EP0493820B1

EP0493820B1 - Driver circuit for liquid crystal display

Info

Publication number: EP0493820B1
Application number: EP19910122321
Authority: EP
Inventors: Misao Matsuura
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1990-12-29
Filing date: 1991-12-27
Publication date: 1994-10-12
Anticipated expiration: 2011-12-27
Also published as: EP0493820A1; DE69104601T2; DE69104601D1; JPH04242788A

Description

BACKGROUND OF THE INVENTION

(1) Field of the invention:

The present invention relates to a driver circuit for a liquid crystal display (hereinafter referred to as an "LCD") which requires a number of display gradations.

(2) Description of the related art:

Fig. 1 is a block diagram showing a conventional driver circuit for a liquid crystal display. This LCD driver circuit comprises k (k being 2 or a greater integer) shift registers SR₁ ∼ SR_k, k latch circuits LATCH₁ ∼ LATCH_k, k select circuits SELECT₁ ∼ SELECT_k and a transistor switch group 2.
Digital image data input voltages V_in inputted to image data input terminals D_in are supplied to the shift registers SR₁ ∼ SR_k, to which a clock pulse V_CLK is also supplied commonly from a clock pulse input terminal CLK.
The outputs of the shift registers SR₁ ∼ SR_k are supplied to the corresponding latch circuits LATCH₁ ∼ LATCH_k, to which a latch pulse V_STB is also supplied commonly from a latch pulse input terminal STB.
The outputs of the latch circuits LATCH₁ ∼ LATCH_k are supplied to the corresponding select circuits SELECT₁ ∼ SELECT_k, to which a frame selection signal V_FRM is also supplied commonly from a frame selection terminal FRM. Each of the select circuits SELECT₁ ∼ SELECT_k has a plurality of output terminals and provides a switch selection signal at a specific output terminal based on the frame selection signal V_FRM and also the outputs of the corresponding latch circuit LATCH₁ ∼ LATCH_k.
The transistor switch group 2 comprises a plurality of transistors T₁₁ ∼ T_km in a matrix form. More specifically, the switch group 2 comprises k (k being 2 or a greater integer) transistor groups, each group comprising m (m being 2 or a greater integer) transistors. These transistors T₁₁ ∼ T_km are on/off operated according to the switch selection signals outputted from the respective select circuits SELECT₁ ∼ SELECT_k, and thus they selectively provide LCD driving voltage: V_LC1 ∼ V_LCm to the output terminals O₁ ∼ O_k.
The operation of the conventional LCD driver circuit having the above circuit configuration is as follows.
From the image data input terminals D_in are supplied the digital image input voltages V_in of n (n being 2 or a greater integer) bits corresponding to the m gradations. The image data input voltages V_in are transferred to the k shift registers SR₁ ∼ SR_k in synchronism with the clock pulse V_CLK supplied to the clock pulse input terminal CLK.
Data having been transferred to the shift registers SR₁ ∼ SR_k are transferred to the corresponding latch circuits LATCH₁ ∼ LATCH_k in synchronism with the latch pulse V_STB supplied to the latch pulse input terminal STB.
Each of the select circuits SELECT₁ ∼ SELECT_k outputs a switch selection signal according to the frame selection signal V_FRM and the data held in the corresponding latch circuit LATCH₁ ∼ LATCH_k, thus selectively turning on a specific transistor in each of the transistors groups T₁₁ ∼ T_1m, ..., T_k1 ∼ T_km. Thus, one of the voltages V_LC1 ∼ V_LCm is selectively provided to each of the output terminals O₁ ∼ O_k. In this way, the voltages corresponding to the m gradations are supplied to the LCD.
The conventional LCD driver circuit explained above has the following defects. Where there are many gradations for display on the LCD, it is necessary for the conventional driver circuit that a low impedance buffer circuit having output terminals corresponding in number to the number of gradations is formed on a semiconductor chip. Therefore, if the number of gradations is to be increased, the chip size and the cost of manufacture of the conventional LCD driver circuit are inevitably increased.

SUMMARY OF THE INVENTION

It is, therefor, an object of the invention to overcome the defects in the conventional LCD driver circuit and to provide an improved LCD driver circuit which permits driving of an LCD in a large number of gradations and which permits high density integration to be realized readily and with a low cost.
To attain this object of the invention, there is provided a driver circuit for a liquid crystal display, the circuit comprises:
a plurality of shift registers for transferring image input data in accordance with a clock pulse;
a plurality of latch circuits for receiving and holding respectively the outputs from the shift registers in accordance with a latch pulse;
a plurality of select circuits each having a plurality of output terminals, each for outputting a switch selection signal to a specific one of the output terminals based on outputs from each of the latch circuits and a frame selection signal;
a multiplexer for receiving m (m being 2 or a greater integer) kinds of drive source voltages whose levels are different from one another and selectively outputting m/2 kinds of drive source voltages based on the frame selection signal;
a plurality of switch groups each having transistor switches for selecting the outputs of the multiplexer based on each of the switch selection signals outputted from the select circuits; and
a plurality of operational amplifiers each provided between each of the switch groups and the liquid crystal display.
According to the present invention, the multiplexer selectively outputs m/2 kinds of voltages among the m kinds of different LCD drive source voltages according to the frame selection signal, and the switch groups select specific ones of these LCD drive source voltages according to the switch selection signals outputted from the select circuits. Thus, the necessary number of the switches can be reduced to one half as compared to that in the conventional circuit, thus permitting high density integration and cost reduction.
In addition, the amplifiers provided between each of the switch groups and the LCD permit switch size reduction, because currents flowing through the switches do not directly drive the LCD. It is thus possible to permit higher density integration of the LCD driver circuit.
As the switches, MOS transistors, for example, may be used. In this case, if each switch is constructed with a single transistor, its "on" resistance is possibly changed by the voltages supplied from the multiplexer. This change in the "on" resistance of the single transistor switch can be avoided by using as the switch a transfer gate, which may be constructed, for example, with a P- and an N-channel transistor connected in parallel. Accordingly, it is desirable that the switch is a transfer gate constituted by a P-channel translator and an N-channel transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention explained with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram showing a conventional LCD driver circuit;
Fig. 2 is a schematic diagram showing a first embodiment of the LCD driver circuit according to the invention;
Fig. 3 is a timing chart for illustrating the operation of the first embodiment of the LCD driver circuit shown in Fig, 2; and
Fig. 4 is a schematic diagram showing a second embodiment of the LCD driver circuit according to the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

Now, preferred embodiments of the invention will be explained with reference to the accompanying drawings. It should be noted that the elements showing the same or like elements in Fig. 1 are indicated with the same reference numerals or symbols in Figs. 2 to 4.
Fig. 2 is a schematic diagram showing a first embodiment of the LCD driver circuit according to the invention.
This LCD driver circuit comprises a plurality of shift registers SR₁ ∼ SR_k, latch circuits LATCH₁ ∼ LATCH_k. select circuits SELECT₁ ∼ SELECT_k, transistor switches T₁₁ ∼ T_k(m/2), a multiplexer MPX, and a plurality of operational amplifiers OP₁ ∼ OP_k.
In the same manner as in the prior art, the image data input voltages V_in inputted to the image data input terminals D_in are transferred to the k shift registers SR₁ ∼ SR_k in synchronism with the clock pulse V_CLK supplied from the clock pulse input terminal CLK. The latch circuits LATCH₁ ∼ LATCH_k hold the respective output signals of the shift registers SR₁ ∼ SR_k in synchronism with the latch pulse V_STB supplied to the latch pulse input terminal STB. Each of the select circuits SELECT₁ ∼ SELECT_k has a plurality of output terminals and outputs a switch selection signal to a specific one of the output terminals based on the frame selection signal V_FRM supplied from the frame selection terminal FRM and also the outputs of each of the latch circuits LATCH₁ ∼ LATCH_k.
To the multiplexer MPX are supplied m kinds of different LCD drive source voltages V_LC1 ∼ V_LCm. It should be noted that the frame selection signal V_FRM is supplied also to the multiplexer MPX. The multiplexer MPX selects m/2 kinds of LCD drive source voltages among the m kinds of different LCD drive source voltages V_LC1 ∼ V_LCm, necessary for m/2 levels of gradation display of image. The transistor switches T₁₁ ∼ T_k(m/2) constitute k transistor switch groups each group consisting of m/2 transistor switches. These transistor switches are on/off operated by the switch selection signals outputted from the respective select circuits SELECT₁ ∼ SELECT_k.
The operational amplifiers OP₁ ∼ OP_k each forming a voltage follower circuit enhance the drive current capacity of the LCD drive source voltages supplied through the transistor switches T₁₁ ∼ T_k(m/2), and supply to the LCD with these enhanced LCD drive source voltages through the output terminals O₁ ∼ O_k.
Fig. 3 is a timing chart for illustrating the operation of this embodiment of the LCD drive circuit shown in Fig. 2. The frame selection signal V_FRM assumes a high and a low level at a predetermined period or frequency. The latch pulse signal V_STB is generated in synchronism with the rising of the frame selection signal V_FRM.
First, the image data input voltages V_in inputted to the image data input terminals D_in are transferred to the shift registers SR₁ ∼ SR_k in synchronism with the clock pulse V_CLK. The latch circuits LATCH₁ ∼ LATCH_k receive signals outputted from the corresponding shift registers SR₁ ∼ SR_k in synchronism with the latch pulse V_STB and hold these input signals as data for the next horizontal scan period.
The select circuits SELECT₁ ∼ SELECT_k selectively turn on specific transistor switches in the individual transistor switch groups of transistor switches T₁₁ ∼ T_k(m/2) based on the frame selection signal V_FRM and the n-bit data outputted from the latch circuits LATCH₁ ∼ LATCH_k. The multiplexer MPX selectively outputs m/2 kinds of voltages among the m kinds of different drive source voltages V_LCl ∼ V_LCm according to the frame selection signal V_FRM. These voltages are selectively supplied to the operational amplifiers OP₁ ∼ OP_k via the aforementioned specific transistor switches selected by the select circuits SELECT₁ ∼ SELECT_k. The operational amplifiers OP₁ ∼ OP_k provide the output voltages V_O1 ∼ V_Ok according to the input voltages V_S1 ∼ V_Sk.
In the conventional LCD driver circuit without any operational amplifiers, the drive current is small if the dimensions of the transistors constituting the transistor switches are small. To increase the drive current, it has been necessary to increase the dimensions of the switching transistors concerned. In the conventional LCD driver circuit, therefore, with an increase in the number of gradations, the number of transistor switches is correspondingly increased to increase the chip size. To the contrary, in this embodiment, the operational amplifiers OP₁ ∼ OP_k are provided as the voltage follower circuits between the transistor switches T₁₁ ∼ T_k(m/2) and the LCD, and they provide drive voltages having large drive current capacity. Therefore, the transistors of the transistor switches T₁₁ ∼ T_k(m/2) may be of smaller dimensions. Thus, the LCD driver circuit in this embodiment may be of a reduced chip size as compared to that in the prior art.
Further, since the LCD driver circuit of the embodiment incorporates the multiplexer MPX, m gradations may be covered by m/2 transistor switches in each of the groups of transistor switches T₁₁ ∼ T_k(m/2), that is, it is possible to reduce the number of transistors used to one half that in the prior art. It is thus possible to further reduce not only the chip size but also the manufacturing cost.
Fig. 4 is a schematic diagram showing a second embodiment of the LCD driver circuit according to the invention. This embodiment is different from the preceding first embodiment in that a transfer gate consisting of a P- and an N-channel transistor and an inverter is provided in stead of each transistor switch constituted by a single transistor. Since the remainder of the structure is basically the same as in the first embodiment, parts like those in Fig. 2 are designated by like reference symbols, and the explanation therefor is not repeated here.
More specifically, in this second embodiment, a plurality of transfer gates each comprising a P- and an N-channel transistor and an inverter are provided as the switching means.
In the first embodiment, in which each switch is constituted by a single transistor, the "on" resistance of the transistor concerned is changed or influenced by the voltages supplied from the multiplexer MPX due to the back gate voltage dependency of the transistor. To the contrary, in this embodiment, a transfer gate consisting of a P- and an N-channel transistor and an inverter is provided as a transistor switch. Thus, In the LCD driver circuit in this embodiment, the P- and N-channel transistors compensate with each other to avoid the change in the "on" resistance caused by the voltage applied thereto.
As has been described in the foregoing, according to the present invention, since the multiplexer receives m kinds of different LCD drive source voltages and provides m/2 kinds of voltages according to the frame selection signal, only one half the number of switches are necessary as compared to the conventional driver circuit. Thus, the LCD driver circuit according to the invention is well suited to the LCD requiring a number of display gradations and permits high density integration and cost reduction. Further, since according to the invention the amplifiers are provided between the switch groups and the LCD, no large current passes through the switches. It is thus possible to reduce the switch dimension and permit higher density integration.

Claims

A driver circuit for a liquid crystal display having: a plurality of shift registers (SR₁ ∼ SR_k) for transferring image input data (V_in) in accordance with a clock pulse (V_CLK); a plurality of latch circuits (LATCH₁ ∼ LATCH_k) for receiving and holding respectively the outputs from said shift registers in accordance with a latch pulse (V_STB); and a plurality of select circuits (SELECT₁ ∼ SELECT_k) each having a plurality of output terminals, each for outputting a switch selection signal to a specific one of said output terminals based on outputs from each of said latch circuits and a frame selection signal (V_FRM), said driver circuit being characterized by comprising:
a multiplexer (MPX) for receiving m, m being 2 or a greater integer, kinds of drive source voltages (V_LC1 ∼ V_LCm) whose levels are different from one another and selectively outputting m/2 kinds of drive source voltages (V_S1 ∼ V_Sk) based on the frame selection signal;
a plurality of switch groups each having switches (T₁₁ ∼ T_k(m/2)) for selecting the outputs of said multiplexer based on each of said switch selection signals outputted from said select circuits; and
a plurality of amplifiers (OP₁ ∼ OP_k) each provided between each of said switch groups and said liquid crystal display.
The driver circuit for a liquid crystal display according to claim 1, wherein each of said switches (T₁₁ ∼ T_k(m/2)) is a transfer gate having a P- and an N-channel MOS transistor and an inverter connected between gates of said P- and N-channel MOS transistors.
The driver circuit for a liquid crystal display according to claim 1, wherein each of said amplifiers (OP₁ ∼ OP_k) is an operational amplifier whose output terminal and inverting input terminal are directly connected so as to operate as a voltage follower circuit.