Method for driving an STN liquid crystal display device

A method for driving an STN liquid crystal display device, which can prevent a printed phenomenon of display on an image plane. In this method, when a display time of a fixed pattern exceeds a set time, ON and OFF of the fixed pattern is forcibly reversed to momentarily provide an inverted pattern, after which the display is returned to the original fixed pattern.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for driving an STN liquid crystal 
display device, and particularly to a driving method for preventing a 
printed phenomenon of a display on an image plane. 
2. Description of the Prior Art 
When a fixed image pattern such as a figure, a character, a picture, etc. 
is continuously displayed on a STN (Super Twisted Nematic) liquid crystal 
display device for a long period of time, a so-called printed phenomenon 
occurs in which even if a display is changed, a pattern having been 
displayed for a long period of time remains visible without completely 
disappearing. This phenomenon conspicuously appears particularly at a high 
temperature. 
For this reason, when the device is used under severe conditions such as in 
motor vehicles, the display printed phenomenon comes into question. It has 
been desired to develop countermeasures to this phenomenon. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method for 
driving an STN liquid crystal display device which can prevent a display 
printed phenomenon. 
According to one feature of the present invention, when a display time of a 
fixed pattern exceeds a set time, ON and OFF of the fixed pattern is 
forcibly reversed to momentarily provide an inverted pattern, after which 
the display is returned to the original fixed pattern. A printed 
phenomenon of a display is prevented, and the quality of the display and 
the contrast are enhanced.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the case where a display pattern A as shown in FIG. 1(A) is fixedly 
displayed on an STN liquid crystal display device for a predetermined 
period of time or more, when the time exceeds a set time (within the 
display time), the pattern A is momentarily reversed to an image pattern 
(an inverted pattern) B as shown in FIG. 1(B). That is, in the fixed 
display image pattern of FIG. 1(A), the dots of ON and the dots of OFF are 
turned OFF and ON, respectively. After a given period of time, for example 
one second, the display is returned to the original fixed display image 
pattern of FIG. 1(A). 
In this case, a relationship between an ambient temperature of the STN 
liquid crystal display device used and a fixed limit time at which 
printing does not occur (FIG. 2) is obtained in advance, and control is 
made on the basis of the aforesaid relationship. 
In FIG. 2, the horizontal axis shows an ambient temperature and the 
vertical axis shows a fixed limit time at which burn in (or printing) 
occurs. Namely, by use of FIG. 2, one can read a time and an ambient 
temperature at which burn in does not occur. The limit time becomes 
shorter as the ambient temperature becomes higher. Accordingly, FIG. 2 
shows that the set time should be less than the limit time, i.e., less 
than the value of the curve of the graph line. 
FIG. 3 is a control block diagram, in which reference numeral 1 designates 
an ambient temperature sensor; 2 a timer; and 3 a control circuit. Data of 
temperature and set time are set in the control circuit 3 so that when a 
limit time at a certain temperature is reached, the display data is 
immediately inverted. The display of the inverted pattern B (FIG. 1(B)) is 
set to approximately one second, and the inverted pattern B is immediately 
returned to the original fixed display image pattern A (FIG. 1(A)). 
In the case where a fixed image pattern is displayed for a long period of 
time, when an inverted pattern is momentarily displayed every set time, 
the printed phenomenon of display is prevented when the inverted pattern 
is changed into the other or original display pattern. Thereby, the 
quality of display and the contrast are improved, and an erroneous display 
is prevented. 
As described above, according to the present invention, in the case where 
one and the same pattern continues to be displayed on the STN liquid 
crystal device for a long period of time, a display of an inverted pattern 
is introduced every set time. Therefore, the improvement in the quality of 
the display and the contrast can be expected, and the erroneous display 
can be effectively prevented.