. The output of the decoder 12 is adjusted by a level shifting circuit 16 before being applied to a driver circuit 18 for the heater resistor RH. A measurement resistor R1 and a comparator circuit 20 are used to determine the voltage applied to the heater resistor RH and to provide a control signal to the level shifting circuit 16. In response to the control signal, the level shifting circuit 16 adjusts the signal applied to the driver circuit 18, which in turn applies the adjusted voltage to the heater resistor RH. Note the amount of circuitry required to determine the voltage applied to the heater resistor. In Fig. 1, a separate resistor R1, a comparator 32, two switches and a level shifting circuit 16 are required to control the voltage applied to the heater resistor RH. In the bipolar case of Fig. 2, even more circuitry is required. Fig. 3 is a simplified schematic diagram of conventional circuits for controlling the energy applied to the heater resistor of thermal inkjet printheads. R P represents the parasitic resistance in the trace and R R represents the resistance in the return lead. As mentioned above, the components required by conventional systems to control the voltage applied to the heater resistor of thermal inkjet printheads are costly to manufacture, consume space on the circuit board, consume power and lower the reliability of the system. Accordingly, it is an object of the present invention to provide a simple, low cost, reliable system for controlling the power applied to the heater resistor of a thermal inkjet printhead which consumes little power. Fig. 4 is a simplified schematic diagram of an energy control circuit for the heater resistor of a thermal inkjet printhead constructed in accordance with the present teachings. Note that the sensing resistor R1, the power control circuitry 20 and the level shifting circuitry 16 are eliminated by the use of a current source I s in place of the driver 18. Fig. 5 is a schematic diagram of the current source I s . The current source includes a transistor Q1, the collector and emitter of which are connected in series with the heater resistor RH and the return path. In the illustrative embodiment, the transistor Q1 is a bipolar NPN transistor. Those skilled in the art will appreciate that the present teachings may be implemented with PNP or MOS technology without departing from the scope of the invention. The voltage applied to the base terminal of the transistor Q1 is controlled by a diode D1 connected between the base and emitter terminals of the transistor Q1. Since Q1 is an NPN transistor, the anode of the diode D1 is connected to the base terminal and the cathode is connected to the emitter of the transistor. A resistor R I is connected between the addressing logic 12 and the junction between the base of the transistor Q1 and the anode of the diode D1. In an integrated circuit implementation, the diode may be created by connecting the collector and base terminals of a transistor. Ideally, the diode is fabricated on the same die as the transistor Q1 in close proximity thereto so that the characteristics of the diode will track those of the transistor Q1 with changes in temperature and manufacturing tolerances over time. Those skilled in the art will appreciate that the matching of the active areas of the diode and the transistor are key considerations as the bandgap of silicon is a constant. If the geometries of the active areas of the diode D1 and the transistor Q1 in the integrated circuit mask are scaled, then the currents will be scaled. Therefore, if the transistor is k times the size of the diode, then the current through the transistor, I 2 , is k times the current, I 1 , through the diode where k is the ratio of the areas A Q1 /A D1 . Multiple transistors may be connected in parallel or multiple diodes may be connected in parallel for optimal matching or to achieve other relationships between the currents I 1 and I 2 . Additional control of the absolute delivered energy may be required when precise control of the operational parameters of the printhead is required by the printing system. These requirements may be beyond the accuracy of the manufacturing tolerances of the components that set the values of I 1 and I 2 , the scale factor Of the areas "k", and the value of the heater resistor. As these components affect the delivered energy according to the following equation, additional control is needed. E = k(I 1 )RHT(Pulse) In the simplest implementation, the source for the programming current source I 1 can be set by the printing system and therefore control I 2 which sets the heater energy. If the printing system is not capable of controlling the programming current, then a system of setting the programming current can be implemented at the time of manufacture. One possible method is similar to the method currently used to program fuse link logic arrays. Fig. 6 is a schematic diagram of an alternative embodiment of an energy control circuit for the heater resistor of a thermal inkjet printhead constructed in accordance with the present teachings which shows how multiple current sources can be used to set the programming current I 1 . By fusing the control junction of the transistors that feed the control node, any combination of currents I a , I b to I n can be set. The unprogrammed current would be the sum of all of these currents or any combination thereof. I 1 = I a +I b + ... I n

Claims: A circuit for controlling the power applied to the heater resistor (RH) of a thermal inkjet printer printhead, said heater resistor being connected to a first source of current (I 2 ), said circuit comprising: a first transistor (Q1) having its collector terminal connected to the heater resistor (RH) and its emitter terminal connected to a return path for the heater resistor (RH); and means for maintaining a constant voltage at a base terminal of said transistor (Q1), said base terminal being the control terminal thereof and said means for maintaining a constant voltage including a diode connected between said emitter terminal and said base terminal to conduct when said junction between said emitter and collector terminals of said transistor (Q1) conduct; wherein said base terminal of said transistor (Q1) is connected to a second source of current (I 1 ). A circuit for controlling the power applied to the heater resistor (RH) of a thermal inkjet printer printhead, said heater resistor being connected to a first source of current (I 2 ), said circuit comprising: a first transistor (Q1) having its drain terminal connected to the heater resistor (RH) and its source terminal connected to a return path for the heater resistor (RH); and means for maintaining a constant voltage at a gate terminal of said transistor (Q1), said gate terminal being the control terminal thereof and said means for maintaining a constant voltage including a diode (D1) connected between said source terminal and said gate terminal to conduct when said junction between said source and drain terminals of said transistor (Q1) conduct; wherein said gate terminal of said transistor (Q1) is connected to a second source of current (I 1 ). The circuit of claim 1 wherein said means for maintaining a constant voltage further includes a resistor (R 1 ) connected between said second source of current (I 1 ) and said base terminal of said transistor (Q1). The circuit of claim 2 wherein said means for maintaining a constant voltage further includes a resistor (R 1 ) connected between said second source of current (I 1 ) and said gate terminal of said transistor (Q1). The circuit of claim 3 wherein the transistor (Q1) is a NPN transistor. The circuit of claim 5 wherein the anode of the diode (D1) is connected to the base terminal of the transistor (Q1). The circuit of claim 6 wherein the diode (D1) is fabricated by connecting the base and collector terminals of a second transistor fabricated on a substrate with the first transistor (Q1).

Inventors: BOHORQUEZ JAIME H, BOHORQUEZ, JAIME H.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1993
Main classification: B41J2
IPC codes: B41J2