JPH04242788A - Liquid crystal driving circuit - Google Patents

Abstract

PURPOSE:To provided a liquid crystal driving circuit corresponding to multiple gradation display and capable of high integration and reduction in cost. CONSTITUTION:A multiplexer MX selects m/2 kinds of liquid crystal driving voltages required for m/2 gradation display from (m) kinds of liquid crystal driving voltages on the basis of a frame selection signal inputted from a frame selecting terminal FRM. Thus, the number of transistor switch can be reduced to 1/2 of the past. Operation amplifier groups OP1-OPk supply voltages having large driving current capacities to a liquid crystal display on the basis of the liquid crystal driving voltages supplied through transistor switch groups T11-Tk(m/2). As it is not necessary to send a large current to the transistor switch, the size of the transistor can be contracted.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal drive circuit for driving a liquid crystal display (hereinafter referred to as an LCD) that requires gradation display.

0002

2. Description of the Related Art FIG. 4 is a block diagram showing a conventional liquid crystal driving circuit. This liquid crystal drive circuit includes a shift register group SR1 to SRk of k stages (k is an integer of 2 or more), a latch circuit group LATCH1 to LATCHk, a select circuit group SELECT1 to SELECTk, and a transistor switch group 2.

Digital image data input voltage Vin input from image data input terminal Din is input to shift register groups SR1 to SRk. A clock pulse VCLK is applied to the shift register group SR1 to SRk from a clock input terminal CLK.

The outputs of the shift register groups SR1 to SRk are input to the latch circuit groups LATCH1 to LATCHk. This latch circuit group LATCH1 ~ LAT
A latch pulse VSTB is applied to CHk from a latch input terminal STB.

[0005] Latch circuit group LATCH1 to LATCH
The output of k is the select circuit group SELECT1 to SE
It is input to LECTk. This select circuit group SEL
ECT1 to SELECTk have a plurality of output terminals, and receive the frame selection signal input from the frame selection terminal FRM and the latch circuit groups LATCH1 to LATCH.
A switch selection signal is output to a specific output terminal based on the output of k.

The transistor switch group 2 is composed of a plurality of transistors T11 to Tkm. That is,
This transistor switch group 2 consists of m (m is an integer of 2 or more) transistors (k is an integer of 2 or more).
It consists of a set of transistors. These transistors T11 to Tkm are connected to the select circuit group SELEC.
The LCD drive voltages VLC1 to VLCm are selectively outputted to the output terminals O1 to Ok by performing an on-off operation based on the switch selection signals outputted from T1 to SELECTk.

Next, the operation of the conventional liquid crystal drive circuit configured as described above will be explained.

An n (n is an integer of 2 or more) bit digital image data input voltage Vin corresponding to m gradations is input from the image data input terminal Din. This image data input voltage Vin is transferred to the k-stage shift register group SR1 to SRk in synchronization with the clock pulse VCLK applied to the clock input terminal CLK.

This shift register group SR1 to SRk
The data transferred to the latch circuit group LATCH1 to LATCHk is synchronized with the latch pulse VSTB applied to the latch input terminal STB.

[0010] Select circuit group SELECT1 to SEL
ECTk is the frame selection signal VFRM input from the frame selection terminal FRM and the latch circuit group LATCH1.
A switch selection signal is output based on the data value held in ~LATCHk, and each transistor group T11~T
For each set, only one specific transistor among 1m to Tk1 to Tkm is selectively turned on. As a result, the voltages VLC1 to Ok are applied to the output terminals O1 to Ok, respectively.
One type of voltage among VLCm is selectively output, and a voltage corresponding to m/2 gradations is supplied to the LCD.

[0011]

However, the conventional liquid crystal drive circuit has the following problems. That is, when the number of gray scales to be displayed on the LCD is large, it is necessary to form a buffer circuit with low impedance and multiple outputs corresponding to the number of gray scales on the semiconductor chip. For this reason, in conventional liquid crystal drive circuits, when attempting to obtain a liquid crystal drive circuit for multi-gradation display, the chip size inevitably increases and the manufacturing cost also increases.

The present invention has been made in view of these problems, and provides a liquid crystal drive circuit that can drive an LCD with multiple gradations, is easily highly integrated, and can be realized at low cost. The purpose is to

[0013]

[Means for Solving the Problems] A liquid crystal drive circuit according to the present invention includes a shift register group that transfers image input data based on a clock pulse, and a shift register group that transfers image input data based on a clock pulse, and a shift register group that transfers image input data based on a clock pulse. It includes a latch circuit group that inputs and holds the output signal of the shift register group based on a latch pulse, and a plurality of output terminals, and outputs a switch selection signal to a specific output terminal based on the output of the latch circuit group. a select circuit group, and a multiplexer that inputs m types of liquid crystal drive voltages (m is an integer of 2 or more) having mutually different voltage values and selectively outputs m/2 types of liquid crystal drive voltages based on a frame selection signal. , a switch group comprising a plurality of switches and selecting the output of the multiplexer based on a switch selection signal output from the select circuit group, and an amplifier interposed between the switch group and the liquid crystal display. It is characterized by having.

[0014]

In the present invention, the multiplexer selects m/2 types of voltages from m types of liquid crystal driving voltages based on a frame selection signal, and selectively outputs the m/2 types of voltages. Then, the switch group selects a specific voltage from these liquid crystal drive voltages based on the switch selection signal output from the selection circuit group. As a result, the number of switches constituting the switch group can be reduced to 1/2 of the conventional number, and high integration and cost reduction can be achieved.

Further, in the present invention, the switch group and L
An amplifier is interposed between the CD and the CD. That is, in the present invention, the current flowing through the switches constituting the switch group does not directly drive the LCD, so the size of the switches can be reduced. This allows the liquid crystal drive circuit to be more highly integrated.

[0016] As the switch, for example, a MOS type transistor can be used. In this case, if one switch is composed of one transistor, the on-resistance of the switch will change depending on the voltage supplied from the multiplexer. However, if a transfer gate composed of, for example, a P-channel transistor and an N-channel transistor is used as the switch, changes in the on-resistance of the switch can be avoided. For this reason, it is preferable to use a transfer gate constituted by a P-channel transistor and an N-channel transistor as the switch.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the implementation of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a liquid crystal drive circuit according to a first embodiment of the present invention.

The liquid crystal drive circuit according to this embodiment includes shift register groups SR1 to SRk and latch circuit groups LATCH.
1 ~ LATCHk, select circuit group SELECT1
~SELECTk, transistor switch group T11
~Tk (m/2), multiplexer MX, and operational amplifier groups OP1 ~OPk.

Select circuit group SELECT1 to SEL
ECTk is connected to the image data input terminal Din as before.
The image data input voltage Vin input from the clock pulse VCLK input from the clock input terminal CLK
It is designed to transfer to stage k in synchronization with . The latch circuit groups LATCH1 to LATCHk are configured to hold the output signals of each stage of the shift register groups SR1 to SRk in synchronization with the latch pulse VSTB input to the latch input terminal STB. Further, the select circuit groups SELECT1 to SELECTk are provided with a plurality of output terminals, and output the frame selection signal inputted from the frame selection terminal FRM and the latch circuit group LATCH1.
Based on the output of ~LATCHk, a switch selection signal is output to a specific output terminal among the output terminals.

The multiplexer MX has m types of LCD driving power supply voltages VLC1 to VLCm having mutually different voltage values.
is given. Then, the multiplexer MX selects the m types of LCDs based on the frame selection signal VFRM.
m/2 of the image from the drive power supply voltage VLC1 to VLCm
Select m/2 types of LCD drive power supply voltages necessary for level gradation display. In addition, the transistor switch group T11
~Tk (m/2) is k consisting of m/2 transistors
These transistors are connected to select circuit groups SELECT1 to SELE.
It is turned on and off by the switch selection signal output from CTk.

The operational amplifier groups OP1 to OPk increase the drive current capacity of the LCD drive power supply voltage supplied through the transistor switch groups T11 to Tk (m/2),
This LCD driving power supply voltage is supplied to the LCD via output terminals O1 to Ok.

FIG. 2 is a timing chart showing the operation of the liquid crystal drive circuit according to this embodiment. Note that the frame selection signal VFRM becomes high level and low level at a predetermined period. Furthermore, the latch pulse VSTB is a signal generated in synchronization with the rise of the frame selection signal VFRM.

First, the image data input voltage Vin input from the image data input terminal Din is clock pulse V.
In synchronization with CLK, shift register groups SR1 to SR
transferred to k. Latch circuit group LATCH1 ~ LA
TCHk receives the signals output from the shift register groups SR1 to SRk in synchronization with the latch pulse VSTB, and holds this input signal as data for the next horizontal period.

Select circuit group SELECT1 to SEL
ECTk is a frame selection signal and latch circuit group LAT
Based on n bits of data output from CH1 to LATCHk, transistor switch groups T11 to T
A specific transistor among each set of k(m/2) transistors is selectively turned on. On the other hand, the multiplexer MX, based on the frame selection signal VFRM,
m types of drive power supply voltages VLC1 to VLCm to m/
Selectively outputs two types of voltage. These voltages are selectively input to the operational amplifier groups OP1 to OPk via the specific transistors selected by the select circuit groups SELECT1 to SELECTk. The operational amplifier group OP1 to OPk is connected to this input voltage VS1 to V
Based on Sk, output voltage VO1 with large drive current capacity
~Output VOk.

In a conventional liquid crystal drive circuit that is not provided with an operational amplifier, the smaller the dimensions of the transistors constituting the transistor switch group, the smaller the drive current becomes. Therefore, in order to increase the drive current, it was necessary to increase the dimensions of the transistor. Therefore, in conventional liquid crystal drive circuits,
When the number of gradations is large, the number of transistor switches increases correspondingly to the number of gradations, resulting in an increase in chip size. However, in this embodiment, an operational amplifier group OP1 to OPk is provided between the transistor switch group T11 to Tk (m/2) and the LCD, and a drive voltage with a large drive current capacity is selected from the operational amplifier group OP1 to OPk. Therefore, the transistors of the transistor switch group T11 to Tk (m/2) need only have small dimensions. Therefore, the liquid crystal drive circuit according to this embodiment can have a smaller chip size than the conventional one.

Furthermore, in this embodiment, since the multiplexer MX is provided, the number of transistor switches constituting the transistor switch group T11 to Tk (m/2) is m/2 in the case of m/2 gradation. The number of transistors can be reduced to 1/2 of the conventional number. Therefore, the chip size can be further reduced and costs can be reduced.

FIG. 3 is a block diagram showing a liquid crystal driving circuit according to a second embodiment of the present invention. This embodiment differs from the first embodiment in that a transfer gate consisting of a P-channel transistor, an N-channel transistor, and an inverter is provided instead of a transistor switch consisting of one transistor, and the other configurations are as follows. Since it is basically the same as the first embodiment, the same components in FIG. 3 as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted.

In this embodiment, the switch group includes:
Transfer gate group 1 composed of P-channel transistors, N-channel transistors, and inverters
is provided.

In the first embodiment in which the switch is composed of one transistor, the on-resistance of the transistor varies depending on the voltage supplied from the multiplexer MX due to the back gate voltage dependence of the transistor. On the other hand, in this embodiment, a transfer gate constituted by a P-channel transistor, an N-channel transistor, and an inverter is provided as a transistor switch. Therefore, in the liquid crystal drive circuit of this embodiment, since the P-channel transistor and the N-channel transistor compensate each other, it is possible to avoid changes in on-resistance due to voltage.

[0031]

As explained above, according to the present invention, the multiplexer inputs m types of liquid crystal drive voltages and outputs m/2 types of voltages based on the frame selection signal.
The number of switches constituting the switch group can be reduced to 1/2 of the conventional number. Therefore, the liquid crystal drive circuit according to the present invention can easily correspond to a multi-gradation display LCD, and
High integration and cost reduction can be achieved. Further, according to the present invention, since the amplifier is provided between the switch group and the LCD, a large current does not flow through the switches forming the switch group. Therefore, the dimensions of the switch can be made smaller than in the past, and even higher integration can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a liquid crystal drive circuit according to a first embodiment of the present invention.

FIG. 2 is a timing chart showing the operation of the liquid crystal drive circuit according to the first embodiment of the present invention.

FIG. 3 is a block diagram showing a liquid crystal drive circuit according to a second embodiment of the present invention.

FIG. 4 is a block diagram showing a conventional liquid crystal drive circuit.

[Explanation of symbols]

CLK; clock input terminal Din; image data input terminal STB; latch input terminal FRM; frame selection terminal O1 to Ok; output terminal SR1 to SRk; shift register group LATCH1
~LATCHk ;Latch circuit group SELECT1 ~S
ELECTk; Select circuit group OP1 to OPk;
Operational amplifier group T11 to Tk (m/2); transistor switch group VC
LK; clock pulse Vin; image data input voltage VSTB; latch pulse VL1 to VLk; latch signal VO1 to VOk; operational amplifier output voltage O1 to Ok; output terminal 1; transfer gate group 2; transistor switch group

Claims (2)

[Claims] 1. A liquid crystal drive circuit for driving a liquid crystal display, which includes a group of shift registers that transfer image input data based on clock pulses, a latch pulse that is input, and an output signal of the shift register group based on the latch pulse. A latch circuit group that inputs and holds the input, and a select circuit group that has a plurality of output terminals and outputs a switch selection signal to a specific output terminal based on the output of the latch circuit group, have mutually different voltage values m( (m is an integer of 2 or more) types of liquid crystal drive voltages are input, and the m
/a multiplexer that selectively outputs two types of liquid crystal drive voltages; a switch group that includes a plurality of switches and selects the output of the multiplexer based on a switch selection signal output from the selection circuit group; and this switch group. and an amplifier interposed between the liquid crystal display and the liquid crystal display. 2. The liquid crystal drive circuit according to claim 1, wherein the switches of the switch group are constituted by transfer gates consisting of a P-channel transistor and an N-channel transistor.