Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-02-02
2002-06-04
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S084000
Reexamination Certificate
active
06400349
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a driving circuit for selectively driving a plurality of driven elements, such as light-emitting diodes used as light sources in an electrophotographic printer, heating elements in a thermal printer, or display elements in a display device.
In the following description, the driven elements are be light-emitting diodes or LEDs employed in an electrophotographic printer.
In a conventional electrophotographic printer, for example, an electrically charged photosensitive drum is selectively illuminated, responsive to the data to be printed, to form a latent electrostatic image, which is developed by application of toner particles to form a toner image. The toner image is then transferred to paper and fused onto the paper.
FIG. 15
is a block diagram of the control circuitry of a conventional electrophotographic printer.
FIG. 16
is a timing diagram illustrating the operation of the conventional electrophotographic printer.
The printing control unit
1
in
FIG. 15
comprises a microprocessor, read-only memory, random-access memory, input-output ports, timers, and other elements disposed in the printing engine of the printer. The printing control unit
1
receives a control signal SG
1
and a dot data signal SG
2
from a higher-order controller, and controls the printing operations performed by the printing engine. The dot data signal SG
2
is a one-dimensional digital signal representing a two-dimensional bit map of picture elements (pixels), referred to below as dots.
Upon receiving a print command via control signal SG
1
, the printing control unit
1
first checks a temperature sensor
23
to determine whether the fuser
22
is within the necessary temperature range. If the fuser
22
is not within the necessary temperature range, the printing control unit
1
activates a heater
22
a
built into the fuser
22
. When the fuser
22
reaches the necessary temperature, the printing control unit
1
activates a driver
2
that drives a stepping motor or pulse motor (PM)
3
used in the developing and transfer process, and activates a charge signal SGC that switches on a high-voltage power source
25
that charges toner particles in a developer unit
27
.
The presence or absence of paper and the size of the paper are detected by a paper sensor
8
and size sensor
9
. If paper is present, the printing control unit
1
activates a driver
4
that drives another pulse motor (PM)
5
. This motor is first driven in reverse by a certain amount, until paper is detected by a pick-up sensor
6
, then driven forward to feed the paper into the printing engine.
When the paper has been fed to the necessary position, the printing control unit
1
sends timing signals SG
3
(including horizontal and vertical synchronization signals) to the higher-order controller, and begins receiving the dot data signal SG
2
, which the higher-order controller generates on a page-at-a-time basis. The dot data signal SG
2
is supplied as a data signal HD-DATA to an LED head
19
comprising a row of LEDs, with one LED per dot. The transfer of dot data into the LED head
19
is synchronized with a clock signal (HD-CLK).
After sufficient dot data (HD-DATA) for one horizontal dot line have been transferred into the LED head
19
, the printing control unit
1
sends the LED head
19
a load signal (HD-LOAD), causing the dot data to be latched in the LED head
19
. The LED head
19
can then print this line while receiving dot data for the next line.
The LED head
19
prints the line by illuminating a photosensitive drum (not visible) which has been precharged to a negative electrical potential. The potential level of illuminated dots rises, creating a latent dot image. The toner in the developer unit
27
is also charged to a negative potential, so toner particles are electrostatically attracted to the illuminated dots, creating a toner image.
The LEDs are turned on and off in synchronization with a strobe signal (HD-STB-N).
FIG. 16
illustrates the timing of this signal and other signals mentioned above. The SG
3
pulses shown at the top of
FIG. 16
are horizontal synchronization pulses.
FIG. 16
illustrates three successive line-printing cycles, for printing lines N−1, N, and N+1 (where N is an arbitrary integer).
Referring again to
FIG. 15
, to transfer the toner image to the paper, the printing control unit
1
activates a transfer signal SG
4
that turns on a high-voltage power source
26
, generating a high positive voltage in a transfer unit
28
. As the paper travels through a narrow gap between the photosensitive drum and transfer unit
28
, the toner image is transferred by electrostatic attraction to the paper.
The paper with the toner image is then transported to the fuser
22
, which has been heated by the heater
22
a.
The heat fuses the toner to the paper, which then passes an exit sensor
7
and is ejected from the printer.
The printing control unit
1
controls these operations so that the high-voltage power source
26
is switched off except while the paper is traveling past the transfer unit
28
, as detected by sensors
6
and
9
. When the paper passes the exit sensor
7
, the printing control unit
1
also switches off the high-voltage power source
25
of the developer unit
27
, and stops the pulse motor (PM)
3
used in the developing and transfer process.
The above sequence is repeated for each page.
FIG. 17
shows the structure of the conventional LED head
19
in more detail. The dot data HD-DATA and clock signal HD-CLK are provided to a shift register comprising, for example, four thousand nine hundred ninety-two flip-flop circuits FF
1
, FF
2
, . . . , FF
4992
(this number of flip-flop circuits is appropriate for printing on A4-size paper at six hundred dots per inch). When four thousand nine hundred ninety-two bits of dot data have been clocked into this shift register, the load signal HD-LOAD is activated, causing the bits to be stored in latches LT
1
, LT
2
, . . . , LT
4992
. When the strobe signal HD-STB-N is driven low, bits set to the high logic level (‘1’) turn on LEDs LD
1
, LD
2
, . . . , LD
4992
by way of an inverter G
0
, pre-buffer circuits G
1
, G
2
, . . . , G
4992
, and p-channel metal-oxide-semiconductor (MOS) transistors Tr
1
, Tr
2
, . . . , Tr
4992
. The transistors Tr
1
, Tr
2
, . . . , Tr
4992
are the driving elements that allow driving current to flow from the power supply (V
DD
) to the anodes of the LEDs LD
1
, LD
2
, . . . , LD
4992
.
In a printer employing the LED head in
FIG. 17
, all of the driven LEDs LD
1
, LD
2
, . . . , LD
4992
are switched on for the same length of time, determined by the strobe signal HD-STB-N. Thus if these LEDs or the driving elements Tr
1
, Tr
2
, . . . , Tr
4992
do not have perfectly uniform electrical properties, the dots will be unevenly illuminated. This will lead to differences in the sizes of the electrostatic dots in the latent image formed on the photosensitive drum, and differences in the sizes of the dots printed on the page.
Referring to
FIG. 18
, the LEDs are disposed on a plurality of LED array chips, which are coupled by bonding wires to integrated driver circuits, referred to below as driver ICs. In the example shown, there are twenty-six LED array chips (CHP
1
to CHP
26
), each with one hundred ninety-two LEDs. Each LED is individually wire-bonded to an output terminal of the corresponding driver IC. The driver ICs (DRV
1
to DRV
26
) are also coupled in a cascaded series to receive the dot data and control signals shown in FIG.
17
.
FIG. 19
shows the internal structure of the first pre-buffer circuit (G
1
) in
FIG. 17
, comprising an AND gate AD
1
, a p-channel MOS transistor TP
1
, and an n-channel MOS transistor TN
1
. The other pre-buffer circuits G
2
to G
4992
are similar.
Each driver IC also has a control-voltage generating circuit
209
. The control-voltage generating circuit
209
comprises an operational amplifier
100
, a p-channel MOS transistor
101
, and a resistor with resistance R
ref
. The output of the operationa
Anyaso Uchendu O.
Oki Data Corporation
Rabin & Berdo P.C.
Saras Steven
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