JPH07266619A - Failed state determination method and led print head - Google Patents

Abstract

PURPOSE: To provide a method for determining the trouble state of each LED in an LED printing bar. CONSTITUTION: A trouble state determining method includes a step for supplying a constant drive current to an LED 12, a step for sequentially monitoring the forward voltage drop of each LED 12 by a multiplexing system under the control of a multiplexer 30, a step for setting a forward voltage drop range showing a trouble state and a step for discriminating the forward voltage drop range and forming a signal showing a trouble state.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to light emitting diode (LED) printbars of the type used in digital printing applications, and more particularly to diagnostic routines for determining the functionality of individual pixels on a printhead. .

[0002]

Image print bars used in image recording systems are well known in the art. The printbar generally consists of a linear array of multiple discrete light emitting sources. Examples of printbars include wire dots, electrostatic, inkjet and thermal printheads. A large number of LEs are arranged so that the scanning movement of the array occurs on the surface of the recording member.
Light emitting diode (LED) printheads have become preferred because of the high resolution enabled by arranging D in a linear array and providing relative movement between the LED printhead and the image recording member. The printbar light output produced by actuation of selected LEDs or pixels in response to video data input is coupled through an elongated lens array disposed between the printbar and the image recording member. The recording member has an electrostatic charge supplied to its surface. In a write white system, selected areas are created by using toner that is discharged by the printbar output and adheres to the areas that are not discharged (the information areas). The generated image areas are transferred to the output medium and heat or pressure fixes the image areas to the medium. A "write black" system is essentially the reverse of the charging and discharging process described above. In other words, the surface of the charging recording member is exposed to provide the desired image one line at a time. Each LED in the linear array, including the printbar,
It is used to expose the corresponding pixel on the recording member to a value determined by the imaging video data information supplied to the LED via the driver circuit. Conventionally, a binary video data signal from a data source, which may be a raster input scanner (RIS), computer, word processor, or other source with digitized image data, is clocked into a shift register assembly. The data bits are then shifted in parallel to the latch circuit where the data bits are temporarily recorded. Shortly after the start of the line signal, the individual LED drive circuits are selectively activated to control the on / off timing of the current flowing through the LEDs. Therefore, the LEDs are selectively turned on and off to form a line exposure pattern on the surface of the photoreceptor (eg, photoreceptor). The complete image is formed by successive line exposures. U.S. Pat. No. 4,689,694
Issue 4, Issue 706, 130, Issue 5, 138, 337
No. 5,126,759, a prior art printhead control circuit is shown.

In order to guarantee copy quality, it is important that each pixel is working properly. Current technology allows printbars of up to 600 LEDs (pixels) per linear inch. Therefore, width 12
An LED printbar spanning inches, with 600 spots per inch allows up to 7200 pixels. To determine the functionality of each pixel (eg, whether the pixel is in the "off" or "on" state, or when the light output is within a specified range when activated). To
It is preferable to be able to test the printbar periodically. Any test method should be performed electrically, automatically, and with minimal interruption to system operation.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an LED
It is to provide a method for determining the fault status of individual LEDs in a printbar.

[0005]

According to the first embodiment of the present invention, the operation of the corresponding LED and the corresponding drive output of each drive transistor are operated sequentially.
Monitored with analog multiplexer. This example determines whether the forward voltage drop of each LED is between certain reference limits. Values above or below the reference limit provide an analog output signal indicative of the particular type of LED failure associated with the detected level. The failure is due to changes in LED manufacturing, junction temperature and forward current. In this example, the LEDs are examined individually and sequentially. In the second embodiment, a digital multiplexer is used with a logic circuit between the transistor and the multiplexer to generate a digital signal that indicates whether the forward voltage of the LED is within or outside a predetermined range. In particular, the present invention relates to LED printbars, and to methods for determining the failure status of individual LEDs. The method comprises the steps of providing a constant drive current to the LEDs, sequentially monitoring the forward voltage drop of each LED in a multiplexing manner under the control of a multiplexer, and a forward direction indicating a fault condition. Setting a voltage drop range, identifying a forward voltage drop outside the forward voltage drop range, and generating a signal representative of a fault condition.

The LED printhead comprises a plurality of LED chips arranged along at least one column and a plurality of driving chips corresponding to the LED chips, the driving chips being connected to a common gate bias voltage. Multiple FEs
T including the drive chip through the FET to the LED
Comprising a constant current source for supplying a drive current to the corresponding LED, the multiplexing means being connected to the output of the FET for monitoring the forward voltage drop of the corresponding LED, the multiplexing means comprising: The multiplexing means including control means for sequentially receiving an analog voltage representative of a forward voltage drop and determining whether the voltage drop is within or outside a predetermined range representative of a failed LED. Producing an output signal representative of the failed LED.

[0007]

DETAILED DESCRIPTION FIG. 1 shows a typical prior art LED printbar 10 including a suitable patterned substrate support having a series of LED chips 12 mounted thereon. As an example of this,
There are 32 LED chips, each chip having 128 LEDs. The LED chips 12 are arranged end to end in a row, and in the 32-chip embodiment, the printhead includes 4,096 LEDs arranged in a row. Each pair of LED chips is provided with a pair of driver chips 14. LEDs are formed of a wafer, such as gallium arsenide, that is appropriately doped to conduct current and conduct light. each
LED chips are formed by cutting or slicing a wafer. Each driver chip has a corresponding LED
Provides a current source for each LED on the chip. Each driver chip includes a field effect transistor (FET) gated by a reference voltage. It is understood that there may be other configurations of LED chips and driver chips. Aluminum or gold wires, using wire bonding techniques known to those skilled in the art, connect the driver chips to the LED chips and the driver chips to the substrate electrical pattern connection pads.

Any of the 4,096 LEDs fail,
It has been found to adversely affect the quality of any output print produced by the printbar. The failure is
It results from the circuit separation or shorted output to the LED. In accordance with the principles of the present invention, a diagnostic test routine is provided to examine the behavior of each pixel to determine if the pixel is operating within a predetermined reference range and, if out of range, what type of failure is responsible. Decide A first embodiment of the invention is shown in FIG. FIG. 2 is a partial schematic diagram of a circuit corresponding to a part of the driver chip 14. The driver chip 14 includes three LEDs on the LED chip 12.
It is shown to provide drive current to the LEDs. From the above, each chip 12 has a total of 128 LEDs. Chip 14 includes a constant current source 20 that provides an input to a plurality of field effect transistors 22A-22N. Where N
Is the 128th LED on the corresponding chip. Each transistor is gated by a pixel on / off control signal input to gates A, B and N. The light output and rise time of all LEDs are approximately the same. The output of each FET transistor is the corresponding LED on the LED chip 12.
Are supplied to the anodes 12A, 12B and 12N.

In accordance with the present invention, the pixel monitoring circuit is formed in a driver chip or in a separate integrated circuit that may be formed adjacent to or on the LED chip.
The circuit includes an electrical connection from the output of each transistor to the analog multiplexer 30. Multiplexer 30 is MPX
(Multiplexer) Controlled by the controller 32,
When pixel 12A is activated, the output of transistor 22A is connected to input A of multiplexer 30. The output of transistor 22A represents the forward voltage drop of LED 12A. The control device 32 monitors (supervises) the input switching selection of the multiplexer 30. The analog output from the multiplexer 30 is sent to an external circuit where the analog comparison method or first the analog output is converted to a digital signal using an A / D (analog / digital) converter and then digitally. The forward voltage drop of the LED is compared with the specified limit by the method of comparing with the specified limit. Typical forward voltage drop (V
_FWD ) must be between 1.7 and 2.2 volts depending on LED manufacturing parameters, junction temperature and forward current. Voltages above approximately 2.2 volts indicate circuit isolation or LED failure for the LED. Voltages below approximately 1.7 volts are most likely to indicate a short circuit. The "error" signal is used to generate a local visual display, or alternatively, a remote diagnostic capability (R
IC) may be integrated into the system. The net result is that the defective pixel or pixels are detected and an appropriate repair action (perhaps replacement of chip 12) is taken.

There is a limit to the embodiment of FIG. 2 that tests the LEDs one at a time to provide an analog output signal regardless of whether the voltage drop is within a predetermined range. FIG. 3 shows a partial schematic diagram of a second digital monitoring system for producing a digital input to a digital multiplexer. Referring to FIG. 3, the circuit operates in a manner similar to that described in the embodiment of FIG. 2 above. The constant current source 20 is
A drive current is supplied to the field effect transistors 22A to 22N. Each transistor is gated by a pixel on / off input. The monitoring circuit of this embodiment includes a logic (AND) circuit 38 connected between the transistor output ( _VFWD ) and the digital multiplexer 40. The circuit 38 is
It includes comparators 38A, 38B biased between two reference voltage levels V _H and V _L. These voltage levels can be adjusted externally to compensate for temperature and forward current variations. The LEDs are operated sequentially with multiplexer 40 under the control of MPX controller 32. With the 2.2 volt or 1.7 volt bias shown, the output of circuit 38 is representative of the status of the active pixel circuit. The output is "1" (high) if the LED forward drop is between 1.7 and 2.2 volts, and "0" (low) outside this range. V _FWD with different values,
See “Truth” Table 1 below for examples of high (H) and low (L) signals using a V _{H of} 2.2 volts and a V _L of 1.7 volts. Multiplexer 40
Processes the digital input pixel by pixel into a digital output. Separate control circuitry 50 looks for the "0" signal and monitors the multiplexer output which produces a malfunctioning or "bad" pixel signal to the local remote indicator.

[0011]

[Table 1]

The printhead may include many individual driver chips, each having its own multiplexer output, so that if a common digital output line is used on all chips, the chip digital outputs are also Must be multiplexed. One method is to provide the circuitry on the driver chip and select which LE on that chip.
Whenever D is not activated either, the multiplexer digital outputs are disconnected or "tri-stated".
te) ”. In this way, only one digital output from the driver chip (if only one pixel is tested on the printhead at a time) is always connected to the common printbar output line.
Since it takes only 200 microseconds to examine an individual pixel, an entire printbar containing 7200 pixels can be examined in less than 2 seconds.

[0013]

The present invention is constructed as described above, and provides a method for determining a failure state of an individual LED in an LED print bar.

[Brief description of drawings]

FIG. 1 is a top view of a prior art LED printbar assembly.

FIG. 2 is a first of the invention showing an analog multiplexer used to generate an analog fault signal for each LED.
FIG. 6 is a simplified block diagram of an embodiment.

FIG. 3 is a simplified block diagram of a second embodiment of the present invention showing a digital multiplexer and corresponding logic circuitry used to provide a digital output representative of the status of each LED.

[Explanation of symbols]

 10 LED Print Bar 12 LED Chip 14 Driver Chip 20 Constant Current Source 22 Field Effect Transistor 30 Analog Multiplexer 32 MPX Controller

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H01L 33/00 L

Claims (2)

[Claims] 1. The LED print bar includes individual LEDs.
Providing a constant drive current to the LEDs, and sequentially monitoring the forward voltage drop of each LED in a multiplexing manner under the control of a multiplexer, the method comprising: Setting a forward voltage drop range representing a state, and identifying a forward voltage drop outside the forward voltage drop range,
And a step of generating a signal representing a fault condition, the fault condition determining method comprising: 2. An LED printhead, comprising: a plurality of LED chips arranged along at least one row; and a plurality of driving chips corresponding to the LED chips, the driving chips having a common gate bias. A plurality of FETs connected to a voltage, the driving chip including a constant current source for supplying a driving current to the LEDs through the FETs, and for monitoring the forward voltage drop of the corresponding LEDs. The above
Comprising multiplexing means connected to the output of the FET, said multiplexing means sequentially receiving an analog voltage representative of the forward voltage drop of said LED,
Including a control means for determining whether the voltage drop is within or outside a predetermined range representing the failed LED, the multiplexing means including the failed LED.
An LED printhead comprising an output signal that represents.