Digital to analog converter having integrated level shifter and method for using same to drive display device

This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0034901 filed in the Korean Intellectual Property Office on Apr. 18, 2006, the entire disclosure of which is incorporated herein by reference.

(a) Field of Invention

The present disclosure of invention relates to digital-to-analog converters (DAC) and particularly to those used for receiving digital input signals operating at a first voltage range and outputting corresponding analog signals to display devices such as Liquid Crystal Displays (LCD's) operating across a wider second voltage range.

(b) Description of Related Art

Recently, flat panel displays such as organic light emitting devices (OLED's), plasma display devices (PDP's), liquid crystal display (LCD's), etc., have been actively developed for replacing the conventionally heavy and large cathode ray tube (CRT) as a form of display, particularly in digital display applications.

The typical PDP uses plasma generated light based on electrically driven gas discharge for displaying characters or images. The typical OLED displays characters or images by using field emission of a particular organic material or polymer. In the LCD, an electrical field is applied to a liquid crystal layer interposed between two display substrates, and transmittance of light passing through the liquid crystal layer is controlled by controlling strength of an electrical field between the substrates to thus obtain a desired image. Voltages used in each of these applications varies according to the underlying technology and according to the interface technologies used for interfacing control electronics (i.e., logic) with drive electronics (i.e., output buffers).

Among the above mentioned flat panel displays, the LCD and the OLED for example, typically include a display substrate on which pixel areas are formed to include switching elements (i.e., thin film transistors) and display signal lines. The typical flat panel also includes a gate-lines driver for transmitting binary gate signals to gate lines that are provided crossing with the display signal lines for turning the switching elements of the pixel areas on or off. The typical flat panel also includes a gray voltages generator for generating a plurality of gray level analog voltages to be used for driving pixel areas at different light intensities. The typical flat panel also includes a data driver for selecting voltages corresponding to video data among the gray voltages as data voltages and applying the selected data voltages to the data lines, and a signal controller for controlling them.

The data driver typically includes a plurality of data driving ICs, and each data driving IC (monolithic integrated circuit) includes a shift register, a latch, a digital-to-analog converter (DAC), and an output buffer.

The typical DAC may convert a digital data signal operating at a first voltage range (i.e., 0-5 volts) into an analog data signal operating across a second and different voltage range (usually larger, i.e., 0-10 volts) by using at least two different power supplies for providing respective low and high power voltages, VDD and VDDH for the digital logic and digital drive circuits. Yet a third power supply is typically provided for supplying the analog voltages used in directly driving the display mechanism (e.g., LCD or PDP). In respect to providing interfacing between logic signaling that operates from the first power voltage, VDD and drive signaling that typically operates from the second and higher power voltage, VDDH, such typically requires use of a DC-to-DC converter type of power supply that generates the second higher power supply voltage, VDDH from the first VDD. It additionally calls for level shifting circuits for converting digital signals operating in the first signaling range (i.e., 0-5V) to the second one (i.e., 0-10V). Use of such additional circuits may lead to an increase in area used by the logic and driving circuits and an increase in power consumption.

The present disclosure of invention provides a DAC that has an integrated level shifter and does not require a separate DC/DC power supply converter for generating the higher digital drive voltages.

An exemplary embodiment of such a digital-to-analog converter (DAC) comprises: a decoder unit for receiving an external data signal operating at a first signaling range of voltages; a resistors array for generating or defining a plurality of different gray voltages; and a voltage selecting unit for selecting one of the gray voltages based on an output of the decoder unit, wherein the decoder unit includes a plurality of decoders each having first and second boost circuits for generating an output of boosted voltage level while using a power supply of lower voltage (VDD).

Each decoder may include first to third stages that are connected sequentially.

A first boost circuit may be positioned between the second and third stages for providing interfacing between lower operating voltages of the first and second stages and higher operating voltages of the third stage.

The first boost circuit may include first to third transistors whose control terminals are commonly connected to the power supply voltage, and a capacitor having one end connected to a contact of the first and second transistors and the other end connected to an input terminal of the third transistor, wherein an input terminal of the first transistor can be connected to the power supply voltage and output terminals of the second and third transistors can be connected to each other.

The first and third transistors may be N-type transistors and the second transistor can be a P-type transistor.

The second boost circuit may generate a high voltage output (e.g., VDDH) based on first and second clock signals each having mutually opposite phases, and the second boost circuit may provide the generated high voltage (e.g., VDDH) to the third stage.

The second boost circuit may include first and second transistors whose control terminals are commonly connected to the second clock signal; a third transistor having a control terminal connected to an output terminal of the first transistor, an input terminal connected to the first clock signal, and an output terminal connected to the output terminal of the second transistor; and a capacitor connected between the control terminal and the output terminal of the third transistor.

The data signal may include a plurality of bits, the first to third stages may include at least a pair of switching units that are mutually connected, respectively, and one of the pair of switching units may receive one of the plurality of bits and the other may receive an inversion signal of the one bit.

The switching units may include two different transistors, respectively, and control terminals of the two transistors may be connected to each other, output terminals of the two transistors may be connected to each other, and input terminals of the two transistors may be connected to first and second voltages, respectively.

The first voltage may be a ground voltage or an output voltage of a previous stage, and the second voltage can be the power supply voltage.

The number of switching units belonging to the second stage or the third stage may be equal to the number of first boost circuits, and the number of switching units belonging to the first stage can be smaller than the number of switching units belonging to the second or third stage.

The first stage may include a pair of switching units, the second stage may include two pairs of switching units, and one of the two pairs of switching units of the second stage can be connected to one switching unit of the first stage and the other of the two pairs of switching units of the second stage can be connected to the remaining switching unit of the first stage.

A method for driving a display device in accordance with the disclosure may include converting external digital video data into corresponding analog voltages where the method comprises: inputting the video data; shifting and latching the video data; decoding the shifted and latched video data; and selecting an analog voltage corresponding to the decoded video data and outputting the selected voltage as an output analog voltage to the display device.

A display device in accordance with the disclosure may include a switching unit connected to a first power supply voltage (VDD), a first boost circuit connected to the switching unit, a second boost circuit for receiving a clock signal, and a selecting unit for selecting an output of the second boost circuit or ground for driving analog section switches (SW).

The decoding of the video data may include generating a first output through the switching unit, amplifying the first output by using the first boost circuit, amplifying the clock signal inputted through the second boost circuit, and selecting an output of the second boost circuit or ground for application to analog section switches (SW).

The display device may include a plurality of pixels and data lines connected thereto, and the method for driving the display device may further include applying an analog voltage selected by the analog section switches (SW) to the data lines of the display device.

The accompanying drawings briefly described below illustrate exemplary embodiments and, together to the description, serve to explain various details of the disclosure.

FIG. 1 is a schematic block diagram of a liquid crystal display (LCD) according to one exemplary embodiment.

FIG. 2 is an equivalent circuit diagram for a single pixel of the LCD according to one exemplary embodiment.

FIG. 3A is a schematic block diagram of a data driver of FIG. 1 .

FIG. 3B is a schematic block diagram of a digital-to-analog converter of FIG. 3A .

FIG. 4 is a schematic block diagram of a decoder unit of FIG. 3B .

FIG. 5 is a circuit diagram of one decoder of FIG. 4 .

FIGS. 6A and 6B are circuit diagrams of a boost circuit of FIG. 5 .

FIG. 6C is waveforms of clock signals shown in FIG. 6B .

FIG. 7 is a circuit diagram of a voltage selecting unit of FIG. 3B .

FIG. 8 is a simulation waveform view of outputs and data voltages of the decoder unit.

FIGS. 9A and 9B are flow charts illustrating the processes of a method for driving a display device according to one exemplary embodiment.

In the drawings, the thickness of layers, films, panels, regions, etc., may be exaggerated for clarity rather than being to scale. Like reference numerals generally designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.

Referring to first to FIGS. 1-2 , a flat panel display device according to one exemplary embodiment, for example a liquid crystal display (LCD), will be described in detail.

FIG. 1 is a schematic block diagram of a liquid crystal display (LCD) according to one exemplary embodiment and FIG. 2 is an equivalent circuit diagram for a single pixel area of the LCD.

As shown in FIG. 1 , an LCD structured in accordance with one exemplary embodiment includes a liquid crystal panel assembly (area 300 ), a gate-lines driver circuit 400 coupled to gate lines of the LCD, a data driver circuit 500 coupled to data lines of the LCD, a gray voltages generator 800 connected to the data driver 500 , and a signal controller 600 coupled to the gate-lines driver circuit 400 and to data driver circuit 500 for controlling them.

In terms of an equivalent circuit, the liquid crystal panel assembly 300 includes a plurality of gate signal lines G 1 -G n and a plurality of data signal lines D 1 -D m and a plurality of pixel units PX each connected to a respective crossing pair among the plurality of signal lines G 1 -G n and D 1 -D m and where the pixel units PX are arranged substantially in a matrix form. In terms of the structure as shown in FIG. 2 , the liquid crystal panel assembly 300 includes lower and upper panel layers 100 and 200 and a liquid crystal material layer 3 interposed therebetween.

The gate lines G 1 -G n are used for transferring gate signals (also called scan signals) and the data lines D 1 -D m are used for transferring data signals. The gate lines G 1 -G n extend substantially in a row direction and are substantially parallel to each other, and the data lines D 1 -D m extend substantially in a column direction and are substantially parallel to each other.

Each pixel unit PXij, i.e., a pixel PX connected to the ith (i=1, 2, . . . , n) gate line G i and the jth (j=1, 2, . . . , m) data line D j , includes a corresponding switching element Qij connected to the signal lines G i and D j , and a liquid crystal capacitor Clc and a storage capacitor Cst connected thereto. The storage capacitor Cst can be omitted if desired.

The switching element Qij can be a three-terminal element such as a thin film field effect transistor (TFT) provided on the lower panel layer 100 , which includes a control terminal connected to the gate line G i , an input terminal (first source/drain) connected to the data line D j , and an output terminal (second source/drain) connected to a pixel-electrode portion 191 of the liquid crystal capacitor Clc and to the storage capacitor Cst.

The liquid crystal capacitor Clc has a pixel electrode 191 formed in the lower panel layer 100 and a common electrode 270 formed in the upper panel layer 200 with the liquid crystal material layer 3 between the two electrodes 191 and 270 serving as a dielectric material. The pixel electrode 191 is connected to the switching element Q, and the common electrode 270 is formed on the entire surface of the upper panel 200 and receives a common voltage Vcom. Unlike what is shown in FIG. 2 , the common electrode 270 can alternatively be provided on the lower panel layer 100 , and in this case, at least one of the two electrodes 191 and 270 can be formed in a linear or bar shape for interdigitation with the other.

The storage capacitor Cst serves as an auxiliary charge storage means to supplement the storage capacity of the liquid crystal capacitor Clc and the Cst may be formed as a separate signal line (not shown) provided on the lower panel layer 100 with the pixel electrode 191 overlapping it and having an insulator interposed therebetween, and a predetermined voltage such as the common voltage Vcom (i.e., ground) or the like is applied to the separate signal line. Also, the storage capacitor Cst can be formed as the pixel electrode 191 overlaps the immediately previous gate line by the medium of the insulator.

In order to implement a color display, each pixel unit PX specifically displays one of the primary colors (spatial division) and/or pixel units PX are activated to alternately display different ones of the primary colors over time (temporal division), so that a desired color can be recognized by the spatial and/or temporal sum of the primary colors. The primary colors can be, for example, the three primary colors of red, green, and blue. FIG. 2 shows one example of spatial division in which each pixel unit PX includes a color filter 230 that optically passes one of the primary colors at a region of the upper panel 200 corresponding to the pixel electrode 191 . Different from the color filter 230 as shown in FIG. 2 , the color filter 230 can be formed above or below the pixel electrode 191 of the lower panel 100 .

At least one polarizer (not shown) may be provided for polarizing light and may be attached on an outer surface of the liquid crystal panel assembly 300 .

Referring back to FIG. 1 , in one embodiment, the gray voltages generator 800 generates two sets of gray voltages (or a set of reference gray voltages) related to light transmittance needs of the pixel units PX. In one embodiment, one of the two sets of the gray voltages has positive values and the other has negative values relative to a given reference or common voltage.

The gate-driver 400 is connected to the gate lines G 1 -G n of the liquid crystal panel assembly 300 and applies digitally-switched gate signals including a combination of a gate-on voltages Von and gate-off voltages Voff to the gate lines G 1 -G n.

The data driver 500 is connected to the data lines D i -D m of the liquid crystal panel assembly 300 , selects an analog gray voltage from the gray voltages generator 800 , and applies it as a data signal to the data lines D 1 -D m . In this respect, if the gray voltages generator 800 does not provide all the gray voltages needed for driving the pixel units PX but instead provides only a predetermined number of reference gray voltages, the data driver 500 uses the reference gray voltages to generate additional gray voltages for all gray scales and selects a data signal from the generated spectrum of gray voltages.

The signal controller 600 controls the gate-driver 400 and the data driver 500 .

The drivers 400 , 500 , 600 , and 800 can be directly mounted in the form of at least one IC chip on the liquid crystal panel assembly 300 , or it can be mounted on a flexible printed circuit film (not shown) and attached in the form of a TCP (tape carrier package) on the liquid crystal panel assembly 300 , or it can be mounted on a separate printed circuit board (not shown). Alternatively, the drivers 400 , 500 , 600 , and 800 can be respectively integrated together with the signal lines G 1 -G n and D 1 -D m , the thin film transistors (TFTs), and the switching elements Q, on the liquid crystal panel assembly 300 . As a further alternative, the drivers 400 , 500 , 600 , and 800 can be integrated as a single chip, and in this case, at least one of them or at least one member among them can be positioned outside the single chip.

The operation of the LCD will now be described in further detail.

The signal controller 600 receives input video signals R, G, and B and input control signals for controlling display of the input video signals from an external graphics controller (not shown). The input control signals include, for example, a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a main clock signal MCLK, a data enable signal DE, etc.

The signal controller 600 appropriately processes the input video signals R, G, and B according to operating conditions of the liquid crystal panel assembly 300 based on the input video signals R, G, and B and the input control signals, and responsively generates a gate control signal CONT 1 and a data control signal CONT 2 , etc., and transmits the gate control signal CONT 1 to the gate-driver 400 and transmits the data control signal CONT 2 and the processed video signal DAT to the data driver 500 .

The gate control signal CONT 1 includes a scanning start signal STV for indicating start of scanning, and at least one clock signal for controlling an output period of the gate-on voltage Von. The gate control signal CONT 1 may additionally include an output enable signal OE for limiting duration of the gate-on voltage Von.

The data control signal CONT 2 includes a horizontal synchronization start signal STH for informing of starting of transmission of the video data with respect to one row of pixels PX, a load signal TP for indicating applying of a data signal to the data lines D 1 -D m , and a data clock signal HCLK. The data control signal CONT 2 may additionally include an inversion signal RVS for inverting the polarity of the voltage of a data signal with respect to the common voltage Vcom (which attribute of the data signal is called “polarity of data signal” hereinafter).

The data driver 500 receives the digital video signal DAT with respect to one row of pixels PX according to the data control signal CONT 2 received from the signal controller 600 , selects a gray voltage corresponding to each digital video signal DAT to convert the digital video signal DAT into a corresponding analog data signal, and applies the corresponding analog data signal to a corresponding one of data lines D 1 -D m .

The gate-driver 400 applies the gate-on voltage Von to the gate lines G 1 -G n according to the gate control signal CONT 1 from the signal controller 600 to turn on the switching elements Q connected to the gate lines G 1 -G n . Then, the data signal, which has been applied to the data lines D 1 -D m , is applied to the pixel-electrode of the corresponding pixel units PX through the turned-on switching elements Q.

A difference between voltage of the data signal applied to the pixel electrodes and the common voltage Vcom appears as a charge voltage, namely, a pixel voltage, of the liquid crystal capacitor Clc of each pixel unit PX. Arrangement of liquid crystal molecules is changed according to the size of the pixel voltage, and polarization of light that transmits through the liquid crystal material layer 3 is changed accordingly. The change in the polarization appears as a change in intensity or transmittance of light by the polarizer attached on the liquid crystal panel assembly 300 .

This process is repeatedly performed over a temporal unit of one horizontal period (namely, “1H” which is equivalent to one period of the horizontal synchronization signal Hsync and the data enable signal DE), whereby when the gate-on voltage Von is sequentially applied over a vertical scan period to all the gate lines G 1 -G n to thereby apply the data signals to all the pixel units PX and to thus display an image of one frame.

When one frame is finished, the next frame is started and in one embodiment, a state of the inversion signal RVS applied to the data driver 500 is controlled (“frame inversion”) such that the polarity of data signals applied to each pixel unit PX can be opposite to the polarity in the previous frame so as to thereby twist the liquid crystal molecules in an opposed direction. In this case, even in one frame, the polarity of a data signal flowing through one data line can be changed according to characteristics of the inversion signal RVS (e.g., row inversion or dot inversion), or polarity of a data signal applied to one pixel row can be different (e.g., column inversion or dot inversion).

Drivers of the LCD according to one exemplary embodiment will now be described in detail with reference to FIGS. 3A to 7 .

FIG. 3A is a schematic block diagram of a data driver of FIG. 1 , FIG. 3B is a schematic block diagram of a digital-to-analog converter (DAC) of FIG. 3A , and FIG. 4 is a schematic block diagram of a decoder unit of FIG. 3B . FIG. 5 is a circuit diagram of one decoder of FIG. 4 , FIGS. 6A and 6B are circuit diagrams of boost circuits of FIG. 5 , and FIG. 7 is a circuit diagram of a voltage selecting unit of FIG. 3B .

Hereinbelow, it is assumed that inputted video data DAT has, for example, 6 bits per pixel so as to thereby specify 64 different gray levels per pixel.

The data driver 500 includes at least one data driving IC 540 as shown in FIG. 3A , and the data driving IC 540 includes a shift register 501 , a latch 502 , a digital-analog converter (DAC) 503 , and an analog output buffer 504 .

When the shift register 501 of the data driving IC 540 receives the horizontal synchronization start signal STH, it sequentially shifts inputted video data DAT according to the data clock signal HCLK and transfers them to the latch 502 . In the case where the data driver 500 includes a plurality of data driving ICs 540 , the shift register 501 may shift the entire video data DAT handled by the shift register 501 itself and it may output a shift clock signal SC to a shift register of a neighboring data driving IC.

The latch 502 includes first and second latches (not shown). The first latch sequentially (serially) receives the video data DAT from the shift register 501 and stores them, and the second latch simultaneously receives (in parallel) the video data DAT from the first latch at a rising edge of a load signal TP, stores them, and outputs them to the DAC 503 at a falling edge of the load signal TP.

The DAC 503 converts the digital video data DAT from the latch 502 into corresponding analog data voltages Vdat, and outputs them to the buffer 504 .

The buffer 504 outputs buffered versions the data voltages from the DAC 503 through output terminals Y 1 -Y r . The output terminals Y 1 -Y r are connected to corresponding data lines D 1 -D m .

As shown in FIG. 3B , the DAC 503 includes a decoder unit 510 , a voltage selecting unit 530 , and a resistor array 550 .

The decoder unit 510 is connected to a power supply voltage VDD and a clock signal CLK, and as mentioned above, it receives the digital data signal DAT.

As shown in FIG. 4 , the decoder unit 510 includes a plurality of 2-to-one-of-4 bit decoders 511 , 513 , and 515 .

The decoders 511 , 513 , and 515 receive a lower two bits (DAT 0 and DAT 1 ), a middle two bits (DAT 2 and DAT 3 ), and an upper two bits (DAT 4 and DAT 5 ), respectively, and output a lower 4 bits DECl, a middle 4 bits DECm, and an upper 4 bits DECh, respectively. This operation will now be described in more detail with reference to FIG. 5 .

FIG. 5 shows the structure of the decoder 511 as an example, and the other decoders 513 and 515 have the same structure, so their descriptions will be omitted.

Here, the data signals DAT or the clock signals CLK with a suffix mark “b” or “B” refer to inversion signals. For example, when DAT 1 has a high or low level, DAT 1 b has a corresponding low or high binary level.

With reference to FIG. 5 , the decoder 511 includes first to fourth stages 511 a , 511 b , 511 c , and 511 d including a plurality of N-type transistors N 11 , N 12 , N 21 , N 22 , N 23 , N 24 , N 31 , N 32 , N 33 , and N 34 , a plurality of P-type transistors P 11 , P 12 , P 21 , P 22 , P 23 , P 24 , P 31 , P 32 , P 33 , and P 34 , and boost circuits BST 11 -BST 14 and BST 2 .

The first stage 511 a , the second stage 511 b , and the fourth stage 511 d include a pair of CMOS switching units SWU and SWUb in which control terminals and output terminals (drains) of the N-type transistors and the P-type transistors are correspondingly connected. The first stage 511 a includes a pair of switching units, and the second and fourth stages 511 b and 511 d include two pairs of switching units, respectively.

For example, in the switching unit SWU of the first stage 511 a , control terminals of the N-type transistor N 11 and the P-type transistor P 11 are connected, and output terminals of the two transistors N 11 and P 11 are also connected. In the two switching units SWU and SWUb, the sources of N-type transistors N 11 and N 12 are connected to ground and the sources of P-type transistors P 11 and P 12 are connected to VDD. A first data signal DAT 1 is inputted to the switching unit SWU, its inverted signal DAT 1 b is inputted to the switching unit SWUb, and a ground voltage or a power supply voltage VDD is transferred to the next stage according to steering by the data signal DAT 1 and the inverted signal DAT 1 b.

The third stage 511 c includes a plurality of boost circuits BST 11 , BST 12 , BST 13 , BST 14 , and BST 2 , and it receives a 4-bit signal OUT 1 from the second stage 511 b , amplifies or shifts the high levels above that of the power supply voltage VDD and outputs the voltage-boosted 4-bit signal OUT 2 to the fourth stage 511 d.

As shown in FIGS. 6A and 6B , for example, the boost circuit BST 11 includes a plurality of transistors N 4 , N 5 , and P 4 , and a capacitor C 1 , and the boost circuit BST 2 includes the plurality of transistors N 6 to N 8 and a capacitor C 2 . The other boost circuits BST 12 , BST 13 , and BST 14 have the same structure as BST 11 .

As shown in FIG. 6A , the control terminals (gates) of the three transistors N 4 , N 5 , and P 4 of the first boost circuit BST 11 are commonly connected to the power supply voltage VDD, and an input terminal (first source/drain) of the transistor N 4 is also connected to the power supply voltage VDD. An output terminal (second source/drain) of the transistor N 4 is connected to an input terminal (first source/drain) of the transistor P 4 , and is also connected via capacitor C 1 to an input terminal (first source/drain) of N 5 . Output terminals of the two transistors P 4 and N 5 are connected, and the output OUT 1 from the second stage 511 b is inputted to the input terminal of the transistor N 5 .

The second boost circuit BST 2 operates as a periodic voltage doubler and periodically has an above-Vdd high level on its OUT 2 terminal when CLK is high even though the second boost circuit BST 2 is powered by the power supply voltage VDD, and connected to have a low level, the same as the ground voltage. The second boost circuit BST 2 receives first and second clock signals CLK and CLKB each having mutually opposite phases.

The boost circuit BST 2 includes the transistors N 6 to N 8 , which are all N-type transistors. An input terminal of the transistor N 6 is connected to the power supply voltage VDD, a control terminal thereof is connected to the second clock signal CLKB, and an output terminal thereof is connected to a control terminal of the transistor N 7 . An input terminal of the transistor N 7 is connected to the first clock signal CLK, and an output terminal thereof is connected to an output terminal of the transistor N 8 . A control terminal of the transistor N 8 is connected to the second clock signal CLKB, and an input terminal thereof is connected to a ground voltage. The capacitor C 2 is connected between the control terminal and the output terminal of the transistor N 7 .

The operation of the decoder unit 511 having the structure as described above will now be described and in the case in which is assumed that the data signals DAT 1 and DAT 0 have a low level, respectively ( 00 ). Accordingly, the inversion signals DAT 1 b and DAT 0 b of the data signals DAT 1 and DAT 0 have a high level, respectively.

First, when the data signal DAT 1 having the low level is inputted to the switching unit SWU of the first stage 511 a , the N-type transistor N 11 is turned off and the P-type transistor P 11 is turned on to output the power supply voltage VDD to the next stage 511 b . Meanwhile, when the inversion signal DAT 1 b having the high level is inputted to the switching unit SWUb of the first stage 511 a , the N-type transistor N 12 is turned on and the P-type transistor P 12 is turned off to transfer a ground voltage to the next stage 511 b.

Likewise, in the second stage 511 b , the data signal DAT 0 has the low level and its inversion signal DAT 0 b has the high level, a first switching unit SWU in the second stage 511 b outputs the power supply voltage VDD to the boost circuit BST 11 of the next stage 511 c and a second switching unit SWUb also outputs the power supply voltage VDD, an input of the transistor N 22 , to the boost circuit BST 12 . A third switching unit SWU in the second stage 511 c outputs the power supply voltage VDD to the boost circuit BST 13 and a fourth switching unit SWUb outputs a ground voltage to the boost circuit BST 14 .

The first boost circuit BST 11 as shown in FIG. 6A receives the power supply voltage VDD, namely, the output OUT 1 from the second stage 511 b.

Regarding the first boost circuit BST 11 , the input terminal and the control terminal of the transistor N 4 are commonly connected to the power supply voltage VDD, so that N 4 is serving as a diode-connected transistor and is in the constantly turned-on state when OUT 1 is low so a to charge capacitor C 1 when OUT 1 is low. (N 4 could be replaced with a diode or a diode resistor series circuit for charging capacitor C 1 when OUT 1 is low.) As a result, the voltage developed at the node (a) when OUT 1 is low has a value obtained by subtracting a threshold voltage of the transistor N 4 from the power supply voltage VDD. (Va=Vdd−Vt when OUT 1 is low.) In the case of the transistor P 4 , when OUT 1 is low, the voltage of the input terminal of P 4 i.e., of node (a) is smaller than the power supply voltage VDD connected to the control terminal thereof, and thus the transistor P 4 is in an OFF state when OUT 1 is low.

In the case of the transistor N 5 , a state of the transistor N 5 is determined depending on the input voltage OUT 1 . Namely, when the power supply voltage VDD is inputted to the transistor N 5 , voltage of its control terminal and voltage of its input terminal become equal so the transistor N 5 is turned off, while if the ground voltage is inputted (when OUT 1 is low) to the transistor N 5 , voltage of its control terminal becomes relatively high so the transistor N 5 is turned on thereby coupling the low voltage of OUT 1 to the OUT 2 terminal when OUT 1 is low.

By contrast, when OUT 1 is high (approximately equal to the power supply voltage VDD), the voltage at the node (a) increases to be as high as the sum of the power supply voltage VDD and the voltage stored by the capacitor C 1 . Accordingly, the voltage at the input terminal of P 4 is higher than the voltage of its control terminal, so the transistor P 4 is turned on to couple the above-Vdd voltage on node (a) to the OUT 2 terminal at the time that OUT 1 is high. As stated above, transistor N 5 is turned off at this time. As a result, a voltage OUT 2 , substantially double the power supply voltage VDD, is outputted at the time that OUT 1 is high. To sum up, when the ground voltage is inputted at the OUT 1 terminal, the ground voltage is outputted at the OUT 2 terminal. When the power supply voltage VDD is inputted at the OUT 1 terminal, a voltage corresponding to substantially double the power supply voltage VDD is outputted at the OUT 2 terminal.

As shown in FIG. 6C , the second boost circuit BST 2 receives the first clock signal CLK having the same magnitude as the power supply voltage VDD. In one embodiment, the first clock signal CLK has a duty ratio of about 75% or greater, and the second clock signal CLKB having the same magnitude as the power supply voltage VDD and a duty ratio of about 25% or smaller.

When the first clock signal CLK has a low level and the second clock signal CLKB has a high level, the transistors N 6 and N 8 are turned on to transfer the input voltage VDD to the node (b), namely, to one end of the capacitor C 2 , and the ground voltage to the other end of the capacitor C 2 , so the power supply voltage VDD is applied across the capacitor C 2 and node (b) of capacitor C 2 charges to the VDD level.

Subsequently, when the first clock signal CLK is changed to a high level and the second clock signal CLKB is changed to a low level, the two transistors N 6 and N 8 are turned off. Accordingly, the upper end of the capacitor C 2 (node (b)) is cut off from the input voltage VDD (because N 6 is nonconductive, having been turned off), and can thus float to a higher voltage state, and the lower end of the capacitor C 2 receives the voltage (VDD) of the high first clock signal CLK, so the voltage at one end of the capacitor C 2 , namely, the voltage at the node (b), increases up to a value obtained by adding the voltage level of the high first clock signal CLK to the previous power supply voltage VDD. That is, as in the boost circuit BST 11 , the voltage corresponding to about double the power supply voltage VDD is outputted at the OUT 3 terminal.

At this time, according to the outputs OUT 2 from the boost circuits BST 11 , BST 12 , BST 13 , and BST 14 , one of the N-type transistors N 31 , N 32 , N 33 , and N 34 and one of the P-type transistors P 31 , P 32 , P 33 , and P 34 belonging to the switching units SWU and SWUb are selected, based on which one of the ground voltage and an output OUT 3 from the boost circuit BST 2 is selected so as to be finally generated as outputs DEC 0 , DEC 1 , DEC 2 , and DEC 3 of the decoder 511 .

For example, when the output OUT 2 of the boost circuit BST 11 has a high voltage level, the transistor N 31 is turned on and the ground voltage is generated as the output DEC 3 of the decoder 511 , and when the output OUT 2 of the boost circuit BST 11 has a low voltage level, the transistor P 31 is turned on and a high voltage is generated as the output DEC 3 of the decoder 511 . To sum up, the decoder 511 simultaneously operates to change the coding format from the multi-bit format of the DTA signal to a one-of-N format (i.e., 1-of-4) and to increase the operating voltage range of each of the 1-of-N bits.

Outputs of the decoders 511 , 513 , and 515 generated in this manner are inputted to the voltage selecting unit 530 .

With reference to FIG. 7 , a plurality of predefined gray voltages, namely, 64 gray voltages V 1 ˜V 64 , whose values may be established by using the resistor array 550 are shown at the left portion, and three switching elements SW are disposed for each row of the gray voltages V 1 ˜V 64 . That is, the switching elements SW are disposed in a matrix form of 64 rows and 3 columns overall. In one embodiment, switching elements SW may include MOS pass transistors. In another embodiment, switching elements SW may include CMOS transmission gates for passing analog voltages of one or more polarities therethrough.

The more significant or upper decoder outputs DEC 0 h , DEC 1 h , DEC 2 h , and DEC 3 h are divided into four groups and 16 switching elements corresponding to one of the four groups are selected. Then for the middle significance decoder outputs DEC 0 m , DEC 1 m , DEC 2 m , and DEC 3 m the 16 selected switching elements SW are divided into four groups and four switching elements SW corresponding to one of the group groups are selected. And finally for lower or least significant decoder outputs DEC 01 , DEC 11 , DEC 21 , and DEC 31 , one of the four selected switching elements SW is selected. (Note that 4×4×4=64.)

A single gray voltage may be selected in this manner to define the analog data voltage Vdat, which is applied to the data lines D 1 -D m . FIG. 8 shows waveforms that may be obtained by simulating the process of selecting the decoder outputs DEC 0 h , DEC 1 h , DEC 2 h , DEC 3 h , DEC 0 m , DEC 1 m , DEC 2 m , DEC 3 m , DEC 01 , DEC 11 , DEC 21 , and DEC 31 , and the data voltage Vdat under the conditions that the levels of the power supply voltage VDD and the clock signals is around 5V.

When a threshold voltage of the two transistors N 6 and N 8 are 0.7V, respectively, voltage generated from the boost circuit BST 2 is theoretically 8.6V which is obtained by subtracting 1.4V, namely, the threshold voltage of the two transistors N 6 and N 8 , from 10V, which is obtained by adding the power supply voltage VDD and the voltage of the high state clock signal CLK. However, when the simulation was performed in consideration of parasitic capacitance existing among the transistors N 6 to N 8 , the 8.6V theoretical value was not obtained and instead an output value of about 7V of decoder outputs DEC 0 h , DEC 1 h , DEC 2 h , DEC 3 h , DEC 0 m , DEC 1 m , DEC 2 m , DEC 3 m , DEC 01 , DEC 11 , DEC 21 , and DEC 31 was obtained. That is, the result was obtained by taking into consideration that the voltages of both ends of the capacitor C 2 were not 4.3V but were about 3.3V, which is lower by 1V than the 4.3V due to the parasitic capacitance, when the second clock signal CLKB has the high level.

However, that level of voltage, i.e., 3.3V, is sufficient to turn on the switching elements SW of FIG. 7 which are shown connected in series in the voltage selecting unit 530 to output the gray voltages V 1 ˜V 64 as the data voltage Vdat.

The operation of the data driver 500 including the digital converter will now be described with reference to FIGS. 9A and 9B .

First, when video data DAT operating in the binary range of about 0v-5V is inputted from the signal controller 600 to the data driver 500 (S 901 ), it passes through the shift register 501 and the latch 502 (S 903 ), and is then inputted to the DAC 503 . The inputted data DAT is decoded in the decoder unit 510 (S 905 ), selected as an analog voltage in the voltage selecting unit 530 (S 907 ), and is then applied as the data voltage Vdat to the data lines D 1 -D m (S 909 ).

In detail, in the decoding step S 905 , the first output is generated through the switching unit SWU connected with the power supply voltage (S 905 a ), and then the switching voltage is amplified by using the first boost circuits BST 11 , BST 12 , BST 13 , and BST 14 (S 905 b ). Also, the clock signals CLK and CLKB inputted through the second boost circuit BST 2 and are amplified (S 905 c ), and then the output of the first boost circuits BST 11 , BST 12 , BST 13 , and BST 14 or the output of the second boost circuit BST 2 is selected (S 905 d ).

Accordingly, the data driver 500 of the display device according to the exemplary embodiment can obtain the desired analog data voltage Vdat of high magnitude by using the boost circuits BST 11 , BST 12 , BST 13 , BST 14 , and BST 2 that use only the one power supply voltage VDD and yet produce digital selection signals of higher magnitude. Therefore, compared with the case where a DC/DC converter is used to form a second power supply, not only can an area for mounting the driving circuits be reduced, but power consumption can be considerably reduced since there is no need for a separate DC/DC converter.

It is to be understood that the present disclosure of invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the disclosure.