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Power Consumption
Low power consumption is a big advantage of STN LCDs, when compared to TFT LCDs. This is especially significant in the handheld instrument marketplace. The Kyocera 3.8 inch color LCDs make excellent examples to study because all required voltages are supplied externally.
KCS038AA1AJ-G21 Power Summary
| Description |
Name |
Voltage |
Current |
Power |
Pwr % |
| Logic |
VDD |
3.3 V |
0.8 mA |
2.64 mW |
0.4 % |
| LCD Driving |
V0 ~ V5 |
30.5 V |
2.4 mA |
70.8 mW |
9.7 % |
| CFL Backlight |
VL |
270 V |
2.5 mA |
675 mW |
89.9 % |
In the simplest Kyocera LCDs like the KCS038AA, the driving voltage is provided as 3 voltage differences (V0-V3, V1-V4, V2-V5) biased slightly apart. When a bias voltage circuit is added to the LCD (for example KCS057QV1AJ), these inputs are replaced by one input voltage named VEE. The purpose of the LCD driving voltage is to provide a field to disrupt the liquid crystal molecules from their light transmitting orientation. The amount of power required to drive the liquid crystal material depends on how many subpixels must be disrupted. Typical LCD drive power is defined as the power required to disrupt half the subpixels. To the extent that energy is lost in the drive transistors, logic power is also affected by the display pattern, with "typical" defined in the same way.
Highly integrated Kyocera LCDs (for example KCS072VG1MB) also have DC-DC converter circuits that generate the LCD drive voltage internally. The user must provide an adjustable contrast voltage VCONT instead of the full VEE. Current flow through VCONT is virtually nil. The LCD drive energy then comes through the logic voltage VDD, with a corresponding increase in the logic current IDD. These additional bias voltage and DC-DC converter circuits are indicated by name in section 2 of the specifications and shown as blocks in the block diagram in section 7.
Clearly, the CFL backlight consumes much more power than the other two functions. Generally, whatever the lighting method, it will consume much more power than the LCD drive and logic. The chart below shows power consumption by the various lighting subsystems available on various models of the 3.8 inch LCD.
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