Display device and driving circuit for display device

A driving circuit for a display device for selectively outputting a driving voltage positive or negative with respect to a given reference voltage of the display device in accordance with an image signal, includes an input stage circuit; and an output stage circuit for outputting a driving voltage between a given high voltage and a first intermediate voltage or a driving voltage between a second intermediate voltage and a given low voltage in accordance with a pair of output stage control signals output from the input stage circuit.

BACKGROUND OF THE INVENTION

The present invention relates to a display device such as a liquid crystal display device and a driving circuit for driving the display device.

In a display device such as a liquid crystal display device, AC driving in which driving voltages positive and negative with respect to a potential of a counter electrode of a display panel are applied to a capacitive load is generally performed. A known example of a driving circuit used for generating such driving voltages is shown in FIG. 1 of Japanese Laid-Open Patent Publication No. 2002-175052.

As shown in this drawing, the driving circuit includes output transistors 11 and 12 connected in series between a high-side power source 8 (VDD) and an intermediate-side power source 10 (VDD/2). It also includes output transistors 13 and 14 connected in series between the intermediate-side power source 10 (VDD/2) and a low-side power source 9 (VSS).

The output transistors 11 and 12 and the output transistors 13 and 14 are respectively controlled by differential input stage circuits 2 and 3 switched by switching elements 6 and 7 so as to alternately apply positive and negative voltages to a capacitive load.

Thus, the capacitive load is charged/discharged by the intermediate-side power source 10 no matter whether the driving voltage is switched to be positive or negative, and thus, power consumption is reduced.

In the case where the differential input stage circuits 2 and 3 are switched by the switching elements 6 and 7 in the aforementioned manner, however, the accuracy in the driving voltages tends to be lowered and the number of necessary switching circuits is disadvantageously large.

SUMMARY OF THE INVENTION

The present invention was devised in consideration of the aforementioned disadvantages, and an object of the invention is reducing power consumed in AC driving a display device while preventing the accuracy lowering caused in switching the output of a differential input stage circuit.

In order to achieve the object, in one aspect of the invention, the driving circuit for a display device for selectively outputting a driving voltage positive or negative with respect to a given reference voltage of the display device in accordance with an image signal, includes an input stage circuit; and an output stage circuit for outputting a driving voltage between a given high voltage and a first intermediate voltage or a driving voltage between a second intermediate voltage and a given low voltage in accordance with a pair of output stage control signals output from the input stage circuit.

The output stage circuit may include a high-voltage-side transistor and a low-voltage-side transistor connected in series to each other; a high-voltage-side voltage supplying circuit for selectively supplying the high voltage or the first intermediate voltage to the high-voltage-side transistor; and a low-voltage-side voltage supplying circuit for selectively supplying the second intermediate voltage or the low voltage to the low-voltage-side transistor.

Alternatively, the output stage circuit may include first and second transistors connected in series to each other between the high voltage and the first intermediate voltage; third and fourth transistors connected in series to each other between the second intermediate voltage and the low voltage; and an output selecting switch circuit for selectively outputting, as the driving voltage, a voltage on a node between the first and second transistors or a voltage on a node between the third and fourth transistors.

Thus, power voltages to be supplied to the high-voltage-side transistor and the low-voltage-side transistor or voltages to be supplied to the first and second transistors and the third and fourth transistors can be suppressed to be low, and hence, the power consumption can be easily suppressed to be small. Furthermore, accuracy of the driving voltage can be easily kept high.

Alternatively, the input stage circuit may include a P-channel transistor and an N-channel transistor connected in parallel to each other and connected, at both ends thereof, respectively to control terminals of the high-voltage-side transistor and the low-voltage-side transistor; and a bias applying circuit for applying, to control terminals of the P-channel transistor and the N-channel transistor, given bias voltages corresponding to selection of voltages supplied to the high-voltage-side transistor and the low-voltage-side transistor of the output stage circuit, or may include a P-channel transistor and an N-channel transistor connected in parallel to each other and connected, at both ends thereof, respectively to control terminals of the first and third transistors and the second and fourth transistors; and a bias applying circuit for applying, to control terminals of the P-channel transistor and the N-channel transistor, given bias voltages corresponding to selection of the voltages on the nodes of the output stage circuit.

Thus, the transient response characteristic of the driving signal can be easily improved.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention will now be described with reference to the accompanying drawings. It is noted that like reference numerals are used to refer to like elements in the respective embodiments so as to omit the description.

FIG. 1is a circuit diagram for showing the architecture of a principal part of a display device driving circuit100according to Embodiment 1 of the invention used for driving a display device such as a liquid crystal display panel. As shown inFIG. 1, the display device driving circuit100includes an input stage circuit101and an output stage circuit104.

The input stage circuit101includes a differential stage circuit102and a cascode stage circuit103.

The differential stage circuit102outputs a signal in accordance with a difference between an image signal IN+ and a driving signal IN− (or OUT) output from the display device driving circuit100.

The cascode stage circuit103includes transistors13,14,15,16,17,18,19,20,21,22,23and24and, under application of bias voltages BN1, BN2, BN3, BP1, BP2and BP3, outputs a pair of output stage control signals used for controlling the output stage circuit104in accordance with the output signal output by the differential stage circuit102.

Also, the output stage circuit104includes transistors11and12serially connected to each other so as to output, as the driving signal OUT, a voltage obtained on the node between these transistors. Thus, a liquid crystal capacitance CL formed between a pixel electrode and a common electrode30is charged/discharged by a charge/discharge current I through, for example, a source line of a liquid crystal display panel. The charge/discharge current I is controlled to be a constant current by, for example, the cascode stage circuit103.

The output stage circuit104further includes switches3,4,5and6respectively controlled in accordance with switch signals SW3, SW4, SW5and SW6, so that a given high voltage Vdd or a first intermediate voltage Vmh can be selectively applied to the transistor11and that a given low voltage Vss or a second intermediate voltage Vml can be selectively applied to the transistor12. At this point, the first and second intermediate voltages Vmh and Vml are set to have absolute values smaller than a voltage of the driving signal OUT at a black level and are preferably set to be as high as possible within this range for reducing the power consumption.

The operation of the display device driving circuit100having the aforementioned architecture will now be described. In the following description, it is assumed for simplification that the high voltage Vdd and the low voltage Vss are given voltages respectively equal to positive and negative white level voltages, that a common electrode voltage Vcom and the first and second intermediate voltages Vmh and Vml are given black level voltages equal to one another and are equal to an average voltage of the high voltage Vdd and the low voltage Vss. It is noted that the positive and negative voltages herein mentioned do not mean absolute potentials being positive and negative but means a relative relationship with a given reference voltage such as the common electrode voltage Vcom.

In the case where a driving signal OUT with positive polarity is output, the switches3and6are placed in an on state and the transistors11and12are operated at voltages between the high voltage Vdd and the common electrode voltage Vcom. At this point, when it is assumed, for example, that the transistor11has low resistance and the transistor12has high resistance in accordance with an image signal, the driving signal OUT corresponds to the high voltage Vdd, that is, a positive white level voltage, as shown inFIG. 2.

Next, when it is assumed, for example, that the transistor11attains high resistance and the transistor12attains low resistance in accordance with an image signal, charge stored in the liquid crystal capacitance CL is discharged by the charge/discharge current I passing through the transistor12, and as a result, the driving signal OUT is linearly lowered to the common electrode voltage Vcom, that is, a black level voltage. Power consumed at this point corresponds to a product of the charge/discharge current I and a difference between the driving signal OUT and the common electrode voltage Vcom. In other words, as compared with the case where the source of the transistor12is connected to the low voltage Vss, the consumed power is reduced. Such reduction of the power consumption is attained similarly in the case where the level of the driving signal OUT is changed in half tone as far as the liquid crystal capacitance CL is discharged.

On the other hand, in the case where a driving signal OUT with negative polarity is output, the switches4and5are placed in an on state, and hence, the transistors11and12are operated at voltages between the common electrode voltage Vcom and the low voltage Vss. In this case, when the driving signal is changed between, for example, the low voltage Vss corresponding to a negative white level voltage and the common electrode voltage Vcom corresponding to a black level voltage as shown inFIG. 3, the consumed power corresponds to a product of the charge/discharge current I and a difference between the common electrode voltage Vcom and the driving signal OUT. Thus, as compared with the case where the source of the transistor11is connected to the high voltage Vdd, the power consumption is also reduced.

As described above, when the power voltages to be supplied to the transistors11and12are switched in accordance with the polarity of a driving signal so as to suppress the voltages on the ends of the transistors11and12to be low, the power consumption can be easily suppressed to be small. Furthermore, transistors used in the switches3through6used for switching the high voltage Vdd, the low voltage Vss and the intermediate voltages Vmh and Vml can be easily made to have comparatively low impedance, and hence, the area of these transistors can be easily made small. Moreover, since there is no need to switch the output of the differential stage circuit102in changing the polarity of the driving signal, accuracy lowering otherwise caused by such switching can be easily suppressed.

FIG. 4is a circuit diagram for showing the architecture of a principal part of a display device driving circuit200according to Embodiment 2 of the invention. The display device driving circuit200includes a bias applying circuit205in addition to the composing elements of the display device driving circuit100of Embodiment 1. The bias applying circuit205includes switches7,8,9and10respectively controlled in accordance with switch signals SW7, SW8, SW9and SW10, so as to switch the bias voltages BN3and BP3to be applied in the cascode stage circuit103in accordance with the polarity of a driving signal OUT.

Specifically, in the case where the switches3and6of the output stage circuit104are placed in an on state so as to output a driving signal OUT with positive polarity, the switches7and9of the bias applying circuit205are placed in an on state so as to apply bias voltages BPH and BNH respectively to gates (control terminals) of the transistors (the P-channel transistor and the N-channel transistor)14and20of the cascode stage circuit103. On the other hand, in the case where a driving signal OUT with negative polarity is output, the switches8and10are placed in an on state so as to apply bias voltages BPL and BNL respectively to the gates of the transistors14and20.

Since the bias voltages to be applied to the transistors14and20are switched in accordance with the polarity of the driving voltage OUT in the aforementioned manner, when these bias voltages are set to attain relationships of, for example, BNH>BNL and BPH>BPL, the resistance characteristics of the transistors14and20can be made uniform, and therefore, the transient response characteristic of the driving signal OUT can be easily improved.

FIG. 5is a circuit diagram for showing the architecture of a principal part of a display device driving circuit300according to Embodiment 3 of the invention. The display device driving circuit300is different from the display device driving circuit100of Embodiment 1 in including an output stage circuit304instead of the output stage circuit104.

The output stage circuit304includes output circuits304aand304band switches1and2respectively controlled in accordance with switch signals SW1and SW2.

The output circuit304aincludes transistors31and32connected in series to each other and is driven by a high voltage Vdd and a first intermediate voltage Vml. On the other hand, the output circuit304bincludes transistors41and42and is driven by a second intermediate voltage Vmh and a low voltage Vss. In other words, the output circuit304ais placed in a state substantially the same as that of the output stage circuit104attained when the switches3and6are in an on state and the output circuit304bis placed in a state substantially the same as that attained when the switches4and5are in an on state.

The switch1is turned on when a driving signal OUT with positive polarity is output and the switch2is turned on when a driving signal OUT with negative polarity is output. Each of these switches1and2may be a path gate including P- and N-channel transistors connected in parallel and preferably has resistance as low as possible when these P- and N-channel transistors are in on state.

Also in this architecture, power consumed when the driving signal OUT with positive polarity is output corresponds to a product of a charge/discharge current I and a difference between the high voltage Vdd and a common electrode voltage Vcom, and power consumed when the driving signal OUT with negative polarity is output corresponds to a product of the charge/discharge current I and a difference between the common electrode voltage Vcom and the low voltage Vss. Accordingly, the power consumption can be also easily suppressed to be small without causing the accuracy lowering otherwise caused in switching the output of the differential stage circuit102.

Also in the architecture of Embodiment 3, the bias applying circuit205described in Embodiment 2 may be provided so as to apply an appropriate bias to each transistor included in the cascode stage circuit103in accordance with the polarity of the driving signal OUT.

Each display device driving circuit described in Embodiments 1 through 3 may be used in, for example, a liquid crystal display panel400shown inFIG. 6. The liquid crystal display panel400includes a liquid crystal display part401, a source driver411, a gate driver412and a plurality of source lines421and gate lines422respectively corresponding to the number of pixels. The source driver411includes a plurality of display device driving circuits100or the like of any of the embodiments described above, so that each driving signal OUT can be supplied through a corresponding source line421to a pixel electrode not shown of a pixel selected by a gate line422.

As described so far, according to the present invention, the power consumed in AC driving a display device can be reduced and the accuracy of the driving voltage can be easily kept high.