US 6,323,631 B1
`(10) Patent No.:
`(12) Unlted States Patent
`11
`.
`V.
`J an
`45 Date of Patent
`No
`27, 2001
`
`
`U8006323631B1
`
`(54) CONSTANT CURRENT DRIVER WITH
`AUTO-CLAMPED PRE-CHARGE FUNCTION
`
`5,886,566 *
`6,043,525 *
`
`........................... 327/536
`3/1999 Park et a1.
`3/2000 Chen .................................... 257/292
`
`(75)
`
`Inventor: Dar-Chang Juang, Hsinchu (TW)
`
`* cited by examiner
`
`(73) Assignee: Sunplus Technology C0., Ltd.,
`Hsin-Chu (TW)
`
`Primary Examiner—Matthew Nguyen
`(74) Attorney, Agent, or Firm—Bacon & Thomas, PLLC
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent 15 extended or adjusted under 35
`U‘S’C’ 154(b) by 0 days.
`
`(21) APPI- N0~3 09/761,685
`(22)
`Filed'
`Jan 18 2001
`'
`'
`’
`51
`Int. Cl.
`........................................................ G05F 1/40
`7
`
`.............. 323/315
`(52)
`(58) Field of Search ..................................... 323/312—315,
`327/535, 537, 538
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`ABSTRACT
`(57)
`A constant current driver With auto-clamped pre-charge
`function includes a reference bias generator and a plurality
`of constant current driver cells, each being connected to the
`reference bias generator to form a respective current mirror.
`Each constant current driver cell has a switch transistor, a
`current output transistor and a pre-charge transistor. When a
`constant current is out utte
`rom t e current out ut tran-
`'
`p
`d f
`h
`p
`sistor for driving an organic light emitting diode, the pre-
`charge transistor is turned OH to PIOVide a drain to source
`current as an additional
`large current for rapidly pre-
`charging the organic light emitting diode until the gate to
`source voltage of the pre-charge transistor is smaller than the
`threshold voltage.
`
`5,473,270 * 12/1995 Denker ................................... 326/95
`
`14 Claims, 12 Drawing Sheets
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`US. Patent
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`Nov. 27, 2001
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`Sheet 8 0f 12
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`US 6,323,631 B1
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`1
`CONSTANT CURRENT DRIVER WITH
`AUTO-CLAMPED PRE-CHARGE FUNCTION
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to a circuit for driving the
`organic light emitting diode (OLED) display panel and,
`more particularly,
`to a constant current driver with auto-
`clamped pre-charge function.
`2. Description of Related Art
`The organic light emitting diode (OLED) is known as an
`organic thin film semiconductor based light emitting device.
`Thus, a display panel can be provided by a two-dimensional
`array of OLEDs.
`In general, an OLED panel may be driven by a constant
`voltage, which is deemed to be less energy consumed.
`However, because the cut-in voltages of the OLEDs on the
`display panel are not uniform, each OLED may de con-
`ducted in different voltage level, which results in that the
`emitted light is not even.
`Furthermore, it is known that the light intensity of the
`OLED is proportional to the current generated by combining
`the electrons and holes at the junction area. This current is
`an exponential function of the junction voltage, so that it is
`very sensitive to the variance of the junction voltage. Hence,
`in order to achieve a uniform light intensity of the whole
`OLED array, it is preferable to drive the OLED panel by
`constant current.
`
`FIG. 8 is a system architecture showing the conventional
`constant current driven OLED display panel and the driver.
`As shown, the driver includes a column driving circuit 81
`and a row driving circuit 82. The column driving circuit 81
`includes a reference bias generator 811 and a plurality of
`constant current column driver cells 812. FIG. 9 is a detailed
`
`circuit diagram of the column driving circuit 81. The refer-
`ence bias generator 811 is coupled to each constant current
`column driver cell 812 to form a current mirror, so as to turn
`on the switch transistor MPS based on an input from a
`column data shift register 83 via an input terminal COLI,
`thereby an output
`transistor MPO providing a constant
`current output on the output terminal COLO. Furthermore,
`a discharge transistor MND, controlled by a discharge
`control terminal DIS, is provided in each constant current
`column driver cell 812 for eliminating the possible residual
`image caused by the junction capacitance and the wiring
`stray capacitance of OLEDs. The discharge transistor MND
`is turned on for a short period of time before the driving
`current is applied, so as to leak out the charge stored in the
`junction capacitors and the wiring stray capacitors of
`OLEDs.
`
`With reference to FIG. 8 again, the row driving circuit 82
`includes a plurality of inverters 821 connected to a row
`scanning shift register 84. Hence, under the control of the
`synchronous signals (HSYNC and VSYNC) and clock sig-
`nal (HCLK), current from the output terminal COLO of a
`selected constant current column driver cell 812 is outputted
`to the OLEDs of a corresponding column. Furthermore, a
`selected inverter 821 drains the conducting current of a row
`of OLEDs, so as to turn on the desired OLEDs to emit light.
`In a typical application, only dozens of micro amperes
`(e.g., 25 yA) of driving current is sufficient for driving a
`pixel having a size of 0.1 mm2 to emit a required light
`intensity under a 1/54 duty cycle operating condition.
`However,
`taking a 64x64 OLED display panel as an
`example, a parasitic capacitance of several hundreds pico
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`farads (e.g., 600 pF) may be generated from the stray
`capacitor on the thin film electrode layout and the junction
`capacitance of the diode array in driving each pixel.
`Therefore, if the constant current driving circuit as shown in
`FIG. 8 is employed for driving, the parasitic capacitor is
`charged by the driving current at first. As shown in FIG. 10,
`in a driving duration of about 200 micro seconds (,us), it
`takes about 150 us to charge the OLED to have an enough
`voltage (e.g., about 7V) for conducting a current of about 25
`yA at the junction. Therefore, the actual duration for emit-
`ting light is greatly reduced, and the intensity of emitting
`light is not satisfactory.
`To eliminate such a problem, a pre-charge capability is
`provided in the constant current driving circuit. A known
`driver with pre-charge circuit is shown in FIG. 11, wherein
`the gate of a PMOS transistor MPPRE, which is used as a
`pre-charge device, is temporarily grounded at the front edge
`of a driving period by a switch, so as to generate a large
`current in a short period of time rapidly charging a stray
`capacitor to a high voltage. However, such a design suffers
`from several disadvantages. With reference to FIG. 12, the
`first disadvantage is that the voltage of stray capacitor may
`be over-charged, resulting in a much larger junction current
`generated in OLED as compared to the predetermined
`driving current at this time period. The second disadvantage
`is that the over-charged voltage of the stray capacitor may be
`slowly discharged through OLED after the pre-charge
`process, resulting in a junction current being difficult to
`control. Particularly, the pre-charge process may produce a
`product of large current and time, i.e., a considerable amount
`of constant charge. As a result, it is difficult to adjust the
`driving current for obtaining a desired intensity of display
`panel. The third disadvantage is that an independent pre-
`charge control pulse signal with a very small width is
`required for alleviating the problem of uneven light emission
`caused by the first disadvantage. In view of above,
`the
`conventional constant current OLED drivers are not
`
`satisfactory, and thus there is a need to have an improved
`constant current driver to mitigate and/or obviate the afore-
`mentioned problems.
`
`SUMMARY OF THE INVENTION
`
`An object of the present invention is to provide a constant
`current driver with auto-clamped pre-charge function, which
`allows the OLED display panel to emit
`light uniformly
`without the need of an additional pre-charge signal, thus
`eliminating the drawbacks of the conventional OLED driver.
`Another object of the present invention is to provide a
`constant current driver with auto-clamped pre-charge
`function, which can be switched into a voltage driven mode
`by a multiplexer, so as to be used in an application requiring
`a low energy consumption,
`instead of requiring uniform
`light illumination.
`the present invention which
`According to one aspect,
`achieves these objects relates to a constant current driver
`with auto-clamped pre-charge function, which comprises: a
`reference bias generator having a bias output terminal for
`providing a reference bias; and a plurality of constant
`current driver cells, each being connected to the reference
`bias generator to form a respective current mirror. The
`constant current driver cell comprises: a switch transistor
`controlled by an input terminal for being turned on or off; a
`current output transistor connected to the switch transistor
`and the bias output terminal of the reference bias generator
`for outputting a constant current when the switch transistor
`is on; and a pre-charge transistor having a gate connected to
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`US 6,323,631 B1
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`3
`the gate of the current output transistor and further con-
`nected to the bias output terminal of the reference bias
`generator, a drain and a source connected to the drain and
`source of the current output
`transistor,
`respectively,
`whereby, when a constant current is outputted from the
`current output transistor for driving an organic light emitting
`diode, the pre-charge transistor is turned on due to the gate
`to source voltage thereof being larger than its threshold
`voltage, so as to provide a drain to source current as an
`additional large current for rapidly pre-charging the organic
`light emitting diode until the gate to source voltage of the
`pre-charge transistor is smaller than the threshold voltage.
`According to another aspect, the present invention which
`achieves these objects relates to a constant current driver
`with auto-clamped pre-charge function, which comprises: a
`reference bias generator having a bias output terminal for
`providing a reference bias; and a plurality of constant
`current driver cells, each being connected to the reference
`bias generator to form a respective current mirror. The
`constant current driver cell comprises: a switch transistor
`controlled by an input terminal for being turned on and off;
`a current output transistor connected to the switch transistor
`and the bias output terminal of the reference bias generator;
`and a diode array having an anode and a cathode connected
`to the drain and the source of the current output transistor,
`respectively, wherein when a constant current is outputted
`from the current output transistor to drive an organic light
`emitting diode, the diode array is turned on for providing an
`additional large current to rapidly pre-charge the organic
`light emitting diode until the voltage of the diode array is
`smaller than its cut-in voltage.
`the present invention
`According to yet another aspect,
`which achieves these objects relates to a constant current
`driver with auto-clamped pre-charge function, wherein a
`multiplexer is connected between the bias output terminal of
`the reference bias generator and the connection point of the
`gates of the pre-charge transistor and the current output
`transistor of the constant current driver cell. The first and
`
`second input terminals of the multiplexer are connected to
`the bias output terminal of the reference bias generator and
`ground respectively, and the output terminal of the multi-
`plexer is connected to the gates of the pre-charge transistor
`and the current output transistor, so as to switch the driving
`circuit to a constant current or a constant voltage driving
`mode. Other objects, advantages, and novel features of the
`invention will become more apparent from the detailed
`description when taken in conjunction with the accompa-
`nying drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a circuit diagram of a first preferred embodiment
`of constant current driver with auto-clamped pre-charge
`function in accordance with the present invention;
`FIG. 2 depicts the driving waveforms of the circuit shown
`in FIG. 1;
`FIG. 3 is a circuit diagram of a second preferred embodi-
`ment of constant current driver with auto-clamped pre-
`charge function in accordance with the present invention;
`FIG. 4 is a circuit diagram of a third preferred embodi-
`ment of constant current driver with auto-clamped pre-
`charge function in accordance with the present invention;
`FIG. 5 depicts the driving waveforms of the circuit shown
`in FIG. 4;
`FIG. 6 is a circuit diagram of a fourth preferred embodi-
`ment of constant current driver with auto-clamped pre-
`charge function in accordance with the present invention;
`
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`FIG. 7 shows the waveforms of the OLED junction
`currents of the present driver and the conventional drivers;
`FIG. 8 is a schematic diagram of the conventional OLED
`display panel driven by a constant current driving circuit;
`FIG. 9 is a circuit diagram of the conventional constant
`current driving circuit for OLED display panel;
`FIG. 10 depicts the driving waveforms of the circuit
`shown in FIG. 9;
`FIG. 11 is a circuit diagram of the conventional constant
`current driving circuit
`for OLED display panel having
`pre-charge function; and
`FIG. 12 depicts the driving waveforms of the circuit
`shown in FIG. 11.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`With reference to FIG. 1, there is shown the constant
`current driver with auto-clamped pre-charge function in
`accordance with a preferred embodiment of the present
`invention. As shown, the column driving circuit 10 includes
`a plurality of constant current driver cells 11 and a reference
`bias generator 12 coupled to a respective constant current
`driver cell 11 to form a current mirror. The constant current
`driver cell 11 includes a PMOS transistor MPS, which is
`used as a switch, and a PMOS transistor MPO, which is used
`as a current output device, connected to the transistor MPS.
`The source of transistor MPS is connected to the voltage
`VDD,
`the drain thereof is connected to the source of
`transistor MPO, and the gate thereof is connected to the
`input terminal COLI. The gate of transistor MP0 is con-
`nected to the bias output terminal VB of the reference bias
`generator 12, and the drain thereof is connected to the output
`terminal COLO. Hence, when the input terminal COLI is of
`a low voltage level, the PMOS transistor MPS is turned on.
`As a result, PMOS transistor MPO outputs a constant current
`on the output terminal COLO.
`The constant current driver cell 11 also includes an
`
`NMOS transistor MND, used as a pre-charge device, which
`has a drain connected to the drain of transistor MPO, a
`source connected to the discharge voltage VDIS, in which
`the discharge VDIS is set to the system’s zero voltage or a
`predetermined voltage for a specific application, and a gate
`connected to a discharge control terminal DIS, so that, when
`discharge control terminal DIS is of a high voltage level,
`transistor MND is turned on to perform a discharge.
`In order to provide an auto-clamped pre-charge capability,
`the present invention utilizes an NMOS transistor MNST,
`which is used as a pre-charge device,
`to connect to the
`current output transistor MPO in parallel, so as to form a
`source follower. That is,
`the gate of transistor MNST is
`connected to the gate of transistor MPO, and further con-
`nected to the bias output terminal VB. The drain of transistor
`MNST is connected to the drain of transistor MPO, and
`further connected to the output terminal COLO. The source
`of transistor MNST is connected to the source of transistor
`
`MPO, and further connected to the voltage VDD via the
`switch transistor MPS, which is controlled by input terminal
`COLI.
`
`Also with reference to FIG. 2, there is shown the driving
`waveforms. In the design of the driver, the DIS signal will
`be pulled to VDD for a short period of time (e.g., about 10
`to 20 us) before driving each horizontal
`line, so as to
`discharge the junction capacitors and wiring stray capacitors
`of OLEDs in the corresponding column, thereby rapidly
`eliminating the residual image effect. Afterwards, the con-
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`US 6,323,631 B1
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`5
`stant current driver cell 11 is controlled by the corresponding
`column data to determine whether to output current or not.
`If it is determined to output current, PMOS transistor MPO
`will output a constant current of 25 MA. At this time, the
`voltage of the OLED 13 to be driven is still 0V, a low voltage
`level, or even a negative voltage level. Because the gate to
`source voltage of transistor MNST VGS=bias voltage VB—
`the voltage of OLED VOLED. Thus, VGS is greater than the
`threshold voltage Vth of transistor MNST, so that
`the
`pre-charge transistor MNST will be turned on and the drain
`to source current IDS of transistor MNST (which is propor-
`tional to the square value of (VGS—Vth)) is provided as
`additional large current for rapidly pre-charging the OLED
`13 to be driven. Thus, voltage VOLED is rapidly charged until
`VGS is smaller than Vth. Furthermore, when considering the
`voltage drop of the row driving circuit 14, the pre-charge
`circuit is automatically disabled after (VOLED+ the voltage
`drop of row drive circuit 14)>(VB—Vth). That is, a clamping
`on the pre-charge circuit is occurred, so as to stop pre-
`charging. As a result, only a 25 yA constant current output-
`ted from transistor MP0 is used to drive the corresponding
`OLED 13 and stray capacitor.
`In the embodiment shown in FIG. 1, a multiplexer 15 is
`used as a single-pole double-throw switch for bias control.
`The multiplexer 15 is connected between the bias output
`terminal VB of reference bias generator 12 of the column
`driving circuit 10 and the gates of transistors MNST and
`MP0 of the constant current driver cells 11. The first input
`terminal 11 and second input terminal 12 of the multiplexer
`15 are coupled to the bias output terminal VB and ground
`respectively. The output terminal Y of the multiplexer 15 is
`connected to the gates of transistors MNST and MP0
`respectively. When control signal ID/VD of the multiplexer
`15 is one, the output terminal Y is switched to the first input
`terminal 11, so that the gate of transistor MNST of constant
`current driver cell 11 is connected to the bias output terminal
`VB. Such a circuit configuration is the same as the previous
`embodiment, which is known as a constant current driving
`mode. When the control signal ID/VD of multiplexer 15 is
`zero,
`the output terminal Y is switched to second input
`terminal 12, and thus the gates of transistors MST and MP0
`of the constant current driver cell 11 are connected to ground
`(i.e., 0V). Hence, transistor MNST is forced to be turned off
`and transistor MP0 is forced to be turned on and behaves as
`
`a low resistor. Thus, such a driving unit is served as a
`constant voltage driving circuit. Accordingly, the user may
`select a desired driving mode of the driver in accordance
`with the present invention depending on a specific applica-
`tion thereby achieving the maximum benefits with the
`minimum cost.
`
`FIG. 3 is the circuit diagram of a second preferred
`embodiment
`in accordance with the present
`invention,
`which is similar to the previous embodiment except that the
`PMOS switch transistor MPS is connected between the
`
`connection point of the source of transistor MNST and the
`drain of transistor MPO and the driving output terminal.
`That is, the source of transistor MP0 is connected to the
`supply voltage VDD, the drain thereof is connected to the
`source of transistor MPS, and the gate thereof is connected
`to the bias output terminal 12 of the reference bias generator
`VB. The gate of transistor MPS is connected to the input
`terminal COLI and the drain thereof is served as a constant
`
`current output terminal COLO. Furthermore, the drain of
`transistor MND is connected to drain of transistor MPS, the
`source thereof is connected to discharge voltage VDIS> an d
`the gate thereof is served a s a discharge control terminal
`DIS. Moreover, the drain of transistor MNST is connected to
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`the output terminal COLO through transistor PS, and both
`the sources of transistors MNST and MP0 are connected to
`
`the supply voltage VDD. With such a circuit configuration,
`the second embodiment can achieve the same advantages as
`the first one.
`
`In other preferred embodiments of the present invention,
`the auto-clamped pre-charge function is achieved by using
`diode arrays. FIG. 4 is a circuit diagram of a third preferred
`embodiment in accordance with the present invention. As
`shown, similar to the above embodiments,
`the constant
`current driver cell 11 of the column driving circuit also
`comprises a PMOS transistor MPS used as a switch device,
`a PMOS transistor MPO used as a voltage output device, and
`a NMOS transistor MND used as a discharge device. The
`gate of PMOS transistor MP0 is connected to the bias output
`terminal VB of a reference bias generator 12 for forming a
`constant current output device. The difference between this
`embodiment and the above ones is that a diode array 41 is
`connected to transistor MPO in parallel, wherein the anode
`of the diode array 41 is connected to the drain of transistor
`MPO and the cathode thereof is connected to the source of
`transistor MPO and also connected in series with switch
`
`transistor MIS which is controlled by input terminal COLI.
`The diode array 41 is comprised by at least one diode. In
`this embodiment, there are two diodes connected in series.
`In the CMOS manufacturing process,
`the diode array is
`preferably implemented by serially-connected diodes manu-
`factured by NMOS or PMOS transistors, as show in the
`figure.
`Also with reference to the driving waveforms shown in
`FIG. 5, the output current of constant current driver cell 11
`is controlled by the corresponding column data to output
`current. If there is current to be output, PMOS transistor
`MPO will output a constant current of 25 MA. At
`this
`moment, the voltage of driven OLED 13 is still 0V, low
`voltage or even negative voltage. Hence, the diode array
`consisting of PMOS transistors MPSTl and MPST2 will be
`turned on for providing an additional
`large current for
`rapidly pre-charging the OLED 13 to be driven. Thus,
`voltage VOLED is rapidly charged until voltage VDLMPO at
`the diode array 41 is smaller than the cut-in voltage of the
`diode array 41. At this moment, the pre-charging circuit is
`disabled. That is, a clamping operation on the pre-charging
`circuit is automatically occurred. As a result, only 25 yA
`constant current from transistor MP0 is used to drive the
`
`corresponding OLED 13 and stray capacitor.
`With reference to FIG. 4 again, it is also applicable to use
`a multiplexer 15 as a single-pole double-throw switch for
`bias control
`in this embodiment. The multiplexer 15 is
`connected between the bias output
`terminal VB of the
`reference bias generator 12 of the column driving circuit 10
`and the gates of transistors MNST and MP0 of the constant
`current driver cell 11, so as to configure the circuit to be a
`constant current driving mode or a constant voltage driving
`mode. Therefore, the user may select a desired operating
`mode of the driver in accordance with the present invention
`depending on a specific application, thereby achieving the
`maximum benefits with the minimum cost.
`
`FIG. 6 is a circuit diagram of a fourth preferred embodi-
`ment of the constant current driver with auto-clamped
`pre-charge function in accordance with the present
`invention, which is similar to the previous embodiment
`except that the PMOS switch transistor MPS is connected
`between the connection point of the anode of the diode array
`41 and the drain of transistor MPO, and the driving output
`terminal COLO. That is, the source of transistor MP0 is
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`US 6,323,631 B1
`
`7
`connected to the supplied voltage VDD, the drain thereof is
`connected to the source of transistor MPS, and the gate
`thereof is connected to the bias output
`terminal of the
`reference bias generator 12. Furthermore, the gate of tran-
`sistor MPS is connected to the input terminal COLI and the
`drain thereof is served as a constant current output terminal
`COLO. Moreover, the source of transistor MND is con-
`nected to the drain of transistor MPS, the drain thereof is
`connected to the discharge voltage VDIS’ and the gate thereof
`is served as a discharge control terminal DIS. In addition, the
`cathode of the diode array 41 is connected to the source of
`transistor MPO and the anode thereof is connected to the
`
`the
`drain of transistor MPO. With Such a configuration,
`fourth embodiment can obtain the same advantages as the
`previous one.
`In view of the foregoing, the constant current driver with
`auto-clamped pre-charge function in accordance with the
`present invention is implemented by utilizing an NMOS
`transistor MNST as a source follower, which is connected
`with transistor MPO in parallel for being used as a pre-
`charging device. Thus, it is able to automatically adjust the
`pre-charging current based on the voltage of OLED, and
`further automatically clamp the voltage to a level of
`VB—VthiMNsr (VthiMNsr denotes the threshold voltage of
`transistor MNST) for preventing the voltage from being
`over-charged. Alternatively, a diode array is connected to the
`constant current output transistor MPS in parallel for being
`used as a pre-charging device. Similarly,
`it
`is able to
`automatically adjust the pre-charging current based on the
`voltage of OLED, and further automatically disable the
`pre-charging circuit when VDLMPO (VDLMPO denotes the
`drain to source voltage of transistor MP0) is smaller than the
`cut-in voltage of the diode array for preventing the voltage
`from being over-charged. Therefore, an independent pre-
`charging control signal as required in the prior art is elimi-
`nated by the present
`invention, so as to avoid all
`the
`drawbacks in the prior art. FIG. 7 shows the waveform of the
`junction current of OLED for the driving circuit of the
`present invention, as denoted by ‘C’, and those for the
`conventional driving circuits without and with pre-charging
`function, as denoted by ‘A’ and ‘B’, respectively. By com-
`paring these waveforms, it is appreciated that the present
`invention does provide better performance and can achieve
`the desired object.
`Although the present invention has been explained in
`relation to its preferred embodiment, it is to be understood
`that many other possible modifications and variations can be
`made without departing from the spirit and scope of the
`invention as hereinafter claimed.
`What is claimed is:
`
`1. A constant current driver with auto-clamped pre-charge
`function, comprising:
`a reference bias generator having a bias output terminal
`for providing a reference bias; and
`a plurality of constant current driver cells, each being
`connected to the reference bias generator to form a
`respective current mirror, wherein each constant cur-
`rent driver cell comprises:
`a switch transistor controlled by an input terminal for
`being turned on or off;
`a current output transistor connected to the switch tran-
`sistor and the bias output terminal of the reference bias
`generator for outputting a constant current when the
`switch transistor is on; and
`a pre-charge transistor having a gate connected to the gate
`of the current output transistor and further connected to
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`the bias output terminal of the reference bias generator,
`a drain and a source connected to the drain and source
`of the current output transistor, respectively, whereby,
`when a constant current is outputted from the current
`output transistor for driving an organic light emitting
`diode, the pre-charge transistor is turned on due to the
`gate to source voltage thereof being larger than its
`threshold voltage, so as to provide a drain to source
`current as an additional large current for rapidly pre-
`charging the organic light emitting diode until the gate
`to source voltage of the pre-charge transistor is smaller
`than the threshold voltage.
`2. The constant current driver with auto-clamped pre-
`charge function, as claimed in claim 1, further comprising a
`multiplexer connected between the bias output terminal of
`the reference bias generator and the gates of the pre-charge
`transistor and the current output transistor of each of the
`constant current driver cells, the multiplexer having a first
`and a second input terminals connected to the bias output
`terminal of the reference bias generator and ground,
`respectively, and an output terminal connected to the gates
`of the pre-charge transistor and the current output transistor.
`3. The constant current driver with auto-clamped pre-
`charge function as claimed in claim 1, wherein the constant
`current driver cell further comprises a discharge transistor
`connected to the current output transistor for discharging
`when the discharge transistor is turned on.
`4. The constant current driver with auto-clamped pre-
`charge function as claimed in claim 3, wherein the switch
`transistor and the current output transistor