Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2000-11-15
2003-03-18
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C327S565000
Reexamination Certificate
active
06535235
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a drive circuit that selectively cyclically drives a row of LEDs (light emitting diodes) used in an electrophotographic printer, a row of heat-generating resistors used in a thermal printer, and a row of display elements used in a display. The invention also relates to an LED head incorporating the drive circuit.
2. Description of the Related Art
In a conventional electrophotographic printer, the surface of a photoconductive drum is charged to a high voltage. An optical writing means such as a light emitting diode (referred to as LED hereinafter) head illuminates the charged surface of the photoconductive drum to form an electrostatic latent image thereon. The electrostatic latent image is then developed with toner into a toner image. The toner image is transferred to a print medium such as paper. The print medium is then advanced to a fixing unit where the toner image on the print medium is fused into a permanent print.
The LED head includes a large number of LED array chips, each of which includes a row of a plurality of LEDs. The LED array chips are aligned in a direction parallel to the rotational axis of the photoconductive drum such that the LEDs of the LED arrays lie on a straight line and the light dots emitted from the LEDs are focused on the surface of the photoconductive drum.
FIG. 17
illustrates a general layout of the conventional driver IC mounted on a printed wiring board and surrounding electrodes formed on the board (e.g., Japanese Patent Preliminary Publication (KOKAI) No. 6-297765/1994). A plurality of input electrodes
31
are arranged on one of opposed long sides of a rectangular driver IC. Two rows of LED drive electrodes
32
are arranged parallel to each other on the other of the opposed long sides such that electrodes in one row are staggered with those in the other.
The driver IC incorporates the row
31
of input electrodes, a row
33
of shift registers, a row
34
of latch circuits, a row
35
of pre-buffers including AND circuits and inverters, and a row
37
of drive transistors in the form of P-channel MOS transistors, all of which are aligned in this order from the input electrode side to the LED drive electrode side. The rows are parallel to one another and extend in the longitudinal direction of the driver IC.
The shift registers, latches, pre-buffers, and the drive transistors are arranged at substantially the same intervals as the LED drive electrodes
32
in the longitudinal direction of the driver IC
11
. All of these circuit components receive their control signals from the input electrode side via wires, not shown.
The LED supply voltage electrode
36
is formed of aluminum and in the shape of a belt having a width W. The LED supply voltage electrode
36
lies between the row of the pre-buffers
35
and the row of the drive transistors
37
and extends in a direction parallel to these rows. The LED supply voltage electrode
36
has a plurality of electrode pads
38
a
-
38
c
(e.g., three pads shown in
FIG. 17
) mounted thereon through which the LED supply voltage VDDH is supplied for driving the LEDs.
Each driver IC
11
drives a total of 192 LEDs through electrode pads DO
1
-DO
192
. The LED drive supply VDDH is supplied through the electrode pads
38
a,
38
b,
and
38
c
provided at locations that correspond to the electrode pads DO
32
, DO
96
, and DO
160
, respectively.
FIG. 18
is a cross-sectional view illustrating a printed wiring board on which the aforementioned the row
31
of input electrodes, LED drive electrodes
32
, and LED supply voltage electrode
36
are formed. The driver IC is not shown in FIG.
18
. The LED drive electrodes
32
are wire-bonded to the LED array, not shown, via the electrode pads DO-DO
192
in the form of an aluminum pattern formed on the printed wiring board.
The electrode pads
38
a
-
38
c
are formed on the LED supply voltage electrode
36
. The input electrodes
31
are in the form of an aluminum electrode and electrode pads
38
a
-
38
c
of the driver IC are connected through bonding wire
40
to electrodes, not shown, on the printed wiring board.
FIG. 19
illustrates a pertinent portion of an equivalent circuit of the driver IC of FIG.
17
and an LED array chip driven by the driver IC.
Referring to
FIG. 19
, one driver IC drives 192 LEDs fabricated on one LED array chip. Resistors R
201
-
203
represent equivalent resistance values of the bonding wires
40
(
FIG. 18
) through which the individual LEDs receive the supply voltage. Nodes S
1
-S
192
indicate the locations on the LED supply voltage electrode
36
to which the sources of the drive transistors M
1
-M
192
are connected. Resistors R
1
-R
19
l represent the resistance values between adjacent nodes.
The electrode pads
38
a,
38
b,
and
38
c
on the LED supply voltage electrode
36
of
FIG. 17
are arranged near the electrode pads DO
32
, DO
96
, and DO
160
, respectively. Therefore, the resistors R
201
, R
202
, and R
203
are connected to the sources of drive transistors M
32
, M
96
, and M
160
, respectively, i.e., the nodes S
32
, S
96
, and S
160
.
The drains of the drive transistors M
1
-M
192
are connected to the anodes of the LEDs D
1
-D
192
. The gates of the drive transistors M
1
-M
192
are connected to a later described controller, for example, shown in
FIG. 20
, which generates a predetermined gate-to-source voltage Vcont that sets the value of drive current Io flowing through each of the LEDs (
FIG. 20
shows only Io flowing through D
1
).
The P-channel MOS transistor M
0
is a reference transistor that generates a reference current Iref. The reference transistor M
0
is aligned with the row of drive transistors M
1
-M
192
. The reference transistor M
0
is located adjacent the drive transistor M
1
and is depicted in solid black (FIG.
17
). The resistors R
0
-R
191
are equivalent resistance values between adjacent nodes of the LED supply voltage electrode
36
formed in the shape of a belt having a width W.
The drive currents Io that flow through individual LEDs are determined in reference to the reference current Iref that flows through the reference transistor M
0
located at an end of the row of drive transistors M
1
-M
192
(FIG.
17
).
FIG. 20
illustrates a part of the control voltage generating circuit that provides the Vcont for driving the drive transistors of FIG.
19
.
Each driver IC has a control voltage-generating circuit
20
and 192 pre-buffers G
1
-G
192
.
FIG. 20
shows the pre-buffer G
1
and an associated circuit that includes a latch circuit LT
1
, a P-channel MOS type drive transistor M
1
, and a light emitting diode D
1
. D
1
is one of the LEDs D
1
-D
192
of FIG.
19
. The drive transistor M
1
is one of the transistors M
1
-M
192
of FIG.
19
. The pre-buffer G
1
includes an AND circuit AD
1
, a P-channel MOS transistor TP
1
, and an N-channel MOS transistor TN
1
.
An OP amplifier
21
generates an output voltage Vcont. A P-channel MOS transistor M
0
has the same gate length as 192 drive transistors M
1
-M
192
. An inverting input of the OP amplifier
21
receives a reference voltage Vref from an external circuit.
The OP amplifier
21
, P-channel MOS transistor M
0
, and a resistor Rref form a feedback control circuit. The current that flows through the resistor Rref, i.e., through the P-channel MOS transistor M
0
is determined by the reference voltage Vref and the resistor Rref if the supply voltage of the P-channel MOS transistor: M
0
is constant.
FIG. 21
illustrates the aforementioned LED array chips; CHP
1
-CHP
26
aligned along the surface of the photoconductive drum in a direction parallel to the rotational axis of the photoconductive drum, and drive currents Io flowing through the LEDs contained in each of the LED array chips CHP
1
-CHP
26
. The LED array chips CHP
1
-CHP
26
are driven by corresponding driver ICs DRV
1
-DRV
26
.
Each of the LED array chips CHP
1
-CHP
26
includes 192 LEDs fabricated therein. Each LED is connected by wire bonding to a corresponding electrode pad DO of
Oki Data Corporation
Pham Hai
Rabin & Berdo, P.C
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