JP2000286707A - Digital/analog converter and liquid crystal display device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the voltage fluctuation of a connection point with simple circuit constitution by injecting current so that voltage at the one connection point of plural resistors connected in series becomes reference voltage and extracting current whose size is similar to said current from the other connection point. SOLUTION: Voltage between power voltages Vdd and Vss is divided by resistors R1-R11 connected in series. Reference voltages V1-V10 are obtained and they are supplied to a digital-analog converter. One connection point voltage in the resistors R1-R11 connected in series, the voltage V1 of high potential, for example, is compared with reference voltage Vref by an amplifier A1. A current source I2 is controlled so that a potential difference becomes zero and the current is injected to the connection point of voltage V1. The current source I1 is connected with a current source I2 through a current mirror circuit and it generates current whose size is similar to current that the current source I2 supplies and extracts it from the other connection point, the connection point of the voltage V10 of low potential, for example. Thus, it compensates the charge/discharge current of parasitic capacity in the circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital-to-analog converter that changes to an analog signal in response to a digital video signal that changes every predetermined period, and a liquid crystal display device using the same.

[0002]

2. Description of the Related Art A general liquid crystal display device is shown in FIG.
As shown in FIG. 5, a display main body 300 in which liquid crystal cells 301 are arranged in a matrix, a plurality of signal lines 304 to which image signals are supplied, and a plurality of scanning lines 305 are arranged to intersect, The liquid crystal display includes a liquid crystal display driving circuit 302 for supplying a signal to a signal line, and a scanning line selection circuit 303 for selectively driving a scanning line.

FIG. 9 shows a configuration example of a display drive circuit in a conventional liquid crystal display device.

As shown in FIG. 1, the display driving circuit has the same number of latches 3022 as the number of pixels required for one horizontal line for storing a digital image signal (hereinafter, RGB signal).
A shift register 3021 for transferring a timing pulse for latching the RGB signal, a latch 3023 for further storing the RGB signal stored in the latch 3022 in a cycle of one horizontal period, and a horizontal line stored in the latch 3023. Digital to convert RGB signals into analog values
-Analog converter (DAC) 3024 and DAC3
In step 024, the RGB signals converted to analog are input, and the signal lines 304 and the liquid crystal cells 30 of the liquid crystal display panel are input.
1 is configured by an amplifier circuit 3025 for driving the driving circuit 1.
The liquid crystal cell 301 to which the RGB signal voltage converted to the analog value is applied determines the brightness of the corresponding pixel by changing the light transmittance according to the voltage value.

A DAC 3024 divides a voltage between a power supply voltage Vdd and Vss by a plurality of resistors R1 to R11 connected in series as shown in FIG.
Generates 10. Further, from among the generated reference voltages, switches SW1 to SW1 are selected by using a selection signal generated by a decoder based on the input digital image signal D-IN.
One is selected by the SW 10 and the selected reference voltage is used as a converted analog signal.

The converted analog signal is applied to a signal line 304 of a liquid crystal display panel and a liquid crystal cell 301 via an amplifier circuit 3025 as shown in FIG.

Here, as shown in FIG.
1 to SW10, the input capacitance Camp of the amplifier circuit 3025, and the parasitic capacitance Cw of the wiring from the switch to the amplifier circuit 3025.

Assuming that the sum of the parasitic capacitances is represented by Cp, at a certain time t1, when the switch SW1 is selected as shown in FIG. 11, Cp is charged and a charge of V1Cp is accumulated in Cp. At the next time t2 (t1 + T, where T indicates the sampling period of the image signal), when the switch SW10 is selected as shown in FIG. 12, Cp is discharged, and the electric charge stored in Cp is V10Cp. Become. Each time this is repeated alternately, (V1-V10) Cp
Charge and discharge are performed, and as shown in FIGS. 13 and 14, the value of the reference voltage changes every time charge and discharge occur.

In a liquid crystal display using a polysilicon thin film transistor having a small on-resistance, the required digital-analog conversion speed is, for example, 200 ns.
13 and 14 because the speed is as fast as c to 350 nsec.
In this case, accurate digital-to-analog conversion cannot be performed due to the temporal change of the reference voltage. Further, even if large capacitances C1 to C10 are added to the respective reference voltage terminals as shown in FIG. 15 to supply charges necessary for charging and discharging, the reference voltage fluctuates with time as shown in FIGS. Problem.

To avoid these problems, FIG.
The reference voltage generated by a plurality of resistors R1 to R11 connected in series as shown in (1) may be used via the buffers B1 to B10. However, the buffer can drive a low output resistance and a large instantaneous current. Since high-speed settling is required, there is a problem that the power consumption becomes extremely high.

[0011]

As described above, in the conventional digital-analog converter, the voltage between the power supply voltage Vdd and Vss is divided by the plurality of resistors R1 to R11 connected in series, and the reference voltage V1 To V10 are generated, and one of the reference voltages is selected by switches SW1 to SW10 selected according to the content of the selection signal obtained by decoding the digital image signal D-IN. It converts to a signal. Therefore, there is a problem that the reference voltage fluctuates with time during high-speed operation. Further, if a buffer is used to avoid this, there is a problem that power consumption increases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art, and has as its object to reduce the fluctuation of a reference voltage with a simple configuration and with a small increase in current consumption. -To provide an analog converter. Another object of the present invention is to provide a liquid crystal display device using a digital-to-analog converter having a small fluctuation of the reference voltage.

[0013]

In order to solve the above-mentioned problems, a digital-to-analog converter according to the present invention selects a voltage between connection points 1 to N of a plurality of resistors connected in series according to a digital signal. In a digital-analog converter that obtains an analog output by performing the above operation, a connection point m (1 ≦ m <N)
Is compared with a predetermined voltage, a current is injected from the connection point m so that the voltage at the connection point m becomes a predetermined voltage, and a current corresponding to the injected current is connected to the connection point k (m <k ≦ N). It is characterized by having control means for pulling out from the.

Further, in a digital-analog converter which obtains an analog output by selecting a voltage at a connection point 1 to N of a plurality of resistors connected in series according to a digital signal, a digital signal at a current time to be inputted is connected to a digital signal. When the absolute value of the difference signal of the digital signal before the sample time is equal to or greater than a predetermined value,
A predetermined current is applied to the connection point m (1 ≦ 1) according to the polarity of the difference signal.
a control means for injecting a predetermined current from the connection point k (m <k ≦ N) or injecting a predetermined current from the connection point k (m <k ≦ N).

By adopting the above configuration, the fluctuation of the reference voltage is detected, and the voltage fluctuation of the reference voltage is reduced by adding a current corresponding to the current flowing by charging / discharging the capacity to be driven by the digital-analog converter. can do.

Further, the power consumption of the current source for correcting the voltage fluctuation and the simple control circuit therefor can be made very small.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a basic configuration of a first embodiment of a digital-analog converter to which the present invention is applied.

In the following description, the digital-to-digital converter of the present invention will be described.
It is assumed that the analog converter is used in the liquid crystal display driving circuit 302 of the liquid crystal display device shown in FIG. 8 as an example.

In FIG. 1, the basic configuration for generating a reference voltage is the same as that shown in FIG. 15, but in the present invention, voltage fluctuation correction control means 1 for correcting voltage fluctuation is further applied. There is a characteristic in the part that did. The voltage fluctuation correction control means 1 includes a current source I2 for injecting a current so that the reference voltage V1 becomes the same as the reference voltage Vref,
A current source I1 for extracting a current corresponding to the injection current from a connection point corresponding to the reference voltage V10, and a reference voltage Vr
It comprises an amplifier A1 for comparing ef with the reference voltage V1 and outputting a signal for controlling the current sources I1 and I2.

In the example of FIG. 1, the highest voltage V1 and the reference voltage Vref among the connection points of a plurality of serially connected resistors for generating a reference voltage are compared, and the result is compared with the level of the output of the amplifier A1. Will be a value corresponding to. When the reference voltage V1 is higher than the reference voltage Vref, the output of the amplifier A1 increases.
Control is performed such that the current flowing through the current sources I1 and I2 becomes zero, while the reference voltage V1 is changed to the reference voltage Vref.
If the output voltage is lower than that, the output of the amplifier A1 becomes lower. Therefore, the current is controlled to flow through the current sources I1 and I2 so that the voltage of the reference voltage V1 becomes equal to the reference voltage Vref.

By detecting the fluctuation of the reference voltage and controlling the currents of the current sources I1 and I2 so as to reduce this voltage fluctuation, the digital-analog converter described with reference to FIGS. Can be supplied from the current sources I1 and I2, and the fluctuation of the reference voltage can be reduced.

FIG. 2 is a diagram showing a specific example of the voltage fluctuation correction control means 1 used in FIG. Amplifier A1 is
It comprises transistors M1 to M4 and a current source Ib1.

The current source I2 comprises a transistor M5 whose drain current is controlled by the output of the amplifier A1, and a current source Ib2 for supplying a bias current.

The current source I2 injects a current corresponding to the current supplied from the current source Ib2 in order to eliminate the fluctuation of the reference voltage V1.

The current source I1 includes a transistor M6 commonly connected to the gate of the transistor M5, transistors M7 and M8 configured as a current mirror, and a current source Ib3 for supplying a bias current.

The current source I1 includes a transistor M6 having a gate commonly connected to the gate of the transistor M5 so as to extract a current corresponding to the current supplied by the current source I2 in order to eliminate the fluctuation of the reference voltage V10. The current flowing in the current source I1 is copied by the
7, and is realized by folding back with a current mirror of M8.

FIGS. 3 and 4 show simulation results (V1_p) performed by the circuits shown in FIGS.
FIG. 18 is a superimposition of a simulation result (V1) of the conventional circuit of FIGS. 16 and 17. As is clear from the simulation results, the conventional circuit fluctuates from a predetermined voltage by several tens of mV, whereas the proposed circuit fluctuates by a few mV.

In the example shown in FIG. 1, the highest voltage V1 among the generated reference voltages is used, but this may be the lowest voltage. Further, as shown in FIG. 5, the same effect can be obtained by using the intermediate voltage V2 for comparison.

The relationship between the position for injecting the current and the position for extracting the current for controlling the fluctuation of the reference voltage is defined as follows.

From the connection points 1 to N of the series-connected resistors, a connection point m, which is a position where current is injected, is 1 ≦ m <N
And the connection point k where the current is drawn is m <k ≦ N
And

FIG. 6 is a diagram showing a configuration of a drive circuit section of a liquid crystal display device using a digital-analog converter according to a second embodiment of the present invention. The difference from the conventional driving circuit section shown in FIG. 9 is that when the difference between the data to be digital-to-analog converted and the previously converted data is equal to or greater than a predetermined value, the digital-to-analog converter is operated in accordance with the polarity of the difference. Control circuit 302 for reducing the fluctuation of the reference voltage
6 is provided.

FIG. 7 shows a control circuit 2036 and a DAC 302.
7 shows the configuration. The control circuit 3026 compares the absolute value of the difference signal between the data (A) to be subjected to the digital-to-analog conversion at the present time and the data (B) to be subjected to the digital-to-analog conversion at the next time with a predetermined threshold value. A latch 71 for sending the result of the judgment by the comparing circuit 70 for one display period, a current source I10 for injecting a predetermined current into the connection point of the reference voltage V1 according to the result of the comparison, And a current source I11 for extracting a predetermined current from the connection point of the reference voltage V10. Also, DAC
Reference numeral 3027 denotes switches SW1 to SW1 for selecting a reference signal.
0 and a decoder for generating a selection signal based on the input digital image signal D-IN.

More specifically, first, in the comparison circuit 70, the data (A) to be digital-analog converted at the current time point
And a difference signal between the data to be subjected to the digital-analog conversion at the next time and the absolute value of the difference signal and a predetermined threshold value (T
H). If it is determined that the absolute value of the difference signal is larger than the threshold value, the signal for enabling the control for suppressing the fluctuation of the reference voltage is changed to the sign of the difference signal for the current source I10 or the current source I11. If the absolute value of the difference signal is determined to be equal to or smaller than the threshold value, the reference voltage change is within the allowable range, and a signal for invalidating the control is set to the current source I10. Output for each of the current sources I11.

The control signal output from the comparison circuit 70 is
One display period is delayed by the latch 71.

The current source I10 or the current source I11 is controlled based on the control signal.

That is, when the reference voltage fluctuates greatly, the control signal becomes valid. According to the control signal, a current corresponding to a current flowing by charging a capacitor attached to the output of the digital-analog converter at the connection point of the reference voltage V1. It supplies ΔI or subtracts ΔI corresponding to the current flowing by discharging the capacitance attached to the output of the digital-analog converter from the reference voltage V10.

In other words, checking the difference between the current data to be subjected to the digital-analog conversion and the previous data is equivalent to checking the difference in the amount of charge charged in the capacity attached to the output of the digital-analog converter. . If the voltage converted to analog in the previous data is low and the voltage converted to analog is high now, supply the current ΔI for charging,
When the voltage converted to analog in the previous data is high and the voltage converted to analog at present is low, the fluctuation of the reference voltage is reduced by subtracting the current ΔI for discharging.

[0038]

As described above, according to the present invention, according to the digital-to-analog converter of the present invention, the capacitance to be driven by the digital-to-analog converter by detecting the fluctuation of the reference voltage. Voltage fluctuation of the reference voltage can be reduced by adding a current corresponding to the current flowing through the charging and discharging. In addition, the power consumption of the current source for voltage fluctuation correction and the simple control circuit therefor can be extremely reduced.

Further, by using the digital-analog converter of the present invention for a driving circuit of a liquid crystal display device, a liquid crystal display device with high image quality can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic configuration of a digital-analog converter according to an embodiment of the present invention.

FIG. 2 is a diagram showing a specific circuit configuration of a voltage fluctuation correction control unit in the embodiment.

FIG. 3 is a diagram showing a waveform of a reference voltage V1 according to the embodiment and a waveform of a reference voltage V1 according to a conventional technique.

FIG. 4 is a diagram showing a waveform of a reference voltage V10 according to the embodiment and a waveform of a reference voltage V10 according to the related art.

FIG. 5 is a diagram showing a configuration of a digital-analog converter according to an embodiment in which a part of the embodiment is modified.

FIG. 6 is a diagram showing a configuration of a drive circuit unit of a liquid crystal display device using a digital-analog converter according to another embodiment of the present invention.

FIG. 7 is a diagram showing a specific example of a voltage fluctuation correction control circuit of a digital-analog converter according to another embodiment.

FIG. 8 is a diagram showing a configuration of a general liquid crystal display device.

FIG. 9 is a diagram showing a configuration of a liquid crystal display driving circuit of a general liquid crystal display device.

FIG. 10 is a diagram showing a configuration of a conventional digital-analog converter.

FIG. 11 is a diagram illustrating voltage fluctuation caused by the operation of the digital-analog converter.

FIG. 12 is a diagram illustrating voltage fluctuation caused by the operation of a digital-analog converter.

FIG. 13 is a diagram showing a waveform of a reference signal V10 in a conventional digital-analog converter.

FIG. 14 is a diagram showing a waveform of a reference signal V1 in a conventional digital-analog converter.

FIG. 15 is a diagram showing a configuration of another conventional digital-analog converter.

FIG. 16 is a diagram showing a waveform of a reference signal V1 in another conventional digital-analog converter.

FIG. 17 is a diagram showing a waveform of a reference signal V10 in another conventional digital-analog converter.

FIG. 18 is a diagram showing a configuration of still another conventional digital-analog converter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Voltage fluctuation correction control means 2 ... Input amplification stage 3 ... Output amplification stage 4 ... Resistance circuit 300 ... Liquid crystal display 301 ... Liquid crystal cell 302 ... Liquid crystal display drive circuit 303 ... Scan line selection circuit 304 ... Signal line 305 ... Scan line A1 ... Amplifier 3021 ... Shift register 3022, 3023, 71 ... Latch 3025 ... Amplification circuit 3026 ... Control circuit 3024, 3027 ... DAC 70 ... Comparison circuit B1-B10 ... Buffer M1-M8 ... Transistor V1-V10 ... Selected by digital signal Vref: Reference voltage I1, I2, I10, I11, Ib1, Ib2, Ib3
... Current source Vdd ... First power supply potential point Vss ... Second power supply potential point C1-C1 ... Capacitance R1-R10 ... Resistance

Claims (6)

[Claims] 1. A digital-analog converter which obtains an analog output by selecting voltages at connection points 1 to N of a plurality of resistors connected in series according to a digital signal, wherein a connection point m (1 ≦ m <N ) Is compared to a given voltage,
A current is injected from the connection point m so that the voltage at the connection point m becomes a predetermined voltage, and a current corresponding to the injected current is connected to the connection point k.
A digital-to-analog converter having control means for extracting from (m <k ≦ N). 2. A digital-to-analog converter which obtains an analog output by selecting voltages at connection points 1 to N of a plurality of series-connected resistors according to a digital signal. When the absolute value of the difference signal of the digital signal before the sampling time is equal to or greater than a predetermined value, a predetermined current is applied to the connection point m (1 ≦ m <) according to the polarity of the difference signal.
A digital-to-analog converter comprising control means for injecting the current into N) or extracting a predetermined current from the connection point k (m <k ≦ N). 3. A liquid crystal display in which a plurality of pixels, a signal line for selectively supplying an image signal to each pixel, and a scanning line intersecting with the signal line are formed. In a liquid crystal display device having a driving circuit for supplying an image signal and a selection circuit for selecting a predetermined scanning line, the driving circuit includes an input terminal, an output terminal, and a digital image signal input from the input terminal. And a digital-to-analog converter that outputs an analog image signal obtained by digital-to-analog conversion to an output terminal, wherein the digital-to-analog converter has a voltage between connection points 1 to N of a plurality of resistors connected in series. Is selected in accordance with the digital image signal to obtain an analog image signal. The digital-analog converter includes a connection point m (1
≦ m <N) is compared with a predetermined voltage, a current is injected from the connection point m so that the voltage at the connection point m becomes the predetermined voltage, and a current corresponding to the injected current is connected to the connection point k (m <K ≦
N) A liquid crystal display device comprising a control means for extracting from a liquid crystal display. 4. A liquid crystal display in which a plurality of pixels, a signal line for selectively supplying an image signal to each pixel, and a scanning line crossing the signal line are arranged, and In a liquid crystal display device having a driving circuit for supplying an image signal and a selection circuit for selecting a predetermined scanning line, the driving circuit includes an input terminal, an output terminal, and a digital image signal input from the input terminal. And a digital-to-analog converter that outputs an analog image signal obtained by digital-to-analog conversion to an output terminal, wherein the digital-to-analog converter has a voltage between connection points 1 to N of a plurality of resistors connected in series. Is selected according to the digital image signal to obtain an analog image signal. The digital-analog converter converts the input digital signal at the current time into When the absolute value of the difference signal of the digital signal one sample time ago is equal to or more than a predetermined value, a predetermined current is injected into the connection point m (1 ≦ m <N) or the connection point k according to the polarity of the difference signal. A liquid crystal display device comprising a control unit for extracting a predetermined current from (m <k ≦ N). 5. A digital-to-analog converter for obtaining an analog output by selecting voltages at connection points 1 to N of a plurality of resistors connected in series according to a digital signal, wherein a connection point m (1 ≦ m <N A) for comparing a voltage with a reference voltage, and a first means for injecting a current so that the voltage at the connection point m becomes equal to the reference voltage according to the comparison result of the comparison means.
And a second current source for extracting a current corresponding to the injected current from a connection point k (m <k ≦ N) according to a comparison result by the comparing unit. Digital
Analog converter. 6. A digital-to-analog converter which obtains an analog output by selecting voltages at connection points 1 to N of a plurality of series-connected resistors according to a digital signal. Comparing means for obtaining a difference signal of the digital signal before the sample time, comparing the absolute value of the difference signal with a predetermined threshold value, and outputting a control signal for controlling voltage fluctuation according to the comparison result; A first current source for injecting a current so that the voltage at the connection point m becomes the same as a reference voltage according to a comparison result by the comparison unit; and a first current source for injecting a current according to the comparison result by the comparison unit. A second current source for extracting a current corresponding to the current from a connection point k (m <k ≦ N).
Analog converter.