Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-03-18
2002-05-07
Lao, Lun-Yi (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S095000, C345S691000
Reexamination Certificate
active
06384806
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention is directed to a digital driver circuit suitably used to drive an electro-optical device, such as a liquid-crystal device, using a TFT active matrix drive scheme, an electro-optical device having the digital driver circuit, and an electronic apparatus having the electro-optical device. More particularly, the invention is directed to a digital driver circuit or the like which receives a digital image signal as an input and uses multi-ramp waves to generate an analog drive signal.
2. Description of Related Art
Digital driver circuits which receive a digital image signal as an input and drive a display panel such as a liquid-crystal panel, to create a gradation display are known. For example, a digital driver circuit including an SC-DAC (Switched Capacitor—Digital to Analog Converter) circuit for selectively performing charge sharing or charge pumping of charges that accumulate in a plurality of capacitors having different capacities by a switching element depending on a digital image signal to generate plural voltage levels have been used. The SC-DAC circuit outputs plural voltage levels to a signal line of the display panel as drive signals corresponding to respective gradation levels, so that a gradation display can be realized. Typically, the digital driver circuit of the form including the SC-DAC circuit is externally connected to a display panel.
As another example of a digital driver circuit for driving a display panel such that a gradation display can be generated, a digital driver circuit of a form including a serial divided-voltage resistor circuit is disclosed in Japanese Unexamined Patent Publication No. 9-54309. In this form, a serial divided-voltage resistor circuit divides a plurality of reference voltages depending on a digital image signal to generate plural voltage levels, and the voltages are output to the signal line of a display panel as drive signals corresponding to respective gradation levels, so that a gradation display can be created.
In addition, as another example of a digital driver circuit for driving a display panel such that a gradation display can be generated, a digital driver circuit having a form including a PWM (pulse width modulation) circuit and using a ramp wave (saw-tooth wave) voltage is disclosed in Japanese Unexamined Patent Publication No. 9-244588. In this formation, the digital image signal is subjected to pulse width modulation by the PWM circuit to generate pulse signals having pulse widths corresponding to the digital image signals. A ramp wave is selected on a time axis depending on the pulse width to generate plural voltage levels, and the voltages are output to the signal line of the display panel as drive signals corresponding to respective gradation levels, so that a gradation display can be created.
In a digital driver circuit of this type, a general demand for a simplified circuit arrangement or for low power consumption is strong, and at the same time, a demand for drive performance to be high even with an increase of the size of a display panel. In particular, &ggr;-correction required depending on a non-linear gradation characteristic for drive signal voltages in a display panel, such as a liquid-crystal panel, must be accurately performed by a circuit arrangement that is as simple as possible.
However, according to a digital driver circuit of a form including the conventional SC-DAC circuit, in order to make drive performance high, a large-capacity capacitor is required. For this reason, for example, the digital driver circuit is actually limited to driving a liquid-crystal panel having a size of about 5″ in diagonal length. More specifically, it is difficult for the digital driver circuit to drive a display panel, such as a liquid-crystal panel, having a size larger than the above size. In particular, in a display panel having a digital driver circuit built therein, this formation in which a large capacitor must be formed on a substrate is not appropriate from the viewpoint of total circuit area or pixel pitch.
According to the conventional digital driver circuit including a serial divided-voltage resistor circuit, in order to improve drive performance, power consumption in each resistor inevitably increases with an increase in current. For this reason, the digital driver circuit cannot have a low power consumption. At the same time, a switching element, such as a thin film transistor, for performing switching control of the resistors must be increased in size to improve drive performance, and the area of the entire circuit increases. In particular, in a display panel having a digital driver circuit being built therein, a large number of resistors and large-size thin film transistors or the like must be formed on a substrate, which is not appropriate from the viewpoint of circuit area or pixel pitch.
In addition, according to the conventional digital driver circuit of a form including a PWM circuit, control of the voltage of a ramp wave with respect to time must be extremely accurately performed to correctly generate a gradation display. Therefore, an amplifier for supplying a ramp wave requires high performance, must be able to rapidly saturate a voltage for a signal line at a correct timing depending on a pulse signal, and also must create a highly accurate waveform for the ramp wave. Practically, it is very difficult to realize this type of circuit. Since a large-power ramp wave must be input at a low output impedance to improve drive performance, a problem that power consumption in the digital driver circuit is considerably high occurs. In particular, when y-correction for a digital image signal is required, problems can occur. More specifically, when as a scheme of &ggr;-correction, any one of (i) a scheme in which the duty of a PWM base clock is changed for a gradation level depending on the characteristics of a display panel, (ii) a scheme in which a ramp wave for a time axis is changed into an S shape depending on the characteristics of a display panel, and (iii) a scheme in which a pseudo-S-shaped ramp waveform depending on the characteristics of a display panel is formed by a voltage which exactly gradually changes is selected, a voltage must be controlled at an accuracy that is higher than that used when &ggr;-correction is not performed. Therefore, it is practically almost impossible that voltages for driving a plurality of signal lines are. assured by the digital driver circuit of this formation. For this reason, the digital driver circuit of this formation is not typically used.
The present invention has been made in consideration of the problems described above, and has as its problem to provide a digital driver circuit having relatively low power consumption and relatively high performance, an electro-optical device having the digital driver circuit, and an electronic apparatus having the electro-optical device.
SUMMARY OF THE INVENTION
The invention provides a digital driver circuit that receives a digital image signal of n (n is a natural number not less than 2) bits as an input and generates an analog drive signal corresponding to the digital image signal to output to a signal line of an electro-optical device. The digital driver circuit includes a series selection circuit that selects one series from plural series of standard multi-ramp waves having voltages which change in steps with the passage of time depending on the value of y (y is a natural number) bits of the n bits. A time selection circuit selects, on a time axis, a voltage which changes in steps in at least the selected series of standard multi-ramp waves depending on the value of x (x is a natural number) bits whose bit position is higher than that of the y bits of the n bits. The drive signal is output based on the selected voltage in the selected series.
One series is selected from the plural series of standard multi-ramp waves depending on the value of the y bits (e.g., medium or least significant three bits, four bits, or the like) of the n bits (e.g., six
Matsueda Yojiro
Quinn Michael J.
Lao Lun-Yi
Oliff & Berridg,e PLC
Seiko Epson Corporation
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