Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-06-11
2004-06-22
Chow, Dennis-Doon (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S098000, C349S040000, C361S018000, C361S020000, C361S021000, C361S824000, C257S355000, C257S356000
Reexamination Certificate
active
06753836
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to liquid crystal devices and, in particular, to a liquid crystal device driver circuit for electrostatic discharge protection.
2. Description of Related Art
In general, a liquid crystal device (hereinafter referred to as “LCD”) driver circuit or an integrated circuit (hereinafter referred to as “IC”) drives a high-level LCD voltage (VLCD) to display information on an LCD panel. Here, the LCD voltage (VLCD) can be externally applied and internally generated using an analog circuit such as an internal charge pump, an operational amplifier, or a band gap circuit. The VLCD is an important factor of the picture quality of an LCD screen.
However, internal circuits in an LCD driver circuit can be damaged by an electrostatic discharge (hereinafter referred to as “ESD”) phenomenon occurring in a voltage input port or a voltage output port. Thus, most semiconductor devices as well as the LCD driver circuit include devices for ESD protection on an input port or output port to protect the semiconductor devices from damage by the ESD phenomenon.
FIG. 1
is a circuit diagram of a conventional LCD driver circuit for ESD protection. The circuit shown in
FIG. 1
is an example of a conventional driver circuit applied in a monochrome LCD and includes an input pad
10
, a resistor R
1
, an ESD protection unit
12
, a voltage generating unit
14
, and an LCD output driver
16
.
In the circuit shown in
FIG. 1
, LCD voltages (VLCDs) V
1
through V
5
are externally applied through each input pad, and high-level voltage is divided by the voltage generating unit
14
to generate the VLCDs V
1
through V
5
. Although not specifically shown, second through fifth voltages V
2
through V
5
can be applied to the LCD output driver
16
by the same method as that used for a first voltage V
1
. During a normal operation, the ESD protection unit
12
does not operate. However, when an ESD pulse is applied through the input pad
10
, the serial resistor R
1
and a first protection device D
1
or a second protection device D
2
are turned on to form a discharge path for discharging a high current of the ESD pulse. Here, the high current of the ESD pulse is lowered by the serial resistor R
1
connected to the input pad
10
, to protect the internal circuits.
However, the amount of change of the LCD voltages (VLCDs) in the LCD driver circuit for driving a color LCD other than the monochrome LCD is strictly stipulated in its design specification. For example, under specific test conditions, when a difference between a current flowing into the pad
10
to which the LCD voltages (VLCDS) are input and a current flowing into the internal voltage generating unit
14
is 10 uA, the amount of change of the VLCDs is less than 10 mV. Thus, in the color LCD driver circuit, other than the circuit of
FIG. 1
, a serial resistor which is a main factor of voltage drop cannot be connected between an input pad and a voltage generating unit. As a result, the high current of the ESD pulse is transferred to the output driver
16
and the voltage generating unit
14
, thereby causing physical damage. That is, when the ESD pulse with positive polarity or negative polarity is applied, first discharge is performed by the first and second protection devices D
1
and D
2
of the ESD protection unit
12
adjacent to the pad
10
, and a remaining current is applied to the LCD output driver
16
.
FIG. 2
a circuit diagram of an output driver applied in a conventional color LCD driver circuit. Each voltage transferring device to which VLCDs V
1
through V
3
having relatively high-voltage levels are transferred, is implemented by CMOS transfer gates TG
21
through TG
23
. The transferring devices for transferring VLCDs V
4
and V
5
having low voltage levels are implemented by NMOS transistors MN
21
and MN
22
. Also, an ESD protection unit
25
is provided to protect internal circuits from an ESD pulse applied through an output pad
22
. The output driver of the color LCD driver circuit is designed to satisfy on-resistance according to its design specification. In other words, on-resistance of each of the transfer gates TG
21
through TG
23
and the NMOS transistors MN
21
and MN
22
is decided in proportion to the VLCDs V
1
through V
5
. Thus, desired on-resistance for driving the VLCDs V
4
and V
5
having low voltage levels is obtained only by the NMOS transistors MN
21
and MN
22
having a small width.
However, in the case of using the NMOS transistors, there is no forward discharge path when the ESD pulse with positive polarity is applied. Also, since the discharge area is very small, the discharge capability is very weak.
Additionally, in the conventional LCD driver circuit, since the discharge efficiency of the protection devices (for example, D
1
and D
2
of
FIG. 1
) connected to the input pads is very low, ESD protection can be deteriorated. That is, since the VLCD voltages are higher than an operating voltage of any other circuit in the LCD driver, the ESD protection unit
12
of
FIG. 1
is formed of a high voltage junction. However, since the operating voltage is high in the high voltage junction, a high current is not driven. Thus, in a case where the high current due to the ESD pulse is applied, the ESD protection can be deteriorated.
SUMMARY OF THE INVENTION
To solve the above and other related problems of the prior art, there is provided a liquid crystal device (LCD) driver circuit for electrostatic discharge protection. The LCD driver circuit is capable of preventing an output driver from being damaged by an ESD pulse in a color LCD driver circuit, and improves the efficiency of protecting against electrostatic discharge.
According to an aspect of the invention, there is provided a liquid crystal device (LCD) driver circuit. The LCD driver circuit includes first through N-th input pads for respectively receiving first through N-th voltages. The first through N-th voltages have different voltage levels and are externally applied to the LCD driver circuit. N is an integer greater than one. First through N-th electrostatic discharge (ESD) protection units are respectively connected to the first through N-th input pads, and form a discharge path when an electrostatic pulse is respectively applied through any of the first through N-th input pads. An output driver has first through N-th resistors. The first through N-th resistors respectively receive the first through N-th voltages input through the first through N-th input pads. The output driver generates a driving voltage for driving an LCD from each of the first through N-th voltages received through the first through N-th resistors, respectively. The first through N-th resistors reduce a current flowing into the output driver when the electrostatic pulse is applied.
According to another aspect of the invention, there is provided a liquid crystal device (LCD) driver circuit. The LCD driver circuit includes first through N-th input pads for respectively receiving first through N-th voltages. The first through N-th voltages have different voltage levels and are externally applied to the LCD driver circuit. N is an integer greater than one. First through N-th electrostatic discharge (ESD) protection units are respectively connected to the first through N-th input pads, and form a discharge path when an electrostatic pulse is respectively applied through any of the first through N-th input pads. An output driver has first through N-th voltage transferring means. The first through N-th voltage transferring means respectively transfer the first through N-th voltages input through the first through N-th input pads, respectively. The output driver generates a driving voltage for driving an LCD from each of the first through N-th voltages transmitted through the first through N-th voltage transferring means, respectively. At least one voltage transferring means of the first through N-th voltage transferring means transfers low-level voltages of the first through N-th voltages and has a paralle
Chow Dennis-Doon
Moyer Michael J.
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