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Temperature Considerations
Extreme temperatures cause a number of difficulties to LCDs. Generally, Kyocera color
STN LCDs are specified for either standard or wide temperature ranges.
Typical Temperature Specs
| LCD Family |
Item |
Celsius |
Fahrenheit |
| Standard STN |
Operating Range |
0 to 50/60 |
32 to 122/140 |
| Standard STN |
Storage Range |
-20 to 60 |
-4 to 140 |
| Standard TFT |
Operating Range |
-10 to 70 |
14 to 158 |
| Standard TFT |
Storage Range |
-20/-30 to 80 |
-4/-22 to 140 |
| Wide Temp STN |
Operating Range |
-20 to 70 |
-4 to 158 |
| Wide Temp STN |
Storage Range |
-25 to 75 |
-13 to 167 |
CFL Effects
At low temperatures, more voltage is required to start a CFL tube. The Backlight
Characteristics section of the LCD specification will give the maximum
no-load voltages which might be required to start the CFL at 25°C and at
the lower operating temperature limit. An inverter should be selected which
provides a 30 per cent margin over the voltage required at the lowest operating
temperature specified for the product containing the LCD. In other words the minimum
open circuit voltage of the inverter should be 30 per cent greater than the maximum
voltage specified for the LCD backlight.
Optical Effects
LCD subpixels switch between light-transmitting and light-blocking states because
the liquid crystal molecules diffuse from one orientation to another. This diffusion
is a strong function of temperature, so the response time is also a strong function
of temperature.
KHS072VG1MB-L99 Temperature Sensitivity
| Temperature |
Time to 90% light |
Time to 90% dark |
Contrast ratio |
| -20°C (-4°F) |
3130 ms |
2100 ms |
3 |
| 25°C (77°F) |
200 ms |
150 ms |
10 |
| 70°C (158°F) |
70 ms |
40 ms |
3 |
Liquid Crystal Material Effects
The liquid crystal material can be permanently damaged by very low temperatures.
Bubbles can form. Molecules can aggregate into solid crystals. Either of these will
disrupt the carefully twisted order of liquid crystal molecules when they are
transmitting light, resulting in a permanent optical defect.
STN Driving Voltage Effects
There is an optimal voltage (Vop) where the maximum contrast ratio is achieved.
In product development environments there is usually an adjustment and we
think of ourselves as controlling Vop. But the reality is that Vop is a function
of temperature and design, and the adjustment is merely being used to find Vop.
The adjusting voltage can be either the full driving voltage VEE or a contrast
adjusting voltage VCONT which is boosted internally to the full driving voltage.
The data below shows the temperature dependency of VEE for one model. The shape
of the curve, with the inflection around 20°C, is typical of all Kyocera
STN LCDs.
For both standard temperature and wide temperature models, the LCD specifications
give values for the operating extremes and for 25°C. The new KCG047QV and
KCG062HV LCD models contain additional temperature compensation circuits which
greatly reduce the variability in VCONT. On other models, the customer should
provide manual adjustment or external temperature compensation. Automatic
temperature compensation designs still should provide a calibration feature because
the typical VEE or VCONT also varies with minute differences in the LCD subpixel
cell thickness.
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