L.E.D. PRINTER APPARATUS WITH IMPROVED TEMPERATURE COMPENSATION

Abstract: A non-impact printhead (20) includes recording elements such as light-emitting diodes (LED's) for recording. An extra LED is also included with each group of LED's. This extra LED is masked (80) to block light from exposing the recording medium. A small constant current (82) is driven to the extra LED and the anode voltage is sensed. This anode voltage is related to the temperature of the chip array (31) carrying the other LED's. In response to this anode voltage an adjusted current is generated in a current mirror that adjusts a voltage bias on a transistor (Q425) regulating current to the recording LED's. Temperature compensation is provided for wholly within a driver chip including the circuitry for driving the LED's without the need for compensation data to flow outside of the driver chip.

Description: L.E.D. PRINTER APPARATUS WITH IMPROVED TEMPERATURE COMPENSATION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-impact recording apparatus such as that using LED's for recording and specifically to such printheads and driver chip therefor which control uniformity of light output automatically even though subjected to temperature gradients along the printhead. 2. Brief Description of the Prior Art In the prior art as exemplified by U.S. Patent 4,885,597, printer apparatus is described which comprises a multiplicity of individually addressable and energizable point-like radiation sources, such as LED's, arranged in a row for exposing points upon a photoreceptor during movement thereof relative to and in a direction normal to the row. Driver circuits are provided for simultaneously energizing the radiation sources responsive to respective data bit input signals applied to the driver circuits during an information line period. The print or recording head includes a support upon which are mounted chips placed end to end and upon each of which are located a group of LED's. The driver circuits are formed as integrated circuits and are incorporated in chips that are located to each side of the linear array of LED chips. The driver circuits in this apparatus each include a shift register for serially reading-in data-bit signals and for driving respective LED's in accordance with the data signals. Associated with each driver chip is a current-level controller that controls the level of current into the LED's of that group during recording. The controller comprises a current mirror having a master control circuit whose current is mirrored in slave circuits to which the LED's are connected. One advantage of this prior art printer apparatus is that current to the LED's may be changed automatically as needed, due to changes in aging or temperature of the printhead. As such changes affect the light output of the LED's, the changes to the current compensate for same so that some uniformity is provided to the recording apparatus. In the current mirror described in this prior art, there are provided two avenues for adjustability. Firstly, there is a "system bias" voltage which is adjustable to compensate for loss in intensity of light output from the LED's due to aging, i.e., hours of use. Since aging will affect most LED's on a printhead to about the same extent, the loss in intensity due to aging may be overcome by changing the system bias voltage which causes additional current to be provided to the LED's. This change in system bias voltage may be characterized as a "global" change since the change in system bias voltage affects all driver chips on the printhead. In order to change system bias voltage, a new digital word is sent to a digital current mirror control that is separate from the driver chips. By enabling the appropriate current-carrying transistors, a new level of system bias may be provided to each driver chip. Incorporated within each driver chip is an additional current mirror that is also subject to digital regulation and can be used to provide "local" regulation or control for such localized effects as temperature and chip to chip nonuniformity. As noted in U.S. Patent 4,831,395 in order to sense temperature to control bias voltage to the driver chip, a temperature sensor, such as a thermistor is located on the printhead at a position or positions that are reasonably representative of the temperature of the LED's. In response to changes in voltage at the forward terminal of the thermistor, bias voltage may be adjusted by the logic and control unit of the printer apparatus. A problem with the above prior art is that it would be desirable to have each driver chip be self-regulating for temperature compensation. A second problem is that variations in voltage reported across the thermistor are related to temperature variations in the printhead but are only approximations of temperatures of the LED array chips. Thus, correction using the thermistor is imprecise. In U.S. Patent 4,952,949 an LED printer apparatus is suggested which employs a dummy LED for monitoring temperature. Current to the other LED's is adjusted in response to measured change in the voltage across the dummy diode which is driven from a current channel that is a part of a current mirror circuit used to drive other LED's. However, because voltage is the parameter being measured, a complex array of sets of capacitors must be adjusted to maintain the relationship between voltage and temperature. It is an object of the invention to improve upon the printer apparatus of the prior art to overcome the above-noted problems. SUMMARY OF THE INVENTION The above and other objects are accomplished by a non-impact printer apparatus for recording, comprising: a plurality of light-emitting elements for emitting light for recording; driving means including a current mirror having a master current driver means and a plurality of slave current driver means for selectively energizing with respective slave driving currents respective elements for recording; an additional element of similar temperature responsiveness to a light-emitting element and including means for precluding light for recording from being emitted therefrom; sensing means responsive to said additional element when it is energized for generating an electrical signal which varies with a temperature of the additional element; adjustment means responsive to said electrical signal for adjusting said respective slave driving currents and characterized by a constant current source means providing a current independent of the levels of currents of said respective slave driving currents for energizing said additional element with said constant current. In accordance with another aspect of the invention, there is provided a non-impact printer apparatus for recording, comprising a plurality of groups of light-emitting elements, a plurality of integrated circuit driver chips, each including means for driving respective groups of light-emitting elements; each driver chip including digitally addressable current-conducting transistor means for selectively establishing a reference current and a voltage bias related to a digital addressing of said digitally addressable current-conducting means; current mirror driver means responsive to said voltage bias for generating a plurality of slave currents that are slaved to said reference current; means coupling said slave currents to respective light-emitting elements; and characterized by temperature sensing means wholly on said driver chip for sensing a temperature related electrical signal generated on said driver chip and in response thereto adjusting said reference current to adjust the slave currents to said light-emitting elements. In accordance with still another aspect of the invention, there is provided a driver chip for use on a non-impact printer apparatus for driving a plurality of light-emitting diodes for recording, the driver chip comprising current mirror means for generating a reference current and respective slave driving currents for driving respective light-emitting diodes to be driven by said driver chip; characterized by means for generating a constant current independent of said reference and slave currents; sensing means responsive to an electrical parameter of a non-light emitting diode when said constant current is driven therethrough for generating an electrical signal which varies with a temperature of the non-light emitting diode; and adjustment means responsive to said electrical signal for adjusting said respective slave driving currents. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of a printer apparatus made in accordance with the invention; FIG. 2 is a block diagram of circuitry used in forming the printhead shown in FIG. 1 in accordance with the invention; FIG. 3 is a block diagram of a driver circuit with data-handling logic for use in one embodiment of the printhead of FIG. 2; and FIGS. 4A, B and C are a schematic of a current driving circuit for the driver circuit of FIG. 3 that includes temperature compensation means in accordance with the invention. FIG. 5 is a schematic of an LED chip array in accordance with the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus of the preferred embodiments will now be described in accordance with an electrophotographic recording member. The invention, however, is not limited to apparatus for creating images on such a member, as other media such as photographic film etc. may also be used with the invention. Because electrophotographic reproduction apparatus are well known, the present description will be directed in particular to elements forming part of or cooperating more directly with the present invention. Apparatus not specifically shown or described herein are selectable from those known in the prior art. With reference now to FIG. 1, an electrophotographic reproduction apparatus 10 includes a recording member such as photoconductive web 11 or other photosensitive medium that is trained about three transport rollers 12, 13 and 14, thereby forming an endless or continuous web. Roller 12 is coupled to a driver motor M in a conventional manner. Motor M is connected to a source of potential when a switch (not shown) is closed by a logic and control unit (LCU) 15. When the switch is closed, roller 12 is driven by motor M and moves web 11 in a clockwise direction as indicated by arrow "A". This movement causes successive image area of web 11 to sequentially pass a series of electrophotographic work stations of the reproduction apparatus. For the purposes of the instant disclosure, several work stations are shown along the web's path. These stations will be briefly described. First, a charging station 17 is provided at which the photoconductive surface 16 of the web 11 is sensitized by applying to such surface a uniform electrostatic primary charge of a predetermined voltage. The output of the charger may be controlled by a grid connected to a programmable power supply (not shown). The supply is in turn controlled by the LCU 15 to adjust the voltage level Vo applied onto the surface 16 by the charger 17. At an exposure station 18 an electrostatic image is formed by modulating the primary charge on an image area of the surface 16 with selective energization of point-like radiation sources in accordance with signals provided by a data source 19. The point-like radiation sources are supported in a printhead 20 to be described in more detail below. A development station 21 includes developer which may consist of iron carrier particles and electroscopic toner particles with an electrostatic charge opposite to that of the latent electrostatic image. Developer is brushed over the photoconductive surface 16 of the web 11 and toner particles adhere to the latent electrostatic image to form a visible toner particle, transferable image. The development station may be of the magnetic brush type with one or two rollers. Alternatively, the toner particles may have a charge of the same polarity as that of the latent electrostatic image and develop the image in accordance with known reversal development techniques. The apparatus 10 also includes a transfer station 25 shown with a corona charger 22 at which the toner image on web 11 is transferred to a copy sheet S; and a cleaning station 28, at which the photoconductive surface 16 of the web 11 is cleaned of any residual toner particles remaining after the toner images have been transferred. After the transfer of the unfixed toner images to a copy sheet S, such sheet is transported to a heated pressure roller fuser 27 where the image is fixed to the copy sheet S. As shown in FIG. 1, a copy sheet S is fed from a supply 23 to driver rollers 24, which then urge the sheet to move forward onto the web 11 in alignment with a toner image at the transfer station 25. To coordinate operation of the various work stations 17, 18, 21, and 25 with movement of the image areas on the web 11 past these stations, the web has a plurality of indicia such as perforations along one of its edges. These perforations generally are spaced equidistantly along the edge of the web 11. At a fixed location along the path of web movement, there is provided suitable means 26 for sensing web perforations. This sensing produces input signals into the LCU 15 which has a digital computer, preferably a microprocessor. The microprocessor has a stored program responsive to the input signals for sequentially actuating, then de-actuating the work stations as well as for controlling the operation of many other machine functions. Additional encoding means may be provided as known in the art for providing more precise timing signals for control of the various functions of the apparatus 10. Programming of a number of commercially available microprocessors is a conventional skill well understood in the art. This disclosure is written to enable a programmer having ordinary skill in the art to produce an appropriate control program for the one or more microprocessors used in this apparatus. The particular details of any such program would, of course, depend on the architecture of the designated microprocessor. With reference to FIGS. 1 and 2 and to U.S.Patent 4,885,597 and to U.S. Patent 4,831,395, the contents of both of which are incorporated herein by this reference, the printhead 20, as noted, is provided with a multiplicity of energizable point-like radiation sources 30, preferably light-emitting diodes (LED's). Optical means 29 may be provided for focusing light from each of the LED's onto the photoconductive surface. The optical means preferably comprises an array of optical fibers such as sold under the name Selfoc, a trademark for a gradient index lens array sold by Nippon Sheet Glass, Limited. Due to the focusing power of the optical means 29, a row of emitters will be imaged on a respective transverse line on the recording medium. With reference to FIG. 2, the printhead 20 comprises a suitable support with a series of LED chips 31 mounted thereon. Each of the chips 31 includes in this example 128 LED's arranged in a single row. Chips 31 are also arranged end-to-end in a row and where twenty-eight LED chips are so arranged, the printhead will extend across the width of the web 11 and include 3584 LED's arranged in a single row. To each side of this row of LED's there are provided twenty-eight identical driver chips 40. Each of these driver chips include circuitry for addressing the logic associated with each of 64 LED's to control whether or not each of the LED's should be energized as well as to determine the level of current to each of the LED's controlled by that driver chip 40. Two driver chips 40 are thus associated with each chip of 128 LED's. Each of the two driver chips will be coupled for driving of alternate LED's. Thus, one driver chip will drive the odd numbered LED's of the 128 LED's and the other will drive the even numbered LED's of these 128 LED's. The driver chips 40 are electrically connected in parallel to a plurality of lines 34-37 providing various electrical control signals. These lines provide electrical energy for operating the various logic devices and current drivers in accordance with their voltage requirements. A series of lines 36 (indicated by a single line in this Fig.) provide clock signals and other pulses for controlling the movement of data to the LED's in accordance with known techniques. A pair of data lines 33a, 33b are also provided for providing data signals in the form of either a high or low logic level. The driver chips each include a data in and data out port so that they serially pass data between them. With reference now to FIG. 3, the architecture for a binary type printhead will be described. Each driver chip 40 includes a 64-bit bidirectional shift register 41. A logic signal carried over line R/LB determines the direction data will flow down this register. Assume that this chip is enabled to cause data to flow down the register from left to right as shown in FIG. 3. Data thus enters shift register 41 over line 33a through the driver chip's data-in port at the left from say the data-out port of a driver chip immediately to the left or from the LCU if the driver chip 40 is the first chip for data to enter. Data exits from this chip at the data-out port to be input to the next adjacent driver chip to the right of driver chip 40. For each line of image to be exposed in the main scanning direction, i.e., transverse to that of movement of the recording medium or web 11, data from the data source suitably rasterized, in accordance with known techniques, streams serially through the shift registers undercontrol of clock pulses provided by the LCU over line 36a. As may be noted, odd and even data may be moved simultaneously since they are provided on separate lines 33a, 33b. Still further reductions in clock speed for moving data through the shift registers may be provided by providing additional lines for distributing data simultaneously. When 3584 bits of data (l's or 0's) are stored by the shift registers of all of the driver chips, a latch signal is provided over line 36b to latch this data into latch registers 42 so that the shift registers 41 may commence filling with data signals for the next line of exposure. Sixty-four latch registers 42 are provided in each driver chip to receive the data shifted out in parallel fashion from the shift register 41. Each latch register is associated with a particular LED and adjacent latch registers are associated with every other LED. A logic AND gate 43 is associated with each latch register and has one input coupled to the output of its respective latch register and its other input coupled to a line 36c for accepting a strobe or timing pulse from the LCU. This strobe pulse determines when to trigger the LED's to turn on in relation to the position of the recording medium and the duration for which the LED's are turned on. All the AND gates have one of their inputs connected to this strobe line. Alternatively, a plurality of strobe lines may be provided with enabling times of different durations; see in this regard U.S. Patent 4,750,010 to Ayers et al, the contents of which are incorporated herein by this reference. The output of each of the AND gates is coupled to a logic circuit that is part of a constant current driver circuit. In a further alternative as noted in U.S. Patent 4,746,941 to Pham et al, the contents of which are incorporated by this reference, the printhead may be of the so-called grey level type wherein multiple data bits per pixel are used to establish the pulsewidth duration of an LED. With reference now to FIGS. 4A, B and C, the current driving circuit portion 110 of each driver chip 40 is shown. The respective outputs of the LATCH registers 42 are fed over respective lines 45 1 , 45 3 , and the following lines not shown 455 t — 45125 > 45127 As may be seen each of these lines is actually a double line one of which carries an enable signal to turn the respective LED on and the other carries a complement of this signal. The lines 45 1 are input to respective control electrodes of transistors Q*^ , Q* 27 . These transistors act as switches and form a part of a current mirror driving circuit that includes a master circuit formed by transistors Q* 24 , Q^ 25 and a series of digitally controlled transistors. More details concerning the digitally controlled transistors will be found below with reference to the discussion of FIGS. 4A and 4B. Briefly, these digitally controlled transistors may be selectively turned on to establish a signal I (CHIP BIAS) to thereby regulate a desired current level for the LED's driven by this driver chip. As may be noted in Figure 4C, circuitry for driving two LED's, i.e., LED- L and LED 3 are illustrated; it being understood that the driver chip would have appropriate circuits typified by those described below for driving say 64 of the odd-numbered LED's in an LED chip array having, for example, 128 LED's. Another driver chip on the other side of the LED chip array would be used to drive the 64 even-numbered LED's. The current through the master circuit establishes a potential V G1 on line 117. Directly in series with LED X are two transistors Q^ 28 , 0* 29 . Transistor Q 428 is biased to be always conductive while transistor is switched on and off and thus is the transistor controlling whether or not current is driven to LED α . The gate or control electrode of transistor Q^ 29 is coupled to the drain-source connection of transistors Q^ 26 , Q* 27 . When LED : is to be turned on, transistor Q 427 is made conductive and when LEDj is to be turned off, transistor Q* 26 is made conductive. The gate of transistor Q* 26 receives a logic signal that is the inverse of that to gate Q* 27 from a data driven enabling means indicated as 116 but is actually the circuitry of FIG. 3 which controls whether or not an LED is to be turned on and for how long. As noted above in a grey level printhead, the LED is to be turned on for a duration determined by the grey level data signals input to the printhead. Also associated with the circuitry for driving LEDj , is an additional current mirror that includes two slave circuits. One slave circuit comprises transistors Q^ 20 , Q, 21 and Q^ 30 . The other slave circuit comprises transistors Q «22 , Q* 23 and 0* 31 . Transistors t 30 , Q* 3 x are N-channel MOSFETS while the other transistors noted above are P-channel MOSFETS. The two additional slave circuits associated with LED : are on continuously and assuming a nominal driving current of say I LED1 -4 ma to LED α , the current through transistor Q, π might be 1/80 I L _ _ x and the current through transistor Q, 23 might be 1/800 x I lXD1 . The currents through these slave circuits establishes a voltage level V G2 on line 114, which is the potential of the drain electrode of transistor Q 427 • In operation with transistor turned off, transistor Q 426 is on and impresses approximately the voltage V c c at the gate of transistor Qjjg . When LED X is to be turned on to record a pixel (picture element) , a signal is provided by the enabling means 116 to the gate of transistor Q t 27 to turn same on, while an inverse signal turns transistor Q 426 off. Before transistor turns on, the capacitive load existing between its gate and substrate must be removed. When transistor * 27 turns on the charge on the gate terminal of transistor Q 429 discharges through transistors Q t 27 and Q 430 • This path for discharge of the gate capacitive load at transistor Q« 29 thereby provides a turn-on time not affected by the number of LED's that are sought to be simultaneously energized. The reason for this is that each control transistor corresponding to transistor Q* 29 bas its own respective path for discharge of its respective capacitive load. Current through transistors <22 , Q <23 a d Q 43 j is proportional to, i.e. mirrors, that through the master circuit because of the identical gate to source terminal biasing (V GS1 ) of transistors Q j 24 and Q < 22 . Thus, current is constant in this slave circuit even though V c c from power supply P 2 varies since the potential difference V GS1 between the gate and source terminal of transistor j 22 remains constant. The current through the slave circuit comprised of transistors Q^ _ _ , (^ 23 and t 3 α is mirrored by that through the slave circuit comprised of transistors Q^ 20 • 0* 21 and Q » 30 due to the identical gate to source biasing of transistors Q^ 30 , Q » 31 . With a constant current being generated in the slave circuit comprised of transistors Q <20 , j 21 and Q* 30 , the potential difference between the gate and source terminals of transistor Q^ o remains fixed as does that of transistor thereby establishing a voltage level V G z on line 114 which varies with V c c although the potential difference V CC -V G2 remains constant. With the transistor Q* 29 turned on and conducting driving current to LED α during an exposure period, the voltage level V G2 is established at the gate of transistor 0* 29 via now conducting transistor (^ 27 • The voltage level at the source terminal of transistor Q* 2 β - s now at a fixed threshold value above that of V G2 . Transistor Qj 29 > acting as a cascode transistor and having its source terminal connected to the drain terminal of transistor Q* 2 β » thereby establishes the drain potential of the transistor as varying with changes in V e e . As noted above, the potential difference V GS1 is constant even though V c c itself varies. The voltage relationships between the various terminals of transistor Q » β are not affected by variations in V c c and the current to LED 2 during a period for recording a pixel stays constant. Thus, stability in driver current to LED. is provided since transient changes in V c c do not cause corresponding changes to the current conducted through LED j and thus do not affect the intensity level of light output by LED α . The tendency in some LED printheads for light output of an LED to diminish when other LED's are turned on can also be reduced with this circuit. As noted above, transistor 0* 29 conducts current to LED a for a time period controlled by the strobe signal or in the case of a grey level printer, for a period controlled by the data bits for recording an appropriate pixel. The level of current for recording this pixel is controlled by the current mirror which is responsive to the current level I(CHIP BIAS). The circuit for generating I(CHIP BIAS) will now be described. When transistor is turned on, the current passing there through mirrors, i.e., is either the same or proportional to, the current passing through transistor Q* 25 , I (CHIP BIAS). With reference now to FIGS. 4A and 4B, this current, I(CHIP BIAS) in turn is controlled by three factors comprising a variable current source 172, a first group of eight digitally controlled NMOSFET transistors Q_ s , Q 26 • • ■ • Q 31 1 Q 32 and a second group of eight digitally controlled NMOSFET transistors Q_ , Qe • • • 1 Qi 1 » Qi 2 • Associated with the f rst group is a non-digitally controlled NMOSFET transistor Q_ 2 . Similarly associated with the second group is non-digitally controlled NMOSFET transistor Q 13 . As may be noted in FIGS. 4A and 4B, not all of the transistors are shown and the number of digitally controlled transistors provided in each group determines the level of control. Transistors Q 25 , ... , Q 32 are parallel connected transistors whose respective gate width to gate length ratios are scaled so that their respective currents are scaled or weighted in powers of two. For example, where eight digitally controlled transistors are provided for this first group (Q 25 "Q 32 ) » respective gate width to gate length ratios may be 256 : 128 : 64 : __2 : 16 : 8 : 4 : 2 and 321. 5 5 5 5 5 5 5 5 5 5 for non-digitally controlled transistor Q 33 . Each digitally controlled transistor is controlled by a logic signal applied to a respective two-transistor switch circuit associated with the transistor. For example, the circuit defined by NMOSFET transistors Q 250 and Qz 5 α cause current to flow through transistor Q 2 ■ when a high level logic signal is applied to the gate of transistor Q 250 and a complementary low logic signal is applied to the gate of transistor Qa 5 _ . The logic signals for controlling which of the current-carrying transistors are to be turned on are controlled by a register j which stores an 8-bit digital word and its 8-bit complement representing a desired current control signal to turn on respective ones of the eight current conducting transistors Qg 5 ,...Q 32 . In conjunction with transistor Q_ 3 , which is on continuously, this group of transistors is used for "localized" control of LED current. By this, it is meant that the digital word stored in register R_ is specific for this driver chip and will be determined by adjustment of driver current to the LED's driven by this driver chip until the LED's each provide a desired light output level. This digital word may be input to the register R2 from memory in the LCU or from a separate memory such as a ROM provided on the printhead. This digital word thus compensates for chip-to-chip nonuniformity. As noted in aforementioned U.S. Patent 4,831,395, the contents of which are incorporated by this reference, the LCU may be programmed to maintain a count of prior activations of each LED and adjust a control voltage according to a program based on the aging characteristics of the printhead. After this initial calibration and as the printhead ages through repeated use, both temperature and age factors operate to degrade light output. The affects due to aging will generally be similar to all LED's and are corrected for by adjustment of an 8-bit digital word and its 8-bit complement stored in register 2 . This digital word controls 8 current-carrying NMOSFET transistors 0^ , ... , Q 12 . Associated with this group of transistors is a continuously conducting NMOSFET transistor Q x 3 . Exemplary gate width to length ratios for weighted digitally controlled transistors Q 5 - Q 12 are for non-digitally controlled transistor Q j 3 . The 8-bit word and its 8-bit complement stored in register R x is the same as that stored in identical registers R 2 on the other driver chips. As the printhead ages, a new 8-bit digital word and its 8-bit complement is calculated by the LCU and input into the registers α . The calculation of this 8-bit word for aging correction may be based on empirical determinations made using similar printheads or based upon a calibration of this printhead using an optical sensor that senses the output from each or selected LED's or by sensing patches recorded on the photoconductor. As noted above, a third factor for adjustment to maintain LED uniformity of light output from chip-to-chip is a variable current source 172 providing an adjustable current I 0 that is adjusted to compensate for temperature nonuniformities on the printhead. The current I 0 generated by the variable current source 172 is responsive to a stable reference voltage, V c e , and to a variable voltage, V A . V A is a calculated voltage representing the voltage necessary to increase I 0 so that increased current is provided to the LED's driven by the driver chip to offset loss of light output due to a rise in temperature on the respective LED chip array 31. Circuits for providing a variable current in response to a reference voltage and a variable voltage are well known. In the preferred embodiment of the invention, an extra LED, LED„ , is provided on each LED chip array 31 carrying the 128 LED's that are arranged in a row. The extra LED includes a mask 80 formed of say metal to block any light from emanating from LED M . In response to a signal from the LCU to initiate a calibration check, a constant current source 82 on the driver chip provides a fixed current, say 100 micro amperes, over the lead connecting the current source 82 with the masked LED, LED M . Because LED M is mounted on the LED chip array it is at a temperature that is approximately the same as that of he other LED's on the same chip array. The low driver current to the extra LED is significantly lower than the nominal driving current, about 10 ma. , to the other LED's so as not to alter the temperature of the extra LED. Additionally, the forward voltage drop of the LED's are related to the temperature thereof. Thus, the anode voltage V τ of LED M is sensed by a sample and hold circuit 84 in response to a signal from the LCU. The voltage V τ is converted to a digital signal by A/D converter 86 and the digitized value for V τ provides an address to look-up table memory 88. This memory may be a ROM or periodically updated RAM that relates the adjusted voltage V A to input voltage V τ . The relationship between V A and V τ may be empirically determined particularly where there is a nonlinear relationship between these variables. The digital output of V A is converted by D/A converter 90 to an analog signal V A that will increase I 0 to maintain the light intensity output of the LED's. It should be appreciated that other circuits than that herein described may be used for converting the anode voltage of LED M to an adjusted current to the LED's. For example, purely analog circuits providing a correct transfer function between V τ and V A may also be used. The operation of the circuit of FIGS. 4A, B and C will now be described. During use of the printhead the temperature of the driver chips will heat up differently in accordance with respective current carrying demands and abilities to dissipate heat caused by such demands through the heat conducting structure to which the chips are mounted. The temperature adjusted current I 0 is conducted to ground via NMOSFET transistor Oj 3 and some or all of the transistors Q 32 , Qg 1 , . . . and H) 2 5 depending upon the digital 8-bit signal and its 8-bit complement stored in register ΕL_ . In accordance with which transistors in this group of transistors are enabled to conduct and recalling that these transistors are scaled or weighted differently in conducting capabilities the voltage level at the source terminal of Q 33 is determined. Note that switching transistors are associated with each of these digitally controlled transistors. For example, transistor Q j 5 is controlled by switching transistors Q^,, and Q 251 in response to a signal causing Q 250 to conduct and Q 251 to turn off. The others are controlled similarly. This voltage level, V IC , is also applied to the gate of transistor Q 13 and thereby controls the current conducted by transistor Q 13 . As noted above, transistor Q : 3 is the non-digitally controlled transistor associated with the digitally controlled transistor group Qs Qiι i Qi 2 • I accordance with the digital word stored in register j selected ones of these transistors are caused to conduct thereby affecting the bias current level I (CHIP BIAS) through PMOSFET transistor Q^ j . Recall that the transistors in this group of transistors also have scaled or weighted current-conducting capabilities. The bias current through PMOSFET transistor Q* 25 controls the current conducted through transistor Qj 21 , which current is replicated or scaled by current mirrors of PMOSFET slave transistors , Q^ 2 g #, ..etc. , i.e., the current controlling transistors to LED α , LED 3 LED 127 , respectively, as well as the extra temperature sensing circuit using channel 65. Transistor Q^ 29 is caused to conduct when its respective logic transistors Q^ 2 fi , Q^ 27 are appropriately signaled by data signals indicating a pixel to be printed. Thus, when a l gic low signal is applied to line 45 1 (AN), transistor j 2 η turns on and biases the gate of transistor Q^ 29 to the level V G2 . Since transistors Q < 24 and Q* 2 - have identical biasing, the current through transistor Q* 29 will mirror or be scaled to that of transistor Q* 2 * for the time period for exposing a pixel as controlled by the duration of the logic low signal on line 45 1 (AN) . As is noted in FIG. 4C, the current through Q* 29 I s fed to LEDi , for the recording of a pixel. Identical current levels willbe developed in the other channels directly providing current to respective other LED's. Thus, all LED's driven by this driver chip receive the same current for periods determined by their respective enablement signals and the currents thereto are appropriately adjusted to maintain constant the intensity of the LED's. ADVANTAGES An improved circuit for a current driver chip used in an LED printhead has been described. The circuit retains the desirable feature of two-way addressability described in the prior art. That is, provision is made for digitally addressing each chip to correct for differences in light output by LED's driven by one chip versus those driven by another chip on the same printhead. These differences can arise due to processing condition differences arising during manufacture of the driver chips and for their respective driven LED's. A second provision for digital addressability is retained to provide for global changes due to aging. By providing both addressable portions on each driver chip problems associated with noise are minimized. In addition, the provision of an extra LED on each chip array that can be monitored for temperature in accordance with the circuit described herein enables current to be varied to the LED's of that chip array by its respective driver chip without necessitating transfer of temperature compensation data of the printhead to the LCU and then back from the LCU to the driver chip. Control is completely provided on each driver chip. The presence of the extra diode on the array ensures that the temperature related parameters that are sensed are in response to the temperature of the LED chip array and not thetemperature of the driver chip or the mounting structure of the printhead. Although the preferred embodiment shows the use of an extra LED that is masked in its broader context, the invention contemplates the use of a recording LED that is not masked or other temperature sensors where the driver chip is used for temperature compensation without the need for the LCU to require data of this type to be transferred to it or in other words temperature compensation is wholly provided for on the driver chip itself with merely certain reference and timing signals being provided to the printhead along with data to be printed. For example, during interframe periods or other non-recording periods, a constant known current may be driven through a recording element and its anode voltage sensed and an adjustment of the driving currents made in accordance with the techniques described above. While the extra LED is shown associated with one driver chip this extra LED may also be associated with a second driver chip that is used to drive the even numbered LED's of the same LED chip array. Alternatively a second extra LED may be located on the same chip array and associated with the other driver chip. The invention is described with respect to pri theads comprised of LED's used as the recording elements in its broader aspects. The invention is also related to other recording elements such as those used for thermal printing, laser etc. Still further modifications include the feeding of the digitized signal representing V τ to the LCU to allow the LCU to monitor the temperature of the LED chip array. This allows flexibility by allowing adjustments to R 2 to provide two controls for temperature compensation. While the preferred embodiment has been described in terms of MOS transistors that have their respective gates controlled, other devices providing an equivalent function such as bipolar or other gate controlled devices are also contemplated. Where bipolar transistors are used, emitter-collector-geometry or doping levels to respective transistors may be modified to provide the current scaling characteristics described herein. The invention has been described in detail with particular reference to preferred embodiments thereof. However, it will be understood that variations and modifications may be effected within the spirit and scope of the invention.

Claims: What is claimed is: 1. A non-impact printer apparatus for recording, comprising: a plurality of light-emitting elements (LED 1,3...127) for emitting light for recording; driving means (110) including a current mirror having a master current driver means (Q424, Q425, Q25-33, Q5-13) and a plurality of slave current driver means (Q428, Q429) for selectively energizing with respective slave driving currents respective elements for recording; an additional element (LEDM) of similar temperature responsiveness to a light-emitting element and including means (80) for precluding light for recording from being emitted therefrom; sensing means (84) responsive to said additional element when it is energized for generating an electrical signal which varies with a temperature of the additional element; adjustment means (86, 88, 90, 172, Q25-33, Q424, Q425) responsive to said electrical signal for adjusting said respective slave driving currents; and characterized by a constant current source means (82) providing a current independent of the levels of currents of said respective slave driving currents for energizing said additional element with said constant current. 2. The apparatus of Claim 1 and wherein the light-emitting elements are grouped on respective chip arrays (31); a plurality of integrated circuit driver chips (40), each driver chip including said driving means for driving respective groups of light-emitting elements and each driver chip further including said sensing means and said adjustment means. 3. The apparatus of Claims 1 or 2 and wherein the driving means further includes digitally addressable current-conducting means (Q25-32, Q5-12) for selectively establishing a voltage bias related to a digital addressing of said digitally addressable current-conducting means; and wherein the adjustment means includes means for generating an adjusted current (172) to flow through said digitally addressable current-conducting means. 4. The apparatus of claim 2 and wherein the adjustment means on each driver chip further includes digitally addressable current-conducting means (Q25-32, Q5-12) for selectively establishing a voltage bias in said driving means related to a digital addressing on said digitally addressable current-conducting means, the digitally addressable current-conducting means including a plurality of digitally addressable transistors. 5. The apparatus of claim 4 and wherein the adjustment means on each driver chip includes a variable current source means (172) responsive to said electrical signal for generating an adjusted current to flow to said digitally addressable transistors. 6. The apparatus of claim 1 and wherein the adjustment means further includes digitally addressable current-conducting means (Q25-Q32, Q5-12) for selectively establishing a voltage bias in said driving means related to a digital addressing on said digitally addressable current-conducting means, the digitally addressable current-conducting means including a plurality of digitally addressable transistors. 7. The apparatus of claim 6 and wherein the adjustment means includes a variable current source means (172) responsive to said electrical signal for generating an adjusted current to flow to said digitally addressable transistors. 8. A non-impact printer apparatus for recording, comprising: a plurality of groups (31) of light-emitting elements, a plurality of integrated circuit driver chips (40), each including means for driving respective groups of light-emitting elements; each driver chip including: digitally addressable current-conducting transistor means (Q25-Q32, Q5-12) for selectively establishing a reference current and a voltage bias related to a digital addressing of said digitally addressable current-conducting means; current mirror driver means (Q424, Q425, Q428, Q429, 117) responsive to said voltage bias for generating a plurality of slave currents that are slaved to said reference current; means coupling said slave currents to respective light-emitting elements; and characterized by temperature sensing means (84, 86, 88, 90, 172) wholly on said driver chip for sensing a temperature related electrical signal generated on said driver chip and in response thereto adjusting said reference current to adjust the slave currents to said light-emitting elements. 9. The apparatus of Claim 8 and wherein said temperature sensing means includes an additional element (LEDM) of similar structure to a light-emitting element but including means (80) for precluding recording radiation from exiting therefrom. 10. The apparatus of claim 9 and including a constant current source (82) for providing a current independent of the reference current and slave currents for energizing said additional element. 11. The apparatus of Claims 3, 8 or 9 and wherein the light-emitting elements are light-emitting diodes. 12. A driver chip (40) for use on a non-impact printer apparatus for driving a plurality of light-emitting diodes for recording, the driver chip comprising: current mirror means (Q424, Q425, 117, Q428, Q429) for generating a reference current (I CH IP BIAS ) and respective slave driving currents for driving respective light-emitting diodes to be driven by said driver chip and characterized by means (82) for generating a constant current independent of said reference and slave currents; sensing means (84) responsive to an electrical parameter of a non-light emitting diode when said constant current is driven therethrough for generating an electrical signal which varies with a temperature of the non-light emitting diode; and adjustment means (86, 88, 90, 172, Q5-12, Q25-32) responsive to said electrical signal for adjusting said respective slave driving currents. 13. The driver chip of claim 12 and wherein the adjustment means further includes digitally addressable current-conducting means (Q5-12, Q25-32) for selectively establishing a voltage bias in said driving means related to a digital addressing on said digitally addressable current-conducting means, the digitally addressable current-conducting means including a plurality of digitally addressable transistors. 14. The driver chip of claim 13 and wherein the adjustment means includes a variable current source means (172) responsive to said electrical signal for generating an adjusted current to flow to said digitally addressable transistors.

Inventors: SMALL JEFFREY A, SMALL, JEFFREY, A.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1991
Main classification: G06K15
IPC codes: B41J2, G06K15