Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-05-31
2003-09-16
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S204000, C345S212000
Reexamination Certificate
active
06621233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power circuit for driving a liquid crystal display panel, and more particularly to a circuit for quickly lowering the electric potential inside the driving power circuit in a very short period of time after having turned off the power circuit for driving the liquid crystal display panel.
2. Related Art
As well known, the liquid crystal display panel is a device which is generally made up of two electrode plates provided with a plurality of electrodes and a liquid crystal as is put between these two electrode plates, and displays an image on the display panel when a predetermined voltage is applied to the liquid crystal through respective those electrodes. In this specification, the electrode provided on the above two electrode plates and led in the lateral direction is called a common electrode (referred to as ‘COM’ hereinafter) while the electrode provided on the above two electrode plates but led in the longitudinal direction is called a segment electrode (referred to as ‘SEG’ hereinafter). When the potential difference between the COM and the SEG is equal to or larger than a predetermined value, the liquid crystal positioned at the intersection of the COM and the SEG is turned on while the above liquid crystal is turned off (non-lit state) when the above potential difference is smaller than the predetermined value.
FIG. 7
is a diagram showing an example of a wave form describing the relation between the potential difference between the COM and the SEG, and the lights-on/lights-out of the liquid crystal. In this example, the voltage level (electric potential) of the COM or the SEG is in a relation of V
1
>V
2
>V
3
>V
4
>V
5
>VSS (VSS: the ground potential GND), and the liquid crystal at the intersection of the COM and the SEG is set to be in the lights-on state if the potential difference between the COM and the SEG is equal to or higher than |V
1
| while the above liquid crystal is set to be in the lights-out state if the potential difference between the COM and the SEG is lower than |V
1
|.
Up to now, the driving power circuit for the liquid crystal display panel has employed a power down short circuit in order to shorten the fall time of the electric potential inside the driving power circuit after turning off the power source of the liquid crystal display panel.
FIG. 8
is a diagram for explaining the change of respective power source potentials V
1
~V
5
within the driving power circuit when operating the power down short circuit, that is, the change of respective power source potentials V
1
~V
5
from the time of starting the operation of the power down short circuit after turning off the power source of the liquid crystal display panel to the time of ending the operation of the power down short circuit.
The power down short circuit as described above will now be explained with reference to FIG.
9
. In the power down short circuit
50
as shown in
FIG. 9
, in order to detect if the power source of the liquid crystal display panel has been certainly turned off, in other words, that the level of the VDD power source has surely dropped down, a certain arbitrary level difference is generated between nodes B and C by means of condensers C
1
and C
2
for generating the level difference and diodes D
1
and D
2
for generating the level difference. A resistance R
1
provided on the VDD power source line, resistances R
2
~R
6
provided on each of power source lines, and a resistance R
7
provided on the power source line of a level comparator
51
are noise absorption resistances for absorbing noises which might be included in the corresponding power source lines. A logical value of a node A is inverted by means of the level comparator
51
, based on the level difference between the nodes B and C.
That is, when detecting if the level of the VDD power source has dropped down (i.e. when the power source of the liquid crystal display panel is turned off), the level of the node A is inverted from the L-level to the H-level. With this inversion of the node A to the H-level, a plurality of N-channel MOS transistors M
1
~M
5
are turned on, thereby short-circuiting the power lines VL
1
~VL
5
provided corresponding to voltage levels V
1
~V
5
to the ground.
By the way, it is a general way that some external condensers are fitted to the driving power circuit of the liquid crystal display panel for stabilization of the voltage level. Furthermore, in case of making use of the power source lines VL
1
~VL
5
of the above-mentioned driving power circuit for instance, it is needed to keep the relative level height relation existing among the voltage levels of the power source lines unchanged, taking account of the relation of the power sources for the lights-on and the lights-out. Accordingly, when applying the driving power circuit to the liquid crystal display device having the characteristic as shown in
FIG. 7
, it is needed to keep the relative level height relation existing among voltage levels, that is, V
1
>V
2
>V
3
>V
4
>V
5
, unchanged.
As described in the above, the power down short circuit starts its operation as soon as the power circuit for driving the liquid crystal display panel is turned off, thereby short-circuiting the power source lines VL
1
~VL
5
to the ground. However, the voltage levels V
1
~V
5
corresponding to respective power source lines VL
1
~VL
5
can not always fall within an adequately short period of time due to the influence of the external condenser, and this causes such a problem that a certain kind of afterglow comes out on the liquid crystal panel. Furthermore, the operation of the power down short circuit can short-circuit the power source lines VL
1
~VL
5
to the ground, but the falling speed of respective voltage levels can not be always uniform but is apt to become unbalanced. This also causes the problem that a certain kind of afterglow comes out on the liquid crystal display panel.
The present invention has been made in view of the above-mentioned problems as have been experienced so far in connection with the prior art power circuit for driving the liquid crystal display panel. Accordingly, the object of the invention is to provide a novel and improved power circuit for driving the liquid crystal display panel, which is able to prevent any afterglow from coming out on the liquid crystal display panel when the power circuit for driving the liquid crystal display panel is turned off.
SUMMARY OF THE INVENTION
In order to solve the problems as described above, according to the first aspect of the invention, there is provided a power circuit for driving a liquid crystal display panel, which includes a voltage generator for generating a plurality of voltage levels for driving a liquid crystal display panel; a plurality of power source lines provided corresponding to the plurality of voltage levels; a detector for detecting if the liquid crystal display panel has been turned off; a grounding circuit for grounding the plurality of power source lines in response to the detection result obtained by the detector; and a circuit for shorting predetermined power source lines each other in response to the detection result obtained by the detector, wherein the short circuit shorts adjacent power source lines from among the plurality of power source lines each other.
According to the driving power circuit having such a structure as described above, when the detector detects if the liquid crystal display panel has been turned off, a plurality of power source lines are grounded to reduce each electric potential thereof and, at the same time, adjacent power source lines can be short-circuited each other. Consequently, it becomes possible to shorten the fall time of the electric potential of each power source lines. In this specification, the expression “adjacent power source lines” indicate such two power source lines from among a plurality of power source lines that are positioned side by side when arranging all of th
Lee Wilson
Oki Electric Industry Co. Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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