Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
2000-05-09
2003-07-08
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S061000, C345S062000, C345S098000, C345S099000, C345S100000, C345S211000, C345S212000, C257S392000, C257S393000, C327S063000, C327S065000, C365S185220, C365S185260, C365S185230
Reexamination Certificate
active
06590570
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a comparator, a display apparatus which uses the comparator for a driving system and a driving method for the comparator, and more particularly to a comparator of the boosting type which outputs, as a comparison output signal, a signal of a higher level than that of a comparison input signal and a display apparatus as represented by a liquid crystal display apparatus or an EL (electroluminescence) display apparatus which uses the comparator as part of a circuit which forms a driving system as well as a driving method for the comparator.
As the process generation of the MOS LSI proceeds and the power supply voltage for a MOS LSI drops, also the output voltage of the LSI drops. As an example, if a peripheral circuit of a liquid crystal display apparatus is incorporated into an LSI, then the level of a driving pulse or display data supplied from the peripheral circuit in the form of an LSI to a driving system of the liquid crystal display apparatus becomes lower. However, in order to satisfy a demand for the picture quality of a liquid crystal display apparatus, it is necessary, under the present conditions, to apply a voltage of approximately 5 V to the liquid crystal.
Meanwhile, in a liquid crystal display apparatus which uses a TFT (thin film transistor) as a switching element for a pixel or the like, in order to apply 5 V to the liquid crystal, a driving voltage of approximately 10 V is required for a driver, a pixel transistor or the like formed from a TFT from characteristics of the TFT. In other words, it is necessary to mount a level shifter, a comparator or the like (conversion into an on-panel device) for converting display data of a small amplitude from the outside into display data of a TFT operation level on a liquid crystal panel (transparent insulating plate on which a pixel section is formed).
However, where a level shifter, a comparator or the like is formed as an on-panel device, there is a problem that characteristics of adjacent or neighboring transistors exhibit some dispersion, which is particularly remarkable with the TFT. Due to such dispersion, it is necessary to make the amplitude of an input signal greater than a certain level.
Meanwhile, reduction of power consumption is required particularly for a liquid crystal panel used for a portable apparatus. If an ordinary level shifter is mounted on a liquid crystal panel, then since it consumes much power, it is necessary also to take a countermeasure for reduction of power consumption. For the level shifter, for example, a comparator of the boosting type is used.
A comparator is a circuit which compares two or more input signals and outputs a comparison result of a high level (hereinafter referred to as “H” level) or a low level (hereinafter referred to as “L” level) in response to a relationship in magnitude among signal levels of the input signals. Usually, at least two transistors are used to compare input signal levels. The circuit system in this instance basically presupposes that characteristics of adjacent transistors such as the threshold voltage Vth, the drain-source current Ids and so forth are almost same. Actually, however, there is some difference between characteristics of adjacent transistors, and from this reason, the minimum comparison voltage is set to a comparatively high value.
FIG. 13
shows a comparator of the boosting type as a related art. Referring to
FIG. 13
, the comparator shown includes an N-channel MOS transistor Q
101
and a P-channel MOS transistor Q
102
connected in series between the ground and a power supply Vdd, and an N-channel MOS transistor Q
103
and a P-channel MOS transistor Q
104
connected in series between the ground and the power supply Vdd similarly. The gate electrode of the MOS transistor Q
102
is connected to a drain common junction S between the MOS transistors Q
103
and Q
104
, and the gate electrode of the MOS transistor Q
104
is connected to a drain common junction T between the MOS transistors Q
101
and Q
102
.
In the comparator described above, comparison input data Vdata is applied to the gate electrode of the MOS transistor Q
101
through a switch SW
101
, and a comparison reference level Vref is applied to the gate electrode of the MOS transistor Q
103
through another switch SW
102
. The switches S
1
WO
1
and SW
102
are driven to be switched between on (closed) and off (open) in response to the same timing pulse signal &phgr;s. A comparison output Vout is extracted from the drain common junction S of the MOS transistors Q
103
and Q
104
.
FIG. 14
illustrates a relationship in timing of the timing pulse signal &phgr;s, comparison input data Vin and comparison output data Vout.
In the comparator described above, if there is a difference in characteristic between the N-channel MOS transistor Q
101
and the N-channel MOS transistor Q
103
, then when the level difference between the two input signals, that is, the comparison input data Vdata and the comparison reference level Vref, is small, a comparation error occurs. Further, since normally an intermediate value between the “H” level and the “L” level of the comparison input data Vdata is set as the comparison reference level Vref, through current flows through the MOS transistors Q
101
and Q
103
. Accordingly, where a comparator of the boosting type is used for a driving system of a liquid crystal display apparatus, a number of such comparators equal to the number of bits of display data are provided for each of pixels in each horizontal row. Consequently, power is consumed as much.
As an example, where 2-bit data is used, if the number of pixels in each horizontal row is 100, then 200 (100×2) comparators are required, and in a color system, since data for three colors of R (red), G (green) and B (blue) are required for each dot, totaling 600 comparators are required. In other words, as the number of bits of display data increases, also the number of comparators increases as much, and accordingly, the power consumption increases. It is to be noted that, if a single comparator substitutes them, since the single comparator drives a great capacity, the power consumption increases.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a comparator which can operate stably against an absolute value distribution of a threshold voltage among MOS transistors and has a wide allowable range against the threshold voltage dispersion and besides allows reduction in power consumption and a display apparatus which uses the comparator for a driving system.
In order to attain the object described above, according to an aspect of the present invention, there is provided a comparator, including a single MOS transistor, inputting means for inputting a comparison reference signal and a comparison input signal in a time series to a source electrode of the MOS transistor, current supplying means connected between a power supply and a drain electrode of the MOS transistor for supplying very small current to the drain electrode, voltage holding means connected between a gate electrode of the MOS transistor and a dc potential point, and switch means connected between the gate electrode and the drain electrode of the MOS transistor.
According to another aspect of the present invention, there is provided a display apparatus, comprising a driving system including the comparator described above.
According to a further aspect of the present invention, there is provided a driving method for a comparator which includes a single MOS transistor, current supplying means connected between a power supply and a drain electrode of the MOS transistor for supplying very small current to the drain electrode, voltage holding means connected between a gate electrode of the MOS transistor and a dc potential point, and switch means connected between the gate electrode and the drain electrode of the MOS transistor, the driving method including the steps of putting the switch means into an on state to short-circuit the gate electrode and the drain electrod
Abdulselam Abbas
Hjerpe Richard
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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